| category | word | Define |
|---|---|---|
| Drilling | exit velocity | The speed the drilling fluid attains when accelerated through bit nozzles. The exit velocity is typically in the low-hundreds of feet per second. It has been reported that in certain shaly formations, an impingement velocity on the order of 250 feet per second is required to effectively remove newly created rock chips from the bottom of the hole. This impingement velocity is not, however, the same as the exit velocity, since the high-energy fluid jet loses velocity through viscous losses and conversions from kinetic energy to forms of potential energy occur once the fluid leaves the bit. For this reason, the well designer generally seeks to maximize the fluid velocity (or other measure of jet energy) to achieve maximum cleaning at the bottom of the hole. |
| Drilling | adjustable choke | A valve usually used in well control operations to reduce the pressure of a fluid from high pressure in the closed wellbore to atmospheric pressure. It may be adjusted (opened or closed) to closely control the pressure drop. Adjustable choke valves are constructed to resist wear while high-velocity, solids-laden fluids are flowing by the restricting or sealing elements. |
| Drilling | air drill | To drill using gases (typically compressed air or nitrogen) to cool the drill bit and lift cuttings out of the wellbore, instead of the more conventional use of liquids. The advantages of air drilling are that it is usually much faster than drilling with liquids and it may eliminate lost circulation problems. The disadvantages are the inability to control the influx of formation fluid into the wellbore and the destabilization of the borehole wall in the absence of the wellbore pressure typically provided by liquids. |
| Drilling | air drilling | A drilling technique whereby gases (typically compressed air or nitrogen) are used to cool the drill bit and lift cuttings out of the wellbore, instead of the more conventional use of liquids. The advantages of air drilling are that it is usually much faster than drilling with liquids and it may eliminate lost circulation problems. The disadvantages are the inability to control the influx of formation fluid into the wellbore and the destabilization of the borehole wall in the absence of the wellbore pressure typically provided by liquids. |
| Drilling | annular blowout preventer | A large valve used to control wellbore fluids. In this type of valve, the sealing element resembles a large rubber doughnut that is mechanically squeezed inward to seal on either pipe (drill collar, drillpipe, casing, or tubing) or the openhole. The ability to seal on a variety of pipe sizes is one advantage the annular blowout preventer has over the ram blowout preventer. Most blowout preventer (BOP) stacks contain at least one annular BOP at the top of the BOP stack, and one or more ram-type preventers below. While not considered as reliable in sealing over the openhole as around tubulars, the elastomeric sealing doughnut is required by API specifications to seal adequately over the openhole as part of its certification process. |
| Drilling | annular BOP | A large valve used to control wellbore fluids. In this type of valve, the sealing element resembles a large rubber doughnut that is mechanically squeezed inward to seal on either pipe (drill collar, drillpipe, casing, or tubing) or the openhole. The ability to seal on a variety of pipe sizes is one advantage the annular blowout preventer has over the ram blowout preventer. Most blowout preventer (BOP) stacks contain at least one annular BOP at the top of the BOP stack, and one or more ram-type preventers below. While not considered as reliable in sealing over the openhole as around tubulars, the elastomeric sealing doughnut is required by API specifications to seal adequately over the openhole as part of its certification process. |
| Drilling | annular gas flow | A flow of formation gas in the annulus between a casing string and the borehole wall. Annular gas flows occur when there is insufficient hydrostatic pressure to restrain the gas. They can occur in uncemented intervals and even in cemented sections if the cement bond is poor. After cementing, as the cement begins to harden, a gel-like structure forms that effectively supports the solid material in the cement slurry. However, during this initial gelling period, the cement has no appreciable strength. Hence, with the solid (weighting) material now supported by the gel structure, the effective density of the slurry that the reservoir experiences falls rather suddenly to the density of the mix water of the cement, which is usually fresh water, whose density is 8.34 lbm/gal, or a gradient of 0.434 psi/ft of vertical column height. Various chemical additives have been developed to reduce annular gas flow. |
| Drilling | annular velocity | The speed at which drilling fluid or cement moves in the annulus. It is important to monitor annular velocity to ensure that the hole is being properly cleaned of cuttings, cavings and other debris while avoiding erosion of the borehole wall. The annular velocity is commonly expressed in units of feet per minute or, less commonly, meters per minute. The term is distinct from volumetric flow. |
| Drilling | annuli | Plural form of annulus. |
| Drilling | annulus | The space between two concentric objects, such as between the wellbore and casing or between casing and tubing, where fluid can flow. Pipe may consist of drill collars, drillpipe, casing or tubing. |
| Drilling | antiwhirl bit | A drill bit, usually polycrystalline diamond compact bit (PDC) type, designed such that the individual cutting elements on the bit create a net imbalance force. This imbalance force pushes the bit against the side of the borehole, which in turn creates a stable rotating condition that resists backwards whirling, wobbling and downhole vibration. Antiwhirl bits allow faster rates of penetration, yet achieve longer bit life than more conventional bits, which are not dynamically biased to run smoothly, are inherently unstable, are vibration-prone and thus have shorter lives. No bit is whirl-proof, however. |
| Drilling | aquifer | Any water-bearing formation encountered while drilling. Drillers often are concerned about aquifers and are required to take special precautions in the design and execution of the well plan to protect fresh water aquifers from contamination by wellbore fluids. Water in aquifers can flow into the wellbore, contaminate drilling fluids and cause well control problems. |
| Drilling | azimuth | The compass direction of a directional survey or of the wellbore as planned or measured by a directional survey. The azimuth is usually specified in degrees with respect to the geographic or magnetic north pole. |
| Drilling | back off | To unscrew drillstring components downhole. The drillstring, including drillpipe and the bottomhole assembly, are coupled by various threadforms known as connections, or tool joints. Often when a drillstring becomes stuck it is necessary to “back off” the string as deep as possible to recover as much of the string as possible. To facilitate the fishing or recovery operation, the backoff is usually accomplished by applying reverse torque and detonating an explosive charge inside a selected threaded connection. The force of the explosion enlarges the female (outer) thread enough that the threaded connection unscrews instantly. A torqueless backoff may be performed as well. In that case, tension is applied, and the threads slide by each other without turning when the explosive detonates. Backing off can also occur unintentionally. |
| Drilling | back wash | Another term for reverse circulation, the intentional pumping of wellbore fluids down the annulus and back up through the drillpipe. This is the opposite of the normal direction of fluid circulation in a wellbore. Since the inside volume of the drillpipe is considerably less than the volume of the annulus outside of the drillpipe, reverse circulation can bring bottomhole fluids to the surface faster than normal circulation for a given flow rate. Two potential hazards of reverse circulation include lifting cuttings and other junk into the drillstring and the rapid flow of reservoir fluids to the surface in a kick situation. |
| Drilling | background gas | An average or baseline measure of gas entrained in circulating mud. This baseline trend pertains to gas that is liberated downhole while drilling through a uniform lithologic interval at a constant rate of penetration. The gas is typically obtained from a suction line above the gas trap located immediately upstream of the shale shaker screens, where the gas evolves out of the mud. Oil-base mud systems tend to produce higher background gas values than do water-base muds. Deviations from the background gas trend likely indicate changes in porosity or permeability, or changes in drilling conditions; any of which merits further investigation. A drift or gradual shift of the background gas trend toward higher values may indicate a slow gas influx into the mud column, which can eventually lead to a kick or blowout. When annotated on mud logs, background gas is usually abbreviated as BGG. |
| Drilling | backoff | To unscrew drillstring components downhole. The drillstring, including drillpipe and the bottomhole assembly, are coupled by various threadforms known as connections, or tool joints. Often when a drillstring becomes stuck it is necessary to “back off” the string as deep as possible to recover as much of the string as possible. To facilitate the fishing or recovery operation, the backoff is usually accomplished by applying reverse torque and detonating an explosive charge inside a selected threaded connection. The force of the explosion enlarges the female (outer) thread enough that the threaded connection unscrews instantly. A torqueless backoff may be performed as well. In that case, tension is applied, and the threads slide by each other without turning when the explosive detonates. Backing off can also occur unintentionally. |
| Drilling | backwash | Another term for reverse circulation, the intentional pumping of wellbore fluids down the annulus and back up through the drillpipe. This is the opposite of the normal direction of fluid circulation in a wellbore. Since the inside volume of the drillpipe is considerably less than the volume of the annulus outside of the drillpipe, reverse circulation can bring bottomhole fluids to the surface faster than normal circulation for a given flow rate. Two potential hazards of reverse circulation include lifting cuttings and other junk into the drillstring and the rapid flow of reservoir fluids to the surface in a kick situation. |
| Drilling | barefoot | Referring to openhole or without casing, as in barefoot completion or barefoot drillstem test. |
| Drilling | basket sub | A tool run into the wellbore to retrieve junk from the bottom of the hole. |
| Drilling | bell nipple | An enlarged pipe at the top of a casing string that serves as a funnel to guide drilling tools into the top of a well. The bell nipple is usually fitted with a side outlet to permit drilling fluids to flow back to the surface mud treating equipment through another inclined pipe called a flowline. |
| Drilling | BGG | An average or baseline measure of gas entrained in circulating mud. This baseline trend pertains to gas that is liberated downhole while drilling through a uniform lithologic interval at a constant rate of penetration. The gas is typically obtained from a suction line above the gas trap located immediately upstream of the shale shaker screens, where the gas evolves out of the mud. Oil-base mud systems tend to produce higher background gas values than do water-base muds. Deviations from the background gas trend likely indicate changes in porosity or permeability, or changes in drilling conditions; any of which merits further investigation. A drift or gradual shift of the background gas trend toward higher values may indicate a slow gas influx into the mud column, which can eventually lead to a kick or blowout. When annotated on mud logs, background gas is usually abbreviated as BGG. |
| Drilling | BHA | The lower portion of the drillstring, consisting of (from the bottom up in a vertical well) the bit, bit sub, a mud motor (in certain cases), stabilizers, drill collar, heavy-weight drillpipe, jarring devices (“jars”) and crossovers for various threadforms. The bottomhole assembly must provide force for the bit to break the rock (weight on bit), survive a hostile mechanical environment and provide the driller with directional control of the well. Oftentimes the assembly includes a mud motor, directional drilling and measuring equipment, measurements-while-drilling tools, logging-while-drilling tools and other specialized devices. A simple BHA consisting of a bit, various crossovers, and drill collars may be relatively inexpensive (less than $100,000 US in 1999), while a complex one may cost ten or more times that amount. |
| Drilling | BHCT | The temperature of the circulating fluid (air, mud, cement or water) at the bottom of the wellbore after several hours of circulation. This temperature is lower than the bottomhole static temperature. Therefore, in extremely harsh environments, a component or fluid that would not ordinarily be suitable under bottomhole static conditions may be used with great care in circulating conditions. Similarly, a high-temperature well may be cooled down in an attempt to allow logging tools to function. The BHCT is also important in the design of operations to cement casing because the setting time for cement is temperature-dependent. The BHCT and bottomhole static temperature (BHST) are important parameters when placing large volumes of temperature-sensitive treatment fluids. |
| Drilling | BHP | The pressure, usually measured in pounds per square inch (psi), at the bottom of the hole. This pressure may be calculated in a static, fluid-filled wellbore with the equation: BHP = MW * Depth * 0.052 where BHP is the bottomhole pressure in pounds per square inch, MW is the mud weight in pounds per gallon, Depth is the true vertical depth in feet, and 0.052 is a conversion factor if these units of measure are used. For circulating wellbores, the BHP increases by the amount of fluid friction in the annulus. The BHP gradient should exceed the formation pressure gradient to avoid an influx of formation fluid into the wellbore. On the other hand, if BHP (including the added fluid friction pressure of a flowing fluid) is too high, a weak formation may fracture and cause a loss of wellbore fluids. The loss of fluid to one formation may be followed by the influx of fluid from another formation. |
| Drilling | BHST | The temperature of the undisturbed formation at the final depth in a well. The formation cools during drilling and most of the cooling dissipates after about 24 hours of static conditions, although it is theoretically impossible for the temperature to return to undisturbed conditions. This temperature is measured under static conditions after sufficient time has elapsed to negate any effects from circulating fluids. Tables, charts and computer routines are used to predict BHST as functions of depth, geographic area and various time functions. The BHST is generally higher than the bottomhole circulating temperature, and can be an important factor when using temperature-sensitive tools or treatments. |
| Drilling | bit | The tool used to crush or cut rock. Everything on a drilling rig directly or indirectly assists the bit in crushing or cutting the rock. The bit is on the bottom of the drillstring and must be changed when it becomes excessively dull or stops making progress. Most bits work by scraping or crushing the rock, or both, usually as part of a rotational motion. Some bits, known as hammer bits, pound the rock vertically in much the same fashion as a construction site air hammer. |
| Drilling | bit box | A container, usually made of steel and fitted with a sturdy lock, to store drill bits, especially higher cost PDC and diamond bits. These bits are extremely costly but often small in size, so they are prone to theft. |
| Drilling | bit breaker | A special tool used by the rig crew to prevent the drill bit from turning while the bit sub on top of it is tightened or loosened. Bits have noncylindrical shapes, so the conventional wrenches used by the rig crew to tighten cylindrical shapes like pipes do not fit the bits properly. In addition, some bits, such as PDC bits, have a wide range of unusual and asymmetric shapes or profiles. The bit breaker must match the bit profile or the bit may be ruined before ever being used. |
| Drilling | bit nozzle | The part of the bit that includes a hole or opening for drilling fluid to exit. The hole is usually small (around 0.25 in. in diameter) and the pressure of the fluid inside the bit is usually high, leading to a high exit velocity through the nozzles that creates a high-velocity jet below the nozzles. This high-velocity jet of fluid cleans both the bit teeth and the bottom of the hole. The sizes of the nozzles are usually measured in 1/32-in. increments (although some are recorded in millimeters), are always reported in “thirty-seconds” of size (i.e., fractional denominators are not reduced), and usually range from 6/32 to 32/32. |
| Drilling | bit record | A historical record of how a bit performed in a particular wellbore. The bit record includes such data as the depth the bit was put into the well, the distance the bit drilled, the hours the bit was being used “on bottom” or “rotating”, the mud type and weight, the nozzle sizes, the weight placed on the bit, the rotating speed and hydraulic flow information. The data are usually updated daily. When the bit is pulled at the end of its use, the condition of the bit and the reason it was pulled out of the hole are also recorded. Bit records are often shared among operators and bit companies and are one of many valuable sources of data from offset wells for well design engineers. |
| Drilling | bit trip | The process of pulling the drillstring out of the wellbore for the purpose of changing a worn or underperforming drill bit. Upon reaching the surface, the bit is usually inspected and graded on the basis of how worn the teeth are, whether it is still in gauge and whether its components are still intact. On drilling reports, this trip may be abbreviated as TFNB (trip for new bit) |
| Drilling | blind ram | A thick, heavy steel component of a conventional ram blowout preventer. In a normal pipe ram, the two blocks of steel that meet in the center of the wellbore to seal the well have a hole (one-half of the hole on each piece) through which the pipe fits. The blind ram has no space for pipe and is instead blanked off in order to be able to close over a well that does not contain a drillstring. It may be loosely thought of as the sliding gate on a gate valve. |
| Drilling | block | A set of pulleys used to gain mechanical advantage in lifting or dragging heavy objects. There are two large blocks on a drilling rig, the crown block and the traveling block. Each has several sheaves that are rigged with steel drilling cable or line such that the traveling block may be raised (or lowered) by reeling in (or out) a spool of drilling line on the drawworks. |
| Drilling | blow out | An uncontrolled flow of reservoir fluids into the wellbore, and sometimes catastrophically to the surface. A blowout may consist of salt water, oil, gas or a mixture of these. Blowouts occur in all types of exploration and production operations, not just during drilling operations. If reservoir fluids flow into another formation and do not flow to the surface, the result is called an underground blowout. If the well experiencing a blowout has significant openhole intervals, it is possible that the well will bridge over (or seal itself with rock fragments from collapsing formations) downhole and intervention efforts will be averted. |
| Drilling | blow out preventer | A large valve at the top of a well that may be closed if the drilling crew loses control of formation fluids. By closing this valve (usually operated remotely via hydraulic actuators), the drilling crew usually regains control of the reservoir, and procedures can then be initiated to increase the mud density until it is possible to open the BOP and retain pressure control of the formation. BOPs come in a variety of styles, sizes and pressure ratings. Some can effectively close over an open wellbore, some are designed to seal around tubular components in the well (drillpipe, casing or tubing) and others are fitted with hardened steel shearing surfaces that can actually cut through drillpipe. Since BOPs are critically important to the safety of the crew, the rig and the wellbore itself, BOPs are inspected, tested and refurbished at regular intervals determined by a combination of risk assessment, local practice, well type and legal requirements. BOP tests vary from daily function testing on critical wells to monthly or less frequent testing on wells thought to have low probability of well control problems. |
| Drilling | blowout |
Uncontrolled flow of formation fluids from a well. An uncontrolled flow of formation fluids from the wellbore or into lower pressured subsurface zones (underground blowout). Uncontrolled flows cannot be contained using previously installed barriers and |
| Drilling | blowout preventer | A large valve at the top of a well that may be closed if the drilling crew loses control of formation fluids. By closing this valve (usually operated remotely via hydraulic actuators), the drilling crew usually regains control of the reservoir, and procedures can then be initiated to increase the mud density until it is possible to open the BOP and retain pressure control of the formation. BOPs come in a variety of styles, sizes and pressure ratings. Some can effectively close over an open wellbore, some are designed to seal around tubular components in the well (drillpipe, casing or tubing) and others are fitted with hardened steel shearing surfaces that can actually cut through drillpipe. Since BOPs are critically important to the safety of the crew, the rig and the wellbore itself, BOPs are inspected, tested and refurbished at regular intervals determined by a combination of risk assessment, local practice, well type and legal requirements. BOP tests vary from daily function testing on critical wells to monthly or less frequent testing on wells thought to have low probability of well control problems. |
| Drilling | BOP | A large valve at the top of a well that may be closed if the drilling crew loses control of formation fluids. By closing this valve (usually operated remotely via hydraulic actuators), the drilling crew usually regains control of the reservoir, and procedures can then be initiated to increase the mud density until it is possible to open the BOP and retain pressure control of the formation. BOPs come in a variety of styles, sizes and pressure ratings. Some can effectively close over an open wellbore, some are designed to seal around tubular components in the well (drillpipe, casing or tubing) and others are fitted with hardened steel shearing surfaces that can actually cut through drillpipe. Since BOPs are critically important to the safety of the crew, the rig and the wellbore itself, BOPs are inspected, tested and refurbished at regular intervals determined by a combination of risk assessment, local practice, well type and legal requirements. BOP tests vary from daily function testing on critical wells to monthly or less frequent testing on wells thought to have low probability of well control problems. |
| Drilling | BOP stack | A set of two or more BOPs used to ensure pressure control of a well. A typical stack might consist of one to six ram-type preventers and, optionally, one or two annular-type preventers. A typical stack configuration has the ram preventers on the bottom and the annular preventers at the top. The configuration of the stack preventers is optimized to provide maximum pressure integrity, safety and flexibility in the event of a well control incident. For example, in a multiple ram configuration, one set of rams might be fitted to close on 5-in. diameter drillpipe, another set configured for 4 1/2-in. drillpipe, a third fitted with blind rams to close on the openhole and a fourth fitted with a shear ram that can cut and hang-off the drillpipe as a last resort. It is common to have an annular preventer or two on the top of the stack since annulars can be closed over a wide range of tubular sizes and the openhole, but are typically not rated for pressures as high as ram preventers. The BOP stack also includes various spools, adapters and piping outlets to permit the circulation of wellbore fluids under pressure in the event of a well control incident. |
| Drilling | borehole | The wellbore itself, including the openhole or uncased portion of the well. Borehole may refer to the inside diameter of the wellbore wall, the rock face that bounds the drilled hole. |
| Drilling | borehole orientation | Borehole direction. Borehole orientation may be described in terms of inclination and azimuth. Inclination refers to the vertical angle measured from the down direction—the down, horizontal and up directions have inclinations of 0°, 90° and 180°, respectively. Azimuth refers to the horizontal angle measured clockwise from true north—the north, east, south and west directions have azimuths of 0°, 90°, 180° and 270°, respectively. |
| Drilling | bottomhole assembly | The lower portion of the drillstring, consisting of (from the bottom up in a vertical well) the bit, bit sub, a mud motor (in certain cases), stabilizers, drill collar, heavy-weight drillpipe, jarring devices (“jars”) and crossovers for various threadforms. The bottomhole assembly must provide force for the bit to break the rock (weight on bit), survive a hostile mechanical environment and provide the driller with directional control of the well. Oftentimes the assembly includes a mud motor, directional drilling and measuring equipment, measurements-while-drilling tools, logging-while-drilling tools and other specialized devices. A simple BHA consisting of a bit, various crossovers, and drill collars may be relatively inexpensive (less than $100,000 US in 1999), while a complex one may cost ten or more times that amount. |
| Drilling | bottomhole circulating temperature | The temperature of the circulating fluid (air, mud, cement or water) at the bottom of the wellbore after several hours of circulation. This temperature is lower than the bottomhole static temperature. Therefore, in extremely harsh environments, a component or fluid that would not ordinarily be suitable under bottomhole static conditions may be used with great care in circulating conditions. Similarly, a high-temperature well may be cooled down in an attempt to allow logging tools to function. The BHCT is also important in the design of operations to cement casing because the setting time for cement is temperature-dependent. The BHCT and bottomhole static temperature (BHST) are important parameters when placing large volumes of temperature-sensitive treatment fluids. |
| Drilling | bottomhole pressure | The pressure, usually measured in pounds per square inch (psi), at the bottom of the hole. This pressure may be calculated in a static, fluid-filled wellbore with the equation: BHP = MW * Depth * 0.052 where BHP is the bottomhole pressure in pounds per square inch, MW is the mud weight in pounds per gallon, Depth is the true vertical depth in feet, and 0.052 is a conversion factor if these units of measure are used. For circulating wellbores, the BHP increases by the amount of fluid friction in the annulus. The BHP gradient should exceed the formation pressure gradient to avoid an influx of formation fluid into the wellbore. On the other hand, if BHP (including the added fluid friction pressure of a flowing fluid) is too high, a weak formation may fracture and cause a loss of wellbore fluids. The loss of fluid to one formation may be followed by the influx of fluid from another formation. |
| Drilling | bottoms up | Pertaining to the mud and cuttings that are calculated or measured to come from the bottom of the hole since the start of circulation. Circulation may be initiated after a static period, such as a trip, or from a given depth while drilling. This latter type is particularly useful to mud loggers and others trying to discern the lithology being drilled, so mud loggers or mud engineers often retrieve what is referred to as a “bottoms-up sample” of the cuttings or the drilling fluid. |
| Drilling | bottoms-up | Pertaining to the mud and cuttings that are calculated or measured to come from the bottom of the hole since the start of circulation. Circulation may be initiated after a static period, such as a trip, or from a given depth while drilling. This latter type is particularly useful to mud loggers and others trying to discern the lithology being drilled, so mud loggers or mud engineers often retrieve what is referred to as a “bottoms-up sample” of the cuttings or the drilling fluid. |
| Drilling | bottoms-up | The sample obtained at the bottoms-up time or a volume of fluid to pump, as in “pump bottoms-up before drilling ahead.” |
| Drilling | bow-spring centralizer | A metal strip shaped like a hunting bow and attached to a tool or to the outside of casing. Bow-spring centralizers are used to keep casing in the center of a wellbore or casing (“centralized”) prior to and during a cement job. |
| Drilling | box | Relating to the female threadform, as in “box end of the pipe.” |
| Drilling | box | A female threadform (internally threaded) for tubular goods and drillstring components. |
| Drilling | brake | To apply the brake to slow the motion of the drawworks, and hence the drilling line and the drillstring. |
| Drilling | brake | The mechanism on the drawworks that permits the driller to control the speed and motion of the drilling line and the drillstring, or the brake handle that the driller operates to control the brake mechanism. |
| Drilling | break circulation | To establish circulation of drilling fluids after a period of static conditions. Circulation may resume after a short break, such as taking a survey or making a mousehole connection, or after a prolonged interruption, such as after a round trip. The operation is of more concern to drillers and well planners with longer static intervals, since immobile drilling fluid tends to become less fluid and more gelatinous or semisolid with time. |
| Drilling | break out | To unscrew drillstring components, which are coupled by various threadforms known as connections, including tool joints and other threaded connections. |
| Drilling | breakout | The process of unscrewing drillstring components, which are coupled by various threadforms known as connections, including tool joints and other threaded connections. |
| Drilling | breakout cathead | A clutching mechanism that permits the driller to apply high torque to a connection using the power of the drawworks motor. |
| Drilling | breakout tongs | Large capacity self-locking wrenches used to grip drillstring components and apply torque. The breakout tongs are the active tongs during breakout operations. A similar set of tongs is tied off to a deadline anchor during breakout operations to provide backup to the connection, not unlike the way a plumber uses two pipe wrenches in an opposing manner to tighten or loosen water pipes, except that breakout tongs are much larger. |
| Drilling | bridge | To intentionally or accidentally plug off pore spaces or fluid paths in a rock formation, or to make a restriction in a wellbore or annulus. A bridge may be partial or total, and is usually caused by solids (drilled solids, cuttings, cavings or junk) becoming lodged together in a narrow spot or geometry change in the wellbore. |
| Drilling | bridge | The gangplank or stairway connecting a jackup rig to a fixed platform. |
| Drilling | brine | Saline liquid usually used in completion operations and, increasingly, when penetrating a pay zone. Brines are preferred because they have higher densities than fresh water but lack solid particles that might damage producible formations. Classes of brines include chloride brines (calcium and sodium), bromides and formates. |
| Drilling | bullhead | To forcibly pump fluids into a formation, usually formation fluids that have entered the wellbore during a well control event. Though bullheading is intrinsically risky, it is performed if the formation fluids are suspected to contain hydrogen sulfide gas to prevent the toxic gas from reaching the surface. Bullheading is also performed if normal circulation cannot occur, such as after a borehole collapse. The primary risk in bullheading is that the drilling crew has no control over where the fluid goes and the fluid being pumped downhole usually enters the weakest formation. In addition, if only shallow casing is cemented in the well, the bullheading operation can cause wellbore fluids to broach around the casing shoe and reach the surface. This broaching to the surface has the effect of fluidizing and destabilizing the soil (or the subsea floor), and can lead to the formation of a crater and loss of equipment and life. |
| Drilling | cable head | An electromechanical device used to connect an electrical tool string to a logging cable, electrical wireline or coiled tubing string equipped with an electrical conductor. It provides attachments to both the mechanical armor wires (which give logging cable its tensile strength) and the outer mechanical housing of a logging tool, usually by means of threads. This connection to the logging tool results in a good electrical path from the electrical conductors of the logging cable to the electrical contacts of the logging tool, and shields this electrical path from contact with conductive fluids, such as certain drilling muds. The basic requirements of most cable heads include providing reliable electrical and mechanical connectivity between the running string and tool string. Another attribute of cable heads is that they serve as a “weak link,” so that if a logging tool becomes irretrievably stuck in a well, the operator may intentionally pull in excess of the breaking strength of the logging cable head, causing the cable to pull out of the cable head in a controlled fashion. |
| Drilling | cable-tool drilling | A method of drilling whereby an impact tool or bit, suspended in the well from a steel cable, is dropped repeatedly on the bottom of the hole to crush the rock. The tool is usually fitted with some sort of cuttings basket to trap the cuttings along the side of the tool. After a few impacts on the bottom of the hole, the cable is reeled in and the cuttings basket emptied, or a bailer is used to remove cuttings from the well. The tool is reeled back to the bottom of the hole and the process repeated. Due to the increasing time required to retrieve and deploy the bit as the well is deepened, the cable-tool method is limited to shallow depths. Though largely obsolete, cable-tool operations are still used to drill holes for explosive charge placement (such as for acquisition of surface seismic data) and water wells. |
| Drilling | caliper log | A representation of the measured diameter of a borehole along its depth. Caliper logs are usually measured mechanically, with only a few using sonic devices. The tools measure diameter at a specific chord across the well. Since wellbores are usually irregular (rugose), it is important to have a tool that measures diameter at several different locations simultaneously. Such a tool is called a multifinger caliper. Drilling engineers or rigsite personnel use caliper measurement as a qualitative indication of both the condition of the wellbore and the degree to which the mud system has maintained hole stability. Caliper data are integrated to determine the volume of the openhole, which is then used in planning cementing operations. |
| Drilling | carbide lag test | A test performed by the mudlogger or wellsite geologist, used to calculate sample lag. The lag period can be measured as a function of time or pump strokes. Acetylene is commonly used as a tracer gas for this purpose. This gas is generated by calcium carbide, a man-made product that reacts with water. Usually, a small paper packet containing calcium carbide is inserted into the drillstring when the kelly is unscrewed from the pipe to make a connection, and the time is noted, along with the pump-stroke count on the mud pump. Once the connection is made and drilling resumes, the packet is pumped downhole with the drilling fluid. Along the way, the drilling fluid breaks down the paper and reacts with the calcium carbide. The resulting acetylene gas circulates with the drilling fluid until it reaches the surface, where it is detected at the gas trap, causing a rapid increase or spike in gas readings. The time and pump-stroke count are again noted, and the cuttings sample lag interval is calculated. |
| Drilling | cased hole | The portion of the wellbore that has had metal casing placed and cemented to protect the openhole from fluids, pressures, wellbore stability problems or a combination of these. |
| Drilling | casing | Large-diameter pipe lowered into an openhole and cemented in place. The well designer must design casing to withstand a variety of forces, such as collapse, burst, and tensile failure, as well as chemically aggressive brines. Most casing joints are fabricated with male threads on each end, and short-length casing couplings with female threads are used to join the individual joints of casing together, or joints of casing may be fabricated with male threads on one end and female threads on the other. Casing is run to protect fresh water formations, isolate a zone of lost returns or isolate formations with significantly different pressure gradients. The operation during which the casing is put into the wellbore is commonly called “running pipe.” Casing is usually manufactured from plain carbon steel that is heat-treated to varying strengths, but may be specially fabricated of stainless steel, aluminum, titanium, fiberglass and other materials. |
| Drilling | casing centralizer | A mechanical device that keeps casing from contacting the wellbore wall. A continuous 360-degree annular space around casing allows cement to completely seal the casing to the borehole wall. There are two distinct classes of centralizers. The older and more common is a simple, low-cost bow-spring design. Since the bow springs are slightly larger than the wellbore, they can provide complete centralization in vertical or slightly deviated wells. However, they do not support the weight of the casing very well in deviated wellbores. The second type is a rigid blade design. This type is rugged and works well even in deviated wellbores, but since the centralizers are smaller than the wellbore, they will not provide as good centralization as bow-spring type centralizers in vertical wells. Rigid-blade casing centralizers are slightly more expensive and can cause trouble downhole if the wellbore is not in excellent condition. |
| Drilling | casing collar | The threaded collar used to connect two joints of casing. The resulting connection must provide adequate mechanical strength to enable the casing string to be run and cemented in place. The casing collar must also provide sufficient hydraulic isolation under the design conditions determined by internal and external pressure conditions and fluid characteristics. |
| Drilling | casing coupling | A short length of pipe used to connect two joints of casing. A casing coupling has internal threads (female threadform) machined to match the external threads (male threadform) of the long joints of casing. The two joints of casing are threaded into opposite ends of the casing coupling. |
| Drilling | casing grade | A system of identifying and categorizing the strength of casing materials. Since most oilfield casing is of approximately the same chemistry (typically steel), and differs only in the heat treatment applied, the grading system provides for standardized strengths of casing to be manufactured and used in wellbores. The first part of the nomenclature, a letter, refers to the tensile strength. The second part of the designation, a number, refers to the minimum yield strength of the metal (after heat treatment) at 1000 psi [6895 KPa]. For example, the casing grade J-55 has minimum yield strength of 55,000 psi [379,211 KPa]. The casing grade P-110 designates a higher strength pipe with minimum yield strength of 110,000 psi [758,422 KPa]. The appropriate casing grade for any application typically is based on pressure and corrosion requirements. Since the well designer is concerned about the pipe yielding under various loading conditions, the casing grade is the number that is used in most calculations. High-strength casing materials are more expensive, so a casing string may incorporate two or more casing grades to optimize costs while maintaining adequate mechanical performance over the length of the string. It is also important to note that, in general, the higher the yield strength, the more susceptible the casing is to sulfide stress cracking (H2S-induced cracking). Therefore, if H2S is anticipated, the well designer may not be able to use tubulars with strength as high as he or she would like. |
| Drilling | casing head | The adapter between the first casing string and either the BOP stack (during drilling) or the wellhead (after completion). This adapter may be threaded or welded onto the casing, and may have a flanged or clamped connection to match the BOP stack or wellhead. |
| Drilling | casing point | The location, or depth, at which drilling an interval of a particular diameter hole ceases, so that casing of a given size can be run and cemented. Establishing correct casing points is important in the design of the drilling fluid program. The casing point may be a predetermined depth, or it may be selected onsite by a pressure hunt team, selected onsite according to geological observations or dictated by problems in the openhole section. In many cases, weak or underpressure zones must be protected by casing to enable mud weight adjustments that control unstable formations or overpressure zones deeper in the wellbore. |
| Drilling | casing shoe | The bottom of the casing string, including the cement around it, or the equipment run at the bottom of the casing string. |
| Drilling | casing string | An assembled length of steel pipe configured to suit a specific wellbore. The sections of pipe are connected and lowered into a wellbore, then cemented in place. The pipe joints are typically approximately 40 ft [12 m] in length, male threaded on each end and connected with short lengths of double-female threaded pipe called couplings. Long casing strings may require higher strength materials on the upper portion of the string to withstand the string load. Lower portions of the string may be assembled with casing of a greater wall thickness to withstand the extreme pressures likely at depth. Casing is run to protect or isolate formations adjacent to the wellbore. The following are the most common reasons for running casing in a well: 1) protect fresh-water aquifers (surface casing) 2) provide strength for installation of wellhead equipment, including BOPs 3) provide pressure integrity so that wellhead equipment, including BOPs, may be closed 4) seal off leaky or fractured formations into which drilling fluids are lost 5) seal off low-strength formations so that higher strength (and generally higher pressure) formations may be penetrated safely 6) seal off high-pressure zones so that lower pressure formations may be drilled with lower drilling fluid densities 7) seal off troublesome formations, such as flowing salt 8) comply with regulatory requirements (usually related to one of the factors listed above). |
| Drilling | casinghead | The adapter between the first casing string and either the BOP stack (during drilling) or the wellhead (after completion). This adapter may be threaded or welded onto the casing, and may have a flanged or clamped connection to match the BOP stack or wellhead. |
| Drilling | cat line | A relatively thin cable used with other equipment to move small rig and drillstring components and to provide tension on the tongs for tightening or loosening threaded connections. |
| Drilling | cathead | A clutched spool connected to the drawworks power system used to tension chains, cables and softline rope. |
| Drilling | catline | A relatively thin cable used with other equipment to move small rig and drillstring components and to provide tension on the tongs for tightening or loosening threaded connections. |
| Drilling | catwalk | A long, rectangular platform about 3 ft [0.9 m] high, usually made of steel and located perpendicular to the vee-door at the bottom of the slide. This platform is used as a staging area for rig and drillstring tools, components that are about to be picked up and run, or components that have been run and are being laid down. A catwalk is also the functionally similar staging area, especially on offshore drilling rigs, that may not be a separate or raised structure. |
| Drilling | cellar | A dug-out area, possibly lined with wood, cement or very large diameter (6 ft [1.8 m]) thin-wall pipe, located below the rig. The cellar serves as a cavity in which the casing spool and casinghead reside. The depth of the cellar is such that the master valve of the Christmas tree are easy to reach from ground level. On smaller rigs, the cellar also serves as the place where the lower part of the BOP stack resides, which reduces the rig height necessary to clear the BOP stack on the top. Prior to setting surface casing, the cellar also takes mud returns from the well, which are pumped back to the surface mud equipment. |
| Drilling | cement bond log | A representation of the integrity of the cement job, especially whether the cement is adhering solidly to the outside of the casing. The log is typically obtained from one of a variety of sonic-type tools. The newer versions, called cement evaluation logs, along with their processing software, can give detailed, 360-degree representations of the integrity of the cement job, whereas older versions may display a single line representing the integrated integrity around the casing. |
| Drilling | cement evaluation log | A representation of the integrity of the cement job, especially whether the cement is adhering solidly to the outside of the casing. The log is typically obtained from one of a variety of sonic-type tools. The newer versions, called cement evaluation logs, along with their processing software, can give detailed, 360-degree representations of the integrity of the cement job, whereas older versions may display a single line representing the integrated integrity around the casing. |
| Drilling | cement head | A device fitted to the top joint of a casing string to hold a cement plug before it is pumped down the casing during the cementing operation. In most operations, a bottom plug is launched before the spacer or cement slurry. The top plug is released from the cement head after the spacer fluid. Most cement heads can hold both the top and bottom plugs. A manifold incorporated into the cement head assembly allows connection of a fluid circulation line. |
| Drilling | cementer | The colloquial term for the crew member in charge of a specialized cementing crew and trucks. |
| Drilling | cementing | To prepare and pump cement into place in a wellbore. Cementing operations may be undertaken to seal the annulus after a casing string has been run, to seal a lost circulation zone, to set a plug in an existing well from which to push off with directional tools or to plug a well so that it may be abandoned. Before cementing operations commence, engineers determine the volume of cement (commonly with the help of a caliper log) to be placed in the wellbore and the physical properties of both the slurry and the set cement needed, including density and viscosity. A cementing crew uses special mixers and pumps to displace drilling fluids and place cement in the wellbore. |
| Drilling | cementing engineer | The colloquial term for the crew member in charge of a specialized cementing crew and trucks. |
| Drilling | cementing plug | A rubber plug used to separate the cement slurry from other fluids, reducing contamination and maintaining predictable slurry performance. Two types of cementing plug are typically used on a cementing operation. The bottom plug is launched ahead of the cement slurry to minimize contamination by fluids inside the casing prior to cementing. A diaphragm in the plug body ruptures to allow the cement slurry to pass through after the plug reaches the landing collar. The top plug has a solid body that provides positive indication of contact with the landing collar and bottom plug through an increase in pump pressure. |
| Drilling | chain tongs | A type of pipe wrench used for hand-tightening various threaded connections around the rigsite. It consists of a handle, a set of gripping die teeth, a length of flat chain and a hooking slot where the chain may be adjusted to fit the pipe. |
| Drilling | choke line | A high-pressure pipe leading from an outlet on the BOP stack to the backpressure choke and associated manifold. During well-control operations, the fluid under pressure in the wellbore flows out of the well through the choke line to the choke, reducing the fluid pressure to atmospheric pressure. In floating offshore operations, the choke and kill lines exit the subsea BOP stack and then run along the outside of the drilling riser to the surface. The volumetric and frictional effects of these long choke and kill lines must be considered to control the well properly. |
| Drilling | choke manifold | A set of high-pressure valves and associated piping that usually includes at least two adjustable chokes, arranged such that one adjustable choke may be isolated and taken out of service for repair and refurbishment while well flow is directed through the other one. |
| Drilling | Christmas tree | The set of valves, spools and fittings connected to the top of a well to direct and control the flow of formation fluids from the well. |
| Drilling | circulate | To pump fluid through the whole active fluid system, including the borehole and all the surface tanks that constitute the primary system. |
| Drilling | circulate out | To pump the drilling fluid until a sample from the bottom of the hole reaches the surface. This is commonly performed when drilling has ceased so that the wellsite geologist may collect a cuttings sample from the formation being drilled, or when the driller suspects that a small amount of gas has entered the wellbore. Thus, by circulating out, the gas bubble is eased out of the wellbore safely. |
| Drilling | circulation | Noun form of circulate. |
| Drilling | circulation loss | The loss of drilling fluid to a formation, usually caused when the hydrostatic head pressure of the column of drilling fluid exceeds the formation pressure. This loss of fluid may be loosely classified as seepage losses, partial losses or catastrophic losses, each of which is handled differently depending on the risk to the rig and personnel and the economics of the drilling fluid and each possible solution. |
| Drilling | circulation system | The complete, circuitous path that the drilling fluid travels. Starting at the main rig pumps, major components include surface piping, the standpipe, the kelly hose (rotary), the kelly, the drillpipe, drill collars, bit nozzles, the various annular geometries of the openhole and casing strings, the bell nipple, the flowline, the mud-cleaning equipment, the mud tanks, the centrifugal precharge pumps and, finally, the positive displacement main rig pumps. |
| Drilling | clear brine | Saline liquid usually used in completion operations and, increasingly, when penetrating a pay zone. Brines are preferred because they have higher densities than fresh water but lack solid particles that might damage producible formations. Classes of brines include chloride brines (calcium and sodium), bromides and formates. |
| Drilling | coiled tubing | A long, continuous length of pipe wound on a spool. The pipe is straightened prior to pushing into a wellbore and rewound to coil the pipe back onto the transport and storage spool. Depending on the pipe diameter (1 in. to 4-1/2 in.) and the spool size, coiled tubing can range from 2,000 ft to 15,000 ft [610 to 4,570 m] or greater length. |
| Drilling | coiled tubing drilling | The use of coiled tubing with downhole mud motors to turn the bit to deepen a wellbore. Coiled tubing drilling operations proceed quickly compared to using a jointed pipe drilling rig because connection time is eliminated during tripping. Coiled tubing drilling is economical in several applications, such as drilling slimmer wells, areas where a small rig footprint is essential, reentering wells and drilling underbalanced. |
| Drilling | combination string | Another term for a tapered string: a string of drillpipe or casing that consists of two or more sizes or weights. In most tapered strings, a larger diameter pipe or casing is placed at the top of the wellbore and the smaller size at the bottom. Note that since the pipe is put into the well bottom first, the smaller pipe is run into the hole first, followed by the larger diameter. Other than the different sizes, which are usually chosen to optimize well economics, there is nothing distinctive about the pipe sections. However, pipe-handling tools must be available for each pipe size, not just one size, as is the typical case. |
| Drilling | come out of the hole | To remove the drillstring from the wellbore. |
| Drilling | company man | The representative of the oil company or operator on a drilling location. For land operations, the company man is responsible for operational issues on the location, including the safety and efficiency of the project. Even administrative managers are expected to respond to the direction of the company man when they are on the rigsite. Offshore, depending on the regulatory requirements, there may be an offshore installation manager, who supervises the company man on safety and vessel integrity issues, but not on operational issues. |
| Drilling | company representative | The representative of the oil company or operator on a drilling location. For land operations, the company man is responsible for operational issues on the location, including the safety and efficiency of the project. Even administrative managers are expected to respond to the direction of the company man when they are on the rigsite. Offshore, depending on the regulatory requirements, there may be an offshore installation manager, who supervises the company man on safety and vessel integrity issues, but not on operational issues. |
| Drilling | completion | The hardware used to optimize the production of hydrocarbons from the well. This may range from nothing but a packer on tubing above an openhole completion (“barefoot” completion), to a system of mechanical filtering elements outside of perforated pipe, to a fully automated measurement and control system that optimizes reservoir economics without human intervention (an “intelligent” completion). |
| Drilling | concentric | Having the same center, such as when the casing and the wellbore have a common center point and, therefore, a uniform annular dimension. |
| Drilling | conductor pipe | The casing string that is usually put into the well first, particularly on land wells, to prevent the sides of the hole from caving into the wellbore. This casing, sometimes called drive pipe, is generally a short length and is sometimes driven into the ground. Conductor pipe is run because the shallow section of most wells onshore is drilled in unconsolidated sediment or soil rather than consolidated strata typically encountered deeper. Offshore, the drive pipe or structural casing may be installed prior to the conductor for similar reasons. |
| Drilling | connection | Any threaded or nonthreaded union or joint that connects two tubular components. |
| Drilling | connection | The act of adding a joint or stand of drillpipe to the top of the drillstring, also described as “making a connection. |
| Drilling | connection gas | A brief influx of gas that is introduced into the drilling fluid when a pipe connection is made. Before making a connection, the driller stops the mud pumps, thereby allowing gas to enter the wellbore at depth. Gas may also be drawn into the wellbore by minor swabbing effects resulting from short movements of the drillstring that occur during the connection. Connection gas usually occurs after one lag interval following the connection. On a mud log, it will appear as a short peak above background levels. This peak often appears at 30-foot intervals, depending on the lengths of drillpipe being connected as the well is drilled. |
| Drilling | contamination gas | Gas that is introduced into the drilling mud from a source other than the formation. Contamination gas normally evolves as a by-product of oil-base mud systems and those using volatile additives such as diesel fuel or other lubricants. |
| Drilling | contract depth | The depth in a drilling well at which the drilling contractor receives a lump-sum payment for reaching a particular milestone. The contract depth is specified in a legal agreement between the operator, who pays for the well, and the drilling contractor, who owns and operates the drilling rig. Contract depth may be the final or total depth (TD) of the well, an intermediate point in the well or another milestone, such as running well-logging tools to the bottom of the hole. In the case of an intermediate contract depth, the work to deepen the well would likely be done on a day rate basis, or a “time and materials” contract. |
| Drilling | core | To deepen the wellbore by way of collecting a cylindrical sample of rock. A core bit is used to accomplish this, in conjunction with a core barrel and core catcher. The bit is usually a drag bit fitted with either PDC or natural diamond cutting structures, but the core bit is unusual in that it has a hole in its center. This allows the bit to drill around a central cylinder of rock, which is taken in through the bit and into the core barrel. The core barrel itself may be thought of as a special storage chamber for holding the rock core. The core catcher serves to grip the bottom of the core and, as tension is applied to the drillstring, the rock under the core breaks away from the undrilled formation below it. The core catcher also retains the core so that it does not fall out the bottom of the drillstring, which is open in the middle at that point. |
| Drilling | crooked hole | Antiquated term for a deviated wellbore, usually used to describe a well deviated accidentally during the drilling process. |
| Drilling | crossflow | The flow of fluid across the bottom of the bit after it exits the bit nozzles, strikes the bottom or sides of the hole and turns upwards to the annulus. Modern, well-designed bits maximize crossflow using an asymmetric nozzle arrangement. |
| Drilling | crossflow | The flow of reservoir fluids from one zone to another. Crossflow can occur when a lost returns event is followed by a well control event. The higher pressured reservoir fluid flows out of the formation, travels along the wellbore to a lower pressured formation, and then flows into the lower pressure formation. |
| Drilling | crown block | The fixed set of pulleys (called sheaves) located at the top of the derrick or mast, over which the drilling line is threaded. The companion blocks to these pulleys are the traveling blocks. By using two sets of blocks in this fashion, great mechanical advantage is gained, enabling the use of relatively small drilling line (3/4 to 1 1/2 in. diameter steel cable) to hoist loads many times heavier than the cable could support as a single strand. |
| Drilling | crushed zone | The rubblized rock just below the tooth of a rock bit. Rock in the crushed zone fails due to the high compressive stress placed on it by the bit tooth (in the case of a roller-cone bit). The effective creation of and removal of crushed zone rock is important to the efficiency of the drill bit. If the rock is not broken and removed efficiently, the result is akin to effectively drilling the hole twice. |
| Drilling | CT | Another term for coiled tubing, a long, continuous length of pipe wound on a spool. The pipe is straightened prior to pushing into a wellbore and rewound to coil the pipe back onto the transport and storage spool. Depending on the pipe diameter (1 in. to 4 1/2 in.) and the spool size, coiled tubing can range from 2,000 ft to 15,000 ft [610 to 4,570 m] or greater length. |
| Drilling | cut and thread fishing technique | A method for recovering wireline stuck in a wellbore. In cut-and-thread operations, the wireline is gripped securely with a special tool and cut at the surface. The cut end is threaded through a stand of drillpipe. While the pipe hangs in the wellbore, the wireline is threaded through another stand of drillpipe, which is screwed onto the stand in the wellbore. The process is repeated until the stuck wireline is recovered. This technique, while dangerous and time-consuming, is known to improve greatly the chances of full recovery of the wireline and the tool at its end in the shortest overall time compared with trying to grab the wireline in the openhole with fishing tools. |
| Drilling | cut-and-thread fishing technique | A method for recovering wireline stuck in a wellbore. In cut-and-thread operations, the wireline is gripped securely with a special tool and cut at the surface. The cut end is threaded through a stand of drillpipe. While the pipe hangs in the wellbore, the wireline is threaded through another stand of drillpipe, which is screwed onto the stand in the wellbore. The process is repeated until the stuck wireline is recovered. This technique, while dangerous and time-consuming, is known to improve greatly the chances of full recovery of the wireline and the tool at its end in the shortest overall time compared with trying to grab the wireline in the openhole with fishing tools. |
| Drilling | cuttings | Small pieces of rock that break away due to the action of the bit teeth. Cuttings are screened out of the liquid mud system at the shale shakers and are monitored for composition, size, shape, color, texture, hydrocarbon content and other properties by the mud engineer, the mud logger and other on-site personnel. The mud logger usually captures samples of cuttings for subsequent analysis and archiving. |
| Drilling | day rate | The daily cost to the operator of renting the drilling rig and the associated costs of personnel and routine supplies. This cost may or may not include fuel, and usually does not include capital goods, such as casing and wellheads, or special services, such as logging or cementing. In most of the world, the day rate represents roughly half of the cost of the well. Similarly, the total daily cost to drill a well (spread rate) is roughly double what the rig day-rate amount is. |
| Drilling | daylight tour | The work shift of a drilling crew that commences at about the sunrise hour. Drilling operations usually take place around the clock because of the cost to rent a rig. As a result, there are usually two separate crews working twelve-hour tours to keep the operation going. Some companies prefer three eight-hour tours: the daylight tour starts at daylight or 8 AM; the graveyard tour is the overnight shift or the shift that begins at midnight. (Pronounced “tower” in many areas.) |
| Drilling | DD | An individual trained in the science and art of intentionally drilling a well along a predetermined path in three-dimensional space, usually involving deviating the well from vertical and directing it in a specific compass direction or heading. The directional driller considers such parameters as rotary speed, weight on bit, control drilling and when to stop drilling and take surveys of the wellpath, and works closely with the toolpusher. |
| Drilling | degasser | A device that removes air or gases (methane, H2S, CO2 and others) from drilling liquids. There are two generic types that work by both expanding the size of the gas bubbles entrained in the mud (by pulling a vacuum on the mud) and by increasing the surface area available to the mud so that bubbles escape (through the use of various cascading baffle plates). If the gas content in the mud is high, a mud gas separator or “poor boy degasser” is used, because it has a higher capacity than standard degassers and routes the evolved gases away from the rig to a flaring area complete with an ignition source. |
| Drilling | derrick | The structure used to support the crown blocks and the drillstring of a drilling rig. Derricks are usually pyramidal in shape, and offer a good strength-to-weight ratio. If the derrick design does not allow it to be moved easily in one piece, special ironworkers must assemble them piece by piece, and in some cases disassemble them if they are to be moved. |
| Drilling | derrick floor | The relatively small work area in which the rig crew conducts operations, usually adding or removing drillpipe to or from the drillstring. The rig floor is the most dangerous location on the rig because heavy iron is moved around there. Drillstring connections are made or broken on the drillfloor, and the driller’s console for controlling the major components of the rig are located there. Attached to the rig floor is a small metal room, the doghouse, where the rig crew can meet, take breaks and take refuge from the elements during idle times. |
| Drilling | derrickman | One of the rig crew members who gets his name from the fact that he works on a platform attached to the derrick or mast, typically 85 ft [26 m] above the rig floor, during trips. On small land drilling crews, the derrickman is second in rank to the driller. Larger offshore crews may have an assistant driller between the derrickman and the driller. In a typical trip out of the hole (TOH), the derrickman wears a special safety harness that enables him to lean out from the work platform (called the monkeyboard) to reach the drillpipe in the center of the derrick or mast, throw a line around the pipe and pull it back into its storage location (the fingerboards) until it is time to run the pipe back into the well. In terms of skill, physical exertion and perceived danger, a derrickman has one of the most demanding jobs on the rig crew. Some modern drilling rigs have automated pipe-handling equipment such that the derrickman controls the machinery rather than physically handling the pipe. In an emergency, the derrickman can quickly reach the ground by an escape line often called the Geronimo line. |
| Drilling | desander | A hydrocyclone device that removes large drill solids from the whole mud system. The desander should be located downstream of the shale shakers and degassers, but before the desilters or mud cleaners. A volume of mud is pumped into the wide upper section of the hydrocylone at an angle roughly tangent to its circumference. As the mud flows around and gradually down the inside of the cone shape, solids are separated from the liquid by centrifugal forces. The solids continue around and down until they exit the bottom of the hydrocyclone (along with small amounts of liquid) and are discarded. The cleaner and lighter density liquid mud travels up through a vortex in the center of the hydrocyclone, exits through piping at the top of the hydrocyclone and is then routed to the mud tanks and the next mud-cleaning device, usually a desilter. Various size desander and desilter cones are functionally identical, with the size of the cone determining the size of particles the device removes from the mud system. |
| Drilling | desilter | A hydrocyclone much like a desander except that its design incorporates a greater number of smaller cones. As with the desander, its purpose is to remove unwanted solids from the mud system. The smaller cones allow the desilter to efficiently remove smaller diameter drill solids than a desander does. For that reason, the desilter is located downstream from the desander in the surface mud system. |
| Drilling | deviated hole | A wellbore that is not vertical. The term usually indicates a wellbore intentionally drilled away from vertical. |
| Drilling | deviation survey | A completed measurement of the inclination and azimuth of a location in a well (typically the total depth at the time of measurement). In both directional and straight holes, the position of the well must be known with reasonable accuracy to ensure the correct wellbore path and to know its position in the event a relief well must be drilled. The measurements themselves include inclination from vertical, and the azimuth (or compass heading) of the wellbore if the direction of the path is critical. These measurements are made at discrete points in the well, and the approximate path of the wellbore computed from the discrete points. Measurement devices range from simple pendulum-like devices to complex electronic accelerometers and gyroscopes used more often as MWD becomes more popular. In simple pendulum measurements, the position of a freely hanging pendulum relative to a measurement grid (attached to the housing of the tool and assumed to represent the path of the wellbore) is captured on photographic film. The film is developed and examined when the tool is removed from the wellbore, either on wireline or the next time pipe is tripped out of the hole. |
| Drilling | diamond bit | A tool for drilling rock that works by scraping industrial grade diamonds against the bottom of the hole. The diamonds are embedded into the metal structure (usually a sintered or powdered carbide base matrix) during the manufacture of the bit. The bit designer has virtually unlimited combinations of bit shape, the placement of hydraulic jetting ports, the amount of diamonds and the size of the diamonds used (usually expressed as diamonds per carat). In general, a diamond bit that drills faster has a shorter lifetime. Similarly, a bit designed for extremely long life will typically drill at a slower rate. If a bit has a relatively high number of diamonds compared with other bits, it is said to be “heavy-set” and has higher durability. A “light-set” bit, on the other hand, drills more aggressively, but wears out faster because fewer diamonds do the work. |
| Drilling | differential pressure | In general, a measurement of fluid force per unit area (measured in units such as pounds per square in.) subtracted from a higher measurement of fluid force per unit area. This comparison could be made between pressures outside and inside a pipe, a pressure vessel, before and after an obstruction in a flow path, or simply between two points along any fluid path, such as two points along the inside of a pipe or across a packer. |
| Drilling | differential pressure | The change in force per unit area measured before and after drilling fluid passes through small-diameter bit nozzles. |
| Drilling | differential pressure | The change in force per unit area measured across various downhole tools such as measurements-while-drilling (MWD) tools, downhole turbines and mud motors. |
| Drilling | differential pressure sticking | A condition whereby the drillstring cannot be moved (rotated or reciprocated) along the axis of the wellbore. Differential sticking typically occurs when high-contact forces caused by low reservoir pressures, high wellbore pressures, or both, are exerted over a sufficiently large area of the drillstring. Differential sticking is, for most drilling organizations, the greatest drilling problem worldwide in terms of time and financial cost. It is important to note that the sticking force is a product of the differential pressure between the wellbore and the reservoir and the area that the differential pressure is acting upon. This means that a relatively low differential pressure (delta p) applied over a large working area can be just as effective in sticking the pipe as can a high differential pressure applied over a small area. |
| Drilling | differential sticking | A condition whereby the drillstring cannot be moved (rotated or reciprocated) along the axis of the wellbore. Differential sticking typically occurs when high-contact forces caused by low reservoir pressures, high wellbore pressures, or both, are exerted over a sufficiently large area of the drillstring. Differential sticking is, for most drilling organizations, the greatest drilling problem worldwide in terms of time and financial cost. It is important to note that the sticking force is a product of the differential pressure between the wellbore and the reservoir and the area that the differential pressure is acting upon. This means that a relatively low differential pressure (delta p) applied over a large working area can be just as effective in sticking the pipe as can a high differential pressure applied over a small area. |
| Drilling | directional driller | An individual trained in the science and art of intentionally drilling a well along a predetermined path in three-dimensional space, usually involving deviating the well from vertical and directing it in a specific compass direction or heading. The directional driller considers such parameters as rotary speed, weight on bit, control drilling and when to stop drilling and take surveys of the wellpath, and works closely with the toolpusher. |
| Drilling | directional survey | A completed measurement of the inclination and azimuth of a location in a well (typically the total depth at the time of measurement). In both directional and straight holes, the position of the well must be known with reasonable accuracy to ensure the correct wellbore path and to know its position in the event a relief well must be drilled. The measurements themselves include inclination from vertical, and the azimuth (or compass heading) of the wellbore if the direction of the path is critical. These measurements are made at discrete points in the well, and the approximate path of the wellbore computed from the discrete points. Measurement devices range from simple pendulum-like devices to complex electronic accelerometers and gyroscopes used more often as MWD becomes more popular. In simple pendulum measurements, the position of a freely hanging pendulum relative to a measurement grid (attached to the housing of the tool and assumed to represent the path of the wellbore) is captured on photographic film. The film is developed and examined when the tool is removed from the wellbore, either on wireline or the next time pipe is tripped out of the hole. |
| Drilling | directional well | A wellbore that requires the use of special tools or techniques to ensure that the wellbore path hits a particular subsurface target, typically located away from (as opposed to directly under) the surface location of the well. |
| Drilling | displacement fluid | The fluid, usually drilling mud, used to force a cement slurry out of the casing string and into the annulus. |
| Drilling | dog house | The steel-sided room adjacent to the rig floor, usually having an access door close to the driller’s controls. This general-purpose shelter is a combination tool shed, office, communications center, coffee room, lunchroom and general meeting place for the driller and his crew. It is at the same elevation as the rig floor, usually cantilevered out from the main substructure supporting the rig. |
| Drilling | dog leg | A particularly crooked place in a wellbore where the trajectory of the wellbore in three-dimensional space changes rapidly. While a dogleg is sometimes created intentionally by directional drillers, the term more commonly refers to a section of the hole that changes direction faster than anticipated or desired, usually with harmful side effects. In surveying wellbore trajectories, a standard calculation of dogleg severity is made, usually expressed in two-dimensional degrees per 100 feet [degrees per 30 m] of wellbore length. There are several difficulties associated with doglegs. First, the wellbore is not located in the planned path. Second is the possibility that a planned casing string may no longer easily fit through the curved section. Third, repeated abrasion by the drillstring in a particular location of the dogleg results in a worn spot called a keyseat, in which the bottomhole assembly components may become stuck as they are pulled through the section. Fourth, casing successfully cemented through the dogleg may wear unusually quickly due to higher contact forces between the drillstring and the inner diameter (ID) of the casing through the dogleg. Fifth, a relatively stiff bottomhole assembly may not easily fit through the dogleg section drilled with a relatively limber BHA. Sixth, excessive doglegs increase the overall friction to the drillstring, increasing the likelihood of getting stuck or not reaching the planned total depth. Usually these problems are manageable. If the dogleg impairs the well, remedial action can be taken, such as reaming or underreaming through the dogleg, or even sidetracking in extreme situations. |
| Drilling | doghouse | The steel-sided room adjacent to the rig floor, usually having an access door close to the driller’s controls. This general-purpose shelter is a combination tool shed, office, communications center, coffee room, lunchroom and general meeting place for the driller and his crew. It is at the same elevation as the rig floor, usually cantilevered out from the main substructure supporting the rig. |
| Drilling | dogleg | A particularly crooked place in a wellbore where the trajectory of the wellbore in three-dimensional space changes rapidly. While a dogleg is sometimes created intentionally by directional drillers, the term more commonly refers to a section of the hole that changes direction faster than anticipated or desired, usually with harmful side effects. In surveying wellbore trajectories, a standard calculation of dogleg severity is made, usually expressed in two-dimensional degrees per 100 feet [degrees per 30 m] of wellbore length. There are several difficulties associated with doglegs. First, the wellbore is not located in the planned path. Second is the possibility that a planned casing string may no longer easily fit through the curved section. Third, repeated abrasion by the drillstring in a particular location of the dogleg results in a worn spot called a keyseat, in which the bottomhole assembly components may become stuck as they are pulled through the section. Fourth, casing successfully cemented through the dogleg may wear unusually quickly due to higher contact forces between the drillstring and the inner diameter (ID) of the casing through the dogleg. Fifth, a relatively stiff bottomhole assembly may not easily fit through the dogleg section drilled with a relatively limber BHA. Sixth, excessive doglegs increase the overall friction to the drillstring, increasing the likelihood of getting stuck or not reaching the planned total depth. Usually these problems are manageable. If the dogleg impairs the well, remedial action can be taken, such as reaming or underreaming through the dogleg, or even sidetracking in extreme situations. |
| Drilling | dope | Pipe dope, a specially formulated blend of lubricating grease and fine metallic particles that prevents thread galling (a particular form of metal-to-metal damage) and seals the roots or void spaces of threads. The American Petroleum Institute (API) specifies properties of pipe dope, including its coefficient of friction. The rig crew applies copious amounts of pipe dope to the drillpipe tool joints every time a connection is made. |
| Drilling | dope | To place lubricant on drillpipe, also known as “doping” the pipe. |
| Drilling | drag bit | A drilling tool that uses polycrystalline diamond compact (PDC) cutters to shear rock with a continuous scraping motion. These cutters are synthetic diamond disks about 1/8-in. thick and about 1/2 to 1 in. in diameter. PDC bits are effective at drilling shale formations, especially when used in combination with oil-base muds. |
| Drilling | drawworks | The machine on the rig consisting of a large-diameter steel spool, brakes, a power source and assorted auxiliary devices. The primary function of the drawworks is to reel out and reel in the drilling line, a large diameter wire rope, in a controlled fashion. The drilling line is reeled over the crown block and traveling block to gain mechanical advantage in a “block and tackle” or “pulley” fashion. This reeling out and in of the drilling line causes the traveling block, and whatever may be hanging underneath it, to be lowered into or raised out of the wellbore. The reeling out of the drilling line is powered by gravity and reeling in by an electric motor or diesel engine. |
| Drilling | drift | A term to describe the inclination from vertical of a wellbore. |
| Drilling | drift | To guarantee the inside diameter of a pipe or other cylindrical tool by pulling a cylinder or pipe (often called a rabbit) of known outside diameter through it. The drift diameter is the inside diameter (ID) that the pipe manufacturer guarantees per specifications. Note that the nominal inside diameter is not the same as the drift diameter but is always slightly larger. The drift diameter is used by the well planner to determine what size tools or casing strings can later be run through the casing, whereas the nominal inside diameter is used for fluid volume calculations such as mud circulating times and cement slurry placement calculations. |
| Drilling | drift | To pass a gauge through casing, tubulars and completion components to ensure minimum-diameter specifications are within tolerance, as described in definition 2. This task is also performed to ensure that there is no junk, dried cement, dirt, rocks or other debris inside the pipe. |
| Drilling | drill bit | The tool used to crush or cut rock. Everything on a drilling rig directly or indirectly assists the bit in crushing or cutting the rock. The bit is on the bottom of the drillstring and must be changed when it becomes excessively dull or stops making progress. Most bits work by scraping or crushing the rock, or both, usually as part of a rotational motion. Some bits, known as hammer bits, pound the rock vertically in much the same fashion as a construction site air hammer. |
| Drilling | drill collar | A component of a drillstring that provides weight on bit for drilling. Drill collars are thick-walled tubular pieces machined from solid bars of steel, usually plain carbon steel but sometimes of nonmagnetic nickel-copper alloy or other nonmagnetic premium alloys. The bars of steel are drilled from end to end to provide a passage to pumping drilling fluids through the collars. The outside diameter of the steel bars may be machined slightly to ensure roundness, and in some cases may be machined with helical grooves (“spiral collars”). Last, threaded connections, male on one end and female on the other, are cut so multiple collars can be screwed together along with other downhole tools to make a bottomhole assembly (BHA). Gravity acts on the large mass of the collars to provide the downward force needed for the bits to efficiently break rock. To accurately control the amount of force applied to the bit, the driller carefully monitors the surface weight measured while the bit is just off the bottom of the wellbore. Next, the drillstring (and the drill bit), is slowly and carefully lowered until it touches bottom. After that point, as the driller continues to lower the top of the drillstring, more and more weight is applied to the bit, and correspondingly less weight is measured as hanging at the surface. If the surface measurement shows 20,000 pounds [9080 kg] less weight than with the bit off bottom, then there should be 20,000 pounds force on the bit (in a vertical hole). Downhole MWD sensors measure weight-on-bit more accurately and transmit the data to the surface. |
| Drilling | drill pipe | Tubular steel conduit fitted with special threaded ends called tool joints. The drillpipe connects the rig surface equipment with the bottomhole assembly and the bit, both to pump drilling fluid to the bit and to be able to raise, lower and rotate the bottomhole assembly and bit. |
| Drilling | drill ship | A maritime vessel modified to include a drilling rig and special station-keeping equipment. The vessel is typically capable of operating in deep water. A drillship must stay relatively stationary on location in the water for extended periods of time. This positioning may be accomplished with multiple anchors, dynamic propulsion (thrusters) or a combination of these. Drillships typically carry larger payloads than semisubmersible drilling vessels, but their motion characteristics are usually inferior. |
| Drilling | drill string | The combination of the drillpipe, the bottomhole assembly and any other tools used to make the drill bit turn at the bottom of the wellbore. |
| Drilling | driller | The supervisor of the rig crew. The driller is responsible for the efficient operation of the rigsite as well as the safety of the crew and typically has many years of rigsite experience. Most drillers have worked their way up from other rigsite jobs. While the driller must know how to perform each of the jobs on the rig, his or her role is to supervise the work and control the major rig systems. The driller operates the pumps, drawworks, and rotary table via the drillers console-a control room of gauges, control levers, rheostats, and other pneumatic, hydraulic and electronic instrumentation. The driller also operates the drawworks brake using a long-handled lever. Hence, the driller is sometimes referred to as the person who is “on the brake. |
| Drilling | drilling break | A sudden increase in the rate of penetration during drilling. When this increase is significant (two or more times the normal speed, depending on local conditions), it may indicate a formation change, a change in the pore pressure of the formation fluids, or both. It is commonly interpreted as an indication of the bit drilling sand (high-speed drilling) rather than shale (low-speed drilling). The fast-drilling formation may or may not contain high-pressure fluids. Therefore, the driller commonly stops drilling and performs a flow check to determine if the formation is flowing. If the well is flowing, or if the results are uncertain, the driller may close the blowout preventers or circulate bottoms-up. Depending on the bit being used and the formations being drilled, a formation, even if sand, may sometimes drill slower rather than faster. This slowing of drilling progress, while technically also a drilling break, is usually referred to as a “reverse drilling break”, or simply “reverse break. |
| Drilling | drilling contractor | The company that owns and operates a drilling rig. The drilling contractor usually charges a fixed daily rate for its hardware (the rig) and software (the people), plus certain extraordinary expenses. Under this arrangement, the cost of the well is largely a function of the time it takes to drill and complete the well. The other primary contracting methods are footage rates (where the contractor receives an agreed upon amount per foot of hole drilled), or turnkey operations, where the contractor may assume substantial risk of the operations and receives a lump sum payment upon supplying a well of a given specification to the operator. |
| Drilling | drilling crew | Personnel who operate the drilling rig. The crew typically consists of roustabouts, roughnecks, floor hands, lead tong operators, motormen, derrickmen, assistant drillers, and the driller. Since drilling rigs operate around the clock, there are at least two crews (twelve hour work shifts called tours, more common when operating offshore), or three crews (eight hour tours, more common onshore). In addition, drilling contractors must be able to supply relief crews from time to time when crew members are unavailable. Though less common now than in years past, the drilling contractor may opt to hire only a driller, and the driller in turn is responsible for hiring everyone reporting to him. |
| Drilling | drilling foreman | The location supervisor for the drilling contractor. The toolpusher is usually a senior, experienced individual who has worked his way up through the ranks of the drilling crew positions. His job is largely administrative, including ensuring that the rig has sufficient materials, spare parts and skilled personnel to continue efficient operations. The toolpusher also serves as a trusted advisor to many personnel on the rigsite, including the operator’s representative, the company man. |
| Drilling | drilling procedure | The engineering plan for constructing the wellbore. The plan includes well geometries, casing programs, mud considerations, well control concerns, initial bit selections, offset well information, pore pressure estimations, economics and special procedures that may be needed during the course of the well. Although drilling procedures are carefully developed, they are subject to change if drilling conditions dictate. |
| Drilling | drilling program | The engineering plan for constructing the wellbore. The plan includes well geometries, casing programs, mud considerations, well control concerns, initial bit selections, offset well information, pore pressure estimations, economics and special procedures that may be needed during the course of the well. Although drilling procedures are carefully developed, they are subject to change if drilling conditions dictate. |
| Drilling | drilling rate | The speed at which the drill bit can break the rock under it and thus deepen the wellbore. This speed is usually reported in units of feet per hour or meters per hour. |
| Drilling | drilling rig | The machine used to drill a wellbore. In onshore operations, the rig includes virtually everything except living quarters. Major components of the rig include the mud tanks, the mud pumps, the derrick or mast, the drawworks, the rotary table or topdrive, the drillstring, the power generation equipment and auxiliary equipment. Offshore, the rig includes the same components as onshore, but not those of the vessel or drilling platform itself. The rig is sometimes referred to as the drilling package, particularly offshore. |
| Drilling | drilling riser | A large-diameter pipe that connects the subsea BOP stack to a floating surface rig to take mud returns to the surface. Without the riser, the mud would simply spill out of the top of the stack onto the seafloor. The riser might be loosely considered a temporary extension of the wellbore to the surface. |
| Drilling | drillpipe | Tubular steel conduit fitted with special threaded ends called tool joints. The drillpipe connects the rig surface equipment with the bottomhole assembly and the bit, both to pump drilling fluid to the bit and to be able to raise, lower and rotate the bottomhole assembly and bit. |
| Drilling | drillship | A maritime vessel modified to include a drilling rig and special station-keeping equipment. The vessel is typically capable of operating in deep water. A drillship must stay relatively stationary on location in the water for extended periods of time. This positioning may be accomplished with multiple anchors, dynamic propulsion (thrusters) or a combination of these. Drillships typically carry larger payloads than semisubmersible drilling vessels, but their motion characteristics are usually inferior. |
| Drilling | drillstem | The combination of the drillpipe, the bottomhole assembly and any other tools used to make the drill bit turn at the bottom of the wellbore. |
| Drilling | drillstem test | A procedure to determine the productive capacity, pressure, permeability or extent (or a combination of these) of a hydrocarbon reservoir. While several different proprietary hardware sets are available to accomplish this, the common idea is to isolate the zone of interest with temporary packers. Next, one or more valves are opened to produce the reservoir fluids through the drillpipe and allow the well to flow for a time. Finally, the operator kills the well, closes the valves, removes the packers and trips the tools out of the hole. Depending on the requirements and goals for the test, it may be of short (one hour or less) or long (several days or weeks) duration and there might be more than one flow period and pressure buildup period. |
| Drilling | drillstring | The combination of the drillpipe, the bottomhole assembly and any other tools used to make the drill bit turn at the bottom of the wellbore. |
| Drilling | dry hole | A wellbore that has not encountered hydrocarbons in economically producible quantities. Most wells contain salt water in some zones. In addition, the wellbore usually encounters small amounts of crude oil and natural gas. Whether the well is a “duster” depends on many factors of the economic equation, including proximity to transport and processing infrastructures, local market conditions, expected completion costs, tax and investment recovery conditions of the jurisdiction and projected oil and gas prices during the productive life of the well. |
| Drilling | duster | Slang term for dry hole |
| Drilling | dynamic positioning | The stationing of a vessel, especially a drillship or semisubmersible drilling rig, at a specific location in the sea by the use of computer-controlled propulsion units called thrusters. Though drilling vessels have varying sea and weather state design conditions, most remain relatively stable even under high wind, wave and current loading conditions. Inability to maintain stationkeeping, whether due to excessive natural forces or failure of one or more electromechanical systems, leads to a “drive off” condition that requires emergency procedures to disconnect the riser from the subsea BOP stack, or worse, drop the riser from the vessel altogether. |
| Drilling | eccentric | Off-center, as when a pipe is off-center within another pipe or the openhole. Eccentricity is usually expressed as a percentage. A pipe would be considered to be fully (100%) eccentric if it were lying against the inside diameter of the enclosing pipe or hole. A pipe would be said to be concentric (0% eccentric) if it were perfectly centered in the outer pipe or hole. Eccentricity becomes important to the well designer in estimating casing wear, wear and tear on the drillstring, and the removal of cuttings from the low side of an inclined hole. In the latter case, if the drillpipe lies on the low side of the hole (100% eccentric), the eccentricity results in low-velocity fluid flow on the low side. Gravity pulls cuttings to the low side of the hole, building a bed of small rock chips on the low side of the hole known as a cuttings bed. This cuttings bed becomes difficult to clean out of the annulus and can lead to significant problems for the drilling operation if the pipe becomes stuck in the cuttings bed. |
| Drilling | eccentricity | The term used to describe how off-center a pipe is within another pipe or the openhole. It is usually expressed as a percentage. A pipe would be considered to be fully (100%) eccentric if it were lying against the inside diameter of the enclosing pipe or hole. A pipe would be said to be concentric (0% eccentric) if it were perfectly centered in the outer pipe or hole. Eccentricity becomes important to the well designer in estimating casing wear, wear and tear on the drillstring, and the removal of cuttings from the low side of an inclined hole. In the latter case, if the drillpipe lies on the low side of the hole (100% eccentric), the eccentricity results in low-velocity fluid flow on the low side. Gravity pulls cuttings to the low side of the hole, building a bed of small rock chips on the low side of the hole known as a cuttings bed. This cuttings bed becomes difficult to clean out of the annulus and can lead to significant problems for the drilling operation if the pipe becomes stuck in the cuttings bed. |
| Drilling | electrodynamic brake | An electric motor that acts as a brake. Braking is accomplished by reversing the electric fields on the motor, effectively turning it into a generator. The usage of the generated power, either in useful applications or dissipation as heat, restrains the motor-turned-generator and provides a braking action. |
| Drilling | elevator | A hinged mechanism that may be closed around drillpipe or other drillstring components to facilitate lowering them into the wellbore or lifting them out of the wellbore. In the closed position, the elevator arms are latched together to form a load-bearing ring around the component. A shoulder or taper on the component to be lifted is larger in size than the inside diameter of the closed elevator. In the open position, the device splits roughly into two halves and may be swung away from the drillstring component. |
| Drilling | embrittlement | The process whereby steel components become less resistant to breakage and generally much weaker in tensile strength. While embrittlement has many causes, in the oil field it is usually the result of exposure to gaseous or liquid hydrogen sulfide [H2S]. On a molecular level, hydrogen ions work their way between the grain boundaries of the steel, where hydrogen ions recombine into molecular hydrogen [H2], taking up more space and weakening the bonds between the grains. The formation of molecular hydrogen can cause sudden metal failure due to cracking when the metal is subjected to tensile stress. This type of hydrogen-induced failure is produced when hydrogen atoms enter high strength steels. The failures due to hydrogen embrittlement normally have a period where no damage is observed, which is called incubation, followed by a sudden catastrophic failure. Hydrogen embrittlement is also called acid brittleness. |
| Drilling | endless tubing | Another term for coiled tubing, a long, continuous length of pipe wound on a spool. The pipe is straightened prior to pushing into a wellbore and rewound to coil the pipe back onto the transport and storage spool. Depending on the pipe diameter (1 in. to 4 1/2 in.) and the spool size, coiled tubing can range from 2,000 ft to 15,000 ft [610 to 4,570 m] or greater length. |
| Drilling | ERD | Abbreviation for extended-reach drilling. Mobil Oil Company first used this term in the early 1980s for drilling directional wells in which the drilled horizontal reach (HR) attained at total depth (TD) exceeded the true vertical depth (TVD) by a factor greater than or equal to two. Extended-reach drilling (ERD) is particularly challenging for directional drilling and requires specialized planning to execute well construction. Since the term was coined, the scope of extended-reach drilling has broadened and the definition, which is now more flexible, includes deep wells with horizontal distance-to-depth, or H:V, ratios less than two. The drilling industry’s ERD database classifies wells, with increasing degree of well construction complexity, into low-, medium-, extended- and very extended-reach wells. Construction complexity depends on many factors, including water depth (for offshore wells), rig capability, geologic constraints and overall TVD. For example, a vertical well with TVD greater than 7,620 m [25,000 ft] is considered an extended-reach well. Also, depending on the conditions, drilling a well in deep water or through salt may be classified as ERD even if the well’s horizontal extent is not more than twice its TVD. |
| Drilling | escape line | A steel cable attached to the rig derrick or mast near the work platform for the derrickman. This cable is anchored at surface level (on a vessel or the Earth) away from the mast in a loose catenary profile, and fitted with a handle and hand brake that is stored at the top. The escape line provides a rapid escape path for the derrickman should well conditions or massive mechanical failure warrant. In such a case the derrickman would disconnect his safety belt from the rig, rehook it over the escape line if time permitted, firmly grip the tee-bar handle and ride the trolley down the cable while holding on to the handle with his hands. The escape line is also known as the “Geronimo line. |
| Drilling | evening tour | The work shift of a drilling crew that starts in the evening or late afternoon. Drilling operations usually occur around the clock because of the cost to rent a rig. As a result, there are usually two separate crews working twelve-hour tours to keep the operation going. Some companies prefer three eight-hour tours: the evening tour starts at 4 PM; the graveyard tour is the overnight shift or the shift that begins at midnight. (Pronounced “tower” in many areas.) |
| Drilling | fingerboard | The working platform approximately halfway up the derrick or mast in which the derrickman stores drillpipe and drill collars in an orderly fashion during trips out of the hole. The entire platform consists of a small section from which the derrickman works (called the monkeyboard), and several steel fingers with slots between them that keep the tops of the drillpipe in place. |
| Drilling | fish | Anything left in a wellbore. It does not matter whether the fish consists of junk metal, a hand tool, a length of drillpipe or drill collars, or an expensive MWD and directional drilling package. Once the component is lost, it is properly referred to as simply “the fish.” Typically, anything put into the hole is accurately measured and sketched, so that appropriate fishing tools can be selected if the item must be fished out of the hole. |
| Drilling | fish | To attempt to retrieve a fish from a wellbore. Where available, specially skilled individuals, aptly called fishermen, are called onto location to direct and assist with the fishing operations. Depending on the type of fish, the manner in which it was lost, regulatory requirements (for example a fish that includes a nuclear source, such as certain well logging tools), and the value of the fish if recovered, fishing operations may be immediately successful or may be attempted unsuccessfully for several days or even weeks. |
| Drilling | fishing tool | A general term for special mechanical devices used to aid the recovery of equipment lost downhole. These devices generally fall into four classes: diagnostic, inside grappling, outside grappling, and force intensifiers or jars. Diagnostic devices may range from a simple impression block made in a soft metal, usually lead, that is dropped rapidly onto the top of the fish so that upon inspection at the surface, the fisherman may be able to custom design a tool to facilitate attachment to and removal of the fish. Other diagnostic tools may include electronic instruments and even downhole sonic or visual-bandwidth cameras. Inside grappling devices, usually called spears, generally have a tapered and threaded profile, enabling the fisherman to first guide the tool into the top of the fish, and then thread the fishing tool into the top of the fish so that recovery may be attempted. Outside grappling devices, usually called overshots, are fitted with threads or another shape that “swallows” the fish and does not release it as it is pulled out of the hole. Overshots are also fitted with a crude drilling surface at the bottom, so that the overshot may be lightly drilled over the fish, sometimes to remove rock or metallic junk that may be part of the sticking mechanism. Jars are mechanical downhole hammers, which enable the fisherman to deliver high-impact loads to the fish, far in excess of what could be applied in a quasi-static pull from the surface. |
| Drilling | fixed-cutter bit | A drilling tool that uses polycrystalline diamond compact (PDC) cutters to shear rock with a continuous scraping motion. These cutters are synthetic diamond disks about 1/8-in. thick and about 1/2 to 1 in. in diameter. PDC bits are effective at drilling shale formations, especially when used in combination with oil-base muds. |
| Drilling | flapper valve | A check valve that has a spring-loaded plate (or flapper) that may be pumped through, generally in the downhole direction, but closes if the fluid attempts to flow back through the drillstring to the surface. This reverse flow might be encountered either due to a U-tube effect when the bulk density of the mud in the annulus is higher than that inside the drillpipe, or a well control event. |
| Drilling | float joint | A full-sized length of casing placed at the bottom of the casing string that is usually left full of cement on the inside to ensure that good cement remains on the outside of the bottom of the casing. If cement were not left inside the casing in this manner, the risk of overdisplacing the cement (due to improper casing volume calculations, displacement mud volume measurements, or both) would be significantly higher. Hence, the well designer plans on a safety margin of cement left inside the casing to guarantee that the fluid left outside the casing is good-quality cement. A float collar is placed at the top of the float joint and a float shoe placed at the bottom to prevent reverse flow of cement back into the casing after placement. There can be one, two or three joints of casing used for this purpose. |
| Drilling | flow line | The large-diameter metal pipe that connects the bell nipple under the rotary table to the possum belly at the mud tanks. The flowline is simply an inclined, gravity-flow conduit to direct mud coming out the top of the wellbore to the mud surface-treating equipment. When drilling certain highly reactive clays, called “gumbo,” the flowline may become plugged and require considerable effort by the rig crew to keep it open and flowing. In addition, the flowline is usually fitted with a crude paddle-type flow-measuring device commonly called a “flow show” that may give the driller the first indication that the well is flowing. |
| Drilling | flowline | The large-diameter metal pipe that connects the bell nipple under the rotary table to the possum belly at the mud tanks. The flowline is simply an inclined, gravity-flow conduit to direct mud coming out the top of the wellbore to the mud surface-treating equipment. When drilling certain highly reactive clays, called “gumbo,” the flowline may become plugged and require considerable effort by the rig crew to keep it open and flowing. In addition, the flowline is usually fitted with a crude paddle-type flow-measuring device commonly called a “flow show” that may give the driller the first indication that the well is flowing. |
| Drilling | formation damage | Alteration of the far-field or virgin characteristics of a producing formation, usually by exposure to drilling fluids. The water or solid particles in the drilling fluids, or both, tend to decrease the pore volume and effective permeability of the producible formation in the near-wellbore region. At least two mechanisms are at work. First, solid particles from the drilling fluid physically plug or bridge across flowpaths in the porous formation. Second, when water contacts certain clay minerals in the formation, the clay typically swells, increasing in volume and decreasing the pore volume. Third, chemical reactions between the drilling fluid and the formation rock and fluids can precipitate solids or semisolids that plug pore spaces. One approach to minimize formation damage is to use drill-in or completion fluids that are specially formulated to avoid damage to the formation when drilling pay zones, rather than ordinary drilling fluids. |
| Drilling | free water | In cementing, any water in the slurry that is in excess of what is required to fully hydrate the Portland cement and other additives. Free water can physically separate as a cement slurry sets. This separation tendency, especially in the presence of a high-pressure gas-bearing formation, can impair zonal isolation, the primary job of the cement. For that reason, the well designer usually specifies a maximum free-water content for the slurry. |
| Drilling | gamma ray log | A common and inexpensive measurement of the natural emission of gamma rays by a formation. Gamma ray logs are particularly helpful because shales and sandstones typically have different gamma ray signatures that can be correlated readily between wells. |
| Drilling | gas cut mud |
Drilling fluid whose bulk, unpressurized density is reduced as a small volume of gas displaces an equivalent volume of liquid. The derrickman periodically measures mud density and communicates the results to the rig team via an intercom. He usually reports something like 9.6 heavy, 10.4, or 13.2 light, indicating more than 9.6 pounds per gallon, 10.4 pounds per gallon, or less than 13.2 pounds per gallon, respectively. Each tenth of a pound per gallon is referred to as a point of mud weight. Note that for this low-accuracy measurement, no direct mention of gas cut is made. A gas cut is inferred only if the mud returning to the surface is significantly less dense than it should be. In the case of the mud logger’s measurement, units of gas (having virtually no absolute meaning) are reported. For the mud logger’s measurement, a direct indication of combustible gases is made, with no direct correlation to mud weight. |
| Drilling | gas show | Gas that rises to the surface, usually detected because it reduces the density of the drilling mud. Gas detectors, which the mud logger monitors, measure combustible gases (methane, ethane, butane and others). The mud logger reports total gas, individual gas components, or both, on the mud log. In extreme cases, gas visibly bubbles out of the mud as it returns to the surface. Because the mud does not circulate to the surface for a considerable time, sometimes lagging several hours after a formation is drilled, a gas show may be representative of what happened in the wellbore hours (or many feet) prior to the current total depth of the well. |
| Drilling | gauge hole | A wellbore that is essentially the same diameter as the bit that was used to drill it. It is common to find well-consolidated sandstones and carbonate rocks that remain gauge after being drilled. For clays, it is common for the hole to slowly enlarge with the passing of time, especially if water-base muds are being used. Bit gauges, rings of defined circumference, are slipped around drill bits to detect and measure wear, which reduces the circumference of the bit during drilling. |
| Drilling | geosteer | To control the direction of a well based on the results of downhole geological logging measurements rather than three-dimensional targets in space, usually to keep a directional wellbore within a pay zone. In mature areas, geosteering may be used to keep a wellbore in a particular section of a reservoir to minimize gas or water breakthrough and maximize economic production from the well. |
| Drilling | geosteering | The intentional directional control of a well based on the results of downhole geological logging measurements rather than three-dimensional targets in space, usually to keep a directional wellbore within a pay zone. In mature areas, geosteering may be used to keep a wellbore in a particular section of a reservoir to minimize gas or water breakthrough and maximize economic production from the well. |
| Drilling | Geronimo line | A steel cable attached to the rig derrick or mast near the work platform for the derrickman. This cable is anchored at surface level (on a vessel or the Earth) away from the mast in a loose catenary profile, and fitted with a handle and hand brake that is stored at the top. The escape line provides a rapid escape path for the derrickman should well conditions or massive mechanical failure warrant. In such a case the derrickman would disconnect his safety belt from the rig, rehook it over the escape line if time permitted, firmly grip the tee-bar handle and ride the trolley down the cable while holding on to the handle with his hands. The escape line is also known as the “Geronimo line. |
| Drilling | goose neck | An inverted “U” shaped section of rigid piping normally used as a conduit for high-pressure drilling fluid. In particular, the term is applied to a structure that connects the top of a vertical standpipe running up the side of a derrick or mast to a flexible kelly hose that in turn is connected to another gooseneck between the flexible line and the swivel. |
| Drilling | gooseneck | An inverted “U” shaped section of rigid piping normally used as a conduit for high-pressure drilling fluid. In particular, the term is applied to a structure that connects the top of a vertical standpipe running up the side of a derrick or mast to a flexible kelly hose that in turn is connected to another gooseneck between the flexible line and the swivel. |
| Drilling | graveyard tour | The overnight work shift of a drilling crew. Drilling operations usually occur around the clock because of the cost to rent a rig. As a result, there are usually two separate crews working twelve-hour tours to keep the operation going. Some companies prefer three eight-hour tours. The graveyard tour is the shift that begins at midnight. (Pronounced “tower” in many areas.) |
| Drilling | gravity toolface | Toolface angle used for deviated wells. Gravity toolface is the angle of the borehole survey instrument within the wellbore measured clockwise relative to up and in the plane perpendicular to the wellbore axis; the high side (maximum build), maximum right, low side (maximum drop) and maximum left directions have gravity toolface angles of 0°, 90°, 180° and 270°, respectively. |
| Drilling | guide shoe | A tapered, often bullet-nosed piece of equipment often found on the bottom of a casing string. The device guides the casing toward the center of the hole and minimizes problems associated with hitting rock ledges or washouts in the wellbore as the casing is lowered into the well. The outer portions of the guide shoe are made from steel, generally matching the casing in size and threads, if not steel grade. The inside (including the taper) is generally made of cement or thermoplastic, since this material must be drilled out if the well is to be deepened beyond the casing point. It differs from a float shoe in that it lacks a check valve. |
| Drilling | gumbo | A generic term for soft, sticky, swelling clay formations that are frequently encountered in surface holes offshore or in sedimentary basins onshore near seas. This clay fouls drilling tools and plugs piping, both severe problems for drilling crews. |
| Drilling | H2S | [H2S] An extraordinarily poisonous gas with a molecular formula of H2S. At low concentrations, H2S has the odor of rotten eggs, but at higher, lethal concentrations, it is odorless. H2S is hazardous to workers and a few seconds of exposure at relatively low concentrations can be lethal, but exposure to lower concentrations can also be harmful. The effect of H2S depends on duration, frequency and intensity of exposure as well as the susceptibility of the individual. Hydrogen sulfide is a serious and potentially lethal hazard, so awareness, detection and monitoring of H2S is essential. Since hydrogen sulfide gas is present in some subsurface formations, drilling and other operational crews must be prepared to use detection equipment, personal protective equipment, proper training and contingency procedures in H2S-prone areas. Hydrogen sulfide is produced during the decomposition of organic matter and occurs with hydrocarbons in some areas. It enters drilling mud from subsurface formations and can also be generated by sulfate-reducing bacteria in stored muds. H2S can cause sulfide-stress-corrosion cracking of metals. Because it is corrosive, H2S production may require costly special production equipment such as stainless steel tubing. Sulfides can be precipitated harmlessly from water muds or oil muds by treatments with the proper sulfide scavenger. H2S is a weak acid, donating two hydrogen ions in neutralization reactions, forming HS- and S-2 ions. In water or water-base muds, the three sulfide species, H2S and HS- and S-2 ions, are in dynamic equilibrium with water and H+ and OH- ions. The percent distribution among the three sulfide species depends on pH. H2S is dominant at low pH, the HS- ion is dominant at mid-range pH and S2 ions dominate at high pH. In this equilibrium situation, sulfide ions revert to H2S if pH falls. Sulfides in water mud and oil mud can be quantitatively measured with the Garrett Gas Train according to procedures set by API. |
| Drilling | hardbanding | A process in which a wear-resistant alloy is applied to the tool joints of drillpipe or drill collars to prolong the life of oilfield tubulars. Hardbanding is applied where rotational and axial friction associated with drilling and tripping create excessive abrasive wear between drillstring and casing, or between drillstring and rock. Hard alloy overlays are applied to the points of greatest contact, typically using advanced welding techniques. Typical alloys used in this process range from ultra-wear resistant tungsten carbide, to less abrasive chromium carbide, titanium carbide and borides. Some hardbanding processes take a different approach to reducing wear in tubulars, using materials that achieve a low coefficient of friction used to protect the drillstring from abrasion. |
| Drilling | heavyweight drillpipe | A type of drillpipe whose walls are thicker and collars are longer than conventional drillpipe. HWDP tends to be stronger and has higher tensile strength than conventional drillpipe, so it is placed near the top of a long drillstring for additional support. |
| Drilling | HHP | A measure of the energy per unit of time that is being expended across the bit nozzles. It is commonly calculated with the equation HHP=P*Q/1714, where P stands for pressure in pounds per square in., Q stands for flow rate in gallons per minute, and 1714 is a conversion factor necessary to yield HHP in terms of horsepower. Bit manufacturers often recommend that fluid hydraulics energy across the bit nozzles be in a particular HHP range, for example 2.0 to 7.0 HHP, to ensure adequate bit tooth and bottom-of-hole cleaning (the minimum HHP) and to avoid premature erosion of the bit itself (the maximum HHP). |
| Drilling | high side toolface | Toolface angle used for deviated wells. High side toolface is the angle of the borehole survey instrument within the wellbore measured clockwise relative to up and in the plane perpendicular to the wellbore axis; the high side (maximum build), maximum right, low side (maximum drop) and maximum left directions have high side toolface angles of 0°, 90°, 180° and 270°, respectively. |
| Drilling | high-pressure, high-temperature | Pertaining to wells that are hotter or higher pressure than most. The term came into use upon the release of the Cullen report on the Piper Alpha platform disaster in the UK sector of the North Sea, along with the contemporaneous loss of the Ocean Odyssey semisubmersible drilling vessel in Scottish jurisdictional waters. In the UK, HPHT is formally defined as a well having an undisturbed bottomhole temperature of greater than 300oF [149oC] and a pore pressure of at least 0.8 psi/ft (~15.3 lbm/gal) or requiring a BOP with a rating in excess of 10,000 psi [68.95 MPa]. Although the term was coined relatively recently, wells meeting the definition have been safely drilled and completed around the world for decades. |
| Drilling | highside toolface | Toolface angle used for deviated wells. Highside toolface is the angle of the borehole survey instrument within the wellbore measured clockwise relative to up and in the plane perpendicular to the wellbore axis; the high side (maximum build), maximum right, low side (maximum drop) and maximum left directions have highside toolface angles of 0°, 90°, 180° and 270°, respectively. |
| Drilling | high-side toolface | Toolface angle used for deviated wells. High-side toolface is the angle of the borehole survey instrument within the wellbore measured clockwise relative to up and in the plane perpendicular to the wellbore axis; the high side (maximum build), maximum right, low side (maximum drop) and maximum left directions have high-side toolface angles of 0°, 90°, 180° and 270°, respectively. |
| Drilling | hook | The high-capacity J-shaped equipment used to hang various other equipment, particularly the swivel and kelly, the elevator bails or topdrive units. The hook is attached to the bottom of the traveling block and provides a way to pick up heavy loads with the traveling block. The hook is either locked (the normal condition) or free to rotate, so that it may be mated or decoupled with items positioned around the rig floor, not limited to a single direction. |
| Drilling | hook load | The total force pulling down on the hook. This total force includes the weight of the drillstring in air, the drill collars and any ancillary equipment, reduced by any force that tends to reduce that weight. Some forces that might reduce the weight include friction along the wellbore wall (especially in deviated wells) and, importantly, buoyant forces on the drillstring caused by its immersion in drilling fluid. If the BOPs are closed, any pressure in the wellbore acting on the cross-sectional area of the drillstring in the BOPs will also exert an upward force. |
| Drilling | hopper | In general, a funnel-shaped device used to transfer products. The hopper is often at the bottom of any container for holding or using bulk products, especially drilling fluid additives and cementing material. |
| Drilling | hopper | The device used to facilitate the addition of drilling fluid additives to the whole mud system. While several types of hoppers exist, they generally have a high velocity stream of mud going through them and a means of mixing either dry or liquid mud additives into the whole mud stream. The resultant mixed mud is then circulated back into the surface mud system. A hopper is generally used to introduce relatively small quantities of additives to the mud system. |
| Drilling | horizontal drilling | A subset of the more general term “directional drilling,” used where the departure of the wellbore from vertical exceeds about 80 degrees. Note that some horizontal wells are designed such that after reaching true 90-degree horizontal, the wellbore may actually start drilling upward. In such cases, the angle past 90 degrees is continued, as in 95 degrees, rather than reporting it as deviation from vertical, which would then be 85 degrees. Because a horizontal well typically penetrates a greater length of the reservoir, it can offer significant production improvement over a vertical well. |
| Drilling | hostile environment | A particularly difficult set of well conditions that may detrimentally affect steel, elastomers, mud additives, electronics, or tools and tool components. Such conditions typically include excessive temperatures, the presence of acid gases (H2S, CO2), chlorides, high pressures and, more recently, extreme measured depths. |
| Drilling | HPHT | Pertaining to wells that are hotter or higher pressure than most. The term came into use upon the release of the Cullen report on the Piper Alpha platform disaster in the UK sector of the North Sea, along with the contemporaneous loss of the Ocean Odyssey semisubmersible drilling vessel in Scottish jurisdictional waters. In the UK, HPHT is formally defined as a well having an undisturbed bottomhole temperature of greater than 300oF [149oC] and a pore pressure of at least 0.8 psi/ft (~15.3 lbm/gal) or requiring a BOP with a rating in excess of 10,000 psi [68.95 MPa]. Although the term was coined relatively recently, wells meeting the definition have been safely drilled and completed around the world for decades. |
| Drilling | HWDP | A type of drillpipe whose walls are thicker and collars are longer than conventional drillpipe. HWDP tends to be stronger and has higher tensile strength than conventional drillpipe, so it is placed near the top of a long drillstring for additional support. |
| Drilling | hydraulic horsepower | A measure of the energy per unit of time that is being expended across the bit nozzles. It is commonly calculated with the equation HHP=P*Q/1714, where P stands for pressure in pounds per square in., Q stands for flow rate in gallons per minute, and 1714 is a conversion factor necessary to yield HHP in terms of horsepower. Bit manufacturers often recommend that fluid hydraulics energy across the bit nozzles be in a particular HHP range, for example 2.0 to 7.0 HHP, to ensure adequate bit tooth and bottom-of-hole cleaning (the minimum HHP) and to avoid premature erosion of the bit itself (the maximum HHP). |
| Drilling | hydraulic horsepower | The power of a positive displacement pump. HHP is important for mud pumps and cement pumps. |
| Drilling | ID | Inside or inner diameter. Casing, tubing and drillpipe are commonly described in terms of inside diameter and outside diameter (OD). |
| Drilling | inclination | The deviation from vertical, irrespective of compass direction, expressed in degrees. Inclination is measured initially with a pendulum mechanism, and confirmed with MWD accelerometers or gyroscopes. For most vertical wellbores, inclination is the only measurement of the path of the wellbore. For intentionally deviated wellbores, or wells close to legal boundaries, directional information is usually also measured. |
| Drilling | inside blowout preventer | A valve in the drillstring that may be used to prevent the well from flowing uncontrollably up the drillstring. |
| Drilling | inside diameter | Inside or inner diameter. Casing, tubing and drillpipe are commonly described in terms of inside diameter and outside diameter (OD). |
| Drilling | intermediate casing string | A length of pipe used below the surface casing string, but before the production casing is run, to isolate one or more zones of the openhole to enable deepening of the well. There may be several intermediate casing strings. Depending on well conditions, these strings may have higher pressure integrity than the prior casing strings, especially when abnormally pressured formations are expected during the drilling of the next openhole section. |
| Drilling | jackup | A self-contained combination drilling rig and floating barge, fitted with long support legs that can be raised or lowered independently of each other. The jackup, as it is known informally, is towed onto location with its legs up and the barge section floating on the water. Upon arrival at the drilling location, the legs are jacked down onto the seafloor, preloaded to securely drive them into the seabottom, and then all three legs are jacked further down. Since the legs have been preloaded and will not penetrate the seafloor further, this jacking down of the legs has the effect of raising the jacking mechanism, which is attached to the barge and drilling package. In this manner, the entire barge and drilling structure are slowly raised above the water to a predetermined height above the water, so that wave, tidal and current loading acts only on the relatively small legs and not the bulky barge and drilling package. |
| Drilling | jackup rig | A self-contained combination drilling rig and floating barge, fitted with long support legs that can be raised or lowered independently of each other. The jackup, as it is known informally, is towed onto location with its legs up and the barge section floating on the water. Upon arrival at the drilling location, the legs are jacked down onto the seafloor, preloaded to securely drive them into the seabottom, and then all three legs are jacked further down. Since the legs have been preloaded and will not penetrate the seafloor further, this jacking down of the legs has the effect of raising the jacking mechanism, which is attached to the barge and drilling package. In this manner, the entire barge and drilling structure are slowly raised above the water to a predetermined height above the water, so that wave, tidal and current loading acts only on the relatively small legs and not the bulky barge and drilling package. |
| Drilling | jar | A mechanical device used downhole to deliver an impact load to another downhole component, especially when that component is stuck. There are two primary types, hydraulic and mechanical jars. While their respective designs are quite different, their operation is similar. Energy is stored in the drillstring and suddenly released by the jar when it fires. The principle is similar to that of a carpenter using a hammer. Kinetic energy is stored in the hammer as it is swung, and suddenly released to the nail and board when the hammer strikes the nail. Jars can be designed to strike up, down, or both. In the case of jarring up above a stuck bottomhole assembly, the driller slowly pulls up on the drillstring but the BHA does not move. Since the top of the drillstring is moving up, this means that the drillstring itself is stretching and storing energy. When the jars reach their firing point, they suddenly allow one section of the jar to move axially relative to a second, being pulled up rapidly in much the same way that one end of a stretched spring moves when released. After a few inches of movement, this moving section slams into a steel shoulder, imparting an impact load. In addition to the mechanical and hydraulic versions, jars are classified as drilling jars or fishing jars. The operation of the two types is similar, and both deliver approximately the same impact blow, but the drilling jar is built such that it can better withstand the rotary and vibrational loading associated with drilling. |
| Drilling | jet | A small-diameter tungsten carbide nozzle used in drill bits to produce a high-velocity drilling fluid stream exiting the bit. |
| Drilling | jet | The high-velocity fluid stream produced by the nozzles in the bit. |
| Drilling | jet | To drill soft, unconsolidated, usually shallow formations by eroding the “rock” below the bit by hydraulic impact loading alone. Though not as common as in the past, a bit may be fitted with asymmetric nozzles, one large and two or more small nozzles. If drillstring rotation is prevented during this jetting operation, the different nozzles tend to cause greater erosion on one side than the other, allowing the well to be intentionally deviated. |
| Drilling | jet | A small-diameter nozzle used to mix cement. |
| Drilling | jet mixer | A mixing system used to mix dry powder materials with a base liquid, such as cement slurry or drilling muds. A funnel for the dry powder is mounted above a profiled bowl that incorporates one or more jets through which the liquid is pumped. The venturi effect created by the jets draws the powder into the turbulent stream, providing a rapid and efficient mixing action. |
| Drilling | jet nozzle | The part of the bit that includes a hole or opening for drilling fluid to exit. The hole is usually small (around 0.25 in. in diameter) and the pressure of the fluid inside the bit is usually high, leading to a high exit velocity through the nozzles that creates a high-velocity jet below the nozzles. This high-velocity jet of fluid cleans both the bit teeth and the bottom of the hole. The sizes of the nozzles are usually measured in 1/32-in. increments (although some are recorded in millimeters), are always reported in “thirty-seconds” of size (i.e., fractional denominators are not reduced), and usually range from 6/32 to 32/32. |
| Drilling | jet velocity | The exit velocity of the drilling fluid after it accelerates through bit nozzles. |
| Drilling | joint | A length of pipe, usually referring to drillpipe, casing or tubing. While there are different standard lengths, the most common drillpipe joint length is around 30 ft [9 m]. For casing, the most common length of a joint is 40 ft [12 m]. |
| Drilling | junk | Anything in the wellbore that is not supposed to be there. The term is usually reserved for small pieces of steel such as hand tools, small parts, bit nozzles, pieces of bits or other downhole tools, and remnants of milling operations. |
| Drilling | A tool run into the wellbore to retrieve junk from the bottom of the hole. | |
| Drilling | junk basket | A large, rectangular steel box, usually with sides made of expanded metal to facilitate seeing what is inside. The junk basket is used by the rig crew to store an assortment of relatively small parts of the drilling rig, ranging from drill bits to crossover subs to lifting subs to spare kellys. Dimensions vary, but a typical junk basket on a land rig is 8 ft wide [2.5 m] by 3 ft [1 m] deep by 30 ft [9 m] long. |
| Drilling | KB | An adapter that serves to connect the rotary table to the kelly. The kelly bushing has an inside diameter profile that matches that of the kelly, usually square or hexagonal. It is connected to the rotary table by four large steel pins that fit into mating holes in the rotary table. The rotary motion from the rotary table is transmitted to the bushing through the pins, and then to the kelly itself through the square or hexagonal flat surfaces between the kelly and the kelly bushing. The kelly then turns the entire drillstring because it is screwed into the top of the drillstring itself. Depth measurements are commonly referenced to the KB, such as 8327 ft KB, meaning 8327 feet below the kelly bushing. |
| Drilling | kelly | A long square or hexagonal steel bar with a hole drilled through the middle for a fluid path. The kelly is used to transmit rotary motion from the rotary table or kelly bushing to the drillstring, while allowing the drillstring to be lowered or raised during rotation. The kelly goes through the kelly bushing, which is driven by the rotary table. The kelly bushing has an inside profile matching the kelly’s outside profile (either square or hexagonal), but with slightly larger dimensions so that the kelly can freely move up and down inside. |
| Drilling | kelly bushing | An adapter that serves to connect the rotary table to the kelly. The kelly bushing has an inside diameter profile that matches that of the kelly, usually square or hexagonal. It is connected to the rotary table by four large steel pins that fit into mating holes in the rotary table. The rotary motion from the rotary table is transmitted to the bushing through the pins, and then to the kelly itself through the square or hexagonal flat surfaces between the kelly and the kelly bushing. The kelly then turns the entire drillstring because it is screwed into the top of the drillstring itself. Depth measurements are commonly referenced to the KB, such as 8327 ft KB, meaning 8327 feet below the kelly bushing. |
| Drilling | kelly down | Referring to the condition that occurs when the kelly is all the way down, so drilling progress cannot continue. A connection must be made, which has the effect of raising the kelly up by the length of the new joint of drillpipe added, so drilling can resume |
| Drilling | kelly hose | A large-diameter (3- to 5-in. inside diameter), high-pressure flexible line used to connect the standpipe to the swivel. This flexible piping arrangement permits the kelly (and, in turn, the drillstring and bit) to be raised or lowered while drilling fluid is pumped through the drillstring. The simultaneous lowering of the drillstring while pumping fluid is critical to the drilling operation. |
| Drilling | kelly spinner | A mechanical device for rotating the kelly. The kelly spinner is typically pneumatic. It is a relatively low torque device, useful only for the initial makeup of threaded tool joints. It is not strong enough for proper torque of the tool joint or for rotating the drillstring itself. The kelly spinner has largely replaced the infamous spinning chains, which were responsible for numerous injuries on the rig floor. |
| Drilling | keyseat | A small-diameter channel worn into the side of a larger diameter wellbore. This can be the result of a sharp change in direction of the wellbore (a dogleg), or if a hard formation ledge is left between softer formations that enlarge over time. In either case, the diameter of the channel is typically similar to the diameter of the drillpipe. When larger diameter drilling tools such as tool joints, drill collars, stabilizers, and bits are pulled into the channel, their larger diameters will not pass and the larger diameter tools may become stuck in the keyseat. Preventive measures include keeping any turns in the wellbore gradual and smooth. The remedy to keyseating involves enlarging the worn channel so that the larger diameter tools will fit through it. |
| Drilling | kick | A flow of formation fluids into the wellbore during drilling operations. The kick is physically caused by the pressure in the wellbore being less than that of the formation fluids, thus causing flow. This condition of lower wellbore pressure than the formation is caused in two ways. First, if the mud weight is too low, then the hydrostatic pressure exerted on the formation by the fluid column may be insufficient to hold the formation fluid in the formation. This can happen if the mud density is suddenly lightened or is not to specification to begin with, or if a drilled formation has a higher pressure than anticipated. This type of kick might be called an underbalanced kick. The second way a kick can occur is if dynamic and transient fluid pressure effects, usually due to motion of the drillstring or casing, effectively lower the pressure in the wellbore below that of the formation. This second kick type could be called an induced kick. |
| Drilling | kick off | Intentionally deviate a vertical well. |
| Drilling | kickoff | The point at which a vertical well is intentionally deviated. |
| Drilling | kill | To stop a well from flowing or having the ability to flow into the wellbore. Kill procedures typically involve circulating reservoir fluids out of the wellbore or pumping higher density mud into the wellbore, or both. In the case of an induced kick, where the mud density is sufficient to kill the well but the reservoir has flowed as a result of pipe movement, the driller must circulate the influx out of the wellbore. In the case of an underbalanced kick, the driller must circulate the influx out and increase the density of the drilling fluid. In the case of a producing well, a kill fluid with sufficient density to overcome production of formation fluid is pumped into the well to stop the flow of reservoir fluids. |
| Drilling | kill line | A high-pressure pipe leading from an outlet on the BOP stack to the high-pressure rig pumps. During normal well control operations, kill fluid is pumped through the drillstring and annular fluid is taken out of the well through the choke line to the choke, which drops the fluid pressure to atmospheric pressure. If the drillpipe is inaccessible, it may be necessary to pump heavy drilling fluid in the top of the well, wait for the fluid to fall under the force of gravity, and then remove fluid from the annulus. In such an operation, while one high pressure line would suffice, it is more convenient to have two. In addition, this provides a measure of redundancy for the operation. In floating offshore operations, the choke and kill lines exit the subsea BOP stack and run along the outside of the riser to the surface. The volumetric and frictional effects of these long choke and kill lines must be taken into account to properly control the well. |
| Drilling | lag gas | Any gas deliberately introduced into the mud system to help a mudlogger or wellsite geologist track the amount of time or the number of mud pump strokes it takes to circulate mud from the kelly downhole through the drillstring to the bit, and back uphole to the gas trap at the shale shaker. This interval is used to calculate the lag period. |
| Drilling | leak off | The magnitude of pressure exerted on a formation that causes fluid to be forced into the formation. The fluid may be flowing into the pore spaces of the rock or into cracks opened and propagated into the formation by the fluid pressure. This term is normally associated with a test to determine the strength of the rock, commonly called a pressure integrity test (PIT) or a leakoff test (LOT). During the test, a real-time plot of injected fluid versus fluid pressure is plotted. The initial stable portion of this plot for most wellbores is a straight line, within the limits of the measurements. The leakoff is the point of permanent deflection from that straight portion. The well designer must then either adjust plans for the well to this leakoff pressure, or if the design is sufficiently conservative, proceed as planned. |
| Drilling | leakoff | The magnitude of pressure exerted on a formation that causes fluid to be forced into the formation. The fluid may be flowing into the pore spaces of the rock or into cracks opened and propagated into the formation by the fluid pressure. This term is normally associated with a test to determine the strength of the rock, commonly called a pressure integrity test (PIT) or a leakoff test (LOT). During the test, a real-time plot of injected fluid versus fluid pressure is plotted. The initial stable portion of this plot for most wellbores is a straight line, within the limits of the measurements. The leakoff is the point of permanent deflection from that straight portion. The well designer must then either adjust plans for the well to this leakoff pressure, or if the design is sufficiently conservative, proceed as planned. |
| Drilling | leakoff test | A test to determine the strength or fracture pressure of the open formation, usually conducted immediately after drilling below a new casing shoe. During the test, the well is shut in and fluid is pumped into the wellbore to gradually increase the pressure that the formation experiences. At some pressure, fluid will enter the formation, or leak off, either moving through permeable paths in the rock or by creating a space by fracturing the rock. The results of the leakoff test dictate the maximum pressure or mud weight that may be applied to the well during drilling operations. To maintain a small safety factor to permit safe well control operations, the maximum operating pressure is usually slightly below the leakoff test result. |
| Drilling | liner | A casing string that does not extend to the top of the wellbore, but instead is anchored or suspended from inside the bottom of the previous casing string. There is no difference between the casing joints themselves. The advantage to the well designer of a liner is a substantial savings in steel, and therefore capital costs. To save casing, however, additional tools and risk are involved. The well designer must trade off the additional tools, complexities and risks against the potential capital savings when deciding whether to design for a liner or a casing string that goes all the way to the top of the well (a “long string”). The liner can be fitted with special components so that it can be connected to the surface at a later time if need be. |
| Drilling | lost circulation | The reduced or total absence of fluid flow up the annulus when fluid is pumped through the drillstring. Though the definitions of different operators vary, this reduction of flow may generally be classified as seepage (less than 20 bbl/hr [3 m3/hr]), partial lost returns (greater than 20 bbl/hr [3 m3/hr] but still some returns), and total lost returns (where no fluid comes out of the annulus). In this severe latter case, the hole may not remain full of fluid even if the pumps are turned off. If the hole does not remain full of fluid, the vertical height of the fluid column is reduced and the pressure exerted on the open formations is reduced. This in turn can result in another zone flowing into the wellbore, while the loss zone is taking mud, or even a catastrophic loss of well control. Even in the two less severe forms, the loss of fluid to the formation represents a financial loss that must be dealt with, and the impact of which is directly tied to the per barrel cost of the drilling fluid and the loss rate over time. |
| Drilling | lost circulation material | Solid material intentionally introduced into a mud system to reduce and eventually prevent the flow of drilling fluid into a weak, fractured or vugular formation. This material is generally fibrous or plate-like in nature, as suppliers attempt to design slurries that will efficiently bridge over and seal loss zones. In addition, popular lost circulation materials are low-cost waste products from the food processing or chemical manufacturing industries. Examples of lost circulation material include ground peanut shells, mica, cellophane, walnut shells, calcium carbonate, plant fibers, cottonseed hulls, ground rubber, and polymeric materials. |
| Drilling | lost returns | The reduced or total absence of fluid flow up the annulus when fluid is pumped through the drillstring. Though the definitions of different operators vary, this reduction of flow may generally be classified as seepage (less than 20 bbl/hr [3 m3/hr]), partial lost returns (greater than 20 bbl/hr [3 m3/hr] but still some returns), and total lost returns (where no fluid comes out of the annulus). In this severe latter case, the hole may not remain full of fluid even if the pumps are turned off. If the hole does not remain full of fluid, the vertical height of the fluid column is reduced and the pressure exerted on the open formations is reduced. This in turn can result in another zone flowing into the wellbore, while the loss zone is taking mud, or even a catastrophic loss of well control. Even in the two less severe forms, the loss of fluid to the formation represents a financial loss that must be dealt with, and the impact of which is directly tied to the per barrel cost of the drilling fluid and the loss rate over time. |
| Drilling | LOT | Abbreviation for leakoff test, a test to determine the strength or fracture pressure of the open formation, usually conducted immediately after drilling below a new casing shoe. During the test, the well is shut in and fluid is pumped into the wellbore to gradually increase the pressure that the formation experiences. At some pressure, fluid will enter the formation, or leak off, either moving through permeable paths in the rock or by creating a space by fracturing the rock. The results of the leakoff test dictate the maximum pressure or mud weight that may be applied to the well during drilling operations. To maintain a small safety factor to permit safe well control operations, the maximum operating pressure is usually slightly below the leakoff test result. |
| Drilling | Love wave | A type of surface wave in which particles oscillate horizontally and perpendicularly to the direction of wave propagation. |
| Drilling | lubricator | A long, high-pressure pipe fitted to the top of a wellhead or Christmas tree so that tools may be put into a high-pressure well. The top of the lubricator assembly includes a high-pressure grease-injection section and sealing elements. The lubricator is installed on top of the tree and tested, the tools placed in the lubricator and the lubricator pressurized to wellbore pressure. Then the top valves of the tree are opened to enable the tools to fall or be pumped into the wellbore under pressure. To remove the tools, the reverse process is used: the tools are pulled up into the lubricator under wellbore pressure, the tree valves are closed, the lubricator pressure is bled off, and then the lubricator may be opened to remove the tools. |
| Drilling | magnetic toolface | Toolface angle used for near-vertical wells. Magnetic toolface is the angle, or azimuth, of the borehole survey instrument within the wellbore measured clockwise relative to magnetic north and in the plane perpendicular to the wellbore axis; the north, east, south and west directions have magnetic toolface angles of 0°, 90°, 180° and 270°, respectively. Magnetic toolface may be corrected to reference either grid north or true north. |
| Drilling | make a connection | To add a length of drillpipe to the drillstring to continue drilling. In what is called jointed pipe drilling, joints of drillpipe, each about 30 ft [9 m] long, are screwed together as the well is drilled. When the bit on the bottom of the drillstring has drilled down to where the kelly or topdrive at the top of the drillstring nears the drillfloor, the drillstring between the two must be lengthened by adding a joint or a stand (usually three joints) to the drillstring. Once the rig crew is ready, the driller stops the rotary, picks up off bottom to expose a threaded connection below the kelly and turns the pumps off. The crew sets the slips to grip the drillstring temporarily, unscrews that threaded connection and screws the kelly (or topdrive) into the additional joint (or stand) of pipe. The driller picks that joint or stand up to allow the crew to screw the bottom of that pipe into the top of the temporarily hanging drillstring. The driller then picks up the entire drillstring to remove the slips, carefully lowers the drillstring while starting the pumps and rotary, and resumes drilling when the bit touches bottom. A skilled rig crew can physically accomplish all of those steps in a minute or two. |
| Drilling | make hole | To deepen a wellbore with the drill bit. To drill ahead. |
| Drilling | make up | To tighten threaded connections. |
| Drilling | makeup cathead | A clutched, rotating spool that enables the driller to use the drawworks motor to apply tension to a chain connected to the makeup tongs. This tensioned chain, acting at right angles to the tong handle, imparts torque to the connection being tightened. |
| Drilling | makeup tongs | Large-capacity, self-locking wrenches used to grip drillstring components and apply torque. As with opposing pipe wrenches for a plumber, the tongs must be used in opposing pairs. As a matter of efficiency, one set of tongs is essentially tied off with a cable or chain to the derrick, and the other is actively pulled with mechanical catheads. The breakout tongs are the active tongs during breakout (or loosening) operations. The makeup tongs are active during makeup (or tightening) operations. |
| Drilling | managed pressure drilling | An adaptive drilling method used to precisely control the annular pressure throughout a wellbore. After determining the downhole pressure environment, drillers manage wellbore pressure constrained by the limits of formation properties. The annular pressure is kept slightly above the pore pressure to prevent the influx of formation fluids into the wellbore, but it is maintained well below the fracture initiation pressure. Rapid corrective actions can often be implemented in order to deal with observed pressure variations. The MPD process may utilize a variety of techniques including control of back pressure, adjusting mud density, modifying fluid rheology, adjusting the annular fluid level, controlling circulating friction and incorporating hole geometry in the well construction. The use of MPD to control the risks and costs of drilling wells that have narrow downhole pressure limits by actively managing the wellbore pressure profile has become a common practice. The dynamic control of annular pressures enables drilling wells that might not otherwise be practical. |
| Drilling | marine drilling riser | A large-diameter pipe that connects the subsea BOP stack to a floating surface rig to take mud returns to the surface. Without the riser, the mud would simply spill out of the top of the stack onto the seafloor. The riser might be loosely considered a temporary extension of the wellbore to the surface. |
| Drilling | mast | The structure used to support the crown block and the drillstring. Masts are usually rectangular or trapezoidal in shape and offer a very good stiffness, important to land rigs whose mast is laid down when the rig is moved. They suffer from being heavier than conventional derricks and consequently are not usually found in offshore environments, where weight is more of a concern than in land operations. |
| Drilling | MD | The length of the wellbore, as if determined by a measuring stick. This measurement differs from the true vertical depth of the well in all but vertical wells. Since the wellbore cannot be physically measured from end to end, the lengths of individual joints of drillpipe, drill collars and other drillstring elements are measured with a steel tape measure and added together. Importantly, the pipe is measured while in the derrick or laying on a pipe rack, in an untensioned, unstressed state. When the pipe is screwed together and put into the wellbore, it stretches under its own weight and that of the bottomhole assembly. Although this fact is well established, it is not taken into account when reporting the well depth. Hence, in virtually all cases, the actual wellbore is slightly deeper than the reported depth. |
| Drilling | measured depth | The length of the wellbore, as if determined by a measuring stick. This measurement differs from the true vertical depth of the well in all but vertical wells. Since the wellbore cannot be physically measured from end to end, the lengths of individual joints of drillpipe, drill collars and other drillstring elements are measured with a steel tape measure and added together. Importantly, the pipe is measured while in the derrick or laying on a pipe rack, in an untensioned, unstressed state. When the pipe is screwed together and put into the wellbore, it stretches under its own weight and that of the bottomhole assembly. Although this fact is well established, it is not taken into account when reporting the well depth. Hence, in virtually all cases, the actual wellbore is slightly deeper than the reported depth. |
| Drilling | measurements while drilling | The evaluation of physical properties, usually including pressure, temperature and wellbore trajectory in three-dimensional space, while extending a wellbore. MWD is now standard practice in offshore directional wells, where the tool cost is offset by rig time and wellbore stability considerations if other tools are used. The measurements are made downhole, stored in solid-state memory for some time and later transmitted to the surface. Data transmission methods vary from company to company, but usually involve digitally encoding data and transmitting to the surface as pressure pulses in the mud system. These pressures may be positive, negative or continuous sine waves. Some MWD tools have the ability to store the measurements for later retrieval with wireline or when the tool is tripped out of the hole if the data transmission link fails. MWD tools that measure formation parameters (resistivity, porosity, sonic velocity, gamma ray) are referred to as logging-while-drilling (LWD) tools. LWD tools use similar data storage and transmission systems, with some having more solid-state memory to provide higher resolution logs after the tool is tripped out than is possible with the relatively low bandwidth, mud-pulse data transmission system. |
| Drilling | measurements-while-drilling | The evaluation of physical properties, usually including pressure, temperature and wellbore trajectory in three-dimensional space, while extending a wellbore. MWD is now standard practice in offshore directional wells, where the tool cost is offset by rig time and wellbore stability considerations if other tools are used. The measurements are made downhole, stored in solid-state memory for some time and later transmitted to the surface. Data transmission methods vary from company to company, but usually involve digitally encoding data and transmitting to the surface as pressure pulses in the mud system. These pressures may be positive, negative or continuous sine waves. Some MWD tools have the ability to store the measurements for later retrieval with wireline or when the tool is tripped out of the hole if the data transmission link fails. MWD tools that measure formation parameters (resistivity, porosity, sonic velocity, gamma ray) are referred to as logging-while-drilling (LWD) tools. LWD tools use similar data storage and transmission systems, with some having more solid-state memory to provide higher resolution logs after the tool is tripped out than is possible with the relatively low bandwidth, mud-pulse data transmission system. |
| Drilling | mechanical specific energy | A measure of drilling efficiency. Mechanical specific energy (MSE) is the energy required to remove a unit volume of rock. For optimum drilling efficiency, the objective is to minimize the MSE and to maximize the rate of penetration (ROP). To control the MSE, drillers can control the weight on bit (WOB), torque, ROP and drillbit revolutions per minute (rpm). |
| Drilling | mechanical sticking | The limiting or prevention of motion of the drillstring by anything other than differential pressure sticking. Mechanical sticking can be caused by junk in the hole, wellbore geometry anomalies, cement, keyseats or a buildup of cuttings in the annulus. |
| Drilling | mist drilling | A variation of air drilling in which a small amount of water trickles into the wellbore from exposed formations and is carried out of the wellbore by the compressed air used for air drilling. The onset of mist drilling often signals the impending end of practical air drilling, at which point the water inflow becomes too great for the compressed air to remove from the wellbore, or the produced water (usually salty) becomes a disposal problem. |
| Drilling | mobile offshore drilling unit | A generic term for several classes of self-contained floatable or floating drilling machines such as jackups, semisubmersibles, and submersibles. |
| Drilling | MODU | A generic term for several classes of self-contained floatable or floating drilling machines such as jackups, semisubmersibles, and submersibles. |
| Drilling | monkey board | The small platform that the derrickman stands on when tripping pipe. |
| Drilling | monkeyboard | The small platform that the derrickman stands on when tripping pipe. |
| Drilling | moon pool | The opening in the hull of a drillship or other offshore drilling vessel through which drilling equipment passes. |
| Drilling | morning tour | The work shift of a drilling crew that starts in the morning. Drilling operations usually occur around the clock because of the cost to rent a rig. As a result, there are usually two separate crews working twelve-hour tours to keep the operation going. Some companies prefer three eight-hour tours: the daylight tour starts at daylight or 8 AM; the graveyard tour is the overnight shift or the shift that begins at midnight. (Pronounced “tower” in many areas.) |
| Drilling | motorman | The member of the rig crew responsible for maintenance of the engines. While all members of the rig crew help with major repairs, the motorman does routine preventive maintenance and minor repairs. |
| Drilling | mousehole | An opening in the rig floor near the rotary table, but between the rotary table and the vee-door, that enables rapid connections while drilling. The mousehole is usually fitted underneath with a length of casing, usually with a bottom. A joint of drillpipe that will be used next in the drilling operation is placed in the mousehole, box end up, by the rig crew at a convenient time (immediately after the previous connection is made). When the bit drills down and the kelly is near the rotary table, another piece of drillpipe must be added for drilling to continue. This next piece of pipe is standing in the mousehole when the kelly is screwed onto it. Then the kelly and the joint of pipe in the mousehole are raised to remove the pipe from the mousehole, the mousehole pipe screwed onto the rest of the drillstring, and the drillstring lowered, rotated, and pumped through to continue drilling. Another piece of pipe is put in the mousehole to await the next connection. |
| Drilling | MPD | Abbreviation for managed pressure drilling. |
| Drilling | MSE | Abbreviation for mechanical specific energy. |
| Drilling | mud pulse telemetry | A method of transmitting LWD and MWD data acquired downhole to the surface, using pressure pulses in the mud system. The measurements are usually converted into an amplitude- or frequency-modulated pattern of mud pulses. The same telemetry system is used to transmit commands from the surface. |
| Drilling | mud return line | Also known as flowline, the large-diameter metal pipe that connects the bell nipple under the rotary table to the possum belly at the mud tanks. The flowline is simply an inclined, gravity-flow conduit to direct mud coming out the top of the wellbore to the mud surface-treating equipment. When drilling certain highly reactive clays, called “gumbo,” the flowline may become plugged and require considerable effort by the rig crew to keep it open and flowing. In addition, the flowline is usually fitted with a crude paddle-type flow-measuring device commonly called a “flow show” that may give the driller the first indication that the well is flowing. |
| Drilling | multilateral | Pertaining to a well that has more than one branch radiating from the main borehole. The term is also used to refer to the multilateral well itself. |
| Drilling | MWD | The evaluation of physical properties, usually including pressure, temperature and wellbore trajectory in three-dimensional space, while extending a wellbore. MWD is now standard practice in offshore directional wells, where the tool cost is offset by rig time and wellbore stability considerations if other tools are used. The measurements are made downhole, stored in solid-state memory for some time and later transmitted to the surface. Data transmission methods vary from company to company, but usually involve digitally encoding data and transmitting to the surface as pressure pulses in the mud system. These pressures may be positive, negative or continuous sine waves. Some MWD tools have the ability to store the measurements for later retrieval with wireline or when the tool is tripped out of the hole if the data transmission link fails. MWD tools that measure formation parameters (resistivity, porosity, sonic velocity, gamma ray) are referred to as logging-while-drilling (LWD) tools. LWD tools use similar data storage and transmission systems, with some having more solid-state memory to provide higher resolution logs after the tool is tripped out than is possible with the relatively low bandwidth, mud-pulse data transmission system. |
| Drilling | neat cement | Cement that has no additives to modify its setting time or rheological properties. |
| Drilling | nipple down | To take apart, disassemble and otherwise prepare to move the rig or blowout preventers. |
| Drilling | nipple up | To put together, connect parts and plumbing, or otherwise make ready for use. This term is usually reserved for the installation of a blowout preventer stack. |
| Drilling | OD | Outside or outer diameter. Casing and tubing are commonly described in terms of inside diameter (ID) and outside diameter |
| Drilling | offset well | An existing wellbore close to a proposed well that provides information for planning the proposed well. In planning development wells, there are usually numerous offsets, so a great deal is known about the subsurface geology and pressure regimes. In contrast, rank wildcats have no close offsets, and planning is based on interpretations of seismic data, distant offsets and prior experience. High-quality offset data are coveted by competent well planners to optimize well designs. When lacking offset data, the well planner must be more conservative in designing wells and include more contingencies. |
| Drilling | open hole | The uncased portion of a well. All wells, at least when first drilled, have openhole sections that the well planner must contend with. Prior to running casing, the well planner must consider how the drilled rock will react to drilling fluids, pressures and mechanical actions over time. The strength of the formation must also be considered. A weak formation is likely to fracture, causing a loss of drilling mud to the formation and, in extreme cases, a loss of hydrostatic head and potential well control problems. An extremely high-pressure formation, even if not flowing, may have wellbore stability problems. Once problems become difficult to manage, casing must be set and cemented in place to isolate the formation from the rest of the wellbore. While most completions are cased, some are open, especially in horizontal or extended-reach wells where it may not be possible to cement casing efficiently. |
| Drilling | openhole | The uncased portion of a well. All wells, at least when first drilled, have openhole sections that the well planner must contend with. Prior to running casing, the well planner must consider how the drilled rock will react to drilling fluids, pressures and mechanical actions over time. The strength of the formation must also be considered. A weak formation is likely to fracture, causing a loss of drilling mud to the formation and, in extreme cases, a loss of hydrostatic head and potential well control problems. An extremely high-pressure formation, even if not flowing, may have wellbore stability problems. Once problems become difficult to manage, casing must be set and cemented in place to isolate the formation from the rest of the wellbore. While most completions are cased, some are open, especially in horizontal or extended-reach wells where it may not be possible to cement casing efficiently. |
| Drilling | operator | The company that serves as the overall manager and decision-maker of a drilling project. Generally, but not always, the operator will have the largest financial stake in the project. At the successful completion of logging the target zones, the decision to complete or plug and abandon generally has partner input and potential override clauses. As far as the drilling contractor and service companies are concerned, the designated operator is paying for the entire operation, and the operator is responsible for recouping some of that expense from the partners. |
| Drilling | outside diameter | Outside or outer diameter. Casing and tubing are commonly described in terms of inside diameter (ID) and outside diameter. |
| Drilling | overbalance | The amount of pressure (or force per unit area) in the wellbore that exceeds the pressure of fluids in the formation. This excess pressure is needed to prevent reservoir fluids (oil, gas, water) from entering the wellbore. However, excessive overbalance can dramatically slow the drilling process by effectively strengthening the near-wellbore rock and limiting removal of drilled cuttings under the bit. In addition, high overbalance pressures coupled with poor mud properties can cause differential sticking problems. Because reservoir pressures vary from one formation to another, while the mud is relatively constant density, overbalance varies from one zone to another. |
| Drilling | p rate | Slang for penetration rate, or the speed that the bit is drilling into the formation. |
| Drilling | P&A | To prepare a well to be closed permanently, usually after either logs determine there is insufficient hydrocarbon potential to complete the well, or after production operations have drained the reservoir. Different regulatory bodies have their own requirements for plugging operations. Most require that cement plugs be placed and tested across any open hydrocarbon-bearing formations, across all casing shoes, across freshwater aquifers, and perhaps several other areas near the surface, including the top 20 to 50 ft [6 to 15 m] of the wellbore. The well designer may choose to set bridge plugs in conjunction with cement slurries to ensure that higher density cement does not fall in the wellbore. In that case, the bridge plug would be set and cement pumped on top of the plug through drillpipe, and then the drillpipe withdrawn before the slurry thickened. |
| Drilling | pack off | A flexible, usually elastomeric sealing element and housing used to seal an irregular surface such as a wireline. |
| Drilling | pack off | To plug the wellbore around a drillstring. This can happen for a variety of reasons, the most common being that either the drilling fluid is not properly transporting cuttings and cavings out of the annulus or portions of the wellbore wall collapse around the drillstring. When the well packs off, there is a sudden reduction or loss of the ability to circulate, and high pump pressures follow. If prompt remedial action is not successful, an expensive episode of stuck pipe can result. The term is also used in gravel packing to describe the act of placing all the sand or gravel in the annulus. |
| Drilling | packer | A device that can be run into a wellbore with a smaller initial outside diameter that then expands externally to seal the wellbore. Packers employ flexible, elastomeric elements that expand. The two most common forms are the production or test packer and the inflatable packer. The expansion of the former may be accomplished by squeezing the elastomeric elements (somewhat doughnut shaped) between two plates, forcing the sides to bulge outward. The expansion of the latter is accomplished by pumping a fluid into a bladder, in much the same fashion as a balloon, but having more robust construction. Production or test packers may be set in cased holes and inflatable packers are used in open or cased holes. They may be run on wireline, pipe or coiled tubing. Some packers are designed to be removable, while others are permanent. Permanent packers are constructed of materials that are easy to drill or mill out. |
| Drilling | pack off | A flexible, usually elastomeric sealing element and housing used to seal an irregular surface such as a wirelin |
| Drilling | pack off | To plug the wellbore around a drillstring. This can happen for a variety of reasons, the most common being that either the drilling fluid is not properly transporting cuttings and cavings out of the annulus or portions of the wellbore wall collapse around the drillstring. When the well packs off, there is a sudden reduction or loss of the ability to circulate, and high pump pressures follow. If prompt remedial action is not successful, an expensive episode of stuck pipe can result. The term is also used in gravel packing to describe the act of placing all the sand or gravel in the annulus. |
| Drilling | pad | A fluid used to initiate hydraulic fracturing that does not contain proppant. |
| Drilling | pad | A temporary drilling site, usually constructed of local materials such as gravel, shell or even wood. For some long-drilling-duration, deep wells, such as the ultradeep wells of western Oklahoma, or some regulatory jurisdictions such as The Netherlands, pads may be paved with asphalt or concrete. After the drilling operation is over, most of the pad is usually removed or plowed back into the ground. |
| Drilling | PDC bit | A drilling tool that uses polycrystalline diamond compact (PDC) cutters to shear rock with a continuous scraping motion. These cutters are synthetic diamond disks about 1/8-in. thick and about 1/2 to 1 in. in diameter. PDC bits are effective at drilling shale formations, especially when used in combination with oil-base muds. |
| Drilling | PDM |
Abbreviation for positive displacement motor, a downhole motor used in the oil field to drive the drill bit or other downhole tools during directional drilling or performance drilling applications. As drilling fluid is pumped through the positive displacement |
| Drilling | penetration rate | The speed at which the drill bit can break the rock under it and thus deepen the wellbore. This speed is usually reported in units of feet per hour or meters per hour. |
| Drilling | pill | Any relatively small quantity (less than 200 bbl) of a special blend of drilling fluid to accomplish a specific task that the regular drilling fluid cannot perform. Examples include high-viscosity pills to help lift cuttings out of a vertical wellbore, freshwater pills to dissolve encroaching salt formations, pipe-freeing pills to destroy filter cake and relieve differential sticking forces and lost circulation material pills to plug a thief zone. |
| Drilling | pin | Relating to the male threadform, as in the “pin end of the pipe. |
| Drilling | pin | A male threadform, especially in tubular goods and drillstring components. |
| Drilling | pipe dope | A specially formulated blend of lubricating grease and fine metallic particles that prevents thread galling (a particular form of metal-to-metal damage) and seals the roots of threads. The American Petroleum Institute (API) specifies properties of pipe dope, including its coefficient of friction. The rig crew applies copious amounts of pipe dope to the drillpipe tool joints every time a connection is made. |
| Drilling | pipe rack | Offshore, the storage bins for drillpipe, drill collars and casing. The offshore pipe rack functions similarly to the onshore version. Due to space limitations, offshore pipe racks tend to be narrower and routinely contain many layers of pipe. The onshore pipe rack tends to have few stacked layers and instead extends laterally as needed to hold the tubular goods because space is not at a premium. |
| Drilling | pipe rack | Onshore, two elevated truss-like structures having triangular cross sections. The pipe rack supports drillpipe, drill collars or casing above the ground. These structures are used in pairs located about 20 ft [6 m] apart and keep the pipe above ground level and closer to the level of the catwalk. Pipe stored horizontally on the pipe racks can have its threads cleaned and inspected and the rig crew may roll the pipe from one end of the pipe racks to the other with relative ease. The pipe racks are usually topped with a wooden board so as to not damage pipe, especially casing, as it is rolled back and forth along the racks. When large amounts of pipe are stored, wooden sills are placed between the layers of pipe to prevent damage. |
| Drilling | pipe ram | A type of sealing element in high-pressure split seal blowout preventers that is manufactured with a half-circle hole on the edge (to mate with another horizontally opposed pipe ram) sized to fit around drillpipe. Most pipe rams fit only one size or a small range of drillpipe sizes and do not close properly around drillpipe tool joints or drill collars. A relatively new style is the variable bore ram, which is designed and manufactured to properly seal on a wider range of pipe sizes. |
| Drilling | pipe trip | The act of pulling the drillstring out of the hole or replacing it in the hole. A pipe trip is usually done because the bit has dulled or has otherwise ceased to drill efficiently and must be replaced. |
| Drilling | PIT | Also known as pressure integrity test or leakoff test, a test to determine the strength or fracture pressure of the open formation, usually conducted immediately after drilling below a new casing shoe. During the test, the well is shut in and fluid is pumped into the wellbore to gradually increase the pressure that the formation experiences. At some pressure, fluid will enter the formation, or leak off, either moving through permeable paths in the rock or by creating a space by fracturing the rock. The results of the leakoff test dictate the maximum pressure or mud weight that may be applied to the well during drilling operations. To maintain a small safety factor to permit safe well control operations, the maximum operating pressure is usually slightly below the leakoff test result. |
| Drilling | plug and abandon | To prepare a well to be closed permanently, usually after either logs determine there is insufficient hydrocarbon potential to complete the well, or after production operations have drained the reservoir. Different regulatory bodies have their own requirements for plugging operations. Most require that cement plugs be placed and tested across any open hydrocarbon-bearing formations, across all casing shoes, across freshwater aquifers, and perhaps several other areas near the surface, including the top 20 to 50 ft [6 to 15 m] of the wellbore. The well designer may choose to set bridge plugs in conjunction with cement slurries to ensure that higher density cement does not fall in the wellbore. In that case, the bridge plug would be set and cement pumped on top of the plug through drillpipe, and then the drillpipe withdrawn before the slurry thickened. |
| Drilling | polycrystalline diamond compact bit | A drilling tool that uses polycrystalline diamond compact (PDC) cutters to shear rock with a continuous scraping motion. These cutters are synthetic diamond disks about 1/8-in. thick and about 1/2 to 1 in. in diameter. PDC bits are effective at drilling shale formations, especially when used in combination with oil-base muds. |
| Drilling | POOH | Abbreviation for pull out of the hole. To remove the drillstring from the wellbore. |
| Drilling | positive displacement motor | A downhole motor used in the oil field to drive the drill bit or other downhole tools during directional drilling or performance drilling applications. As drilling fluid is pumped through the positive displacement motor, it converts the hydraulic power of the fluid into mechanical power to cause the bit to rotate. During directional drilling, this capability is used while drilling in sliding mode, when the drillstring is not rotated from the surface. Positive displacement motors can also be used for performance drilling, straight hole drilling, coring, underreaming, and milling operations. In straight hole drilling, the motor functions as a drilling performance tool to increase the rate of penetration and reduce casing wear by minimizing drillstring rotation. |
| Drilling | p-rate | Slang for penetration rate, or the speed that the bit is drilling into the formation. |
| Drilling | pressure hunt | The evaluation of various well parameters in an attempt to identify when the pore pressure in a drilling well is changing. A team consisting of geologists, engineers and most of the rigsite personnel usually conducts the hunt. The purpose of a pressure hunt is to detect the pore pressure transition (usually from lower to higher pressure) and safely set casing in the transition zone to maximize wellbore strength. A casing string set too shallow, while eliminating some problems associated with drilling fluid contacting the wellbore wall, may not add strength or aid in drilling deeper, perhaps abnormally pressured formations. On the other hand, if drilling is continued too deep into a transition zone, a kick may be taken that cannot be contained in the open wellbore, causing an underground blowout. The hunt team, therefore, seeks to get into the transition zone far enough to gain wellbore strength without taking a kick. |
| Drilling | pressure integrity test | Also known as leakoff test, a test to determine the strength or fracture pressure of the open formation, usually conducted immediately after drilling below a new casing shoe. During the test, the well is shut in and fluid is pumped into the wellbore to gradually increase the pressure that the formation experiences. At some pressure, fluid will enter the formation, or leak off, either moving through permeable paths in the rock or by creating a space by fracturing the rock. The results of the leakoff test dictate the maximum pressure or mud weight that may be applied to the well during drilling operations. To maintain a small safety factor to permit safe well control operations, the maximum operating pressure is usually slightly below the leakoff test result. Synonyms: leakoff test |
| Drilling | prime mover | The source of power for the rig location. On modern rigs, the prime mover consists of one to four or more diesel engines. These engines commonly produce several thousand horsepower. Typically, the diesel engines are connected to electric generators. The electrical power is then distributed by a silicon-controlled-rectifier (SCR) system around the rigsite. Rigs that convert diesel power to electricity are known as diesel electric rigs. Older designs transmit power from the diesel engines to certain rig components (drawworks, pumps and rotary table) through a system of mechanical belts, chains and clutches. On these rigs, a smaller electric generator powers lighting and small electrical requirements. These older rigs are referred to as mechanical rigs or more commonly, simply power rigs. |
| Drilling | pull out of the hole | To remove the drillstring from the wellbore. |
| Drilling | pusher | The location supervisor for the drilling contractor. The toolpusher is usually a senior, experienced individual who has worked his way up through the ranks of the drilling crew positions. His job is largely administrative, including ensuring that the rig has sufficient materials, spare parts and skilled personnel to continue efficient operations. The toolpusher also serves as a trusted advisor to many personnel on the rigsite, including the operator’s representative, the company man. |
| Drilling | racking back pipe | To place a stand of drillpipe in the derrick when coming out of the hole on a trip. The rig crew racks back pipe after the stand is unscrewed from the rest of the drillstring. The floor crew then pushes the lower part of the stand away from the rotary table to a position on one side of the vee-door. While the floor crew is pushing the pipe, the derrickman gets ready to pull the top of the stand over into the fingerboards. Once the rig crew has the pipe in the correct location, the driller slacks off on the drawworks, allowing the stand to rest on the drillfloor. This takes weight off of the elevators previously supporting the pipe at the top, so the derrickman can then unlatch the elevators and pull the top of the pipe into the fingerboards for storage. Modern rig designs have automated pipe-handling equipment that moves the pipe. When tripping the pipe out of the hole, racking back pipe may occur every two to five minutes for hours at a time. |
| Drilling | ram blowout preventer | A device that can be used to quickly seal the top of the well in the event of a well control event (kick). A ram blowout preventer (BOP) consists of two halves of a cover for the well that are split down the middle. Large-diameter hydraulic cylinders, normally retracted, force the two halves of the cover together in the middle to seal the wellbore. These covers are constructed of steel for strength and fitted with elastomer components on the sealing surfaces. The halves of the covers, formally called ram blocks, are available in a variety of configurations. In some designs, they are flat at the mating surfaces to enable them to seal over an open wellbore. Other designs have a circular cutout in the middle that corresponds to the diameter of the pipe in the hole to seal the well when pipe is in the hole. These pipe rams effectively seal a limited range of pipe diameters. Variable-bore rams are designed to seal a wider range of pipe diameters, albeit at a sacrifice of other design criteria, notably element life and hang-off weight. Still other ram blocks are fitted with a tool steel-cutting surface to enable the ram BOPs to completely shear through drillpipe, hang the drillstring off on the ram blocks themselves and seal the wellbore. Obviously, such an action limits future options and is employed only as a last resort to regain pressure control of the wellbore. The various ram blocks can be changed in the ram preventers, enabling the well team to optimize BOP configuration for the particular hole section or operation in progress. |
| Drilling | rate of penetration | The speed at which the drill bit can break the rock under it and thus deepen the wellbore. This speed is usually reported in units of feet per hour or meters per hour. |
| Drilling | rathole | Extra hole drilled at the bottom of the hole to leave expendable completion equipment, such as the carriers for perforating gun charges. |
| Drilling | rathole | Extra hole drilled at the end of the well (beyond the last zone of interest) to ensure that the zone of interest can be fully evaluated. The logging tool string may be as much as 120 ft [36.5 m] in length, so the rathole allows tools at the top of the logging string to reach and measure the deepest zone of interest. In addition, there is usually a small amount of extra hole drilled to allow for junk, hole fill-in and other conditions that may reduce the effective depth of the well prior to running logging tools. |
| Drilling | rathole | A storage place for the kelly, consisting of an opening in the rig floor fitted with a piece of casing with an internal diameter larger than the outside diameter of the kelly, but less than that of the upper kelly valve so that the kelly may be lowered into the rathole until the upper kelly valve rests on the top of the piece of casing. |
| Drilling | RCD | Abbreviation for rotating control device, a pressure-control device used during drilling for the purpose of making a seal around the drillstring while the drillstring rotates. The RCD is intended to contain hydrocarbons or other wellbore fluids and prevent their release to the atmosphere. |
| Drilling | ream | To enlarge a wellbore. Reaming may be necessary for several reasons. Perhaps the most common reason for reaming a section of a hole is that the hole was not drilled as large as it should have been at the outset. This can occur when a bit has been worn down from its original size, but might not be discovered until the bit is tripped out of the hole, and some undergauge hole has been drilled. Last, some plastic formations may slowly flow into the wellbore over time, requiring the reaming operation to maintain the original hole size. |
| Drilling | reciprocate | To alternately raise and lower the drillstring, casing string or liner in the wellbore. Reciprocation is usually limited to 30 to 60 ft [9 to 18 m] of vertical travel in the derrick. The purpose of reciprocating the drillstring is usually to clean cuttings and other debris from the wellbore. Reciprocating the strings can improve the chances of a good cement job in casing or liners |
| Drilling | recycled gas | Residual gas that remains entrained in the drilling fluid despite being circulated to surface. At the surface, it remains in the mudstream, which is suctioned from the mud pit and recirculated into the wellbore. |
| Drilling | reeled tubing | Another term for coiled tubing, a long, continuous length of pipe wound on a spool. The pipe is straightened prior to pushing into a wellbore and rewound to coil the pipe back onto the transport and storage spool. Depending on the pipe diameter (1 in. to 4 1/2 in.) and the spool size, coiled tubing can range from 2,000 ft to 15,000 ft [610 to 4,570 m] or greater length. |
| Drilling | reserve pit | In onshore operations, an earthen-bermed storage area for discarded drilling fluid. These small reservoirs are used for several reasons. First, when properly arranged, most of the solids in the mud settle out and a suction hose may be placed in the reserve pit to have additional fluid available to pump into the wellbore in an emergency. In addition, in arid areas, a considerable amount of evaporation occurs, thus minimizing mud disposal volumes. At the end of drilling operations, and perhaps at intermediate times during drilling, the fluids and solids in the reserve pit must be carefully discarded, usually by transfer to a properly certified landfill. If the mud is benign, the solids (mostly clay), and liquids (water), may be plowed and tilled back into the local soil. This technique of disposal and reclamation is known as land farming. |
| Drilling | reservoir pressure | The pressure of the subsurface formation fluids, commonly expressed as the density of fluid required in the wellbore to balance that pore pressure. A normal pressure gradient might require 9 lbm/galUS [1.08 kg/m3], while an extremely high gradient may need 18 lbm/galUS [2.16 kg/m3] or higher. |
| Drilling | reverse circulation | The intentional pumping of wellbore fluids down the annulus and back up through the drillpipe. This is the opposite of the normal direction of fluid circulation in a wellbore. Since the inside volume of the drillpipe is considerably less than the volume of the annulus outside of the drillpipe, reverse circulation can bring bottomhole fluids to the surface faster than normal circulation for a given flow rate. Two potential hazards of reverse circulation include lifting cuttings and other junk into the drillstring and the rapid flow of reservoir fluids to the surface in a kick situation. |
| Drilling | reversing out | Another term for reverse circulation, the intentional pumping of wellbore fluids down the annulus and back up through the drillpipe. This is the opposite of the normal direction of fluid circulation in a wellbore. Since the inside volume of the drillpipe is considerably less than the volume of the annulus outside of the drillpipe, reverse circulation can bring bottomhole fluids to the surface faster than normal circulation for a given flow rate. Two potential hazards of reverse circulation include lifting cuttings and other junk into the drillstring and the rapid flow of reservoir fluids to the surface in a kick situation. |
| Drilling | rig | The machine used to drill a wellbore. In onshore operations, the rig includes virtually everything except living quarters. Major components of the rig include the mud tanks, the mud pumps, the derrick or mast, the drawworks, the rotary table or topdrive, the drillstring, the power generation equipment and auxiliary equipment. Offshore, the rig includes the same components as onshore, but not those of the vessel or drilling platform itself. The rig is sometimes referred to as the drilling package, particularly offshore. |
| Drilling | rig down | To take apart equipment for storage and portability. Equipment typically must be disconnected from power sources, decoupled from pressurized systems, disassembled and moved off the rig floor or even off location. |
| Drilling | rig floor | The relatively small work area in which the rig crew conducts operations, usually adding or removing drillpipe to or from the drillstring. The rig floor is the most dangerous location on the rig because heavy iron is moved around there. Drillstring connections are made or broken on the drillfloor, and the driller’s console for controlling the major components of the rig are located there. Attached to the rig floor is a small metal room, the doghouse, where the rig crew can meet, take breaks and take refuge from the elements during idle times. |
| Drilling | rig up | To make ready for use. Equipment must typically be moved onto the rig floor, assembled and connected to power sources or pressurized piping systems. |
| Drilling | RIH | To connect pipe together and lower the connected length into the borehole in a controlled fashion. The pipe lengths are usually screwed together either with rotary-shouldered connections for the drillstring, or threaded and coupled connections for casing, liners and most tubing. |
| Drilling | roller cone bit | A tool designed to crush rock efficiently while incurring a minimal amount of wear on the cutting surfaces. Invented by Howard Hughes, the roller-cone bit has conical cutters or cones that have spiked teeth around them. As the drillstring is rotated, the bit cones roll along the bottom of the hole in a circle. As they roll, new teeth come in contact with the bottom of the hole, crushing the rock immediately below and around the bit tooth. As the cone rolls, the tooth then lifts off the bottom of the hole and a high-velocity fluid jet strikes the crushed rock chips to remove them from the bottom of the hole and up the annulus. As this occurs, another tooth makes contact with the bottom of the hole and creates new rock chips. Thus, the process of chipping the rock and removing the small rock chips with the fluid jets is continuous. The teeth intermesh on the cones, which helps clean the cones and enables larger teeth to be used. There are two main types of roller-cone bits, steel milled-tooth bits and carbide insert bits. |
| Drilling | roller-cone bit | A tool designed to crush rock efficiently while incurring a minimal amount of wear on the cutting surfaces. Invented by Howard Hughes, the roller-cone bit has conical cutters or cones that have spiked teeth around them. As the drillstring is rotated, the bit cones roll along the bottom of the hole in a circle. As they roll, new teeth come in contact with the bottom of the hole, crushing the rock immediately below and around the bit tooth. As the cone rolls, the tooth then lifts off the bottom of the hole and a high-velocity fluid jet strikes the crushed rock chips to remove them from the bottom of the hole and up the annulus. As this occurs, another tooth makes contact with the bottom of the hole and creates new rock chips. Thus, the process of chipping the rock and removing the small rock chips with the fluid jets is continuous. The teeth intermesh on the cones, which helps clean the cones and enables larger teeth to be used. There are two main types of roller-cone bits, steel milled-tooth bits and carbide insert bits. |
| Drilling | ROP | Abbreviation for rate of penetration. The speed at which the drill bit can break the rock under it and thus deepen the wellbore. This speed is usually reported in units of feet per hour or meters per hour. |
| Drilling | rotary bushing | Another term for kelly bushing, an adapter that serves to connect the rotary table to the kelly. The kelly bushing has an inside diameter profile that matches that of the kelly, usually square or hexagonal. It is connected to the rotary table by four large steel pins that fit into mating holes in the rotary table. The rotary motion from the rotary table is transmitted to the bushing through the pins, and then to the kelly itself through the square or hexagonal flat surfaces between the kelly and the kelly bushing. The kelly then turns the entire drillstring because it is screwed into the top of the drillstring itself. Depth measurements are commonly referenced to the KB, such as 8327 ft KB, meaning 8327 feet below the kelly bushing. |
| Drilling | rotary drilling | method, made popular after the discovery of the East Texas Field by “Dad” Joiner in 1930, is much more efficient than the alternative, cable tool drilling. Rotary drilling is a nearly continuous process, because cuttings are removed as drilling fluids circulate through the bit and up the wellbore to the surface. Cable tool operations are discontinuous and cuttings removal is inefficient. This difference in efficiency becomes particularly significant as hole depth increases. |
| Drilling | rotary hose | A large-diameter (3- to 5-in. inside diameter), high-pressure flexible line used to connect the standpipe to the swivel. This flexible piping arrangement permits the kelly (and, in turn, the drillstring and bit) to be raised or lowered while drilling fluid is pumped through the drillstring. The simultaneous lowering of the drillstring while pumping fluid is critical to the drilling operation. |
| Drilling | rotary table | The revolving or spinning section of the drillfloor that provides power to turn the drillstring in a clockwise direction (as viewed from above). The rotary motion and power are transmitted through the kelly bushing and the kelly to the drillstring. When the drillstring is rotating, the drilling crew commonly describes the operation as simply, “rotating to the right,” “turning to the right,” or, “rotating on bottom.” Almost all rigs today have a rotary table, either as primary or backup system for rotating the drillstring. Topdrive technology, which allows continuous rotation of the drillstring, has replaced the rotary table in certain operations. A few rigs are being built today with topdrive systems only, and lack the traditional kelly system. |
| Drilling | rotating control device | A pressure-control device used during drilling for the purpose of making a seal around the drillstring while the drillstring rotates. This device is intended to contain hydrocarbons or other wellbore fluids and prevent their release to the atmosphere. |
| Drilling | roughneck | Generically, any member of the drilling crew. In conversational use, one might claim to have “roughnecked” in one’s youth. This might actually refer to roughneck duties, or to one of the other crew positions, such as lead tong operator, motorman, derrickman, assistant driller or even driller. |
| Drilling | roughneck | A floor hand, or member of the drilling crew who works under the direction of the driller to make or break connections as drillpipe is tripped in or out of the hole. On most drilling rigs, roughnecks are also responsible for maintaining and repairing much of the equipment found on the drill floor and derrick. The roughneck typically ranks above a roustabout and beneath a derrickman, and reports to the driller. |
| Drilling | round trip | The complete operation of removing the drillstring from the wellbore and running it back in the hole. This operation is typically undertaken when the bit becomes dull or broken, and no longer drills the rock efficiently. After some preliminary preparations for the trip, the rig crew removes the drillstring 90 ft [27 m] at a time, by unscrewing every third drillpipe or drill collar connection. When the three joints are unscrewed from the rest of the drillstring, they are carefully stored upright in the derrick by the fingerboards at the top and careful placement on wooden planks on the rig floor. After the drillstring has been removed from the wellbore, the dull bit is unscrewed with the use of a bit breaker and quickly examined to determine why the bit dulled or failed. Depending on the failure mechanism, the crew might choose a different type of bit for the next section. If the bearings on the prior bit failed, but the cutting structures are still sharp and intact, the crew may opt for a faster drilling (less durable) cutting structure. Conversely, if the bit teeth are worn out but the bearings are still sealed and functioning, the crew should choose a bit with more durable (and less aggressive) cutting structures. Once the bit is chosen, it is screwed onto the bottom of the drill collars with the help of the bit breaker, the drill collars are run into the hole (RIH), and the drillpipe is run in the hole. Once on bottom, drilling commences again. The duration of this operation depends on the total depth of the well and the skill of the rig crew. A general estimate for a competent crew is that the round trip requires one hour per thousand feet of hole, plus an hour or two for handling collars and bits. At that rate, a round trip in a ten thousand-foot well might take twelve hours. A round trip for a 30,000-ft [9230 m] well might take 32 or more hours, especially if intermediate hole-cleaning operations must be undertaken. |
| Drilling | roustabout | Any unskilled manual laborer on the rigsite. A roustabout may be part of the drilling contractor’s employee workforce, or may be on location temporarily for special operations. Roustabouts are commonly hired to ensure that the skilled personnel that run an expensive drilling rig are not distracted by peripheral tasks, ranging from cleaning up location to cleaning threads to digging trenches to scraping and painting rig components. Although roustabouts typically work long hard days, this type of work can lead to more steady employment on a rig crew. |
| Drilling | safety joint | A weak spot in the drillstring. Such a weak spot sometimes is intentionally put into the drillstring so that if tension in the drillstring exceeds a predetermined amount, the safety joint will part and the rest of the drillstring will be salvageable. A safety joint is commonly included in fishing strings and drillstem testing equipment, where the fish may be successfully caught by the fishing assembly, but tension to free the fish may prove insurmountable. By having the safety joint in the hole, the fishing company representative knows where the fishing string will part and what will be needed to latch onto the top of this additional fish. |
| Drilling | saver sub | A short length of drill collar that has male threads on one end and female on the other. It is screwed onto the bottom of the kelly or topdrive and onto the rest of the drillstring. When the hole must be deepened, and pipe added to the drillstring, the threads are unscrewed between the saver sub and the rest of the drillstring, as opposed to between the kelly or topdrive and the saver sub. This means that the connection between the kelly or topdrive and the saver sub rarely is used, and suffers minimal wear and tear, whereas the lower connection is used in almost all cases and suffers the most wear and tear. The saver sub is expendable and does not represent a major investment. However, the kelly or topdrive component threads are spared by use of a saver sub, and those components represent a significant capital cost and considerable downtime when replaced. |
| Drilling | scratcher | A device for cleaning mud and mud filter cake off of the wellbore wall when cementing casing in the hole to ensure good contact and bonding between the cement and the wellbore wall. The scratcher is a simple device, consisting of a band of steel that fits around a joint of casing, and stiff wire fingers or cable loops sticking out in all directions around the band (360-degree coverage). A scratcher resembles a bottlebrush, but its diameter is greater than its height. Importantly, for scratchers to be effective, the casing must be moved. This movement may be reciprocal motion in and out of the wellbore, rotary motion, or both. In general, the more motion, the better the cement job will be. |
| Drilling | semisubmersible | A particular type of floating vessel that is supported primarily on large pontoon-like structures submerged below the sea surface. The operating decks are elevated perhaps 100 or more feet above the pontoons on large steel columns. This design has the advantage of submerging most of the area of components in contact with the sea and minimizing loading from waves and wind. Semisubmersibles can operate in a wide range of water depths, including deep water. They are usually anchored with six to twelve anchors tethered by strong chains and wire cables, which are computer controlled to maintain stationkeeping. Semisubmersibles (called semisubs or simply semis) can be used for drilling, workover operations, and production platforms, depending on the equipment with which they are equipped. When fitted with a drilling package, they may be called semisubmersible drilling rigs. |
| Drilling | settling pit | A drilling mud filled open steel or earthen berm tank that is not stirred or circulated. By having mud slowly pass through such a container, most large drilling solids sink to the bottom, cleaning the mud somewhat. If the settling pit is small, as in the case of steel mud tanks, it must be cleaned out frequently as cuttings pile up on the bottom of the tank. In the early days of rotary drilling, some rigs had no more solids control than a large settling pit into which mud was discharged after coming back from the wellbore and suction for the mud pumps was taken at the other end of the pit. A major drawback to this type of “cleaning” is that solids intentionally put into the mud, such as barite, may settle to the bottom and be discarded rather than circulated back into the wellbore. |
| Drilling | settling tank | Another term for settling pit, a drilling mud filled open steel or earthen berm tank that is not stirred or circulated. By having mud slowly pass through such a container, most large drilling solids sink to the bottom, cleaning the mud somewhat. If the settling pit is small, as in the case of steel mud tanks, it must be cleaned out frequently as cuttings pile up on the bottom of the tank. In the early days of rotary drilling, some rigs had no more solids control than a large settling pit into which mud was discharged after coming back from the wellbore and suction for the mud pumps was taken at the other end of the pit. A major drawback to this type of “cleaning” is that solids intentionally put into the mud, such as barite, may settle to the bottom and be discarded rather than circulated back into the wellbore. |
| Drilling | shaker | Abbreviation for shale shaker, the primary and probably most important device on the rig for removing drilled solids from the mud. This vibrating sieve is simple in concept, but a bit more complicated to use efficiently. A wire-cloth screen vibrates while the drilling fluid flows on top of it. The liquid phase of the mud and solids smaller than the wire mesh pass through the screen, while larger solids are retained on the screen and eventually fall off the back of the device and are discarded. Obviously, smaller openings in the screen clean more solids from the whole mud, but there is a corresponding decrease in flow rate per unit area of wire cloth. Hence, the drilling crew should seek to run the screens (as the wire cloth is called), as fine as possible, without dumping whole mud off the back of the shaker. Where it was once common for drilling rigs to have only one or two shale shakers, modern high-efficiency rigs are often fitted with four or more shakers, thus giving more area of wire cloth to use, and giving the crew the flexibility to run increasingly fine screens. |
| Drilling | shale shaker | The primary and probably most important device on the rig for removing drilled solids from the mud. This vibrating sieve is simple in concept, but a bit more complicated to use efficiently. A wire-cloth screen vibrates while the drilling fluid flows on top of it. The liquid phase of the mud and solids smaller than the wire mesh pass through the screen, while larger solids are retained on the screen and eventually fall off the back of the device and are discarded. Obviously, smaller openings in the screen clean more solids from the whole mud, but there is a corresponding decrease in flow rate per unit area of wire cloth. Hence, the drilling crew should seek to run the screens (as the wire cloth is called), as fine as possible, without dumping whole mud off the back of the shaker. Where it was once common for drilling rigs to have only one or two shale shakers, modern high-efficiency rigs are often fitted with four or more shakers, thus giving more area of wire cloth to use, and giving the crew the flexibility to run increasingly fine screens. |
| Drilling | sheave | A pulley. In oilfield usage, the term usually refers to either the pulleys permanently mounted on the top of the rig (the crown blocks), or the pulleys used for running wireline tools into the wellbore. In the case of the crown blocks, the drilling line, a heavy wire rope, is threaded between the crown blocks and the traveling blocks in a block and tackle arrangement to gain mechanical advantage. A relatively weak drilling line, with a breaking strength of perhaps 100,000 pounds [45,400 kg], may be used to lift much larger loads, perhaps in excess of one million pounds [454,000 kg]. During wireline operations, two sheaves are temporarily hung in the derrick, and the wireline is run from the logging truck through the sheaves and then down to the logging tool in the wellbore. |
| Drilling | shoe | A short assembly, typically manufactured from a heavy steel collar and profiled cement interior, that is screwed to the bottom of a casing string. The rounded profile helps guide the casing string past any ledges or obstructions that would prevent the string from being correctly located in the wellbore. |
| Drilling | shoe | The bottom of the casing string, including the cement around it, or the equipment run at the bottom of the casing string. |
| Drilling | shoe joint | Another term for float joint, a full-sized length of casing placed at the bottom of the casing string that is usually left full of cement on the inside to ensure that good cement remains on the outside of the bottom of the casing. If cement were not left inside the casing in this manner, the risk of overdisplacing the cement (due to improper casing volume calculations, displacement mud volume measurements, or both) would be significantly higher. Hence, the well designer plans on a safety margin of cement left inside the casing to guarantee that the fluid left outside the casing is good-quality cement. A float collar is placed at the top of the float joint and a float shoe placed at the bottom to prevent reverse flow of cement back into the casing after placement. There can be one, two or three joints of casing used for this purpose. |
| Drilling | shoe track | Another term for float joint, a full-sized length of casing placed at the bottom of the casing string that is usually left full of cement on the inside to ensure that good cement remains on the outside of the bottom of the casing. If cement were not left inside the casing in this manner, the risk of overdisplacing the cement (due to improper casing volume calculations, displacement mud volume measurements, or both) would be significantly higher. Hence, the well designer plans on a safety margin of cement left inside the casing to guarantee that the fluid left outside the casing is good-quality cement. A float collar is placed at the top of the float joint and a float shoe placed at the bottom to prevent reverse flow of cement back into the casing after placement. There can be one, two or three joints of casing used for this purpose. |
| Drilling | short trip | An abbreviated recovery of pipe out of, and then the replacement of same back into the wellbore. Such a trip is normally limited to 10 or 20 stands of drillpipe. Since the short trip is drillpipe only (no bottomhole assembly for the drilling crew to handle), and is limited in length, it can be accomplished quickly and sometimes results in additional information or improved operating conditions. A short trip often is used to gauge whether a hole is clean or whether the mud weight is sufficient to permit a full trip out of the hole. |
| Drilling | show | A surface observation of hydrocarbons, usually observed as florescent liquid on cuttings when viewed with an ultraviolet or black light (oil show) or increased gas readings from the mud logger’s gas-detection equipment (gas show). |
| Drilling | shut in bottomhole pressure | The force per unit area exerted at the bottom of a wellbore when it is closed at either the Christmas tree or the BOP stack. The SIBP is generated by a combination of the hydrostatic pressure from the weight of the liquid in the well and any additional applied pressure. The applied pressure component may be from the formation or from an external source at the surface |
| Drilling | shut-in bottomhole pressure | The force per unit area exerted at the bottom of a wellbore when it is closed at either the Christmas tree or the BOP stack. The SIBP is generated by a combination of the hydrostatic pressure from the weight of the liquid in the well and any additional applied pressure. The applied pressure component may be from the formation or from an external source at the surface. |
| Drilling | shut-in pressure | The surface force per unit area exerted at the top of a wellbore when it is closed at either the Christmas tree or the BOP stack. The pressure may be from the formation or an external and intentional source. The SIP may be zero, indicating that any open formations are effectively balanced by the hydrostatic column of fluid in the well. If the pressure is zero, the well is considered to be dead, and can normally be opened safely to the atmosphere. |
| Drilling | SIBHP | The force per unit area exerted at the bottom of a wellbore when it is closed at either the Christmas tree or the BOP stack. The SIBP is generated by a combination of the hydrostatic pressure from the weight of the liquid in the well and any additional applied pressure. The applied pressure component may be from the formation or from an external source at the surface. |
| Drilling | SIBP | The force per unit area exerted at the bottom of a wellbore when it is closed at either the Christmas tree or the BOP stack. The SIBP is generated by a combination of the hydrostatic pressure from the weight of the liquid in the well and any additional applied pressure. The applied pressure component may be from the formation or from an external source at the surface |
| Drilling | sidetrack | To drill a secondary wellbore away from an original wellbore. A sidetracking operation may be done intentionally or may occur accidentally. Intentional sidetracks might bypass an unusable section of the original wellbore or explore a geologic feature nearby. In the bypass case, the secondary wellbore is usually drilled substantially parallel to the original well, which may be inaccessible due to an irretrievable fish, junk in the hole, or a collapsed wellbore. |
| Drilling | SIP | The surface force per unit area exerted at the top of a wellbore when it is closed at either the Christmas tree or the BOP stack. The pressure may be from the formation or an external and intentional source. The SIP may be zero, indicating that any open formations are effectively balanced by the hydrostatic column of fluid in the well. If the pressure is zero, the well is considered to be dead, and can normally be opened safely to the atmosphere. |
| Drilling | skid | A steel frame on which portable equipment is mounted to facilitate handling with cranes or flatbed trucks. The skid is robust, is usually designed with attachment points for hooks, chains, or cables, and has at least two lengthwise beams to facilitate sliding the equipment into place on the rigsite. |
| Drilling | slant hole rig | A specially designed drilling rig capable of drilling directional wells. |
| Drilling | slant rig | A specially designed drilling rig capable of drilling directional wells. |
| Drilling | slant-hole rig | A specially designed drilling rig capable of drilling directional wells. |
| Drilling | slide | The escape device for workers on the rig floor should an emergency require prompt evacuation. It is similar to a child’s playground slide, only longer and perhaps faster. |
| Drilling | slide | To drill with a mud motor rotating the bit downhole without rotating the drillstring from the surface. This operation is conducted when the bottomhole assembly has been fitted with a bent sub or a bent housing mud motor, or both, for directional drilling. Sliding is the predominant method to build and control or correct hole angle in modern directional drilling operations. Directional drilling is conceptually simple: Point the bit in the desired direction. This pointing is accomplished through the bent sub, which has a small angle offset from the axis of the drillstring, and a measurement device to determine the direction of offset. Without turning the drillstring, the bit is rotated with a mud motor, and drills in the direction it points. With steerable motors, when the desired wellbore direction is attained, the entire drillstring is rotated and drills straight rather than at an angle. By controlling the amount of hole drilled in the sliding versus the rotating mode, the wellbore trajectory can be controlled precisely. |
| Drilling | slim hole well | An inexact term describing a borehole (and associated casing program) significantly smaller than a standard approach, commonly a wellbore less than 6 in. in diameter. The slimhole concept has its roots in the observed correlation between well costs and volume of rock extracted. If one can extract less rock, then well costs should fall. One form of slimhole work involves using more or less conventional equipment and procedures, but simply reducing the hole and casing sizes for each hole interval. A second form involves technology used for exploration boreholes in the hard rock mining industry. In the mining rig operations, the drillstem serves a dual purpose. After the hole is drilled, the drillstem remains in the hole and is cemented in place. Then a new drillstem is used for the new hole section, and also cemented in place. The drillstring for mining rig operations is rotated like that for conventional oilfield rotary rig operations, but typically at a much higher speed. |
| Drilling | slimhole well | An inexact term describing a borehole (and associated casing program) significantly smaller than a standard approach, commonly a wellbore less than 6 in. in diameter. The slimhole concept has its roots in the observed correlation between well costs and volume of rock extracted. If one can extract less rock, then well costs should fall. One form of slimhole work involves using more or less conventional equipment and procedures, but simply reducing the hole and casing sizes for each hole interval. A second form involves technology used for exploration boreholes in the hard rock mining industry. In the mining rig operations, the drillstem serves a dual purpose. After the hole is drilled, the drillstem remains in the hole and is cemented in place. Then a new drillstem is used for the new hole section, and also cemented in place. The drillstring for mining rig operations is rotated like that for conventional oilfield rotary rig operations, but typically at a much higher speed. |
| Drilling | slip and cut | To replace the drilling line wrapped around the crown block and traveling block. As a precaution against drilling line failure due to fatigue, the work done by the drilling line is closely monitored and limited. The work is commonly measured as the cumulative product of the load lifted (in tons) and the distance lifted or lowered (in miles). After a predetermined limit of ton-miles, new line is unspooled from the storage reel and slipped through the crown block and traveling block sheaves and drawworks spool, with the excess on the drawworks spool end cut off and discarded. |
| Drilling | slip joint | A telescoping joint at the surface in floating offshore operations that permits vessel heave (vertical motion) while maintaining a riser pipe to the seafloor. As the vessel heaves, the slip joint telescopes in or out by the same amount so that the riser below the slip joint is relatively unaffected by vessel motion. |
| Drilling | slip-and-cut | To replace the drilling line wrapped around the crown block and traveling block. As a precaution against drilling line failure due to fatigue, the work done by the drilling line is closely monitored and limited. The work is commonly measured as the cumulative product of the load lifted (in tons) and the distance lifted or lowered (in miles). After a predetermined limit of ton-miles, new line is unspooled from the storage reel and slipped through the crown block and traveling block sheaves and drawworks spool, with the excess on the drawworks spool end cut off and discarded. |
| Drilling | slips | Any self-gripping toothed device functioning substantially as above, but gripping components other than drillstring, such as wireline, metal sinker bars, or drill collars. |
| Drilling | slips | A device used to grip the drillstring in a relatively nondamaging manner and suspend it in the rotary table. This device consists of three or more steel wedges that are hinged together, forming a near circle around the drillpipe. On the drillpipe side (inside surface), the slips are fitted with replaceable, hardened tool steel teeth that embed slightly into the side of the pipe. The outsides of the slips are tapered to match the taper of the rotary table. After the rig crew places the slips around the drillpipe and in the rotary, the driller slowly lowers the drillstring. As the teeth on the inside of the slips grip the pipe, the slips are pulled down. This downward force pulls the outer wedges down, providing a compressive force inward on the drillpipe and effectively locking everything together. Then the rig crew can unscrew the upper portion of the drillstring (kelly, saver sub, a joint or stand of pipe) while the lower part is suspended. After some other component is screwed onto the lower part of the drillstring, the driller raises the drillstring to unlock the gripping action of the slips, and the rig crew removes the slips from the rotary. |
| Drilling | snub | To put drillpipe into the wellbore when the blowout preventers (BOPs) are closed and pressure is contained in the well. Snubbing is necessary when a kick is taken, since well kill operations should always be conducted with the drillstring on bottom, and not somewhere up the wellbore. If only the annular BOP has been closed, the drillpipe may be slowly and carefully lowered into the wellbore, and the BOP itself will open slightly to permit the larger diameter tool joints to pass through. If the well has been closed with the use of ram BOPs, the tool joints will not pass by the closed ram element. Hence, while keeping the well closed with either another ram BOP or the annular BOP, the ram must be opened manually, then the pipe lowered until the tool joint is just below the ram, and then closing the ram again. This procedure is repeated whenever a tool joint must pass by a ram BOP. In snubbing operations, the pressure in the wellbore acting on the cross-sectional area of the tubular can exert sufficient force to overcome the weight of the drillstring, so the string must be pushed (or “snubbed”) back into the wellbore. In ordinary stripping operations, the pipe falls into the wellbore under its own weight, and no additional downward force or pushing is required. |
| Drilling | snubbing | The act of putting drillpipe into the wellbore when the blowout preventers (BOPs) are closed and pressure is contained in the well. Snubbing is necessary when a kick is taken, since well kill operations should always be conducted with the drillstring on bottom, and not somewhere up the wellbore. If only the annular BOP has been closed, the drillpipe may be slowly and carefully lowered into the wellbore, and the BOP itself will open slightly to permit the larger diameter tool joints to pass through. If the well has been closed with the use of ram BOPs, the tool joints will not pass by the closed ram element. Hence, while keeping the well closed with either another ram BOP or the annular BOP, the ram must be opened manually, then the pipe lowered until the tool joint is just below the ram, and then closing the ram again. This procedure is repeated whenever a tool joint must pass by a ram BOP. In snubbing operations, the pressure in the wellbore acting on the cross-sectional area of the tubular can exert sufficient force to overcome the weight of the drillstring, so the string must be pushed (or “snubbed”) back into the wellbore. In ordinary stripping operations, the pipe falls into the wellbore under its own weight, and no additional downward force or pushing is required. |
| Drilling | soft line | Oilfield slang term for rope not made of steel, such as nylon, cotton, or especially standard manila hemp rope |
| Drilling | softline | Oilfield slang term for rope not made of steel, such as nylon, cotton, or especially standard manila hemp rope. |
| Drilling | spacer fluid | Any liquid used to physically separate one special-purpose liquid from another. Special-purpose liquids are typically prone to contamination, so a spacer fluid compatible with each is used between the two. The most common spacer is simply water. However, chemicals are usually added to enhance its performance for the particular operation. Spacers are used primarily when changing mud types and to separate mud from cement during cementing operations. In the former, an oil-base fluid must be kept separate from a water-base fluid. In this case, the spacer may be base oil. In the latter operation, a chemically treated water spacer usually separates drilling mud from cement slurry. For proper performance and to prevent unanticipated problems, the spacer should be tested with each fluid in small-scale pilot tests. Some spacer fluids are designed to induce a particular flow regime. Ideally, a cement slurry should have turbulent flow to efficiently displace drilling fluids, but there might be pumping restrictions on fluid velocity. Therefore, a spacer that can achieve turbulent or pseudolaminar flow might be selected. |
| Drilling | spinning chain | A length of ordinary steel link chain used by the drilling crew to cause pipe being screwed together to turn rapidly. This is accomplished by first carefully wrapping the chain around the lower half of the tool joint that is hanging off in the slips, stabbing another joint into that one, and then throwing the chain in such a manner that it wraps itself on the new upper joint. At the proper time, the driller must pull tension on the chain while a member of the floor crew holds some tension on the free end of the chain. This causes the new drillpipe joint to act like a spool, and as the driller pulls the chain on one end using the drawworks, the spool (or new pipe joint) turns and screws into the joint hung off in the slips. If the floor crew members are not extremely careful, loose clothing or worse, fingers, may become trapped in the unspooling chain and be severely damaged or cut off. Most rig contractors have discontinued the use of spinning chains because of high accident rates. The chains are still available on the rigs, but are not routinely used, having been replaced with other mechanical spinning devices. |
| Drilling | spud | To apply weight to a troublesome drilling section, usually by moving the drilling string up and down, in hopes that the section will drill faster. |
| Drilling | spud | To start the well drilling process by removing rock, dirt and other sedimentary material with the drill bit. |
| Drilling | stab | To place the male threads of a piece of the drillstring, such as a joint of drillpipe, into the mating female threads, prior to making up tight. |
| Drilling | stand | Two or three single joints of drillpipe or drill collars that remain screwed together during tripping operations. Most modern medium- to deep-capacity drilling rigs handle three-joint stands, called “trebles” or “triples.” Some smaller rigs have the capacity for only two-joint stands, called “doubles.” In each case, the drillpipe or drill collars are stood back upright in the derrick and placed into fingerboards to keep them orderly. This is a relatively efficient way to remove the drillstring from the well when changing the bit or making adjustments to the bottomhole assembly, rather than unscrewing every threaded connection and laying the pipe down to a horizontal position. |
| Drilling | stand pipe | A rigid metal conduit that provides the high-pressure pathway for drilling mud to travel approximately one-third of the way up the derrick, where it connects to a flexible high-pressure hose (kelly hose). Many large rigs are fitted with dual standpipes so that downtime is kept to a minimum if one standpipe requires repair. |
| Drilling | steerable motor | A mud motor incorporating a bent housing that may be stabilized like a rotary bottomhole assembly. A steerable motor can be used to steer the wellbore without drillstring rotation in directional drilling operations, or to drill ahead in a rotary drilling mode |
| Drilling | stripping | The act of putting drillpipe into the wellbore when the blowout preventers (BOPs) are closed and pressure is contained in the well. This is necessary when a kick is taken, since well kill operations should always be conducted with the drillstring on bottom, and not somewhere up the wellbore. If only the annular BOP has been closed, the drillpipe may be slowly and carefully lowered into the wellbore, and the BOP itself will open slightly to permit the larger diameter tool joints to pass through. If the well has been closed with the use of ram BOPs, the tool joints will not pass by the closed ram element. Hence, while keeping the well closed with either another ram or the annular BOP, the ram must be opened manually, then the pipe lowered until the tool joint is just below the ram, and then the ram closed again. This procedure is repeated whenever a tool joint must pass by a ram BOP. Rig crews are usually required to practice ram-to-ram and ram-to-annular stripping operations as part of their well control certifications. In stripping operations, the combination of the pressure in the well and the weight of the drillstring is such that the pipe falls in the hole under its own weight, whereas in snubbing operations the pipe must be pushed into the hole. |
| Drilling | structural steering | Structural steering is a method of directing the wellbore trajectory of horizontal wells using 3D visualization. It is the process of combining structural analysis and modeling capabilities with borehole images to optimize well placement, often in real-time. Structural steering integrates deep-reading LWD resistivity tools and high-resolution imaging devices to create structural models of often complex geologic conditions encountered by the drill bit. This technique helps operators understand the formations already drilled and allows them proactively to correct wellbore trajectories for anticipated changes. |
| Drilling | stuck | Referring to the varying degrees of inability to move or remove the drillstring from the wellbore. At one extreme, it might be possible to rotate the pipe or lower it back into the wellbore, or it might refer to an inability to move the drillstring vertically in the well, though rotation might be possible. At the other extreme, it reflects the inability to move the drillstring in any manner. Usually, even if the stuck condition starts with the possibility of limited pipe rotation or vertical movement, it will degrade to the inability to move the pipe at all. |
| Drilling | stuck pipe | The portion of the drillstring that cannot be rotated or moved vertically. |
| Drilling | sub | Any small component of the drillstring, such as a short drill collar or a thread crossover. |
| Drilling | sub | Slang for substructure, which is the part of the rig that supports the derrick, rig floor and associated equipment. |
| Drilling | submersible drilling rig | A particular type of floating vessel, usually used as a mobile offshore drilling unit (MODU), that is supported primarily on large pontoon-like structures submerged below the seasurface. The operating decks are elevated 100 or more feet [30 m] above the pontoons on large steel columns. Once on the desired location, this type of structure is slowly flooded until it rests on the seafloor. After the well is completed, the water is pumped out of the buoyancy tanks, the vessel refloated and towed to the next location. Submersibles, as they are known informally, operate in relatively shallow water, since they must actually rest on the seafloor. |
| Drilling | suction pit | A mud tank, usually made of steel, connected to the intake of the main rig pumping system. The connection is commonly formed with a centrifugal pump charging the main rig pumps to increase efficiency. Since it is the last tank in the surface mud system, the suction pit should contain the cleanest and best-conditioned mud on location. It is also the most representative of mud characteristics in the hole, except for temperature. |
| Drilling | supply vessel | In offshore operations, any barge, boat or ship that brings materials and personnel to and from the rigsite. |
| Drilling | surface casing | A large-diameter, relatively low-pressure pipe string set in shallow yet competent formations for several reasons. First, the surface casing protects fresh-water aquifers onshore. Second, the surface casing provides minimal pressure integrity, and thus enables a diverter or perhaps even a blowout preventer (BOP) to be attached to the top of the surface casing string after it is successfully cemented in place. Third, the surface casing provides structural strength so that the remaining casing strings may be suspended at the top and inside of the surface casing. |
| Drilling | surface pipe | Another term for surface casing, a large-diameter, relatively low-pressure pipe string set in shallow yet competent formations for several reasons. First, the surface casing protects fresh-water aquifers onshore. Second, the surface casing provides minimal pressure integrity, and thus enables a diverter or perhaps even a blowout preventer (BOP) to be attached to the top of the surface casing string after it is successfully cemented in place. Third, the surface casing provides structural strength so that the remaining casing strings may be suspended at the top and inside of the surface casing. |
| Drilling | survey | A completed measurement of the inclination and azimuth of a location in a well (typically the total depth at the time of measurement). In both directional and straight holes, the position of the well must be known with reasonable accuracy to ensure the correct wellbore path and to know its position in the event a relief well must be drilled. The measurements themselves include inclination from vertical, and the azimuth (or compass heading) of the wellbore if the direction of the path is critical. These measurements are made at discrete points in the well, and the approximate path of the wellbore computed from the discrete points. Measurement devices range from simple pendulum-like devices to complex electronic accelerometers and gyroscopes used more often as MWD becomes more popular. In simple pendulum measurements, the position of a freely hanging pendulum relative to a measurement grid (attached to the housing of the tool and assumed to represent the path of the wellbore) is captured on photographic film. The film is developed and examined when the tool is removed from the wellbore, either on wireline or the next time pipe is tripped out of the hole. |
| Drilling | survey | To make the measurements as in definitions 1 or 2. |
| Drilling | survey | A precise and legally binding measurement of surface locations, referenced to known benchmark locations. |
| Drilling | swab | To reduce pressure in a wellbore by moving pipe, wireline tools or rubber-cupped seals up the wellbore. If the pressure is reduced sufficiently, reservoir fluids may flow into the wellbore and towards the surface. Swabbing is generally considered harmful in drilling operations, because it can lead to kicks and wellbore stability problems. In production operations, however, the term is used to describe how the flow of reservoir hydrocarbons is initiated in some completed wells. |
| Drilling | swivel | A mechanical device that suspends the weight of the drillstring. It is designed to allow rotation of the drillstring beneath it conveying high volumes of high-pressure drilling mud between the rig’s circulation system and the drillstring. |
| Drilling | tapered string | A string of drillpipe or casing that consists of two or more sizes or weights. In most tapered strings, a larger diameter pipe or casing is placed at the top of the wellbore and the smaller size at the bottom. Note that since the pipe is put into the well bottom first, the smaller pipe is run into the hole first, followed by the larger diameter. Other than the different sizes, which are usually chosen to optimize well economics, there is nothing distinctive about the pipe sections. However, pipe-handling tools must be available for each pipe size, not just one size, as is the typical case. |
| Drilling | TD | Abbreviation for total depth. The depth of the bottom of the well. Usually, it is the depth where drilling has stopped. |
| Drilling | Texas deck | On an offshore jackup drilling rig, the deck below the rotary table and rig floor where workers can access the BOP stack. This platform surrounds the base of the BOP stack and is suspended from the cantilever (where the rig floor is located) by adjustable cables. It is accessed from the main deck of the jackup barge by a semipermanent stairwell. The Texas deck is used primarily for installing the wellhead and nippling the BOP stack up and down. |
| Drilling | TFNB | An abbreviation on drilling reports or mud logs signifying trip for new bit. |
| Drilling | TG | Gas entrained in the drilling fluid during a pipe trip, which typically results in a significant increase in gas that is circulated to surface. This increase arises from a combination of two factors: lack of circulation when the mud pumps are turned off, and swabbing effects caused by pulling the drillstring to surface. These effects may be seen following a short trip into casing or a full trip to surface. |
| Drilling | thief zone | A formation encountered during drilling into which circulating fluids can be lost. |
| Drilling | thread protector | A cheap, expendable, perhaps even disposable threaded shape to mate with threads on drillstring and casing components. Thread protectors prevent harmful impacts and other contact to the metal thread surfaces. Some protectors are strong enough and are fitted with lifting eyes so that they may be screwed into a joint of drillpipe, a drill collar or another component and a chain tied to the eye for lifting the joint. Except for this type, most of the other available styles of thread protectors are relatively inexpensive, being made from thermoplastics and various epoxy resins |
| Drilling | threadform | A particular style or type of threaded connection, especially as used for rotary shouldered connections. Threadforms come in a variety of sizes, pitches, tapers, threads per in., and individual thread profiles. Fortunately, each of these varieties has a published standard, either considered public and maintained by the American Petroleum Institute (API) or maintained by operating or service companies as proprietary information. |
| Drilling | three component seismic data | A type of multicomponent seismic data acquired in a land, marine, or borehole environment by using three orthogonally oriented geophones or accelerometers. 3C is particularly appropriate when the addition of a hydrophone (the basis for 4C seismic data) adds no value to the measurement, as for example, on land. This technique allows determination of both the type of wave and its direction of propagation. |
| Drilling | tight hole | A well that the operator requires be kept as secret as possible, especially the geologic information. Exploration wells, especially rank wildcats, are often designated as tight. Unfortunately, this designation is of questionable benefit in keeping the data secret. |
| Drilling | tight hole | A section of a wellbore, usually openhole, where larger diameter components of the drillstring, such as drillpipe tool joints, drill collars, stabilizers, and the bit, may experience resistance when the driller attempts to pull them through these sections. |
| Drilling | tongs | Large-capacity, self-locking wrenches used to grip drillstring components and apply torque. As with opposing pipe wrenches for a plumber, the tongs must be used in opposing pairs. As a matter of efficiency, one set of tongs is essentially tied off with a cable or chain to the derrick, and the other is actively pulled with mechanical catheads. The breakout tongs are the active tongs during breakout (or loosening) operations. The makeup tongs are active during makeup (or tightening) operations. |
| Drilling | tool joint | The enlarged and threaded ends of joints of drillpipe. These components are fabricated separately from the pipe body and welded onto the pipe at a manufacturing facility. The tool joints provide high-strength, high-pressure threaded connections that are sufficiently robust to survive the rigors of drilling and numerous cycles of tightening and loosening at threads. Tool joints are usually made of steel that has been heat treated to a higher strength than the steel of the tube body. The large-diameter section of the tool joints provides a low stress area where pipe tongs are used to grip the pipe. Hence, relatively small cuts caused by the pipe tongs do not significantly impair the strength or life of the joint of drillpipe. |
| Drilling | toolface | The angle measured in a plane perpendicular to the drillstring axis that is between a reference direction on the drillstring and a fixed reference. For near-vertical wells, north is the fixed reference and the angle is the magnetic toolface. For more-deviated wells, the top of the borehole is the fixed reference and the angle is the gravity toolface, or high side toolface. |
| Drilling | toolpusher | The location supervisor for the drilling contractor. The toolpusher is usually a senior, experienced individual who has worked his way up through the ranks of the drilling crew positions. His job is largely administrative, including ensuring that the rig has sufficient materials, spare parts and skilled personnel to continue efficient operations. The toolpusher also serves as a trusted advisor to many personnel on the rigsite, including the operator’s representative, the company man. |
| Drilling | top drive | A device that turns the drillstring. It consists of one or more motors (electric or hydraulic) connected with appropriate gearing to a short section of pipe called a quill, that in turn may be screwed into a saver sub or the drillstring itself. The topdrive is suspended from the hook, so the rotary mechanism is free to travel up and down the derrick. This is radically different from the more conventional rotary table and kelly method of turning the drillstring because it enables drilling to be done with three joint stands instead of single joints of pipe. It also enables the driller to quickly engage the pumps or the rotary while tripping pipe, which cannot be done easily with the kelly system. While not a panacea, modern topdrives are a major improvement to drilling rig technology and are a large contributor to the ability to drill more difficult extended-reach wellbores. In addition, the topdrive enables drillers to minimize both frequency and cost per incident of stuck pipe. |
| Drilling | topdrive | A device that turns the drillstring. It consists of one or more motors (electric or hydraulic) connected with appropriate gearing to a short section of pipe called a quill, that in turn may be screwed into a saver sub or the drillstring itself. The topdrive is suspended from the hook, so the rotary mechanism is free to travel up and down the derrick. This is radically different from the more conventional rotary table and kelly method of turning the drillstring because it enables drilling to be done with three joint stands instead of single joints of pipe. It also enables the driller to quickly engage the pumps or the rotary while tripping pipe, which cannot be done easily with the kelly system. While not a panacea, modern topdrives are a major improvement to drilling rig technology and are a large contributor to the ability to drill more difficult extended-reach wellbores. In addition, the topdrive enables drillers to minimize both frequency and cost per incident of stuck pipe. |
| Drilling | total depth | The depth of the bottom of the well. Usually, it is the depth where drilling has stopped. |
| Drilling | tour | A work shift of a drilling crew. Drilling operations usually occur around the clock because of the cost to rent a rig. As a result, there are usually two separate crews working twelve-hour tours to keep the operation going. Some companies prefer three eight-hour tours: the daylight tour starts at daylight or 8 AM; the graveyard tour is the overnight shift or the shift that begins at midnight. (Pronounced “tower” in many areas.) |
| Drilling | travel joint | Also known as slip joint, a telescoping joint at the surface in floating offshore operations that permits vessel heave (vertical motion) while maintaining a riser pipe to the seafloor. As the vessel heaves, the slip joint telescopes in or out by the same amount so that the riser below the slip joint is relatively unaffected by vessel motion. |
| Drilling | traveling block | The set of sheaves that move up and down in the derrick. The wire rope threaded through them is threaded (or “reeved”) back to the stationary crown blocks located on the top of the derrick. This pulley system gives great mechanical advantage to the action of the wire rope drilling line, enabling heavy loads (drillstring, casing and liners) to be lifted out of or lowered into the wellbore. |
| Drilling | trip | The act of pulling the drillstring out of the hole or replacing it in the hole. A pipe trip is usually done because the bit has dulled or has otherwise ceased to drill efficiently and must be replaced. |
| Drilling | trip gas | Gas entrained in the drilling fluid during a pipe trip, which typically results in a significant increase in gas that is circulated to surface. This increase arises from a combination of two factors: lack of circulation when the mud pumps are turned off, and swabbing effects caused by pulling the drillstring to surface. These effects may be seen following a short trip into casing or a full trip to surface. |
| Drilling | trip out | To remove the drillstring from the wellbor |
| Drilling | tripping pipe | The act of pulling the drillstring out of the hole or replacing it in the hole. A pipe trip is usually done because the bit has dulled or has otherwise ceased to drill efficiently and must be replaced. |
| Drilling | true vertical depth | The vertical distance from a point in the well (usually the current or final depth) to a point at the surface, usually the elevation of the rotary kelly bushing (RKB). This is one of two primary depth measurements used by the drillers, the other being measured depth. TVD is important in determining bottomhole pressures, which are caused in part by the hydrostatic head of fluid in the wellbore. For this calculation, measured depth is irrelevant and TVD must be used. For most other operations, the driller is interested in the length of the hole or how much pipe will fit into the hole. For those measurements, measured depth, not TVD, is used. While the drilling crew should be careful to designate which measurement they are referring to, if no designation is used, they are usually referring to measured depth. Note that measured depth, due to intentional or unintentional curves in the wellbore, is always longer than true vertical depth. |
| Drilling | tubulars | A generic term pertaining to any type of oilfield pipe, such as drill pipe, drill collars, pup joints, casing, production tubing and pipeline. |
| Drilling | turnkey | A type of financing arrangement for the drilling of a wellbore that places considerable risk and potential reward on the drilling contractor. Under such an arrangement, the drilling contractor assumes full responsibility for the well to some predetermined milestone such as the successful running of logs at the end of the well, the successful cementing of casing in the well or even the completion of the well. Until this milestone is reached, the operator owes nothing to the contractor. The contractor bears all risk of trouble in the well, and in extreme cases, may have to abandon the well entirely and start over. In return for assuming such risk, the price of the well is usually a little higher than the well would cost if relatively trouble free. Therefore, if the contractor succeeds in drilling a trouble-free well, the fee added as contingency becomes profit. Some operators, however, have been required by regulatory agencies to remedy problem wells, such as blowouts, if the turnkey contractor does not. |
| Drilling | TVD | The vertical distance from a point in the well (usually the current or final depth) to a point at the surface, usually the elevation of the rotary kelly bushing (RKB). This is one of two primary depth measurements used by the drillers, the other being measured depth. TVD is important in determining bottomhole pressures, which are caused in part by the hydrostatic head of fluid in the wellbore. For this calculation, measured depth is irrelevant and TVD must be used. For most other operations, the driller is interested in the length of the hole or how much pipe will fit into the hole. For those measurements, measured depth, not TVD, is used. While the drilling crew should be careful to designate which measurement they are referring to, if no designation is used, they are usually referring to measured depth. Note that measured depth, due to intentional or unintentional curves in the wellbore, is always longer than true vertical depth. |
| Drilling | twist off | To part or break the drillstring downhole due to either fatigue or excessive torque. |
| Drilling | twist-off | Parting or breaking of the drillstring downhole due to fatigue or excessive torque. |
| Drilling | underbalance | The amount of pressure (or force per unit area) exerted on a formation exposed in a wellbore below the internal fluid pressure of that formation. If sufficient porosity and permeability exist, formation fluids enter the wellbore. The drilling rate typically increases as an underbalanced condition is approached. |
| Drilling | underbalanced | Referring to a situation when the pressure (or force per unit area) exerted on a formation exposed in a wellbore is less than the internal fluid pressure of that formation. If sufficient porosity and permeability exist, formation fluids enter the wellbore. The drilling rate typically increases as an underbalanced condition is approached. |
| Drilling | underground blowout | The uncontrolled flow of reservoir fluids from one reservoir into the wellbore, along the wellbore, and into another reservoir. This crossflow from one zone to another can occur when a high-pressure zone is encountered, the well flows, and the drilling crew reacts properly and closes the blowout preventers (BOPs). Pressure in the annulus then builds up to the point at which a weak zone fractures. Depending on the pressure at which the fracturing occurs, the flowing formation can continue to flow and losses continue to occur in the fractured zone. Underground blowouts are historically the most expensive problem in the drilling arena, eclipsing the costs of even surface blowouts. It may prove necessary to drill a second kill well in order to remedy an underground blowout. |
| Drilling | underream | To enlarge a wellbore past its original drilled size. Underreaming is sometimes done for safety or efficiency reasons. Some well planners believe it is safer to drill unknown shallow formations with a small-diameter bit, and if no gas is encountered, to then enlarge the pilot hole. An underreaming operation may also be done if a small additional amount of annular space is desired, as might be the case in running a liner if surge pressures were problematic. |
| Drilling | upset | A part at the end of tubulars, such as drillpipe, casing or other tubing, which has extra thickness and strength to compensate for the loss of metal in the threaded ends. |
| Drilling | vee door | The upside down V-shaped opening in one side of the derrick that enables long pipes and tools to be lifted into the interior of the derrick. This opening is aligned with the slide and catwalk of the rig. |
| Drilling | vee door | The open location on a mast-type rig (nonderrick) that functions like the vee-door. At least two sides are open on most mast rigs. Hence, the open side adjacent to the slide and catwalk is considered the vee-door. The vee-door is really a hole and has no true door that can be closed or locked, so inexperienced visitors to a rigsite are sometimes asked by the rig crew to find the key to the vee-door as a joke. |
| Drilling | vee-door | The upside down V-shaped opening in one side of the derrick that enables long pipes and tools to be lifted into the interior of the derrick. This opening is aligned with the slide and catwalk of the rig. |
| Drilling | vee-door | The open location on a mast-type rig (nonderrick) that functions like the vee-door. At least two sides are open on most mast rigs. Hence, the open side adjacent to the slide and catwalk is considered the vee-door. The vee-door is really a hole and has no true door that can be closed or locked, so inexperienced visitors to a rigsite are sometimes asked by the rig crew to find the key to the vee-door as a joke. |
| Drilling | volumetric efficiency | The ratio of the actual output volume of a positive displacement pump divided by the theoretical geometric maximum volume of liquid that the pump could output under perfect conditions. Inefficiencies are caused by gaseous components (air and methane) being trapped in the liquid mud, leaking and noninstantaneously sealing valves in the pumps, fluid bypass of pump swab seals, and mechanical clearances and “play” in various bearings and connecting rods in the pumps. This efficiency is usually expressed as a percentage, and ranges from about 92% to 99% for most modern rig pumps and cement pumps. For critical calculations, this efficiency can be determined by a rigsite version of the “bucket and stopwatch” technique, whereby the rig crew will count the number of pump strokes required to pump a known volume of fluid. In cementing operations, displacement is often measured by alternating between two 10-bbl displacement tanks. |
| Drilling | wait on cement | To suspend drilling operations while allowing cement slurries to solidify, harden and develop compressive strength. The drilling crew usually uses this time to catch up on maintenance items, to rig down one BOP and rig up another one for the new casing, to get tools and materials ready for the next hole section, and other non-drilling tasks. The WOC time ranges from a few hours to several days, depending on the difficulty and criticality of the cement job in question. WOC time allows cement to develop strength, and avert development of small cracks and other fluid pathways in the cement that might impair zonal isolation. |
| Drilling | wall sticking | Also known as differential sticking, a condition whereby the drillstring cannot be moved (rotated or reciprocated) along the axis of the wellbore. Differential sticking typically occurs when high-contact forces caused by low reservoir pressures, high wellbore pressures, or both, are exerted over a sufficiently large area of the drillstring. Differential sticking is, for most drilling organizations, the greatest drilling problem worldwide in terms of time and financial cost. It is important to note that the sticking force is a product of the differential pressure between the wellbore and the reservoir and the area that the differential pressure is acting upon. This means that a relatively low differential pressure (delta p) applied over a large working area can be just as effective in sticking the pipe as can a high differential pressure applied over a small area. |
| Drilling | wash out | An enlarged region of a wellbore. A washout in an openhole section is larger than the original hole size or size of the drill bit. Washout enlargement can be caused by excessive bit jet velocity, soft or unconsolidated formations, in-situ rock stresses, mechanical damage by BHA components, chemical attack and swelling or weakening of shale as it contacts fresh water. Generally speaking, washouts become more severe with time. Appropriate mud types, mud additives and increased mud density can minimize washouts. |
| Drilling | washout | A hole in a pressure-containing component caused by erosion. A washout is relatively common where a high-velocity stream of dry gas carries abrasive sand. The severity generally decreases with sand content, velocity and liquid content. |
| Drilling | washout | An enlarged region of a wellbore. A washout in an openhole section is larger than the original hole size or size of the drill bit. Washout enlargement can be caused by excessive bit jet velocity, soft or unconsolidated formations, in-situ rock stresses, mechanical damage by BHA components, chemical attack and swelling or weakening of shale as it contacts fresh water. Generally speaking, washouts become more severe with time. Appropriate mud types, mud additives and increased mud density can minimize washouts. |
| Drilling | washover pipe | In fishing operations, a large-diameter pipe fitted with an internal grappling device and tungsten carbide cutting surfaces on the bottom. The washover pipe can be lowered over a fish in the wellbore and to latch onto and retrieve the fish. Since the washover pipe is relatively thin-walled and large in diameter, and may be prone to sticking itself, the washover operation is usually reserved as a measure of last resort before abandoning the fish altogether. |
| Drilling | washover shoe | A downhole tool routinely used in fishing operations to prepare the top and outside surface of a fish, generally to allow an overshot or similar fishing tool to engage cleanly on the fish. In some cases, the outer portion of a fish may be milled out to allow the body and remaining debris to be pushed to the bottom of the wellbore. |
| Drilling | weevil | A new, completely inexperienced member of the drilling crew. Such a crewmember is stereotyped as prone to making mistakes and being injured, and typically endures pranks played on him by the drilling crew. While the terms weevil and its close cousin, worm, are used widely, they are labels of inexperience, rather than derogatory terms. |
| Drilling | weight indicator | One of the instruments that the driller uses to monitor and improve the operating efficiencies of the drilling operation. The actual measurement of weight is made with a hydraulic gauge attached to the dead line of the drilling line. As tension increases in the drilling line, more hydraulic fluid is forced through the instrument, turning the hands of the indicator. The weight that is measured includes everything exerting tension on the wire rope, including the traveling blocks and cable itself. Hence, to have an accurate weight measurement of the drillstring, the driller must first make a zero offset adjustment to account for the traveling blocks and items other than the drillstring. Then the indicated weight will represent the drillstring (drillpipe and bottomhole assembly). However, the driller is only nominally interested in this weight for most operations. The weight of interest is the weight applied to the bit on the bottom of the hole. The driller could simply take the rotating and hanging off bottom weight, say 300,000 pounds [136,200 kg], and subtract from that the amount of rotating on bottom weight, say 250,000 pounds [113,500 kg], to get a bit weight of 50,000 pounds [22,700 kg]. However, most rigs are equipped with a weight indicator that has a second indicator dial that can be set to read zero (“zeroed”) with the drillstring hanging free, and works backwards from the main indicator dial. After proper zeroing, any weight set on bottom (that takes weight away from the main dial), has the effect of adding weight to this secondary dial, so that the driller can read weight on bit directly from the dial. |
| Drilling | well control | The technology focused on maintaining pressure on open formations (that is, exposed to the wellbore) to prevent or direct the flow of formation fluids into the wellbore. This technology encompasses the estimation of formation fluid pressures, the strength of the subsurface formations and the use of casing and mud density to offset those pressures in a predictable fashion. Also included are operational procedures to safely stop a well from flowing should an influx of formation fluid occur. To conduct well-control procedures, large valves are installed at the top of the well to enable wellsite personnel to close the well if necessary. |
| Drilling | wellbore | The drilled hole or borehole, including the openhole or uncased portion of the well. Borehole may refer to the inside diameter of the wellbore wall, the rock face that bounds the drilled hole. |
| Drilling | wellbore orientation | Wellbore direction. Wellbore orientation may be described in terms of inclination and azimuth. Inclination refers to the vertical angle measured from the down direction—the down, horizontal and up directions have inclinations of 0°, 90° and 180°, respectively. Azimuth refers to the horizontal angle measured clockwise from true north—the north, east, south and west directions have azimuths of 0°, 90°, 180° and 270°, respectively. |
| Drilling | wellhead | The system of spools, valves and assorted adapters that provide pressure control of a production well. |
| Drilling | wildcat | An exploration well. The significance of this type of well to the drilling crew and well planners is that by definition, little if anything about the subsurface geology is known with certainty, especially the pressure regime. This higher degree of uncertainty necessitates that the drilling crews be appropriately skilled, experienced and aware of what various well parameters are telling them about the formations they drill. The crews must operate top-quality equipment, especially the blowout preventers, since a kick could occur at virtually any time. If a wildcat is especially far from another wellbore, it may be described as a “rank wildcat. |
| Drilling | wiper plug | Another term for cementing plug, a rubber plug used to separate the cement slurry from other fluids, reducing contamination and maintaining predictable slurry performance. Two types of cementing plug are typically used on a cementing operation. The bottom plug is launched ahead of the cement slurry to minimize contamination by fluids inside the casing prior to cementing. A diaphragm in the plug body ruptures to allow the cement slurry to pass through after the plug reaches the landing collar. The top plug has a solid body that provides positive indication of contact with the landing collar and bottom plug through an increase in pump pressure. |
| Drilling | wiper trip | An abbreviated recovery and replacement of the drillstring in the wellbore that usually includes the bit and bottomhole assembly passing by all of the openhole, or at least all of the openhole that is thought to be potentially troublesome. This trip varies from the short trip or the round trip only in its function and length. Wiper trips are commonly used when a particular zone is troublesome or if hole-cleaning efficiency is questionable. |
| Drilling | WOC | To suspend drilling operations while allowing cement slurries to solidify, harden and develop compressive strength. The drilling crew usually uses this time to catch up on maintenance items, to rig down one BOP and rig up another one for the new casing, to get tools and materials ready for the next hole section, and other non-drilling tasks. The WOC time ranges from a few hours to several days, depending on the difficulty and criticality of the cement job in question. WOC time allows cement to develop strength, and avert development of small cracks and other fluid pathways in the cement that might impair zonal isolation. |
| Drilling | workover | The repair or stimulation of an existing production well for the purpose of restoring, prolonging or enhancing the production of hydrocarbons. |
| Drilling | worm | A new, completely inexperienced member of the drilling crew. Such a crewmember is stereotyped as prone to making mistakes and being injured, and typically endures pranks played on him by the drilling crew. While the terms weevil and its close cousin, worm, are used widely, they are labels of inexperience, rather than derogatory terms. |
| Drilling | yield | The volume occupied by one sack of dry cement after mixing with water and additives to form a slurry of a desired density. Yield is commonly expressed in US units as cubic feet per sack (cu. ft./sk). |
| Drilling | yield | The specified minimum yield strength of steel used in pipe. For example, the yield of N-80 casing is 80,000 psi [552 MPa]. |
| Drilling | zip collars | Drill collars (usually straight drill collars) that have been machined with a reduced diameter at the box (up) end so that they may be more easily handled with open-and-close elevators. The elevators close around the reduced-diameter section, latch securely, and a shoulder on the elevators prevents the larger diameter end of the collar from passing through the elevators, so the collars can be lifted. If zip grooves are not used on the collars, special lifting subs must be threaded into each stand of collars for lifting, which is time-consuming and less efficient than zip grooves. The primary drawback to zip grooves is that they may reduce the life of the collar by putting an effective limit on how many times the collar threads may be recut. |
| Drilling | zip groove | A reduced-diameter section that has been machined at the box (up) end of a drill collar (usually a straight drill collar) so that the collar may be more easily handled with open-and-close elevators. The elevators close around the reduced-diameter section, latch securely, and a shoulder on the elevators prevents the larger diameter end of the collar from passing through the elevators, so the collars can be lifted. If zip grooves are not used on the collars, special lifting subs must be threaded into each stand of collars for lifting, which is time-consuming and less efficient than zip grooves. The primary drawback to zip grooves is that they may reduce the life of the collar by putting an effective limit on how many times the collar threads may be recut. |
| Drilling Fluids | base exchange | Quantity of positively charged ions (cations) that a clay mineral (or similar material) can accommodate on its negative charged surface, expressed as milliequivalents per 100 grams. CEC of solids in drilling muds is measured on a whole mud sample by a methylene blue capacity (MBC) test, which is typically performed to specifications established by API. CEC for a mud sample is reported as MBC, methylene blue test (MBT) or bentonite equivalent, lbm/bbl or kg/m3. |
| Drilling Fluids | chemical barrel | A 20- to 50-gallon [3.2- to 7.9 m3] container for liquid mud additives, usually located above the suction pit on a drilling rig. The chemical barrel is used to slowly dispense various types of liquids into the active mud system. It has traditionally been used to add caustic (NaOH or KOH) solution at a slow and steady rate in order to maintain a uniform pH throughout a circulating mud system. Adding caustic solution is an especially risky operation and the proper design and use of the chemical barrel for safety is vitally important. Derrickmen must be informed of the dangers, proper protective clothing and safety rules to follow when using the chemical barrel. |
| Drilling Fluids | plastic viscosity | A parameter of the Bingham plastic model. PV is the slope of the shear stress/shear rate line above the yield point. PV represents the viscosity of a mud when extrapolated to infinite shear rate on the basis of the mathematics of the Bingham model. (Yield point, YP, is the other parameter of that model.) A low PV indicates that the mud is capable of drilling rapidly because of the low viscosity of mud exiting at the bit. High PV is caused by a viscous base fluid and by excess colloidal solids. To lower PV, a reduction in solids content can be achieved by dilution of the mud. |
| Drilling Fluids | abrasion test | A laboratory test to evaluate drilling-grade weighting material for potential abrasiveness. The test measures weight loss of a specially shaped, stainless-steel mixer blade after 20 minutes at 11,000 rpm running in a laboratory-prepared mud sample. Abrasiveness is quantified by the rate of weight loss, reported in units of mg/min. Mineral hardness, particle size and shape are the main parameters that affect abrasiveness of weighting materials. Some crystalline forms of hematite grind to a higher percentage of large particles than do other forms and are therefore more abrasive. Hematites are harder than barites, grind courser and are more abrasive. Thus, a hematite that is proposed as a weighting material for mud is typically a candidate for abrasion testing. |
| Drilling Fluids | absolute volume | The volume a solid occupies or displaces when added to water divided by its weight, or the volume per unit mass. In the oil field, absolute volume is typically given in units of gallons per pound (gal/lbm) or cubic meters per kilogram (m3/kg). |
| Drilling Fluids | accretion | The mechanism by which partially hydrated cuttings stick to parts of the bottomhole assembly and accumulate as a compacted, layered deposit. |
| Drilling Fluids | acid | Pertaining to an aqueous solution, such as a water-base drilling fluid, which has more hydrogen ions (H+) than hydroxyl ions (OH-) and pH less than 7. |
| Drilling Fluids | acidity | A chemical property of an aqueous system that implies that there are more hydrogen ions (H+) in the system, or a potential to produce more hydrogen ions, than there are hydroxyl ions (OH-), or potential to produce hydroxyl ions. |
| Drilling Fluids | acrylamide acrylate polymer | A linear copolymer of acrylate (anionic) and acrylamide (nonionic) monomers, also called partially-hydrolyzed polyacrylamide (PHPA). The ratio of acrylic acid to acrylamide groups on the polymer chain can be varied in manufacturing, as can molecular weight. Another variable is the base used to neutralize the acrylic acid groups, usually NaOH or KOH, or sometimes NH4OH. A concentration of approximately 10 to 30% acrylate groups provides optimal anionic characteristics for most drilling applications. High-molecular weight PHPA is used as a shale-stabilizing polymer in PHPA mud systems. It is also used as clay extender, either dry-mixed into clay or added at the rig to a low-bentonite mud. PHPA can also be used to flocculate colloidal solids during clear-water drilling and for wastewater cleanup. Low molecular-weight PHPA is a clay deflocculant. |
| Drilling Fluids | acrylamide polymer | A linear, nonionic polymer made of acrylamide monomers, CH2=CHCONH2 . High molecular-weight polyacrylamides are used as selective flocculants in clear-water drilling, low-solids muds and wastewater cleanup. Polymers made of smaller molecules are used as clay deflocculants in water muds, which can contain hardness ions. Polyacrylamides are not nearly as sensitive to salinity and hardness as the anionic polyacrylates (SPA). Also, being nonionic, they are not as powerful for flocculation or deflocculation applications. Acrylamide polymers are, however, susceptible to hydrolysis and release ammonia under hot, alkaline conditions. |
| Drilling Fluids | acrylamide-acrylate polymer | A linear copolymer of acrylate (anionic) and acrylamide (nonionic) monomers, also called partially-hydrolyzed polyacrylamide (PHPA). The ratio of acrylic acid to acrylamide groups on the polymer chain can be varied in manufacturing, as can molecular weight. Another variable is the base used to neutralize the acrylic acid groups, usually NaOH or KOH, or sometimes NH4OH. A concentration of approximately 10 to 30% acrylate groups provides optimal anionic characteristics for most drilling applications. High-molecular weight PHPA is used as a shale-stabilizing polymer in PHPA mud systems. It is also used as clay extender, either dry-mixed into clay or added at the rig to a low-bentonite mud. PHPA can also be used to flocculate colloidal solids during clear-water drilling and for wastewater cleanup. Low molecular-weight PHPA is a clay deflocculant. |
| Drilling Fluids | acrylamido methyl propane sulfonate polymer | A copolymer of 2-acrylamido-2methyl propane sulfonate and acrylamide. AMPS polymers are highly water-soluble anionic additives designed for high-salinity and high-temperature water-mud applications. (Alkyl-substituted acrylamide can be used instead of ordinary acrylamide, which lessens its vulnerability to hydrolysis at high temperature and high pH.) Polymers from 0.75 to 1.5 MM molecular weight are suggested for fluid-loss control in these difficult muds. Reference: Perricone AC, Enright DP and Lucas JM: “Vinyl Sulfonate Copolymers for High-Temperature Filtration Control of Water-Base Muds,” SPE Drilling Engineering 1, no. 5 (October 1986): 358-364. |
| Drilling Fluids | acrylamido-methyl-propane sulfonate polymer | A copolymer of 2-acrylamido-2methyl propane sulfonate and acrylamide. AMPS polymers are highly water-soluble anionic additives designed for high-salinity and high-temperature water-mud applications. (Alkyl-substituted acrylamide can be used instead of ordinary acrylamide, which lessens its vulnerability to hydrolysis at high temperature and high pH.) Polymers from 0.75 to 1.5 MM molecular weight are suggested for fluid-loss control in these difficult muds. |
| Drilling Fluids | acrylate polymer | Linear, anionic polymer made from the monomer acrylic acid, CH2=CHCOO- H+. The acrylic acid groups are evenly spaced along the chain. Acrylic acid polymer neutralized with NaOH is sodium polyacrylate (SPA). Polyacrylates are best utilized in soft water with low salinity to achieve the best dispersion and full chain elongation. Even low concentrations of hardness ions, for example, Ca+2, precipitate polyacrylates. Low molecular-weight polyacrylates are used as clay deflocculants. High molecular weight polymers are used for fluid-loss control and as a clay extender. As an extender, SPA is added to bentonite at the grinding plant. It is also used at the rig in low-solids mud. Divalent cations can negate its benefits as a clay extender. SPA is highly efficient when used to flocculate colloids in native-solids muds, clear-water muds and wastewater cleanup. The polymer chain links together colloidal solids that can be removed by gravity settling in shallow pits or by applying hydrocyclone, centrifuge or filtration techniques. |
| Drilling Fluids | active sulfide | A compound of sulfur that contains the S-2 ion. Sulfides can be generated from soluble iron sulfide minerals or from sulfate-reducing bacteria. The term “active sulfide” is used to denote compounds that revert to the highly toxic H2S gas when acidified with 2-molar citric acid solution, as opposed to inert sulfide, which is stable. Active sulfides include calcium sulfide and bisulfide formed when H2S reacts with lime in an oil-base mud. Their accumulation constitutes a safety concern at the rig because of the risk of reverting to H2S gas should an acidic influx occur. They may be converted to inert sulfides by adding zinc oxide. |
| Drilling Fluids | activity of aqueous solutions | The escaping tendency, or vapor pressure, of water molecules in an aqueous solution compared with that of pure water, typically abbreviated aw. Activity is expressed mathematically as the ratio of two vapor pressures: aw = p/po, where p is vapor pressure of the solution and po is vapor pressure of pure water. The ratio ranges from near 0 to 1.0 and corresponds to percent relative humidity (% RH) of air in equilibrium with the aqueous solution. For pure water, aw = po/po = 1.00 and RH = 100%. By increasing the concentration of salt (or other solutes) in the solution, aw decreases, because vapor pressure of the solution decreases. However, aw never reaches zero. Known-activity, saturated-salt solutions are used to calibrate RH meters. Measuring RH of air above an oil mud is a simple way to measure the activity (salinity) of its water phase. Adjusting the salinity of the water phase is a way to control movement of water into or out of shales that are being drilled with an oil mud. Chenevert related aw in oil mud to RH above the mud sample and devised a practical test using an electrohygrometer to measure RH, called the “Chenevert Method. |
| Drilling Fluids | acyclic compound | One of a group of organic compounds of carbon (C) and hydrogen (H) in which the carbon atoms have linear, branched chain (open), or both types of structures. Aliphatics, as they are informally called, can be divided into paraffinic (saturated) and olefinic (unsaturated) chain types. The simplest paraffinic aliphatic is methane, CH4. The simplest olefinic aliphatic is ethylene, C2H6. In drilling fluids, particularly oil-base muds, the amounts and types of hydrocarbon in the mud can be an important parameter in overall performance of the mud. |
| Drilling Fluids | aerobic | Referring to a condition or a situation in which free oxygen exists in an environment. |
| Drilling Fluids | aerobic | Referring to a condition or a situation or a living creature, such as a bacteria, in which oxygen is required to sustain life. |
| Drilling Fluids | agglomeration | The formation of groups or clusters of particles (aggregates) in a fluid. In water or in water-base drilling fluid, clay particles form aggregates in a dehydrated, face-to-face configuration. This occurs after a massive influx of hardness ions into freshwater mud or during changeover to a lime mud or gyp mud. Agglomeration results in drastic reductions in plastic viscosity, yield point and gel strength. It is part of wastewater cleanup and water clarification. Alum or polymers cause colloidal particles to aggregate, allowing easier separation. |
| Drilling Fluids | aggregate | Group or cluster of particles in a fluid. In water or in water-base drilling fluid, clay particles form aggregates in a dehydrated, face-to-face configuration. This occurs after a massive influx of hardness ions into freshwater mud or during changeover to a lime mud or gyp mud. Aggregation results in drastic reductions in plastic viscosity, yield point and gel strength. It is part of wastewater cleanup and water clarification. Alum or polymers cause colloidal particles to aggregate, allowing easier separation. |
| Drilling Fluids | aggregation | The formation of groups or clusters of particles (aggregates) in a fluid. In water or in water-base drilling fluid, clay particles form aggregates in a dehydrated, face-to-face configuration. This occurs after a massive influx of hardness ions into freshwater mud or during changeover to a lime mud or gyp mud. Aggregation results in drastic reductions in plastic viscosity, yield point and gel strength. It is part of wastewater cleanup and water clarification. Alum or polymers cause colloidal particles to aggregate, allowing easier separation. |
| Drilling Fluids | aliphatic compound | One of a group of organic compounds of carbon (C) and hydrogen (H) in which the carbon atoms have linear, branched chain (open), or both types of structures. Aliphatics, as they are informally called, can be divided into paraffinic (saturated) and olefinic (unsaturated) chain types. The simplest aliphatic, paraffinic hydrocarbon is methane, CH4. The simplest aliphatic, olefinic hydrocarbon is ethylene, C2H6. In drilling fluids, particularly oil-base muds, the amounts and types of hydrocarbon in the mud can be an important parameter in overall performance of the mud. |
| Drilling Fluids | alkaline | Pertaining to an aqueous solution, such as a water-base drilling fluid, which has more hydroxyl ions (OH-) than hydrogen ions (H+) and pH greater than 7. |
| Drilling Fluids | alkalinity | A chemical property of an aqueous system that implies that there are more hydroxyl ions (OH-) in the system, or a potential to produce more hydroxyl ions, than there are hydrogen ions (H+), or potential to produce hydrogen ions. |
| Drilling Fluids | alkalinity test | A measure of the total amount of hydroxyl ions in a solution as determined by titration with standardized acid. This test is a well-known water-analysis procedure to estimate hydroxyl, carbonate ion and bicarbonate ion concentrations. There are two pH endpoints, P and M, in this titration, corresponding to phenolphthalein and methyl orange indicators. The “P” endpoint is at pH 8.3 and the “M” endpoint is at pH 4.3. Each is reported in units of cm3 acid/cm3 sample. For water samples and very simple mud filtrates, P and M data indicate OH-, HCO3- and CO3-2 concentrations, but an alkalinity test is unreliable for analyzing complex mud filtrates. The API has established standards for conducting alkalinity tests. |
| Drilling Fluids | alum | A series of double salts of aluminum sulfate and potassium sulfate with the formula Al2(SO4)3·K2SO4·nH2O. Alum is used as a colloidal flocculant in wastewater cleanup. |
| Drilling Fluids | aluminum stearate | The salt of aluminum hydroxide and stearic acid (saturated C-18 fatty acid) with the formula Al(O2C18H35)3. It is a grease-like solid. When mixed with oil (for example, diesel oil) and the mixture sprayed onto the surface of a foamy water mud, it helps the gas bubbles break out of the mud. |
| Drilling Fluids | amides | A group of organic chemicals with the general formula RCO-NH2 formed from reactions of ammonia (NH3) and a carboxylic acid, RCOO-H+. “R” groups range from hydrogen to various linear and ring structures. Amides and polyamides are emulsifiers and surfactants, many of which are made from fatty acids. |
| Drilling Fluids | amines | A group of organic chemicals that are analogs of ammonia (NH3), in which either one, two or three hydrogen atoms of ammonia are replaced by organic radicals. General formulas are: (1) primary amines, RNH2, (2) secondary amines, R1R2NH, (3) tertiary amines, R1R2R3N and quaternary amines, R1R2R3 R4N+X (where X represents an anion). Amines are organic bases (mildly alkaline) and react with acids to form nitrogenous, organic salts. Amines made from fatty acids are emulsifiers and oil-wetting agents for oilfield chemicals. |
| Drilling Fluids | AMPS | A copolymer of 2-acrylamido-2methyl propane sulfonate and acrylamide. AMPS polymers are highly water-soluble anionic additives designed for high-salinity and high-temperature water-mud applications. (Alkyl-substituted acrylamide can be used instead of ordinary acrylamide, which lessens its vulnerability to hydrolysis at high temperature and high pH.) Polymers from 0.75 to 1.5 MM molecular weight are suggested for fluid-loss control in these difficult muds. Reference: Perricone AC, Enright DP and Lucas JM: “Vinyl Sulfonate Copolymers for High-Temperature Filtration Control of Water-Base Muds,” SPE Drilling Engineering 1, no. 5 (October 1986): 358-364. |
| Drilling Fluids | anaerobic | Pertaining to systems, reactions or life processes of species, such as bacteria, in which atmospheric oxygen is not present or not required for survival. |
| Drilling Fluids | aniline point test | A test to evaluate base oils that are used in oil mud. The test indicates if an oil is likely to damage elastomers (rubber compounds) that come in contact with the oil. The aniline point is called the “aniline point temperature,” which is the lowest temperature (°F or °C) at which equal volumes of aniline (C6H5NH2) and the oil form a single phase. The aniline point (AP) correlates roughly with the amount and type of aromatic hydrocarbons in an oil sample. A low AP is indicative of higher aromatics, while a high AP is indicative of lower aromatics content. Diesel oil with AP below 120°F [49°C] is probably risky to use in oil-base mud. The API has developed test procedures that are the standard for the industry. |
| Drilling Fluids | anion | A negatively charged ion. Clay surfaces, groups on polymer chains, colloids and other materials have distinct, negatively charged areas or ions. Anionic characteristics affect performance of additives and contaminants in drilling fluids, especially water muds, in which clays and polymers are used extensively. |
| Drilling Fluids | anionic | Related to negatively charged ions. Clay surfaces, groups on polymer chains, colloids and other materials have distinct, negatively charged areas or ions. Anionic characteristics affect performance of additives and contaminants in drilling fluids, especially water muds, in which clays and polymers are used extensively. |
| Drilling Fluids | anode | The half of a battery that is positively charged and to which anions migrate by electrostatic attraction. Half of an electrolytic corrosion cell in metal is called the “anode,” from which metal dissolves, often leaving pits. The anode is the part of a corrosion cell in which oxidation occurs. |
| Drilling Fluids | antifoam | A mud additive used to lower interfacial tension so that trapped gas will readily escape from mud. Mechanical degassing equipment is commonly used along with defoamer. Octyl alcohol, aluminum stearate, various glycols, silicones and sulfonated hydrocarbons are used as defoamers. |
| Drilling Fluids | antifoam agent | A mud additive used to lower interfacial tension so that trapped gas will readily escape from mud. Mechanical degassing equipment is commonly used along with defoamer. Octyl alcohol, aluminum stearate, various glycols, silicones and sulfonated hydrocarbons are used as defoamers. |
| Drilling Fluids | API cement | One of several classes of cement manufactured to the specifications of the American Petroleum Institute (API) Specification 10A. Classes of API cement are A, B, C, D, E, F, G and H. |
| Drilling Fluids | API fluid loss test | A test to measure static filtration behavior of water mud at ambient (room) temperature and 100-psi differential pressure, usually performed according to specifications set by API, using a static filter press. The filter medium is filter paper with 7.1 sq. in. filtering area. A half-size cell is sometimes used, in which case the filtrate volume is doubled. |
| Drilling Fluids | API fluid-loss test | A test to measure static filtration behavior of water mud at ambient (room) temperature and 100-psi differential pressure, usually performed according to specifications set by API, using a static filter press. The filter medium is filter paper with 7.1 sq. in. filtering area. A half-size cell is sometimes used, in which case the filtrate volume is doubled. |
| Drilling Fluids | API water | The amount of mixing water specified in API Specification 10A for specification testing of cement to meet API requirements. This amount is not intended to be a guide for mix water requirements in field applications. |
| Drilling Fluids | apparent viscosity | The viscosity of a fluid measured at a given shear rate at a fixed temperature. In order for a viscosity measurement to be meaningful, the shear rate must be stated or defined. |
| Drilling Fluids | apparent viscosity | The viscosity of a fluid measured at the shear rate specified by API. In the Bingham plastic rheology model, apparent viscosity (AV) is one-half of the dial reading at 600 rpm (1022 sec-1 shear rate) using a direct-indicating, rotational viscometer. For example, a 600-rpm dial reading is 50 and the AV is 50/2, or 25 cp. |
| Drilling Fluids | aromatic content test | One of two quantitative analysis procedures for measuring aromatic content of base oils for use in oil mud as proscribed by the API. Results of the two aromatic content tests may differ because of the way the two gas-chromatography techniques separate and identify aromatics. The Institute of Petroleum (IP) instruments are more widely available than the American Society for Testing and Materials (ASTM) instruments, and the Institute of Petroleum method measures trace levels better than the more complex ASTM method. |
| Drilling Fluids | aromatic hydrocarbon | A type of compound containing hydrogen and carbon atoms arranged in a symmetrical 6-carbon ring structure with single (C-C) and double (C=C) bonds alternating around the ring. Rings are single, multiple or fused and can have other chemical groups attached in place of hydrogen. Benzene, C6H6 is the simplest single-ring aromatic, napthalene, C10H8, the simplest fused-ring aromatic and toluene is the simplest aromatic, having an alkyl side chain, C6H5-CH3. Xylene, a common oilfield chemical, has two methyl side chains, C6H4-(CH3)2. Aniline is the simplest aromatic amine, C6H5-NH2 and is used in the aniline point test. Aromatic hydrocarbons in oils used to prepare oil-base mud can damage elastomers and increase the toxicity of the fluid. The aniline point test is used for screening oils to infer aromatic content. |
| Drilling Fluids | asphaltic mud additive | A group of high-viscosity or solid hydrocarbons obtained from naturally occurring deposits or from the residue of petroleum refining, commonly used as additives for oil-base and water-base muds. Molten asphalt can be further processed by heating and passing air through the melt to oxidize and polymerize its components. Cooled, air-blown asphalt is glassy and can be ground. It has a high softening point and polar sites that offer emulsion-stabilizing qualities and affinity for clays and shales. |
| Drilling Fluids | ASTM | The designation of a standard developed by ASTM International. Until 2001, ASTM was an acronym for the American Society for Testing and Materials, but the organization changed its name to ASTM International to reflect its global scope as a forum for development of international voluntary consensus standards Some API procedures for drilling fluids are similar to ASTM procedures. |
| Drilling Fluids | attapulgite | A needle-like clay mineral composed of magnesium-aluminum silicate. Major deposits occur naturally in Georgia, USA. Attapulgite and sepiolite have similar structures and both can be used in saltwater mud to provide low-shear rate viscosity for lifting cuttings out of the annulus and for barite suspension. Attapulgite and sepiolite are sometimes called “salt gel.” Attapulgite has no capability to control the filtration properties of the mud. For use as an oil mud additive, the clay is coated with quaternary amine, which makes it oil-dispersible and provides gel structure but does not improve the filter cake, unlike organophilic bentonite clay. |
| Drilling Fluids | AV | Abbreviation for apparent viscosity. The viscosity of a fluid measured at a given shear rate at a fixed temperature. In order for a viscosity measurement to be meaningful, the shear rate must be stated or defined. |
| Drilling Fluids | bactericide | 1. n. [Drilling Fluids] An additive that kills bacteria. Bactericides are commonly used in water muds containing natural starches and gums that are especially vulnerable to bacterial attack. Bactericide choices are limited and care must be taken to find those that are effective yet approved by governments and by company policy. Bactericides, also called biocides, can be used to control sulfate-reducing bacteria, slime-forming bacteria, iron-oxidizing bacteria and bacteria that attacks polymers in fracture and secondary recovery fluids. In polymers, the degradation of the fluid is controlled, thus avoiding the formation of a large biomass, which could plug the formation and reduce permeability. |
| Drilling Fluids | balanced activity oil mud | An oil-base mud in which the activity, or vapor pressure, of the brine phase is balanced with that of the formations drilled. Although long shale sections may not have a constant value for vapor pressure, aw, the oil mud will adjust osmotically to achieve an “average” aw value. Dynamic (autopilot) balance of mud salinity and drilled shales is maintained because as water moves into or out of the mud, it also moves out of or into the shale. As water transfer continues during drilling, the mud’s water phase will be either diluted or concentrated in CaCl2 as needed to match the average aw value of the shale section and cuttings exposed to the mud. Reference: Chenevert ME: “Shale Control With Balanced-Activity Oil-Continuous Muds,” Journal of Petroleum Technology 33, no. 11 (November 1970): 1370-1378. |
| Drilling Fluids | balanced-activity oil mud | An oil-base mud in which the activity, or vapor pressure, of the brine phase is balanced with that of the formations drilled. Although long shale sections may not have a constant value for vapor pressure, aw, the oil mud will adjust osmotically to achieve an “average” aw value. Dynamic (autopilot) balance of mud salinity and drilled shales is maintained because as water moves into or out of the mud, it also moves out of or into the shale. As water transfer continues during drilling, the mud’s water phase will be either diluted or concentrated in CaCl2 as needed to match the average aw value of the shale section and cuttings exposed to the mud. |
| Drilling Fluids | barite | A dense mineral comprising barium sulfate [BaSO4]. Commonly used as a weighting agent for all types of drilling fluids, barites are mined in many areas worldwide and shipped as ore to grinding plants in strategic locations, where API specifies grinding to a particle size of 3 to74 microns. Pure barium sulfate has a specific gravity of 4.50 g/cm3, but drilling-grade barite is expected to have a specific gravity of at least 4.20 g/cm3 to meet API specifications. Contaminants in barite, such as cement, siderite, pyrrhotite, gypsum and anhydrite, can cause problems in certain mud systems and should be evaluated in any quality assurance program for drilling-mud additives. |
| Drilling Fluids | barrel equivalent | A volume of 350 cm3. In mud laboratory experiments, 350 cm3 is the volume chosen to represent 42 US gallons (1 oilfield barrel) [0.159 m3], so that 1.0 gram mass represents 1.0 lbm. This is a convenient concept for mud technicians to use when mixing or pilot-testing mud samples. For example, in preparing a mud formulation or for pilot-testing purposes, adding 1.0 gram to 350 cm3 of fluid is the experimental equivalent of adding 1.0 lbm to 42 US gallons (1.0 bbl) of fluid. |
| Drilling Fluids | baryte | Alternate spelling of barite. |
| Drilling Fluids | base oil | Refers to the continuous phase in oil-base drilling fluids. Oil-base drilling fluids are water-in-oil emulsions in which water is the dispersed phase and oil is the dispersion, or continuous, phase. Oil-to-water ratios (OWR) in oil-base drilling fluids vary from 65/35 to 95/5. |
| Drilling Fluids | base slurry | A conventional cement slurry used as the cementitious component of a foamed cement slurry. |
| Drilling Fluids | BBL or bbl | An abbreviation for oilfield barrel, a volume of 42 US gallons [0.16 m3]. |
| Drilling Fluids | Bc | The pumpability or consistency of a slurry, measured in Bearden units of consistency (Bc), a dimensionless quantity with no direct conversion factor to more common units of viscosity |
| Drilling Fluids | Bearden units of consistency | The pumpability or consistency of a slurry, measured in Bearden units of consistency (Bc), a dimensionless quantity with no direct conversion factor to more common units of viscosity. |
| Drilling Fluids | beneficiate | To improve a mineral or ore for its designed use through chemical treatments or mechanical processes. For example, barite and bentonite clay minerals are beneficiated in order to help them meet specifications for use in drilling fluids. Reference: Garrett RL: Quality Requirements for Industrial Minerals Used in Drilling Fluids, Mining Engineering 39, no. 11 (November 1987): 1011-1016. |
| Drilling Fluids | beneficiation | Chemical treatment or mechanical processes that improve a mineral or ore for its designed use. For example, barite and bentonite clay minerals are beneficiated in order to help them meet certain specifications for use in drilling fluids. Reference: Garrett RL: Quality Requirements for Industrial Minerals Used in Drilling Fluids, Mining Engineering 39, no. 11 (November 1987): 1011-1016. |
| Drilling Fluids | bentonite | A clay mineral that is composed principally of three-layer clays, such as montmorillonite, and widely used as a mud additive for viscosity and filtration control. Commercial bentonite ores vary widely in amount and quality of the swelling clay, sodium montmorillonite. Ores of lower quality, those with more calcium-type montmorillonite, are treated during grinding by adding one or more of the following: sodium carbonate, long-chain synthetic polymers, carboxymethylcellulose (CMC), starch or polyphosphates. These help make the final product meet quality specifications. Unfortunately, the additives may not remain effective in “the real mud world” when in use at the rig due to hardness ions in the water, high temperature, bacterial attack, mechanical shear-degradation and other factors that can render these additives ineffective. |
| Drilling Fluids | bentonite equivalent | A term used to express the results of the methylene blue test, which determines the amount of clay-like materials in a water-base drilling fluid based on the amount of methylene blue dye absorbed by the sample. Results are reported as “MBT” and also as “lbm/bbl, bentonite equivalent” when performed to API specifications. |
| Drilling Fluids | bicarb | A compound containing the bicarbonate ion [HCOO-]. The term is commonly used to refer to the ion itself. Bicarbonates are common constituents of drilling fluids. The ions are in equilibrium with carbonate and CO2 gas. |
| Drilling Fluids | bicarbonate | A compound containing the bicarbonate ion [HCOO-]. The term is commonly used to refer to the ion itself. Bicarbonates are common constituents of drilling fluids. The ions are in equilibrium with carbonate and CO2 gas. |
| Drilling Fluids | bicenter bit | A type of salt in which chromium atoms are in the plus-7 valence state, such as potassium dichromate, K2Cr2O7. |
| Drilling Fluids | Bingham plastic model |
A two-parameter rheological model widely used in the drilling fluids industry to describe flow characteristics of many types of muds. It can be described mathematically as follows: ? = YP + PV(?), where ? = shear stress ? = shear rate YP = yield point |
| Drilling Fluids | bioaccumulation | The concentration of a particular substance in a living organism, possibly with harmful effects. The likelihood of this occurring is expressed as the bioaccumulation potential and can be estimated by the octanol/water partition coefficient, expressed as logPOW. This test is commonly required on drilling fluid additives in the North Sea area and other countries following the Oslo and Paris Commission (OSPAR) regulations. Values of logPOW below 3 indicate no bioaccumulation tendency; values between 3 and 6 indicate that bioaccumulation is possible, providing the substance is small enough to pass through the cell wall (mol. wt. < 600). This may be confirmed by a bioconcentration test in which a population of animals is exposed to the product. |
| Drilling Fluids | bioassay | A laboratory test or other assessment utilizing a living organism, such as mysid shrimp, to determine the effect of a condition to which the organism is exposed. Such tests are performed under controlled environmental conditions and duration. Bioassay tests of drilling fluids are required by governmental agencies throughout the world prior to discharge of mud or cuttings. The organisms used in bioassays are those found in the area that would be most affected by contact with the proposed drilling fluid. The dosage of interest is typically the lethal concentration, known as LC50, that will kill 50% of the population of organisms in a given period of time. Chronic bioassay tests indicate sublethal effects, such as changes in growth or reproduction of the organism over a longer period of time. |
| Drilling Fluids | biochemical oxygen demand | The amount of oxygen consumed by biodegradation processes during a standardized test. The test usually involves degradation of organic matter in a discarded waste or an effluent. |
| Drilling Fluids | biocide | An additive that kills bacteria. Bactericides are commonly used in water muds containing natural starches and gums that are especially vulnerable to bacterial attack. Bactericide choices are limited and care must be taken to find those that are effective yet approved by governments and by company policy. Bactericides, also called biocides, can be used to control sulfate-reducing bacteria, slime-forming bacteria, iron-oxidizing bacteria and bacteria that attacks polymers in fracture and secondary recovery fluids. In polymers, the degradation of the fluid is controlled, thus avoiding the formation of a large biomass, which could plug the formation and reduce permeability. |
| Drilling Fluids | bioconcentration | The concentration of a particular substance in a living organism, possibly with harmful effects. The likelihood of this occurring is expressed as the bioaccumulation potential and can be estimated by the octanol/water partition coefficient, expressed as logPOW. This test is commonly required on drilling fluid additives in the North Sea area and other countries following the Oslo and Paris Commission (OSPAR) regulations. Values of logPOW below 3 indicate no bioaccumulation tendency; values between 3 and 6 indicate that bioaccumulation is possible, providing the substance is small enough to pass through the cell wall (mol. wt. < 600). This may be confirmed by a bioconcentration test in which a population of animals is exposed to the product. |
| Drilling Fluids | biodegradation | The process by which complex molecules are broken down by micro-organisms to produce simpler compounds. Biodegradation can be either aerobic (with oxygen) or anaerobic (without oxygen). The potential for biodegradation is commonly measured on drilling-fluid products to ensure that they do not persist in the environment. A variety of tests exist to assess biodegradation. |
| Drilling Fluids | biopolymer | A polymer produced by a strain of bacteria. The most common type, used in drilling and completion operations, is a polysaccharide biopolymer known as XC polymer. |
| Drilling Fluids | black list | List of products considered unsuitable by the Oslo and Paris Commission (OSPAR) for discharge, including mercury, cadmium and ‘persistent oils and hydrocarbons of a petroleum origin.’ OSPAR, formerly known as PARCOM, is a group of experts who advise North Sea countries on environmental policy and legislation. The group has been influential in establishing North Sea legislation on drilling fluids that has served as the model for other operating areas. The Commission has published lists of environmentally acceptable and unacceptable products, referred to as the “green,” “grey” and “black” lists. The inclusion of hydrocarbons in the black list has been the driving force behind the reduction of oil discharges in the North Sea and elsewhere and has serious implications for the use of oil and synthetic fluids. |
| Drilling Fluids | Blaine fineness | The particle size or fineness of a cement in cm2/g or m2/kg, usually determined from air permeability tests using a device known as a Blaine permeameter. Fineness affects the hydration rate (setting) and the requirements for the amounts of water, retarder and dispersant. |
| Drilling Fluids | bland coring fluid | A coring fluid formulated with components that are not likely to alter the wettability in the pores of the rock sample and that has low dynamic filtration characteristics. These qualities help retain the core’s native properties and can retain some (or all) of the reservoir’s fluids [water, oil and gas (gas only if kept under pressure)]. Bland water-base fluid is formulated to make the filtrate resemble the connate water in the reservoir. Keeping ionic composition and especially the pH matched to the reservoir water is most important. Thus, strong alkaline agents and clay deflocculants are avoided when designing bland coring fluids. Bland oil-base fluids should contain no water phase, and the base oil should resemble the reservoir oil. (Reservoir crude is used in some cases.) Amine, amide, phosphonated and sulfonated emulsifiers and the powerful oil-wetting agents are also avoided. Fatty acid soaps are chosen to emulsify the trace of water that is likely to be encountered. Additives that minimize dynamic filtration rate must be chosen. Setting mud density and bit hydraulics to give equivalent circulating density close to the reservoir pressure helps avoid filtrate invasion into the core. Designing core bits to core as fast as possible also limits filtrate invasion ahead of the bit. |
| Drilling Fluids | BOD | The amount of oxygen consumed by biodegradation processes during a standardized test. The test usually involves degradation of organic matter in a discarded waste or an effluent. |
| Drilling Fluids | bomb | Slang term for a type of pressure vessel. |
| Drilling Fluids | bond log | A log that uses the variations in amplitude of an acoustic signal traveling down the casing wall between a transmitter and receiver to determine the quality of cement bond on the exterior casing wall. |
| Drilling Fluids | bottomhole circulating temperature | The temperature at the bottom of a well while fluid is being circulated, abbreviated BHCT. This is the temperature used for most tests of cement slurry in a liquid state (such as thickening time and fluid loss). In most cases, the BHCT is lower than the bottomhole static temperature (BHST), but in some cases, such as in deep water or in the arctic, the BHCT may be higher than the BHST. |
| Drilling Fluids | bottomhole static temperature | The undisturbed temperature at the bottom of a well, abbreviated as BHST. After circulation and after the well is shut in, the temperature approaches the BHST after about 24 to 36 hours, depending on the well conditions. The BHST is the temperature used in most tests in which the cement slurry is required to set or is set. |
| Drilling Fluids | bottoms up mud sample | A sample of mud from the deepest or current drilling depth of a well. The term refers particularly to a mud sample that has experienced stagnant conditions at the bottom of the hole, including the temperature, pressure and other conditions at that depth. A bottoms-up sample is commonly collected after a trip out of the hole or if an influx of formation fluid is suspected |
| Drilling Fluids | bottoms-up mud sample | A sample of mud from the deepest or current drilling depth of a well. The term refers particularly to a mud sample that has experienced stagnant conditions at the bottom of the hole, including the temperature, pressure and other conditions at that depth. A bottoms-up sample is commonly collected after a trip out of the hole or if an influx of formation fluid is suspected. |
| Drilling Fluids | breaker | A chemical that reduces the viscosity of a fluid by breaking long-chain molecules into shorter segments. Drilling fluids are commonly emulsified or contain long-chain molecules that have sufficient viscosity to carry cuttings to surface. After the drilling fluid has done its job, a breaker may be added to reduce the viscosity of the fluid by breaking down the long chain molecules into shorter molecules. A surfactant may be added to an emulsion to reduce its viscosity. |
| Drilling Fluids | bridging agent | Solids added to a drilling fluid to bridge across the pore throat or fractures of an exposed rock thereby building a filter cake to prevent loss of whole mud or excessive filtrate. Bridging materials are commonly used in drilling fluids and in lost circulation treatments. For reservoir applications, the bridging agent should be removable-common products include calcium carbonate (acid-soluble), suspended salt (water-soluble) or oil-soluble resins. For lost-circulation treatments, any suitably sized products can be used, including mica, nutshells and fibers. These products are more commonly referred to as lost-circulation material (LCM). |
| Drilling Fluids | bridging material | Solids added to a drilling fluid to bridge across the pore throat or fractures of an exposed rock thereby building a filter cake to prevent loss of whole mud or excessive filtrate. Bridging materials are commonly used in drilling fluids and in lost circulation treatments. For reservoir applications, the bridging agent should be removable-common products include calcium carbonate (acid-soluble), suspended salt (water-soluble) or oil-soluble resins. For lost-circulation treatments, any suitably sized products can be used, including mica, nutshells and fibers. These products are more commonly referred to as lost-circulation material (LCM). |
| Drilling Fluids | brine | A general term that refers to various salts and salt mixtures dissolved in an aqueous solution. Brine can be used more strictly, however, to refer to solutions of sodium chloride. We prefer to use brine as a general term. The emulsified calcium chloride [CaCl2] solution (or any other saline phase) in an oil mud is referred to as “brine” or “brine phase.” The oil/brine ratio, abbreviated OBR, is used to compare solids content and salinities of oil muds. Clear brines are salt solutions that have few or no suspended solids. |
| Drilling Fluids | bromide brine | An aqueous solution of sodium, calcium or zinc bromide salt or mixtures of these salts. These dense aqueous solutions are used for well completion and workover purposes. |
| Drilling Fluids | bromocresol green | An indicator used in place of methyl orange in alkalinity tests. It is green at pH values over 4.3, but yellow when pH is less than 4.3. |
| Drilling Fluids | Brookfield viscometer | A cone-and-plate rheometer designed to measure viscosity of non-Newtonian fluids at low shear rates and with more accuracy than is attainable with a 6-speed, direct-indicating viscometer. Such low shear-rate data are needed for designing muds with improved hole-cleaning properties and to minimize sag of weighting material. (Brookfield is a mark of Brookfield Engineering Laboratories, Inc.) |
| Drilling Fluids | buffer | A chemical system that resists a change in pH. It comprises three components: water, weak acid (or weak base) and salt of the weak acid (or salt of weak base). In a buffered system, the concentration of H+ and OH- ions remain relatively constant because they are in equilibrium with one or more of the other two components, even with the addition of acids or bases. |
| Drilling Fluids | buffer solution | Any aqueous solution that contains a buffer mixture (weak acid or weak base and salt of the weak acid or base) to maintain constant or almost constant pH of the system. |
| Drilling Fluids | buffered mud | A type of mud that contains the three components that form a chemical buffer, whether by design or by coincidence. Buffering results from components that react with the added OH- ions (or added H+ ions) forming slightly soluble or slightly ionized compounds. Water is one component of a buffer and various ions are the other components, such as bicarbonates, carbonates, lignite, lignosulfonate, silicate and sulfide. Clay solids are buffers because of their ability to accept or donate H+ ions on their surface. The pH of a buffered mud will not increase as fast as expected after addition of caustic soda, for example. |
| Drilling Fluids | buffered solution | A solution used in analyses to hold pH at or above or below a certain value, as in the titration for magnesium versus calcium ions. |
| Drilling Fluids | buffered solution | A solution of constant pH used in calibration of pH meters. |
| Drilling Fluids | bulk volume | The volume per unit mass of a dry material plus the volume of the air between its particles. |
| Drilling Fluids | BWOC | Describing the amount (in percent) of a material added to cement, and is often abbreviated as BWOC. BWOC is the method used to describe the amount of most additives in the dry form |
| Drilling Fluids | BWOW | Describing the amount (in percent) of a material added to a cement slurry based on the weight of water used to mix the slurry. Commonly abbreviated as BWOW, this convention normally is used only for salt [NaCl]. |
| Drilling Fluids | by weight of blend | Describing the amount (in percent) of a material added to cement when the material is added based on the total amount of a specific blend, often abbreviated as BWOB. |
| Drilling Fluids | by weight of cement | Describing the amount (in percent) of a material added to cement, and is often abbreviated as BWOC. BWOC is the method used to describe the amount of most additives in the dry form. |
| Drilling Fluids | by weight of water | Describing the amount (in percent) of a material added to a cement slurry based on the weight of water used to mix the slurry. Commonly abbreviated as BWOW, this convention normally is used only for salt [NaCl]. |
| Drilling Fluids | bypass | The act of passing the mud around a piece of equipment, such as passing mud returns around the shale shaker screens or going around a hydrocyclone device. From a mud-engineering viewpoint, this can be a bad practice because it can allow drill solids to degrade and accumulate as fines to the degree that they might cause mud problems. |
| Drilling Fluids | bypassed mud | Mud that is left somewhere in the wellbore when some other fluid is pumped into the well. This can occur when pumping an oil mud into a well to displace a water mud. The bypassed water mud becomes a contaminant in the oil mud when it gets mixed into the circulating system. Drilling mud may be bypassed behind a casing or a liner when pumping cement into the casing or wellbore annular region. This mud-contaminated cement might not set up and might not isolate zones satisfactorily |
| Drilling Fluids | cake thickness | A measurement of the thickness of the filter cake, usually recorded in 32nds-inch. Under dynamic conditions, filter-cake thickness depends on rate of deposition versus erosion caused by fluid circulation and mechanical abrasion by the rotating drillstring. Typically, the filter cake will reach an equilibrium thickness in the wellbore. In laboratory tests, however, filter cake is built under static conditions with no erosion. |
| Drilling Fluids | calcium bromide | A compound of formula CaBr2 used in conjunction with calcium chloride [CaCl3] in completion operations to make solids-free brines with densities in the range 11.5 to 14.5 ppg. |
| Drilling Fluids | calcium carbonate | A compound with formula CaCO3 that occurs naturally as limestone. Ground and sized calcium carbonate is used to increase mud density to about 12 lbm/gal [1.44 kg/m3], and is preferable to barite because it is acid-soluble and can be dissolved with hydrochloric acid to clean up production zones. Its primary use today is as a bridging material in drill-in, completion and workover fluids. Sized calcium carbonate particles, along with polymers, control fluid loss in brines or drill-in, completion and workover fluids. Insoluble calcium carbonate is the precipitated byproduct of mud treatments used for removal of either Ca+2 or CO3-2 by addition of the other ion. |
| Drilling Fluids | calcium chloride | A highly soluble calcium salt of formula CaCl2 used to make drilling and workover fluids or brines with a density range from 8.33 to 11.6 lbm/gal [1.39 g/cm3] at saturation. CaCl2 can be blended with other brines, including sodium chloride [NaCl], calcium bromide [CaBr2] and zinc bromide [ZnBr2]. Emulsification of CaCl2 brine as the internal phase of oil-base or synthetic-base mud is an important use because the brine provides osmotic wellbore stability while drilling water-sensitive shale zones. |
| Drilling Fluids | calcium contamination | A contamination problem caused by Ca+2 ions, usually occurring in fresh water, seawater and other low-salinity and low-hardness mud systems. Soluble calcium comes into a mud from various sources: gypsum- or anhydrite-bearing strata, unset cement and hardness ions in make-up water or from an influx of formation water. Ca+2 can flocculate colloidal clays and precipitate large anionic polymers that contain carboxylate groups, such as an acrylate polymer. On the other hand, some mud types tolerate calcium, in which case calcium is not considered a contaminant. |
| Drilling Fluids | calcium hydroxide | A chemical with formula Ca(OH)2, commonly called slaked lime. Lime is used in lime muds and as a treatment to remove carbonate ions. It is used as a stabilizing ingredient in oil- and synthetic-base mud, essential to formation of fatty-acid soap emulsifiers. It is an alkaline material that can be carried in excess to neutralize hydrogen sulfide [H2S] and carbon dioxide [CO2]. |
| Drilling Fluids | calcium mud | A class of water-base drilling fluid that utilize dissolved Ca+2 as a component. Examples are lime mud, gyp mud and calcium chloride [CaCl2] mud. The latter is rarely used, but is based on solutions of CaCl2 that, in high concentration, can impart density up to 11.6 lbm/gal (1.39 g/cm3) and has been touted as providing shale inhibition. |
| Drilling Fluids | calcium oxide | A chemical with formula CaO, commonly called quick lime or hot lime. When hydrated with one mole of water, it forms slaked lime, Ca(OH)2. Quick lime is used in preference to slaked lime at oil mud mixing plants because it generates heat when it becomes slaked with water and therefore speeds up emulsification by the reaction to form calcium fatty-acid soap. |
| Drilling Fluids | calcium sulfate | The chemical CaSO4, which occurs naturally as the mineral anhydrite. Gypsum is the dihydrate mineral form, CaSO4·2H2O. Anhydrite and gypsum (commonly called gyp) are found in the subsurface and drilling even small stringers of these minerals can upset a freshwater or seawater mud. Gyp muds, lime muds and oil muds tolerate these salts best. CaSO4 is used as a mud treatment when no pH increase is needed to remove carbonate ion contamination in freshwater and seawater muds. (Lime increases pH when added for this purpose.) Gypsum and lime treatments are often used together to keep pH in the proper range. The test for determining the dissolved and undissolved calcium sulfate in a gyp mud requires two titrations with the strong EDTA reagent and Calver II® indicator when performed to API standards. It also requires a retort analysis for water content in the mud in order to calculate CaSO4 content, lbm/bbl. |
| Drilling Fluids | calcium test | A quantitative analytical procedure for water-mud filtrate and for calcium in an oil mud. |
| Drilling Fluids | capillary suction time test | A type of static filtration test for water-base drilling fluid that measures the filtration rate (time for free water to pass between two electrodes) using filter paper as the medium. It is used primarily to indicate filter-cake permeability, but data from the test have been used to study how clays and shales react in filter cakes and how brines of various types affect clays in a filter cake. |
| Drilling Fluids | capillary tube viscometer | An instrument for measuring the viscosity of a fluid by passing the fluid at a known pressure gradient or velocity through a length of tubing of known diameter. The viscosity of base oils for oil muds, which are Newtonian fluids, is measured using a glass capillary tube in a thermostatic bath, when performed according to API procedures |
| Drilling Fluids | capillary-suction-time test | A type of static filtration test for water-base drilling fluid that measures the filtration rate (time for free water to pass between two electrodes) using filter paper as the medium. It is used primarily to indicate filter-cake permeability, but data from the test have been used to study how clays and shales react in filter cakes and how brines of various types affect clays in a filter cake. |
| Drilling Fluids | carbon dioxide | The compound with the formula CO2. An odorless gas, carbon dioxide [CO2] is widely distributed in nature and is a minor component of air. It is highly soluble in water and oil, especially under pressure. In water, it occurs as carbonic acid, a weak acid that can donate one or two hydrogen ions in neutralization reactions that produce bicarbonate [HCO3-] and carbonate [CO3-2] salts or ions. CO2, being an acid in water, reacts instantly with NaOH or KOH in an alkaline water mud, forming carbonate and bicarbonate ions. Similarly, it reacts with Ca(OH)2 (lime) to form insoluble calcium carbonate and water. |
| Drilling Fluids | carbonate ion | An anion with formula CO3-2. Carbonate chemistry involves a pH-dependent equilibrium between H2O, H+, OH-, CO2, HCO3- and CO3-2. At low pH, carbon dioxide [CO2] dominates. As pH rises from acidic toward neutral, HCO3- ions dominate. As pH rises above neutral, CO3-2 ions dominate. If no component is lost from the system (such as CO2 gas evolving), changing pH up and down continually reverses the relative proportion of the carbonate species. Carbonates play several important roles in water mud chemistry. One role is the corrosion of metals by acidic CO2. A second is the formation of calcium carbonate [CaCO3] scale on surfaces by carbonate and calcium ion reactions. Another role is in the chemistry of deflocculated mud, where bicarbonate ions prevent attachment of deflocculants such as lignosulfonate, onto clay edge charges. |
| Drilling Fluids | carbonate test | An analytical procedure to determine the concentration of carbonate species using the Garrett Gas Train (GGT) when performed to API specifications. A water mud filtrate sample is put into the GGT. N2 or N2O is the carrier gas. A CO2 Drdger tube is used to measure the total carbonates released as CO2 when sulfuric acid is added to the chamber containing the sample. Total carbonates are measured by the amount of CO2 evolved in the test. |
| Drilling Fluids | carboxymethyl hydroxyethylcellulose | A cellulose polymer that contains anionic carboxymethyl and nonionic hydroxyethyl groups added by ether linkages to the OHs on the cellulose backbone. This polymer has seen limited use in drilling mud, but more use in brines and completion fluids. |
| Drilling Fluids | carboxymethyl starch | A natural starch derivative. CMS is used primarily for fluid-loss control in drilling muds, drill-in, completion and workover fluids. It is slightly anionic and can be affected by hardness and other electrolytes in a mud. CMS is similar to CMC (carboxymethylcellulose) in method of manufacture and many of its uses. The linear and branched starch polymers in natural starch react with monochloroacetic acid in alkaline solution, adding carboxymethyl groups at the OH positions by an ether linkage. By adding the carboxymethyl groups, the starch becomes more resistant to thermal degradation and bacterial attack. |
| Drilling Fluids | carboxymethylcellulose | A drilling-fluid additive used primarily for fluid-loss control, manufactured by reacting natural cellulose with monochloroacetic acid and sodium hydroxide [NaOH] to form CMC sodium salt. Up to 20 wt % of CMC may be NaCl, a by-product of manufacture, but purified grades of CMC contain only small amounts of NaCl. To make CMC, OH groups on the glucose rings of cellulose are ether-linked to carboxymethyl (-OCH2-COO-) groups. (Note the negative charge.) Each glucose ring has three OH groups capable of reaction, degree-of-substitution = 3. Degree of substitution determines water solubility and negativity of the polymer, which influences a CMC’s effectiveness as a mud additive. Drilling grade CMCs used in muds typically have degree-of-substitution around 0.80 to 0.96. Carboxymethylcellulose is commonly supplied either as low-viscosity (“CMC-Lo Vis”) or high-viscosity (“CMC-Hi Vis”) grades, both of which have API specifications. The viscosity depends largely on the molecular weight of the starting cellulose material. |
| Drilling Fluids | carrying capacity | The ability of a circulating drilling fluid to transport rock fragments out of a wellbore. Carrying capacity is an essential function of a drilling fluid, synonymous with hole-cleaning capacity and cuttings lifting. Carrying capacity is determined principally by the annular velocity, hole angle and flow profile of the drilling fluid, but is also affected by mud weight, cuttings size and pipe position and movement. |
| Drilling Fluids | cathode | The negative terminal of an electrolytic cell or battery. |
| Drilling Fluids | cation | A positively charged ion. |
| Drilling Fluids | cation exchange capacity | Quantity of positively charged ions (cations) that a clay mineral (or similar material) can accommodate on its negative charged surface, expressed as milliequivalents per 100 grams. CEC of solids in drilling muds is measured on a whole mud sample by a methylene blue capacity (MBC) test, which is typically performed to specifications established by API. CEC for a mud sample is reported as MBC, methylene blue test (MBT) or bentonite equivalent, lbm/bbl or kg/m3. |
| Drilling Fluids | cation-exchange capacity | Quantity of positively charged ions (cations) that a clay mineral (or similar material) can accommodate on its negative charged surface, expressed as milliequivalents per 100 grams. CEC of solids in drilling muds is measured on a whole mud sample by a methylene blue capacity (MBC) test, which is typically performed to specifications established by API. CEC for a mud sample is reported as MBC, methylene blue test (MBT) or bentonite equivalent, lbm/bbl or kg/m3. |
| Drilling Fluids | cationic | Related to positively charged ions. |
| Drilling Fluids | caustic extraction test | A test used to determine if a barite sample contains caustic-soluble sulfide or carbonate minerals. Reference: Binder GG, Carlton LA and Garrett RL: Evaluating Barite as a Source of Soluble Carbonate and Sulfide Contamination in Drilling Fluids, Journal of Petroleum Technology 33, no. 12 (December 1981): 2371-2376. Garrett RL: Quality Requirements for Industrial Minerals Used in Drilling Fluids, Mining Engineering 39, no. 11 (November 1987): 1011-1016. |
| Drilling Fluids | caustic potash | The common name for potassium hydroxide [KOH]. Caustic potash is used in potassium-based water muds to increase pH and alkalinity and to help maintain the K+ ion concentration. As the name implies, it is highly caustic and gives off heat when dissolved in water. Caustic potash is hazardous to use without proper training and equipment. |
| Drilling Fluids | caustic soda | The common name for sodium hydroxide [NaOH]. Caustic soda is used in most water-base muds to increase and maintain pH and alkalinity. It is a hazardous material to handle because it is very caustic and gives off heat when dissolved in water. Proper training and equipment are needed to handle it safely. |
| Drilling Fluids | cavings | Pieces of rock that came from the wellbore but that were not removed directly by the action of the drill bit. Cavings can be splinters, shards, chunks and various shapes of rock, usually spalling from shale sections that have become unstable. The shape of the caving can indicate why the rock failure occurred. The term is typically used in the plural form. |
| Drilling Fluids | CEC | Quantity of positively charged ions (cations) that a clay mineral (or similar material) can accommodate on its negative charged surface, expressed as milliequivalents per 100 grams. CEC of solids in drilling muds is measured on a whole mud sample by a methylene blue capacity (MBC) test, which is typically performed to specifications established by API. CEC for a mud sample is reported as MBC, methylene blue test (MBT) or bentonite equivalent, lbm/bbl or kg/m3. |
| Drilling Fluids | cellulosic polymer | A drilling-fluid additive used primarily for fluid-loss control, manufactured by reacting natural cellulose with monochloroacetic acid and sodium hydroxide [NaOH] to form CMC sodium salt. Up to 20 wt % of CMC may be NaCl, a by-product of manufacture, but purified grades of CMC contain only small amounts of NaCl. To make CMC, OH groups on the glucose rings of cellulose are ether-linked to carboxymethyl (-OCH2-COO-) groups. (Note the negative charge.) Each glucose ring has three OH groups capable of reaction, degree-of-substitution = 3. Degree of substitution determines water solubility and negativity of the polymer, which influences a CMC’s effectiveness as a mud additive. Drilling grade CMCs used in muds typically have degree-of-substitution around 0.80 to 0.96. Carboxymethylcellulose is commonly supplied either as low-viscosity (“CMC-Lo Vis”) or high-viscosity (“CMC-Hi Vis”) grades, both of which have API specifications. The viscosity depends largely on the molecular weight of the starting cellulose material. Reference: Hughes TL, Jones TG and Houwen OW: “The Chemical Characterization of CMC and Its Relationship to Drilling-Mud Rheology and Fluid Loss,” SPE Drilling & Completion 8, no. 3 (September 1993): 157-164. |
| Drilling Fluids | centipoise | A unit of measurement for viscosity equivalent to one-hundredth of a poise and symbolized by cP. Viscosity is the ratio of shear stress to shear rate, giving the traditional unit of dyne-sec/cm2 for Poise. In metric (SI) units, one cP is one millipascal-second. |
| Drilling Fluids | centrifuge | An item of solids-removal equipment that removes fine and ultrafine solids. It consists of a conical drum that rotates at 2000 to 4000 rpm. Drilling fluid is fed into one end and the separated solids are moved up the bowl by a rotating scroll to exit at the other end. Centrifuges generally have limited processing capacity (50 to 250 gpm) but are useful for processing weighted drilling fluids and can remove finer solids than can a hydrocyclone or shaker screens. They can also be used for water clarification or for processing oily cuttings. |
| Drilling Fluids | CERCLA | Abbreviation for “Comprehensive Environmental Response, Compensation and Liability Act” of 1980. CERCLA is an expansion of RCRA, “Resources Conservation and Recovery Act” of 1976. These acts of the US Congress outline responsibilities of operators for transportation, storage, treatment or disposal of regulated “hazardous substances,” which include certain oilfield materials. |
| Drilling Fluids | cesium acetate | A salt of cesium hydroxide and acetic acid, with formula CH3COO-Cs+, used to make high-density completion fluids. It has neutral to alkaline pH in water solutions and has better temperature stability than cesium formate. |
| Drilling Fluids | cesium formate | A neutral to slightly alkaline salt of cesium hydroxide and formic acid having the formula HCOO-Cs+. It is extremely soluble in water. An 82 wt.% cesium formate solution has a density of 2.4 g/cm3 [19.9 lbm/gal]. It has shown favorable health, safety and environmental (HSE) characteristics in laboratory tests and has applications as a drill-in, completion or workover fluid. Cesium formate can be mixed with less expensive potassium formate to make clear brine mixtures with a density range from 1.8 to 2.4 g/cm3. Formates have temperature stability in the range of 375°F [190°C], depending on the duration of exposure to such a temperature. |
| Drilling Fluids | chemical oxygen demand | The amount of oxygen needed to oxidize reactive chemicals in a water system, typically determined by a standardized test procedure. COD is used to estimate the amount of a pollutant in an effluent. Compare to biochemical oxygen demand, BOD. |
| Drilling Fluids | Chenevert Method | The name given by API to the electrohygrometer method for testing oil mud and cuttings samples for water-phase activity, aw. |
| Drilling Fluids | chloride test | A titration procedure standardized by the API to quantitatively determine Cl- (chloride ion) concentration by using silver nitrate as titrant with potassium chromate as the endpoint indicator. |
| Drilling Fluids | chromate salt | A type of salt in which chromium atoms are in the plus-6 valence state, such as potassium chromate, K2CrO4. Chromium compounds of various types have been used in lignite and lignosulfonate and other mud additives to enhance thermal stability. Since the late 1970s, they are prohibited in muds to be discarded offshore and in other environmentally sensitive areas of the US. |
| Drilling Fluids | chrome free | Pertaining to a mud additive (usually lignosulfonate or lignite) that does not contain any chromium compounds. |
| Drilling Fluids | chrome lignite | A lignite that has been treated (admixed or reacted with) chromic or chromate salt, such as potassium or sodium chromate or dichromate or chromic chloride. Also, chrome lignite may contain added (sometimes reacted) KOH or NaOH. Chromed mud additives have largely been eliminated from usage in the US because of environmental concerns. Chrome lignite was more temperature-stable than plain lignite in clay-based water muds. |
| Drilling Fluids | chrome lignosulfonate | A lignosulfonate that has been treated by mixing or reacting into the molecular structure some form of chromium (either chromate or chromic salt). Although still used today in less environmentally sensitive areas, it has been replaced by iron or calcium lignosulfonates. Ferro-chrome lignosulfonate is a popular type of deflocculant that contains iron and chromium salts. |
| Drilling Fluids | chrome-free | Pertaining to a mud additive (usually lignosulfonate or lignite) that does not contain any chromium compounds. |
| Drilling Fluids | chromic salt | A salt of chromium in which chromium atoms are in the plus-3 valence state, such as chromic chloride, CrCl3. Chromic compounds are considered less harmful to the environment than chromates (plus-6 valence) because they are at a low oxidation state and not highly reactive. |
| Drilling Fluids | circulation time | The elapsed time for mud to circulate from the suction pit, down the wellbore and back to surface. Circulation time allows the mud engineer to catch “in” and “out” samples that accurately represent the same element of mud in a circulating system. Circulation time is calculated from the estimated hole volume and pump rate and can be checked by using tracers such as carbide or rice granules. |
| Drilling Fluids | citric acid | An organic acid, properly called 2-Hydroxy-1,2,3-propanetricarboxylic acid, with formula C6H8O7. Citric acid is used to reduce the pH of drilling fluids and hence for treatment of cement contamination. It also acts as a polymer stabilizer. |
| Drilling Fluids | clathrate | A crystalline solid consisting of water with gas molecules in an ice-like cage structure. The general term for this type of solid is clathrate. Water molecules form a lattice structure into which many types of gas molecules can fit. Most gases, except hydrogen and helium, can form hydrates. C1 to nC5 hydrocarbons, H2S and CO2 readily form hydrates at low temperature and high pressure. Heavier hydrocarbons may also enter the structure but do not form hydrates by themselves. Gas-cut muds can form hydrates in deepwater drilling operations, plugging BOP lines, risers and subsea wellheads, causing a well-control risk. Gas hydrates are thermodynamically suppressed by adding antifreeze materials such as salts or glycols. A common practice is to use 20 to 23 wt.% NaCl. Nucleation and growth of hydrates can be dynamically inhibited by certain polymers or surfactants. Gas hydrates are found in nature, on the bottom of cold seas and in arctic permafrost regions. Drilling into these can be hazardous, but they offer another source of hydrocarbons for future exploitation. |
| Drilling Fluids | clay extender | A class of polymers added to a drilling-grade clay mineral during grinding, or added directly into a clay-based mud system, to enhance the clay’s rheological performance. In concept, clay-extender polymers achieve the type of rheology needed for fast drilling with fewer colloidal solids and lower viscosity at high shear rate (at the bit). This is the concept of a “low-solids, nondispersed mud” system. Extenders are usually long-chain anionic or nonionic polymers that link clay platelets together in large networks. Anionic polymers are highly effective but can be precipitated by hardness ions. Nonionic polymers are less effective but also much less sensitive to hardness ions. Excessively long, linear polymers may break up under mechanical shearing. Either by precipitation or breakup, extender polymers can quickly become ineffective if poorly chosen and used improperly. A drilling-grade clay that has no extender is that which meets the standard for API nontreated bentonite. API bentonite and OCMA-grade API bentonite usually contain extender polymers. |
| Drilling Fluids | clay water interaction | An all-inclusive term to describe various progressive interactions between clay minerals and water. In the dry state, clay packets exist in face-to-face stacks like a deck of playing cards, but clay packets begin to change when exposed to water. Five descriptive terms describe the progressive interactions that can occur in a clay-water system, such as a water mud. 1) Hydration occurs as clay packets absorb water and swell. 2) Dispersion (or disaggregation) causes clay platelets to break apart and disperse into the water due to loss of attractive forces as water forces the platelets farther apart. 3) Flocculation begins when mechanical shearing stops and platelets previously dispersed come together due to the attractive force of surface charges on the platelets. 4) Deflocculation, the opposite effect, occurs by addition of chemical deflocculant to flocculated mud; the positive edge charges are covered and attraction forces are greatly reduced. 5) Aggregation, a result of ionic or thermal conditions, alters the hydrational layer around clay platelets, removes the deflocculant from positive edge charges and allows platelets to assume a face-to-face structure. |
| Drilling Fluids | clay-water interaction | An all-inclusive term to describe various progressive interactions between clay minerals and water. In the dry state, clay packets exist in face-to-face stacks like a deck of playing cards, but clay packets begin to change when exposed to water. Five descriptive terms describe the progressive interactions that can occur in a clay-water system, such as a water mud. 1) Hydration occurs as clay packets absorb water and swell. 2) Dispersion (or disaggregation) causes clay platelets to break apart and disperse into the water due to loss of attractive forces as water forces the platelets farther apart. 3) Flocculation begins when mechanical shearing stops and platelets previously dispersed come together due to the attractive force of surface charges on the platelets. 4) Deflocculation, the opposite effect, occurs by addition of chemical deflocculant to flocculated mud; the positive edge charges are covered and attraction forces are greatly reduced. 5) Aggregation, a result of ionic or thermal conditions, alters the hydrational layer around clay platelets, removes the deflocculant from positive edge charges and allows platelets to assume a face-to-face structure. |
| Drilling Fluids | clear brine | A general term that refers to various salts and salt mixtures dissolved in an aqueous solution. Brine can be used more strictly, however, to refer to solutions of sodium chloride. We prefer to use brine as a general term. Clear brines are salt solutions that have few or no suspended solids. |
| Drilling Fluids | clear water drilling | Drilling operations using a water-base drilling fluid that contains few solids. Clear-water drilling is done in “hard rocks” in which density and fluid loss are not critical. Rapid drilling rate is the incentive for using a solids-free mud. Fluid returned to the surface must be screened and processed by hydrocyclones and centrifuges to remove larger solids. Colloidal solids can be agglomerated by adding polymers and removing the aggregates. Polymers such as acrylates, acrylamides and partially-hydrolyzed polyacrylamides are used. They are added at the flowline as mud exits the well or added in pits downstream from the flowline. |
| Drilling Fluids | clear-water drilling | Drilling operations using a water-base drilling fluid that contains few solids. Clear-water drilling is done in “hard rocks” in which density and fluid loss are not critical. Rapid drilling rate is the incentive for using a solids-free mud. Fluid returned to the surface must be screened and processed by hydrocyclones and centrifuges to remove larger solids. Colloidal solids can be agglomerated by adding polymers and removing the aggregates. Polymers such as acrylates, acrylamides and partially-hydrolyzed polyacrylamides are used. They are added at the flowline as mud exits the well or added in pits downstream from the flowline. |
| Drilling Fluids | closed mud system | A mud and solids-control system in which the only discarded waste is moist, drilled-up rock materials. Such systems are used for drilling wells in environmentally sensitive areas. No reserve-mud pit is used in a truly closed mud system. Mud is continually processed primarily by mechanical means, such as screening, hydrocycloning and centrifuging to remove solids initially. A second stage to remove colloidal solids is by wastewater cleanup techniques. |
| Drilling Fluids | cloud point | The temperature at which a solution of a surfactant or glycol starts to form micelles (molecular agglomerates), thus becoming cloudy. This behavior is characteristic of nonionic surfactants, which are often soluble at low temperatures but “cloud out” at some point as the temperature is raised. Glycols demonstrating this behavior are known as “cloud-point glycols” and are used as shale inhibitors. The cloud point is affected by salinity, being generally lower in more saline fluids. |
| Drilling Fluids | cloud point glycol | A glycol that is soluble at low temperatures but starts to form micelles (molecular agglomerates), thus becoming cloudy, as the temperature is raised. The temperature at which this phenomenon occurs the cloud point is affected by salinity, being generally lower in more saline fluids. Cloud point glycols are used as shale inhibitors. The purported mechanism is that the glycol clouds out at higher downhole temperatures, coating the surface of clays and preventing hydration. |
| Drilling Fluids | CMC | A drilling-fluid additive used primarily for fluid-loss control, manufactured by reacting natural cellulose with monochloroacetic acid and sodium hydroxide [NaOH] to form CMC sodium salt. Up to 20 wt % of CMC may be NaCl, a by-product of manufacture, but purified grades of CMC contain only small amounts of NaCl. To make CMC, OH groups on the glucose rings of cellulose are ether-linked to carboxymethyl (-OCH2-COO-) groups. (Note the negative charge.) Each glucose ring has three OH groups capable of reaction, degree-of-substitution = 3. Degree of substitution determines water solubility and negativity of the polymer, which influences a CMC’s effectiveness as a mud additive. Drilling grade CMCs used in muds typically have degree-of-substitution around 0.80 to 0.96. Carboxymethylcellulose is commonly supplied either as low-viscosity (“CMC-Lo Vis”) or high-viscosity (“CMC-Hi Vis”) grades, both of which have API specifications. The viscosity depends largely on the molecular weight of the starting cellulose material. |
| Drilling Fluids | CMC-Hi Vis | A high viscosity grade of drilling-fluid additive used primarily for fluid-loss control, manufactured by reacting natural cellulose with monochloroacetic acid and sodium hydroxide [NaOH] to form CMC sodium salt. Up to 20 wt % of CMC may be NaCl, a by-product of manufacture, but purified grades of CMC contain only small amounts of NaCl. To make CMC, OH groups on the glucose rings of cellulose are ether-linked to carboxymethyl (-OCH2-COO-) groups. (Note the negative charge.) Each glucose ring has three OH groups capable of reaction, degree-of-substitution = 3. Degree of substitution determines water solubility and negativity of the polymer, which influences a CMC’s effectiveness as a mud additive. Drilling grade CMCs used in muds typically have degree-of-substitution around 0.80 to 0.96. Carboxymethylcellulose is commonly supplied either as low-viscosity (CMC-Lo Vis) or high-viscosity (CMC-Hi Vis) grades, both of which have API specifications. The viscosity depends largely on the molecular weight of the starting cellulose material. |
| Drilling Fluids | CMC-HVT | A high viscosity grade of drilling-fluid additive used primarily for fluid-loss control, manufactured by reacting natural cellulose with monochloroacetic acid and sodium hydroxide [NaOH] to form CMC sodium salt. Up to 20 wt % of CMC may be NaCl, a by-product of manufacture, but purified grades of CMC contain only small amounts of NaCl. To make CMC, OH groups on the glucose rings of cellulose are ether-linked to carboxymethyl (-OCH2-COO-) groups. (Note the negative charge.) Each glucose ring has three OH groups capable of reaction, degree-of-substitution = 3. Degree of substitution determines water solubility and negativity of the polymer, which influences a CMC’s effectiveness as a mud additive. Drilling grade CMCs used in muds typically have degree-of-substitution around 0.80 to 0.96. Carboxymethylcellulose is commonly supplied either as low-viscosity (“CMC-Lo Vis”) or high-viscosity (“CMC-Hi Vis”) grades, both of which have API specifications. The viscosity depends largely on the molecular weight of the starting cellulose material. |
| Drilling Fluids | CMC-Lo Vis | A low viscosity grade of drilling-fluid additive used primarily for fluid-loss control, manufactured by reacting natural cellulose with monochloroacetic acid and sodium hydroxide [NaOH] to form CMC sodium salt. Up to 20 wt % of CMC may be NaCl, a by-product of manufacture, but purified grades of CMC contain only small amounts of NaCl. To make CMC, OH groups on the glucose rings of cellulose are ether-linked to carboxymethyl (-OCH2-COO-) groups. (Note the negative charge.) Each glucose ring has three OH groups capable of reaction, degree-of-substitution = 3. Degree of substitution determines water solubility and negativity of the polymer, which influences a CMC’s effectiveness as a mud additive. Drilling grade CMCs used in muds typically have degree-of-substitution around 0.80 to 0.96. Carboxymethylcellulose is commonly supplied either as low-viscosity (CMC-Lo Vis) or high-viscosity (CMC-Hi Vis) grades, both of which have API specifications. The viscosity depends largely on the molecular weight of the starting cellulose material. |
| Drilling Fluids | CMC-LVT | A low viscosity grade of drilling-fluid additive used primarily for fluid-loss control, manufactured by reacting natural cellulose with monochloroacetic acid and sodium hydroxide [NaOH] to form CMC sodium salt. Up to 20 wt % of CMC may be NaCl, a by-product of manufacture, but purified grades of CMC contain only small amounts of NaCl. To make CMC, OH groups on the glucose rings of cellulose are ether-linked to carboxymethyl (-OCH2-COO-) groups. (Note the negative charge.) Each glucose ring has three OH groups capable of reaction, degree-of-substitution = 3. Degree of substitution determines water solubility and negativity of the polymer, which influences a CMC’s effectiveness as a mud additive. Drilling grade CMCs used in muds typically have degree-of-substitution around 0.80 to 0.96. Carboxymethylcellulose is commonly supplied either as low-viscosity (CMC-Lo Vis) or high-viscosity (CMC-Hi Vis) grades, both of which have API specifications. The viscosity depends largely on the molecular weight of the starting cellulose material. |
| Drilling Fluids | CMHEC | A cellulose polymer that contains anionic carboxymethyl and nonionic hydroxyethyl groups added by ether linkages to the OHs on the cellulose backbone. This polymer has seen limited use in drilling mud, but more use in brines and completion fluids. |
| Drilling Fluids | CMS | A natural starch derivative. CMS is used primarily for fluid-loss control in drilling muds, drill-in, completion and workover fluids. It is slightly anionic and can be affected by hardness and other electrolytes in a mud. CMS is similar to CMC (carboxymethylcellulose) in method of manufacture and many of its uses. The linear and branched starch polymers in natural starch react with monochloroacetic acid in alkaline solution, adding carboxymethyl groups at the OH positions by an ether linkage. By adding the carboxymethyl groups, the starch becomes more resistant to thermal degradation and bacterial attack. |
| Drilling Fluids | coalesce | To grow, as in the process of droplet growth, through small drops merging when they come in contact. If this occurs repeatedly, a continuous liquid phase forms. Through this phenomenon, emulsions break and form two distinct liquid phases that tend to separate. In oil-base mud, the water phase is dispersed as small droplets, with oil as the continuous (external) phase. A stable oil mud will remain dispersed under normal drilling conditions because when droplets contact each other, they do not coalesce due to the strong emulsifier film around each droplet. However, when the emulsion film around each droplet becomes weakened, droplets will begin to coalesce. If not corrected, this can lead to total emulsion breakdown with solids becoming water-wetted. |
| Drilling Fluids | coalescence | The process of droplet growth as small drops merge together when they come in contact. If this occurs repeatedly, a continuous liquid phase forms. Through this phenomenon, emulsions break and form two distinct liquid phases that tend to separate. In oil-base mud, the water phase is dispersed as small droplets, with oil as the continuous (external) phase. A stable oil mud will remain dispersed under normal drilling conditions because when droplets contact each other, they do not coalesce due to the strong emulsifier film around each droplet. However, when the emulsion film around each droplet becomes weakened, droplets will begin to coalesce. If not corrected, this can lead to total emulsion breakdown with solids becoming water-wetted. |
| Drilling Fluids | coarse | Referring in the strict sense (API Bulletin 13C) to any particle larger than 2000 microns. |
| Drilling Fluids | COD | The amount of oxygen needed to oxidize reactive chemicals in a water system, typically determined by a standardized test procedure. COD is used to estimate the amount of a pollutant in an effluent. Compare to biochemical oxygen demand, BOD. |
| Drilling Fluids | colloid | A finely divided, solid material, which when dispersed in a liquid medium, scatters a light beam and does not settle by gravity; such particles are usually less than 2 microns in diameter. Some drilling fluid materials become colloidal when used in a mud, such as bentonite clay, starch particles and many polymers. Oil muds contain colloidal emulsion droplets, organophilic clays and fatty-acid soap micelles. |
| Drilling Fluids | colloidal | Referring to a finely divided, solid material, which when dispersed in a liquid medium scatters a light beam and does not settle by gravity; such particles are usually less than 2 microns in diameter. Some drilling fluid materials such as bentonite clay, starch particles and many polymers become colloidal when used in a mud. Oil muds contain |
| Drilling Fluids | colloidal solids | Solid particles of size less than 2 microns equivalent spherical diameter, also identified as clay by definitions in International Standards Organization ISO/CD 13501, par. 3.1.17. Because of extremely small size, these solids largely defy direct removal by mechanical devices that rely on screening or gravitational forces. Their removal is aided by chemical aggregation prior to gravity separation or filtration. |
| Drilling Fluids | colloidal suspension | A finely divided, solid material dispersed in a liquid medium. The solid particles scatter a light beam and do not settle by gravity; they are usually less than 2 microns in diameter. Some drilling fluid materials such as bentonite clay, starch particles and many polymers become colloidal when used in a mud. Oil muds contain colloidal emulsion droplets, organophilic clays and fatty-acid soap micelles. |
| Drilling Fluids | consistency | A rheological property of matter related to the cohesion of the individual particles of a given material, its ability to deform and its resistance to flow. The consistency of cement slurries is determined by thickening time tests in accordance with API Recommended Practice 10B and is expressed in Bearden units of consistency (Bc). |
| Drilling Fluids | contact time | The elapsed time required for a specific fluid to pass a designated depth or point in the annulus during pumping operations. Contact time is normally used as a design criterion for mud removal in turbulent flow. |
| Drilling Fluids | continuous phase | The continuous phase of an emulsion. The internal phase is the dispersed droplets of emulsified fluid. |
| Drilling Fluids | conventional mud | A term that, in the past, referred to a mud containing bentonite clay, water, caustic soda and perhaps barite (as needed for density) usually with lignite or lignosulfonate present. Modern drilling does not necessarily recognize this as a conventional mud because polymer muds, special drill-in fluids and synthetic-base muds are now in common use. There may not be a “conventional mud” today. |
| Drilling Fluids | copolymer | A polymer that is formed from two or more different structural units. |
| Drilling Fluids | copper carbonate | A compound, CuCO3, that was used as a sulfide scavenger for water-base muds. However, it was found to be corrosive due to spontaneous plating of metallic copper onto metal surfaces, causing pitting corrosion; it has largely been replaced by zinc compounds. Reference: Perricone AC and Chesser BG: Corrosive Aspects of Copper Carbonate in Drilling Fluids, Oil & Gas Journal 68, no. 37 (September 14, 1970): 82-85. |
| Drilling Fluids | coring fluid | A specially designed fluid that is used for cutting cores with a core barrel and core bit. Sometimes the fluid used is the drilling mud, but if cores are for special studies, the coring fluid must be carefully designed to avoid damaging or altering the rock sample, such as a bland coring fluid. |
| Drilling Fluids | corrosion coupon | A specimen of test material to be used in a corrosion test, usually a metal strip or ring shaped to fit into a testing cell or between joints of drillpipe. Rings, or coupons, are weighed before and after exposure, and weight loss is measured. They are also examined for pits and cracks. Corrosion products are analyzed to define the type of corrosion reaction. |
| Drilling Fluids | coupon | An abbreviation for corrosion coupon, a specimen of test material to be used in a corrosion test, usually a metal strip or ring shaped to fit into a testing cell or between joints of drillpipe. Rings, or coupons, are weighed before and after exposure, and weight loss is measured. They are also examined for pits and cracks. Corrosion products are analyzed to define the type of corrosion reaction. |
| Drilling Fluids | creaming | The separation of phases of an emulsion with the lighter phase on top and denser phase on bottom. When oil muds are stagnant, the less dense oil phase rises and the denser aqueous phase settles. This behavior is not necessarily related to emulsion weakness, nor does it portend breaking, as does coalescence. |
| Drilling Fluids | critical rate | The minimum rate required to achieve turbulent flow. |
| Drilling Fluids | curing | The aging of cement under specific temperature and pressure conditions. |
| Drilling Fluids | cut point | The particle size that has a specified chance of being removed by an item of solids control equipment. Most commonly, D10, D50 and D90 cut points are specified corresponding to 10, 50 and 90% chances of removal, respectively. Taken together, they approximate the separation curve. If the percent is not specified, it is normally taken to be the D50 value. For example, if the D50 of a shaker screen is 100 microns, then a particle of this size has an equal chance of being removed or staying in the system. Larger particles are more likely to be removed and smaller ones more likely to be retained in the underflow. |
| Drilling Fluids | Cutt point | The spherical diameter corresponding to the ellipsoidal volume distribution of the screen opening sizes as measured by image analysis techniques. Named after Al Cutt of Amoco who developed the technique. Not to be confused with cut point. Reference: Cutt AR: Shaker Screen Characterization Through Image Analysis, paper SPE 22570, presented at the 66th SPE Annual Technical Conference and Exhibition, Dallas, Texas, USA, October 6-9, 1991. |
| Drilling Fluids | cuttings | Rock pieces dislodged by the drill bit as it cuts rock in the hole. Cuttings are distinct from cavings, rock debris that spalls as a result of wellbore instability. Visual inspection of rock at the shale shaker usually distinguishes cuttings from cavings. |
| Drilling Fluids | cuttings lifting | Transport of rock fragments out of a wellbore by a circulating drilling fluid. Carrying capacity is an essential function of a drilling fluid, synonymous with hole-cleaning capacity and cuttings lifting capacity. Carrying capacity is determined principally by the annular velocity, hole angle and flow profile of the drilling fluid, but is also affected by mud weight, cuttings size and pipe position and movement. |
| Drilling Fluids | CWA | Abbreviation for “Clean Water Act,” a law passed by the US Congress to control the discharge of contaminants, particularly oil, into the waters of the US. |
| Drilling Fluids | cycle time | The elapsed time for mud to circulate from the suction pit, down the wellbore and back to surface. Cycle time allows the mud engineer to catch “in” and “out” samples that accurately represent the same element of mud in a circulating system. Cycle time is calculated from the estimated hole volume and pump rate and can be checked by using tracers such as carbide or rice granules. |
| Drilling Fluids | DD | A surfactant-type mud additive intended to prevent formation shales and clays from sticking to the drilling assembly and also to prevent gumbo shale from agglomerating and plugging the annulus and flowlines. Some DDs are claimed to be mud lubricants that lessen the torque and drag of the drillstring as it is rotated and moved up and down in the hole. |
| Drilling Fluids | deflocculant | A thinning agent used to reduce viscosity or prevent flocculation; incorrectly called a “dispersant.” Most deflocculants are low-molecular weight anionic polymers that neutralize positive charges on clay edges. Examples include polyphosphates, lignosulfonates, quebracho and various water-soluble synthetic polymers. |
| Drilling Fluids | deflocculate | To reduce the viscosity of a suspension by adding a thinning agent, also known as a deflocculant. |
| Drilling Fluids | deflocculated mud | A clay-based, water mud that has had its viscosity reduced with a chemical treatment; incorrectly, called a “dispersed” mud. The chemical used is a deflocculant, not a dispersant. A well-known and effective clay deflocculant is lignosulfonate. The mud, after being deflocculated, usually shows much improved filter-cake qualities with lower yield point and gel strengths. Filter-cake quality is improved because when clays are deflocculated, the platelets become detached from each other and can lie flat to form a thin, low-permeability filter cake. Lowering yield point and gel strength may not always be desired and can be adjusted by the amount of deflocculant added in each treatment. If yield point and gels are lowered too far, suspension and cutting capacity of the mud are impaired. |
| Drilling Fluids | deflocculation | The act of reducing the viscosity of a suspension by adding a thinning agent, also known as a deflocculant. |
| Drilling Fluids | defoamer | A mud additive used to lower interfacial tension so that trapped gas will readily escape from mud. Mechanical degassing equipment is commonly used along with defoamer. Octyl alcohol, aluminum stearate, various glycols, silicones and sulfonated hydrocarbons are used as defoamers. |
| Drilling Fluids | dehydration | The loss of water from cement slurry or drilling fluid by the process of filtration. Dehydration results in the deposition of a filter cake and loss of the slurrys internal fluid into a porous matrix. The cement is not completely dehydrated because sufficient water remains to allow setting of the cement. |
| Drilling Fluids | demulsifier | A chemical used to break emulsions (that is, to separate the two phases). The type of demulsifier selected depends on the type of emulsion, either oil-in-water or water-in-oil. Demulsifiers are used in the chemical analysis of oil and synthetic muds and to treat produced hydrocarbons. |
| Drilling Fluids | dewatering | The process of removing water from water-base drilling mud. Dewatering can involve chemical treatment for the flocculation and aggregation of solids followed by mechanical separation, such as centrifugation, or mechanical treatments alone. |
| Drilling Fluids | dichromate salt | A type of salt in which chromium atoms are in the plus-7 valence state, such as potassium dichromate, K2Cr2O7. |
| Drilling Fluids | diesel oil mud | An oil-base mud with diesel oil as its external phase. Diesel-oil mud is the traditional oil mud and has a history of excellent performance for drilling difficult wells. It has been used because the base oil is low-cost and widely available motor fuel. In-gauge holes can be drilled through all types of shales, salt, gypsum and other difficult strata using diesel-oil mud systems. It is often the mud of choice for drilling high-pressure, high-temperature zones. Diesel-oil muds usually contain from 5 to 40 vol.% emulsified brine water (except those that are specially designed to have none). The water phase usually contains 20 to 40 wt.% dissolved calcium chloride for shale control. Diesel-oil muds have been replaced in land drilling by mineral-oil muds and offshore by synthetic-fluid muds. These newer muds have fewer health, safety and environmental concerns compared to diesel oil. |
| Drilling Fluids | diesel-oil mud | An oil-base mud with diesel oil as its external phase. Diesel-oil mud is the traditional oil mud and has a history of excellent performance for drilling difficult wells. It has been used because the base oil is low-cost and widely available motor fuel. In-gauge holes can be drilled through all types of shales, salt, gypsum and other difficult strata using diesel-oil mud systems. It is often the mud of choice for drilling high-pressure, high-temperature zones. Diesel-oil muds usually contain from 5 to 40 vol.% emulsified brine water (except those that are specially designed to have none). The water phase usually contains 20 to 40 wt.% dissolved calcium chloride for shale control. Diesel-oil muds have been replaced in land drilling by mineral-oil muds and offshore by synthetic-fluid muds. These newer muds have fewer health, safety and environmental concerns compared to diesel oil. |
| Drilling Fluids | diesel-oil plug | Another term for gunk plug, a slurry that consists of bentonite, cement or polymers mixed into an oil; bentonite in diesel oil is commonly used as a gunk plug. A small batch of the slurry is pumped down a well that has lost circulation to seal the leaky zone. The gunk plug may or may not be squeezed by pressure into the zone. Water downhole interacts with the bentonite, cement or polymers to make a sticky gunk. |
| Drilling Fluids | difficult to disperse | Pertaining to a cement that is not easily dispersed by a material known as a dispersant. This term is commonly abbreviated as DTD. |
| Drilling Fluids | difficult to disperse in salt | Pertaining to cement that is not easily dispersed by a material known as a dispersant when the slurry is mixed with water containing a high concentration of salt. The term is commonly abbreviated DTDS. |
| Drilling Fluids | dilution | The process of adding fresh mud (or liquid phase) in order to reduce the solids content and maintain the properties of the drilling fluid in the active system. |
| Drilling Fluids | dilution water | Also called make-up water, which is water added to maintain or dilute a water-mud system. The added water may be fresh water, seawater or salt water, as appropriate for the mud. Make-up water volume is an important parameter in a material balance check on solids content and solids removal efficiency for a mud system. The amount of dilution strongly influences mud economics. If soft make-up water is needed, treatments to remove hardness ions should be done prior to adding the water to the mud to avoid clay flocculation and polymer precipitation. |
| Drilling Fluids | direct emulsifier | A chemical used in preparation and maintenance of an emulsion mud, which is a water mud containing dispersed oil (or a synthetic hydrocarbon). Numerous types of emulsifiers will disperse oil into water muds, including sulfonated hydrocarbons, ethyoxylated nonylphenols, alkali-metal fatty-acid soaps, lignosulfonate, lignite and lignin at high pH. Even clays, starch and carboxymethylcellulose aid emulsion mud stability. Reference: Rogers WF: “Oil-in-Water Emulsion Muds,” in Composition and Properties of Oil Well Drilling Fluids, 3rd ed. Houston, Texas, USA: Gulf Publishing Company, 1963. |
| Drilling Fluids | direct indicating viscometer | The instrument used to measure viscosity and gel strength of drilling mud. The direct-indicating viscometer is a rotational cylinder and bob instrument, also known as a V-G meter. Two speeds of rotation, 300 and 600 rpm, are available in all instruments, but some are 6- or variable-speed. It is called “direct-indicating” because at a given speed, the dial reading is a true centipoise viscosity. For example, at 300 rpm, the dial reading (511 sec-1) is a true viscosity. Bingham plastic rheological parameters are easily calculated from direct-indicating viscometer readings: PV (in units of cp) = 600 dial – 300 dial and YP (in units of lb/100 ft2) = 300 dial – PV. Gel strength is also directly read as dial readings in oilfield units of lb/100 ft2. |
| Drilling Fluids | direct-indicating viscometer | The instrument used to measure viscosity and gel strength of drilling mud. The direct-indicating viscometer is a rotational cylinder and bob instrument, also known as a V-G meter. Two speeds of rotation, 300 and 600 rpm, are available in all instruments, but some are 6- or variable-speed. It is called “direct-indicating” because at a given speed, the dial reading is a true centipoise viscosity. For example, at 300 rpm, the dial reading (511 sec-1) is a true viscosity. Bingham plastic rheological parameters are easily calculated from direct-indicating viscometer readings: PV (in units of cp) = 600 dial – 300 dial and YP (in units of lb/100 ft2) = 300 dial – PV. Gel strength is also directly read as dial readings in oilfield units of lb/100 ft2. |
| Drilling Fluids | dispersant | A chemical that aids in breaking up solids or liquids as fine particles or droplets into another medium. This term is often applied incorrectly to clay deflocculants. Clay dispersants are various sodium phosphates and sodium carbonates aided by heat, mechanical shearing and time. Powdered polymers are dispersed by precoating the particles with a type of glycol to prevent formation of “fish-eye” globules. For dispersing (emulsification) of oil into water (or water into oils), surfactants selected on the basis of hydrophile-lipophile balance (HLB) number can be used. For foam drilling fluids, synthetic detergents and soaps are used, along with polymers, to disperse foam bubbles into the air or gas. |
| Drilling Fluids | dispersion | The act of breaking up large particles into smaller ones and distributing them throughout a liquid or gaseous medium. For example, in water-base drilling mud, dispersion is the act of degrading clay materials, starches, carboxymethylcellulose, biopolymer, synthetic polymers or oils into submicroscopic particles by applying mechanical energy, heat or chemical means. In oil-mud emulsion terminology, dispersion is the act of forming a fine-grained emulsion of an aqueous phase into an oil. This is usually accomplished with by mechanical shearing or heating in the presence of surfactants. It also includes dispersing solids into an oil mud, such as barite or organophilic clays. Mud viscosity increases as more and more platelets are dispersed. No gel structure can form as long as mechanical shearing is in progress. |
| Drilling Fluids | dispersion | A colloidal system, such as a drilling fluid, that has been dispersed. More generally, the term applies to any two (or more) phase system in which one phase exists as small particles or droplets dispersed in the second phase. |
| Drilling Fluids | dissolved solids | In water analysis, the soluble components in a sample or the residue left after evaporation of a sample. Dissolved solids are reported as ppm or mg/L. Dissolved solids are included in retort solids and can be calculated from chemical analysis results by assuming that all dissolved solids are either NaCl or CaCl2, or a mixture of the two. |
| Drilling Fluids | Draeger tube | A type of gas detector tube that quantitatively measures a gas that is passed through the tube by the length of the stain it generates chemically in the tube. Dräger tubes are used in Garrett Gas Train tests for sulfides and carbonates. |
| Drilling Fluids | Dräger tube | A type of gas detector tube that quantitatively measures a gas that is passed through the tube by the length of the stain it generates chemically in the tube. Dräger tubes are used in Garrett Gas Train tests for sulfides and carbonates. |
| Drilling Fluids | drill in fluid | A special fluid designed exclusively for drilling through the reservoir section of a wellbore. The reasons for using a specially designed mud are: (1) to drill the reservoir zone successfully, often a long, horizontal drainhole. (2) to minimize damage and maximize production of exposed zones. (3) to facilitate the well completion needed, which can include complicated procedures. A drill-in fluid should resemble a completion fluid. It may be a brine containing only selected solids of appropriate particle size ranges (salt crystals or calcium carbonate) and polymers. Only additives essential for filtration control and cuttings carrying are present in a drill-in fluid. |
| Drilling Fluids | drill solids | Formation solids contained in a mud system, generally considered to be detrimental to the drilling operation because they produce high plastic viscosity, yield point and gel strengths and build poor-quality filter cakes. They also occupy space that is needed for barite in high-density muds. Drill solids cause excessive wear in the mud pumps and other rig equipment. Solids control is aimed at economically and efficiently removing drill solids. This implies removal as soon as possible after they enter the mud system, while the particles are at their largest size. |
| Drilling Fluids | drilled solids | Formation solids contained in a mud system, generally considered to be detrimental to the drilling operation because they produce high plastic viscosity, yield point and gel strengths and build poor-quality filter cakes. They also occupy space that is needed for barite in high-density muds. Drill solids cause excessive wear in the mud pumps and other rig equipment. Solids control is aimed at economically and efficiently removing drill solids. This implies removal as soon as possible after they enter the mud system, while the particles are at their largest size. |
| Drilling Fluids | drill-in fluid | A special fluid designed exclusively for drilling through the reservoir section of a wellbore. The reasons for using a specially designed mud are: (1) to drill the reservoir zone successfully, often a long, horizontal drainhole. (2) to minimize damage and maximize production of exposed zones. (3) to facilitate the well completion needed, which can include complicated procedures. A drill-in fluid should resemble a completion fluid. It may be a brine containing only selected solids of appropriate particle size ranges (salt crystals or calcium carbonate) and polymers. Only additives essential for filtration control and cuttings carrying are present in a drill-in fluid. |
| Drilling Fluids | drilling detergent | A surfactant-type mud additive intended to prevent formation shales and clays from sticking to the drilling assembly and also to prevent gumbo shale from agglomerating and plugging the annulus and flowlines. Some DDs are claimed to be mud lubricants that lessen the torque and drag of the drillstring as it is rotated and moved up and down in the hole. |
| Drilling Fluids | drilling fluid | Any of a number of liquid and gaseous fluids and mixtures of fluids and solids (as solid suspensions, mixtures and emulsions of liquids, gases and solids) used in operations to drill boreholes into the earth. Synonymous with “drilling mud” in general usage, although some prefer to reserve the term “drilling fluid” for more sophisticated and well-defined “muds.” Classifications of drilling fluids has been attempted in many ways, often producing more confusion than insight. One classification scheme, given here, is based only on the mud composition by singling out the component that clearly defines the function and performance of the fluid: (1) water-base, (2) non-water-base and (3) gaseous (pneumatic). Each category has a variety of subcategories that overlap each other considerably. |
| Drilling Fluids | drilling mud | Any of a number of liquid and gaseous fluids and mixtures of fluids and solids (as solid suspensions, mixtures and emulsions of liquids, gases and solids) used in operations to drill boreholes into the earth. Synonymous with “drilling fluid” in general usage, although some prefer to reserve the term “drilling fluid” for more sophisticated and well-defined “muds.” Classifications of drilling fluids has been attempted in many ways, often producing more confusion than insight. One classification scheme, given here, is based only on the mud composition by singling out the component that clearly defines the function and performance of the fluid: (1) water-base, (2) non-water-base and (3) gaseous (pneumatic). Each category has a variety of subcategories that overlap each other considerably. |
| Drilling Fluids | drilling-fluid engineer | A person responsible for testing the mud at a rig and for prescribing mud treatments to maintain mud weight, properties and chemistry within recommended limits. The mud engineer works closely with the rig supervisor to disseminate information about mud properties and expected treatments and any changes that might be needed. The mud engineer also works closely with the rig’s derrickman, who is charged with making scheduled additions to the mud during his work period. |
| Drilling Fluids | dynamic filter press | Equipment used to measure filtration under dynamic conditions. Two commercial dynamic-filtration testers are available, one of which uses a thick-walled cylinder with rock-like characteristics as the filter medium to simulate radial flow into a wellbore. The other tester uses flat porous disks, such as paper or fused ceramic plates, as filter media. In a dynamic test, filter cake is continually eroded and deposited. Data from this test include a steady-state filtration rate measured during the test, and cake thickness, cake quality and return permeability of the filter medium measured at the conclusion of a test. There is no API standardized test equipment or procedure. |
| Drilling Fluids | dynamic filtration | A filtration process in which the slurry being filtered is being circulated over the filter cake, so that the cake is simultaneously eroded and deposited. The erosion rate depends on the shear rate of the fluid at the face of the cake. If the shear rate remains constant, cake thickness and filtration rate reach steady state, usually in a matter of hours. When the conditions change, a new steady state will be established. |
| Drilling Fluids | dynamic-aging test | A mud test in which the mud sample is mildly agitated by rolling (or tumbling) for the duration of the test, usually performed at a selected high temperature. Typically, the mud sample is sealed in a mud-aging cell and placed in an oven that will roll (or tumble) the mud cells continually for a given period of time (often 16 hours or overnight). The cooled mud is tested for properties. A rolled (or tumbled) mud sample simulates circulation in the hole by pumping. |
| Drilling Fluids | easy to disperse | Pertaining to cement that is highly sensitive to the concentration of dispersant, often leading to slurry-stability problems. The term is commonly abbreviated ETD. |
| Drilling Fluids | easy to disperse in salt | Pertaining to cement that is highly sensitive to the concentration of dispersant when the slurry is mixed with water containing a high concentration of salt. Overdispersion often leads to slurry-stability problems. The term is commonly abbreviated ETDS. |
| Drilling Fluids | ECD | The effective density exerted by a circulating fluid against the formation that takes into account the pressure drop in the annulus above the point being considered. The ECD is calculated as: d + P/0.052*D, where d is the mud weight (ppg), P is the pressure drop in the annulus between depth D and surface (psi), and D is the true vertical depth (feet). The ECD is an important parameter in avoiding kicks and losses, particularly in wells that have a narrow window between the fracture gradient and pore-pressure gradient. |
| Drilling Fluids | EDTA | Ethylenediamine tetraacetic acid, the reagent used to titrate for calcium and magnesium ions (hardness ions) in water samples. It is also known as versenate or titraver. |
| Drilling Fluids | effective laminar flow | A technique for displacing drilling mud from the annulus using a laminar-flow regime. |
| Drilling Fluids | electrical stability test | A test for oil-base and synthetic-base muds that indicates the emulsion and oil-wetting qualities of the sample. The test is performed by inserting the ES probe into a cup of 120°F [48.9°C] mud and pushing a test button. The ES meter automatically applies an increasing voltage (from 0 to 2000 volts) across an electrode gap in the probe. Maximum voltage that the mud will sustain across the gap before conducting current is displayed as the ES voltage. The modern ES meter has sine-wave circuitry, whereas older meters used square-wave circuits. (The older units should not be used because they do not correctly address the theory described in the reference below.) The ES sine-wave design and meaning of ES readings have been studied and were found to relate to an oil mud’s oil-wetting of solids and to stability of the emulsion droplets in a complex fashion not yet understood. Reference: Growcock FB, Ellis CF and Schmidt DD: Electrical Stability, Emulsion Stability, and Wettability of Invert Oil-Based Muds, SPE Drilling & Completion 9, no. 1 (March 1994): 39-46. |
| Drilling Fluids | electrohygrometer | A device for measuring the moisture in a gaseous atmosphere, such as the air, usually as percent relative humidity. Mechanical hygrometers detect moisture by elongation and shrinkage of a fiber or sheet or by a device attached to a needle on a dial. Electrohygrometers measure changes in an electrical property of a moisture-sensitive sensing probe and are more reliable. Determination of the aqueous-phase activity of oil muds by the Chenevert Method requires an electrohygrometer and a series of salt solutions for calibration. |
| Drilling Fluids | emulsifier | A chemical additive that creates an emulsion, a dispersion of one immiscible liquid into another, by reducing the interfacial tension between the two liquids to achieve stability. Two emulsion types are used as muds: (1) oil-in-water (or direct) emulsion, known as an “emulsion mud” and (2) water-in-oil (or invert) emulsion, known as an “invert emulsion mud.” The former is classified as a water-base mud and the latter as an oil-base mud. |
| Drilling Fluids | emulsion | A dispersion of one immiscible liquid into another through the use of a chemical that reduces the interfacial tension between the two liquids to achieve stability. Two emulsion types are used as muds: (1) oil-in-water (or direct) emulsion, known as an “emulsion mud” and (2) water-in-oil (or invert) emulsion, known as an “invert emulsion mud.” The former is classified as a water-base mud and the latter as an oil-base mud. |
| Drilling Fluids | emulsion mud | A water-base drilling fluid that contains dispersed oil or synthetic hydrocarbon as an internal phase. Early emulsion muds used diesel or crude oil dispersed into alkaline water-base muds. Synthetic liquids are now being substituted for oils in emulsion muds. Water-base muds containing certain synthetic liquids can be discharged in the Gulf of Mexico because they are environmentally safe and pass the EPA static sheen test and mysid shrimp toxicity tests. Reference: Rogers WF: Oil-in-Water Emulsion Muds, in Composition and Properties of Oil Well Drilling Fluids, 3rd ed. Houston, Texas, USA: Gulf Publishing Company, 1963. |
| Drilling Fluids | encapsulation | In drilling fluid parlance, the absorption of a polymer film onto cuttings and wellbore walls to form a coat or barrier. The term is usually applied to shale encapsulation by long-chain, acrylamide-acrylate (PHPA) polymers. Negative sites on PHPA may bond to positive sites on the clays in shales, although it is not well-defined how (or how well) encapsulation works. A viscous polymer film, according to some test results, slows diffusion of water molecules into the shale and thus slows hydration and disintegration. This does not prevent wellbore problems but can delay their onset. |
| Drilling Fluids | end point | During a titration procedure in analytical chemistry, the point at which reagent addition should be immediately stopped and the volume of reagent recorded. The endpoint represents the end of a specific chemical reaction, such as precipitation, and may be indicated by a color change, a voltage or pH reading or an inflection point on a graphical plot of the data. |
| Drilling Fluids | endpoint | During a titration procedure in analytical chemistry, the point at which reagent addition should be immediately stopped and the volume of reagent recorded. The endpoint represents the end of a specific chemical reaction, such as precipitation, and may be indicated by a color change, a voltage or pH reading or an inflection point on a graphical plot of the data. |
| Drilling Fluids | EPA | Abbreviation for Environmental Protection Agency, a branch of the US government that administers laws passed by the US Congress on environmental matters. |
| Drilling Fluids | epm | Abbreviation for equivalents-per-million. This unit of concentration is determined by dividing the number of equivalents of a substance by the weight of the solution that contains the substance, expressing the result as parts per million, and then dividing by the valency. For example, a solution containing one milligram of calcium in one kilogram of solution contains 1 ppm calcium. By dividing the ppm by the valency of calcium, which is 2 in this case, the result is the epm of calcium, 0.5 epm. |
| Drilling Fluids | equivalent circulating density | The effective density exerted by a circulating fluid against the formation that takes into account the pressure drop in the annulus above the point being considered. The ECD is calculated as: d + P/0.052*D, where d is the mud weight (ppg), P is the pressure drop in the annulus between depth D and surface (psi), and D is the true vertical depth (feet). The ECD is an important parameter in avoiding kicks and losses, particularly in wells that have a narrow window between the fracture gradient and pore-pressure gradient. |
| Drilling Fluids | equivalent sack | The weight of any cementitious material or blend based on the absolute volume of the cement. The term is normally used to define a sack of cement blend in which part of the cement has been replaced, on an absolute volume basis, by a pozzolanic material such as fly ash. |
| Drilling Fluids | equivalent weight | The molecular weight of an element, molecule or ion divided by its valence (or valence change for a redox reaction). For example, the molecular weight of calcium hydroxide, or “slaked lime,” [Ca(OH)2] is 72.10. Because the valency of calcium in this case is 2, the equivalent weight of lime is 36.05. Mud analyses give concentrations in various units: ppm, mg/L, wt.% and epm. Mud engineers should recognize the meaning of epm and equivalent weight of a mud chemical. |
| Drilling Fluids | ES test | A test for oil-base and synthetic-base muds that indicates the emulsion and oil-wetting qualities of the sample. The test is performed by inserting the ES probe into a cup of 120°F [48.9°C] mud and pushing a test button. The ES meter automatically applies an increasing voltage (from 0 to 2000 volts) across an electrode gap in the probe. Maximum voltage that the mud will sustain across the gap before conducting current is displayed as the ES voltage. The modern ES meter has sine-wave circuitry, whereas older meters used square-wave circuits. (The older units should not be used because they do not correctly address the theory described in the reference below.) The ES sine-wave design and meaning of ES readings have been studied and were found to relate to an oil mud’s oil-wetting of solids and to stability of the emulsion droplets in a complex fashion not yet understood. Reference: Growcock FB, Ellis CF and Schmidt DD: “Electrical Stability, Emulsion Stability, and Wettability of Invert Oil-Based Muds,” SPE Drilling & Completion 9, no. 1 (March 1994): 39-46. |
| Drilling Fluids | expanding cement | A cement system exhibiting a bulk volumetric increase after setting. Expanding cement is commonly used to eliminate or minimize the effects of microannuli. |
| Drilling Fluids | external phase | The continuous phase of an emulsion. The internal phase is the dispersed droplets of emulsified fluid. |
| Drilling Fluids | Fann viscometer | Also known as direct-indicating viscometer or V-G meter, an instrument used to measure viscosity and gel strength of drilling mud. The direct-indicating viscometer is a rotational cylinder and bob instrument. Two speeds of rotation, 300 and 600 rpm, are available in all instruments, but some are 6- or variable-speed. It is called “direct-indicating” because at a given speed, the dial reading is a true centipoise viscosity. For example, at 300 rpm, the dial reading (511 sec-1) is a true viscosity. Bingham plastic rheological parameters are easily calculated from direct-indicating viscometer readings: PV (in units of cp) = 600 dial – 300 dial and YP (in units of lb/100 ft2) = 300 dial – PV. Gel strength is also directly read as dial readings in oilfield units of lb/100 ft2. |
| Drilling Fluids | fatty acid | A type of organic acid derived from animal and vegetable fats and oils. Fatty acids are the raw materials used in the manufacture of many drilling-fluid additives, such as emulsifiers, oil-wetting agents and lubricants. Tall-oil fatty acids are distilled from conifer trees. Animal and vegetable fats and oils are triglycerides, which are hydrolyzed to give fatty acids (and glycerol). Fatty acids from animals are mostly saturated acids, having single bonds between carbon atoms. Tall oils and vegetable oils yield both saturated and unsaturated (double- and triple-bond) fatty acids. |
| Drilling Fluids | fatty acid soap | A salt formed when a fatty acid reacts with a metal oxide or hydroxide. Fatty acids and lime, Ca(OH)2, form emulsifiers for oil muds. Fatty acids reacted with sodium hydroxide [NaOH] or potassium hydroxide [KOH] are laundry soaps, some used as foamers for air drilling. Fatty acids and aluminum hydroxide form soaps used as greases and as defoamer chemicals. |
| Drilling Fluids | fatty-acid soap | A salt formed when a fatty acid reacts with a metal oxide or hydroxide. Fatty acids and lime, Ca(OH)2, form emulsifiers for oil muds. Fatty acids reacted with sodium hydroxide [NaOH] or potassium hydroxide [KOH] are laundry soaps, some used as foamers for air drilling. Fatty acids and aluminum hydroxide form soaps used as greases and as defoamer chemicals. |
| Drilling Fluids | ferro-chrome lignosulfonate | A popular type of deflocculant that contains iron and chromium salts. |
| Drilling Fluids | fiber LCM | A type of lost circulation material (LCM) that is long, slender and flexible and occurs in various sizes and lengths of fiber. Fiber LCM is added to mud and placed downhole to help retard mud loss into fractures or highly permeable zones. Ideally, fiber LCM should be insoluble and inert to the mud system in which it is used. Examples are cedar bark, shredded cane stalks, mineral fiber and hair. Often, granular, flake and fiber LCMs are mixed together into an LCM pill and pumped into the well next to the zone of fluid loss to seal the formation that is taking mud from the system. |
| Drilling Fluids | fiber lost circulation material | A type of lost circulation material that is long, slender and flexible and occurs in various sizes and lengths of fiber. Fiber LCM is added to mud and placed downhole to help retard mud loss into fractures or highly permeable zones. Ideally, fiber LCM should be insoluble and inert to the mud system in which it is used. Examples are cedar bark, shredded cane stalks, mineral fiber and hair. Often, granular, flake and fiber LCM are mixed together into an LCM pill and pumped into the well next to the zone of fluid loss to seal the formation that is taking mud from the system. |
| Drilling Fluids | fiber lost-circulation material | A type of lost circulation material (LCM) that is long, slender and flexible and occurs in various sizes and lengths of fiber. Fiber LCM is added to mud and placed downhole to help retard mud loss into fractures or highly permeable zones. Ideally, fiber LCM should be insoluble and inert to the mud system in which it is used. Examples are cedar bark, shredded cane stalks, mineral fiber and hair. Often, granular, flake and fiber LCMs are mixed together into an LCM pill and pumped into the well next to the zone of fluid loss to seal the formation that is taking mud from the system. |
| Drilling Fluids | fill cement | A cement system used to provide zonal isolation across generally nonproductive zones located above the zones of interest. The fill cement is also called the lead cement. |
| Drilling Fluids | film forming amine | Also known as quaternary amine, a cationic amine salt in which the nitrogen atom has four groups bonded to it and carries a positive charge. Quaternary amines are used as oil-wetting agents, corrosion and shale inhibitors and bactericides. |
| Drilling Fluids | film-forming amine | Also known as quaternary amine, a cationic amine salt in which the nitrogen atom has four groups bonded to it and carries a positive charge. Quaternary amines are used as oil-wetting agents, corrosion and shale inhibitors and bactericides. |
| Drilling Fluids | filter cake | The residue deposited on a permeable medium when a slurry, such as a drilling fluid, is forced against the medium under a pressure. Filtrate is the liquid that passes through the medium, leaving the cake on the medium. Drilling muds are tested to determine filtration rate and filter-cake properties. Cake properties such as cake thickness, toughness, slickness and permeability are important because the cake that forms on permeable zones in the wellbore can cause stuck pipe and other drilling problems. Reduced oil and gas production can result from reservoir damage when a poor filter cake allows deep filtrate invasion. A certain degree of cake buildup is desirable to isolate formations from drilling fluids. In openhole completions in high-angle or horizontal holes, the formation of an external filter cake is preferable to a cake that forms partly inside the formation. The latter has a higher potential for formation damage. |
| Drilling Fluids | filter cake quality | A subjective description of a filter cake, especially its toughness, slickness and hardness. A mud engineer makes these observations, which are recorded on the mud report along with filtrate volume and cake thickness. With increasing experience, the engineer’s observations can become less subjective. |
| Drilling Fluids | filter cake thickness | A measurement of the thickness of the filter cake, usually recorded in 32nds-inch. Under dynamic conditions, filter-cake thickness depends on rate of deposition versus erosion caused by fluid circulation and mechanical abrasion by the rotating drillstring. Typically, the filter cake will reach an equilibrium thickness in the wellbore. In laboratory tests, however, filter cake is built under static conditions with no erosion. |
| Drilling Fluids | filter cell | A pressurized cell, fitted with a filter medium, used for evaluating filtration characteristics of a drilling fluid while it is either static or stirred (to simulate circulation) in the test cell. Generally, either low-pressure, low-temperature or high-pressure, high-temperature devices are used. |
| Drilling Fluids | filter loss | The volume of mud filtrate measured after 30 minutes in API static filtration tests. The volume and cake thickness are the two data points in the test. |
| Drilling Fluids | filter media | Plural of filter medium. |
| Drilling Fluids | filter medium | A permeable material used in a filtration device through which filtrate passes and on which the filter cake is deposited, commonly a specifically designed filter paper or permeable disk used in a static filter press that meets API standards. The filter medium can be the cylindrical, permeable core or disk used in a dynamic filtration test or permeable rock downhole on which a filter cake is deposited in a wellbore. |
| Drilling Fluids | filter press | A pressurized cell, fitted with a filter medium, used for evaluating filtration characteristics of a drilling fluid while it is either static or stirred (to simulate circulation) in the test cell. Generally, either low-pressure, low-temperature or high-pressure, high-temperature devices are used. |
| Drilling Fluids | filtercake | The residue deposited on a permeable medium when a slurry, such as a drilling fluid, is forced against the medium under a pressure. Filtrate is the liquid that passes through the medium, leaving the cake on the medium. Drilling muds are tested to determine filtration rate and filtercake properties. Cake properties such as cake thickness, toughness, slickness and permeability are important because the cake that forms on permeable zones in the wellbore can cause stuck pipe and other drilling problems. Reduced oil and gas production can result from reservoir damage when a poor filtercake allows deep filtrate invasion. A certain degree of cake buildup is desirable to isolate formations from drilling fluids. In openhole completions in high-angle or horizontal holes, the formation of an external filtercake is preferable to a cake that forms partly inside the formation. The latter has a higher potential for formation damage. |
| Drilling Fluids | filtercake quality | A subjective description of a filtercake, especially its toughness, slickness and hardness. A mud engineer makes these observations, which are recorded on the mud report along with filtrate volume and filtercake thickness. With increasing experience, the engineer’s observations can become less subjective. |
| Drilling Fluids | filter-cake quality | A subjective description of a filter cake, especially its toughness, slickness and hardness. A mud engineer makes these observations, which are recorded on the mud report along with filtrate volume and cake thickness. With increasing experience, the engineer’s observations can become less subjective. |
| Drilling Fluids | filtercake thickness | A measurement of the thickness of the filtercake, usually recorded in 32nds-inch. Under dynamic conditions, filtercake thickness depends on rate of deposition versus erosion caused by fluid circulation and mechanical abrasion by the rotating drillstring. Typically, the filtercake will reach an equilibrium thickness in the wellbore. In laboratory tests, however, filtercake is built under static conditions with no erosion. |
| Drilling Fluids | filter-cake thickness | A measurement of the thickness of the filter cake, usually recorded in 32nds-inch. Under dynamic conditions, filter-cake thickness depends on rate of deposition versus erosion caused by fluid circulation and mechanical abrasion by the rotating drillstring. Typically, the filter cake will reach an equilibrium thickness in the wellbore. In laboratory tests, however, filter cake is built under static conditions with no erosion. |
| Drilling Fluids | filtrate tracer | A chemical or isotopic marker that is uniformly distributed in the continuous phase of a drilling, coring, drill-in or completion fluid and used to later identify the filtrate in cores or in fluids sampled from permeable strata. A tracer must become a part of the filtrate, remaining in true solution and moving with the filtrate into permeable zones. It must not be a component in the strata that is expected to migrate, be adsorbed on clays, or degraded. It should be measurable in trace amounts and safe to handle. Examples of filtrate tracers include: (1) Radioactively tagged compounds (isotopes of elements). Tritium, a weakly-emitting radioisotope of hydrogen, can be a safe and effective tracer in both oil and water (as T2O) muds. It is measured by scintillation counts. (2) Bromide or iodide compounds are practical to use because they do not occur naturally in most muds or reservoirs. They are detectable in small amounts by electron-capture gas chromatography. (3) Fatty acids (or their derivatives) normally present in an oil-mud emulsifier can serve as oil-filtrate tracers and are analyzed by gas chromatography. (4) Nitrate (NO3-) anion, added as sodium, potassium or calcium nitrate, is one of the earliest tracers used. It is limited by being difficult to analyze and lost by degradation. |
| Drilling Fluids | filtrate volume | The volume of mud filtrate measured after 30 minutes in API static filtration tests. The volume and cake thickness are the two data points in the test. |
| Drilling Fluids | filtration | The process of separating components of a slurry by leaving the suspended solids as filter cake on a filter medium while the liquid passes through. The process can be either static or dynamic. |
| Drilling Fluids | filtration test cell | A pressurized cell, fitted with a filter medium, used for evaluating filtration characteristics of a drilling fluid while it is either static or stirred (to simulate circulation) in the test cell. Generally, either low-pressure, low-temperature or high-pressure, high-temperature devices are used. |
| Drilling Fluids | filtration tester | Equipment used to measure filtration under dynamic conditions. Two commercial dynamic-filtration testers are available, one of which uses a thick-walled cylinder with rock-like characteristics as the filter medium to simulate radial flow into a wellbore. The other tester uses flat porous disks, such as paper or fused ceramic plates, as filter media. In a dynamic test, filter cake is continually eroded and deposited. Data from this test include a steady-state filtration rate measured during the test, and cake thickness, cake quality and return permeability of the filter medium measured at the conclusion of a test. There is no API standardized test equipment or procedure. |
| Drilling Fluids | fine | A particle size term referring in the strict sense (API Bulletin 13C) to any particle in the size range 44 to 74 microns. More generally it is used to indicate any particle not removed by the shaker screens. |
| Drilling Fluids | fines | In a broad sense, very small particles, either in a mud or a mud additive sample. |
| Drilling Fluids | fish eye | A slang term for a globule of partly hydrated polymer caused by poor dispersion during the mixing process (commonly a result of adding the product too fast). Fish eyes are typically 0.2 to 0.5 inches in size and consist of a granule of unhydrated polymer surrounded by a gelatinous covering of hydrated polymer, which prevents water from entering to complete the hydration process. Thus, once formed, fish eyes do not disperse and the product is removed on the shaker screens and wasted. |
| Drilling Fluids | flake LCM | A type of lost-circulation material (LCM) that is thin and flat in shape, with a large surface area. Flake LCM can be prepared in various sizes. It should be insoluble and inert to the mud system in which it is used. Its purpose is to seal off fluid loss zones in a well and help stop lost circulation. Mica flakes and pieces of plastic (cellophane) sheeting are commonly used. Often, granular, flake and fiber LCMs will be mixed into one LCM pill and pumped into the zone where losses are occurring. |
| Drilling Fluids | flake lost circulation material | A type of lost-circulation material that is thin and flat in shape, with a large surface area. Flake LCM can be prepared in various sizes. It should be insoluble and inert to the mud system in which it is used. Its purpose is to seal off fluid loss zones in a well and help stop lost circulation. Mica flakes and pieces of plastic (cellophane) sheeting are commonly used. Often, granular, flake and fiber LCMs will be mixed into one LCM pill and pumped into the zone where losses are occurring. |
| Drilling Fluids | flake lost-circulation material | A type of lost-circulation material that is thin and flat in shape, with a large surface area. Flake LCM can be prepared in various sizes. It should be insoluble and inert to the mud system in which it is used. Its purpose is to seal off fluid loss zones in a well and help stop lost circulation. Mica flakes and pieces of plastic (cellophane) sheeting are commonly used. Often, granular, flake and fiber LCMs will be mixed into one LCM pill and pumped into the zone where losses are occurring. |
| Drilling Fluids | flash point | The lowest temperature at which application of a flame to the test chamber of a tester causes vapors of the sample in the chamber to ignite. The test can be applied to base fluids being considered for use in an oil mud or a synthetic mud or to any flammable liquid to determine at what temperature an explosion hazard exists. Test methods, established by API and ASTM, include open-cup and closed-cup tests. |
| Drilling Fluids | flat gels | The situation in which 10-second and 10-minute gel strengths for a drilling mud have similar values. Flat gels indicates that the mud will remain pumpable with time if left static in the hole. However, if gel values are too low, barite sag or solids settling is likely. |
| Drilling Fluids | floc | A coagulated mass of particles in a liquid. Flocs can occur naturally but often are generated from a dispersed colloidal system to which a flocculant chemical has been added. Clay particles and polymers in water can be flocculated to form flocs. |
| Drilling Fluids | flocculant | A chemical that causes a dispersed colloidal system (such as clay) to coagulate and form flocs. Most flocculants are either multivalent cations such as calcium, magnesium and aluminum, or long-chain polymers. High pH, high salinity and high temperature can also cause clay flocculation. |
| Drilling Fluids | flocculate | The term used to describe what clays, polymers or other small charged particles do when they become attached and form a fragile structure, a floc. In dispersed clay slurries, flocculation occurs after mechanical agitation ceases and the dispersed clay platelets spontaneously form flocs because of attractions between negative face charges and positive edge charges. |
| Drilling Fluids | flocculation | A condition in which clays, polymers or other small charged particles become attached and form a fragile structure, a floc. In dispersed clay slurries, flocculation occurs after mechanical agitation ceases and the dispersed clay platelets spontaneously form flocs because of attractions between negative face charges and positive edge charges. |
| Drilling Fluids | flowline mud sample | A mud sample that exits directly out of the well from the annulus and is caught before it passes through the shale-shaker screens. A flowline mud sample contains drill cuttings entrained in the mud. |
| Drilling Fluids | flowline sample | A mud sample that exits directly out of the well from the annulus and is caught before it passes through the shale-shaker screens. A flowline mud sample contains drill cuttings entrained in the mud. |
| Drilling Fluids | fluid loss control | The act or means of controlling (usually lowering) the volume of filtrate that passes through a filter medium. Control of fluid loss for a mud is achieved by several means, one of which is by addition of fluid-loss-control materials to the mud system. Another is to change the mud chemistry to make the materials already present work better. Adding a clay deflocculant to freshwater mud typically improves fluid-loss control. |
| Drilling Fluids | fluid loss control material | A group of mud additives specifically designed to lower the volume of filtrate that passes through a filter medium. Specific materials are available for all types of water- and oil-base mud systems and are evaluated in static filtration tests or in various dynamic filtration tests. |
| Drilling Fluids | fluid-loss additive | A group of mud additives specifically designed to lower the volume of filtrate that passes through a filter medium. Specific materials are available for all types of water- and oil-base mud systems and are evaluated in static filtration tests or in various dynamic filtration tests. |
| Drilling Fluids | fluid-loss control | The act or means of controlling (usually lowering) the volume of filtrate that passes through a filter medium. Control of fluid loss for a mud is achieved by several means, one of which is by addition of fluid-loss-control materials to the mud system. Another is to change the mud chemistry to make the materials already present work better. Adding a clay deflocculant to freshwater mud typically improves fluid-loss control. |
| Drilling Fluids | fluid-loss-control material | A group of mud additives specifically designed to lower the volume of filtrate that passes through a filter medium. Specific materials are available for all types of water- and oil-base mud systems and are evaluated in static filtration tests or in various dynamic filtration tests. |
| Drilling Fluids | fly ash | The noncombustible residue from the burning of pulverized coal. Fly ash is pozzolanic and is frequently used to replace a portion of the cement and reduce its density. |
| Drilling Fluids | foam breaker | A mud additive used to lower interfacial tension so that trapped gas will readily escape from mud. Mechanical degassing equipment is commonly used along with defoamer. Octyl alcohol, aluminum stearate, various glycols, silicones and sulfonated hydrocarbons are used as defoamers. |
| Drilling Fluids | foamed cement | A homogeneous, ultralightweight cement system consisting of base cement slurry, gas (usually nitrogen) and surfactants. Foamed cements are commonly used to cement wells that penetrate weak rocks or formations with low formation-fracture gradients. |
| Drilling Fluids | formaldehyde | The simplest aldehyde, having the formula HCHO. Formaldehyde is used in aqueous solutions as a preservative. In muds, paraformaldehyde is added to protect against bacterial attack. The formaldehyde test determines the paraformaldehyde (bactericide) content of a drilling fluid by a P-alkalinity titration of a sulfite-oxidized mud filtrate. |
| Drilling Fluids | formate | A class of salts made from neutralization of formic acid with a metal hydroxide or oxide. Three alkali-metal formates are used in drilling, drill-in and completion fluids, (1) sodium formate, HCOO-Na+, (2) potassium formate, HCOO-K+ and (3) cesium formate, HCOO-Cs+. Clear solutions of each can reach densities of 1.32, 1.58 and 2.4 g/cm3, respectively. They are near neutral pH and meet HSE standards. Most formates can be mixed together over broad ranges of concentration or temperature without solubility or crystallization problems. |
| Drilling Fluids | formation damage | A reduction in the natural capability of a reservoir to produce its fluids, such as a decrease in porosity or permeability, or both. Damage can occur near the wellbore face (easier to repair) or deep into the rock (harder to repair). Damage is caused by several mechanisms: (1) physical plugging of pores by mud solids, (2) alteration of reservoir rock wettability, (3) precipitation of insoluble materials in pore spaces, (4) clay swelling in pore spaces, (5) migration of fines into pore throats, (6) introduction of an immobile phase, and (7) emulsion formation and blockage. Damage can occur when sensitive formations are exposed to drilling fluids. |
| Drilling Fluids | free fluid | The volume of fluid (expressed in percent) that separates from a cement slurry when the slurry is left static. The free fluid can be measured as specified in API Recommended Practice 10B. Free fluid is also known as free water. |
| Drilling Fluids | funnel viscosity | Time, in seconds for one quart of mud to flow through a Marsh funnel. This is not a true viscosity, but serves as a qualitative measure of how thick the mud sample is. The funnel viscosity is useful only for relative comparisons. |
| Drilling Fluids | galena | The mineral form of lead sulfide, PbS, and the most common ore for lead smelting. In drilling muds, powdered galena has been used, although rarely, to construct extremely high density (kill-weight) muds by taking advantage of its 7.5 g/cm3 density. |
| Drilling Fluids | Garrett Gas Train | An instrument used for quantitative analyses of sulfides and carbonates. Specific test methods have been published by API. The oil-mud procedure analyzes active sulfides and uses whole mud samples, whereas the water-base drilling fluid procedure tests filtrate. The GGT unit is a clear, plastic block (2.5 in. x 4 in. x 6 in.) that contains three interconnected chambers. A carrier gas is used to flow an inert gas through the chambers. The sample is placed in chamber #1 and is acidified to release sulfides as H2S and carbonates as CO2. The appropriate Drdger tube is used to measure the effluent gas that is evolved from the sample. The device is named after Bob Garrett, who invented it while at Exxon Production Research. |
| Drilling Fluids | gas hydrate | A crystalline solid consisting of water with gas molecules in an ice-like cage structure. The general term for this type of solid is clathrate. Water molecules form a lattice structure into which many types of gas molecules can fit. Most gases, except hydrogen and helium, can form hydrates. C1 to nC5 hydrocarbons, H2S and CO2 readily form hydrates at low temperature and high pressure. Heavier hydrocarbons may also enter the structure but do not form hydrates by themselves. Gas-cut muds can form hydrates in deepwater drilling operations, plugging BOP lines, risers and subsea wellheads, causing a well-control risk. Gas hydrates are thermodynamically suppressed by adding antifreeze materials such as salts or glycols. A common practice is to use 20 to 23 wt.% NaCl. Nucleation and growth of hydrates can be dynamically inhibited by certain polymers or surfactants. Gas hydrates are found in nature, on the bottom of cold seas and in arctic permafrost regions. Drilling into these can be hazardous, but they offer another source of hydrocarbons for future exploitation. |
| Drilling Fluids | gas migration | A generic term referring to all possible routes for annular gas entry and propagation through and around the cement sheath. Gas migration is also known as annular gas flow. |
| Drilling Fluids | gel | The term used by drilling rig personnel to refer to bentonite clay. |
| Drilling Fluids | gel | A colloidal form of solids suspended in a liquid medium. |
| Drilling Fluids | gel | A name used to refer to one of the gel-strength numbers. |
| Drilling Fluids | gel strength | The shear stress measured at low shear rate after a mud has set quiescently for a period of time (10 seconds and 10 minutes in the standard API procedure, although measurements after 30 minutes or 16 hours may also be made). |
| Drilling Fluids | gelation | The process of a mud becoming “gelled-up” or developing high gel strength. |
| Drilling Fluids | gelled mud | A mud that is excessively viscous, having high gel strengths and high yield point. A gelled-up mud may not be pumpable without exceeding limits on pump pressure. Often caused by excessive solids content, especially colloidal solids, or, in the case of oil or synthetic muds, by low temperature. |
| Drilling Fluids | gelled up mud | A mud that is excessively viscous, having high gel strengths and high yield point. A gelled-up mud may not be pumpable without exceeding limits on pump pressure. Often caused by excessive solids content, especially colloidal solids, or, in the case of oil or synthetic muds, by low temperature. |
| Drilling Fluids | gelled-up mud | A mud that is excessively viscous, having high gel strengths and high yield point. A gelled-up mud may not be pumpable without exceeding limits on pump pressure. Often caused by excessive solids content, especially colloidal solids, or, in the case of oil or synthetic muds, by low temperature. |
| Drilling Fluids | gels | Jargon referring to the two gel-strength values for a mud. The 10-second and 10-minute “gels,” often written as one number over the other. For example, 6/16 means 6 lb/100 ft2 and is 10-second gel, and 16 lb/100 ft2 is the 10-minute gel. |
| Drilling Fluids | GGT | An instrument used for quantitative analyses of sulfides and carbonates. Specific test methods have been published by API. The oil-mud procedure analyzes active sulfides and uses whole mud samples, whereas the water-base drilling fluid procedure tests filtrate. The GGT unit is a clear, plastic block (2.5 in. x 4 in. x 6 in.) that contains three interconnected chambers. A carrier gas is used to flow an inert gas through the chambers. The sample is placed in chamber #1 and is acidified to release sulfides as H2S and carbonates as CO2. The appropriate Drdger tube is used to measure the effluent gas that is evolved from the sample. The device is named after Bob Garrett, who invented it while at Exxon Production Research. Reference: Garrett RL: “A New Field Method for the Quantitative Determination of Sulfides in Water-Based Drilling Fluids,” Journal of Petroleum Technology 29, no. 9 (September 1977): 1195-1201. Garrett RL: “A New Field Method for the Quantitative Determination of Carbonates in Water-Base Drilling Fluids,” Journal of Petroleum Technology 30, no. 7 (July 1978): 860-868. Garrett RL, Carlton LA and Denekas MO: “Methods for Field Monitoring of Oil-Based Drilling Fluids for Hydrogen Sulfide and Water Intrusions,” SPE Drilling Engineering 3, no.3 (September 1988): 296-302. |
| Drilling Fluids | gilsonite | A generic name widely used for a black, lustrous, carbonaceous resin classified as an asphaltite. Its proper name is uintaite, and it is found in Utah, USA. An important characteristic of gilsonite is its softening-point temperature. In oil-base muds, it is used as a fluid-loss control agent. Being a hydrocarbon, it is naturally wetted by the oil. In water-base muds, it is used as a shale-stabilizing additive and is difficult to evaluate unless tested at or above its softening point. As a hydrocarbon, the powder must be coupled to water by using a glycol or similar water-wetter. |
| Drilling Fluids | glass jar test | A qualitative pilot test used to determine whether water-wet solids (for example, barite) exist in an oil-base mud. An oil mud is put into a clean, round, glass quart jar filled about 1/3 full. The mud is stirred on a commercial-grade mixer or blender for 15 minutes. The mud is poured out and the jar, drained and examined. Solids strongly adhering to the glass are indicative of the presence of water-wet solids in the mud. The test has been correlated with inside plugging of drillpipe, which occurs when an oil mud becomes under-treated with oil-wetting agent. The electrical stability test cam be used to evaluate wettability and emulsion quality along with evidence of free water in high-pressure, high-temperature filtrate. |
| Drilling Fluids | glycol | A series of alcohols with general formula C2nH4n+2On+1. The simplest member is ethylene glycol C2H6O2, widely used as antifreeze. Glycols may be used in drilling fluids as gas hydrate inhibitors. |
| Drilling Fluids | granular LCM | A type of lost-circulation material (LCM) that is chunky in shape and prepared in a range of particle sizes. Granular LCM is added to mud and placed downhole to help retard the loss of mud into fractures or highly permeable formations. Ideally, granular LCM should be insoluble and inert to the mud system in which it is used. Examples are ground and sized limestone or marble, wood, nut hulls, Formica, corncobs and cotton hulls. Often, granular, flake and fiber LCMs are mixed together into an LCM pill and pumped into the well next to the loss zone to seal the formation into which circulation is lost. |
| Drilling Fluids | granular lost circulation material | A type of lost-circulation material that is chunky in shape and prepared in a range of particle sizes. Granular LCM is added to mud and placed downhole to help retard the loss of mud into fractures or highly permeable formations. Ideally, granular LCM should be insoluble and inert to the mud system in which it is used. Examples are ground and sized limestone or marble, wood, nut hulls, Formica, corncobs and cotton hulls. Often, granular, flake and fiber LCMs are mixed together into an LCM pill and pumped into the well next to the loss zone to seal the formation into which circulation is lost. |
| Drilling Fluids | granular lost-circulation material | A type of lost-circulation material that is chunky in shape and prepared in a range of particle sizes. Granular LCM is added to mud and placed downhole to help retard the loss of mud into fractures or highly permeable formations. Ideally, granular LCM should be insoluble and inert to the mud system in which it is used. Examples are ground and sized limestone or marble, wood, nut hulls, Formica, corncobs and cotton hulls. Often, granular, flake and fiber LCMs are mixed together into an LCM pill and pumped into the well next to the loss zone to seal the formation into which circulation is lost. |
| Drilling Fluids | gray list | Products ‘requiring strong regulatory control,’ as determined by the Oslo and Paris Commission (OSPAR). The list includes heavy metals such as zinc, lead and chromium. OSPAR, formerly known as PARCOM, is a group of experts who advise North Sea countries on environmental policy and legislation. The group has been influential in establishing North Sea legislation on drilling fluids that has served as the model for other operating areas. The Commission has published lists of environmentally acceptable and unacceptable products, referred to as the “green,” “grey” and “black” lists. |
| Drilling Fluids | greasing out | The action in which mud particles (barite) become coated with an oily substance that causes the particles to agglomerate. This can cause barite settlement or removal by shaker screens. The problem can occur when mud additives, such as fatty-acid soaps or mud lubricants, are incompatible with the mud system or react adversely to contaminating ions. |
| Drilling Fluids | green list | Products posing relatively little harm to the environment (specifically the marine environment), as determined by the Oslo and Paris Commission (OSPAR). Examples of products that comprise the Green or A list include inert minerals such as bentonite, inorganic salts that are common constituents of seawater such as sodium and potassium chloride, and simple organic products such as starch and carboxymethylcellulose (CMC). OSPAR, formerly known as PARCOM, is a group of experts who advise North Sea countries on environmental policy and legislation. The group has been influential in establishing North Sea legislation on drilling fluids that has served as the model for other operating areas. The Commission has published lists of environmentally acceptable and unacceptable products, referred to as the “green,” “grey” and “black” lists. |
| Drilling Fluids | grey list | Products ‘requiring strong regulatory control,’ as determined by the Oslo and Paris Commission (OSPAR). The list includes heavy metals such as zinc, lead and chromium. OSPAR, formerly known as PARCOM, is a group of experts who advise North Sea countries on environmental policy and legislation. The group has been influential in establishing North Sea legislation on drilling fluids that has served as the model for other operating areas. The Commission has published lists of environmentally acceptable and unacceptable products, referred to as the “green,” “grey” and “black” lists. |
| Drilling Fluids | grind | The fineness to which cement is ground. Grind also may refer to a specific production of cement, such as the lot number. |
| Drilling Fluids | guar gum | A hydrophilic polysaccharide from the seed of the guar plant. It is a galactomannan type of saccharide that is highly dispersible into water and brines of various types and salinity. Its water solutions are non-Newtonian and also can be cross-linked by borax to give very high gel strength for suspension. Such a structure is easily broken by breakers in fracturing fluids, so it serves as a carrier for placing sand far back into fractures. It is also used as a top-hole drilling fluid. Disadvantages of using guar gum include its lack of thermal stability and sensitivity to high pH and bacterial fermentation. |
| Drilling Fluids | gumbo | A nonspecific type of shale that becomes sticky when wet and adheres aggressively to surfaces. It forms mud rings and balls that can plug the annulus, the flowline and shale-shaker screens. Gumbo is likely to contain appreciable amounts of Ca+2 smectite clays. It is dispersed in a water mud, causing rapid accumulations of colloidal solids. |
| Drilling Fluids | gun the pits | To strongly agitate the mud pits, particularly on bottom and in corners, with the mud guns. Gunning the pits lifts settled barite, which can result in sudden, perhaps undesirable, increase in mud density in the pits. |
| Drilling Fluids | gunk plug | A slurry that consists of bentonite, cement or polymers mixed into an oil; bentonite in diesel oil is commonly used as a gunk plug. A small batch of the slurry is pumped down a well that has lost circulation to seal the leaky zone. The gunk plug may or may not be squeezed by pressure into the zone. Water downhole interacts with the bentonite, cement or polymers to make a sticky gunk. |
| Drilling Fluids | gunk squeeze | The operation of squeezing a gunk plug into a zone of lost circulation. When spotted next to the zone, the annular blowout preventers are closed and pressure is applied by further pumping to force the gunk into the loss zone. |
| Drilling Fluids | gunning the pits | The act of strongly agitating the mud pits, particularly on bottom and in corners, with the mud guns. Gunning the pits lifts settled barite, which can result in sudden, perhaps undesirable, increase in mud density in the pits. |
| Drilling Fluids | gyp mud | A calcium-based water mud system containing gypsum. Gyp mud can be used for drilling shales, but it is also well-suited for drilling gypsum, anhydrite and salt stringers. An advantage of gyp over lime muds is that the pH of gyp mud need not be so high because it contains more soluble Ca+2 to inhibit shale swelling. Gypsum, CaSO4·2H2O, content is measured by an API test, and more can be added as needed. A calcium-tolerant clay deflocculant may be needed to control viscosity. Carboxymethylcellulose (CMC) and starch are used for fluid loss control along with a small amount of prehydrated bentonite. |
| Drilling Fluids | haematite | The mineral form of ferric oxide [Fe2O3]. The hematite ore used as a weighting material in drilling muds has a mica-like crystal structure that grinds to particle size suitable for use in drilling fluids. To check for potential wear, an abrasion test is usually run on hematite as a quality control pilot test. |
| Drilling Fluids | hard water | Water that contains hardness ions. |
| Drilling Fluids | hardness ion | One of three divalent cations that can be present in water, including calcium (Ca+2), magnesium (Mg+2) and ferrous (Fe+2, a form of iron). Hardness ions develop from dissolved minerals, bicarbonate, carbonate, sulfate and chloride. Bicarbonate salts cause temporary hardness, which can be removed by boiling the water and leaving behind a calcium carbonate solid. Mg+2 and Fe+2 ions can be removed by raising the pH (with NaOH or KOH) and then allowing the precipitated Fe(OH)2 and Mg(OH)2 to settle out. Calcium hardness can be removed by adding excess sodium carbonate to precipitate Ca+2 as CaCO3. Hard water can be passed through an ion exchange column where hardness ions are captured on the resin. Removal of hardness is the process called water softening. |
| Drilling Fluids | HE starch | A nonionic starch derivative, analogous to hydroxyethylcellulose in its method of manufacture and most applications for drilling, workover and completion fluids. Rather than using a cellulosic starting material, such as hydroxyethylcellulose (HEC), starch is used instead, and reacted with ethylene oxide in an alkaline environment. |
| Drilling Fluids | header box | A small box mounted on a shaker screen that takes drilling fluid from the return flow line and distributes it across the surface of the screens via adjustable weirs. |
| Drilling Fluids | heavy metal | A term used by US Environmental Protection Agency (EPA) to specify the elements cadmium (Cd) and mercury (Hg). In a broader sense, the term can be used to specify other metals for which environmental concerns exist, such as copper, lead, chromium, nickel, arsenic and zinc. NPDES permits for offshore drilling set limits on Cd and Hg concentrations in barite that go into drilling fluids to be discharged offshore. Cadmium sulfide and mercury sulfide are minerals associated with barite ores. Maximum concentrations are Cd >= 1 and Hg >= 3 ppm (mg/kg). |
| Drilling Fluids | heavy metal | In general chemistry, the term refers to metals that are more dense than iron, although some texts and chemical dictionaries do not recognize this as a chemical term. |
| Drilling Fluids | HEC | A nonionic cellulose derivative with hydroxyethyl groups attached to the polymer structure. HEC is used as a viscosifier in brines and saline fracturing fluids, workover fluids, completion fluids and drill-in fluids. It gives pseudoplastic rheology but essentially no gel strength development. HEC offers little fluid-loss control, other than its rheological effects. HEC is seldom used in drilling fluids. Cellulose fibers are reacted with caustic soda and ethylene oxide to form HEC. Hydroxyethyl groups attach to the OH groups of the polysaccharide structure by ether linkages. A high degree of substitution (from 1.5 to 2.5 out of 3 maximum) gives HEC superior solubility in water and various brines. Being nonionic, it is not precipitated by hardness ions and disperses well at high salinity. HEC is not degraded by common bacteria. |
| Drilling Fluids | hectorite | A clay mineral similar in structure to bentonite but with more negative charges on its surface. Organophilic hectorite, made by the wet process, is a premium performance additive for use in oil-base drilling mud. |
| Drilling Fluids | hematite | The mineral form of ferric oxide [Fe2O3]. The hematite ore used as a weighting material in drilling muds has a mica-like crystal structure that grinds to particle size suitable for use in drilling fluids. To check for potential wear, an abrasion test is usually run on hematite as a quality control pilot test. |
| Drilling Fluids | Herschel Bulkley fluid | A fluid described by a three-parameter rheological model. A Herschel-Bulkley fluid can be described mathematically as follows: ? = ?0 + k(?)n, where ? = shear stress ?0 = yield stress k = consistency factor ? = shear rate n = flow index, a power law exponent. The Herschel-Bulkley equation is preferred to power law or Bingham relationships because it results in more accurate models of rheological behavior when adequate experimental data are available. The yield stress is normally taken as the 3 rpm reading, with the n and K values then calculated from the 600 or 300 rpm values or graphically. |
| Drilling Fluids | Herschel-Bulkley fluid | A fluid described by a three-parameter rheological model. A Herschel-Bulkley fluid can be described mathematically as follows: ? = ?0 + k(?)n, where ? = shear stress ?0 = yield stress k = consistency factor ? = shear rate n = flow index, a power law exponent. The Herschel-Bulkley equation is preferred to power law or Bingham relationships because it results in more accurate models of rheological behavior when adequate experimental data are available. The yield stress is normally taken as the 3 rpm reading, with the n and K values then calculated from the 600 or 300 rpm values or graphically. Reference: Hemphill T, Campos W and Pilehvari A: Yield-Power Law Model More Accurately Predicts Mud Rheology, Oil & Gas Journal 91, no. 34 (August 23, 1993): 45–50. |
| Drilling Fluids | HGS | Dense solids, such as barite or hematite, which are added to a mud to increase its density, also known as weighting material. The concentration of high-gravity solids in a weighted mud is measured by the mud engineer daily using mud weight, retort data, chloride titration data and other information. Solids are reported as lbm/bbl or vol.%. The specific gravity of water is 1.00, barite is 4.20, and hematite 5.505 g/cm3. Drill solids and other low-gravity solids are normally assumed to be 2.60 g/cm3. |
| Drilling Fluids | high-gravity solids | Dense solids, such as barite or hematite, which are added to a mud to increase its density, also known as weighting material. The concentration of high-gravity solids in a weighted mud is measured by the mud engineer daily using mud weight, retort data, chloride titration data and other information. Solids are reported as lbm/bbl or vol.%. The specific gravity of water is 1.00, barite is 4.20, and hematite 5.505 g/cm3. Drill solids and other low-gravity solids are normally assumed to be 2.60 g/cm3. |
| Drilling Fluids | high-pressure, high-temperature filtration test | A test to measure static filtration behavior of water mud or oil mud at elevated temperature, up to about 380°F [193°C] maximum (450°F [227°C] maximum if a special cell is used), usually according to the specifications of API. Although the test can simulate downhole temperature conditions, it does not simulate downhole pressure. Total pressure in a cell should not exceed 700 psi [4900 kPa], and the differential pressure across the filter medium is specified as 500 psi [3500 kPa]. Because these cells are half the size of the ambient filtration area, HPHT filtrate volumes after 30 minutes are doubled. |
| Drilling Fluids | high-pressure, high-temperature viscometer | A type of viscometer generally used in laboratories to test drilling fluids at simulated downhole conditions. |
| Drilling Fluids | high-specific-gravity solids | Dense solids, such as barite or hematite, which are added to a mud to increase its density, also known as weighting material. The concentration of high-gravity solids in a weighted mud is measured by the mud engineer daily using mud weight, retort data, chloride titration data and other information. Solids are reported as lbm/bbl or vol.%. The specific gravity of water is 1.00, barite is 4.20, and hematite 5.505 g/cm3. Drill solids and other low-gravity solids are normally assumed to be 2.60 g/cm3. |
| Drilling Fluids | HLB number | A number on the scale of one to 40 according to the HLB system, introduced by Griffin (1949 and 1954). The HLB system is a semi-empirical method to predict what type of surfactant properties a molecular structure will provide. The HLB system is based on the concept that some molecules have hydrophilic groups, other molecules have lipophilic groups, and some have both. Weight percentage of each type of group on a molecule or in a mixture predicts what behavior the molecular structure will exhibit. Water-in-oil emulsifiers have a low HLB numbers, typically around 4. Solubilizing agents have high HLB numbers. Oil-in-water emulsifiers have intermediate to high HLB numbers. Reference: Griffin WC: “Classification of Surface-Active Agents by ‘HLB,'” Journal of the Society of Cosmetic Chemists 1 (1949): 311. Reference: Griffin WC: “Calculation of HLB Values of Non-Ionic Surfactants,” Journal of the Society of Cosmetic Chemists 5 (1954): 259. |
| Drilling Fluids | hole cleaning | The ability of a circulating drilling fluid to transport rock fragments out of a wellbore. Carrying capacity is an essential function of a drilling fluid, synonymous with hole-cleaning capacity and cuttings lifting. Carrying capacity is determined principally by the annular velocity, hole angle and flow profile of the drilling fluid, but is also affected by mud weight, cuttings size and pipe position and movement. |
| Drilling Fluids | hot lime | A chemical with formula CaO, commonly called quick lime or hot lime. When hydrated with one mole of water, it forms slaked lime, Ca(OH)2. Quick lime is used in preference to slaked lime at oil mud mixing plants because it generates heat when it becomes slaked with water and therefore speeds up emulsification by the reaction to form calcium fatty-acid soap. |
| Drilling Fluids | HP starch | Hydroxypropyl starch is a derivative of natural starch, used primarily for fluid-loss control in drilling muds, drill-in, completion and workover fluids. Being nonionic, it is only slightly affected by salinity and hardness in fluids. Linear and branched carbohydrate polymers in natural starch have three reactive OH groups on each glucose unit. During manufacture, these polymers are reacted with propylene oxide, adding hydroxypropyl (CH(OH)CH2CH3) groups at the OH positions by an ether linkage. By adding the hydroxypropyl groups, the HP starch becomes more resistant to thermal degradation and bacterial attack. |
| Drilling Fluids | HPHT filtration test | A test to measure static filtration behavior of water mud or oil mud at elevated temperature, up to about 380°F [193°C] maximum (450°F [227°C] maximum if a special cell is used), usually according to the specifications of API. Although the test can simulate downhole temperature conditions, it does not simulate downhole pressure. Total pressure in a cell should not exceed 700 psi [4900 kPa], and the differential pressure across the filter medium is specified as 500 psi [3500 kPa]. Because these cells are half the size of the ambient filtration area, HPHT filtrate volumes after 30 minutes are doubled. |
| Drilling Fluids | HPHT viscometer | A type of viscometer generally used in laboratories to test drilling fluids at simulated downhole conditions. |
| Drilling Fluids | HSE | Abbreviation for health, safety and environmental. These three issues are of paramount importance to the drilling and drilling fluids community, as they are to the entire petroleum industry. Adherence to HSE guidelines is a requirement for operators worldwide and is also dictated by internal policies of most corporations. |
| Drilling Fluids | humic acid | Organic carboxylic acids of complex molecular structure (aromatic and phenolic) that comprise 10 to 90% of lignite. Humic acids in lignite react with caustic ingredients (NaOH and KOH) in mud. The water solubility of lignite depends on its humic acid content. Decarboxylation of humic acid groups by hydrolysis in alkaline muds is a major source of carbonate and bicarbonate anions in water muds. |
| Drilling Fluids | humidity | Moisture (water vapor) in a gaseous atmosphere, such as in air. It is quantified as relative humidity. |
| Drilling Fluids | humidity meter | A device to measure humidity. |
| Drilling Fluids | hydrate | For a hygroscopic material such as a clay or polymer to absorb water. Hydration is the first stage of clay-water (or polymer-water) interaction. When dry bentonite is stirred into water, hydration is observed as swelling. |
| Drilling Fluids | hydration | Absorption of water by a hygroscopic material such as a clay or polymer. Hydration is the first stage of clay-water (or polymer-water) interaction. When dry bentonite is stirred into water, hydration is observed as swelling. |
| Drilling Fluids | hydraulic cement | A substance which, when mixed with water, hardens like stone because of a chemical reaction with the water. Hydraulic cement is capable of setting under water. |
| Drilling Fluids | hydrocyclone | An item of solids-control equipment consisting of an inverted cone, the mud being fed tangentially into the upper (larger diameter) part. The resulting spinning effect forces solids to the wall of the device and they exit from the bottom (apex) of the cone, while the cleaned liquid exits at the top. Hydrocyclones are classified by the size of the cone as either desanders (typically 12 inches in diameter) or desilters (4 to 6 inches in diameter) and will separate particles in the medium-, fine- and ultrafine-size ranges. The efficiency of hydrocyclones is poor in viscous weighted muds and many units are being replaced by more efficient, high-speed shakers. |
| Drilling Fluids | hydrocycloning | Using a hydrocyclone, an item of solids-control equipment consisting of an inverted cone, the mud being fed tangentially into the upper (larger diameter) part. The resulting spinning effect forces solids to the wall of the device and they exit from the bottom (apex) of the cone, while the cleaned liquid exits at the top. Hydrocyclones are classified by the size of the cone as either desanders (typically 12 inches in diameter) or desilters (4 to 6 inches in diameter) and will separate particles in the medium-, fine- and ultrafine-size ranges. The efficiency of hydrocyclones is poor in viscous weighted muds and many units are being replaced by more efficient, high-speed shakers. |
| Drilling Fluids | hydrolysis | Any chemical reaction with water (H2O), such as degradation of lignite by decarboxylation of humic acid (a major component of lignite), which is driven by hydrolysis at high pH and begins at modest temperature. |
| Drilling Fluids | hydrometer | A weighted, hollow glass bulb with a long, graduated tube attached for measuring the density of a liquid. A hydrometer is placed in the liquid and the bulb sinks according to the density of the liquid. Graduations on the tube indicate the density. Hydrometers are used in fluids that have no gel strength, such as brine, but are not reliable in drilling fluids because of gelation. |
| Drilling Fluids | hydroxyethyl starch | A nonionic starch derivative, analogous to hydroxyethylcellulose in its method of manufacture and most applications for drilling, workover and completion fluids. Rather than using a cellulosic starting material, such as hydroxyethylcellulose (HEC), starch is used instead, and reacted with ethylene oxide in an alkaline environment. |
| Drilling Fluids | hydroxyethylcellulose | A nonionic cellulose derivative with hydroxyethyl groups attached to the polymer structure. HEC is used as a viscosifier in brines and saline fracturing fluids, workover fluids, completion fluids and drill-in fluids. It gives pseudoplastic rheology but essentially no gel strength development. HEC offers little fluid-loss control, other than its rheological effects. HEC is seldom used in drilling fluids. Cellulose fibers are reacted with caustic soda and ethylene oxide to form HEC. Hydroxyethyl groups attach to the OH groups of the polysaccharide structure by ether linkages. A high degree of substitution (from 1.5 to 2.5 out of 3 maximum) gives HEC superior solubility in water and various brines. Being nonionic, it is not precipitated by hardness ions and disperses well at high salinity. HEC is not degraded by common bacteria. |
| Drilling Fluids | hydroxypropyl starch | Hydroxypropyl starch is a derivative of natural starch, used primarily for fluid-loss control in drilling muds, drill-in, completion and workover fluids. Being nonionic, it is only slightly affected by salinity and hardness in fluids. Linear and branched carbohydrate polymers in natural starch have three reactive OH groups on each glucose unit. During manufacture, these polymers are reacted with propylene oxide, adding hydroxypropyl (CH(OH)CH2CH3) groups at the OH positions by an ether linkage. By adding the hydroxypropyl groups, the HP starch becomes more resistant to thermal degradation and bacterial attack. |
| Drilling Fluids | hygrometer | A device for measuring the moisture in a gaseous atmosphere, such as the air, usually as percent relative humidity. Mechanical hygrometers detect moisture by elongation and shrinkage of a fiber or sheet or by a device attached to a needle on a dial. Electrohygrometers measure changes in an electrical property of a moisture-sensitive sensing probe and are more reliable. Determination of the aqueous-phase activity of oil muds by the Chenevert Method requires an electrohygrometer and a series of salt solutions for calibration. |
| Drilling Fluids | hygroscopic | Pertaining to a property of a substance that allows the substance to take up water from the surrounding atmosphere. Many materials used in drilling muds are hygroscopic, for example, high-purity grades of calcium chloride. Bentonite clay is also hygroscopic and absorbs water from the atmosphere. Care must be taken in packaging and handling such materials to avoid waste by premature hydration. |
| Drilling Fluids | IEOM | An outdated distinction between two types of oil muds. In the past, invert-emulsion oil muds were those with more than 5 vol.% emulsified water, and oil-base muds were those with less than 5 vol.% water. Today, this distinction is not pertinent because the general term oil mud covers all water concentrations. |
| Drilling Fluids | ilmenite | A dense mineral whose specific gravity is 4.67 g/cm3, composed of FeO·TiO2. Ilmenite is used as a weighting agent for cement and mud. |
| Drilling Fluids | indicator | A visible sign of a chemical reaction. Examples include a color change, a voltage or pH reading or an inflection point on a graphical plot of the data. |
| Drilling Fluids | inert sulfide | A stable compound of sulfur that contains the S-2 ion. Sulfides can be generated from soluble iron sulfide minerals or from sulfate-reducing bacteria. Active sulfides, which revert to H2S gas when acidified and consequently pose a potential danger, may be converted to inert sulfides by adding zinc oxide. |
| Drilling Fluids | inhibit | To prevent, arrest or slow down any action. For example, one can inhibit a corrosion process by coating drillpipe with amine films to arrest pipe corrosion in air. In drilling fluids, the terms inhibit, inhibition and inhibitive mud system refer to arresting or slowing the hydration, swelling and disintegration of clays and shales. |
| Drilling Fluids | inhibition | Prevention, arrest or slowing down of any action. For example, one can inhibit a corrosion process by coating drillpipe with amine films to arrest pipe corrosion in air. In drilling fluids, the terms inhibit, inhibition and inhibitive mud system refer to arresting or slowing the hydration, swelling and disintegration of clays and shales. |
| Drilling Fluids | inhibitive mud | A mud that slows or stops hydration, swelling and disintegration of shales. A variety of mud types have been labeled as “inhibitive muds.” The degree of inhibition is not quantitative, but qualitatively they range from highly inhibitive (balanced-activity oil muds), moderately inhibitive (potassium muds and silicate muds), fairly inhibitive (calcium-based fluids) to slightly inhibitive (lignosulfonate, lignite water muds) to non-inhibitive (freshwater, nontreated muds). |
| Drilling Fluids | intermediate | Referring to any particle in the size range 250 to 2000 microns. |
| Drilling Fluids | internal phase | The discontinuous phase of an emulsion, the dispersed droplets of emulsified fluid. |
| Drilling Fluids | invert emulsion | An emulsion in which oil is the continuous or external phase and water is the internal phase. Invert emulsion usually refers to an oil-base mud, and the terms are considered synonyms. Invert-emulsion muds can be run with 5 to 50% water in the liquid phase, although there are systems that are 100% oil. |
| Drilling Fluids | invert emulsion oil mud | An outdated distinction between two types of oil muds. In the past, invert-emulsion oil muds were those with more than 5 vol.% emulsified water, and oil-base muds were those with less than 5 vol.% water. Today, this distinction is not pertinent because the general term oil mud covers all water concentrations. |
| Drilling Fluids | invert-emulsion oil mud | An outdated distinction between two types of oil muds. In the past, invert-emulsion oil muds were those with more than 5 vol.% emulsified water, and oil-base muds were those with less than 5 vol.% water. Today, this distinction is not pertinent because the general term oil mud covers all water concentrations. |
| Drilling Fluids | IO | A synthetic hydrocarbon liquid made by the polymerization of ethylene, H2C=CH2. IOs are one of several synthetic fluids that have recently been used as base for synthetic-base muds and in other applications where refined oils might otherwise be used except for HSE concerns. IOs are linear structures that have their olefin double bond in the center of the chain length. They are made by isomerization of linear alphaolefins (LAO), which have their double bond at the end of the chain. Because the olefin bond is in the central area of the chain, the physical properties of IOs are different (for example, they are generally lower viscosity) compared with the LAOs from which they are made. |
| Drilling Fluids | ion exchange | A process of exchanging one ion for another ion on a charged, solid substrate, such as a natural clay, zeolite or resin. Cation exchange on clay minerals occurs in muds and during mud testing. In muds, various positive ions (for example, Ca+2) on clay surfaces can be replaced by other positive ions (for example, Na+) that occur in higher concentration or have a higher affinity for the substrate. In water muds, cation exchange is an ongoing and dynamic process. In mud testing, the methylene blue dye is a cationic dye that goes onto clays in the mud sample being tested to show the extent of cation-exchange capacity. Ion exchange is the basis for clay peptization, which can improve a poorly performing clay. |
| Drilling Fluids | IP | Abbreviation for the Institute of Petroleum, a standardization body for the petroleum industry in Europe. Several industry-standard drilling-fluid tests are adopted from IP and ASTM procedures. |
| Drilling Fluids | IPA | A simple alcohol (C3H7OH) used as a solvent in some mud analyses. A 50/50 xylene/IPA mixture was used in the past as an emulsion breaker for oil mud but has been replaced with propylene glycol normal propyl ether (PNP). |
| Drilling Fluids | iron carbonate | A mineral composed of ferrous carbonate, FeCO3, and having 3.8 g/cm3 specific gravity. It is found as an accessory mineral in some shales and carbonate rocks and also in some barite and hematite ores. FeCO3 is readily soluble in acids and breaks down slowly in alkaline muds, particularly at high temperature to form a gelatinous solid, Fe(OH)2, and soluble CO3-2 anions. |
| Drilling Fluids | iron oxide | A group of minerals and inorganic compounds made up of iron that is in +2 (ferrous) and +3 (ferric) valence states and oxygen in the -2 valence state, such as ferrous oxide, FeO, and ferric oxide, Fe2O3. Fe3O4 is a mixture of ferric oxide and ferrous oxide that commonly occurs in a fine-grained, magnetic crystalline form. Hematite, Fe2O3, the most common iron oxide, exists in several crystalline forms. Other forms of hematite are too abrasive to use as weighting material in drilling fluids. |
| Drilling Fluids | iron sulfide | A group of compounds containing iron in +2 (ferrous) and +3 (ferric) valence states and sulfur in -2 (sulfide) valence states. Examples are ferric sulfide [Fe2S3], ferrous sulfide [FeS] and iron disulfide (FeS2, which is found in the minerals marcasite and pyrite). Fe2S3 and FeS2 are insoluble, but FeS dissolves in alkaline muds to form a gelatinous solid, Fe(OH)2, and soluble S-2 ions. FeS is the primary component in the mineral pyrrhotite, one of many possible sources of sulfides in water mud. Pyrrhotite might also coexist with BaSO4 in some barite ores. Even in trace amounts, FeS in a barite supply could generate enough sulfides in a weighted mud system to cause stress-corrosion cracking of drillpipe. |
| Drilling Fluids | ISO | Abbreviation for the International Organization for Standardization, a developer and publisher of international standards. ISO is a network of national-standards institutes from 157 countries, with one member per country and a Central Secretariat that coordinates the system from offices in Geneva, Switzerland. (Recognizing that the acronym for International Organization for Standardization would vary widely from one member country to the next, ISO founders decided to standardize the organization acronym. ISO is derived from the Greek isos, meaning equal.) API and ISO joint committees are developing standards for worldwide use. |
| Drilling Fluids | isomerized olefin | A synthetic hydrocarbon liquid made by the polymerization of ethylene, H2C=CH2. IOs are one of several synthetic fluids that have recently been used as base for synthetic-base muds and in other applications where refined oils might otherwise be used except for HSE concerns. IOs are linear structures that have their olefin double bond in the center of the chain length. They are made by isomerization of linear alphaolefins (LAO), which have their double bond at the end of the chain. Because the olefin bond is in the central area of the chain, the physical properties of IOs are different (for example, they are generally lower viscosity) compared with the LAOs from which they are made. |
| Drilling Fluids | isopropanol | A simple alcohol (C3H7OH) used as a solvent in some mud analyses. A 50/50 xylene/IPA mixture was used in the past as an emulsion breaker for oil mud but has been replaced with propylene glycol normal propyl ether (PNP). |
| Drilling Fluids | isopropyl alcohol | A simple alcohol (C3H7OH) used as a solvent in some mud analyses. A 50/50 xylene/IPA mixture was used in the past as an emulsion breaker for oil mud but has been replaced with propylene glycol normal propyl ether (PNP). |
| Drilling Fluids | jar test | A qualitative pilot test used to determine whether water-wet solids (for example, barite) exist in an oil-base mud. An oil mud is put into a clean, round, glass quart jar filled about 1/3 full. The mud is stirred on a commercial-grade mixer or blender for 15 minutes. The mud is poured out and the jar, drained and examined. Solids strongly adhering to the glass are indicative of the presence of water-wet solids in the mud. The test has been correlated with inside plugging of drillpipe, which occurs when an oil mud becomes under-treated with oil-wetting agent. The electrical stability test cam be used to evaluate wettability and emulsion quality along with evidence of free water in high-pressure, high-temperature filtrate. |
| Drilling Fluids | jet hopper | A mud-flow device, also called a mud hopper, through which materials are put into the circulating mud system. The mud hopper is powered by a centrifugal pump that flows the mud at high velocity through a venturi nozzle (jet) below the conical-shaped hopper. Dry materials are added through the mud hopper to provide dispersion, rapid hydration and uniform mixing. Liquids are sometimes fed into the mud by a hose placed in the hopper. |
| Drilling Fluids | kill weight fluid | A mud whose density is high enough to produce a hydrostatic pressure at the point of influx in a wellbore and shut off flow into the well. Kill-weight mud, when needed, must be available quickly to avoid loss of control of the well or a blowout. Thus, it is usually made by weighting up some of the mud in the system or in storage by adding barite or hematite. Unless diluted in advance, the mud may become too thick and perhaps un-pumpable due to high solids loading. A weight-up pilot test can identify if and how much dilution will be needed in advance of adding weighting material to the mud in the pits. |
| Drilling Fluids | kill-weight fluid | A mud with density high enough to produce a hydrostatic pressure at the point of influx in a wellbore that is sufficient to shut off flow into the well. Kill-weight mud, when needed, must be available quickly to avoid loss of control of the well or a blowout. Thus, it is usually made by weighting up some of the mud (link to ID 2107) in the system or in storage by adding barite or hematite. Unless diluted in advance, the mud may become too thick and perhaps un-pumpable due to high solids loading. A weight-up pilot test can identify if and how much dilution will be needed in advance of adding weighting material to the mud in the pits. |
| Drilling Fluids | kill-weight mud | A mud with density high enough to produce a hydrostatic pressure at the point of influx in a wellbore that is sufficient to shut off flow into the well. Kill-weight mud, when needed, must be available quickly to avoid loss of control of the well or a blowout. Thus, it is usually made by weighting up some of the mud (link to ID 2107) in the system or in storage by adding barite or hematite. Unless diluted in advance, the mud may become too thick and perhaps un-pumpable due to high solids loading. A weight-up pilot test can identify if and how much dilution will be needed in advance of adding weighting material to the mud in the pits. |
| Drilling Fluids | kilogram per cubic meter | The SI unit of measurement for density. Mud weights are typically expressed in kg/m3. The conversion factor from lbm/gal to kg/m3 is 120. For example, 12 lbm/gal = 1440 kg/m3. |
| Drilling Fluids | kilopascal | A unit of measurement for pressure in the International System of Units (SI), symbolized by kPa. The conversion factor from lb/in2 to kPa is 6.9 kPa per lbf/in2 (psi). For example, 5000 psi = 34,500 kPa. |
| Drilling Fluids | lag time | The time taken for cuttings to reach the surface. The term is also used in place of cycle time. |
| Drilling Fluids | LAO | A synthetic hydrocarbon liquid made by the polymerization of ethylene, H2C=CH2. LAOs and other synthetic fluids are used in synthetic-base drilling fluids and other applications in which refined oils might otherwise be used if not for HSE concerns. LAOs have a linear structure with a double bond (olefin) at the end of the chain, making them more biodegradable than other olefins. LAOs can be catalytically reacted to move the double bond toward the center of the chain length to convert them to isomerized olefins, IOs. |
| Drilling Fluids | latex | A colloidal suspension or emulsion of specific organic materials. Certain latices may be used as cement additives. Latex is used to provide gas-migration control, improve durability and improve bonding. It also offers excellent fluid-loss control. Latex additives also impart some acid resistance to cement. |
| Drilling Fluids | lbm/bbl | The abbreviation for concentration in US oilfield units, pounds per barrel. One lbm/bbl is the equivalent of one pound of additive in 42 US gallons of mud. The “m” is used to denote mass to avoid possible confusion with pounds force (denoted by “lbf”). Sometimes, lbm/bbl is written as ppb, but must not be confused with parts per billion. In SI units, the conversion factor is one pound per barrel equals 2.85 kilograms per cubic meter. For example, 10 lbm/bbl = 28.5 kg/m3. |
| Drilling Fluids | LC50 | The lethal concentration of a substance, reported in ppm, that kills 50% of a population of test organisms, such as mysid shrimp, in a standard, controlled laboratory bioassay test. In offshore drilling operations, the LC50 number is used to determine whether waste mud or cuttings can be discharged into the water. The larger the LC50 ppm number from the test, the less toxic the sample is to the organism. For example, if LC50 number is 1,000,000 ppm, the sample is presumably nontoxic according to the test protocol. |
| Drilling Fluids | lead cement | A cement system used to provide zonal isolation across generally nonproductive zones located above the zones of interest. |
| Drilling Fluids | leonardite | A mineral component of lignite, which is similar to brown coal. Lignite is found in surface deposits worldwide. Lignite is mined and put into piles where it can oxidize in the air before it is dried, ground and bagged for use in drilling fluids. The humic acid content of lignite, which varies widely, controls its solubility. The soluble and colloidal lignite components both help in fluid-loss control. Soluble components serve as clay deflocculants and improve filter cake quality. Colloidal lignite helps plug off the permeable parts of filter cake. When straight lignite is added to a mud, caustic soda is also needed to make it dissolve. Precaustisized lignite is available, which contains NaOH or KOH already mixed. Adding chromium salts improves high-temperature performance, but their use is limited by HSE concerns. Organophilic lignite is a straight lignite that has been treated with quaternary amine compounds to make it oil dispersible in oil- and synthetic-base muds. |
| Drilling Fluids | LGS | A type of drilling-fluid solid having a lower density than the barite or hematite that is used to weight up a drilling fluid, including drill solids plus the added bentonite clay. The mud engineer calculates the concentration of these and other types of solids on the basis of mud weight, retort analysis, chloride titrations and other information. Solids are reported as lbm/bbl or vol.%. Water is 1.0, barite 4.20, and hematite 5.505 g/cm3. Low-gravity solids are normally assumed to have a density of 2.60 g/cm3.. |
| Drilling Fluids | lignin | The component of a tree that is extracted in the paper-manufacturing process and used as an additive in drilling fluids. Specifically, lignin is a highly polymerized, amorphous material that makes up the middle lamella of woody fibers and cements the fibers together. Methoxy groups are abundant on the lignin structure, giving lignin many reactive sites and promoting its water solubility. In paper manufacturing, lignin is dissolved from wood chips. In the sulfite paper process, the liquor byproduct contains wood sugars and lignosulfonate. The wood sugars are removed and the lignosulfonate is used as a clay deflocculant. In the kraft paper process, lignin is solubilized by caustic soda. Kraft lignin must be further reacted to make a functional drilling-fluid additive. |
| Drilling Fluids | lignite | A type of coal. Lignite is found in surface deposits worldwide. Lignite is mined and put into piles where it can oxidize in the air before it is dried, ground and bagged for use in drilling fluids. The humic acid content of lignite, which varies widely, controls its solubility. The soluble and colloidal lignite components both help in fluid-loss control. Soluble components serve as clay deflocculants and improve filter cake quality. Colloidal lignite helps plug off the permeable parts of filter cake. When straight lignite is added to a mud, caustic soda is also needed to make it dissolve. Precaustisized lignite is available, which contains NaOH or KOH already mixed. Adding chromium salts improves high-temperature performance, but their use is limited by HSE concerns. Organophilic lignite is a straight lignite that has been treated with quaternary amine compounds to make it oil dispersible in oil- and synthetic-base muds. |
| Drilling Fluids | lignosulfonate | A highly anionic polymer used to deflocculate clay-based muds. Lignosulfonate is a byproduct of the sulfite method for manufacturing paper from wood pulp. Sometimes it is called sulfonated lignin. Lignosulfonate is a complex mixture of small- to moderate-sized polymeric compounds with sulfonate groups attached to the molecule. LS became a popular deflocculant in the late 1950s as a replacement for quebracho. Chromium and iron compounds were admixed to get stability benefits at higher temperature. These were called chrome lignosulfonates (CLS) and ferro-chrome lignosulfonates (FCLS). There is less chrome is in most CLS than in the past (now about 2.5 to 3 %), and chrome-free products are available. |
| Drilling Fluids | lime mud | A type of water-base mud that is saturated with lime, Ca(OH)2, and has excess, undissolved lime solids maintained in reserve. Lime muds are classified according to excess lime content: (1) low-lime, 0.5 to 2 lbm/bbl, (2) medium-lime, 2 to 4 lbm/bbl and (3) high-lime, over 4 lbm/bbl. All lime muds have pH in the range of 12, and the filtrate is saturated with lime. Fluid-loss additives include starch, HP-starch, carboxymethylcellulose (CMC) or polyanionic cellulose (PAC). Prehydrated bentonite can improve the fluid loss and rheology of a lime mud. A maltodextrin in lime muds has been used as a clay deflocculant, a shale stabilizer and to increase calcium solubility. KCl in lime muds has been another innovation for successful drilling of hydratable shales. The ability to carry very high mud alkalinity (as excess lime) to neutralize acid gases is one reason lime muds are used. H2S zones can be drilled with more safety and copious amounts of CO2 can be neutralized by a large excess of lime. |
| Drilling Fluids | linear alphaolefin | A synthetic hydrocarbon liquid made by the polymerization of ethylene, H2C=CH2. LAOs and other synthetic fluids are used in synthetic-base drilling fluids and other applications in which refined oils might otherwise be used if not for HSE concerns. LAOs have a linear structure with a double bond (olefin) at the end of the chain, making them more biodegradable than other olefins. LAOs can be catalytically reacted to move the double bond toward the center of the chain length to convert them to isomerized olefins, IOs. |
| Drilling Fluids | liquid additive | A material used in a liquid form to modify the properties of cement for use in oil- or gas-well cementing. |
| Drilling Fluids | lost circulation | A lack of mud returning to the surface after being pumped down a well. Lost circulation occurs when the drill bit encounters natural fissures, fractures or caverns, and mud flows into the newly available space. Lost circulation may also be caused by applying more mud pressure (that is, drilling overbalanced) on the formation than it is strong enough to withstand, thereby opening up a fracture into which mud flows. |
| Drilling Fluids | lost returns | Another term for lost circulation, a lack of mud returning to the surface after being pumped down a well. Lost circulation occurs when the drill bit encounters natural fissures, fractures or caverns, and mud flows into the newly available space. Lost circulation may also be caused by applying more mud pressure (that is, drilling overbalanced) on the formation than it is strong enough to withstand, thereby opening up a fracture into which mud flows. |
| Drilling Fluids | lost-circulation material | The collective term for substances added to drilling fluids when drilling fluids are being lost to the formations downhole. Commonly used lost-circulation materials include are fibrous (cedar bark, shredded cane stalks, mineral fiber and hair), flaky (mica flakes and pieces of plastic or cellophane sheeting) or granular (ground and sized limestone or marble, wood, nut hulls, Formica, corncobs and cotton hulls). Laymen have likened lost-circulation materials to the “fix-a-flat” materials for repair of automobile tires. |
| Drilling Fluids | low colloid oil mud | An oil mud designed and maintained with a minimum of colloid-sized solids, typically by omitting fatty-acid soap and lime, and minimizing organophilic clays and fluid-loss additives. Low-colloid oil mud, also called a relaxed filtrate oil mud, increases drilling rate. A disadvantage is that filter cake formed on sands is not tight, can quickly become very thick, and can cause pipe to stick by differential pressure. |
| Drilling Fluids | low gravity solids | A type of drilling-fluid solid having a lower density than the barite or hematite that is used to weight up a drilling fluid, including drill solids plus the added bentonite clay. The mud engineer calculates the concentration of these and other types of solids on the basis of mud weight, retort analysis, chloride titrations and other information. Solids are reported as lbm/bbl or vol.%. Water is 1.0, barite 4.20, and hematite 5.505 g/cm3. Low-gravity solids are normally assumed to have a density of 2.60 g/cm3. |
| Drilling Fluids | low solids mud | A mud that has fewer solids than conventional clay-based muds of the same density and similar use. Low-solids mud design and maintenance is accomplished primarily by substituting one or more polymers for the ordinary bentonite clay. Viscosity can be obtained either entirely by polymers or by using a premium quality (nontreated) bentonite along with the appropriate extender polymer. Together, these give rheology comparable to that of a higher concentration of ordinary bentonite. Polyanionic cellulose (PAC) may be needed for fluid-loss control. XC polymer can be effective for cuttings carrying. By combining premium bentonite and the right extender polymer, PAC and XC polymer, solids can be kept low, if solids control is required. This concept applies best to low-density muds, below about 13 lbm/gal, but has some validity in all muds . |
| Drilling Fluids | low solids nondispersed mud | A low-solids mud in which there is no clay deflocculant chemical. |
| Drilling Fluids | low specific gravity solids | A type of drilling-fluid solid having a lower density than the barite or hematite that is used to weight up a drilling fluid, including drill solids plus the added bentonite clay. The mud engineer calculates the concentration of these and other types of solids on the basis of mud weight, retort analysis, chloride titrations and other information. Solids are reported as lbm/bbl or vol.%. Water is 1.0, barite 4.20, and hematite 5.505 g/cm3. Low-gravity solids are normally assumed to have a density of 2.60 g/cm3. |
| Drilling Fluids | low yield clay | Native clays that are generally unsuitable for use in a clay-based drilling mud. Low-yield clays are considered to be drill solids, although they may give high values for bentonite-equivalent in a mud according to the methylene blue test. |
| Drilling Fluids | low-colloid oil mud | An oil mud designed and maintained with a minimum of colloid-sized solids, typically by omitting fatty-acid soap and lime, and minimizing organophilic clays and fluid-loss additives. Low-colloid oil mud, also called a relaxed filtrate oil mud, increases drilling rate. A disadvantage is that filter cake formed on sands is not tight, can quickly become very thick, and can cause pipe to stick by differential pressure. |
| Drilling Fluids | low-gravity solids | A type of drilling-fluid solid having a lower density than the barite or hematite that is used to weight up a drilling fluid, including drill solids plus the added bentonite clay. The mud engineer calculates the concentration of these and other types of solids on the basis of mud weight, retort analysis, chloride titrations and other information. Solids are reported as lbm/bbl or vol.%. Water is 1.0, barite 4.20, and hematite 5.505 g/cm3. Low-gravity solids are normally assumed to have a density of 2.60 g/cm3. |
| Drilling Fluids | low-pressure, low-temperature filtration test | A test to measure static filtration behavior of water mud at ambient (room) temperature and 100-psi differential pressure, usually performed according to specifications set by API, using a static filter press. The filter medium is filter paper with 7.1 sq. in. filtering area. A half-size cell is sometimes used, in which case the filtrate volume is doubled. |
| Drilling Fluids | low-solids mud | A mud that has fewer solids than conventional clay-based muds of the same density and similar use. Low-solids mud design and maintenance is accomplished primarily by substituting one or more polymers for the ordinary bentonite clay. Viscosity can be obtained either entirely by polymers or by using a premium quality (nontreated) bentonite along with the appropriate extender polymer. Together, these give rheology comparable to that of a higher concentration of ordinary bentonite. Polyanionic cellulose (PAC) may be needed for fluid-loss control. XC polymer can be effective for cuttings carrying. By combining premium bentonite and the right extender polymer, PAC and XC polymer, solids can be kept low, if solids control is required. This concept applies best to low-density muds, below about 13 lbm/gal, but has some validity in all muds . |
| Drilling Fluids | low-solids, nondispersed mud | A low-solids mud in which there is no clay deflocculant chemical. |
| Drilling Fluids | low-specific-gravity solids | A type of drilling-fluid solid having a lower density than the barite or hematite that is used to weight up a drilling fluid, including drill solids plus the added bentonite clay. The mud engineer calculates the concentration of these and other types of solids on the basis of mud weight, retort analysis, chloride titrations and other information. Solids are reported as lbm/bbl or vol.%. Water is 1.0, barite 4.20, and hematite 5.505 g/cm3. Low-gravity solids are normally assumed to have a density of 2.60 g/cm3. |
| Drilling Fluids | low-yield clay | Native clays that are generally unsuitable for use in a clay-based drilling mud. Low-yield clays are considered to be drill solids, although they may give high values for bentonite-equivalent in a mud according to the methylene blue test. |
| Drilling Fluids | LS | A highly anionic polymer used to deflocculate clay-based muds. Lignosulfonate is a byproduct of the sulfite method for manufacturing paper from wood pulp. Sometimes it is called sulfonated lignin. Lignosulfonate is a complex mixture of small- to moderate-sized polymeric compounds with sulfonate groups attached to the molecule. LS became a popular deflocculant in the late 1950s as a replacement for quebracho. Chromium and iron compounds were admixed to get stability benefits at higher temperature. These were called chrome lignosulfonates (CLS) and ferro-chrome lignosulfonates (FCLS). There is less chrome is in most CLS than in the past (now about 2.5 to 3 %), and chrome-free products are available. |
| Drilling Fluids | lubricant | A mud additive for lowering torque (rotary friction) and drag (axial friction) in the wellbore and to lubricate bit bearings if not sealed. Lubricants may be solids, such as plastic beads, glass beads, nut hulls and graphite, or liquids, such as oils, synthetic fluids, glycols, modified vegetable oils, fatty-acid soaps and surfactants. |
| Drilling Fluids | lubricity | A measure of the degree of lubrication. |
| Drilling Fluids | lyophilic | A descriptive term for the strong affinity that a solid material (usually a colloid) has for the liquid in which the solid is dispersed. For example, clay is a lyophilic colloid in water. |
| Drilling Fluids | lyophilic colloid | A colloid with a strong affinity for the liquid in which it is dispersed. For example, clay is a lyophilic colloid in water. |
| Drilling Fluids | lyophobic | A descriptive term for the lack of affinity (or repulsion) that a solid material has for the liquid in which the solid is dispersed. For example clay is lyophobic to oil. |
| Drilling Fluids | lyophobic colloid | A colloid that repels or lacks affinity for the liquid in which it is dispersed. For example, clay is lyophobic to oil. |
| Drilling Fluids | Mf | A measure of the total amount of hydroxyl ions in a solution as determined by titration with standardized acid and methyl orange indicator. The alkalinity test is a well-known water-analysis procedure to estimate hydroxyl ion, carbonate ion and bicarbonate ion concentrations. There are two pH endpoints, P and M, in this titration, corresponding to phenolphthalein and methyl orange indicators. The “P” endpoint is at pH 8.3 and the “M” endpoint is at pH 4.3. Each is reported in units of cm3 acid/cm3 sample. Mf therefore represents the amount of acid required, per unit volume of the sample, to reduce the pH to 4.3. For water samples and very simple mud filtrates, P and M data indicate OH-, HCO3- and CO3-2 concentrations, but an alkalinity test is unreliable for analyzing complex mud filtrates. The API has established standards for conducting alkalinity tests. |
| Drilling Fluids | magnesium test | A procedure for determining magnesium ion (Mg+2) concentration in a water-base drilling fluid based upon analyses for both calcium and total hardness. The standard test has been proscribed by API. Magnesium ion (Mg+2) concentration is calculated by subtracting calcium (Ca+2) analysis results from total hardness analysis results. |
| Drilling Fluids | make up water | Water added to maintain or dilute a water-mud system. The added water may be fresh water, seawater or salt water, as appropriate for the mud. Make-up water volume is an important parameter in a material balance check on solids content and solids removal efficiency for a mud system. The amount of dilution strongly influences mud economics. If soft make-up water is needed, treatments to remove hardness ions should be done prior to adding the water to the mud to avoid clay flocculation and polymer precipitation. |
| Drilling Fluids | make-up water | Water added to maintain or dilute a water-mud system. The added water may be fresh water, seawater or salt water, as appropriate for the mud. Make-up water volume is an important parameter in a material balance check on solids content and solids removal efficiency for a mud system. The amount of dilution strongly influences mud economics. If soft make-up water is needed, treatments to remove hardness ions should be done prior to adding the water to the mud to avoid clay flocculation and polymer precipitation. |
| Drilling Fluids | Marsh funnel | A conical-shaped funnel, fitted with a small-bore tube on the bottom end through which mud flows under a gravity head. A screen over the top removes large particles that might plug the tube. In the test standardized by API for evaluating water-base and oil-base muds, the funnel viscosity measurement is the time (in seconds) required for one quart of mud to flow out of a Marsh funnel into a graduated mud cup. Funnel viscosity is reported in seconds (for a quart). Water exits the funnel in about 26 seconds. This test was one of the earliest mud measurements for field use. Simple, quick and fool-proof, it still serves as a useful indicator of change in the mud by comparing mud-in and mud-out sample funnel viscosities. Hallan N. Marsh of Los Angeles published the design and use of his funnel viscometer in 1931, and it is worth the time to read the detailed, often humorous, discussion that followed. Mr. Marsh was a forward thinking mud technologist in his day, as can be seen from the following words from his 1931 AIME paper: The subject of mud sounds so simple, uninteresting and unimportant that it has failed to receive the attention that it deserves, at least as applied to the drilling of oil wells. As a matter of fact, it is one of the most complicated, technical, important and interesting subjects in connection with rotary drilling. Mr. Marsh was quoted by someone who knew him as saying (paraphrased), Of all the things I have done in mud technology, I am remembered for inventing this d*** funnel. |
| Drilling Fluids | Marsh funnel viscosity | Time, in seconds for one quart of mud to flow through a Marsh funnel. This is not a true viscosity, but serves as a qualitative measure of how thick the mud sample is. The funnel viscosity is useful only for relative comparisons. |
| Drilling Fluids | mass balance | Equality between the mass (density × volume) of a mixture or blend such as a drilling mud or completion fluid and the sum of the masses (density × corresponding volume) of its components. Assumptions are: (1) masses and volumes of components are additive and (2) material is neither generated nor lost from the system. As a simple example, below are the two material-balance equations for a three-component mixture of oil (o), water (w) and solids (s), where V = volume percent, D = specific density and MW = mixture weight. (This could represent a simple, weighted oil-base mud formulation.) MW = DsVs + DoVo + DwVw 100% = Vs + Vo + Vw. By solving these equations simultaneously, an unknown parameter can be found if other parameters are known or can be estimated accurately. Material-balance equations are used to derive formulations of muds, to calculate the amount of barite needed to weight-up a mud, to determine the amount of water needed to dilute a mud, and to find the volume of two or more muds to mix together to achieve a new mud weight and volume. Material balance is also the basis for calculating solids content of muds based on mud testing data. |
| Drilling Fluids | material balance equation | Mathematical relationship between the densities and the corresponding volumes of mixtures of liquid-solid slurries and clear fluid blends, such as drilling muds and completion fluids. Assumptions are: (1) masses and volumes of components are additive and (2) material is neither generated nor lost from the system. As a simple example, below are the two material-balance equations for a three-component mixture of oil (o), water (w) and solids (s), where V = volume percent, D = specific density and MW = mixture weight. (This could represent a simple, weighted oil-base mud formulation.) MW = DsVs + DoVo + DwVw 100% = Vs + Vo + Vw. By solving these equations simultaneously, an unknown parameter can be found if other parameters are known or can be estimated accurately. Material-balance equations are used to derive formulations of muds, to calculate the amount of barite needed to weight-up a mud, to determine the amount of water needed to dilute a mud, and to find the volume of two or more muds to mix together to achieve a new mud weight and volume. Material balance is also the basis for calculating solids content of muds based on mud testing data. |
| Drilling Fluids | Material Safety Data Sheet | A specific document that shows important physical and chemical characteristics of a chemical or product to alert a user, transporter or other interested party to potential safety hazards that may be associated with the material. The MSDS also contains treatments for exposure or ingestion as well as the type of equipment needed for safe handling. An MSDS is a legal requirement in most countries for all aspects of commerce involving chemicals. |
| Drilling Fluids | material-balance equation | Mathematical relationship between the densities and the corresponding volumes of mixtures of liquid-solid slurries and clear fluid blends, such as drilling muds and completion fluids. Assumptions are: (1) masses and volumes of components are additive and (2) material is neither generated nor lost from the system. As a simple example, below are the two material-balance equations for a three-component mixture of oil (o), water (w) and solids (s), where V = volume percent, D = specific density and MW = mixture weight. (This could represent a simple, weighted oil-base mud formulation.) MW = DsVs + DoVo + DwVw 100% = Vs + Vo + Vw. By solving these equations simultaneously, an unknown parameter can be found if other parameters are known or can be estimated accurately. Material-balance equations are used to derive formulations of muds, to calculate the amount of barite needed to weight-up a mud, to determine the amount of water needed to dilute a mud, and to find the volume of two or more muds to mix together to achieve a new mud weight and volume. Material balance is also the basis for calculating solids content of muds based on mud testing data. |
| Drilling Fluids | MBT test | A test to determine the amount of clay-like materials in a water-base drilling fluid based on the amount of methylene blue dye absorbed by the sample. Results are reported as “MBT” and also as “lbm/bbl, bentonite equivalent” when performed to API specifications. |
| Drilling Fluids | medium | Referring to any particle in the size range 74 to 250 microns. |
| Drilling Fluids | meniscus | The curved interface between two immiscible phases in a tube, such as in a pipette or graduated cylinder. Liquid volumes should be read at the bottom of a curved meniscus by alignment of the bottom of the meniscus. For water and liquids that wet the glass, the meniscus is concave. For nonwetting liquids, such as mercury, the meniscus is convex. |
| Drilling Fluids | methyl orange | A pH indicator used in alkalinity titration of mud filtrate and water samples. The indicator is yellow in solutions above pH 4.3, and red below pH 4.3. “M” alkalinity is the titration volume measured using the methyl orange indicator. Methyl orange is often replaced in test kits by bromocresol green. |
| Drilling Fluids | methyl orange alkalinity | A measure of the total amount of hydroxyl ions in a solution as determined by titration with standardized acid and methyl orange indicator. The alkalinity test is a well-known water-analysis procedure to estimate hydroxyl ion, carbonate ion and bicarbonate ion concentrations. There are two pH endpoints, P and M, in this titration, corresponding to phenolphthalein and methyl orange indicators. The “P” endpoint is at pH 8.3 and the “M” endpoint is at pH 4.3. Each is reported in units of cm3 acid/cm3 sample. Mf therefore represents the amount of acid required, per unit volume of the sample, to reduce the pH to 4.3. For water samples and very simple mud filtrates, P and M data indicate OH-, HCO3- and CO3-2 concentrations, but an alkalinity test is unreliable for analyzing complex mud filtrates. The API has established standards for conducting alkalinity tests. |
| Drilling Fluids | methylene blue dye | A blue dye with a cationic charge on the molecule used as the reagent for the methylene blue test used to estimate cation-exchange capacity (CEC) of solids in a water-base drilling mud. |
| Drilling Fluids | methylene blue test | A test to determine the amount of clay-like materials in a water-base drilling fluid based on the amount of methylene blue dye absorbed by the sample. Results are reported as “MBT” and also as “lbm/bbl, bentonite equivalent” when performed to API specifications. |
| Drilling Fluids | methylglucoside drilling fluid | A water-base drilling fluid containing a high concentration of methylglucoside. The mud has been used to drill water-sensitive shales with less hole enlargement and fewer drilling problems. Methylglucoside is a polysaccharide containing methyl groups on the ring-like sugar structure. Being a rather large, nonionic molecule (resembling starch but highly water soluble), it ties up water molecules in concentrated solutions and it is thought to act as a low-efficiency osmotic membrane. |
| Drilling Fluids | mica | [Muscovite mica K2Al4(Si6Al2O20(OH,F)4] A group of sheet silicates characterized by a platy appearance and basal cleavage most common in igneous and metamorphic rocks. Several clay minerals, such as chlorite and glauconite, are closely related to the mica group. |
| Drilling Fluids | microgel | A small fish eye, typically invisible, but which can nevertheless cause formation damage by polymer plugging of pore throats. Microgels may be formed by adding polymer too quickly when viscosifying a completion brine. |
| Drilling Fluids | milk emulsion mud | A term used to describe an emulsion in a water-base mud in which the oil phase is internal (as in milk), and water is external. |
| Drilling Fluids | milligrams per kilogram | On the basis of weight, the equivalent of parts per million, usually applied to small amounts of one solid admixed with another solid, such as 100 mg/kg of siderite in barite, the same as 100 ppm. |
| Drilling Fluids | milligrams per liter | On a weight per volume basis, the SI unit of concentration, abbreviated mg/L, usually applied to dissolved material in a solution. This unit is used in water analyses and in mud and mud-filtrate analyses. Increasingly, mg/L and ppm are used interchangeably in mud analyses. Actually, mg/L and ppm can only be interchanged when the sample has the exact density of water, which is only approximated by very dilute solutions. |
| Drilling Fluids | Minerals Management Service | A branch of the US Lands and Mineral Management Department formerly charged with supervising national resources. MMS had oversight of oil and gas leasing, royalty collection and other operations in US-owned areas. It was responsible for closely monitoring operations in Federal waters, overseeing leasing of acreage, issuing drilling permits and monitoring operators for permit violations. From 2010 to 2011, the MMS was replaced by three independent agencies within the US Department of Interior: the Office of Natural Resources Revenue (ONRR) for revenue collection, Bureau of Ocean Energy Management (BOEM) for US offshore resources management and Bureau of Safety and Environmental Enforcement (BSEE) for safety and environmental regulations enforcement. |
| Drilling Fluids | mixed metal hydroxide | mixed-metal hydroxide 1. n. [Drilling Fluids] ID: 2102 A compound containing hydroxide anions in association with two or more metal cations. MMH particles are extremely small and carry multiple positive charges. They can associate with bentonite to form a strong complex that exhibits highly shear-thinning properties, with high and fragile gel strengths, high yield point (YP), and low plastic viscosity (PV). MMH is described as a mixed-metal layered hydroxide (MMLH). In the crystal layers, Al+3 , Mg+2 and OH- ions reside, but due to symmetry considerations, there is not enough room for sufficient OH- ions to electrically offset the charges of the two cations. Therefore, a net positive charge exists on the crystal surfaces. Exchangeable anions sit on the positive surface (much the same as cations sit on negative clay surfaces). MMH muds are used as nondamaging drilling fluids, metal-reaming fluids (to carry out metal cuttings) and for wellbore shale control. Being cationic, MMH mud is sensitive to anionic deflocculants and small anionic polymers, such as polyphosphates, lignosulfonate or lignite. Reference: Burba JL III and Crabb CR: “Laboratory and Field Evaluation of Novel Inorganic Drilling Fluid Additive,” paper IADC/SPE 17198, presented at the IADC/SPE Drilling Conference, Dallas, Texas, USA, February 28-March 2, 1988. Fraser L and Enriquez F: “Mixed-Metal Hydroxides Fluid Research Widens Applications,” Petroleum Engineer International 64, no. 6 (June 1992): 43-46. |
| Drilling Fluids | mixed metal silicate | A product similar to mixed-metal hydroxide, but based on silicate chemistry. |
| Drilling Fluids | mixed-metal hydroxide | A compound containing hydroxide anions in association with two or more metal cations. MMH particles are extremely small and carry multiple positive charges. They can associate with bentonite to form a strong complex that exhibits highly shear-thinning properties, with high and fragile gel strengths, high yield point (YP), and low plastic viscosity (PV). MMH is described as a mixed-metal layered hydroxide (MMLH). In the crystal layers, Al+3 , Mg+2 and OH- ions reside, but due to symmetry considerations, there is not enough room for sufficient OH- ions to electrically offset the charges of the two cations. Therefore, a net positive charge exists on the crystal surfaces. Exchangeable anions sit on the positive surface (much the same as cations sit on negative clay surfaces). MMH muds are used as nondamaging drilling fluids, metal-reaming fluids (to carry out metal cuttings) and for wellbore shale control. Being cationic, MMH mud is sensitive to anionic deflocculants and small anionic polymers, such as polyphosphates, lignosulfonate or lignite. |
| Drilling Fluids | mixed-metal silicate | A product similar to mixed-metal hydroxide, but based on silicate chemistry. |
| Drilling Fluids | MMH | A compound containing hydroxide anions in association with two or more metal cations. MMH particles are extremely small and carry multiple positive charges. They can associate with bentonite to form a strong complex that exhibits highly shear-thinning properties, with high and fragile gel strengths, high yield point (YP), and low plastic viscosity (PV). MMH is described as a mixed-metal layered hydroxide (MMLH). In the crystal layers, Al+3 , Mg+2 and OH- ions reside, but due to symmetry considerations, there is not enough room for sufficient OH- ions to electrically offset the charges of the two cations. Therefore, a net positive charge exists on the crystal surfaces. Exchangeable anions sit on the positive surface (much the same as cations sit on negative clay surfaces). MMH muds are used as nondamaging drilling fluids, metal-reaming fluids (to carry out metal cuttings) and for wellbore shale control. Being cationic, MMH mud is sensitive to anionic deflocculants and small anionic polymers, such as polyphosphates, lignosulfonate or lignite. Reference: Burba JL III and Crabb CR: “Laboratory and Field Evaluation of Novel Inorganic Drilling Fluid Additive,” paper IADC/SPE 17198, presented at the IADC/SPE Drilling Conference, Dallas, Texas, USA, February 28-March 2, 1988. Fraser L and Enriquez F: “Mixed-Metal Hydroxides Fluid Research Widens Applications,” Petroleum Engineer International 64, no. 6 (June 1992): 43-46. |
| Drilling Fluids | MMS | Abbreviation for Minerals Management Service, a branch of the US Lands and Mineral Management Department formerly charged with supervising national resources. MMS had oversight of oil and gas leasing, royalty collection and other operations in US-owned areas. It was responsible for closely monitoring operations in Federal waters, overseeing leasing of acreage, issuing drilling permits and monitoring operators for permit violations. From 2010 to 2011, the MMS was replaced by three independent agencies within the US Department of Interior: the Office of Natural Resources Revenue (ONRR) for revenue collection, Bureau of Ocean Energy Management (BOEM) for US offshore resources management and Bureau of Safety and Environmental Enforcement (BSEE) for safety and environmental regulations enforcement. |
| Drilling Fluids | MMS | A product similar to mixed-metal hydroxide, but based on silicate chemistry |
| Drilling Fluids | monomer | The chemical unit from which a polymer is made. |
| Drilling Fluids | montmorillonite | A hydratable, dispersible clay mineral of the smectite group. Montmorillonite is a three-layer, expanding clay with a large surface area and high cation-exchange capacity. Na+ and Ca+2 are the typical exchangeable cations. Sodium montmorillonite, also called sodium bentonite, is a premium clay mud additive. Natural deposits are found in Wyoming, North Dakota, South Dakota and Utah, USA. Calcium montmorillonite is a low-yield bentonite that is more widely distributed and used in many commercial applications, including drilling fluid. |
| Drilling Fluids | MSDS | A specific document that shows important physical and chemical characteristics of a chemical or product to alert a user, transporter or other interested party to potential safety hazards that may be associated with the material. The MSDS also contains treatments for exposure or ingestion as well as the type of equipment needed for safe handling. An MSDS is a legal requirement in most countries for all aspects of commerce involving chemicals. |
| Drilling Fluids | mud | A term that is generally synonymous with drilling fluid and that encompasses most fluids used in hydrocarbon drilling operations, especially fluids that contain significant amounts of suspended solids, emulsified water or oil. Mud includes all types of water-base, oil-base and synthetic-base drilling fluids. Drill-in, completion and workover fluids are sometimes called muds, although a fluid that is essentially free of solids is not strictly considered mud. |
| Drilling Fluids | mud additive | A material added to a drilling fluid to perform one or more specific functions, such as a weighting agent, viscosifier or lubricant. |
| Drilling Fluids | mud aging cell | A cylindrical vessel in which a mud sample can be heated under pressure. Cells, often called bombs, are routinely used for static-aging and hot-roll aging of mud samples. Cells are usually made of metal or metal alloy, such as stainless steel or aluminum bronze, and have open tops. Caps should be fitted with a valve so that gas pressure can be applied and then released before opening the cell. Common sizes are 260 and 500 cm3, to accommodate half- and one-barrel equivalent volumes, plus space for thermal expansion. Glass or plastic jars can be used judiciously when pressure is nil and temperature is limited to below about 150°F [66°C]. |
| Drilling Fluids | mud balance | A device to measure density (weight) of mud, cement or other liquid or slurry. A mud balance consists of a fixed-volume mud cup with a lid on one end of a graduated beam and a counterweight on the other end. A slider-weight can be moved along the beam, and a bubble indicates when the beam is level. Density is read at the point where the slider-weight sits on the beam at level. Accuracy of mud weight should be within +/- 0.1 lbm/gal (+/- 0.01 g/cm3). A mud balance can calibrated with water or other liquid of known density by adjusting the counter weight. Most balances are not pressurized, but a pressurized mud balance operates in the same manner. |
| Drilling Fluids | mud cell | Another term for mud-aging cell, a cylindrical vessel in which a mud sample can be heated under pressure. Cells, often called bombs, are routinely used for static-aging and hot-roll aging of mud samples. Cells are usually made of metal or metal alloy, such as stainless steel or aluminum bronze, and have open tops. Caps should be fitted with a valve so that gas pressure can be applied and then released before opening the cell. Common sizes are 260 and 500 cm3, to accommodate half- and one-barrel equivalent volumes, plus space for thermal expansion. Glass or plastic jars can be used judiciously when pressure is nil and temperature is limited to below about 150°F [66°C]. |
| Drilling Fluids | mud cleaner | A desilter unit in which the underflow is further processed by a fine vibrating screen, mounted directly under the cones. The liquid underflow from the screens is fed back into the mud, thus conserving weighting agent and the liquid phase but at the same time returning many fine solids to the active system. Mud cleaners are used mainly with oil- and synthetic-base muds where the liquid discharge from the cone cannot be discharged, either for environmental or economic reasons. It may also be used with weighted water-base fluids to conserve barite and the liquid phase. |
| Drilling Fluids | mud cup | A graduated cup used to take samples and to crudely measure volumes of mud for testing at the rig. A mud cup is used primarily with the Marsh funnel to measure one quart of flow out of the funnel. It is also used as a container for performing simple pilot tests with an electric mixer that clamps onto the top of the cup. |
| Drilling Fluids | mud density | The mass per unit volume of a drilling fluid, synonymous with mud weight. Weight is reported in lbm/gal (also known as ppg), kg/m3 or g/cm3 (also called specific gravity or SG), lb/ft3 or in hydrostatic gradient, lb/in2/ft (psi/ft) or pptf (psi/1000 ft). Mud weight controls hydrostatic pressure in a wellbore and prevents unwanted flow into the well. The weight of the mud also prevents collapse of casing and the openhole. Excessive mud weight can cause lost circulation by propagating, and then filling, fractures in the rock. Mud weight (density) test procedures using a mud balance have been standardized and published by the API. |
| Drilling Fluids | mud engineer | A person responsible for testing the mud at a rig and for prescribing mud treatments to maintain mud weight, properties and chemistry within recommended limits. The mud engineer works closely with the rig supervisor to disseminate information about mud properties and expected treatments and any changes that might be needed. The mud engineer also works closely with the rig’s derrickman, who is charged with making scheduled additions to the mud during his work period. |
| Drilling Fluids | mud hopper | A mud-flow device, also called a jet hopper, in which materials are put into the circulating mud system. The mud hopper is powered by a centrifugal pump that flows the mud at high velocity through a venturi nozzle (jet) below the conical-shaped hopper. Dry materials are added through the mud hopper to provide dispersion, rapid hydration and uniform mixing. Liquids are sometimes fed into the mud by a hose placed in the hopper. |
| Drilling Fluids | mud house | The place where mud additives are kept at the rig, also known as the sack room. |
| Drilling Fluids | mud in | The mud flowing into the well. The mud-in sample is taken from the suction pit (the last pit in the flow series) just before the mud goes into the pump and down the wellbore. The in sample is also called the suction-pit sample, or “mud in” on a drilling fluid report. This mud has been treated and properly weighted and is in good condition to encounter downhole pressures, temperatures and contamination. Comparisons are made between properties of this mud-in sample and the “out” or mud-out sample taken at surface prior to solids removal. |
| Drilling Fluids | mud in sample | A mud sample taken from the suction pit (the last pit in the flow series) just before the mud goes into the pump and down the wellbore. The in sample is also called the suction-pit sample, or “mud in” on a drilling fluid report. This mud has been treated and properly weighted and is in good condition to encounter downhole pressures, temperatures and contamination. Comparisons are made between properties of this mud-in sample and the “out” or mud-out sample taken at surface prior to solids removal. |
| Drilling Fluids | mud motor | A positive displacement drilling motor that uses hydraulic horsepower of the drilling fluid to drive the drill bit. Mud motors are used extensively in directional drilling operations. |
| Drilling Fluids | mud out | The mud returning through the flowline. This mud has experienced the downhole pressures, temperatures and contamination that cause degradation. It is evaluated for needed treatments and compared, on a lagged time basis, with the corresponding “in” or mud-in sample. |
| Drilling Fluids | mud out sample | A mud sample taken after it has passed from the flowline and through the shale shaker screens to remove large cuttings. The out sample is also called the shale shaker sample. This mud has experienced the downhole pressures, temperatures and contamination that cause degradation. It is evaluated for needed treatments and compared, on a lagged time basis, with the corresponding “in” or mud-in sample. |
| Drilling Fluids | mud oven | An oven into which mud-testing cells are placed. Ovens usually have a set of horizontal rollers inside and are also called roller ovens. Mud-aging cells are placed on the rollers. In pilot tests, rolling the cells allows a film of mud to continually contact the hot wall of the cell. Another type of oven tumbles cells end-to-end. Most ovens can also be used for static-aging tests. |
| Drilling Fluids | mud pit | A large tank that holds drilling fluid on the rig or at a mud-mixing plant. For land rigs, most mud pits are rectangular steel construction, with partitions that hold about 200 barrels each. They are set in series for the active mud system. On most offshore rigs, pits are constructed into the drilling vessel and are larger, holding up to 1000 barrels. Circular pits are used at mixing plants and on some drilling rigs to improve mixing efficiency and reduce dead spots that allow settling. Earthen mud pits were the earliest type of mud pit, but environmental protection concern has led to less frequent use of open pits in the ground. Today, earthen pits are used only to store used or waste mud and cuttings prior to disposal and remediation of the site of the pit. |
| Drilling Fluids | mud program | A formal plan developed for a specific well with predictions and requirements at various intervals of the wellbore depth. The mud program gives details on mud type, composition, density, rheology, filtration and other property requirements and general and specific maintenance needs. Mud densities are especially important because they must fit with the casing design program and rock mechanics required in openhole to ensure wellbore pressures are properly controlled as the well is drilled deeper. |
| Drilling Fluids | mud report | The report sheets filled out by the mud engineer at the wellsite on a daily basis. The mud report supplies results of tests performed several times per day as well as details about mud product usage, inventory, recommendations and other pertinent information. Multiple-copy forms in a format approved by the API, which are provided by the mud service company, are the traditional type of mud report. Today, mud reports are more likely to be computerized and transmitted electronically. |
| Drilling Fluids | mud retort | A mud distillation unit used to measure the water, oil and solids content of a mud. It consists of a cylindrical body fitted with a mud sample holder, a heater element (or an oven) and an aluminum condenser. A graduated glass receiver catches and measures the volumes of water and oil that condense from the mud. Retort devices are available in three sizes, 10-, 20- and 50-cm3 , which are the volumes of mud placed in the retort sample cup. Data from the test are volume percent water, oil and retort solids. |
| Drilling Fluids | mud scale | Another term for mud balance, a device to measure density (weight) of mud, cement or other liquid or slurry. A mud balance consists of a fixed-volume mud cup with a lid on one end of a graduated beam and a counterweight on the other end. A slider-weight can be moved along the beam, and a bubble indicates when the beam is level. Density is read at the point where the slider-weight sits on the beam at level. Accuracy of mud weight should be within +/- 0.1 lbm/galUS (+/- 0.01 g/cm3). A mud balance can be calibrated with water or other liquid of known density by adjusting the counter weight. Most balances are not pressurized, but a pressurized mud balance operates in the same manner. |
| Drilling Fluids | mud still | A mud distillation unit used to measure the water, oil and solids content of a mud. It consists of a cylindrical body fitted with a mud sample holder, a heater element (or an oven) and an aluminum condenser. A graduated glass receiver catches and measures the volumes of water and oil that condense from the mud. Retort devices are available in three sizes, 10-, 20- and 50-cm3 , which are the volumes of mud placed in the retort sample cup. Data from the test are volume percent water, oil and retort solids. |
| Drilling Fluids | mud tracer | A type of nonreactive, easily differentiated material placed in a small portion of a circulating mud system at a certain time to be identified when it later returns to the surface from downhole. Mud tracers are used to determine mud cycle time (circulation time). Dyes, paints, beans, oats, chips, glitter or any material that will follow the mud and not be lost or destroyed can be used as a tracer. Care must be taken to use materials that do not dissolve, disperse or plug the bit or downhole motor. Mud tracers are distinct from mud-filtrate tracers. |
| Drilling Fluids | mud up | To add commercial materials to convert water or a water-clay slurry into a mud. Mudding up is usually done after drilling a well to a certain depth with relatively inexpensive spud mud or other native-clay mud, or with water or air. By delaying the use of drilling fluid, operators can save money in the initial stages of drilling a well. |
| Drilling Fluids | mud weight | The mass per unit volume of a drilling fluid, synonymous with mud density. Weight is reported in lbm/gal (also known as ppg), kg/m3 or g/cm3 (also called specific gravity or SG), lb/ft3 or in hydrostatic gradient, lb/in2/ft (psi/ft) or pptf (psi/1000 ft). Mud weight controls hydrostatic pressure in a wellbore and prevents unwanted flow into the well. The weight of the mud also prevents collapse of casing and the openhole. Excessive mud weight can cause lost circulation by propagating, and then filling, fractures in the rock. Mud weight (density) test procedures using a mud balance have been standardized and published by the API. |
| Drilling Fluids | mud-aging cell | A cylindrical vessel in which a mud sample can be heated under pressure. Cells, often called bombs, are routinely used for static-aging and hot-roll aging of mud samples. Cells are usually made of metal or metal alloy, such as stainless steel or aluminum bronze, and have open tops. Caps should be fitted with a valve so that gas pressure can be applied and then released before opening the cell. Common sizes are 260 and 500 cm3, to accommodate half- and one-barrel equivalent volumes, plus space for thermal expansion. Glass or plastic jars can be used judiciously when pressure is nil and temperature is limited to below about 150°F [66°C]. |
| Drilling Fluids | mudding off | The action of coating rock grains and plugging off the permeability of a productive reservoir during drilling. The term is seldom used today, but refers to formation damage by mud solids. By proper selection of solids, such as bridging materials and drill-in fluids, mudding off can be minimized. |
| Drilling Fluids | mudding up | The act of adding commercial materials to convert water or a water-clay slurry into a mud. Mudding up is usually done after drilling a well to a certain depth with relatively inexpensive spud mud or other native-clay mud, or with water or air. By delaying the use of drilling fluid, operators can save money in the initial stages of drilling a well. |
| Drilling Fluids | mud-in sample | A mud sample taken from the suction pit (the last pit in the flow series) just before the mud goes into the pump and down the wellbore. The in sample is also called the suction-pit sample, or “mud in” on a drilling fluid report. This mud has been treated and properly weighted and is in good condition to encounter downhole pressures, temperatures and contamination. Comparisons are made between properties of this mud-in sample and the “out” or mud-out sample taken at surface prior to solids removal. |
| Drilling Fluids | mud-out sample | A mud sample taken after it has passed from the flowline and through the shale shaker screens to remove large cuttings. The out sample is also called the shale shaker sample. This mud has experienced the downhole pressures, temperatures and contamination that cause degradation. It is evaluated for needed treatments and compared, on a lagged time basis, with the corresponding “in” or mud-in sample. |
| Drilling Fluids | mysid shrimp | The common name for the small shrimp species Mysidopsis bahia, which is used as the test organism in a US EPA bioassay test protocol. |
| Drilling Fluids | naphthenic hydrocarbon | A type of organic compound of carbon and hydrogen that contains one or more saturated cyclic (ring) structures, or contains such structures as a major portion of the molecule. The general formula is CnH2n. Naphthenic compounds are sometimes called naphthenes, cycloparaffins or hydrogenated benzenes. Naphtha is a refined petroleum fraction that contains a high percentage of these types of hydrocarbons. In drilling fluids, particularly oil-base muds, the amounts and types of hydrocarbons in the mud can be an important parameter in the overall performance of the mud. |
| Drilling Fluids | native clay | Clays incorporated into a so-called native-solids mud when drilling shallow formations. Native clays are undesirable in muds that are (or will be) weighted with barite. The viscosity of weighted fluids can rise quickly with added native clays, making it difficult to control and pump the mud. Better mud properties result when the drilled solids level, including the level of native clays, is kept low. |
| Drilling Fluids | native solids mud | A mud in which the suspended solids are dispersed clays, sand, chert and other rock that originated from formations being drilled. A spud mud is commonly a type of native-solids mud. Native muds can be economically diluted with water and passed through banks of desilters and desanders to keep solids down. No expensive weighting materials are being discarded and replaced in such a process. At the depth that higher density is required, native mud is usually totally or partially discarded and new mud is made using commercially prepared mud additives and barite. |
| Drilling Fluids | native-solids mud | A mud in which the suspended solids are dispersed clays, sand, chert and other rock that originated from formations being drilled. A spud mud is commonly a type of native-solids mud. Native muds can be economically diluted with water and passed through banks of desilters and desanders to keep solids down. No expensive weighting materials are being discarded and replaced in such a process. At the depth that higher density is required, native mud is usually totally or partially discarded and new mud is made using commercially prepared mud additives and barite. |
| Drilling Fluids | neutralization | A chemical reaction between an acid and a base to form a salt and water. Neutralization is used in the manufacture of mud products, removal of acidic contaminants from muds and formation of emulsifiers in oil mud. Neutralization is used in the test for alkalinity of mud and mud filtrate. |
| Drilling Fluids | neutralize | To form a salt and water by chemical reaction between an acid and a base. Neutralization is used in the manufacture of mud products, removal of acidic contaminants from muds, formation of emulsifiers in oil mud and in the test for alkalinity of mud and mud filtrate. |
| Drilling Fluids | Newtonian fluid | A typical fluid. A fluid is said to be Newtonian if its viscosity—a measure of a fluid’s ability to resist flow—only varies in response to changes in temperature or pressure. A Newtonian fluid will take the shape of its container. Under constant temperature and pressure conditions, the viscosity of a Newtonian fluid is the constant of proportionality, or the ratio, between the shear stress that builds in the fluid to resist flow and the shear rate applied to the fluid to induce flow; the viscosity is the same for all shear rates applied to the fluid. Water, sugar solutions, glycerin, silicone oils, light-hydrocarbon oils, air and other gases are Newtonian fluids. Most drilling fluids are non-Newtonian fluids. |
| Drilling Fluids | nonconductive drilling fluid | A mud that does not conduct electricity sufficiently well to allow spontaneous potential (SP) logging or resistivity logging. Oil- and synthetic-base muds are nonconductive drilling fluids. Water muds are not in this category. |
| Drilling Fluids | non-Newtonian fluid | Not a typical fluid. Unlike a Newtonian fluid, which displays liquid behavior, a non-Newtonian fluid has properties of a liquid and of a solid. Under certain conditions, a non-Newtonian fluid flows as a liquid and under other conditions, it exhibits elasticity, plasticity and strength similar to a solid. In addition, unlike Newtonian fluids, the viscosity of many non-Newtonian fluids varies with shear rate. Four classes of non-Newtonian fluids depend on how the fluid viscosity—a measure of a fluid’s ability to resist flow—varies in response to the duration and magnitude of applied shear rate. The viscosity of: Thixotropic fluids decreases over time under shearing. For example, solid honey becomes a liquid after continuous stirring. Rheopectic fluids increases over time under shearing. For example, cream will thicken after continuous stirring. Pseudoplastic fluids decreases with increasing shear rate; these fluids exhibit shear thinning behavior. For example, ketchup will squirt through a hole in a bottle top at high velocity but stand still as a dollop on a plate. Dilatant fluids increases with increasing shear rate; these fluids exhibit shear thickening behavior. Most successful drilling fluids are non-Newtonian and exhibit behaviors that are described by rheological mathematical models of shear stress, or resistance, as a function of shear rate. In the Bingham plastic model, flow will not begin until the shear stress attains a minimum value, the yield stress, after which the flow is similar to that of a Newtonian fluid because the viscosity is constant and does not vary with shear rate. Pseudoplasticity, or shear thinning, is a non-Newtonian behavior that is desirable for drilling fluids. Power-law and Herschel-Bulkley models describe pseudoplastic behavior, in which the slope—the viscosity—of the shear stress versus shear rate curve decreases as the shear rate decreases. |
| Drilling Fluids | normality | A unit of concentration for solutions of reagent chemicals used in testing mud chemistry. Normality provides a simple relationship between the volume in cm3 of reagent added during a titration and the chemical equivalents of a material with which the reagent reacts. A one-normal (1N) solution contains the equivalent weight in grams dissolved in one liter of solution. |
| Drilling Fluids | NPDES | Abbreviation for “National Pollutant Discharge Elimination System.” The US Congress passed this Clean Water Act to control discharges of contaminants. Discharges are allowed in to US water only by NPDES permits. Drilling fluids, drill cuttings, produced water, drilling rig deck drainage and blowout preventor fluids are covered specifically. Barite must be monitored for heavy metals to allow mud discharges. Oils are banned from discharge. |
| Drilling Fluids | OBR | Ratio of the volume percent oil to the volume percent brine in an oil mud, in which each is expressed as a percent of the total liquid in the mud. OBR is determined by retort analyses of oil and water content, but the brine content is calculated from the water content by using the chloride and calcium titration data. For example, if a mud contains 60 vol.% oil and 20 vol.% brine, the oil percentage is [60/(60 + 20)] 100 = 75 %, and the brine percentage is [20/(60 +20)] = 25%. That OBR is written as 75/25. |
| Drilling Fluids | OCMA | Pertaining to a drilling-grade bentonite clay with API/ISO specifications. API specifications for this clay are similar to those of OCMA. |
| Drilling Fluids | OCMA | Abbreviation for the now-defunct “Oil Company Materials Association,” an organization that for years set standards based primarily on what oil companies operating in the Middle East wanted in mud materials. Suppliers had little voice in OCMA. OCMA’s specifications for bentonite clay were modified and taken over by API/ISO. |
| Drilling Fluids | octahedral layer | One of the layers that constitute the atomic structure of the clay group of layered silicate minerals. The structure of these minerals can consist of two, three or four layers. The octahedral layer is a plane of aluminum hydroxide octahedra (aluminum at the center and hydroxides at all six corners). Another structural layer is a plane of silicon dioxide tetrahedra (silicon at the center and oxygen at all four corners of the tetrahedron). The tetrahedral and octahedral layers fit one on top of the other, with oxygen atoms being shared as oxide and hydroxide groups. |
| Drilling Fluids | octanol | An eight-carbon alcohol. Iso-octanol is used as a defoamer for water muds. |
| Drilling Fluids | octyl alcohol | Also known as octanol, an eight-carbon alcohol. Iso-octanol is used as a defoamer for water muds. |
| Drilling Fluids | oil base mud | An invert-emulsion mud, or an emulsion whose continuous phase is oil. In the past, the term referred to an oil mud containing less than about 5 vol.% water. This definition, at the time, distinguished mud with less than 5 vol.% water from invert-emulsion oil muds, which had more than 5 vol.% water. Today, this distinction is not practical because most commercial oil muds can be formulated with more or less than 5 vol.% water using essentially the same types of products. |
| Drilling Fluids | oil content | The volume percent of oil in a mud. The term should not be used to refer to the amount of synthetic fluid. |
| Drilling Fluids | oil emulsion mud | A water-base drilling fluid that contains dispersed oil or synthetic hydrocarbon as an internal phase. Early emulsion muds used diesel or crude oil dispersed into alkaline water-base muds. Synthetic liquids are now being substituted for oils in emulsion muds. Water-base muds containing certain synthetic liquids can be discharged in the Gulf of Mexico because they are environmentally safe and pass the EPA static sheen test and mysid shrimp toxicity tests. |
| Drilling Fluids | oil in water emulsion | A dispersion of oil droplets into an aqueous medium. This describes an emulsion mud, as contrasted with an invert-emulsion mud. The term should not be used to refer to synthetic-in-water emulsion because a synthetic fluid is not a true oi |
| Drilling Fluids | oil mud | More generally, a mud system that has any type of nonaqueous fluid as the external phase. This definition would include the newer variety of oil muds that are more exactly defined as synthetic-base muds. Synthetic mud is analogous to oil mud. |
| Drilling Fluids | oil mud | A mud in which the external phase is a product obtained from an oil, such as diesel oil or mineral oil. |
| Drilling Fluids | oil on cuttings | The oil content of cuttings, normally expressed as grams of oil adsorbed per kilogram of dry cuttings. The value has been used as the basis for discharge regulations for oil-base muds in several operating areas. |
| Drilling Fluids | oil water interface | The surface of contact between a water layer and an oil layer. |
| Drilling Fluids | oil/brine ratio | Ratio of the volume percent oil to the volume percent brine in an oil mud, in which each is expressed as a percent of the total liquid in the mud. OBR is determined by retort analyses of oil and water content, but the brine content is calculated from the water content by using the chloride and calcium titration data. For example, if a mud contains 60 vol.% oil and 20 vol.% brine, the oil percentage is [60/(60 + 20)] 100 = 75 %, and the brine percentage is [20/(60 +20)] = 25%. That OBR is written as 75/25. |
| Drilling Fluids | oil/water ratio | Ratio of the volume percent oil to the volume percent water in an oil mud, where each is a percent of the total liquid in the mud. OWR is calculated directly from the retort analysis of an oil mud. For example, if a mud contains 60 vol.% oil and 18 vol.% water, the oil percentage is [60/(60 + 18)]100 = 77% and the water percent is [18/(60 +18)] = 23%. That OWR is written as 77/23. |
| Drilling Fluids | oil-base mud | An invert-emulsion mud, or an emulsion whose continuous phase is oil. In the past, the term referred to an oil mud containing less than about 5 vol.% water. This definition, at the time, distinguished mud with less than 5 vol.% water from invert-emulsion oil muds, which had more than 5 vol.% water. Today, this distinction is not practical because most commercial oil muds can be formulated with more or less than 5 vol.% water using essentially the same types of products. |
| Drilling Fluids | oil-emulsion mud | A water-base drilling fluid that contains dispersed oil or synthetic hydrocarbon as an internal phase. Early emulsion muds used diesel or crude oil dispersed into alkaline water-base muds. Synthetic liquids are now being substituted for oils in emulsion muds. Water-base muds containing certain synthetic liquids can be discharged in the Gulf of Mexico because they are environmentally safe and pass the EPA static sheen test and mysid shrimp toxicity tests. |
| Drilling Fluids | oil-in-water emulsion | A dispersion of oil droplets into an aqueous medium. This describes an emulsion mud, as contrasted with an invert-emulsion mud. The term should not be used to refer to synthetic-in-water emulsion because a synthetic fluid is not a true oil. |
| Drilling Fluids | oil-mud emulsifier | A chemical used in preparation and maintenance of an oil- or synthetic-base drilling fluid that forms a water-in-oil emulsion (invert emulsion). An oil-mud emulsifier lowers the interfacial tension between oil and water, which allows stable emulsions with small drops to be formed. Historically, oil-mud emulsifiers have been classified as primary and secondary. Secondary emulsifiers are generally not used alone to make a stable oil mud. Emulsifiers can be calcium fatty-acid soaps made from various fatty acids and lime, or derivatives such as amides, amines, amidoamines and imidazolines made by reactions of fatty acids and various ethanolamine compounds. These emulsifiers surround water droplets, like an encapsulating film, with the fatty acid component extending into the oil phase. Emulsifier molecules that cannot fit around drops form clusters (micelles) in the oil phase or adsorb onto solids. Oil-mud emulsion drops each behave like a small osmotic cell. The emulsifier around the drops acts like a semipermeable membrane through which water can move but ions cannot pass. Thus, oil muds have the special capability (which water muds do not have) to control water transfer to and from the drops simply by adjusting salinity within the water phase of the oil mud. |
| Drilling Fluids | oil-water interface | The surface of contact between a water layer and an oil layer. |
| Drilling Fluids | olefinic hydrocarbon | The group of hydrocarbon compounds that has one or more double or triple bonds between carbon atoms in the linear chain. Ethylene, C2H4, is the smallest olefin. Synthetic olefinic hydrocarbons are made by polymerization of ethylene under catalytic conditions. They are used in synthetic-base mud and as lubricants for water muds. |
| Drilling Fluids | oligomer | A low-molecular-weight polymer typically with two to five monomer units. |
| Drilling Fluids | OOC | Abbreviation for Offshore Operators Committee, a group of more than 85 oil and gas producers who operate in the Gulf or Mexico. OOC was formed in the 1950s. It interacts with various governmental organizations, such as the Environmental Protection Agency (EPA) and Minerals Management Service (MMS), to keep regulations up to date and economically sound. Many rules about discharge of mud and cuttings into Gulf waters have been developed jointly by OOC and the EPA. |
| Drilling Fluids | OOC | The oil content of cuttings, normally expressed as grams of oil adsorbed per kilogram of dry cuttings. The value has been used as the basis for discharge regulations for oil-base muds in several operating areas. |
| Drilling Fluids | organophilic | Pertaining to a material that associates with organic and oily surfaces and liquids and rejects aqueous systems. An example of organophilic behavior in drilling fluids is the organophilic coating applied to clays and lignites to make them dispersible in oil-base muds. |
| Drilling Fluids | organophilic clay | Clay minerals whose surfaces have been coated with a chemical to make them oil-dispersible. Bentonite and hectorite (plate-like clays) and attapulgite and sepiolite (rod-shaped clays) are treated with oil-wetting agents during manufacturing and are used as oil-mud additives. Quaternary fatty-acid amine is applied to the clay. Amine may be applied to dry clay during grinding or it can be applied to clay dispersed in water. The latter process is much more expensive, requiring filtering, drying and other manufacturing steps. Organophilic bentonite and hectorite, “bentones,” are used in oil muds to build rheology for cuttings lifting and solids suspension. They also contribute to low-permeability filter cake. Organophilic attapulgite and sepiolite are used in oil muds strictly to build gel structure, which may not be long lasting due to shear degradation as the mud is pumped through the bit. |
| Drilling Fluids | organophilic lignite | A lignite that has been coated with a chemical that renders it dispersible in oil. The treatment is usually accomplished with a type of quaternary amine compound. |
| Drilling Fluids | OSHA | Abbreviation for Occupational Safety and Health Act of the US. OSHA laws protect safety and health of workers and give workers the right to know what materials they are handling. Safe handling of mud chemicals and the general safety of all drilling operations are of prime concern to OSHA, the operator and the mud engineer. Monitoring for toxic hydrogen sulfide [H2S] in air around the rig and testing mud for sulfides are an essential part of rig safety. |
| Drilling Fluids | osmosis | The movement of water from one aqueous system to another through a semipermeable membrane. Osmotic movement is driven by activity differences between the two systems and can be considered as a vapor-phase transfer. An oil mud acts as an osmotic system. Emulsion film surrounding each brine droplet in an oil mud acts as semipermeable osmotic membrane and allows water molecules to pass back and forth, but restricts ions and larger molecules. Clays in shale formations also are aqueous systems that interact by osmosis with oil-mud droplets. |
| Drilling Fluids | osmotic pressure | The pressure that must be applied to the low-salinity side of an osmotic system to prevent water movement into the high-salinity side by osmosis. Conversely, it is the suction pressure that a high-salinity system exerts on the low-salinity system across a semipermeable membrane in an osmotic system. |
| Drilling Fluids | OSPAR | The Oslo and Paris Commission, formerly known as PARCOM. The commission is a group of experts who advise North Sea countries on environmental policy and legislation. OSPAR has been influential in establishing North Sea legislation on drilling fluids that has served as the model for other operating areas. OSPAR has published lists of environmentally acceptable and unacceptable products, referred to as the “green,” “grey” and “black” lists. The Green or A list consists of products posing relatively little harm to the environment (specifically the marine environment). Examples include inert minerals such as bentonite, inorganic salts that are common constituents of seawater such as sodium and potassium chloride, and simple organic products such as starch and carboxymethylcellulose (CMC). The Grey List consists of products ‘requiring strong regulatory control’ and includes heavy metals such as zinc, lead and chromium. The Black list covers products considered unsuitable for discharge and includes mercury, cadmium and ‘persistent oils and hydrocarbons of a petroleum origin.’ The inclusion of hydrocarbons in the black list has been the driving force behind the reduction of oil discharges in the North Sea and elsewhere and has serious implications for the use of oil and synthetic fluids. |
| Drilling Fluids | OWR | Ratio of the volume percent oil to the volume percent water in an oil mud, where each is a percent of the total liquid in the mud. OWR is calculated directly from the retort analysis of an oil mud. For example, if a mud contains 60 vol.% oil and 18 vol.% water, the oil percentage is [60/(60 + 18)]100 = 77% and the water percent is [18/(60 +18)] = 23%. That OWR is written as 77/23. |
| Drilling Fluids | oxygen scavenger | A chemical that reacts with dissolved oxygen (O2) to reduce corrosion, such as sulfite (SO3-2) and bisulfite (HSO3-) ions that combine with oxygen to form sulfate (SO4-2). This is a redox reaction and requires a nickel or cobalt catalyst. Removal of air from a mud by defoaming and mechanical degassing is an essential first step before a scavenger can lower the dissolved oxygen content. |
| Drilling Fluids | Pf | A chemical property of an aqueous system that implies that there are more hydroxyl ions (OH-) in the system, or a potential to produce more hydroxyl ions, than there are hydrogen ions (H+), or potential to produce hydrogen ions. |
| Drilling Fluids | Pm | A chemical property of an aqueous system that implies that there are more hydroxyl ions (OH-) in the system, or a potential to produce more hydroxyl ions, than there are hydrogen ions (H+), or potential to produce hydrogen ions. |
| Drilling Fluids | PAC | A cellulose derivative similar in structure, properties and usage in drilling fluids to carboxymethylcellulose. PAC is considered to be a premium product because it typically has a higher degree of carboxymethyl substitution and contains less residual NaCl than technical grade carboxymethylcellulose, although some PACs contain considerable NaCl. |
| Drilling Fluids | packer fluid | A fluid that is left in the annular region of a well between tubing and outer casing above a packer. The main functions of a packer fluid are: (1) to provide hydrostatic pressure in order to lower differential pressure across the sealing element, (2) to lower differential pressure on the wellbore and casing to prevent collapse and (3) to protect metals and elastomers from corrosion. |
| Drilling Fluids | PAG | A polymer or copolymer of an alkalene oxide, such as polyethylene glycol (PEG), a polymer of ethylene oxide with general formula HO(CH2CH2O)nH, or polypropylene glycol (PPG), which is a polymer of propylene oxide. PAGs are effective shale inhibitors and have effectively replaced the earlier polyglycerols. |
| Drilling Fluids | PAO | One of the synthetic hydrocarbon liquids manufactured from the monomer ethylene, H2C=CH2. Polyalphaolefins have a complex branched structure with an olefin bond in the alpha position of one of the branches. Hydrogenated polyalphaolefins have olefin-carbons saturated with hydrogen, which lends excellent thermal stability to the molecule. Synthetic-base fluids (similar to oil muds) are made with the various types of synthetic liquids because the cuttings can be discharged in offshore waters, whereas discharge of cuttings coated with refined oils would be disallowed. |
| Drilling Fluids | paraffinic hydrocarbon | The group of hydrocarbons consisting of linear molecules with the formula CnH2n+2. Methane, CH4, is the simplest member. Higher members, starting at about C18, are wax-like and are called paraffin. Excessive amounts of paraffinic hydrocarbons in an oil mud adversely affect mud flow and oil removal from cuttings at cool temperatures. |
| Drilling Fluids | paraformaldehyde | A commonly used preservative for starch, xanthan gum, guar gum and other natural polymers that are prone to attack by bacteria. It is as a trimer of formaldehyde and has the formula O-CH2-O-CH2-O-CH2. Paraformaldehyde is a white, water soluble powder. When added to a mud in advance of a bacterial inoculation and maintained, paraformaldehyde can effectively control many strains of bacteria. The amount or paraformaldehyde in a mud can be estimated by oxidizing it with sulfite into formic acid and performing an alkalinity titration, according to a procedure published by API. |
| Drilling Fluids | PARCOM | The Oslo and Paris Commission, formerly known as PARCOM. The commission is a group of experts who advise North Sea countries on environmental policy and legislation. OSPAR has been influential in establishing North Sea legislation on drilling fluids that has served as the model for other operating areas. OSPAR has published lists of environmentally acceptable and unacceptable products, referred to as the “green,” “grey” and “black” lists. The Green or A list consists of products posing relatively little harm to the environment (specifically the marine environment). Examples include inert minerals such as bentonite, inorganic salts that are common constituents of seawater such as sodium and potassium chloride, and simple organic products such as starch and carboxymethylcellulose (CMC). The Grey List consists of products ‘requiring strong regulatory control’ and includes heavy metals such as zinc, lead and chromium. The Black list covers products considered unsuitable for discharge and includes mercury, cadmium and ‘persistent oils and hydrocarbons of a petroleum origin.’ The inclusion of hydrocarbons in the black list has been the driving force behind the reduction of oil discharges in the North Sea and elsewhere and has serious implications for the use of oil and synthetic fluids. |
| Drilling Fluids | particle plugging apparatus | A specialized apparatus used in the particle-plugging test. The PPA is used to determine the ability of particles in the drilling fluid to effectively bridge the pores in the filter medium and, therefore, the ability of the mud to reduce formation damage in the reservoir. The apparatus resembles a high-pressure, high-temperature filtration cell that has been modified to operate upside down (to remove the effects of gravity) and to accept filter media of different permeabilities (sintered metal, which is chosen for higher temperature conditions, aloxite, which is a porous ceramic material, or rock). The medium is selected to match the permeability of the reservoir to be drilled. The filter medium is at the top so that sediment will not affect the filter cake. Pressure is applied hydraulically from below. |
| Drilling Fluids | particle size distribution | The weight, or net volume, of solid particles that fall into each of the various size ranges, given as a percentage of the total solids of all sizes in the sample of interest. Particle size can be determined by sieve analysis, light scattering, passage through an electrically charged orifice, settling rate or other methods. Data are typically shown as a histogram chart with percentage-smaller-than on the y-axis and size ranges on the x-axis. Mud engineers use such data to operate solids-control equipment effectively. Particle-size distributions are used to evaluate bridging materials for drill-in and completion fluids. Barite and hematite samples are examined to ensure performance without excessive wear on equipment and as an API/ISO quality specification. |
| Drilling Fluids | particle-plugging apparatus | A specialized apparatus used in the particle-plugging test. The PPA is used to determine the ability of particles in the drilling fluid to effectively bridge the pores in the filter medium and, therefore, the ability of the mud to reduce formation damage in the reservoir. The apparatus resembles a high-pressure, high-temperature filtration cell that has been modified to operate upside down (to remove the effects of gravity) and to accept filter media of different permeabilities (sintered metal, which is chosen for higher temperature conditions, aloxite, which is a porous ceramic material, or rock). The medium is selected to match the permeability of the reservoir to be drilled. The filter medium is at the top so that sediment will not affect the filter cake. Pressure is applied hydraulically from below. |
| Drilling Fluids | particle-plugging test | A test performed in a specialized filtration-type apparatus (particle-plugging apparatus) to determine the effectiveness of additives to prevent fluid loss into a permeable medium. |
| Drilling Fluids | particle-size distribution | The weight, or net volume, of solid particles that fall into each of the various size ranges, given as a percentage of the total solids of all sizes in the sample of interest. Particle size can be determined by sieve analysis, light scattering, passage through an electrically charged orifice, settling rate or other methods. Data are typically shown as a histogram chart with percentage-smaller-than on the y-axis and size ranges on the x-axis. Mud engineers use such data to operate solids-control equipment effectively. Particle-size distributions are used to evaluate bridging materials for drill-in and completion fluids. Barite and hematite samples are examined to ensure performance without excessive wear on equipment and as an API/ISO quality specification. |
| Drilling Fluids | parts per billion | A unit of concentration. |
| Drilling Fluids | parts per million | A unit of concentration, frequently abbreviated to ppm. For solid and liquid concentrations, ppm refers to weight (mass) units. For example: (1) Calcite in a ground barite sample may be 400 ppm. (2) Calcium chloride in a water solution may be 250,000 ppm. Note that the relationship of ppm to weight percentage is 10,000 ppm = 1 wt.%. For gases, ppm refers to volume (or mole) units. For example, H2S in an air sample may be 10 ppm (both by volume and by moles). |
| Drilling Fluids | pascal | A unit of measurement for pressure in the International System of Units (SI), symbolized by Pa. The conversion factor from lbf/in2 to Pa is 6,895 Pa per lbf/in2 (psi). For example, 5,000 psi = 34.5 × 106 Pa. |
| Drilling Fluids | peptization | Dispersal of a substance into a colloidal form or dispersal of a clay in water to form a colloidal suspension. |
| Drilling Fluids | peptize | To disperse a substance into a colloidal form or to disperse a clay in water to form a colloidal suspension. |
| Drilling Fluids | peptized clay | A clay that has been treated during manufacturing to enhance its dispersion. |
| Drilling Fluids | peptizing agent | A product that enhances dispersion of a substance (such as clay) into colloidal form. Peptizing agents for drilling-mud clays are sodium carbonate, sodium metaphosphates, sodium polyacrylates, sodium hydroxide and other water-soluble sodium compounds, even common table salt, NaCl, if added at low concentration. The divalent cations on a clay are replaced by the sodium cations, aiding clay hydration and dispersion. Greater benefit is attained by an agent that contributes an anion (for example, carbonate, phosphate or polyacrylate) that precipitates divalent cations and removes them from solution. This process is successful only when the water first contacted is free of hardness ions, otherwise the anion in the peptizing salt (or polymer) will be precipitated by the hard water and make the peptizing agent much less effective. |
| Drilling Fluids | permafrost | The permanently frozen subsoil that lies below the upper layer (the upper several inches to feet) of soil in arctic regions. |
| Drilling Fluids | pH | Hydrogen ion potential, which is the log10 of the reciprocal of hydrogen ion, H+, concentration. Mathematically, pH = log10 (1/[H+]), where [ ] represents mole/L. pH is derived from the ion-product constant of water, which at room temperature is 1 x 10-14 = [H+] x [OH-]. Pure water (at neutral pH) has equal concentrations of its two ions: [H+] = [OH-] = 10-7 mole/L. Log10 1/[H+] is 7, which is the pH of a neutral solution. The pH scale ranges from 0 to 14, and values below 7 are acidic and above 7 are basic. |
| Drilling Fluids | pH test | A drilling-fluid test to measure pH of muds and mud filtrates, usually performed according to API specifications. The pH test uses a pH meter equipped with a glass-membrane measuring electrode and reference electrode, which read from 0 to 14. The preferred pH meter automatically compensates for temperature. Buffer solutions of pH = 4, 7 and 10 are specified for calibration of the meter. Color-matching pH paper and sticks are not recommended except for simple muds. |
| Drilling Fluids | phenolphthalein | A pH indicator that is clear below pH 8.3 and red above 8.3. It is the indicator used in various alkalinity titrations. |
| Drilling Fluids | phosphate salt | A group of salts formed by neutralization of phosphorous or phosphoric acid with a base, such as NaOH or KOH. Orthophosphates are phosphoric acid (H3PO4) salts, where 1, 2 or 3 of the hydrogen ions are neutralized. Neutralization with NaOH gives three sodium orthophosphates: (a) monosodium phosphate (MSP), (b) disodium phosphate (DSP) or (c) trisodium phosphate (TSP). Their solutions are buffers in the 4.6 to 12 pH range. TSP is an excellent degreaser. All will precipitate hardness ions such as calcium. Polyphosphates are polymers made from various orthophosphates by dehydration with heat. Sodium acid pyrophosphate (SAPP) is a clay deflocculant and treatment for cement contamination. For clay deflocculation, polyphosphates are limited by the temperature at which they hydrolyze back to orthophosphates, although several that performed up to 280°F [138°C] have been documented in the literature (see reference). |
| Drilling Fluids | PHPA | A class of water muds that use partially-hydrolyzed polyacrylamide (PHPA) as a functional additive, either to control wellbore shales or to extend bentonite clay in a low-solids mud. As a shale-control mud, PHPA is believed to seal microfractures and coat shale surfaces with a film that retards dispersion and disintegration. KCl is used as a shale inhibitor in most PHPA mud designs. In low-solids muds, PHPA interacts with minimal concentrations of bentonite to link particles together and improve rheology without increased colloidal solids loading. |
| Drilling Fluids | PHPA mud | A class of water muds that use partially-hydrolyzed polyacrylamide (PHPA) as a functional additive, either to control wellbore shales or to extend bentonite clay in a low-solids mud. As a shale-control mud, PHPA is believed to seal microfractures and coat shale surfaces with a film that retards dispersion and disintegration. KCl is used as a shale inhibitor in most PHPA mud designs. In low-solids muds, PHPA interacts with minimal concentrations of bentonite to link particles together and improve rheology without increased colloidal solids loading. |
| Drilling Fluids | pill | A small volume of mud used for a specific purpose in a drilling operation. Various types of pills are needed from time to time on the rig, such as to stop circulation loss or free stuck drillpipe. |
| Drilling Fluids | pilot test | An experimental test, or series of tests, used to predict mud behavior and guide future actions by the mud engineer. Rather than experimenting on the full mud volume and risking serious and expensive mistakes, pilot tests such as those listed below give valuable guidance: |
| Drilling Fluids | plastic fluid | A fluid in which the shear force is not proportional to the shear rate (non-Newtonian) and that requires a finite shear stress to start and maintain flow. Most drilling muds are characterized as either plastic or pseudoplastic fluids. |
| Drilling Fluids | plugging material | A solid or gel in a workover or drilling fluid that blocks off permeable zones to prevent loss of fluid into those permeable zones or to protect those zones from damage. The plugging may be temporary or permanent. |
| Drilling Fluids | PNP | A solvent used with water to break the emulsion of an oil-base or synthetic-base drilling fluid to prepare the sample for chemical titrations to determine lime, calcium or chloride content according to API testing procedures. PNP is an abbreviation for propylene glycol normal propyl ether. It is an environmentally friendlier replacement of a xylene-isopropynol mixture previously used in certain titrations. |
| Drilling Fluids | POD | A term used to describe the beginning of thickening of a cement slurry during the thickening-time test, often abbreviated as POD. For some slurries, the POD is used as the thickening time. |
| Drilling Fluids | point of departure | A term used to describe the beginning of thickening of a cement slurry during the thickening-time test, often abbreviated as POD. For some slurries, the POD is used as the thickening time. |
| Drilling Fluids | poise | A unit of measurement for viscosity, symbolized by P. Viscosity is the ratio of shear stress to shear rate, giving the traditional unit of dyne-sec/cm2 for poise. In metric (SI) units, 1 P is 0.1 Pa.s (pascal-second). |
| Drilling Fluids | polar | In chemistry, referring to a compound in which electrons are not shared equally in the chemical bond, resulting in partial electrical charges. The best example is water, H2O, where the oxygen atom “pulls” the electrons more strongly than the hydrogen atoms and has a partial negative charge. The hydrogen atoms thus carry a partial positive charge. Polar compounds may ionize partially when dissolved in water. |
| Drilling Fluids | polar compound | A compound whose electrons are not shared equally in chemical bonds. A polar compound is not necessarily ionized. Water is a polar compound. Polymers can have ionizing polar groups on their complex structures. |
| Drilling Fluids | polyacrylate | Linear, anionic polymer made from the monomer acrylic acid, CH2=CHCOO- H+. The acrylic acid groups are evenly spaced along the chain. Acrylic acid polymer neutralized with NaOH is sodium polyacrylate (SPA). Polyacrylates are best utilized in soft water with low salinity to achieve the best dispersion and full chain elongation. Even low concentrations of hardness ions, for example, Ca+2, precipitate polyacrylates. Low molecular-weight polyacrylates are used as clay deflocculants. High molecular weight polymers are used for fluid-loss control and as a clay extender. As an extender, SPA is added to bentonite at the grinding plant. It is also used at the rig in low-solids mud. Divalent cations can negate its benefits as a clay extender. SPA is highly efficient when used to flocculate colloids in native-solids muds, clear-water muds and wastewater cleanup. The polymer chain links together colloidal solids that can be removed by gravity settling in shallow pits or by applying hydrocyclone, centrifuge or filtration techniques. |
| Drilling Fluids | polyalkalene glycol | A polymer or copolymer of an alkalene oxide, such as polyethylene glycol (PEG), a polymer of ethylene oxide with general formula HO(CH2CH2O)nH, or polypropylene glycol (PPG), which is a polymer of propylene oxide. PAGs are effective shale inhibitors and have effectively replaced the earlier polyglycerols. |
| Drilling Fluids | polyalphaolefin | One of the synthetic hydrocarbon liquids manufactured from the monomer ethylene, H2C=CH2. Polyalphaolefins have a complex branched structure with an olefin bond in the alpha position of one of the branches. Hydrogenated polyalphaolefins have olefin-carbons saturated with hydrogen, which lends excellent thermal stability to the molecule. Synthetic-base fluids (similar to oil muds) are made with the various types of synthetic liquids because the cuttings can be discharged in offshore waters, whereas discharge of cuttings coated with refined oils would be disallowed. |
| Drilling Fluids | polyanionic cellulose | A cellulose derivative similar in structure, properties and usage in drilling fluids to carboxymethylcellulose. PAC is considered to be a premium product because it typically has a higher degree of carboxymethyl substitution and contains less residual NaCl than technical grade carboxymethylcellulose, although some PACs contain considerable NaCl. |
| Drilling Fluids | polyanionic lignin | A fluid-loss control additive used in high-temperature, water-base muds. It shows good salt tolerance and temperature tolerance. |
| Drilling Fluids | polyelectrolyte | A polymer that ionizes in solution. Charged groups strongly affect behavior and interactions with colloidal clays, other polymers and solvents. |
| Drilling Fluids | polyglycerol | A series of alcohols with glycerol, C3H5(OH)3, (usually referred to as glycerin in the USA) being the simplest member. Polyglycerols have been used as shale inhibitors in water-base drilling fluids. |
| Drilling Fluids | polymer | A large molecule made up of repeating units. Some polymers are naturally occurring, such as xanthan gum, guar gum and starch. Other polymers are modified natural polymers, such as carboxymethylcellulose (CMC) and hydropropyl starch and lignosulfonate. Some are synthetic such as polyacrylates, polyacrylamides and polyalphaolefins. Polymers may be classified by their structure and may be linear, branched or less commonly cyclic. Copolymers contain two or more different monomers that can be arranged randomly or in blocks. In solution, entangled polymer chains can create networks, giving complex viscosity behavior. Polymers that ionize in solution are called polyelectrolytes. Charged groups strongly affect behavior and interactions with colloidal clays, other polymers and solvents. Molecular size (weight) influences how a specific polymer type performs in a given type of mud. A small polymer may be a deflocculant, whereas a large polymer of the same type may be a flocculant. Some are viscosifiers and others are fluid-loss control additives while others are multifunctional. |
| Drilling Fluids | polyol | A generic name for low molecular weight, water-soluble polymers and oligomers containing a large number of hydroxyl groups. Specific examples include glycols, polyglycols and polyglycerols. Polyols are used in water-base fluids as shale inhibitors and gas hydrate inhibitors. |
| Drilling Fluids | polyolefin | Also known as isomerized olefin or IO, a synthetic hydrocarbon liquid made by the polymerization of ethylene, H2C=CH2. IOs are one of several synthetic fluids that have recently been used as base for synthetic-base muds and in other applications where refined oils might otherwise be used except for HSE concerns. IOs are linear structures that have their olefin double bond in the center of the chain length. They are made by isomerization of linear alphaolefins (LAO), which have their double bond at the end of the chain. Because the olefin bond is in the central area of the chain, the physical properties of IOs are different (for example, they are generally lower viscosity) compared with the LAOs from which they are made. |
| Drilling Fluids | polyphosphates | Also known as pyrophosphates, polymers made from various orthophosphates by dehydration with heat. Orthophosphates are phosphoric acid (H3PO4) salts, where 1, 2 or 3 of the hydrogen ions are neutralized. Sodium acid pyrophosphate (SAPP) is a clay deflocculant and treatment for cement contamination. For clay deflocculation, polyphosphates are limited by the temperature at which they hydrolyze back to orthophosphates, although several that performed up to 280°F [138°C] have been documented in the literature (see reference). |
| Drilling Fluids | pore pressure transmission | A laboratory test used to determine if a drilling fluid blocks movement of filtrate through pore spaces of a shale sample. The PPT device monitors the increase in pore pressure in a shale when exposed to a drilling fluid over a period of time. Shale cores from 1 to 3-inches long are fitted into a modified Hassler cell that has sensitive pressure transducers in reservoirs on each end of the cell. Reference: van Oort E, Hale AH, Mody FK and Roy S: “Transport in Shales and the Design of Improved Water-Based Shale Drilling Fluids,” in SPE Drilling and Completion 11, no. 3 (September 1996): 137-146. |
| Drilling Fluids | pore-pressure transmission | A laboratory test used to determine if a drilling fluid blocks movement of filtrate through pore spaces of a shale sample. The PPT device monitors the increase in pore pressure in a shale when exposed to a drilling fluid over a period of time. Shale cores from 1 to 3-inches long are fitted into a modified Hassler cell that has sensitive pressure transducers in reservoirs on each end of the cell. |
| Drilling Fluids | portland cement | The product obtained by pulverizing clinker consisting essentially of hydraulic calcium silicates. Portland cement is the most common type of cement used for oil- and gas-well cementing. |
| Drilling Fluids | portland cement clinker | Hard granular nodules composed essentially of hydraulic calcium silicates, with smaller quantities of calcium aluminates and ferrites. Portland cement clinker is produced by the heat treatment of cement raw materials in a kiln. Clinker is pulverized with gypsum in the manufacture of portland cement. |
| Drilling Fluids | potassium ion | The ion of potassium, K+. There are tests used to monitor high (>5000 mg/L) or low ( <5000 mg/L) potassium ion (K+) concentration in water-base muds. The test for high concentration, as specified by the API, is based on the insolubility of potassium perchlorate. A volume of mud filtrate is mixed with an excess of NaClO4 in a centrifuge tube. White KClO4 is precipitated by the reaction. After the tube has spun in a centrifuge to settle the white sediment in the tube, the amount of precipitate is read and compared to a calibration chart that relates sediment to concentration of K+ in the filtrate sample. The test to monitor low potassium ion (K+) concentration in water-base muds, as prescribed by API, is a titration procedure using quaternary ammonium salt solution (QAS) as reagent. Potassium ion is first precipitated as the tetraphenylborate (TPB) salt by adding an excess of sodium tetraphenylborate. After filtering out the solid, the amount of TPB not reacted with K+ ion is titrated with a standard QAS solution. The endpoint is purple-to-light blue color change. The test for high potassium ion concentration was first applied to drilling fluids by Ron Steiger at Exxon Production Research Co. and has proven to be a reliable way to measure K+ ion concentration in a mud at the wellsite, which allows the mud engineer to maintain the proper level when drilling through hydratable shales. Before a direct test for K+ ion was available, KCl in a mud was monitored by chloride analysis. Although simple to perform, this analysis was misleading and counterproductive because after drilling shale with a new KCl mud for awhile, the beneficial K+ ion was consumed by the clays but the Cl- ion remained in the mud. Eventually, the wellbore shales hydrated, fell into the hole and created severe mud and drilling problems. As a result of improper analytical methods, early potassium muds earned an undeservedly negative reputation. For optimal shale stability, K+ ion must be continually maintained by adding KCl (or some other K salt) as fast as it is consumed. The advent of a direct field-worthy method for K+ analysis, the result of Dr. Steiger 's efforts, was a breakthrough in the use of water mud for drilling troublesome shale. |
| Drilling Fluids | potassium mud | A class of muds that contain potassium ion (K+) dissolved in the water phase. Potassium muds are the most widely accepted water mud system for drilling water-sensitive shales, especially hard, brittle shales. K+ ions attach to clay surfaces and lend stability to shale exposed to drilling fluids by the bit. The ions also help hold the cuttings together, minimizing dispersion into finer particles. The presence of Na+ ions counteracts the benefits of K+ ions and should be minimized by using fresh water (not sea water) for make-up water. With time, Na+, Ca+2 and other ions accumulate from ion exchange with clays, making the mud less effective, but regular treatment to remove Ca+2 improves polymer function. Potassium chloride, KCl, is the most widely used potassium source. Others are potassium acetate, potassium carbonate, potassium lignite, potassium hydroxide and potassium salt of PHPA. Use of bentonite clay is restricted because of its strong affinity for K+. Instead, various polymers are used. XC polymer and PHPA are used for rheology. For fluid-loss control, mixtures of starch and polyanionic cellulose are often used. CM starch, HP starch, carboxymethylcellulose and sodium polyacrylate (SPA) are also used. PHPA is widely used for shale encapsulation. Potassium, lime and starch-like polymers have also been used as potassium mud systems. Although three API methods exist for determining the K+ ion concentration, the centrifuge method (for K+ >5000 mg/L) is the most accepted field method, and essential for daily monitoring of potassium in a mud. Regular additions of potassium salt maintain shale stability. K+ ion is rapidly consumed while drilling shallow, soft and highly dispersive (gumbo) shales, but maintaining sufficient K+ ion to stabilize gumbo can become expensive when drilling large holes. Researchers, notably Dr. Dennis O’Brien and Dr. Martin Chenevert (while at Exxon Production Research), evaluated different shales, their clay mineralogy and the concentration of K+ needed to stabilize them. Potassium muds above about 1 wt.% K+ ion usually fail the mysid shrimp (US EPA) bioassay test. Therefore, K-muds currently find low acceptance in offshore drilling in USA waters. |
| Drilling Fluids | pounds per barrel | US oilfield unit of concentration, usually abbreviated to lbm/bbl. One lbm/bbl is the equivalent of one pound of additive in 42 US gallons of mud. The “m” is used to denote mass to avoid possible confusion with pounds force (denoted by “lbf”). Sometimes lbm/bbl is written as ppb, but must not be confused with parts per billion. In SI units, the conversion factor is one pound per barrel equals 2.85 kilograms per cubic meter. For example, 10 lbm/bbl = 28.5 kg/m3. |
| Drilling Fluids | pour point | The lowest temperature (in °F or °C) at which a liquid remains pourable (meaning it still behaves as a fluid). Oil or synthetic muds with high pour points may suffer from poor screening and excessive pressure, surges in deepwater wells or other operations subject to low temperatures. In oils, the pour point is generally increased by a high paraffin content. The pour point of liquid additives is an important consideration for arctic drilling operations. |
| Drilling Fluids | power law fluid | A fluid described by the two-parameter rheological model of a pseudoplastic fluid, or a fluid whose viscosity decreases as shear rate increases. Water-base polymer muds, especially those made with XC polymer, fit the power-law mathematical equation better than the Bingham plastic or any other two-parameter model. Power-law fluids can be described mathematically as follows: |
| Drilling Fluids | power-law fluid | A fluid described by the two-parameter rheological model of a pseudoplastic fluid, or a fluid whose viscosity decreases as shear rate increases. Water-base polymer muds, especially those made with XC polymer, fit the power-law mathematical equation better than the Bingham plastic or any other two-parameter model. Power-law fluids can be described mathematically as follows: |
| Drilling Fluids | pozzolan | A siliceous or siliceous and aluminous material that possesses little or no cementitious value. In a finely divided form and in the presence of moisture, however, pozzolan reacts chemically with calcium hydroxide to form compounds possessing cementitious properties. |
| Drilling Fluids | pozzolanic | Pertaining to material that possesses little or no cementitious value, but that is capable of reacting chemically with calcium hydroxide at ordinary temperatures to form compounds with cementitious properties. |
| Drilling Fluids | PPA | A specialized apparatus used in the particle-plugging test. The PPA is used to determine the ability of particles in the drilling fluid to effectively bridge the pores in the filter medium and, therefore, the ability of the mud to reduce formation damage in the reservoir. The apparatus resembles a high-pressure, high-temperature filtration cell that has been modified to operate upside down (to remove the effects of gravity) and to accept filter media of different permeabilities (sintered metal, which is chosen for higher temperature conditions, aloxite, which is a porous ceramic material, or rock). The medium is selected to match the permeability of the reservoir to be drilled. The filter medium is at the top so that sediment will not affect the filter cake. Pressure is applied hydraulically from below. |
| Drilling Fluids | ppb | Abbreviation for the US oilfield unit of concentration, pounds per barrel, more correctly written as lbm/bbl. One lbm/bbl is the equivalent of one pound of additive in 42 US gallons of mud. The “m” is used to denote mass to avoid possible confusion with pounds force (denoted by “lbf”). In SI units, the conversion factor is one pound per barrel equals 2.85 kilograms per cubic meter. For example, 10 lbm/bbl = 28.5 kg/m3. |
| Drilling Fluids | ppb | Abbreviation for concentration, parts-per-billion. For example, lead in a water sample may be 10 ppb. |
| Drilling Fluids | ppg | Abbreviation for density, pounds-per-gallon, more correctly written lbm/gal. For example, the density of water is 8.33 ppg at 60°F [16°C]. |
| Drilling Fluids | ppm | Abbreviation for the expression of concentration, parts-per-million. For solid and liquid concentrations, ppm refers to weight (mass) units. For example: (1) Calcite in a ground barite sample may be 400 ppm. (2) Calcium chloride in a water solution may be 250,000 ppm. Note that the relationship of ppm to weight percentage is 10,000 ppm = 1 wt.%. For gases, ppm refers to volume (or mole) units. For example, H2S in an air sample may be 10 ppm (both by volume and by moles). |
| Drilling Fluids | PPT | A test performed in a specialized filtration-type apparatus (particle-plugging apparatus) to determine the effectiveness of additives to prevent fluid loss into a permeable medium |
| Drilling Fluids | PPT | A laboratory test used to determine if a drilling fluid blocks movement of filtrate through pore spaces of a shale sample. The PPT device monitors the increase in pore pressure in a shale when exposed to a drilling fluid over a period of time. Shale cores from 1 to 3-inches long are fitted into a modified Hassler cell that has sensitive pressure transducers in reservoirs on each end of the cell. |
| Drilling Fluids | precipitate | To form an insoluble material in a fluid. Precipitation can occur by a chemical reaction of two or more ions in solution or by changing the temperature of a saturated solution. There are many examples of this important phenomenon in drilling fluids. Precipitation occurs in the reaction between calcium cations and carbonate anions to form insoluble calcium carbonate: Ca+2 + CO3-2 –> CaCO3. When a saturated clear brine first crystallizes, the solid is a precipitate, and is often caused by changing temperature. |
| Drilling Fluids | precipitation | The formation of an insoluble material in a fluid. Precipitation can occur by a chemical reaction of two or more ions in solution or by changing the temperature of a saturated solution. There are many examples of this important phenomenon in drilling fluids. Precipitation occurs in the reaction between calcium cations and carbonate anions to form insoluble calcium carbonate: Ca+2 + CO3-2 –> CaCO3. When a saturated clear brine first crystallizes, the solid is a precipitate, and is often caused by changing temperature. |
| Drilling Fluids | pregelatinized starch | A water-soluble starch that has undergone irreversible changes by heating in water or steam. |
| Drilling Fluids | prehydrate | To mix with water and allow to react or yield in the water before use. Prehydrating is a common technique for incorporating bentonite in cement slurry or drilling mud. Prehydration may also be done for convenience in cementing operations to allow mixing of water containing the additives with powdered neat cement. Additives also may be prehydrated with mix water to avoid dry-blending the additives with cement. |
| Drilling Fluids | prehydrated bentonite | A concentrated slurry of bentonite clay mixed in fresh water. The maximum practical concentration of bentonite is about 30 to 40 lbm/bbl because greater concentrations of bentonite are difficult to mix and pump. Water is put into the rig’s prehydration tank and the pH raised to 10 or 11 with caustic soda. Soda ash is added as required to remove hardness. Bentonite is slowly added through the mud hopper. Continual energetic mixing and stirring helps the clay particles fully disperse. In some muds, lignosulfonate should be added shortly before mixing the slurry into the active system to protect the colloidal clay particles from flocculation. |
| Drilling Fluids | prehydration | The addition of a mud product to fresh water prior to adding it into the mud system. Bentonite clay and XC polymers are two additives whose performance improves by hydration in fresh water before adding them to a highly-treated or salty mud system. |
| Drilling Fluids | preservative | Another term for bactericide, an additive that kills bacteria. Bactericides are commonly used in water muds containing natural starches and gums that are especially vulnerable to bacterial attack. Bactericide choices are limited and care must be taken to find those that are effective yet approved by governments and by company policy. Bactericides, also called biocides, can be used to control sulfate-reducing bacteria, slime-forming bacteria, iron-oxidizing bacteria, and bacteria that attacks polymers in fracture and secondary recovery fluids. In polymers, the degradation of the fluid is controlled, thus avoiding the formation of a large biomass, which could plug the formation and reduce permeability. |
| Drilling Fluids | pressurized mud balance | A device to measure density (weight) of a mud, cement or other liquid or slurry under sufficient pressure that the effect of gas bubbles in the liquid is eliminated. The balance consists of a fixed-volume mud cup on one end of a graduated beam and a counterweight on the other end. The beam has a knife-edge as a balance point and a bubble to show when it is level. The mud cup has a screw-on, sealed cap with a valve in the cap to allow connection of a small piston-type hand pump. Operation of the pressurized balance is identical to an ordinary mud balance after pressurization. |
| Drilling Fluids | primary cementing | The first cementing operation performed to place a cement sheath around a casing or liner. The main objectives of primary cementing include zonal isolation to prevent fluid migration in the annulus, support for the casing or liner, and protection of the casing from corrosive fluids. |
| Drilling Fluids | progressive gels | The situation in which 10-second and 10-minute gel strengths for a drilling mud have dissimilar values, with the 10-minute number being much higher than the 10-second number. This indicates that the gelation of the mud is rapidly gaining strength with time, which generally is an undesirable feature of a mud. The mud may require excessive pump pressures to break circulation. If gels appear to be too progressive, a 30-minute gel-strength measurement may be warranted as a third check of progress. |
| Drilling Fluids | propylene glycol normal propyl ether | A solvent used with water to break the emulsion of an oil-base or synthetic-base drilling fluid to prepare the sample for chemical titrations to determine lime, calcium or chloride content according to API testing procedures. PNP is an abbreviation for propylene glycol normal propyl ether. It is an environmentally friendlier replacement of a xylene-isopropynol mixture previously used in certain titrations. |
| Drilling Fluids | pseudoplastic | A descriptive term for a fluid with shear-thinning characteristics that does not exhibit thixotropy. Most effective drilling fluids are shear thinning, although most also exhibit some gel-building characteristics. Pseudoplastic rheology, low viscosity at high shear rates and high viscosity at low shear rates, benefits several aspects of drilling-higher drilling rate and improved cuttings lifting. Bingham plastic fluids, power-law fluids and Herschel-Bulkley fluids fall in the psuedoplastic category of rheology. |
| Drilling Fluids | pumpability | The ability of the slurry to be pumped. Pumpability is usually measured by the API thickening-time test. |
| Drilling Fluids | pumping time | The total time required for pumping the cement slurry into the well, plus a safety factor. Pumping time can also be the time required to reach a consistency deemed to be unpumpable (generally 70 Bc) during an API thickening-time test. |
| Drilling Fluids | PV | A parameter of the Bingham plastic model. PV is the slope of the shear stress/shear rate line above the yield point. PV represents the viscosity of a mud when extrapolated to infinite shear rate on the basis of the mathematics of the Bingham model. (Yield point, YP, is the other parameter of that model.) A low PV indicates that the mud is capable of drilling rapidly because of the low viscosity of mud exiting at the bit. High PV is caused by a viscous base fluid and by excess colloidal solids. To lower PV, a reduction in solids content can be achieved by dilution of the mud. |
| Drilling Fluids | PVT | A shorthand term for pressure, volume, temperature dependencies for fluid properties. In oil-base drilling fluids, PVT effects on viscosity and density must be understood to develop density and hydraulics programs. Downhole pressure makes the base oil more viscous and dense, whereas temperature has the opposite effect. Brines for downhole use also require an understanding of PVT behavior. |
| Drilling Fluids | pyrophosphate | Also known as polyphosphate, a polymer made from an orthophosphate by dehydration with heat. Orthophosphates are phosphoric acid (H3PO4) salts, where 1, 2 or 3 of the hydrogen ions are neutralized. Sodium acid pyrophosphate (SAPP) is a clay deflocculant and treatment for cement contamination. For clay deflocculation, polyphosphates are limited by the temperature at which they hydrolyze back to orthophosphates, although several that performed up to 280°F [138°C] have been documented in the literature (see reference). |
| Drilling Fluids | pyrrhotite | A mineral containing ferrous sulfide, FeS, that typically contains inclusions of free sulfur and other minerals. It is commonly present in shales, and may occur as a trace mineral in some barite ores. Pyrrhotite can liberate sulfides in alkaline muds, with adverse consequences for safety and corrosion. |
| Drilling Fluids | QA | Planned and systematic monitoring, testing and documenting of practices to show that a product or procedure meets established standards. Companies that manufacture, process, sell, handle, ship or buy drilling fluid materials typically establish QA programs. Some oil company labs perform QA testing. API and ISO jointly issue specifications for mud materials. A mud supplier must meet specifications and adhere to quality testing practices in order to place an API monogram (logo) on a product. Suppliers are required to pay for use of the API monogram program. Quality specifications that the API has adopted through joint user and supplier efforts are not always suitable for all users of the materials |
| Drilling Fluids | quality assurance | Planned and systematic monitoring, testing and documenting of practices to show that a product or procedure meets established standards. Companies that manufacture, process, sell, handle, ship or buy drilling fluid materials typically establish QA programs. Some oil company labs perform QA testing. API and ISO jointly issue specifications for mud materials. A mud supplier must meet specifications and adhere to quality testing practices in order to place an API monogram (logo) on a product. Suppliers are required to pay for use of the API monogram program. Quality specifications that the API has adopted through joint user and supplier efforts are not always suitable for all users of the materials. |
| Drilling Fluids | quat | Slang term for quaternary amine. |
| Drilling Fluids | quat amine | Also known as quaternary amine, a cationic amine salt in which the nitrogen atom has four groups bonded to it and carries a positive charge. Quaternary amines are used as oil-wetting agents, corrosion and shale inhibitors and bactericides. |
| Drilling Fluids | quaternary amine | A cationic amine salt in which the nitrogen atom has four groups bonded to it and carries a positive charge. Quaternary amines are used as oil-wetting agents, corrosion and shale inhibitors and bactericides. |
| Drilling Fluids | quebracho | A powdered form of tannic acid extract from the bark of the quebracho tree, used as a high-pH and lime mud deflocculant. It was in widespread use until the 1950s, at which time lignosulfonate became widely available and performed the same function better and cheaper than quebracho. High pH was needed to neutralize tannic acids to form the tannates, which are red. The oilfield name red mud was applied to tannate-dispersed muds. A legend of the oil patch holds that, back in the days when quebracho mud was all the rage (in the days when Hobby Airport was Houston’s main airport), a mud man parked his old field car in the airport parking lot for several days. It was raining when he left, but sunny when he got back. The police were waiting to talk to him when he returned because someone noticed a dried blood-red stain under his car. The liquid had obviously leaked out of the trunk. When the mud man opened the trunk for the police, they found two wet sacks of quebracho. As you may have guessed, his trunk lid leaked. |
| Drilling Fluids | quick lime | A chemical with formula CaO, commonly called quick lime or hot lime. When hydrated with one mole of water, it forms slaked lime, Ca(OH)2. Quick lime is used in preference to slaked lime at oil mud mixing plants because it generates heat when it becomes slaked with water and therefore speeds up emulsification by the reaction to form calcium fatty-acid soap. |
| Drilling Fluids | Rh | The water content of air compared to the water content that the air could hold if it were saturated, expressed as a percentage. Air in equilibrium with fresh water is saturated with water vapor, so its RH = 100%. Air above a saturated NaCl solution has RH = 75%. Air above a saturated CaCl2 solution has RH = 31%. RH can, therefore, be used as an indicator of the water activity of a solution with which air is in equilibrium. RH can also reflect the aqueous-phase activity of an oil-emulsion mud, the basis for the Chenevert Method for testing oil muds. |
| Drilling Fluids | rate of shear | The velocity gradient measured across the diameter of a fluid-flow channel, be it a pipe, annulus or other shape. Shear rate is the rate of change of velocity at which one layer of fluid passes over an adjacent layer. As an example, consider that a fluid is placed between two parallel plates that are 1.0 cm apart, the upper plate moving at a velocity of 1.0 cm/sec and the lower plate fixed. The fluid layer at the lower plate is not moving and the layer nearest the top plate is moving at 1.0 cm/sec. Halfway between the plate, a layer is moving at 0.5 cm/sec. The velocity gradient is the rate of change of velocity with distance from the plates. This simple case shows the uniform velocity gradient with shear rate (v1 – v2)/h = shear rate = (cm/sec)/(cm/1) = 1/sec. Hence, shear rate units are reciprocal second |
| Drilling Fluids | red mud | A clay-based water mud that used tannates (from tannic acid) as clay deflocculant and mined lignite for fluid-loss control, usually with lime. The tannates were usually quebracho, which is red at high pH. Red muds were used extensively in the 1940s and 1950s. |
| Drilling Fluids | redox | A contraction of reduction-oxidation, a type of chemical reaction in which one reactant is reduced (gains electrons) while the other is oxidized (loses electrons). Examples of redox mud chemistry are: (1) sulfite anions to remove molecular oxygen, (2) sulfide removal by oxygen or peroxide, (3) air oxidation of lignite to create more humic acid, (4) sulfate reducing bacteria that generate sulfide ions by biological redox reactions, (5) chromate ions being converted to chromic ion in a mud system. |
| Drilling Fluids | reduced water slurry | A cement slurry made with less mix water than is customarily used without modifying additives. |
| Drilling Fluids | reduced-water slurry | A cement slurry made with less mix water than is customarily used without modifying additives. |
| Drilling Fluids | relative filtrate volume | Quantity that is double the filtrate volume collected from a filtration test between 7.5 to 30 minutes. This ignores the spurt of filtrate that comes out of the filter press before a cake is established. Relative filtrate volume is used to evaluate certain mud materials. Relative filtrate volume can be expressed as Vrel, = (V30 – V7.5) x 2. Static filtration theory holds that filtrate volume increases as the square root of time. Thus, the volume from 0 to 7.5 minutes should be the same as from 7.5 to 30 minutes in the case of no spurt loss in the first interval. Because spurt often occurs in the 0 to 7.5 minutes, those data can be ignored by doubling the volume in the second interval. This practice has no bearing on dynamic filtration or on high-pressure, high-temperature filtration tests. |
| Drilling Fluids | relative humidity | The water content of air compared to the water content that the air could hold if it were saturated, expressed as a percentage. Air in equilibrium with fresh water is saturated with water vapor, so its RH = 100%. Air above a saturated NaCl solution has RH = 75%. Air above a saturated CaCl2 solution has RH = 31%. RH can, therefore, be used as an indicator of the water activity of a solution with which air is in equilibrium. RH can also reflect the aqueous-phase activity of an oil-emulsion mud, the basis for the Chenevert Method for testing oil muds. |
| Drilling Fluids | relaxed filtrate oil mud | An oil mud designed and maintained with a minimum of colloid-sized solids, typically by omitting fatty-acid soap and lime, and minimizing organophilic clays and fluid-loss additives. Relaxed filtrate oil mud increases drilling rate. A disadvantage is that filter cake formed on sands is not tight, can quickly become very thick, and can cause pipe to stick by differential pressure |
| Drilling Fluids | relaxed-filtrate oil mud | An oil mud designed and maintained with a minimum of colloid-sized solids, typically by omitting fatty-acid soap and lime, and minimizing organophilic clays and fluid-loss additives. Relaxed filtrate oil mud increases drilling rate. A disadvantage is that filter cake formed on sands is not tight, can quickly become very thick, and can cause pipe to stick by differential pressure. |
| Drilling Fluids | reserve mud pit | Any pit not part of the active (circulatory) system. The reserve pit may be used to store spare or waste mud, base oil or brine. In operations on land, the reserve pit is usually a plastic-lined, earthen pit, in which waste mud is stored until final disposal. |
| Drilling Fluids | reserve-mud pit | Any pit not part of the active (circulatory) system. The reserve pit may be used to store spare or waste mud, base oil or brine. In operations on land, the reserve pit is usually a plastic-lined, earthen pit, in which waste mud is stored until final disposal. |
| Drilling Fluids | resin | Organic material having low solubility. Resins are usually large and complex polymeric molecules with noncrystalline structure and no distinct melting point or other definitive properties. Resins are used as additives to improve filter cake, provide lubricity or stop lost circulation as lost-circulation material. Resins are derived from plant sources (such as pine trees), some are residues of manufacturing processes and some resins are mined material. |
| Drilling Fluids | retort | A mud distillation unit used to measure the water, oil and solids content of a mud. It consists of a cylindrical body fitted with a mud sample holder, a heater element (or an oven) and an aluminum condenser. A graduated glass receiver catches and measures the volumes of water and oil that condense from the mud. Retort devices are available in three sizes, 10-, 20- and 50-cm3 , which are the volumes of mud placed in the retort sample cup. Data from the test are volume percent water, oil and retort solids. |
| Drilling Fluids | retort solids | The volume percent (or fraction) of a mud that is not captured in the receiver when performing the water, oil and solids test as prescribed by API, as given in the equation below. Retort solids thus include suspended solids, dissolved solids (salts), charred organic materials and volatile materials that do not condense. For calculations, retort solids are normally assumed to be only suspended and dissolved solids, as in the equation below. Volume percent suspended solids (weighting material plus drill solids) is of particular interest to mud engineers. To calculate that percentage, the volume increase caused by the dissolved salts is determined from filtrate analyses of chloride and calcium ions. For oil muds, the calculations are more complicated. |
| Drilling Fluids | returns | Mud that comes back to the surface and exits through the flowline after being pumped down the drillpipe. “Lost returns” is the situation in which some or all of the mud does not come back to the surface, which indicates that mud is being lost into weak, fractured or vugular formations downhole. |
| Drilling Fluids | RH | The water content of air compared to the water content that the air could hold if it were saturated, expressed as a percentage. Air in equilibrium with fresh water is saturated with water vapor, so its RH = 100%. Air above a saturated NaCl solution has RH = 75%. Air above a saturated CaCl2 solution has RH = 31%. RH can, therefore, be used as an indicator of the water activity of a solution with which air is in equilibrium. RH can also reflect the aqueous-phase activity of an oil-emulsion mud, the basis for the Chenevert Method for testing oil muds. |
| Drilling Fluids | rheological | Pertaining to rheology, the science and study of the deformation and flow of matter. The term rheology is also used to indicate the properties of a given fluid, as in mud rheology. Rheology is an extremely important property of drilling muds, drill-in fluids, workover and completion fluids, cements and specialty fluids and pills. Mud rheology is measured on a continual basis while drilling and adjusted with additives or dilution to meet the needs of the operation. In water-base fluids, water quality plays an important role in how additives perform. Temperature affects behavior and interactions of the water, clay, polymers and solids in a mud. Downhole pressure must be taken into account in evaluating the rheology of oil muds. |
| Drilling Fluids | rheological property | One of several flow characteristics of a material, such as a drilling fluid, completion fluid, workover fluid or cement. Shear-stress measurements made at a minimum of two shear rates are needed to define the properties of these oilfield fluids. Three parameters are sometimes used to better define fluid behavior. “Rheological properties” most often refers to the Bingham plastic fluid parameters, PV (plastic viscosity) and YP (yield point), as measured by the direct-indicating rheometer. The power-law fluid model parameters, exponent (n) and consistency (k), apply to polymer muds, although the three-parameter Herschel-Bulkley model is a better fit to polymer muds. Brookfield viscometers measure flow properties at low shear rates to determine suspension and transport of cuttings in high-angle holes. |
| Drilling Fluids | rheology | The science and study of the deformation and flow of matter. The term is also used to indicate the properties of a given fluid, as in mud rheology. Rheology is an extremely important property of drilling muds, drill-in fluids, workover and completion fluids, cements and specialty fluids and pills. Mud rheology is measured on a continual basis while drilling and adjusted with additives or dilution to meet the needs of the operation. In water-base fluids, water quality plays an important role in how additives perform. Temperature affects behavior and interactions of the water, clay, polymers and solids in a mud. Downhole pressure must be taken into account in evaluating the rheology of oil muds. |
| Drilling Fluids | rheology modifier | An additive for oil- and synthetic-base muds that provides high viscosity at low shear rates, which is useful when drilling high-angle and horizontal wells and can be critical for cuttings carrying and to prevent sag and settling of weighting material. Products used include dimeric and trimeric fatty acids, imidazolines, amides and synthetic polymers. |
| Drilling Fluids | right angle set | The characteristic of a cement slurry whose consistency changes from the point of departure or 30 Bc to 100 Bc in a short time. The term refers to the characteristic 90-degree bend in a plot of cement consistency versus time. |
| Drilling Fluids | right-angle set | The characteristic of a cement slurry whose consistency changes from the point of departure or 30 Bc to 100 Bc in a short time. The term refers to the characteristic 90-degree bend in a plot of cement consistency versus time |
| Drilling Fluids | RM | An additive for oil- and synthetic-base muds that provides high viscosity at low shear rates, which is useful when drilling high-angle and horizontal wells and can be critical for cuttings carrying and to prevent sag and settling of weighting material. Products used include dimeric and trimeric fatty acids, imidazolines, amides and synthetic polymers |
| Drilling Fluids | rolling aging test | A mud test in which the mud sample is mildly agitated by rolling (or tumbling) for the duration of the test, usually performed at a selected high temperature. Typically, the mud sample is sealed in a mud-aging cell and placed in an oven that will roll (or tumble) the mud cells continually for a given period of time (often 16 hours or overnight). The cooled mud is tested for properties. A rolled (or tumbled) mud sample simulates circulation in the hole by pumping. |
| Drilling Fluids | rolling-aging test | A mud test in which the mud sample is mildly agitated by rolling (or tumbling) for the duration of the test, usually performed at a selected high temperature. Typically, the mud sample is sealed in a mud-aging cell and placed in an oven that will roll (or tumble) the mud cells continually for a given period of time (often 16 hours or overnight). The cooled mud is tested for properties. A rolled (or tumbled) mud sample simulates circulation in the hole by pumping. |
| Drilling Fluids | RP | Abbreviation for Recommended Practice. |
| Drilling Fluids | sack | A unit of measure for portland cement. In the United States, a sack refers the amount of cement that occupies a bulk volume of 1.0 ft3. For most portland cement, including API classes of cement, a sack weighs 94 pounds. The sack is the basis for slurry design calculations and is often abbreviated as sk. |
| Drilling Fluids | sacrificial anode | A protective device to prevent electrolytic corrosion. Anodes (often made of Mg or Al metal) are sacrificed intentionally to protect a steel system, such as a buried pipeline or offshore platform. |
| Drilling Fluids | sag | Settling of particles in the annulus of a well, which can occur when the mud is static or being circulated. Because of the combination of secondary flow and gravitational forces, weighting materials can settle (sag) in a flowing mud in a high-angle well. If settling is prolonged, the upper part of a wellbore will lose mud density, which lessens the hydrostatic pressure in the hole, so an influx (a kick) of formation fluid can enter the well. |
| Drilling Fluids | salt | The product formed by neutralization of an acid and a base. The term is more specifically applied to sodium chloride. Neutralization is an important reaction in many aspects of mud control and treatment. |
| Drilling Fluids | saltwater flow | An influx of formation water, usually salty and sometimes hard, into the mud in the wellbore. Saltwater flows contaminate freshwater or seawater muds, making it expensive, difficult and time-consuming to regain the mud properties. Influxes pose a lesser problem for saltwater muds. Saltwater contamination flocculates the bentonite clay in fresh- or seawater muds. Flocculated, thick, filter cake on a permeable zone frequently results in differential-pressure sticking. |
| Drilling Fluids | saltwater mud | A water mud containing varying amounts of dissolved sodium chloride, NaCl, as a major component. Undissolved salt may also be present in saturated salt muds to increase density beyond 10 lbm/gal or to act as a bridging agent over permeable zones. Starch and starch derivatives for fluid-loss control and xanthan gums for hole-cleaning are among the few highly effective additives for saltwater muds. Attapulgite and sepiolite are used in saltwater muds only for cuttings lifting. The primary use of saltwater mud is to drill salt strata that are prone to dissolution when exposed to other types of drilling fluid. A saturated salt mud is used to drill salt to prevent hole enlargement. In hot, plastic, salt zones, the hole may close inward unless extremely high mud weight is maintained. As an alternative to high mud weight, maintaining undersaturation in the fluid allows controlled leaching to offset hole closure by plastic flow. Sized salt particles in saturated saltwater muds are used, along with polymers, to bridge over permeable production zones. The salt can be removed later with a water flush. Salt solids can increase density beyond 10 lbm/gal, up to about 13 lbm/gal, if needed. |
| Drilling Fluids | sand | A category of size used to describe particles in a mud that will not pass through a 200-mesh screen (74 micrometers and larger). |
| Drilling Fluids | sand test | A test to determine the volume percent of solids in a mud that are retained on 200-mesh screen. A glass, sand-content tube with a tapered lower end and a 200-mesh screen are used in the test. The test measures percent solids above 74 micrometers, which include those that could be abrasive to pumps and piping. When performed according to the API protocol for water-base muds, the sand-content tube is filled to the first mark with mud. Water is added to the next mark and the tube is shaken. The diluted slurry is poured through the 200-mesh screen, discarding the liquid. The screen is washed and the residue on the screen is poured back into the tube. Volume percent “sand” is measured from divisions on the tapered tube. |
| Drilling Fluids | sand trap | A small pit, typically located immediately after the shaker screens, which is used as a settling pit to separate coarser solids that accidentally bypass the shakers. Mud enters the pit at one side and exits via an overflow at the other. Sand traps are dumped periodically to remove the settled solids, or alternatively the contents can be processed over a fine screen or with a centrifuge. |
| Drilling Fluids | SAPP | Abbreviation for sodium acid pyrophosphate, a sequestering agent used to treat cement contamination and a deflocculant for low-temperature water muds. |
| Drilling Fluids | saturated solution | A solution that contains as much dissolved materials as it can hold at a given temperature. Precipitation of some components will likely occur if a more soluble compound is introduced or if the temperature is changed. |
| Drilling Fluids | SBR | Ratio of the volume percent synthetic fluid to the volume percent brine in a synthetic mud, where each is expressed as a percent of the total liquid in the mud. The SBR is calculated in an analogous way to the oil/brine ratio using data from the retort test. |
| Drilling Fluids | scale | A deposit or coating formed on the surface of metal, rock or other material. Scale is caused by a precipitation due to a chemical reaction with the surface, precipitation caused by chemical reactions, a change in pressure or temperature, or a change in the composition of a solution. The term is also applied to a corrosion product. Typical scales are calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate, iron sulfide, iron oxides, iron carbonate, the various silicates and phosphates and oxides, or any of a number of compounds insoluble or slightly soluble in water. |
| Drilling Fluids | scavenger | A treating chemical that is added to a drilling mud or other fluid to react with a contaminant to change the contaminant to a less harmful compound. If a contaminant is harmful at very low concentration, a scavenger must be able to remove the contaminant to an even lower concentration to ensure safety. |
| Drilling Fluids | SCC | A form of corrosion in which susceptible types of metals will break by a combination of stress within the metal and the specific type of corrosion. Sulfide corrosion of ferrous alloys and chloride corrosion of stainless steels are two common type of SCC. When high-strength steel remains in contact with hydrogen sulfide (or sulfide ion) in a water-mud environment, sulfide SCC may occur. Tool joints, hardened parts of blowout preventers and valve trim are particularly susceptible to brittle failure caused by sulfide SCC. For this reason, along with toxicity risks of hydrogen sulfide gas, it is essential that water muds be kept entirely free of soluble sulfides and especially hydrogen sulfide at low pH. |
| Drilling Fluids | seawater mud | A water-base mud designed for offshore drilling whose make-up water is taken from the ocean. Sea water contains relatively low salinity, about 3 to 4 wt. % NaCl, but has a high hardness because of Mg+2 and Ca+2 ions. Hardness is removed from sea water by adding NaOH, which precipitates Mg+2 as Mg(OH)2, and by adding Na2CO3, which removes Ca+2 as CaCO3. Mud additives are the same as those used in freshwater mud-bentonite clay, lignosulfonate, lignite, carboxymethylcellulose or polyanionic cellulose and caustic soda. XC polymer may also be used in place of bentonite. Higher concentrations of each additive are required because of salinity effects. Bentonite (if used) should be prehydrated in fresh water. |
| Drilling Fluids | sedimentation | The process of separation of the components of a cement slurry during which the solids settle. Sedimentation is one of the characterizations used to define slurry stability. |
| Drilling Fluids | sepiolite | A clay mineral with long, slender, needle-like structure, similar to attapulgite. It contains a mixture of fibrous and amorphous clay-like materials. API and ISO specifications exist for sepiolite used in drilling fluids. |
| Drilling Fluids | sequestering agent | A chemical whose molecular structure can envelop and hold a certain type of ion in a stable and soluble complex. Divalent cations, such as hardness ions, form stable and soluble complex structures with several types of sequestering chemicals. When held inside the complex, the ions have a limited ability to react with other ions, clays or polymers. Ethylenediamine tetraacetic acid (EDTA) is a well-known sequestering agent for the hardness ions, such as Ca+2, and is the reagent solution used in the hardness test protocol published by API. Polyphosphates can also sequester hardness ions. The addition of sodium acid pyrophosphate (SAPP) to a cement-contaminated mud renders the calcium ions essentially nonreactive with clays in the mud. As a side benefit, SAPP also lowers mud pH. Sequestering is not the same as precipitation because sequestering does not form a solid. |
| Drilling Fluids | shaly | Containing shale, a fine-grained, impermeable, sedimentary rock composed of clays and other minerals, usually with a high percentage of quartz. Shale is the most common, and certainly the most troublesome, rock type that must be drilled in order to reach oil and gas deposits. The characteristic that makes shales most troublesome to drillers is its water sensitivity, due in part to its clay content and the ionic composition of the clay. For this reason, oil-base drilling fluids are the mud of choice to drill the most water-sensitive shales. |
| Drilling Fluids | shear rate | The velocity gradient measured across the diameter of a fluid-flow channel, be it a pipe, annulus or other shape. Shear rate is the rate of change of velocity at which one layer of fluid passes over an adjacent layer. As an example, consider that a fluid is placed between two parallel plates that are 1.0 cm apart, the upper plate moving at a velocity of 1.0 cm/sec and the lower plate fixed. The fluid layer at the lower plate is not moving and the layer nearest the top plate is moving at 1.0 cm/sec. Halfway between the plate, a layer is moving at 0.5 cm/sec. The velocity gradient is the rate of change of velocity with distance from the plates. This simple case shows the uniform velocity gradient with shear rate (v1 – v2)/h = shear rate = (cm/sec)/(cm/1) = 1/sec. Hence, shear rate units are reciprocal seconds. |
| Drilling Fluids | shear strength | Another term for gel strength in a fluid. Shear strength opposes the movement of mud, whether by pumping or movement of pipe in a wellbore. Excessive shear strength can develop after a mud has been quiescent in the hole at high temperature for a period of time. Shear strength can be measured according to procedures published by the API. |
| Drilling Fluids | shear strength measurement test | A test procedure published by the API that specifies the use of a shearometer tube and a set of weights to measure the shear strength of a mud (lbf/100 ft2 or kPa). The typical use for this test is for evaluation of a static-aged mud sample left at high temperature for several hours. The shear tube is placed on the surface of the gelled mud and weights are applied until the tube sinks to a marked depth. The applied weight indicates shear strength of the mud sample. Reference: Watkins TE and Nelson ME: “High Temperature Gellation of Drilling Fluids,” Transactions of the AIME 193 (1953): 213-218. |
| Drilling Fluids | shear stress |
The force per unit area required to sustain a constant rate of fluid movement. Mathematically, shear stress can be defined as: ? = F/A, where ? = shear stess F = shear force A = Area acted on by the shear force. If a fluid is placed between two parallel |
| Drilling Fluids | shearometer | An instrument used together with a set of weights to conduct a shear-strength measurement test. This test procedure published by the API measures the shear strength of a mud (lbf/100 ft2 or kPa). The typical use for this test is for evaluation of a static-aged mud sample left at high temperature for several hours. The shearometer tube is placed on the surface of the gelled mud and weights are applied until the tube sinks to a marked depth. The applied weight indicates shear strength of the mud sample. Reference: Watkins TE and Nelson ME: High Temperature Gellation of Drilling Fluids, Transactions of the AIME 193 (1953): 213-218. |
| Drilling Fluids | shear-strength measurement test | A test procedure published by the API that specifies the use of a shearometer tube and a set of weights to measure the shear strength of a mud (lbf/100 ft2 or kPa). The typical use for this test is for evaluation of a static-aged mud sample left at high temperature for several hours. The shear tube is placed on the surface of the gelled mud and weights are applied until the tube sinks to a marked depth. The applied weight indicates shear strength of the mud sample. Reference: Watkins TE and Nelson ME: High Temperature Gellation of Drilling Fluids, Transactions of the AIME 193 (1953): 213-218. |
| Drilling Fluids | sheen | A luster, brightness or radiance. A sheen appears as a spectrum of colors and is commonly caused by a thin film on a surface that diffracts light. A film of diesel oil on water has a multicolored luster and is an indicator of an oil spill or oil slick. |
| Drilling Fluids | sheen test | A test intended to indicate the presence of free oil when drilling fluid, drilled cuttings, deck drainage, well treatment fluids, completion and workover fluids, produced water or sand or excess cement slurry are discharged into offshore waters. Two types of sheen tests are mandated by EPA under NPDES permits. The visual sheen test consists of an observation made when surface and atmospheric conditions permit watching the ocean water for a sheen around the point where the discharge entered the water. When the conditions do not permit visual observations, a static sheen test is mandated by NPDES permits and the protocol published by US EPA. This test uses sea water in a shallow pan (not more than 30 cm deep) with 1000 cm2 surface area. Either 15 cm3 of fresh mud or 15 g fresh cuttings are injected below the surface of the water. An observer watches for up to 1.0 hour for a silvery, metallic, colored or iridescent sheen. If sheen covers 50% of the area, the mud or cuttings cannot be discharged. |
| Drilling Fluids | shrimp test | A laboratory test or other assessment utilizing a living organism, such as mysid shrimp, to determine the effect of a condition to which the organism is exposed. Such tests are performed under controlled environmental conditions and duration. Bioassay tests of drilling fluids are required by governmental agencies throughout the world prior to discharge of mud or cuttings. The organisms used in bioassays are those found in the area that would be most affected by contact with the proposed drilling fluid. The dosage of interest is typically the lethal concentration, known as LC50, that will kill 50% of the population of organisms in a given period of time. Chronic bioassay tests indicate sublethal effects, such as changes in growth or reproduction of the organism over a longer period of time. |
| Drilling Fluids | siderite | A mineral composed of ferrous carbonate, FeCO3, and having 3.8 g/cm3 specific gravity. It is found as an accessory mineral in some shales and carbonate rocks and also in some barite and hematite ores. FeCO3 is readily soluble in acids and breaks down slowly in alkaline muds, particularly at high temperature to form a gelatinous solid, Fe(OH)2, and soluble CO3-2 anions. Reference: Binder GG, Carlton LA and Garrett RL: Evaluating Barite as a Source of Soluble Carbonate and Sulfide Contamination in Drilling Fluids, Journal of Petroleum Technology 33, no. 12 (December 1981): 2371-2376. |
| Drilling Fluids | silica layer | One of the layers that constitute the atomic structure of the clay group of layered silicate minerals. The structure of these minerals can consist of two, three or four layers. The silica layer is a plane of silicon dioxide tetrahedra (silicon at the center and oxygen at all four corners of the tetrahedron). Another structural layer is a plane of aluminum hydroxide octahedra (aluminum at the center and hydroxides at all six corners). The tetrahedral and octahedral layers fit one on top of the other, with oxygen atoms being shared as oxide and hydroxide groups. |
| Drilling Fluids | silicate | A type of salt derived from silicic acid |
| Drilling Fluids | silicate anion | The anion, SiO4-4, found in solutions of sodium and potassium silicate, formed by dissolving silica or silicate minerals in NaOH or KOH solutions. Silicate anions form polysilicates, or colloidal silica gel. |
| Drilling Fluids | silicate mud | A type of shale-inhibitive water base drilling fluid that contains sodium silicate or potassium silicate polymeric ions. These ions adsorb on the shale surface and form a semipermeable osmotic membrane that prevents the transport of water and ions internal to the shale structure. This physicochemical barrier helps improve wellbore stability and provides in-gauge holes through troublesome shale sections that otherwise might require a nonaqueous drilling fluid. Silicate-gel drilling muds were first used in the 1930s to control problematic shales. In the 1990s, silicate nondispersed polymer drilling fluids were reintroduced to provide a high-performance shale-inhibitive water-base fluid, as an alternative to oil-base fluids. The highly inhibitive silicate fluid not only provides wellbore stability but also improves solids control performance with minimal environmental impact. |
| Drilling Fluids | silicic acid | A group of seven hydrated forms of SiO2, including the following silicic acids: tetra, H2Si4O9, meta-di, H2Si2O5, meta-tri, H4Si3O8, meta, H2SiO3, ortho-tri, H8Si3O10, ortho-di, H6Si2O7 and ortho, H4SiO4. The latter formula is often written as Si(OH)4. Silicic acids and silicate anions polymerize through formation of multiple Si-O-Si bonds. The polysilic structure can be linear or cyclic and is not uniform in size. |
| Drilling Fluids | silt | A term used to describe particle whose size is between 2 and 74 micrometers (200 mesh). |
| Drilling Fluids | silver nitrate | A chemical used as a titrant with potassium chromate as the endpoint indicator, for a chloride test, a titration procedure standardized by the API to quantitatively determine Cl- (chloride ion) concentration. |
| Drilling Fluids | sintered | Pertaining to a type of filter medium in which the particles are fused together to give a designed permeability. Sintered filters are used in the API high-pressure, high-temperature filtration test above about 375°F [190°C] and in laboratory tests of formation damage. Large-scale sintered filters are also used to clean up clear brines after use. |
| Drilling Fluids | sized calcium carbonate | Calcium carbonate, such as limestone, marble or oyster shells, that has a specified minimum and maximum range of particle sizes and may also have a specified distribution of sizes. It is used as a bridging agent in drill-in, workover and completion fluids to positively seal permeable zones by plugging pores at the wellbore face. It has the additional advantage that it can be dissolved by acid treatment to clean up the zone afterwards. |
| Drilling Fluids | sized salt | NaCl solid particles that have a specified minimum and maximum range of particle sizes and may also have a specified distribution of sizes. Sized salt is used as a bridging agent in saturated saltwater systems used as drill-in, workover and completion fluids. Sized salt can positively seal permeable zones by plugging pores at the wellbore face. It is a preferred bridging agent because it can be dissolved by low-salinity water treatment to clean up the zone afterwards. |
| Drilling Fluids | slaked lime | Common term for calcium hydroxide, a chemical with formula Ca(OH)2. Lime is used in lime muds and as a treatment to remove carbonate ions. It is used as a stabilizing ingredient in oil- and synthetic-base mud, essential to formation of fatty-acid soap emulsifiers. It is an alkaline material that can be carried in excess to neutralize hydrogen sulfide [H2S] and carbon dioxide [CO2]. |
| Drilling Fluids | sloughing shale | A fine-grained, impermeable, sedimentary rock composed of clays and other minerals, usually with a high percentage of quartz. Shale is the most common, and certainly the most troublesome, rock type that must be drilled in order to reach oil and gas deposits. The characteristic that makes shales most troublesome to drillers is its water sensitivity, due in part to its clay content and the ionic composition of the clay. For this reason, oil-base drilling fluids are the mud of choice to drill the most water-sensitive shales. |
| Drilling Fluids | slug | A volume of mud that is more dense than the mud in the drillpipe and wellbore annulus. A slug is used to displace mud out of the upper part of the drillpipe before pulling pipe out of the hole and is mixed in the pill pit by adding additional weighting material (barite) to a few barrels of mud from the surface pits. The pill is pumped into the top of the drillstring to push mud downward, out of the pipe, thus keeping the upper stands of pipe empty. |
| Drilling Fluids | slugging pill | Also called a slug, a volume of mud that is more dense than the mud in the drillpipe and wellbore annulus. A slug is used to displace mud out of the upper part of the drillpipe before pulling pipe out of the hole and is mixed in the pill pit by adding additional weighting material (barite) to a few barrels of mud from the surface pits. The pill is pumped into the top of the drillstring to push mud downward, out of the pipe, thus keeping the upper stands of pipe empty. |
| Drilling Fluids | slurries | Plural form of slurry |
| Drilling Fluids | slurry | A mixture of suspended solids and liquids. Muds in general are slurries, but are seldom called that. Cement is a slurry and is often referred to as such. |
| Drilling Fluids | slurry density | The weight per unit volume of a cement slurry, usually given in units of kg/m3 or lbm/gal. Typical oil- or gas-well slurries have densities of 1380 kg/m3 to 2280 kg/m3 [11.5 lbm/gal to 19.0 lbm/gal], although special techniques, such as foamed cementing and particle-size distribution cementing, extend this range to 840 kg/m3 to 2760 kg/m3 [7 lbm/gal to 23 lbm/gal]. |
| Drilling Fluids | slurry stability | The ability of a cement slurry to maintain homogeneity. Two tests are used as a measure of slurry stability: the free-fluid test and the sedimentation test. |
| Drilling Fluids | slurry yield | The volume of slurry obtained when one sack of cement is mixed with the desired amount of water and other additives, usually given in units of m3/kg or ft3/sk (sack). |
| Drilling Fluids | smectite clay | A category of clay minerals that have a three-layer crystalline structure (one alumina and two silica layers) and that exhibit a common characteristic of hydrational swelling when exposed to with water. Montmorillonite is a well-known smectite clay mineral to those working in drilling and drilling fluids. Its sodium form, bentonite, is a widely-used water mud additive. It is also used as an oil-mud additive when made oil-dispersible by surface treatment. Smectite clays that occur naturally in shales cause wellbore and mud-control problems due to their hydrational swelling and colloidal dispersion characteristics. |
| Drilling Fluids | soap | A collective term for organic salts made by reacting an aliphatic carboxylic acid with a base. The base can be an alkali-metal hydroxide (NaOH or KOH), alkaline-earth hydroxide (Ca(OH)2 or Mg(OH)2) or oxide (CaO or MgO). Fatty acids are the carboxylic acids often used to make soaps for oilfield applications, such as emulsifiers for oil muds. Aluminum soaps are used as defoamers in drilling fluids. Sodium and potassium soaps are detergents to emulsify oil into water. |
| Drilling Fluids | soda ash | Another term for sodium carbonate, a chemical with the formula Na2CO3. It is called soda ash at the drilling rig and is used to treat most types of calcium ion contamination in freshwater and seawater muds. For cement contamination, sodium bicarbonate is used. Calcium ions from drilling gypsum or anhydrite, CaSO4, cause clay flocculation and polymer precipitation and lower pH. A soda-ash treatment is appropriate for gypsum contamination because caustic soda, NaOH, is not needed to raise pH. This is also generally the case with hard water influxes into water muds. |
| Drilling Fluids | sodium acid pyrophosphate | A sequestering agent used to treat cement contamination and a deflocculant for low-temperature water muds. The term is sometimes abbreviated to SAPP. |
| Drilling Fluids | sodium bicarbonate | A chemical with the formula NaHCO3. It is called bicarb at the drilling rig and is used to treat cement contamination in water mud. When cement hydrates, substantial amounts of lime, Ca(OH)2, are produced. As the cement sets, less free lime is available. When partially set cement is drilled with a water mud, Ca+2 and OH- ions are leached into the mud, often causing problems associated with clay flocculation and polymer precipitation. Bicarb can be added, either as a pretreatment or over a period of time, to remove the Ca+2 in the form of insoluble CaCO3 while simultaneously neutralizing OH- ions with the H+ ion in the bicarb molecule. |
| Drilling Fluids | sodium carbonate | A chemical with the formula Na2CO3. It is called soda ash at the drilling rig and is used to treat most types of calcium ion contamination in freshwater and seawater muds. For cement contamination, sodium bicarbonate is used. Calcium ions from drilling gypsum or anhydrite, CaSO4, cause clay flocculation and polymer precipitation and lower pH. A soda-ash treatment is appropriate for gypsum contamination because caustic soda, NaOH, is not needed to raise pH. This is also generally the case with hard water influxes into water muds. |
| Drilling Fluids | sodium chromate | A sodium salt, Na2CrO4, in which chromium atoms are in the plus-6 valence state. Chromium compounds of various types have been used in lignite and lignosulfonate and other mud additives to enhance thermal stability. Since the late 1970s, they are prohibited in muds to be discarded offshore and in other environmentally sensitive areas of the US. |
| Drilling Fluids | sodium dichromate | A sodium salt, Na2Cr2O7, in which chromium atoms are in the plus-7 valence state. |
| Drilling Fluids | sodium hydroxide | A compound with the chemical formula NaOH. It is commonly known as caustic soda and used in most water-base muds to increase and maintain pH and alkalinity. It is a hazardous material to handle because it is very caustic and gives off heat when dissolved in water. Proper training and equipment are needed to handle it safely. |
| Drilling Fluids | sodium phosphate | A group of salts formed by neutralization of phosphorous or phosphoric acid with sodium hydroxide (NaOH). Orthophosphates are phosphoric acid (H3PO4) salts, where 1, 2 or 3 of the hydrogen ions are neutralized. Neutralization with NaOH gives three sodium orthophosphates: (a) monosodium phosphate (MSP), (b) disodium phosphate (DSP) or (c) trisodium phosphate (TSP). Their solutions are buffers in the 4.6 to 12 pH range. TSP is an excellent degreaser. All will precipitate hardness ions such as calcium. Polyphosphates are polymers made from various orthophosphates by dehydration with heat. Sodium acid pyrophosphate (SAPP) is a clay deflocculant and treatment for cement contamination. For clay deflocculation, polyphosphates are limited by the temperature at which they hydrolyze back to orthophosphates, although several that performed up to 280°F [138°C] have been documented in the literature (see reference). Reference: Sikorski CF and Weintritt DJ: Polyphosphate Drilling-Mud Thinners Deserve Second Look, Oil & Gas Journal 81, no. 27 (July 4, 1983): 71-78. |
| Drilling Fluids | sodium polyacrylate | Made by neutralizing a linear, anionic polymer made from the monomer acrylic acid, CH2=CHCOO- H+, with sodium hydroxide (NaOH). Sodium polyacrylate is often abbreviated to SPA. Polyacrylates are best utilized in soft water with low salinity to achieve the best dispersion and full chain elongation. Even low concentrations of hardness ions, for example, Ca+2, precipitate polyacrylates. Low molecular-weight polyacrylates are used as clay deflocculants. High molecular weight polymers are used for fluid-loss control and as a clay extender. As an extender, SPA is added to bentonite at the grinding plant. It is also used at the rig in low-solids mud. Divalent cations can negate its benefits as a clay extender. SPA is highly efficient when used to flocculate colloids in native-solids muds, clear-water muds and wastewater cleanup. The polymer chain links together colloidal solids that can be removed by gravity settling in shallow pits or by applying hydrocyclone, centrifuge or filtration techniques. |
| Drilling Fluids | sodium silicate | Formed by dissolving silica or silicate minerals in NaOH solutions. The silicate anion, SiO4-4, is found in solutions of sodium silicate. Silicate anions form polysilicates, or colloidal silica gel. A silicate mud is a type of shale-inhibitive water base drilling fluid that contains sodium silicate or potassium silicate polymeric ions. These ions adsorb on the shale surface and form a semipermeable osmotic membrane that prevents the transport of water and ions internal to the shale structure. This physicochemical barrier helps improve wellbore stability and provides in-gauge holes through troublesome shale sections that otherwise might require a nonaqueous drilling fluid. Silicate-gel drilling muds were first used in the 1930s to control problematic shales. In the 1990s, silicate nondispersed polymer drilling fluids were reintroduced to provide a high-performance shale-inhibitive water-base fluid, as an alternative to oil-base fluids. The highly inhibitive silicate fluid not only provides wellbore stability but also improves solids control performance with minimal environmental impact. |
| Drilling Fluids | soft water | Water that does not contain divalent cations, such as Ca+2, Mg+2 or Fe+2 and is therefore suitable for prehydrating bentonite or polymers. |
| Drilling Fluids | sour gas | A general term for those gases that are acidic either alone or when associated with water. Two sour gases associated with oil and gas drilling and production are hydrogen sulfide, H2S, and carbon dioxide, CO2. Sulfur oxides and nitrogen oxides, generated by oxidation of certain sulfur- or nitrogen-bearing materials, are also in this category but not found in the anaerobic conditions of the subsurface. |
| Drilling Fluids | SPA | Abbreviation for sodium polyacrylate. SPA is the result of neutralizing a linear, anionic polymer made from the monomer acrylic acid, CH2=CHCOO- H+, with sodium hydroxide (NaOH). Polyacrylates are best utilized in soft water with low salinity to achieve the best dispersion and full chain elongation. Even low concentrations of hardness ions, for example, Ca+2, precipitate polyacrylates. Low molecular-weight polyacrylates are used as clay deflocculants. High molecular weight polymers are used for fluid-loss control and as a clay extender. As an extender, SPA is added to bentonite at the grinding plant. It is also used at the rig in low-solids mud. Divalent cations can negate its benefits as a clay extender. SPA is highly efficient when used to flocculate colloids in native-solids muds, clear-water muds and wastewater cleanup. The polymer chain links together colloidal solids that can be removed by gravity settling in shallow pits or by applying hydrocyclone, centrifuge or filtration techniques. |
| Drilling Fluids | spot | To place a small volume or pill of fluid in a wellbore annulus to free differentially stuck pipe. Oil-base mud is the traditional stuck-pipe spotting fluid. Speed in mixing and placing the spot is of primary importance to successfully freeing pipe. Because of concern about mud disposal, spots used offshore are either synthetic-based emulsions or benign water-base formulations. Each type is supplied as prepackaged concentrate designed for rapid access and mixing at the rig. A spot frees pipe by covering the stuck region. It presumably breaks up the filter cake, allowing the spot to migrate into cracks in the cake and between the pipe and the cake, reducing the stuck area and allowing pipe to be pulled free. |
| Drilling Fluids | spotting | Placing a small volume or pill of fluid in a wellbore annulus to free differentially stuck pipe. Oil-base mud is the traditional stuck-pipe spotting fluid. Speed in mixing and placing the spot is of primary importance to successfully freeing pipe. Because of concern about mud disposal, spots used offshore are either synthetic-based emulsions or benign water-base formulations. Each type is supplied as prepackaged concentrate designed for rapid access and mixing at the rig. A spot frees pipe by covering the stuck region. It presumably breaks up the filter cake, allowing the spot to migrate into cracks in the cake and between the pipe and the cake, reducing the stuck area and allowing pipe to be pulled free. |
| Drilling Fluids | spotting fluid | A small volume or pill of fluid placed in a wellbore annulus to free differentially stuck pipe. Oil-base mud is the traditional stuck-pipe spotting fluid. Speed in mixing and placing the spot is of primary importance to successfully freeing pipe. Because of concern about mud disposal, spots used offshore are either synthetic-based emulsions or benign water-base formulations. Each type is supplied as prepackaged concentrate designed for rapid access and mixing at the rig. A spot frees pipe by covering the stuck region. It presumably breaks up the filter cake, allowing the spot to migrate into cracks in the cake and between the pipe and the cake, reducing the stuck area and allowing pipe to be pulled free. |
| Drilling Fluids | spud mud | Mud used to drill a well from surface to a shallow depth. Guar gum or salt gel are commonly used offshore as spud mud. Onshore spud mud is usually a water-base mud containing bentonite clay that is flocculated with lime. In a large-diameter surface hole, a flocculated clay-based mud can remove large gravel cuttings encountered at shallow depths and is simple and inexpensive. |
| Drilling Fluids | spurt loss | The instantaneous volume (spurt) of liquid that passes through a filter medium prior to deposition of a competent and controlling filter cake. In static filtration, the spurt volume can be a disproportionately large percentage of the total 30-minute filtrate in an API-type test. This indicates that filter cake is slow in being deposited compared to a similar mud with lower spurt loss. To avoid uncertainties of spurt-loss volume, relative filtrate volume is used in static tests. |
| Drilling Fluids | squeeze cementing | The forcing, by pressure, of cement slurry into a specified location in a well, such as channels or perforations, for the purpose of achieving zonal isolation. Squeeze cementing is a remedial cementing technique used to repair flaws in primary cement or damage incurred by corrosive fluids. |
| Drilling Fluids | SRB | A common anaerobic bacterium, commonly abbreviated SRB, that can convert sulfate ions, SO4-2, into S-2 and HS-, with the concomitant oxidation of a carbon source. The lignite, lignin, tannins, cellulose, starches and fatty acids found in many mud systems are carbon food sources for SRB. Where mud is stored, precautions should always be taken when handling or reconditioning water muds containing lignosulfonates, gypsum (sulfate sources) and starches, cellulose, xanthan gum and lignite (food sources). These muds can harbor SRB and can have high sulfide accumulations. Mud filtrate should be tested with the Garrett Gas Train to determine sulfide concentration in a stored mud, followed by treatments with caustic soda to raise pH and zinc-based scavengers to remove sulfides as ZnS. Before storage of mud, treatment with a bactericide can inhibit SRB growth. Also, circulating mud from time to time, with air entrainment, can retard development of anaerobic conditions. Anaerobic bacteria can convert the sulfate or sulfite present in water handling facilities to hydrogen sulfide [H2S]. This by-product, combined with iron, can form iron sulfide, a scale that is very difficult to remove. SRB occur naturally in surface waters, including seawater. Bacteria accumulation can lead to pitting of steel, and the buildup of H2S increases the corrosiveness of the water, thus increasing the possibility of hydrogen blistering or sulfide stress cracking. |
| Drilling Fluids | SSMA | A copolymer of polystyrene (containing sulfonate groups on the ring) and anhydrous maleic acid (a di-hydroxy acid). The sulfonated ring-structure polymer component is anionic and usually low to moderate in chain length and molecular weight. As such, with negative groups on the structure (amount of negativity depending on degree of sulfonation), it is used as a clay deflocculant for bentonite-based water mud. It is especially stable to temperature up to around 400°F [204°C], and often used in high-density muds to stabilize rheology. Lignosulfonate is used for this purpose up to about 300°F [149°C] and then SSMA polymeric deflocculant is often phased into the mud system for drilling deeper and hotter zones. |
| Drilling Fluids | SSMA copolymer | A copolymer of polystyrene (containing sulfonate groups on the ring) and anhydrous maleic acid (a di-hydroxy acid). The sulfonated ring-structure polymer component is anionic and usually low to moderate in chain length and molecular weight. As such, with negative groups on the structure (amount of negativity depending on degree of sulfonation), it is used as a clay deflocculant for bentonite-based water mud. It is especially stable to temperature up to around 400°F [204°C], and often used in high-density muds to stabilize rheology. Lignosulfonate is used for this purpose up to about 300°F [149°C] and then SSMA polymeric deflocculant is often phased into the mud system for drilling deeper and hotter zones. |
| Drilling Fluids | stability meter | A piece of equipment required to conduct the electrical stability (ES) test, a test for oil-base and synthetic-base muds that indicates the emulsion and oil-wetting qualities of the sample. The test is performed by inserting the ES probe into a cup of 120°F [48.9°C] mud and pushing a test button. The ES meter automatically applies an increasing voltage (from 0 to 2000 volts) across an electrode gap in the probe. Maximum voltage that the mud will sustain across the gap before conducting current is displayed as the ES voltage. The modern ES meter has sine-wave circuitry, whereas older meters used square-wave circuits. (The older units should not be used because they do not correctly address the theory described in the reference below.) The ES sine-wave design and meaning of ES readings have been studied and were found to relate to an oil mud’s oil-wetting of solids and to stability of the emulsion droplets in a complex fashion not yet understood. |
| Drilling Fluids | starch | A drilling-mud additive used to control fluid loss in water muds ranging from freshwater to saturated-salt to high-pH lime muds. Starches have thermal stability to about 250°F [121°C]. They are subject to bacterial attack unless protected by high salinity or bactericide. Drilling-grade natural starch has API/ISO specifications for quality. Starches are carbohydrates of a general formula (C6H10O5)n and are derived from corn, wheat, oats, rice, potatoes, yucca and similar plants and vegetables. They consist of about 27% linear polymer (amylose) and about 73% branched polymer (amylopectin). The two polymers are intertwined within starch granules. Granules are insoluble in cold water, but soaking in hot water or under steam pressure ruptures their covering and the polymers hydrate into a colloidal suspension. This product is a pregelatinized starch and has been used in muds for many years. Amylose and amylopectin are nonionic polymers that do not interact with electrolytes. Derivatized starches, such as hydroxypropyl and carboxymethyl starches, are used in drill-in fluids, completion fluids and various brine systems as well as in drilling-mud systems. The use of starch typically causes a minimal increase in viscosity while effectively controlling fluid loss. |
| Drilling Fluids | static aging test | A mud test in which the mud sample is not agitated. This test is usually performed at a selected high temperature. Typically, the mud sample is sealed in a mud-aging cell and placed in an oven for a given period of time (often 16 hours, overnight). The cooled mud is tested before it is stirred. Commonly, the sample is retested after it has been stirred. Static-aged mud, before it is stirred, simulates a mud that is static in a well while pipe is out of the hole during a bit trip or logging run. The stirred mud simulates the same mud after arriving at the surface and after being agitated by mud guns and through centrifugal pumps. The amount and kind of mud treatment are determined from these tests. |
| Drilling Fluids | static filter press | A pressurized cell, fitted with a filter medium, used for evaluating filtration characteristics of a drilling fluid while it is static in the test cell. Generally, either low-pressure, low-temperature or high-pressure, high-temperature devices are used. |
| Drilling Fluids | static filtration | A filtration process in which the slurry being filtered remains static. Filter cake continues to grow thicker as filtration continues. Under static conditions, no cake erosion occurs. In theory, the filtrate volume increases as the square root of elapsed time, ignoring spurt loss. |
| Drilling Fluids | static-aging test | A mud test in which the mud sample is not agitated. This test is usually performed at a selected high temperature. Typically, the mud sample is sealed in a mud-aging cell and placed in an oven for a given period of time (often 16 hours, overnight). The cooled mud is tested before it is stirred. Commonly, the sample is retested after it has been stirred. Static-aged mud, before it is stirred, simulates a mud that is static in a well while pipe is out of the hole during a bit trip or logging run. The stirred mud simulates the same mud after arriving at the surface and after being agitated by mud guns and through centrifugal pumps. The amount and kind of mud treatment are determined from these tests. |
| Drilling Fluids | stearate | A salt made with stearic acid, which is a fatty acid, a type of organic acid derived from animal and vegetable fats and oils. Fatty acids are the raw materials used in the manufacture of many drilling-fluid additives, such as emulsifiers, oil-wetting agents and lubricants. |
| Drilling Fluids | strength retrogression | A decline of cement strength at elevated temperatures. This decline is pronounced at temperatures above 230°F [110°C], but it may be controlled by the addition of silica to the cement. |
| Drilling Fluids | stress corrosion cracking | A form of corrosion in which susceptible types of metals will break by a combination of stress within the metal and the specific type of corrosion. Sulfide corrosion of ferrous alloys and chloride corrosion of stainless steels are two common type of SCC. When high-strength steel remains in contact with hydrogen sulfide (or sulfide ion) in a water-mud environment, sulfide SCC may occur. Tool joints, hardened parts of blowout preventers and valve trim are particularly susceptible to brittle failure caused by sulfide SCC. For this reason, along with toxicity risks of hydrogen sulfide gas, it is essential that water muds be kept entirely free of soluble sulfides and especially hydrogen sulfide at low pH. |
| Drilling Fluids | stress-corrosion cracking | A form of corrosion in which susceptible types of metals will break by a combination of stress within the metal and the specific type of corrosion. Sulfide corrosion of ferrous alloys and chloride corrosion of stainless steels are two common type of SCC. When high-strength steel remains in contact with hydrogen sulfide (or sulfide ion) in a water-mud environment, sulfide SCC may occur. Tool joints, hardened parts of blowout preventers and valve trim are particularly susceptible to brittle failure caused by sulfide SCC. For this reason, along with toxicity risks of hydrogen sulfide gas, it is essential that water muds be kept entirely free of soluble sulfides and especially hydrogen sulfide at low pH. |
| Drilling Fluids | styrene | The compound C6H5-HC=CH2, also known as styrolene, cinnamene and phenethylene. The phenyl radical, C6H5-, replaces one of the hydrogen atoms on ethylene. Styrene polymers are analogous to vinyl polymers in structure except that phenyl radicals replace the corresponding H atom. (Due to larger size of the phenyl radical as compared to the H atom, not all corresponding polymers are possible.) A phenyl radical has the benzene ring structure-missing one H-with alternating double bonds between adjacent carbons. It is an aromatic group that is nonionic. |
| Drilling Fluids | sulfate reducing bacteria | A common anaerobic bacterium, commonly abbreviated SRB, that can convert sulfate ions, SO4-2, into S-2 and HS-, with the concomitant oxidation of a carbon source. The lignite, lignin, tannins, cellulose, starches and fatty acids found in many mud systems are carbon food sources for SRB. Where mud is stored, precautions should always be taken when handling or reconditioning water muds containing lignosulfonates, gypsum (sulfate sources) and starches, cellulose, xanthan gum and lignite (food sources). These muds can harbor SRB and can have high sulfide accumulations. Mud filtrate should be tested with the Garrett Gas Train to determine sulfide concentration in a stored mud, followed by treatments with caustic soda to raise pH and zinc-based scavengers to remove sulfides as ZnS. Before storage of mud, treatment with a bactericide can inhibit SRB growth. Also, circulating mud from time to time, with air entrainment, can retard development of anaerobic conditions. Anaerobic bacteria can convert the sulfate or sulfite present in water handling facilities to hydrogen sulfide [H2S]. This by-product, combined with iron, can form iron sulfide, a scale that is very difficult to remove. SRB occur naturally in surface waters, including seawater. Bacteria accumulation can lead to pitting of steel, and the buildup of H2S increases the corrosiveness of the water, thus increasing the possibility of hydrogen blistering or sulfide stress cracking. |
| Drilling Fluids | sulfate resistance | The ability of set cement to resist deterioration in the presence of sulfate ions. |
| Drilling Fluids | sulfate resistant cement | A cement in which the amount of tricalcium aluminate is controlled as specified by API Specification 10A. |
| Drilling Fluids | sulfate-reducing bacteria | A common anaerobic bacterium, commonly abbreviated SRB, that can convert sulfate ions, SO4-2, into S-2 and HS-, with the concomitant oxidation of a carbon source. The lignite, lignin, tannins, cellulose, starches and fatty acids found in many mud systems are carbon food sources for SRB. Where mud is stored, precautions should always be taken when handling or reconditioning water muds containing lignosulfonates, gypsum (sulfate sources) and starches, cellulose, xanthan gum and lignite (food sources). These muds can harbor SRB and can have high sulfide accumulations. Mud filtrate should be tested with the Garrett Gas Train to determine sulfide concentration in a stored mud, followed by treatments with caustic soda to raise pH and zinc-based scavengers to remove sulfides as ZnS. Before storage of mud, treatment with a bactericide can inhibit SRB growth. Also, circulating mud from time to time, with air entrainment, can retard development of anaerobic conditions. Anaerobic bacteria can convert the sulfate or sulfite present in water handling facilities to hydrogen sulfide [H2S]. This by-product, combined with iron, can form iron sulfide, a scale that is very difficult to remove. SRB occur naturally in surface waters, including seawater. Bacteria accumulation can lead to pitting of steel, and the buildup of H2S increases the corrosiveness of the water, thus increasing the possibility of hydrogen blistering or sulfide stress cracking. |
| Drilling Fluids | sulfate-resistant cement | A cement in which the amount of tricalcium aluminate is controlled as specified by API Specification 10A. |
| Drilling Fluids | sulfide | A compound of sulfur that contains the S-2 ion. H2S is the gaseous and highly toxic molecular form often found in the subsurface. Sulfide, S-2, and bisulfide, HS-, are the corresponding ionic forms. Sulfides can be generated from soluble iron sulfide minerals or from sulfate-reducing bacteria. The term active sulfide is used to denote compounds that revert to H2S gas when acidified with 2-molar citric acid solution, as opposed to inert sulfides, which are stable. Active sulfides include calcium sulfide and bisulfide formed when H2S reacts with lime in an oil mud. Their accumulation constitutes a safety concern at the rig because of the risk of reverting to H2S gas should an acidic influx occur. They may be converted to inert sulfides by adding zinc oxide. |
| Drilling Fluids | sulfide scavenger | A chemical that removes all three soluble sulfide species, H2S, S-2 and HS-, and forms a product that is nonhazardous and noncorrosive. Zinc compounds are commonly used to precipitate ZnS and decrease the concentration of all three sulfides that are in equilibrium in a solution to a very low concentration. For water mud, zinc basic carbonate, and, for oil mud, zinc oxide, are recognized to be effective sulfide scavengers. Oxidation of sulfides to form other types of sulfur compounds will remove sulfides from a mud, but slowly and with less certainty. |
| Drilling Fluids | sulfide test | A quantitative analysis of sulfides in the drilling fluid. Specific test methods have been published by API. The oil-mud procedure analyzes active sulfides and uses whole mud samples, whereas the water-base drilling fluid procedure tests filtrate. The instrument used is called a Garrett gas train (GGT), a clear, plastic block (2.5 in. x 4 in. x 6 in.) that contains three interconnected chambers. A carrier gas is used to flow an inert gas through the chambers. The sample is placed in chamber #1 and is acidified to release sulfides as H2S (and carbonates as CO2). The appropriate Drdger tube is used to measure the effluent gas that is evolved from the sample. The device is named after Bob Garrett, who invented it while at Exxon Production Research. |
| Drilling Fluids | sulfonated polystyrene-maleic anhydride copolymer | A copolymer of polystyrene (containing sulfonate groups on the ring) and anhydrous maleic acid (a di-hydroxy acid). The sulfonated ring-structure polymer component is anionic and usually low to moderate in chain length and molecular weight. As such, with negative groups on the structure (amount of negativity depending on degree of sulfonation), it is used as a clay deflocculant for bentonite-based water mud. It is especially stable to temperature up to around 400°F [204°C], and often used in high-density muds to stabilize rheology. Lignosulfonate is used for this purpose up to about 300°F [149°C] and then SSMA polymeric deflocculant is often phased into the mud system for drilling deeper and hotter zones. |
| Drilling Fluids | surface tension | Surface free energy that exists between a liquid and air. Surface tension can be observed as a curved meniscus in a small tube of the liquid. This energy barrier prevents a liquid (such as water) from spontaneously mixing with air to form a foam. To make a foam, as used for a drilling fluid, the liquid’s surface tension must be lowered by adding a third component (a foamer) that accumulates at the interface. Foam preparation usually requires mechanical energy to break up the bulk liquid into thin films around each gas bubble. |
| Drilling Fluids | suspended solids | Dispersed particles in a slurry that can be separated by filtration and are not dissolved. In the water, oil and solids test (retort test), the retort solids are divided into two types, dissolved and suspended solids. Suspended solids are the particulates. In calculating solids content of water- or oil-base muds, suspended solids are divided into high-gravity and low-gravity solids (HGS and LGS). LGS are sometimes further subdivided into active (clay) and inactive solids. |
| Drilling Fluids | SWR | Ratio of the volume percent synthetic fluid to the volume percent water in a synthetic-base mud, where each is a percent of the total liquid in the mud. The SWR is calculated in an analogous way to the oil/brine ratio using data from the retort test. |
| Drilling Fluids | syn | The slang abbreviation for synthetic. The term can be confusing to the uninitiated, so its use is avoided. |
| Drilling Fluids | synthetic base fluid | Any of a number of fluids (liquids) manufactured from starting products of known composition and purity. Popular fluid types include several olefin oligomers of ethylene. Esters made from vegetable fatty acid and alcohol were among the first such fluids. Ethers and polyethers, made from alcohols and polyalcohols, have been used, along with paraffinic hydrocarbons and linear alkyl benzenes. Mixtures of these fluids are also used to make synthetic-base muds. |
| Drilling Fluids | synthetic base mud | Nonaqueous, water-internal (invert) emulsion muds in which the external phase is a synthetic fluid rather than an oil. This and other more minor changes in formulations have made synthetic fluids in muds more environmentally acceptable for offshore use. Synthetic muds are popular in most offshore drilling areas, despite high initial mud costs, because of their environmental acceptance and approval to dispose of cuttings into the water. “Oil mud” should not be used to describe synthetic-base muds. |
| Drilling Fluids | synthetic/brine ratio | Ratio of the volume percent synthetic fluid to the volume percent brine in a synthetic mud, where each is expressed as a percent of the total liquid in the mud. The SBR is calculated in an analogous way to the oil/brine ratio using data from the retort test. |
| Drilling Fluids | synthetic/water ratio | Ratio of the volume percent synthetic fluid to the volume percent water in a synthetic-base mud, where each is a percent of the total liquid in the mud. The SWR is calculated in an analogous way to the oil/brine ratio using data from the retort test. |
| Drilling Fluids | synthetic-base fluid | Any of a number of fluids (liquids) manufactured from starting products of known composition and purity. Popular fluid types include several olefin oligomers of ethylene. Esters made from vegetable fatty acid and alcohol were among the first such fluids. Ethers and polyethers, made from alcohols and polyalcohols, have been used, along with paraffinic hydrocarbons and linear alkyl benzenes. Mixtures of these fluids are also used to make synthetic-base muds. |
| Drilling Fluids | synthetic-base mud | Nonaqueous, water-internal (invert) emulsion muds in which the external phase is a synthetic fluid rather than an oil. This and other more minor changes in formulations have made synthetic fluids in muds more environmentally acceptable for offshore use. Synthetic muds are popular in most offshore drilling areas, despite high initial mud costs, because of their environmental acceptance and approval to dispose of cuttings into the water. “Oil mud” should not be used to describe synthetic-base muds. |
| Drilling Fluids | tail cement | The last cement system pumped during primary cementing. The tail cement covers the lower sections of the well, especially planned completion intervals, and is typically more dense than the lead slurry that precedes it. |
| Drilling Fluids | tall oil | An oil produced by conifer trees that yields both saturated and unsaturated (double- and triple-bond) fatty acids. These acids are the raw materials used in the manufacture of many drilling-fluid additives, such as emulsifiers, oil-wetting agents and lubricants. |
| Drilling Fluids | TAME | A term describing the application of a cloud point glycol or polyglycol as a shale inhibitor. The purported mechanism is that the glycol clouds out at the higher downhole temperatures, coating onto the surface of clays and preventing hydrationA term describing the application of a cloud point glycol or polyglycol as a shale inhibitor. The purported mechanism is that the glycol clouds out at the higher downhole temperatures, coating onto the surface of clays and preventing hydration |
| Drilling Fluids | tannic acid | The acids found in tannin. Quebracho contains tannic acid. |
| Drilling Fluids | tannin | Chemical extracted from the bark of trees and used as clay deflocculant in water muds. Tannins are moderate molecular weight, anionic polymers with complex structures. Quebracho is a tannin. |
| Drilling Fluids | temperature stability | The characteristic of a drilling fluid or a mud product pertaining to its response to prolonged heating, usually in a controlled mud composition in a rolling- or static-aging test. |
| Drilling Fluids | ten minute gel strength | The shear stress measured at low shear rate after a mud has set quiescently for 10 minutes as per the standard API procedure. |
| Drilling Fluids | ten second gel strength | The shear stress measured at low shear rate after a mud has set quiescently for 10 seconds as per the standard API procedure. |
| Drilling Fluids | ten-minute gel strength | The shear stress measured at low shear rate after a mud has set quiescently for 10 minutes as per the standard API procedure. |
| Drilling Fluids | ten-second gel strength | The shear stress measured at low shear rate after a mud has set quiescently for 10 seconds as per the standard API procedure. |
| Drilling Fluids | tensile strength | The force per unit cross-sectional area required to pull a substance apart. |
| Drilling Fluids | tetrahedral layer | One of the layers that constitute the atomic structure of the clay group of layered silicate minerals. The structure of these minerals can consist of two, three or four layers. The tetrahedral layer is a plane of silicon dioxide tetrahedra (silicon at the center and oxygen at all four corners of the tetrahedron). Another structural layer is a plane of aluminum hydroxide octahedra (aluminum at the center and hydroxides at all six corners). The tetrahedral and octahedral layers fit one on top of the other, with oxygen atoms being shared as oxide and hydroxide groups. |
| Drilling Fluids | thermally activated mud emulsion | A term describing the application of a cloud point glycol or polyglycol as a shale inhibitor. The purported mechanism is that the glycol clouds out at the higher downhole temperatures, coating onto the surface of clays and preventing hydration. |
| Drilling Fluids | thickening time | A measurement of the time during which a cement slurry remains in a fluid state and is capable of being pumped. Thickening time is assessed under simulated downhole conditions using a consistometer that plots the consistency of a slurry over time at the anticipated temperature and pressure conditions. The end of the thickening time is considered to be 50 or 70 Bc for most applications. |
| Drilling Fluids | thinner | Another term for deflocculant, a thinning agent used to reduce viscosity or prevent flocculation; incorrectly called a “dispersant.” Most deflocculants are low-molecular weight anionic polymers that neutralize positive charges on clay edges. Examples include polyphosphates, lignosulfonates, quebracho and various water-soluble synthetic polymers. |
| Drilling Fluids | thixotropic | Pertaining to the ability of a fluid, such as cement or drilling mud, to develop gel strength over time when not subject to shearing, and then to liquefy when agitated. |
| Drilling Fluids | thixotropy | The characteristic of a fluid, such as a drilling mud, to form a gelled structure over time when not subject to shearing and then to liquefy when agitated. The viscosity of a thixotropic fluid changes with time under constant shear rate until reaching equilibrium. Most drilling muds exhibit thixotropy, which is necessary for fast drilling, efficient cuttings lifting and to support weighting material when mud flow stops. Gel strength measured at various time intervals indicates the relative thixotropy of a mud. Thixotropy is sometimes desirable to provide resistance to flowing, such as to avoid or reduce losses or flow into a weak formation. |
| Drilling Fluids | titrate | To perform a titration, a procedure to determine the amount of a constituent in a sample by adding a measured volume of reagent until the reaction between the constituent of interest and the reagent is completed, as shown by an appropriate endpoint indicator. For mud and mud filtrate analyses, titration is a common procedure for determining alkalinity, chloride, total hardness, methylene blue capacity and formaldehyde. |
| Drilling Fluids | titration | In chemical analysis, a procedure to determine the amount of a constituent in a sample by adding a measured volume of reagent until the reaction between the constituent of interest and the reagent is completed, as shown by an appropriate endpoint indicator. For mud and mud filtrate analyses, titration is a common procedure for determining alkalinity, chloride, total hardness, methylene blue capacity and formaldehyde. |
| Drilling Fluids | total hardness test | A chemical analysis to measure the hardness ions in water-mud filtrates or in make-up water. Hardness is quantitatively determined by titration using standardized EDTA (versenate) reagent and ammonium hydroxide (weak) buffer, typically according to procedures of API. Results are reported as calcium ion in mg/L. The hardness ion Ca+2 can be analyzed alone by another EDTA titration method described by the API. |
| Drilling Fluids | total solids | Also known as retort solids, the volume percent (or fraction) of a mud that is not captured in the receiver when performing the water, oil and solids test as prescribed by API, as given in the equation below. Retort solids thus include suspended solids, dissolved solids (salts), charred organic materials and volatile materials that do not condense. For calculations, retort solids are normally assumed to be only suspended and dissolved solids, as in the equation below. Volume percent suspended solids (weighting material plus drill solids) is of particular interest to mud engineers. To calculate that percentage, the volume increase caused by the dissolved salts is determined from filtrate analyses of chloride and calcium ions. For oil muds, the calculations are more complicated. |
| Drilling Fluids | tracer | A chemical or other material placed in the borehole fluid and later detected to infer information about the borehole or the drilled formations. The two main types of tracers used during drilling are the mud tracer and the filtrate tracer. |
| Drilling Fluids | trip pill | Also called a slug, a volume of mud that is more dense than the mud in the drillpipe and wellbore annulus. A slug is used to displace mud out of the upper part of the drillpipe before pulling pipe out of the hole and is mixed in the pill pit by adding additional weighting material (barite) to a few barrels of mud from the surface pits. The pill is pumped into the top of the drillstring to push mud downward, out of the pipe, thus keeping the upper stands of pipe empty. |
| Drilling Fluids | turbulent flow | A type of fluid flow characterized by swirling or chaotic motion as the fluid moves along the flow path. This is a preferred flow regime for mud removal during primary cementing because it is perceived to result in better removal of mud, especially of mud at the formation wall. |
| Drilling Fluids | ultrafine | Referring to any particle in the size range 2 to 44 microns. |
| Drilling Fluids | undersaturated fluid | A solution that could contain more solute than is presently dissolved in it. In brines, an undersaturated solution will not form crystals as easily as if it were saturated or supersaturated. In saltwater muds, an under-saturated fluid is used to allow salt to leach into the mud, keeping the hole from closing in on the drilling assembly. For moisture in air, a relative humidity of less than 100% is under-saturated. |
| Drilling Fluids | unweighted mud | A mud that contains no commercial weighting material. Native-solids muds are unweighted muds, containing no barite. More solids-control techniques are available for unweighted muds than for weighted muds. In fact, dilution of unweighted muds is highly economical. |
| Drilling Fluids | U-tube effect | In a U-tube manometer, the height of one leg of fluid changed by altering the density of some of the fluid in the other leg. In a well with drillpipe in the hole, the string of drillpipe is one leg and the annulus between the drillpipe and the wellbore is the other. If a denser mud goes into the drillpipe, mud flows up the annulus, and vice versa. The practice of putting a dense slugging pill in the drillpipe in order to pull a dry string makes use of the U-tube effect. |
| Drilling Fluids | VAMA | A copolymer of vinyl acetate (ethylenic polymer) and anhydrous maleic acid (a di-hydroxy acid). The vinyl acetate polymer component is usually high molecular weight. As such, with polar groups on the structure, it is used as a flocculant or bentonite extender. |
| Drilling Fluids | vapor pressure | The pressure exerted by a vapor escaping from a liquid. It quantifies the tendency of molecules to enter the gaseous phase. The vapor pressure of water increases as temperature increases and reaches one atmosphere pressure (760 mm Hg or 14.7 psia) at the boiling point (100°C or 212°F). The activity of an aqueous solution is the ratio of vapor pressures: aw = p/po, where p = vapor pressure of a solution and po is vapor pressure of pure water. Since this is a ratio of vapor pressures, activity is not a strong function of temperature. |
| Drilling Fluids | versenate | Ethylenediamine tetraacetic acid, the reagent used to titrate for calcium and magnesium ions (hardness ions) in water samples. It is also known as EDTA or titraver. |
| Drilling Fluids | VG meter | Viscosity-gel meter. This jargon is used to describe the direct-indicating viscometer, the instrument commonly used to test flow properties of drilling muds. |
| Drilling Fluids | V-G meter | Viscosity-gel meter. This jargon is used to describe the direct-indicating viscometer, the instrument commonly used to test flow properties of drilling muds. |
| Drilling Fluids | vinyl acetate-maleic anhydride copolymer | A copolymer of vinyl acetate (ethylenic polymer) and anhydrous maleic acid (a di-hydroxy acid). The vinyl acetate polymer component is usually high molecular weight. As such, with polar groups on the structure, it is used as a flocculant or bentonite extender. |
| Drilling Fluids | vinyl polymer | A class of polymers constructed with the monomer ethylene, H2C=CH2, with hydrogen replaced by various chemical groups. Among the many vinyl-based polymers and copolymers are acrylates, methacrylates, acrylamides, acrylate-acrylamide (PHPA), vinyl acetate and the various oligomers of ethylene, polyalphaolefins, linear alphaolefins and isomerized olefins. The prefix “vinyl” is more correctly “ethenyl” and sometimes “vinylene.” “Polyvinyl” is synonymous with vinyl polymers, but not specific to an exact polymer. For example, polyvinyl acetate, polyvinyl alcohol, polyvinyl sulfide are commonly used polymers based on ethylene monomers. |
| Drilling Fluids | vis | Abbreviation for viscosity. |
| Drilling Fluids | viscosity | A property of fluids and slurries that indicates their resistance to flow, defined as the ratio of shear stress to shear rate. Viscosity can be expressed mathematically as follows: Poise is the unit for viscosity, equivalent to dyne-sec/cm2. Because one poise represents a high viscosity, 1/100 poise, or one centipoise (cp), is used for mud measurements. One centipoise equals one millipascal-second. Viscosity must have a stated or an understood shear rate in order to be meaningful. Measurement temperature also must be stated or understood. |
| Drilling Fluids | viscosity and gel-strength test | Measurement of the viscosity and gel strength of a drilling fluid. The gel strength is the shear stress measured at low shear rate after a mud has set quiescently for a period of time (10 seconds and 10 minutes in the standard API procedure, although measurements after 30 minutes or 16 hours may also be made). Viscosity is measured using a Marsh funnel, a conical-shaped funnel, fitted with a small-bore tube on the bottom end through which mud flows under a gravity head. A screen over the top removes large particles that might plug the tube. In the test standardized by API for evaluating water-base and oil-base muds, the funnel viscosity measurement is the time (in seconds) required for one quart of mud to flow out of a Marsh funnel into a graduated mud cup. Funnel viscosity is reported in seconds (for a quart). Water exits the funnel in about 26 seconds. This test was one of the earliest mud measurements for field use. Simple, quick and fool-proof, it still serves as a useful indicator of change in the mud by comparing mud-in and mud-out sample funnel viscosities. |
| Drilling Fluids | wastewater cleanup | A process in which dirty water is stripped of its solids and made suitable for recycling into a mud system or disposal into sewer systems or other places. In closed mud systems, water containing colloidal matter can be cleaned and recycled. Efficient agglomeration of colloidal solids is achieved by pH adjustment, small additions of alum or a high-molecular-weight polymer. Agglomerated solids are then filtered or centrifuged from the fluid. |
| Drilling Fluids | water base drilling fluid | A drilling fluid (mud) in which water or saltwater is the major liquid phase as well as the wetting (external) phase. General categories of water-base muds are fresh water, seawater, salt water, lime, potassium and silicate. Subcategorizes of these abound. |
| Drilling Fluids | water base mud | A drilling fluid (mud) in which water or saltwater is the major liquid phase as well as the wetting (external) phase. General categories of water-base muds are fresh water, seawater, salt water, lime, potassium and silicate. Subcategorizes of these abound. |
| Drilling Fluids | water clarification | The process of removing colloidal materials from water. A chemical coagulant (for example, alum) or a chemical flocculant (for example, polymer) or both are added to the water. Colloidal particles attach to each other and to the additives and clumps grow to sufficient size that they can be separated from the water by gravity settling, centrifuging, hydrocycloning or filtration. Clarification is a final step in a closed mud system when a clear effluent is needed. |
| Drilling Fluids | water in oil emulsion | A fluid with water or brine as droplets dispersed into an external phase of oil. |
| Drilling Fluids | water loss | Referring to the volume of liquid measured in the filtration tests performed according to API specifications, in units of cm3/30 minutes. Although applied to water mud, in which the liquid is truly water, the term is sometimes applied to oil mud, in which the liquid is oil. |
| Drilling Fluids | water loss | Jargon applied to a mud additive used to control fluid loss. |
| Drilling Fluids | water mud | A drilling fluid (mud) in which water or saltwater is the major liquid phase as well as the wetting (external) phase. General categories of water-base muds are fresh water, seawater, salt water, lime, potassium and silicate. Subcategorizes of these abound. |
| Drilling Fluids | water mud emulsifier | A chemical used in preparation and maintenance of an emulsion mud, which is a water mud containing dispersed oil (or a synthetic hydrocarbon). Numerous types of emulsifiers will disperse oil into water muds, including sulfonated hydrocarbons, ethyoxylated nonylphenols, alkali-metal fatty-acid soaps, lignosulfonate, lignite and lignin at high pH. Even clays, starch and carboxymethylcellulose aid emulsion mud stability. Reference: Rogers WF: “Oil-in-Water Emulsion Muds,” in Composition and Properties of Oil Well Drilling Fluids, 3rd ed. Houston, Texas, USA: Gulf Publishing Company, 1963. |
| Drilling Fluids | water to cement ratio | In a cement slurry, the ratio of water to cement expressed as percent; the number of parts of water used to mix with 100 parts of cement. |
| Drilling Fluids | water, oil and solids test | A test for water mud or oil mud, generally known as the retort test. Proper procedures for retort tests have been published by API. The test is a distillation of a mud sample that measures condensed oil and water collected from the retort. Data obtained are: (1) vol. % water, (2) vol. % oil and (3) vol. % retort solids. Retort solids is the volume that was not recovered as a liquid. Three sizes of retort apparatus are available: 10-, 20- and 50-cm3 mud sample size. Some designs have a small oven in the carrying case to heat the sample (the preferred method for oil muds) while others use a blade heater that goes into the mud sample. Retorts should be heated to around 700°F [371°C] to be effective. |
| Drilling Fluids | water-base drilling fluid | A drilling fluid (mud) in which water or saltwater is the major liquid phase as well as the wetting (external) phase. General categories of water-base muds are fresh water, seawater, salt water, lime, potassium and silicate. Subcategorizes of these abound. |
| Drilling Fluids | water-base mud | A drilling fluid (mud) in which water or saltwater is the major liquid phase as well as the wetting (external) phase. General categories of water-base muds are fresh water, seawater, salt water, lime, potassium and silicate. Subcategorizes of these abound. |
| Drilling Fluids | water-in-oil emulsion | A fluid with water or brine as droplets dispersed into an external phase of oil. |
| Drilling Fluids | water-mud emulsifier | A chemical used in preparation and maintenance of an emulsion mud, which is a water mud containing dispersed oil (or a synthetic hydrocarbon). Numerous types of emulsifiers will disperse oil into water muds, including sulfonated hydrocarbons, ethyoxylated nonylphenols, alkali-metal fatty-acid soaps, lignosulfonate, lignite and lignin at high pH. Even clays, starch and carboxymethylcellulose aid emulsion mud stability. |
| Drilling Fluids | water-to-cement ratio | In a cement slurry, the ratio of water to cement expressed as percent; the number of parts of water used to mix with 100 parts of cement. |
| Drilling Fluids | weight | Referring to mud weight, the mass per unit volume of a drilling fluid, synonymous with mud density. Weight is reported in lbm/gal (also known as ppg), kg/m3 or g/cm3 (also called specific gravity or SG), lb/ft3 or in hydrostatic gradient, lb/in2/ft (psi/ft) or pptf (psi/1000 ft). Mud weight controls hydrostatic pressure in a wellbore and prevents unwanted flow into the well. The weight of the mud also prevents collapse of casing and the openhole. Excessive mud weight can cause lost circulation by propagating, and then filling, fractures in the rock. Mud weight (density) test procedures using a mud balance have been standardized and published by the API. |
| Drilling Fluids | weight material | Also known as weighting material, a high-specific gravity and finely divided solid material used to increase density of a drilling fluid. (Dissolved salts that increase fluid density, such as calcium bromide in brines, are not called weighting materials.) Barite is the most common, with minimum specific gravity of 4.20 g/cm3. Hematite is a more dense material, with minimum specific gravity of 5.05 g/cm3, per API and ISO specifications. Calcium carbonate, specific gravity 2.7 to 2.8, is considered weighting material but is used more for its acid solubility than for density. Siderite, specific gravity around 3.8, has been used to densify mud, but can cause problems by dissolving into the mud at high pH. Ilmenite, specific gravity of 4.6 has been used in drilling fluid and cement. Only barite and hematite have API/ISO standards. |
| Drilling Fluids | weighted mud | A mud that contains commercial weighting material such as barite or hematite. The economic difference in weighted and unweighted muds is the cost of replacing weighting material according to the solids control practices used. Solids control techniques, such as dilution or hydrocycloning, that can be economical in unweighted muds are not necessarily economical for weighted muds, although centrifugation (incorrectly called “barite recovery”) is typically performed when using weighted muds to control mud viscosity. |
| Drilling Fluids | weighting agent | Also known as weighting material, a high-specific gravity and finely divided solid material used to increase density of a drilling fluid. (Dissolved salts that increase fluid density, such as calcium bromide in brines, are not called weighting materials.) Barite is the most common, with minimum specific gravity of 4.20 g/cm3. Hematite is a more dense material, with minimum specific gravity of 5.05 g/cm3, per API and ISO specifications. Calcium carbonate, specific gravity 2.7 to 2.8, is considered weighting material but is used more for its acid solubility than for density. Siderite, specific gravity around 3.8, has been used to densify mud, but can cause problems by dissolving into the mud at high pH. Ilmenite, specific gravity of 4.6 has been used in drilling fluid and cement. Only barite and hematite have API/ISO standards. |
| Drilling Fluids | weighting material | A high-specific gravity and finely divided solid material used to increase density of a drilling fluid. (Dissolved salts that increase fluid density, such as calcium bromide in brines, are not called weighting materials.) Barite is the most common, with minimum specific gravity of 4.20 g/cm3. Hematite is a more dense material, with minimum specific gravity of 5.05 g/cm3, per API and ISO specifications. Calcium carbonate, specific gravity 2.7 to 2.8, is considered weighting material but is used more for its acid solubility than for density. Siderite, specific gravity around 3.8, has been used to densify mud, but can cause problems by dissolving into the mud at high pH. Ilmenite, specific gravity of 4.6 has been used in drilling fluid and cement. Only barite and hematite have API/ISO standards. |
| Drilling Fluids | whole mud dilution | A dilution process which involves selective dumping of the active system (such as sand traps and “bottoms up” mud) and replacement of the lost volume with fresh mud. This process has proved economical with inhibitive water-base systems and is the only method that actually removes colloidal size particles. |
| Drilling Fluids | whole-mud dilution | A dilution process which involves selective dumping of the active system (such as sand traps and “bottoms up” mud) and replacement of the lost volume with fresh mud. This process has proved economical with inhibitive water-base systems and is the only method that actually removes colloidal size particles. |
| Drilling Fluids | xanthan gum | A polysaccharide secreted by the bacteria genus Xanthomonas campestris, also known as XC polymer. XC in water muds provides non-Newtonian mud rheology, highly desirable because of the flat velocity profile it produces in annular flow, which is required for efficient cuttings lifting in lower density muds. XC polymer is anionic, with tolerance for salinity and fair tolerance for hardness ions. XC is a finely powdered material that can vary in the amount of residual bacteria debris and the ease with which it disperses into water. Temperature tolerance varies with water-phase components, but starts to degrade around 200 to 250°F [93 to 121°C]. Extreme pH or hardness are not well-tolerated by XC polymer and it is susceptible to bacterial attack. |
| Drilling Fluids | XC polymer | A polysaccharide secreted by the bacteria genus Xanthomonas campestris, also known as xanthan gum. XC in water muds provides non-Newtonian mud rheology, highly desirable because of the flat velocity profile it produces in annular flow, which is required for efficient cuttings lifting in lower density muds. XC polymer is anionic, with tolerance for salinity and fair tolerance for hardness ions. XC is a finely powdered material that can vary in the amount of residual bacteria debris and the ease with which it disperses into water. Temperature tolerance varies with water-phase components, but starts to degrade around 200 to 250°F [93 to 121°C]. Extreme pH or hardness are not well-tolerated by XC polymer and it is susceptible to bacterial attack. |
| Drilling Fluids | xylene | An aromatic hydrocarbon molecule containing a benzene ring with two methyl side chains, formula C6H4(CH3)2. Xylene is an excellent solvent, especially for aromatic solids such as asphaltic materials. It is used as a solvent and emulsion breaker in workover operations to clean up reservoirs. In drilling mud testing, a 50/50 xylene/isopropanol (IPA) mixture had been used to break oil-mud emulsions prior to titrations to measure alkalinity, chloride and calcium. However, the xylene mixture has been replaced by a single material, propylene glycol normal propyl ether (PNP), to break oil mud emulsions. |
| Drilling Fluids | yield | A term used to specify the quality of a clay according to the number of barrels of 30-cp viscosity mud that one ton of the clay would produce. Although seldom used today, clays were classified as high-, medium- or low-yield bentonites. A “good” bentonite produced at least 85 bbl of mud per ton. High-yield clays produced 30 to 50 bbl/ton, and low-yield clays produced 15 to 30 bbl/ton. (Note that yield and yield point are not related terms.) |
| Drilling Fluids | yield point | A parameter of the Bingham plastic model. YP is the yield stress extrapolated to a shear rate of zero. (Plastic viscosity, PV, is the other parameter of the Bingham-plastic model.) A Bingham plastic fluid plots as a straight line on a shear rate (x-axis) versus shear stress (y-axis) plot, in which YP is the zero-shear-rate intercept. (PV is the slope of the line.) YP is calculated from 300- and 600-rpm viscometer dial readings by subtracting PV from the 300-rpm dial reading. YP is used to evaluate the ability of a mud to lift cuttings out of the annulus. A high YP implies a non-Newtonian fluid, one that carries cuttings better than a fluid of similar density but lower YP. YP is lowered by adding deflocculant to a clay-based mud and increased by adding freshly dispersed clay or a flocculant, such as lime. |
| Drilling Fluids | yield stress | The stress that must be applied to a material to make it begin to flow (or to yield). Yield stress is a parameter in the Herschel-Bulkley rheological model. |
| Drilling Fluids | YP | A parameter of the Bingham plastic model. YP is the yield stress extrapolated to a shear rate of zero. (Plastic viscosity, PV, is the other parameter of the Bingham-plastic model.) A Bingham plastic fluid plots as a straight line on a shear rate (x-axis) versus shear stress (y-axis) plot, in which YP is the zero-shear-rate intercept. (PV is the slope of the line.) YP is calculated from 300- and 600-rpm viscometer dial readings by subtracting PV from the 300-rpm dial reading. YP is used to evaluate the ability of a mud to lift cuttings out of the annulus. A high YP implies a non-Newtonian fluid, one that carries cuttings better than a fluid of similar density but lower YP. YP is lowered by adding deflocculant to a clay-based mud and increased by adding freshly dispersed clay or a flocculant, such as lime. |
| Drilling Fluids | zero zero gels | Gel strengths that are very low, with both values near zero, when measured at 10 seconds and 10 minutes according to standardized test procedures. Settling of barite and cuttings may occur in a zero-zero gels mud. |
| Drilling Fluids | zero-zero gels | Gel strengths that are very low, with both values near zero, when measured at 10 seconds and 10 minutes according to standardized test procedures. Settling of barite and cuttings may occur in a zero-zero gels mud. |
| Drilling Fluids | zinc basic carbonate | A neutral double salt of zinc carbonate and zinc hydroxide in 2-to-3 stoichiometric proportions, 2ZnCO3·3Zn(OH)2. Zinc basic carbonate is used as a sulfide scavenger for water-base drilling fluid in a concentration of about 0.1 lbm/bbl per 50 mg/L GGT sulfides (determined by Garrett Gas Train sulfide analysis of the filtrate). The zinc cation reacts with the sulfide anion to form inert zinc sulfide (ZnS). |
| Drilling Fluids | zinc bromide | A acidic salt, ZnBr2, which is used to prepare dense, clear (solids-free) brine for well completion and workover operations. The density of saturated zinc bromide is approximately 20 lbm/gal, but its pH is low and can cause acidic corrosion and handling problems. Its HSE problems must be weighed carefully against its advantages. Zinc bromide can be used to scavenge sulfides from a mud system in cases where the addition of that salt will not cause detrimental effects on mud performance, such as in a saltwater mud. |
| Drilling Fluids | zinc carbonate | A neutral zinc salt, ZnCO3, which can be used as a sulfide scavenger in water-base muds. Zinc carbonate is less soluble than zinc basic carbonate and perhaps slower to react with sulfide ions. Treatment level is about 0.1 lbm/bbl per 50 mg/L sulfide ion (determined by Garrett Gas Train sulfide analysis of the filtrate). |
| Drilling Fluids | zinc chloride | An acidic salt, ZnCl2, used as one of the standard saturated salt solutions for calibration of the electrohygrometer. Saturated zinc chloride solution at room temperature has 10% relative humidity. Zinc chloride can be used to scavenge sulfides from a mud system in cases where the addition of a chloride salt will not cause detrimental effects on mud performance, such as in a saltwater mud. |
| Drilling Fluids | zinc oxide | A very weak base, ZnO, which can be used as a sulfide scavenger in oil-base or synthetic-base muds. |
| Drilling Fluids, Drilling | air cut mud | A drilling fluid (or mud) that has gas (air or natural gas) bubbles in it, resulting in a lower bulk, unpressurized density compared with a mud not cut by gas. The density of gas-cut mud can be measured accurately using a pressurized mud balance. Defoamer chemicals added to the mud or a mechanical vacuum pump degasser can liberate the trapped gas. The derrickman periodically measures mud density and communicates the results to the driller via an intercom, typically reporting something like “9.6 heavy,” “10.4,” or “13.2 light,” indicating more than 9.6 pounds per gallon, 10.4 pounds per gallon, or less than 13.2 pounds per gallon, respectively. Each tenth of a pound per gallon is referred to as a “point” of mud weight. Note that for this low-accuracy measurement, no direct mention of gas cut is made. A gas cut is inferred only if the mud returning to the surface is significantly less dense than it should be. In the case of the mud logger’s measurement, “units” of gas (having virtually no absolute meaning) are reported. For the mud logger’s measurement, a direct indication of combustible gases is made, with no direct correlation to mud weight. |
| Drilling Fluids, Enhanced Oil Recovery | foaming agent | An additive used in preparation of foam used as a drilling fluid. Drilling foam is water containing air or gas bubbles, much like shaving foam, and it must withstand high salinity, hard water, solids, entrained oil and high temperature. Foaming agents are usually nonionic surfactants and contain polymeric materials. |
| Drilling Fluids, Enhanced Oil Recovery | hydrophilic | Pertaining to an attraction for water by the surface of a material or a molecule. Clays and most other natural minerals used in drilling fluids, such as barite and hematite, are hydrophilic. They are spontaneously wet by water. To render them oleophilic, they can be treated with an oil-wetting chemical. |
| Drilling Fluids, Enhanced Oil Recovery | hydrophobic | Pertaining to a repulsion of water by the surface of a material or a molecule. |
| Drilling Fluids, Formation Evaluation | cake | The residue deposited on a permeable medium when a slurry, such as a drilling fluid, is forced against the medium under a pressure. Filtrate is the liquid that passes through the medium, leaving the cake on the medium. Drilling muds are tested to determine filtration rate and filter-cake properties. Cake properties such as cake thickness, toughness, slickness and permeability are important because the cake that forms on permeable zones in the wellbore can cause stuck pipe and other drilling problems. Reduced oil and gas production can result from reservoir damage when a poor filter cake allows deep filtrate invasion. A certain degree of cake buildup is desirable to isolate formations from drilling fluids. In openhole completions in high-angle or horizontal holes, the formation of an external filter cake is preferable to a cake that forms partly inside the formation. The latter has a higher potential for formation damage. |
| Drilling Fluids, Formation Evaluation | mudcake | The residue deposited on a permeable medium when a slurry, such as a drilling fluid, is forced against the medium under a pressure. Filtrate is the liquid that passes through the medium, leaving the cake on the medium. Drilling muds are tested to determine filtration rate and filter-cake properties. Cake properties such as cake thickness, toughness, slickness and permeability are important because the cake that forms on permeable zones in the wellbore can cause stuck pipe and other drilling problems. Reduced oil and gas production can result from reservoir damage when a poor filter cake allows deep filtrate invasion. A certain degree of cake buildup is desirable to isolate formations from drilling fluids. In openhole completions in high-angle or horizontal holes, the formation of an external filter cake is preferable to a cake that forms partly inside the formation. The latter has a higher potential for formation damage. |
| Drilling Fluids, Formation Evaluation | wall cake | The residue deposited on a permeable medium when a slurry, such as a drilling fluid, is forced against the medium under a pressure. Filtrate is the liquid that passes through the medium, leaving the cake on the medium. Drilling muds are tested to determine filtration rate and filter-cake properties. Cake properties such as cake thickness, toughness, slickness and permeability are important because the cake that forms on permeable zones in the wellbore can cause stuck pipe and other drilling problems. Reduced oil and gas production can result from reservoir damage when a poor filter cake allows deep filtrate invasion. A certain degree of cake buildup is desirable to isolate formations from drilling fluids. In openhole completions in high-angle or horizontal holes, the formation of an external filter cake is preferable to a cake that forms partly inside the formation. The latter has a higher potential for formation damage. |
| Drilling Fluids, Geology | thermal gradient | Also known as geothermal gradient, the rate of increase in temperature per unit depth in the Earth. Although the geothermal gradient varies from place to place, it averages 25 to 30 °C/km [15 °F/1000 ft]. Temperature gradients sometimes increase dramatically around volcanic areas. It is particularly important for drilling fluids engineers to know the geothermal gradient in an area when they are designing a deep well. The downhole temperature can be calculated by adding the surface temperature to the product of the depth and the geothermal gradient. |
| Drilling Fluids, Well Workover and Intervention | fluid loss | The leakage of the liquid phase of drilling fluid, slurry or treatment fluid containing solid particles into the formation matrix. The resulting buildup of solid material or filter cake may be undesirable, as may the penetration of filtrate through the formation. Fluid-loss additives are used to control the process and avoid potential reservoir damage. |
| Drilling Fluids, Well Workover and Intervention | paddle blender | A type of fluid-mixing tank used in the preparation of treatment fluids or slurries that provides the agitation to achieve a well-dispersed mixture. Paddle mixers are generally equipped with rotating paddles that provide turbulence for mixing fluids and an action that prevents the settling of solids prior to being pumped. |
| Drilling Fluids, Well Workover and Intervention | paddle mixer | A type of fluid-mixing tank used in the preparation of treatment fluids or slurries that provides the agitation to achieve a well-dispersed mixture. Paddle mixers are generally equipped with rotating paddles that provide turbulence for mixing fluids and an action that prevents the settling of solids prior to being pumped. |
| Drilling Fluids, Well Workover and Intervention, Well Completions | sulfide stress cracking | A type of spontaneous brittle failure in steels and other high-strength alloys when they are in contact with moist hydrogen sulfide and other sulfidic environments. Tool joints, hardened parts of blowout preventers and valve trim are particularly susceptible. For this reason, along with toxicity risks of hydrogen sulfide gas, it is essential that water muds be kept entirely free of soluble sulfides and especially hydrogen sulfide at low pH. Sulfide stress cracking is also called hydrogen sulfide cracking, sulfide cracking, sulfide corrosion cracking and sulfide stress-corrosion cracking. The variation of the name is due to the lack of agreement in the mechanism of failure. Some researchers consider sulfide-stress cracking a type of stress-corrosion cracking, while others consider it a type of hydrogen embrittlement. |
| Drilling Fluids | BWOB | Describing the amount (in percent) of a material added to cement when the material is added based on the total amount of a specific blend, often abbreviated as BWOB. |
| Drilling Fluids | RCRA | Abbreviation for Resources Conservation and Recovery Act passed by the US Congress in 1976 and expanded in 1980 as CERCLA. Both acts are related to the transportation, storage, treatment or disposal of hazardous substances. |
| Drilling, Drilling Fluids | air-cut mud | A drilling fluid (or mud) that has gas (air or natural gas) bubbles in it, resulting in a lower bulk, unpressurized density compared with a mud not cut by gas. The density of gas-cut mud can be measured accurately using a pressurized mud balance. Defoamer chemicals added to the mud or a mechanical vacuum pump degasser can liberate the trapped gas. The derrickman periodically measures mud density and communicates the results to the driller via an intercom, typically reporting something like “9.6 heavy,” “10.4,” or “13.2 light,” indicating more than 9.6 pounds per gallon, 10.4 pounds per gallon, or less than 13.2 pounds per gallon, respectively. Each tenth of a pound per gallon is referred to as a “point” of mud weight. Note that for this low-accuracy measurement, no direct mention of gas cut is made. A gas cut is inferred only if the mud returning to the surface is significantly less dense than it should be. In the case of the mud logger’s measurement, “units” of gas (having virtually no absolute meaning) are reported. For the mud logger’s measurement, a direct indication of combustible gases is made, with no direct correlation to mud weight. |
| Drilling, Drilling Fluids | gas-cut mud | A drilling fluid (or mud) that has gas (air or natural gas) bubbles in it, resulting in a lower bulk, unpressurized density compared with a mud not cut by gas. The density of gas-cut mud can be measured accurately using a pressurized mud balance. Defoamer chemicals added to the mud or a mechanical vacuum pump degasser can liberate the trapped gas. The derrickman periodically measures mud density and communicates the results to the drilling crew via an intercom, typically reporting something like “9.6 heavy,” “10.4,” or “13.2 light,” indicating more than 9.6 pounds per gallon, 10.4 pounds per gallon, or less than 13.2 pounds per gallon, respectively. Each tenth of a pound per gallon is referred to as a “point” of mud weight. Note that for this low-accuracy measurement, no direct mention of gas cut is made. A gas cut is inferred only if the mud returning to the surface is significantly less dense than it should be. In the case of the mud logger’s measurement, “units” of gas (having virtually no absolute meaning) are reported. For the mud logger’s measurement, a direct indication of combustible gases is made, with no direct correlation to mud weight. |
| Drilling, Drilling Fluids | LCM | Solid material intentionally introduced into a mud system to reduce and eventually prevent the flow of drilling fluid into a weak, fractured or vugular formation. This material is generally fibrous or plate-like in nature, as suppliers attempt to design slurries that will efficiently bridge over and seal loss zones. In addition, popular lost circulation materials are low-cost waste products from the food processing or chemical manufacturing industries. Examples of lost circulation material include ground peanut shells, mica, cellophane, walnut shells, calcium carbonate, plant fibers, cottonseed hulls, ground rubber, and polymeric materials. |
| Drilling, Drilling Fluids, Production Facilities, Well Testing, Well Workover and Intervention, Well Completions | hydrogen sulfide | [H2S] An extraordinarily poisonous gas with a molecular formula of H2S. At low concentrations, H2S has the odor of rotten eggs, but at higher, lethal concentrations, it is odorless. H2S is hazardous to workers and a few seconds of exposure at relatively low concentrations can be lethal, but exposure to lower concentrations can also be harmful. The effect of H2S depends on duration, frequency and intensity of exposure as well as the susceptibility of the individual. Hydrogen sulfide is a serious and potentially lethal hazard, so awareness, detection and monitoring of H2S is essential. Since hydrogen sulfide gas is present in some subsurface formations, drilling and other operational crews must be prepared to use detection equipment, personal protective equipment, proper training and contingency procedures in H2S-prone areas. Hydrogen sulfide is produced during the decomposition of organic matter and occurs with hydrocarbons in some areas. It enters drilling mud from subsurface formations and can also be generated by sulfate-reducing bacteria in stored muds. H2S can cause sulfide-stress-corrosion cracking of metals. Because it is corrosive, H2S production may require costly special production equipment such as stainless steel tubing. Sulfides can be precipitated harmlessly from water muds or oil muds by treatments with the proper sulfide scavenger. H2S is a weak acid, donating two hydrogen ions in neutralization reactions, forming HS- and S-2 ions. In water or water-base muds, the three sulfide species, H2S and HS- and S-2 ions, are in dynamic equilibrium with water and H+ and OH- ions. The percent distribution among the three sulfide species depends on pH. H2S is dominant at low pH, the HS- ion is dominant at mid-range pH and S2 ions dominate at high pH. In this equilibrium situation, sulfide ions revert to H2S if pH falls. Sulfides in water mud and oil mud can be quantitatively measured with the Garrett Gas Train according to procedures set by API. |
| Drilling, Production | nipple | Any short piece of pipe, especially if threaded at both ends with male threads. |
| Drilling, Shale Gas | azimuth | The direction in which a deviated or horizontal well is drilled relative to magnetic north. Most horizontal wells in shale reservoirs are drilled in the direction of the minimum horizontal stress. This allows for the creation of multiple hydraulic fractures that are normal to the wellbore. |
| Drilling, Shale Gas | directional drilling | The intentional deviation of a wellbore from the path it would naturally take. This is accomplished through the use of whipstocks, bottomhole assembly (BHA) configurations, instruments to measure the path of the wellbore in three-dimensional space, data links to communicate measurements taken downhole to the surface, mud motors and special BHA components and drill bits, including rotary steerable systems, and drill bits. The directional driller also exploits drilling parameters such as weight on bit and rotary speed to deflect the bit away from the axis of the existing wellbore. In some cases, such as drilling steeply dipping formations or unpredictable deviation in conventional drilling operations, directional-drilling techniques may be employed to ensure that the hole is drilled vertically. While many techniques can accomplish this, the general concept is simple: point the bit in the direction that one wants to drill. The most common way is through the use of a bend near the bit in a downhole steerable mud motor. The bend points the bit in a direction different from the axis of the wellbore when the entire drillstring is not rotating. By pumping mud through the mud motor, the bit turns while the drillstring does not rotate, allowing the bit to drill in the direction it points. When a particular wellbore direction is achieved, that direction may be maintained by rotating the entire drillstring (including the bent section) so that the bit does not drill in a single direction off the wellbore axis, but instead sweeps around and its net direction coincides with the existing wellbore. Rotary steerable tools allow steering while rotating, usually with higher rates of penetration and ultimately smoother boreholes. Directional drilling is common in shale reservoirs because it allows drillers to place the borehole in contact with the most productive reservoir rock. |
| Drilling, Shale Gas | formation evaluation while drilling | Also known as logging while drilling or LWD, the measurement of formation properties during the excavation of the hole, or shortly thereafter, through the use of tools integrated into the bottomhole assembly. LWD, while sometimes risky and expensive, has the advantage of measuring properties of a formation before drilling fluids invade deeply. Further, many wellbores prove to be difficult or even impossible to measure with conventional wireline tools, especially highly deviated wells. In these situations, the LWD measurement ensures that some measurement of the subsurface is captured in the event that wireline operations are not possible. Timely LWD data can also be used to guide well placement so that the wellbore remains within the zone of interest or in the most productive portion of a reservoir, such as in highly variable shale reservoirs. |
| Drilling, Shale Gas | logging while drilling | The measurement of formation properties during the excavation of the hole, or shortly thereafter, through the use of tools integrated into the bottomhole assembly. LWD, while sometimes risky and expensive, has the advantage of measuring properties of a formation before drilling fluids invade deeply. Further, many wellbores prove to be difficult or even impossible to measure with conventional wireline tools, especially highly deviated wells. In these situations, the LWD measurement ensures that some measurement of the subsurface is captured in the event that wireline operations are not possible. Timely LWD data can also be used to guide well placement so that the wellbore remains within the zone of interest or in the most productive portion of a reservoir, such as in highly variable shale reservoirs. |
| Drilling, Shale Gas | logging-while-drilling | The measurement of formation properties during the excavation of the hole, or shortly thereafter, through the use of tools integrated into the bottomhole assembly. LWD, while sometimes risky and expensive, has the advantage of measuring properties of a formation before drilling fluids invade deeply. Further, many wellbores prove to be difficult or even impossible to measure with conventional wireline tools, especially highly deviated wells. In these situations, the LWD measurement ensures that some measurement of the subsurface is captured in the event that wireline operations are not possible. Timely LWD data can also be used to guide well placement so that the wellbore remains within the zone of interest or in the most productive portion of a reservoir, such as in highly variable shale reservoirs. |
| Drilling, Shale Gas | LWD | Abbreviation for logging while drilling. The measurement of formation properties during the excavation of the hole, or shortly thereafter, through the use of tools integrated into the bottomhole assembly. LWD, while sometimes risky and expensive, has the advantage of measuring properties of a formation before drilling fluids invade deeply. Further, many wellbores prove to be difficult or even impossible to measure with conventional wireline tools, especially highly deviated wells. In these situations, the LWD measurement ensures that some measurement of the subsurface is captured in the event that wireline operations are not possible. Timely LWD data can also be used to guide well placement so that the wellbore remains within the zone of interest or in the most productive portion of a reservoir, such as in highly variable shale reservoirs. |
| Drilling, Shale Gas | rotary steerable system | A tool designed to drill directionally with continuous rotation from the surface, eliminating the need to slide a steerable motor. Rotary steerable systems typically are deployed when drilling directional, horizontal, or extended-reach wells. State-of-the-art rotary steerable systems have minimal interaction with the borehole, thereby preserving borehole quality. The most advanced systems exert consistent side force similar to traditional stabilizers that rotate with the drillstring or orient the bit in the desired direction while continuously rotating at the same number of rotations per minute as the drillstring. |
| Drilling, Well Completions | bottomhole static temperature | The temperature of the undisturbed formation at the final depth in a well. The formation cools during drilling and most of the cooling dissipates after about 24 hours of static conditions, although it is theoretically impossible for the temperature to return to undisturbed conditions. This temperature is measured under static conditions after sufficient time has elapsed to negate any effects from circulating fluids. Tables, charts and computer routines are used to predict BHST as functions of depth, geographic area and various time functions. The BHST is generally higher than the bottomhole circulating temperature, and can be an important factor when using temperature-sensitive tools or treatments. |
| Drilling, Well Completions | positive-displacement pump | A type of fluid pump in which the displacement volume of the pump is fixed for each rotation of the pump. Generally associated with high-pressure applications, positive-displacement pumps are commonly used in drilling operations to circulate the drilling fluid and in a range of oil and gas well treatments, such as cementing, matrix treatments and hydraulic fracturing. |
| Drilling, Well Completions | swellable packer | An isolation device that relies on elastomers to expand and form an annular seal when immersed in certain wellbore fluids. The elastomers used in these packers are either oil- or water-sensitive. Their expansion rates and pressure ratings are affected by a variety of factors. Oil-activated elastomers, which work on the principle of absorption and dissolution, are affected by fluid temperature as well as the concentration and specific gravity of hydrocarbons in a fluid. Water-activated elastomers are typically affected by water temperature and salinity. This type of elastomer works on the principle of osmosis, which allows movement of water particles across a semi-permeable membrane based on salinity differences in the water on either side of the membrane. |
| Drilling, Well Workover and Intervention | mill out | To use a mill or similar downhole tool to cut and remove metal downhole. A mill is usually used to remove junk in the hole or to grind away all or part of a casing string. When milling out casing, the intent is to cut a window through the side of the casing or to remove a continuous section of the casing so that the wellbore may be deviated from the original well through the window or section removed. Successful milling operations require appropriate selection of milling tools, fluids and techniques. The mills, or similar cutting tools, must be compatible with the fish or casing materials and wellbore conditions. The circulated fluids should be capable of removing the milled material from the wellbore. Finally, the techniques employed should be appropriate to the anticipated conditions and the likely time required to reach the operation objectives. |
| Drilling, Well Workover and Intervention, Well Completions | positive displacement pump | A type of fluid pump in which the displacement volume of the pump is fixed for each rotation of the pump. Generally associated with high-pressure applications, positive-displacement pumps are commonly used in drilling operations to circulate the drilling fluid and in a range of oil and gas well treatments, such as cementing, matrix treatments and hydraulic fracturing. |
| Drilling | bottoms up | The sample obtained at the bottoms-up time or a volume of fluid to pump, as in “pump bottoms-up before drilling ahead.” |
| Drilling | run in hole | To connect pipe together and lower the connected length into the borehole in a controlled fashion. The pipe lengths are usually screwed together either with rotary-shouldered connections for the drillstring, or threaded and coupled connections for casing, liners and most tubing. |
| Drilling | standpipe | A rigid metal conduit that provides the high-pressure pathway for drilling mud to travel approximately one-third of the way up the derrick, where it connects to a flexible high-pressure hose (kelly hose). Many large rigs are fitted with dual standpipes so that downtime is kept to a minimum if one standpipe requires repair. |
| Enhanced Oil Recovery | acid number | A measure of the amount of acidic components present in a crude oil. This measurement is the mass of potassium hydroxide (KOH) in milligrams titrated into a one-gram sample of oil—such as stock-tank oil—that is required reach a neutral pH of 7. The test is performed under ASTM Standard D664. |
| Enhanced Oil Recovery | adhesion tension | In a system with two immiscible fluids in contact with a solid, the difference in the two fluid-solid surface tensions. In thermodynamic equilibrium this difference is equivalent as a result of the Young-Laplace equation to the product of the interfacial tension between the two fluids and the cosine of the contact angle at the fluid/fluid/solid interface. As the combination of these two individual interfacial terms, adhesion tension is a useful measure of the wetting character of a petroleum reservoir’s pore system. |
| Enhanced Oil Recovery | alkaline-surfactant-polymer flooding | A chemical enhanced oil recovery flood that uses two sources of surfactant and a polymer. Alkaline chemicals such as sodium carbonate react with acidic oil components in situ to create petroleum soap, which is one of the surfactants. A synthetic surfactant is injected simultaneously with the alkali. A water-soluble polymer is also injected, both in mixture with the alkali and surfactant and as a slug following the mixture, to increase the viscosity of the injectant, thereby improving mobility control of the flood fronts. |
| Enhanced Oil Recovery | areal displacement efficiency | In a reservoir waterflood or other fluid injection using a well pattern, the fraction of the pattern area from which reservoir fluid is displaced by the injected phase at the time of breakthrough. Parameters such as formation dip angle and dip azimuth, presence of fractures, mobility ratio, injection pattern and directional permeability affect areal displacement efficiency (EA). |
| Enhanced Oil Recovery | areal sweep efficiency | In a reservoir waterflood or other fluid injection using a well pattern, the fraction of the pattern area from which reservoir fluid is displaced by the injected phase at the time of breakthrough. Parameters such as formation dip angle and dip azimuth, presence of fractures, mobility ratio, injection pattern and directional permeability affect areal displacement efficiency (EA). |
| Enhanced Oil Recovery | ASP flooding | A chemical enhanced oil recovery flood that uses two sources of surfactant and a polymer. Alkaline chemicals such as sodium carbonate react with acidic oil components in situ to create petroleum soap, which is one of the surfactants. A synthetic surfactant is injected simultaneously with the alkali. A water-soluble polymer is also injected, both in mixture with the alkali and surfactant and as a slug following the mixture, to increase the viscosity of the injectant, thereby improving mobility control of the flood fronts. |
| Enhanced Oil Recovery | asphaltene onset concentration | The minimum concentration of solvent injected into a reservoir oil at a given test pressure and temperature that causes asphaltene particles to precipitate from the oil. |
| Enhanced Oil Recovery | asphaltene onset pressure | As pressure decreases, the pressure at a given test temperature that first causes asphaltene to precipitate from a reservoir fluid. |
| Enhanced Oil Recovery | asphaltene precipitation | The flocculation of asphaltene particles from reservoir fluid. The precipitation is typically measured at specific conditions of temperature and pressure, such as at reservoir or flowline conditions. |
| Enhanced Oil Recovery | asphaltenes | Organic materials consisting of aromatic and naphthenic ring compounds containing nitrogen, sulfur and oxygen molecules. The asphaltene fraction of crude is defined as the organic part of the oil that is not soluble in straight-chain solvents such as pentane or heptane. Asphaltenes exist as a colloidal suspension stabilized by resin molecules (aromatic ring systems) in the oil. The stability of asphaltic dispersions depends on the ratio of resin to asphaltene molecules. The determination of the quantity of resin is important in estimating the potential damage created by asphaltenes. Asphaltene precipitates as a result of pressure drop, shear (turbulent flow), acids, solution carbon dioxide [CO2], injected condensate, mixing of incompatible crude oils or other conditions or materials that break the stability of the asphaltic dispersion. For example, in matrix acidizing, iron ions in solution favor the precipitation of asphaltene deposits. |
| Enhanced Oil Recovery | backward multiple-contact test | A laboratory test to determine the phase envelope between oil and enriched gas. The test is conducted by equilibrating an oil sample several times with fresh samples of gas. Intermediate components are stripped from the gas by multiple contacts with the oil. The test also indicates how many contacts are required before the oil with added intermediate components becomes miscible with the gas. The molar ratios at each contact step are typically designed using PVT simulation software that incorporates the fluid composition from the previous contact. |
| Enhanced Oil Recovery | batch treatment | The pumping of a specific amount of treatment fluid, such as cement slurry, stimulation fluid, well completion fluid or chemical corrosion inhibitor. In corrosion control, there are several batch-treating techniques, such as tubing displacement and standard batch treatments, which are used to place the corrosion inhibitor in an oil or gas well. |
| Enhanced Oil Recovery | billion cubic feet per day | A type of corrosion in which two different metals are placed in contact in a corrosive environment. A small electric current flows from one piece of metal to the other, accelerating the corrosion rate of the more reactive of the two metals. Bimetallic corrosion is sometimes found when new pipe is added to old pipelines. The old pipeline covered by oxide and rust is cathodic to the new pipe, thus accelerating the corrosion rate in the new pipe. Another type of bimetallic corrosion is ringworm corrosion. |
| Enhanced Oil Recovery | Bond number |
A dimensionless group used in analysis of fluid flow that characterizes the ratio of gravitational forces to surface or interfacial tension forces. It is usually denoted Nb in the oil field and Bo in chemical engineering. A value of Nb |
| Enhanced Oil Recovery | brownfield | An oil or gas accumulation that has matured to a production plateau or even progressed to a stage of declining production. Operating companies seek to extend the economic producing life of the field using cost-effective, low-risk technologies. Stimulation or refracturing operations, completing additional zones, and installing artificial lift equipment are a few technologies commonly applied in brownfields before any drilling options are attempted. |
| Enhanced Oil Recovery | capillary number | A dimensionless group used in analysis of fluid flow that characterizes the ratio of viscous forces to surface or interfacial tension forces. It is usually denoted NC in the oil field and Ca in chemical engineering. For a flowing liquid, if NC >>1, then viscous forces dominate over interfacial forces; however if NC <<1, then viscous forces are negligible compared with interfacial forces. Capillary numbers are usually large for high-speed flows and low for low-speed flows; thus, typically for flow through pores in the reservoir NC is ~10?6, and for flow in production tubulars NC is ~1. Capillary number equation: NC=C a=( ?U) / ? where NC=C a=c apillary number ?=f luid viscosity U=f luid velocity ?=s urface or interfacial tension. |
| Enhanced Oil Recovery | cathodic protection | A technique used to minimize the rate of corrosion of a structure. Cathodic protection does not eliminate corrosion, it transfers corrosion from the structure under protection to a known location where artificial anodes (plates or metal bars) are placed and could be replaced easily. Cathodic protection is used for floating vessels, platforms, storage tanks and pipelines The cathodic protection principle is based on the electrochemical nature of the corrosion phenomena; the anodic area corrodes (current is discharged) and the cathodic area does not corrode (current is received). Cathodic protection overrides the naturally occurring anodic areas inside a structure, thus turning the structure under protection completely cathodic, which means it receives current from the surrounding electrolyte (for example, soils, water) and does not corrode. Cathodic protection is achieved by passing enough direct current electricity from an external source (a more powerful anode), which could be a galvanic anode or an impressed current anode. |
| Enhanced Oil Recovery | chemical potential | The change in the Gibbs free energy (G) of a system when an infinitesimally small amount of a component is added under constant pressure (P) and temperature (T) while keeping the mass of the other components of the system unchanged. Concentration variation within a system tends to drive a particle along a gradient from higher to lower chemical potential (?). Chemical potential can also be defined in terms of Helmholtz free energy (A) under conditions of constant volume (V) and temperature. Chemical potential equation: ?i = (?G / ?Ni)T, P, Nj ? i, alternatively: ?i = (?A / ?Ni)T, V, Nj ? i, where ?i = chemical potential of component i G = Gibbs free energy A = Helmholtz free energy Ni = number of particles or moles of component i T = temperature P = pressure V = volume ? = differential operator i, j = chemical components. |
| Enhanced Oil Recovery | clay swelling | A type of damage in which formation permeability is reduced because of the alteration of clay equilibrium. Clay swelling occurs when water-base filtrates from drilling, completion, workover or stimulation fluids enter the formation. Clay swelling can be caused by ion exchange or changes in salinity. However, only clays that are directly contacted by the fluid moving in the rock will react; these include authigenic clays, some detrital clays on the pore boundaries and unprotected clay cement. The nature of the reaction depends on the structure of the clays and their chemical state at the moment of contact. The most common swelling clays are smectite and smectite mixtures that create an almost impermeable barrier for fluid flow when they are located in the larger pores of a reservoir rock. In some cases, brines such as potassium chloride [KCl] are used in completion or workover operations to avoid clay swelling. |
| Enhanced Oil Recovery | coating | Any thin material, liquid or powder, which, applied over a structure, forms a continuous film to protect against corrosion. Corrosion coatings should possess flexibility, resistance against impact and moisture, good adhesion and cohesion, and chemical resistance to the exposure conditions (such as temperature, hydrogen sulfide). Organic coatings such as polyethylenes (plastic) are normally used for external protection of pipelines while asphalt and coal tar enamels are used to protect buried pipes or undersides of oilfield tanks. Inorganic coating such as zinc-silicate is used to protect drilling and production platforms above the splash zone and nickel phosphate coating is used to protect packer body parts. |
| Enhanced Oil Recovery | coating flaw | A void in the pipe coating. Coating flaws are detected by either mechanical or visual inspections and must be repaired to avoid significant corrosion problems. A coating flaw is also called a holiday. |
| Enhanced Oil Recovery | COFCAW | Abbreviation for a combination of forward combustion and waterflooding, also called wet combustion or in situ steam generation. COFCAW is an in situ combustion technique in which water is injected simultaneously or alternately with air into a formation. Wet combustion actually refers to wet forward combustion and was developed to use the great amount of heat that would otherwise be lost in the formation. The injected water recovers the heat from behind the burning front and transfers it to the oil bank ahead. Because of this additional energy, the oil displacement is more efficient and requires less air. In spite of these advantages, a wet combustion process cannot avoid liquid-blocking problems and use of wet combustion is limited by the oil viscosity. |
| Enhanced Oil Recovery | coke | An insoluble organic deposit that has low hydrogen content. Coke, also known as pyrobitumen, is formed by thermal cracking and distillation during in-situ combustion. |
| Enhanced Oil Recovery | compatibility | In matrix stimulation, a characteristic of rock that indicates formation permeability is not reduced when treating fluids and their additives contact the formation minerals or fluids inside the reservoir. Compatibility is especially important in sandstone treatments, in which potentially damaging reactions may occur. The treatment fluid should remove existing damage without creating additional damage, such as precipitates or emulsions, through interactions with the formation rock or fluids. |
| Enhanced Oil Recovery | condensing drive | A gasflood process in which an injection gas enriched with components of intermediate molecular weight, for example butane, is injected into a reservoir to achieve multiple-contact miscibility. Upon contact with the oil, intermediate molecular-weight hydrocarbons transfer from the injected gas phase into the oil phase, a process in which those components are said to condense into the oil. Formation of miscibility may require several contacts between fresh enriched gas and the oil containing condensed components. If the reservoir oil becomes sufficiently enriched with these components that miscibility results between the injection gas and the enriched oil, then the enriched gas and oil have multiple-contact miscibility. A backward multiple-contact test is a laboratory evaluation of a condensing drive process. In the field, both forward- and backward-contact processes can occur during a given gasflood. |
| Enhanced Oil Recovery | constant composition expansion | A laboratory test usually performed as part of a routine PVT analysis that measures the change in volume of a reservoir fluid as a function of pressure. This change is determined by measuring the total volume of a sample of reservoir fluid at various pressures above and below the saturation pressure. The pressure-dependent volumes are normalized to the volume of the sample at the saturation pressure. |
| Enhanced Oil Recovery | contact angle | The angle of intersection of the interface between two fluids at a solid surface. The angle is measured from the solid surface through the aqueous phase, or in an oil and gas test through the oil phase. The contact angle displays hysteresis based on direction of motion of the interface. Surface roughness affects the equilibrium contact angle, so measurements are typically made on smooth, flat surfaces. A contact-angle test uses carefully captured and preserved samples of reservoir oil to determine the wetting preference. A droplet of the crude oil is suspended between two parallel plates of quartz or calcite inside a simulated formation water bath at reservoir temperature and sometimes at reservoir pressure. By periodically displacing one of the plates sideways, a contact angle is determined at the side of the droplet where water is forcing the oil from the solid. A small angle indicates water-wetting preference, while a large angle indicates oil-wetting. Angles near 90 degrees are intermediate-wetting. Different minerals display different wetting preferences, although most are more likely to be water-wet. |
| Enhanced Oil Recovery | coreflooding | A laboratory test in which a fluid or combination of fluids is injected into a sample of rock. Objectives include measurement of permeability, relative permeability, saturation change, formation damage caused by the fluid injection, or interactions between the fluid and the rock. The core material often comes from an oil reservoir, but some tests use outcrop rock. The fluid in place at the start of the test is typically either a simulated formation brine, oil (either crude oil or refined oil), or a combination of brine and oil. Injected fluids may include crude oil, simulated reservoir brine, refined fluids, drilling mud filtrate, acids, foam or other chemicals used in the oil field. Depending on the purpose of the test, conditions may be either ambient temperature and low confining pressure or high temperature and pressure of a subject reservoir. Pressures and flow rates at both ends of the core are measured, and the core can also be investigated using other measurements such as nuclear magnetic resonance (NMR) during the test. A coreflood is typically used to determine the optimum development option for an oil reservoir and often helps evaluate the effect of injecting fluids specially designed to improve or enhance oil recovery. |
| Enhanced Oil Recovery | corrosion | The loss of metal due to chemical or electrochemical reactions, which could eventually destroy a structure. Corrosion can occur anywhere in the production system, either at bottomhole or in surface lines and equipment. The corrosion rate will vary with time depending on the particular conditions of the oil field, such as the amount of water produced, secondary recovery operations and pressure variations. |
| Enhanced Oil Recovery | corrosion control | The measures used to prevent or considerably reduce the effects of corrosion. Corrosion can occur anywhere in the production system, either at bottomhole or in surface lines and equipment. Some practices for corrosion control involve: cathodic protection, chemical inhibition, chemical control (removal of dissolved gases such as hydrogen sulfide, carbon dioxide and oxygen), oxygen scavenging, pH adjustment, deposition control (for example, scales) and coatings. One of the most difficult environments for corrosion control is high bottomhole temperatures, such as 400 to 500oF [200 to 260oC]. The corrosion rate will vary with time depending on the particular conditions of the oil field, such as the amount of water produced, secondary recovery operations and pressure variations. Therefore, corrosion control is a continuous process in oil and gas production operations. |
| Enhanced Oil Recovery | corrosion fatigue | A type of corrosion in which the metal components of a structure fail due to cyclic stresses applied in a corrosive environment, such as salt water. Consequently, a break in the metal will occur at stresses considerably lower than the tensile strength of the material. Corrosion fatigue is the main cause of sucker-rod and drillstring failures. |
| Enhanced Oil Recovery | corrosion inhibitor | In matrix treatments, a chemical added to acid that adsorbs on the pipe surface to form a protective film. This decreases the destructive reaction of acid with metals. The inhibitor does not completely stop the corrosion reaction, but it eliminates more than 99% of the metal losses that would occur if the inhibitor were not present. The inhibitor has little or no effect on the reaction rate of acid with limestone, dolomite or acid-soluble minerals. Specific corrosion inhibitors are environmentally compatible, effective in hydrogen sulfide [H2S] environments, effective on high chrome steel, and effective on special steel alloys, such as coiled tubing. These inhibitors may be used at temperatures approaching 500oF [260oC]. |
| Enhanced Oil Recovery | corrosion rate | The weight loss of a corrosion coupon after exposure to a corrosive environment, expressed as mils (thousandths of an inch) per year penetration. Corrosion rate is calculated assuming uniform corrosion over the entire surface of the coupon. mpy = (weight loss in grams) * (22,300)/(Adt) mpy = corrosion rate (mils per year penetration) A = area of coupon (sq. in.) d = metal density of coupon (g/cm3) t = time of exposure in corrosive environment (days). It is important to note that the calculated values using this formula are not representative in cases of severe pitting. Therefore, a complete report, including a visual inspection, is required to determine either the type of attack or the appropriate corrosion control program. Corrosion rate is also known as corrosion ratio. |
| Enhanced Oil Recovery | cosolvent | A chemical used in small quantities to improve the effectiveness of a primary solvent in a chemical process. |
| Enhanced Oil Recovery | cosurfactant | A chemical added to a process to enhance the effectiveness of a surfactant. In the oil industry, cosurfactants are often used to increase the oil-solubilizing capacity of microemulsion surfactant systems. An example of such a cosurfactant is a long-chain alcohol. Pure surfactants often organize well at a liquid/liquid boundary, which leads to relatively stiff interfaces or even liquid-crystal phases. To achieve ultralow interfacial tension for enhanced oil recovery applications, a cosurfactant is added to disturb this organization at the liquid/liquid interface. Cosurfactants can also be used to fine-tune the formulation phase behavior, for example, by expanding the temperature or salinity range of microemulsion formation. |
| Enhanced Oil Recovery | critical matrix | A near-wellbore area where injected fluids such as acids can restore original permeability. Most of the reservoir pressure drop during production occurs in this near-wellbore part of the reservoir. |
| Enhanced Oil Recovery | damage | Natural or induced production impairments that can develop in the reservoir, the near-wellbore area, the perforations, the gravel-pack completion or the production pipelines, such as the tubing. Natural damage occurs as produced reservoir fluids move through the reservoir, while induced damage is the result of external operations and fluids in the well, such as drilling, well completion, workover operations or stimulation treatments. Some induced damage triggers natural damage mechanisms. Natural damage includes phenomena such as fines migration, clay swelling, scale formation, organic deposition, including paraffins or asphaltenes, and mixed organic and inorganic deposition. Induced damage includes plugging caused by foreign particles in the injected fluid, wettability changes, emulsions, precipitates or sludges caused by acid reactions, bacterial activity and water blocks. Wellbore cleanup or matrix stimulation treatments are two different operations that can remove natural or induced damage. Selecting the proper operation depends on the location and nature of the damage. |
| Enhanced Oil Recovery | detergency | The ability of a chemical agent to remove a contaminant from a solid surface. For example, in enhanced oil recovery, a surfactant can be used to remove an oil phase from a mineral surface. At least two mechanisms can occur: a) The surfactant adsorbs on the contaminated surface and presents its hydrophilic group to the contacting liquid. Thus, the surface behaves hydrophilically and repels macroscopic oil drops. b) The surfactant adsorbs to the contaminant. It is energetically more favorable for the combination of surfactant and contaminant to be in solution than to be attached to the surface so the contaminant is solubilized, exposing the mineral surface. |
| Enhanced Oil Recovery | disbonding | A common coating problem in which the protective coating detaches from the pipeline. |
| Enhanced Oil Recovery | dispersion | Spatial separation of components within a fluid. This separation is often driven by diffusion, mixing or differential flow. In an oil field components might be separated because of heterogeneity of permeability, or simply because of different paths taken by the fluid through the pore structure. Hydrodynamic dispersion includes both of these mechanical effects and molecular diffusion. The components of an enhanced oil recovery formulation can also be dispersed within a porous rock via differential adsorption properties (chromatographic effects). |
| Enhanced Oil Recovery | displacement efficiency | The fraction of oil that has been recovered from a zone swept by a waterflood or other displacement process. Displacement efficiency equation: E = (Voi ? Vor) / Voi, where Voi = volume of oil at start of flood Vor = volume of oil remaining after flood. |
| Enhanced Oil Recovery | displacement front | The interface between an injectant and the fluid it is displacing. |
| Enhanced Oil Recovery | dynamic miscibility | A dynamic fluid-mixing process in which an injected gas exchanges components with in situ oil until the phases achieve a state of miscibility within the mixing zone of the flood front. In a vaporizing drive, light and intermediate components from the oil phase enter the gas phase. By contrast, in a condensing drive, intermediate components from the gas phase enter the oil phase. The process may be a combination of vaporizing and condensing drives. |
| Enhanced Oil Recovery | electrical resistance probe | An instrument used in a corrosion testing to determine metal loss. The probe directly measures the increase in resistance of a metal as its cross-sectional area is reduced by corrosion. At suitable times, once the readings are obtained, these numbers are converted into corrosion rates (mpy). An electrical resistance probe is also called an electrical coupon. |
| Enhanced Oil Recovery | electromagnetic heating | A process to increase thermal energy in a reservoir using electromagnetic means. The two types of electromagnetic heating are conductive and radio frequency (RF). In conductive heating, a current passes from one electrode through the formation to a second electrode. Electrical resistance of the reservoir brine generates thermal energy, heating the reservoir. In RF heating, transmitters in the wellbore generate electromagnetic waves with frequencies in the microwave range that are directed into the formation. The waves interact with water molecules, generating heat in much the same way a microwave oven does. Since both methods rely on water to transfer thermal energy to the reservoir, higher water saturation increases the efficiency of the heat transfer. |
| Enhanced Oil Recovery | emulsion | A type of damage in which there is a combination of two or more immiscible fluids, including gas, that will not separate into individual components. Emulsions can form when fluid filtrates or injected fluids and reservoir fluids (for example oil or brine) mix, or when the pH of the producing fluid changes, such as after an acidizing treatment. Acidizing might change the pH from 6 or 7 to less than 4. Emulsions are normally found in gravel packs and perforations, or inside the formation. Most emulsions break easily when the source of the mixing energy is removed. However, some natural and artificial stabilizing agents, such as surfactants and small particle solids, keep fluids emulsified. Natural surfactants, created by bacteria or during the oil generation process, can be found in many waters and crude oils, while artificial surfactants are part of many drilling, completion or stimulation fluids. Among the most common solids that stabilize emulsions are iron sulfide, paraffin, sand, silt, clay, asphalt, scale and corrosion products. Emulsions are typically treated using mutual solvents. |
| Enhanced Oil Recovery | enhanced oil recovery | An oil recovery enhancement method using sophisticated techniques that alter the original properties of oil. Once ranked as a third stage of oil recovery that was carried out after secondary recovery, the techniques employed during enhanced oil recovery can actually be initiated at any time during the productive life of an oil reservoir. Its purpose is not only to restore formation pressure, but also to improve oil displacement or fluid flow in the reservoir. The three major types of enhanced oil recovery operations are chemical flooding (alkaline flooding or micellar-polymer flooding), miscible displacement (carbon dioxide [CO2] injection or hydrocarbon injection), and thermal recovery (steamflood or in-situ combustion). The optimal application of each type depends on reservoir temperature, pressure, depth, net pay, permeability, residual oil and water saturations, porosity and fluid properties such as oil API gravity and viscosity. Enhanced oil recovery is also known as improved oil recovery or tertiary recovery and it is abbreviated as EOR. |
| Enhanced Oil Recovery | EOR | Abbreviation for enhanced oil recovery, an oil recovery enhancement method using sophisticated techniques that alter the original properties of oil. Once ranked as a third stage of oil recovery that was carried out after secondary recovery, the techniques employed during enhanced oil recovery can actually be initiated at any time during the productive life of an oil reservoir. Its purpose is not only to restore formation pressure, but also to improve oil displacement or fluid flow in the reservoir. The three major types of enhanced oil recovery operations are chemical flooding (alkaline flooding or micellar-polymer flooding), miscible displacement (carbon dioxide [CO2] injection or hydrocarbon injection), and thermal recovery (steamflood or in-situ combustion). The optimal application of each type depends on reservoir temperature, pressure, depth, net pay, permeability, residual oil and water saturations, porosity and fluid properties such as oil API gravity and viscosity. Enhanced oil recovery is also known as improved oil recovery or tertiary recovery. |
| Enhanced Oil Recovery | Eötvös number | A dimensionless group used in analysis of fluid flow that characterizes the ratio of gravitational forces to surface or interfacial tension forces. The Eötvös number is denoted Eo and is equivalent to the Bond number (Nb = Bo). The Eötvös or Bond number is given by Eo = Nb = Bo = (?? g b2) / ?, where Eo = Eötvös number Nb = Bo = Bond number ?? = density difference between the two phases g = acceleration due to gravity b = a characteristic length scale of the flow geometry ? = surface or interfacial tension. |
| Enhanced Oil Recovery | erosion corrosion | A type of corrosion produced when easily removed scales (such as iron carbonate) that were initially protecting the metals in the pipe are eroded and the underlying metals are corroded. Erosion-corrosion is a common cause of failure in oilfield equipment. The attack is normally localized at changes of pipe sections, bends or elbows where there is high velocity or turbulent flow. |
| Enhanced Oil Recovery | erosion-corrosion | A type of corrosion produced when easily removed scales (such as iron carbonate) that were initially protecting the metals in the pipe are eroded and the underlying metals are corroded. Erosion-corrosion is a common cause of failure in oilfield equipment. The attack is normally localized at changes of pipe sections, bends or elbows where there is high velocity or turbulent flow. |
| Enhanced Oil Recovery | ferrous sulfide | A corrosion by-product [FeS2] formed when hydrogen sulfide [H2S] contacts the iron [Fe] present in steel. Ferrous sulfide is a black crystalline material at bottomhole conditions. However, when it contacts air at surface, it will be converted into iron oxide, which is a red-brown compound. Ferrous sulfide is also called iron sulfide. |
| Enhanced Oil Recovery | fingering | A condition whereby the interface of two fluids, such as oil and water, bypasses sections of reservoir as it moves along, creating an uneven, or fingered, profile. Fingering is a relatively common condition in reservoirs with water-injection wells. The result of fingering is an inefficient sweeping action that can bypass significant volumes of recoverable oil and, in severe cases, an early breakthrough of water into adjacent production wellbores. |
| Enhanced Oil Recovery | flood front | The interface between an injectant and the fluid it is displacing. |
| Enhanced Oil Recovery | flooding pattern | Also known as injection pattern, the particular arrangement of production and injection wells. The injection pattern for an individual field or part of a field is based on the location of existing wells, reservoir size and shape, cost of new wells and the recovery increase associated with various injection patterns. The flood pattern can be altered during the life of a field to change the direction of flow in a reservoir with the intent of contacting unswept oil. It is common to reduce the pattern size by infill drilling, which improves oil recovery by increasing reservoir continuity between injectors and producers. Common injection patterns are direct line drive, staggered line drive, two-spot, three-spot, four-spot, five-spot, seven-spot and nine-spot. Normally, the two-spot and three-spot patterns are used for pilot testing purposes. The patterns are called normal or regular when they include only one production well per pattern. Patterns are described as inverted when they include only one injection well per pattern. |
| Enhanced Oil Recovery | flue gas | A gas generated by burning hydrocarbons with air; it is sometimes used as an enhanced oil recovery (EOR) injectant. The composition consists mainly of nitrogen, carbon dioxide, water vapor and excess oxygen with some impurities, such as carbon monoxide, nitrogen oxides and sulfur oxides. Generally, more carbon dioxide in the flue gas results in a better recovery factor for EOR. By contrast, using more nitrogen results in a lower recovery factor for EOR. However, high concentration of impurities, such as oxygen, nitrous oxides and carbon monoxide, can cause corrosion in production tubulars and surface equipment. |
| Enhanced Oil Recovery | fluoboric acid | An acid mixture that generates more hydrofluoric [HF] acid as the HF is consumed. In the field, fluoboric acid [HBF4] is easily prepared by mixing boric acid [H3BO3], ammonium bifluoride [NH3F.HF] and hydrochloric acid. Fluoboric acid was developed to counteract the shortcomings associated with mud-acid treatments. It is a retarded fluid that can penetrate deep into the reservoir before spending, especially at high temperatures, and does not contain high HF at any given time. Thus, it is less reactive than mud acid, but its total dissolving power is comparable: HBF4 + H3O –> HBF3OH + HF. The limited amount of HF at any given time decreases the probability of forming precipitates of fluosilicates, fluoaluminates or silica. Fluoboric acid provides permanent stabilization of clays and fines through reactions related to borate and fluoborate ions. For example, borosilicates coat and bind undissolved clays and fines, preventing further mobility of these particles that might plug the formation and impair production. Mud acid does not provide this coating feature. Fluoboric acid also eliminates water sensitivity and is especially recommended in formations containing potassium minerals. Fluoboric acid can be used as a preflush, an overflush or as a main stage in a sandstone matrix acidizing. As a main fluid, a fluoboric acid treatment requires a preflush (weak HCl acid or brine) and should not be overflushed to obtain the maximum stabilization effect in the critical matrix area. Fluoboric acid treatments are the only acid formulations that require long shut-in times because of their long reaction times. |
| Enhanced Oil Recovery | foam flooding | An enhanced oil recovery process in which foam is injected into a reservoir to improve the sweep efficiency of a driving fluid. Foam can be generated either in the reservoir pore space or at the surface before injection. Foam flooding mitigates sweep inhomogeneities such as those caused by layers with higher permeability than the surrounding formations, or those caused by gravity override. |
| Enhanced Oil Recovery | formation damage | Natural or induced production impairments that can develop in the reservoir, the near-wellbore area or the perforations. Natural damage occurs as produced reservoir fluids move through the reservoir, while induced damage is the result of external operations and fluids in the well, such as drilling, well completion, workover operations or stimulation treatments. Some induced damage triggers natural damage mechanisms. Natural damage includes phenomena such as fines migration, clay swelling, scale formation, organic deposition, including paraffins or asphaltenes, and mixed organic and inorganic deposition. Induced damage includes plugging caused by foreign particles in the injected fluid, wettability changes, emulsions, precipitates or sludges caused by acid reactions, bacterial activity and water blocks. Wellbore cleanup or matrix stimulation treatments are two different operations that can remove natural or induced damage. Selecting the proper operation depends on the location and nature of the damage. |
| Enhanced Oil Recovery | forward multiple-contact test | A laboratory test to determine the phase envelope between lean gas and oil by equilibrating a gas sample several times with fresh samples of oil. In a forward-contact test, light and intermediate components are stripped from the oil by multiple contacts with the gas. The test also indicates how many contacts are required before the gas with added components becomes miscible with the oil. The molar ratios at each contact step are typically designed using PVT simulation software that incorporates the fluid composition at each contact. |
| Enhanced Oil Recovery | fracture acidizing | A well-stimulation operation in which acid, usually hydrochloric [HCl], is injected into a carbonate formation at a pressure above the formation-fracturing pressure. Flowing acid tends to etch the fracture faces in a nonuniform pattern, forming conductive channels that remain open without a propping agent after the fracture closes. The length of the etched fracture limits the effectiveness of an acid-fracture treatment. The fracture length depends on acid leakoff and acid spending. If acid fluid-loss characteristics are poor, excessive leakoff will terminate fracture extension. Similarly, if the acid spends too rapidly, the etched portion of the fracture will be too short. The major problem in fracture acidizing is the development of wormholes in the fracture face; these wormholes increase the reactive surface area and cause excessive leakoff and rapid spending of the acid. To some extent, this problem can be overcome by using inert fluid-loss additives to bridge wormholes or by using viscosified acids. Fracture acidizing is also called acid fracturing or acid-fracture treatment. |
| Enhanced Oil Recovery | galvanic anodes | Materials used to provide cathodic protection. Galvanic anodes are made of metals such as zinc, magnesium or aluminum, which corrode more easily than the structure, thus developing enough electric current flow through the electrolyte (such as soils or water). Galvanic anodes, also called sacrificial anodes, are commonly used when the current required for cathodic protection is small. |
| Enhanced Oil Recovery | gas cap drive | A type of reservoir-drive mechanism in which the energy for the transport and production of reservoir fluids is provided by the expansion of gas either in the gas cap or inside the oil phase. |
| Enhanced Oil Recovery | gas-cap drive | A type of reservoir-drive mechanism in which the energy for the transport and production of reservoir fluids is provided by the expansion of gas either in the gas cap or inside the oil phase. |
| Enhanced Oil Recovery | gravity drainage | The least common primary recovery mechanism in which the force of gravity pushes hydrocarbons out of the reservoir, into the wellbore and up to surface. Gravity force is always present in the reservoir, but its effect is greater in thick gas-condensate reservoirs and in shallow, highly permeable, steeply dipping reservoirs. |
| Enhanced Oil Recovery | gravity override | A phenomenon of multiphase flow in a reservoir in which a less dense fluid flows preferentially on the top of a reservoir unit and a more dense fluid flows at the bottom. For example, in a steamflood, steam flows on the top and condensed liquid flows at the bottom of the zone. Gravity override causes sweep inhomogeneities that can be mitigated through foam flooding. |
| Enhanced Oil Recovery | gravity-stable displacement | A frontal advance in which gravity and viscous forces are in equilibrium, resulting in a stable, highly efficient frontal advance. |
| Enhanced Oil Recovery | HCl | The chemical formula for the compound hydrogen chloride. A solution of hydrogen chloride [HCl] in water is hydrochloric acid. |
| Enhanced Oil Recovery | high-pressure air injection | An enhanced oil recovery process utilizing compressed air that is injected into a reservoir. Oxygen in the gas reacts exothermically with some of the oil, producing highly mobile flue gas. The flue gas advances ahead of the reaction front and achieves an efficient displacement of the in situ oil. Scientists believe that the high displacement efficiency of high-pressure air injection is due to a combination of processes that include immiscible gas displacement, improved miscibility caused by the presence of CO2 in the flue gas, reduction in interfacial tension, oil swelling and reservoir repressurization. The process is typically used for deep, tight, relatively light-oil reservoirs where water injectivity is low. |
| Enhanced Oil Recovery | hydrofluoric acid | A poisonous liquid acid composed of hydrogen and fluorine. Hydrofluoric acid [HF] is used primarily because it is the only common, inexpensive mineral acid that can dissolve siliceous minerals. HF is typically mixed with hydrochloric acid [HCl] or organic acid to keep the pH low when it spends, thereby preventing detrimental precipitates. These mixtures, also called mud acids, are considered the main fluid in a sandstone acid treatment because they remove formation damage. Hydrofluoric acid should not be used in sandstone formations with high carbonate content because of the high risk of calcium fluoride precipitation [CaF2]. |
| Enhanced Oil Recovery | hydrogen blistering | A type of hydrogen-induced failure produced when hydrogen atoms enter low-strength steels that have macroscopic defects, such as laminations. The defects in the steel (void spaces) provide places for hydrogen atoms to combine, forming gaseous molecular hydrogen [H2] that can build enough pressure to produce blistering. Hydrogen blistering is a problem mainly in sour environments. Frequently, it does not cause a brittle failure, but it can produce rupture or leakages. |
| Enhanced Oil Recovery | hydrogen embrittlement | The process whereby hydrogen causes steel components to become less resistant to breakage and generally much weaker in tensile strength. While embrittlement has many causes, in the oil field it is usually the result of exposure to gaseous or liquid hydrogen sulfide [H2S]. On a molecular level, hydrogen ions work their way between the grain boundaries of the steel, where hydrogen ions recombine into molecular hydrogen [H2], taking up more space and weakening the bonds between the grains. The formation of molecular hydrogen can cause sudden metal failure due to cracking when the metal is subjected to tensile stress. This type of hydrogen-induced failure is produced when hydrogen atoms enter high strength steels. The failures due to hydrogen embrittlement normally have a period where no damage is observed, which is called incubation, followed by a sudden catastrophic failure. Hydrogen embrittlement is also called acid brittleness. |
| Enhanced Oil Recovery | hydrogen induced failures | A type of corrosion produced when a metal absorbs hydrogen atoms. This phenomenon can cause undesirable effects such as blistering, cracking, methane formation above 400oF [204oC] and hydrogen embrittlement. |
| Enhanced Oil Recovery | hydrogen probe | A corrosion test instrument mainly used in sour systems (for example, hydrogen sulfide or other sulfide rich environments) to determine qualitatively or semiquantitatively the corrosion of a structure. A hydrogen probe is also called a hydrogen patch probe. |
| Enhanced Oil Recovery | impressed current anodes | Materials to provide cathodic protection. Impressed current anodes are relatively inert to corrosion and require an external power source to generate the electric current that will bring cathodic protection to the structure. Impressed current systems are used mainly when the current required for cathodic protection is large. |
| Enhanced Oil Recovery | improved oil recovery | A method for recovering additional oil beyond fluid expansion, rock compressibility, gravitational drainage, pressure decline and natural waterdrive or gasdrive. This term is used in both a restricted sense and a more general sense. In its restricted sense, it is a process, such as waterflooding or gasflooding, that adds energy to a reservoir to stimulate oil production and increase recovery factor. In its more general sense, it is any activity that increases oil production and increases the recovery factor. This sense can also include, for example, enhanced oil recovery methods, infill drilling, hydraulic fracturing, and drilling horizontal and multilateral wells. |
| Enhanced Oil Recovery | induced particle plugging | A type of damage in which foreign particles injected during normal well operations, such as drilling, completion, workover, stimulation or enhanced recovery, block the near-wellbore formation, reducing well productivity. Potentially damaging particles in drilling fluids include clays, cuttings, weighting agents and fluid-loss control materials. In workover and stimulation fluids, suspended solids include bacteria and polymer residues. Foreign plugging particles can also be introduced as a result of poor water-handling practices. These foreign particles include debris from tanks and tubing. |
| Enhanced Oil Recovery | infill drilling | The addition of wells in a field that decreases average well spacing. This practice both accelerates expected recovery and increases estimated ultimate recovery in heterogeneous reservoirs by improving the continuity between injectors and producers. As well spacing is decreased, the shifting well patterns alter the formation-fluid flow paths and increase sweep to areas where greater hydrocarbon saturations exist. |
| Enhanced Oil Recovery | injection pattern | The particular arrangement of production and injection wells. The injection pattern for an individual field or part of a field is based on the location of existing wells, reservoir size and shape, cost of new wells and the recovery increase associated with various injection patterns. The flood pattern can be altered during the life of a field to change the direction of flow in a reservoir with the intent of contacting unswept oil. It is common to reduce the pattern size by infill drilling, which improves oil recovery by increasing reservoir continuity between injectors and producers. Common injection patterns are direct line drive, staggered line drive, two-spot, three-spot, four-spot, five-spot, seven-spot and nine-spot. Normally, the two-spot and three-spot patterns are used for pilot testing purposes. The patterns are called normal or regular when they include only one production well per pattern. Patterns are described as inverted when they include only one injection well per pattern. |
| Enhanced Oil Recovery | iron-oxidizing bacteria | Aerobic bacteria that convert iron from the ferrous [Fe+2] to the ferric [Fe+3] state and produce ferric hydroxide [Fe(OH)3], which is a highly insoluble by-product that will damage the formation. Iron-oxidizing bacteria also produce some corrosion, but they are considered harmful mainly because they cover sulfate-reducing bacteria colonies and protect them from attack with bactericides. |
| Enhanced Oil Recovery | line drive | An injection pattern in which the injection wells are located in a straight line parallel to the production wells. In a line drive pattern, the injected fluid, which is normally water, steam or gas, creates a nearly linear frontal movement. A line drive pattern is also called direct line drive. |
| Enhanced Oil Recovery | low-salinity waterflooding | An enhanced oil recovery method that uses water with a low concentration of dissolved salts as a flooding medium. The sources of low-salinity water are typically rivers, lakes or aquifers associated with meteoric water. |
| Enhanced Oil Recovery | micellar polymer flooding | An enhanced oil recovery technique in which a micelle solution is pumped into a reservoir through specially distributed injection wells. The chemical solution reduces the interfacial and capillary forces between oil and water and triggers an increase in oil production. The procedure of a micellar-polymer flooding includes a preflush (low-salinity water), a chemical solution (micellar or alkaline), a mobility buffer and, finally, a driving fluid (water), which displaces the chemicals and the resulting oil bank to production wells. |
| Enhanced Oil Recovery | micellar-polymer flooding | An enhanced oil recovery technique in which a micelle solution is pumped into a reservoir through specially distributed injection wells. The chemical solution reduces the interfacial and capillary forces between oil and water and triggers an increase in oil production. The procedure of a micellar-polymer flooding includes a preflush (low-salinity water), a chemical solution (micellar or alkaline), a mobility buffer and, finally, a driving fluid (water), which displaces the chemicals and the resulting oil bank to production wells. |
| Enhanced Oil Recovery | micelle | An ordered aggregate of surfactant molecules formed when the surfactant concentration in a solution reaches a critical point, thus lowering the free energy of the system. Within an aqueous phase, the molecules in a micelle organize such that the hydrophilic head group is the outermost part of the micelle and the hydrophobic tail group is inside the micellar surface. Within an oil phase a reverse, or inverse, micelle can form: The surfactant molecules then organize such that the hydrophobic tail group is outermost, and the hydrophilic head group is inside the surface. A micelle can solubilize oil in water; a reverse micelle can solubilize water in oil. |
| Enhanced Oil Recovery | microbial enhanced oil recovery | An enhanced recovery process in which microorganisms are used in a reservoir to improve oil recovery. The microorganism can either be injected into the reservoir, or the population of an existing microorganism in the reservoir can be enhanced by injection of nutrients preferred by that microorganism. The microorganisms improve oil recovery by various means: (1) by releasing gases and increasing the pressure of the reservoir; (2) by breaking the heavier molecules into smaller chain components, resulting in the reduction of viscosity of oil; and 3) by producing natural surfactants that can improve oil flow by altering the interfacial properties of the system comprising the crude oil, brine and rock. |
| Enhanced Oil Recovery | microemulsion | A thermodynamically stable emulsion consisting of a mixture of oil, water and surfactant. In contrast to a simple emulsion formed under shear, a microemulsion is a minimum energy state. It does not require an input of energy into the system to form; instead, it forms spontaneously. Depending on the structure of the surfactant and the presence or absence of cosurfactant, an oil-in-water system (Winsor Type I), a water-in-oil system (Winsor Type II) or a bicontinuous system (Winsor Type III) may form. Various structures of micelles and reverse micelles are possible, ranging from spherical through cylindrical to lamellar. A typical microemulsion will have micelle diameters in the range of 3 to 20 nm. |
| Enhanced Oil Recovery | mils per year penetration | A unit of measurement for the corrosion rate of a coupon, abbreviated as mpy. A mil is one thousandth of an inch. |
| Enhanced Oil Recovery | minimum miscibility concentration | At constant temperature and pressure, the minimum quantity of additional components, such as intermediate-chain gases or CO2, that must be added to an injection gas to reach first-contact miscibility with a reservoir fluid at a given temperature and pressure. At minimum miscibility concentration conditions, the interfacial tension is zero and no interface exists between the fluids. |
| Enhanced Oil Recovery | minimum miscibility enrichment | Another term for minimum miscibility concentration, the minimum quantity of additional components, such as intermediate-chain gases or CO2, that must be added to an injection gas to reach first-contact miscibility with a reservoir fluid at a given temperature and pressure. At minimum miscibility concentration conditions, the interfacial tension is zero and no interface exists between the fluids. |
| Enhanced Oil Recovery | minimum miscibility pressure | At constant temperature and composition, the lowest pressure at which first- or multiple-contact miscibility (dynamic miscibility) can be achieved. At minimum miscibility pressure, the interfacial tension is zero and no interface exists between the fluids. |
| Enhanced Oil Recovery | miscible displacement | A general term for injection processes that introduce miscible gases into the reservoir. A miscible displacement process maintains reservoir pressure and improves oil displacement because the interfacial tension between oil and water is reduced. The effect of gas injection is similar to that of a solution gasdrive. Miscible displacement is a major branch of enhanced oil recovery processes. Injected gases include liquefied petroleum gas (LPG), such as propane, methane under high pressure, methane enriched with light hydrocarbons, nitrogen under high pressure, and carbon dioxide [CO2] under suitable reservoir conditions of temperature and pressure. The fluid most commonly used for miscible displacement is carbon dioxide because it reduces the oil viscosity and is less expensive than liquefied petroleum gas. Miscible displacement is also called miscible gasdrive, miscible drive or miscible flood. |
| Enhanced Oil Recovery | mixed deposits | A blend of organic and inorganic compounds such as scales, silts or clays. Migrating fines that become oil-wet often become targets for organic deposits, thereby creating a mixed deposit. Mixed deposits are considered a type of damage. Treating this type of deposit requires a dual-solvent system composed of an aromatic hydrocarbon and an acid. |
| Enhanced Oil Recovery | MMC | Minimum miscibility enrichment or minimum miscibility concentration, the minimum quantity of additional components, such as intermediate-chain gases or CO2, that must be added to an injection gas to reach first-contact miscibility with a reservoir fluid at a given temperature and pressure. At minimum miscibility concentration conditions, the interfacial tension is zero and no interface exists between the fluids. |
| Enhanced Oil Recovery | MME | Minimum miscibility enrichment or minimum miscibility concentration, the minimum quantity of additional components, such as intermediate-chain gases or CO2, that must be added to an injection gas to reach first-contact miscibility with a reservoir fluid at a given temperature and pressure. At minimum miscibility concentration conditions, the interfacial tension is zero and no interface exists between the fluids. |
| Enhanced Oil Recovery | mobility buffer | In chemical flooding, a fluid stage, normally water thickened with a polymer, pumped between the micellar or alkaline chemical solution and the final water injection. Mobility buffers are prepared with polyacrylamides or polysaccharides and are frequently employed in micellar-polymer flooding operations because they improve sweep efficiency, which increases oil production. The high viscosity of the mobility buffer aids in the displacement of chemicals into the reservoir and also minimizes the channeling of the final water injection into the chemical solution or into the resulting oil bank. |
| Enhanced Oil Recovery | mobility control | A condition in oil recovery processes whereby the mobility of the injectant is lower than that of the oil or preceding chemical slug, leading to a stable displacement by the injectant. Commonly the injectant is water containing a soluble polymer that increases its viscosity. Micellar-polymer floods incorporate a mobility buffer to maximize the sweep efficiency of the injected chemical and associated oil bank. |
| Enhanced Oil Recovery | mpy | Abbreviation for mils (thousandths of an inch) per year penetration, a unit of measurement for the corrosion rate of a coupon. |
| Enhanced Oil Recovery | multiple-contact miscibility | A dynamic fluid-mixing process in which an injected gas exchanges components with in situ oil until the phases achieve a state of miscibility within the mixing zone of the flood front. In a vaporizing drive, light and intermediate components from the oil phase enter the gas phase. By contrast, in a condensing drive, intermediate components from the gas phase enter the oil phase. The process may be a combination of vaporizing and condensing drives. |
| Enhanced Oil Recovery | nitrogen injection | A process whereby nitrogen gas is injected into an oil reservoir to increase the oil recovery factor. Below the minimum miscibility pressure (MMP), this is an immiscible process in which recovery is increased by oil swelling, viscosity reduction and limited crude-oil vaporization. Above the MMP, nitrogen injection is a miscible vaporizing drive. Miscibility of nitrogen can be achieved only with light oils that are at high pressures; therefore, the miscible method is suitable only in deep reservoirs. |
| Enhanced Oil Recovery | off pattern well | A production or injection well that has a lateral or diagonal displacement with respect to the other wells in an injection pattern. The existence of an off-pattern well affects oil recovery and water/oil ratio. |
| Enhanced Oil Recovery | off-pattern well | A production or injection well that has a lateral or diagonal displacement with respect to the other wells in an injection pattern. The existence of an off-pattern well affects oil recovery and water/oil ratio. |
| Enhanced Oil Recovery | oil bank | The portion of a reservoir where the oil saturation is increased because of the application of an improved oil recovery method. |
| Enhanced Oil Recovery | oil mining | A strip-mining process involving the removal and subsequent processing of tar sand from shallow reservoirs containing heavy, viscous oil. The oil is mostly bitumen and does not flow at reservoir conditions. For strip mining to be economic, the reservoir must be shallow and have high oil saturation. Strip mining is energy intensive, both in mining the tar sand and in its subsequent processing. Large amounts of waste material, called gangue, are produced. Companies are usually required to landscape the area when mining is completed. |
| Enhanced Oil Recovery | oil swelling | An expansion in oil volume that can occur when a solvent contacts a reservoir fluid. The swelling is due to the complete or partial dissolution of the solvent molecules into the reservoir fluid. The amount of swelling is dependent on the pressure, temperature, composition and physical properties of the solvent and the reservoir fluid. Reservoir oil swelling can result in improved oil recovery by mobilizing residual oil trapped in inaccessible pore spaces. |
| Enhanced Oil Recovery | organic acid | A type of organic fluid, such as acetic or formic acid, used in oil and gas well-stimulation treatments. The use of inhibited HCl at elevated temperatures is limited to relatively short exposure times because of the risk of increased corrosion. Inhibited organic acids are much less reactive with metals than are HCl or mixtures of HCl and HF. For this reason, organic acids are commonly used successfully at high bottomhole temperatures or when long contact times between acid and pipe are needed, such as during perforating. Organic acids also are used to protect exotic alloys, such as aluminum or chrome-plated parts. |
| Enhanced Oil Recovery | organic deposit | A type of damage in which heavy hydrocarbons precipitate when temperature or pressure is reduced. These deposits are commonly located in the tubing, gravel pack and perforations, or inside the formation. The injection of cold treating fluids promotes the formation of organic deposits. Organic deposits such as paraffins or asphaltenes are resolubilized using aromatic organic solvents such as toluene or xylene. Small amounts of alcohol help to further dissolve asphaltenes. |
| Enhanced Oil Recovery | paraffin control | A set of techniques used to prevent or considerably reduce paraffin deposition. Paraffin control might involve the following options: · use of paraffin inhibitors. · maintaining pipe surfaces in a water-wet condition because paraffin will not adhere to water. However, the presence of natural surfactants in some crude oils converts water-wet surfaces to an oil-wet condition, making this technique effective only temporarily. · coating the pipe with plastic to provide a smooth surface and reduce paraffin adhesion. · reducing heat transfer to maintain the oil temperature above its cloud point. Filling the annulus of a well with a fluid that has poorer heat transfer properties than the oil maintains the temperature of the flowing crude oil above its cloud point. |
| Enhanced Oil Recovery | paraffin inhibitor | A chemical injected into the wellbore to prevent or minimize paraffin deposition. The effectiveness of paraffin inhibitors is strongly dependent on crude oil composition. Paraffin inhibitors must be introduced into the oil before the oil cools to its cloud point. In additional, asphaltene composition should be determined before treatment because it can reduce the effectiveness of the paraffin inhibitor. In some cases, the use of a paraffin inhibitor can actually increase the rate of paraffin deposition because the stability of colloidal asphaltenes is disturbed. |
| Enhanced Oil Recovery | partitioning | The degree of solubilization of a solute into each of multiple immiscible phases at equilibrium. For example, a water-soluble surfactant injected as part of an enhanced oil recovery flood will partially solubilize, or partition, in the oil phase. The degree of partitioning will influence the efficiency of the enhanced oil recovery agent. |
| Enhanced Oil Recovery | pendant-drop tensiometer | A standard laboratory instrument used to measure interfacial tension. The method is particularly applicable to relatively high interfacial tensions, but with care can measure down to approximately 1 mN/m. A drop of the denser liquid is poised at the end of a square-ended syringe needle. The drop is of sufficient size that its shape is deformed by gravity, but not so large that it detaches from the syringe. Its shape is determined by the balance of interfacial tension and gravity. The interfacial tension can be obtained from the drop shape and the densities of the two liquids. The method works equally well for a drop of the less-dense liquid by inverting the syringe. This inverted configuration can be useful if the less-dense liquid is opaque. |
| Enhanced Oil Recovery | pitting | A type of corrosion in which there is loss of metal in localized areas. The corrosion rate in the pits is many times greater than the corrosion rate on the entire surface. The resultant pits can be large and shallow or narrow and deep. Pitting is a more dangerous problem than general corrosion because the pitted areas can be easily penetrated. |
| Enhanced Oil Recovery | polyacrylamide | A polymer with a high molecular weight. The basic repeating unit or monomer of polyacrylamide is a combination of carbon, hydrogen, oxygen and nitrogen. Polyacrylamides increase the viscosity of the water slug that precedes the final water injection. Polyacrylamides are frequently used as mobility-control buffers in micellar-polymer flooding operations. |
| Enhanced Oil Recovery | polymer flooding | An enhanced oil recovery technique using water viscosified with soluble polymers. Viscosity is increased until the mobility of the injectant is less than that of the oil phase in place, so the mobility ratio is less than unity. This condition maximizes oil-recovery sweep efficiency, creating a smooth flood front without viscous fingering. Polymer flooding is also applied to heterogeneous reservoirs; the viscous injectant flows along high-permeability layers, decreasing the flow rates within them and enhancing sweep of zones with lower permeabilities. The two polymers that are used most frequently in polymer flooding are partially hydrolyzed polyacrylamide and xanthan. |
| Enhanced Oil Recovery | polysaccharide | A carbohydrate composed of many monosaccharides. Polysaccharides increase the viscosity of the water slug that precedes the final water injection. However, they are not frequently used in chemical flooding operations because they generate numerous by-products that can potentially plug filters or well sandfaces, especially when they contact polyvalent cations or bacteria. Polysaccharides are also called biopolymers. |
| Enhanced Oil Recovery | precipitate | A reaction by-product. In sandstone acidizing, the reaction between hydrofluoric acids [HF] or spent HF acids with formation minerals can precipitate nondamaging products, such as silica, borosilicates or fluoborates. However, other insoluble or difficult to remove by-products can create formation damage. Ferric iron (Fe+3) and ferrous iron (Fe+2) are potential sources for precipitates. Ferric iron present in some formation minerals, including chlorite and glauconite clays, and in tubing rust (iron oxide) can precipitate as ferric hydroxide [Fe(OH)3], which is a gelatinous, highly insoluble mass that can plug pore channels and reduce permeability. The precipitation of ferric hydroxide or ferrous hydroxide [Fe(OH)2] depends on the pH of the spent acid. The former needs a pH higher than 2.2, while the latter requires a pH higher than 7.7. Since the maximum pH for a spent acid is approximately 5.3, the precipitation of ferric hydroxide is more common. Iron-sequestering or iron-reducing agents can be used in acid to maintain the ferric iron in solution. Calcium fluoride [CaF2] precipitates when HF contacts calcite or any other calcium source, and alkali-fluosilicates or iron sulfide form crystal-like by-products that can bridge pore throats. Additionally, some sequestering agents, corrosion inhibitors or friction reducers can also form residues that may plug formation pores. The formation of precipitates can be avoided or reduced by using a preflush, which dissolves calcareous material, iron rust or iron scales, and displaces formation brines (K, Na, Ca ions) away from the wellbore, thereby reducing the formation of CaF2, ferric hydroxide and alkali-fluosilicates. |
| Enhanced Oil Recovery | preflush | A fluid stage, normally hydrochloric acid [HCl], pumped ahead of the main treating fluid (mixture of hydrofluoric [HF] and hydrochloric [HCl] or organic acids) in a sandstone matrix-stimulation treatment. One of the purposes of a preflush is to displace formation brines that contain K, Na, Ca ions away from the wellbore, decreasing the possibility of crystallizing alkali-fluosilicates that could plug the pores. The other purpose of a preflush is to dissolve calcareous materials to minimize calcium fluoride [CaF2] precipitation, and to dissolve iron scale or rust to avoid the precipitation of the gelatinous, highly insoluble ferric hydroxide [Fe(OH)3]. Multiple preflush stages using brines such as ammonium chloride [NH4Cl] or solvents are used when multiple damage types are present. A preflush is sometimes called a spearhead. |
| Enhanced Oil Recovery | pressure-composition diagram | A graphical representation indicating phase behavior for variation of saturation pressure and injection gas concentration at a given temperature. The diagram indicates conditions for single-phase and two-phase behavior and, within the two-phase region, lines of constant volume fraction, termed quality lines. The diagram is constructed using swelling test saturation pressures and liquid volumes. |
| Enhanced Oil Recovery | PVT | An abbreviation for pressure, volume, temperature. The term is used in fluid properties evaluations. |
| Enhanced Oil Recovery | recovery factor | The recoverable amount of hydrocarbon initially in place, normally expressed as a percentage. The recovery factor is a function of the displacement mechanism. An important objective of enhanced oil recovery is to increase the recovery factor. |
| Enhanced Oil Recovery | reducing agent | A chemical added to an acid to stabilize iron. The injected acid dissolves iron from rust, millscale, iron scales or iron-containing minerals in the formation. Iron can exist as ferric iron [Fe+3] or ferrous iron [Fe+2]. If the iron is not controlled, it will precipitate insoluble products such as ferric hydroxide and, in sour environments, ferrous sulfide [FeS], which will damage the formation. Reducing agents change or reduce Fe+3 to Fe+2 to avoid precipitation. Erythorbic acid is an effective reducing agent. |
| Enhanced Oil Recovery | retort test | A test for water mud or oil mud, also known as the water, oil and solids test. Proper procedures for retort tests have been published by API. The test is a distillation of a mud sample that measures condensed oil and water collected from the retort. Data obtained are: (1) vol. % water, (2) vol. % oil and (3) vol. % retort solids. Retort solids is the volume that was not recovered as a liquid. Three sizes of retort apparatus are available: 10-, 20- and 50-cm3 mud sample size. Some designs have a small oven in the carrying case to heat the sample (the preferred method for oil muds) while others use a blade heater that goes into the mud sample. Retorts should be heated to around 700°F [371°C] to be effective. |
| Enhanced Oil Recovery | ringworm corrosion | A type of bimetallic corrosion. Ringworm corrosion has the shape of a ring and is located a few inches from the pipe upset. The ring can either be very smooth or have severe pitting. Ringworm corrosion is caused by the upsetting process, in which the heat required for upsetting creates two different grain structures, one in the upset and another in the rest of the pipe. This condition can be avoided by fully normalizing the pipe after upsetting. To normalize the pipe, heat is applied to change the grains to a uniform structure. |
| Enhanced Oil Recovery | rising-bubble apparatus | A laboratory device used to indicate miscibility between reservoir oil and injection gas. A gas bubble is injected into an oil-filled visual cell at a given temperature and test pressure. The change in shape of the rising bubble indicates its miscibility with the oil at those conditions. Below the minimum miscibility pressure (MMP), the bubble holds its shape as it rises. Above the MMP, the bubble shape changes as it rises; it may disintegrate, dissolve or disappear into the oil. Testing at several pressures helps determine the MMP between the gas and oil. The rising-bubble test represents a forward-contacting miscibility process and therefore may not accurately estimate the MMP for a backward or combined contact mechanism. |
| Enhanced Oil Recovery | sandstone petrography | The study of and description of sandstones, including the mineral content. In matrix stimulation, only the mineral surfaces contacted by the stimulation fluid will be dissolved, so a petrographic study often helps anticipate the rocks response to fluid injection. Sandstone reservoirs are made of silicate grains such as quartz, feldspar, chert and mica, which are deposited as sand; secondary minerals may be deposited in the original pore spaces. Secondary quartz or carbonate minerals often bind sand grains together. Authigenic clays, mainly composed of silicon and aluminum, may also form in the pores. The reactivity of a given mineral depends on three factors: surface area, chemical composition and temperature. Clays have greater specific surface area compared with other matrix minerals, which makes them the most reactive components during well-stimulation operations. |
| Enhanced Oil Recovery | SCAL | Special core analysis laboratory. |
| Enhanced Oil Recovery | scale inhibitor squeeze | A type of inhibition treatment used to control or prevent scale deposition. In a scale-inhibitor squeeze, the inhibitor is pumped into a water-producing zone. The inhibitor is attached to the formation matrix by chemical adsorption or by temperature-activated precipitation and returns with the produced fluid at sufficiently high concentrations to avoid scale precipitation. Some chemicals used in scale-inhibitor squeezes are phosphonated carboxylic acids or polymers. |
| Enhanced Oil Recovery | scale removal | A common well-intervention operation involving a wide variety of mechanical scale-inhibitor treatments and chemical options. Mechanical removal is done by means of a pig or by abrasive jetting that cuts scale but leaves the tubing untouched. Scale-inhibition treatments involve squeezing a chemical inhibitor into a water-producing zone for subsequent commingling with produced fluids, preventing further scale precipitation. Chemical removal is performed with different solvents according to the type of scale: · Carbonate scales such as calcium carbonate or calcite [CaCO3] can be readily dissolved with hydrochloric acid [HCl] at temperatures less than 250oF [121oC]. · Sulfate scales such as gypsum [CaSO4·2H2O] or anhydrite [CaSO4] can be readily dissolved using ethylenediamine tetraacetic acid (EDTA). The dissolution of barytine [BaSO4] or strontianite [SrSO4] is much more difficult. · Chloride scales such as sodium chloride [NaCl] are easily dissolved with fresh water or weak acidic solutions, including HCl or acetic acid. · Iron scales such as iron sulfide [FeS] or iron oxide [Fe2O3] can be dissolved using HCl with sequestering or reducing agents to avoid precipitation of by-products, for example iron hydroxides and elemental sulfur. · Silica scales such as crystallized deposits of chalcedony or amorphous opal normally associated with steamflood projects can be dissolved with hydrofluoric acid [HF]. |
| Enhanced Oil Recovery | scale-inhibitor squeeze | A type of inhibition treatment used to control or prevent scale deposition. In a scale-inhibitor squeeze, the inhibitor is pumped into a water-producing zone. The inhibitor is attached to the formation matrix by chemical adsorption or by temperature-activated precipitation and returns with the produced fluid at sufficiently high concentrations to avoid scale precipitation. Some chemicals used in scale-inhibitor squeezes are phosphonated carboxylic acids or polymers. |
| Enhanced Oil Recovery | scraper trap | Equipment placed in a pipeline for inserting or retrieving a pipeline scraper (pig) |
| Enhanced Oil Recovery | screening | A preliminary assessment of the suitability of a reservoir for a particular process or development methodology. The assessment compares the reservoir characteristics to a number of screening criteria. The criteria are developed by studying the reservoir characteristics of similar past projects and identifying the ones that influenced success or failure of the process or methodology, or are consistently present where the process or methodology succeeded or failed. |
| Enhanced Oil Recovery | sensitivity | In matrix stimulation, a characteristic of rock that indicates the degree of reaction between the rock minerals and a given treating fluid. A formation is described as sensitive if a given stimulating fluid damages the formation. The detrimental reactions include disaggregation and collapse of the matrix, release of fines or formation of precipitates. Sensitivity depends on the overall reactivity of the formation minerals with the fluid; reactivity depends on the structure of the rock and the distribution of minerals within the rock. Sandstone sensitivity also depends on permeability; low-permeability formations are normally more sensitive than high-permeability sandstones for a given mineralogy because certain types of damage, such as formation of precipitates, are more harmful in small pore throats (as in low-permeability formations). |
| Enhanced Oil Recovery | sequestering agent | Another term for chelating agent, a chemical added to an acid to stabilize iron. The injected acid dissolves iron from rust, millscale, iron scales or iron-containing minerals in the formation. Iron can exist as ferric iron [Fe+3] or ferrous iron [Fe+2]. If the iron is not controlled, it will precipitate insoluble products such as ferric hydroxide and, in sour environments, ferrous sulfide [FeS], which will damage the formation. Chelating agents associate with iron [Fe+3 or Fe+2] to form soluble complexes. Citric acid, acetic acid and EDTA are effective chelating agents and can be used at temperatures up to 400oF [204oC]. |
| Enhanced Oil Recovery | slime forming bacteria | Bacteria that can live with or without oxygen and produce mats of high-density slime that cover surfaces. Their primary detrimental effects are the protection of sulfate-reducing bacteria and pore plugging. |
| Enhanced Oil Recovery | slime-forming bacteria | Bacteria that can live with or without oxygen and produce mats of high-density slime that cover surfaces. Their primary detrimental effects are the protection of sulfate-reducing bacteria and pore plugging. |
| Enhanced Oil Recovery | slim-tube test | A laboratory test used to estimate the minimum miscibility pressure (MMP) or minimum miscibility concentration (MMC) of a given injection solvent and reservoir oil. The slim tube is a long coiled tube filled with sand of a specific mesh size or similar porous media. The tube is saturated at the beginning of each test with reservoir fluid at a given temperature. Solvent injection is performed at several test pressures. Effluent production, density and composition are measured as functions of the injected volume. Oil recovery after injection of a specific number of pore volumes (PV) such as 1.2 PV of solvent is the test criterion for miscibility. Two trend lines appear on a plot of recovery versus pore pressure for several slim-tube tests. The point of intersection of those trend lines is the estimated MMP for the given oil-solvent system. The data from a slim tube test can also be used as input to fine-tune a fluid equation of state for reservoir simulation. |
| Enhanced Oil Recovery | sludge | A thick, viscous emulsion containing oil, water, sediment and residue that forms because of the incompatibility of certain native crude oils and strong inorganic acids used in well treatments. Use of certain additives, such as surfactants, or the presence of dissolved iron can promote sludge formation, especially if asphaltenes are present in the crude oil. Therefore, it is important to test a sample of crude with the treating fluid before injecting a treatment into a reservoir. |
| Enhanced Oil Recovery | solubility | The maximum amount of a substance that will dissolve in a given amount of solvent at a given temperature and pressure, or the degree to which a substance will dissolve in a particular solvent. The solubility of a substance is its concentration in a saturated solution. Two fluids that are soluble in one another in all proportions are also referred to as miscible. |
| Enhanced Oil Recovery | solution gasdrive | A type of reservoir-drive mechanism in which the energy for the transport and production of reservoir fluids is provided by the gas dissolved in the liquid. As reservoir fluids enter the wellbore, changing pressure conditions cause the gas to break from solution to create a commingled flow of gas and liquid that aids production. |
| Enhanced Oil Recovery | spinning-drop tensiometer | A standard laboratory instrument to measure interfacial tension. The method is particularly applicable to values of interfacial tension below 1 mN/m and especially below 10-2 mN/m, as may occur when employing surfactants for enhanced oil recovery. The method utilizes a tube containing a drop of the less-dense phase within the more-dense phase. When the tube is spun along its long axis at high speed, the resulting forces center the drop on the tube axis and deform it. The interfacial tension is a function of the shape of the deformed drop, the liquid densities and the rotation speed. Advanced versions of the instrument can periodically vary the rotation rate. The phase lag between the change of rotation rate and the drop deformation can be used to determine both interfacial elasticity and interfacial viscosity. |
| Enhanced Oil Recovery | standard batch | A type of batch-treating technique used in corrosion control. The batch of corrosion inhibitor is displaced through the annulus to the bottom of the well. Once the inhibitor is at the bottom, it is circulated up the tubing and returned back into the annulus, leaving a considerable amount of inhibitor in the annulus for further circulation. A standard batch treatment is used mainly in pumping wells and could last from a day to several months depending on the specific corrosion inhibitor used. |
| Enhanced Oil Recovery | steam trap | A barrier or resistance to the flow of injected steam formed by a volume around a producing well in a steamflood that contains high oil and liquid water saturation. This is typically maintained by choking the production well to keep the surrounding formation just below saturated steam temperature and pressure conditions. It is used in the steam-assisted gravity-drainage process. |
| Enhanced Oil Recovery | stock-tank oil initially in place | The volume of oil in a reservoir prior to production. |
| Enhanced Oil Recovery | stock-tank original oil in place | The volume of oil in a reservoir prior to production. |
| Enhanced Oil Recovery | STOIIP | Abbreviation for stock-tank oil initially in place, the volume of oil in a reservoir prior to production. |
| Enhanced Oil Recovery | STOOIP | Abbreviation for stock-tank original oil in place, the volume of oil in a reservoir prior to production. |
| Enhanced Oil Recovery | surfactant flooding | An enhanced oil recovery process in which a small amount of surfactant is added to an aqueous fluid injected to sweep the reservoir. The presence of surfactant reduces the interfacial tension between the oil and water phases and also alters the wettability of the reservoir rock to improve oil recovery. |
| Enhanced Oil Recovery | surfactant-alternating gas | An enhanced oil recovery process in which alternating slugs of a surfactant solution and gas are injected into a reservoir. The injected surfactant and gas mix and generate foam that reduces the gas mobility, especially in previously swept or high-permeability regions of the reservoir. This improves sweep efficiency by mitigating gravity override and viscous fingering during gas injection. The presence of the surfactant in the injectant can also improve recovery by reducing interfacial tension between reservoir oil and the injection phases. |
| Enhanced Oil Recovery | sweep efficiency | A measure of the effectiveness of an enhanced oil recovery process that depends on the volume of the reservoir contacted by the injected fluid. The volumetric sweep efficiency is an overall result that depends on the injection pattern selected, off-pattern wells, fractures in the reservoir, position of gas-oil and oil/water contacts, reservoir thickness, permeability and areal and vertical heterogeneity, mobility ratio, density difference between the displacing and the displaced fluid, and flow rate. |
| Enhanced Oil Recovery | sweet corrosion | The corrosion caused by contact with carbon dioxide [CO2] dissolved in water. In gas condensate wells, sweet corrosion takes the form of deep pitting inside the tubing walls. The pitting is produced only at depths where the acidic gas contacts condensed water droplets. |
| Enhanced Oil Recovery | ternary diagram | A graphical representation of concentrations in a system with three components. Since the sum of the component percentages is unity, any composition can be uniquely mapped to a single point within a triangular space. In many cases, a mixture of fluids with more than three components is divided into three pseudocomponents, such as light, intermediate and heavy components of a hydrocarbon phase. These diagrams are used to illustrate the phase behavior of a fluid. |
| Enhanced Oil Recovery | tertiary recovery | Traditionally, the third stage of hydrocarbon production, comprising recovery methods that follow waterflooding or pressure maintenance. The principal tertiary recovery techniques used are thermal methods, gas injection and chemical flooding. The term is sometimes used as a synonym for enhanced oil recovery (EOR), but because EOR methods today may be applied at any stage of reservoir development, the term tertiary recovery is less commonly used than in the past. |
| Enhanced Oil Recovery | tie line | In a ternary diagram, a graphical representation of two fluids being mixed. The ends of the tie line indicate the compositional concentrations of the two mixed fluids. The composition of the mixture lies on the line, with its position dependent on the concentration ratio of the two end-point fluids. |
| Enhanced Oil Recovery | trapped oil | Oil in pore spaces that cannot be moved because of capillary forces. Typical trapped or residual oil saturation is in the range of 10% to 50% of the pore space, and it is higher in tighter formations where the pore spaces are small. The amount of trapped oil is a function of the displacement method and conditions, making this oil a target for enhanced oil recovery (EOR) processes. EOR methods introduce fluids that reduce viscosity, interfacial tension or mobility ratio, and thus improve flow and sweep efficiency to release the residual oil. |
| Enhanced Oil Recovery | tubing displacement | A type of batch-treating technique used in corrosion control in which a batch of corrosion inhibitor is displaced through the tubing to the bottom of the well. The well is shut in for 2 to 15 hr and then put back on production. The tubing-displacement technique, also called a kiss squeeze, is used mainly in wells with packers and in gas-lift wells. The treatment could last from a week to several months depending on the specific corrosion inhibitor used. |
| Enhanced Oil Recovery | ultralow interfacial tension | Values of interfacial tension (IFT) less than about 10-2 mN/m. Mixed surfactant systems, as used in enhanced oil recovery, adsorb at the oil/water interface and can be designed to generate an interface that is flexible and that has an ultralow IFT. Ultralow IFT implies (1) a significant increase in the capillary number for a given flow velocity and therefore that viscous forces generated during a flood can mobilize additional oil; and (2) a significant increase in the Bond number and therefore that gravitational forces (i.e., buoyancy) can mobilize additional oil. Micellar-polymer and alkaline-surfactant-polymer flooding are techniques used to achieve ultralow IFT. |
| Enhanced Oil Recovery | vaporizing drive | A gasflood process in which a lean gas, for example methane, nitrogen or carbon dioxide, is injected into a reservoir to achieve multiple-contact miscibility. Upon contact with the oil, light and intermediate molecular-weight hydrocarbons transfer from the oil into the gas phase, thus vaporizing into the gas. Formation of miscibility may require several contacts between gas containing vaporized components and fresh reservoir oil. If the injected gas becomes sufficiently enriched with these components that miscibility results with the oil, then the lean gas and oil have multiple-contact miscibility. A forward multiple-contact test is a laboratory evaluation of a vaporizing drive process. In the field, both forward- and backward-contact processes can occur during a given gasflood. |
| Enhanced Oil Recovery | vertical displacement efficiency | In a displacement process, the ratio of the cumulative height of the vertical sections of the pay zone that are contacted by injection fluid to the total vertical pay zone height. Vertical displacement efficiency (EI) strongly depends on parameters such as mobility ratio and total volume of fluid injected. Nonuniform permeability may cause an irregular front that affects the vertical displacement efficiency because the injected fluid flows faster in high-permeability zones than in low-permeability zones. |
| Enhanced Oil Recovery | vertical sweep efficiency | Another term for vertical displacement efficiency, in a displacement process, the ratio of the cumulative height of the vertical sections of the pay zone that are contacted by injection fluid to the total vertical pay zone height. Vertical displacement efficiency (EI) strongly depends on parameters such as mobility ratio and total volume of fluid injected. Nonuniform permeability may cause an irregular front that affects the vertical displacement efficiency because the injected fluid flows faster in high-permeability zones than in low-permeability zones. |
| Enhanced Oil Recovery | viscous fingering | A condition whereby the interface of two fluids, such as oil and water, bypasses sections of reservoir as it moves along, creating an uneven, or fingered, profile. Fingering is a relatively common condition in reservoirs with water-injection wells. The result of fingering is an inefficient sweeping action that can bypass significant volumes of recoverable oil and, in severe cases, an early breakthrough of water into adjacent production wellbores. |
| Enhanced Oil Recovery | viscous force | A measure of a fluid’s resistance to flow. Viscous forces in a fluid are proportional to the rate at which the fluid velocity is changing in space; the proportionality constant is the viscosity. For Newtonian liquids (liquids that show no variation of viscosity with shear or extension rate), the ratio of extensional viscosity to shear viscosity is 3. This value is Trouton’s ratio. For more complex liquids, for example, polymer solutions, Trouton’s ratio can be different from 3 and can vary with shear or extension rate. |
| Enhanced Oil Recovery | volumetric displacement efficiency | Another term for volumetric sweep efficiency, a measure of the effectiveness of an enhanced oil recovery process that depends on the volume of the reservoir contacted by the injected fluid. The volumetric sweep efficiency is an overall result that depends on the injection pattern selected, off-pattern wells, fractures in the reservoir, position of gas-oil and oil/water contacts, reservoir thickness, permeability and areal and vertical heterogeneity, mobility ratio, density difference between the displacing and the displaced fluid, and flow rate. |
| Enhanced Oil Recovery | volumetric sweep efficiency | A measure of the effectiveness of an enhanced oil recovery process that depends on the volume of the reservoir contacted by the injected fluid. The volumetric sweep efficiency is an overall result that depends on the injection pattern selected, off-pattern wells, fractures in the reservoir, position of gas-oil and oil/water contacts, reservoir thickness, permeability and areal and vertical heterogeneity, mobility ratio, density difference between the displacing and the displaced fluid, and flow rate. |
| Enhanced Oil Recovery | waterflood kick | The first indication of increased crude-oil production as the result of a waterflooding project. |
| Enhanced Oil Recovery | Winsor phase behavior | A distinction among three phase behaviors of oil, water and surfactant systems when they form a microemulsion. The salinity of the brine phase is an important parameter influencing which type of behavior occurs. To test for the type of system, surfactant is added to an oil-water system. In a Winsor Type I system, the surfactant forms an oil-in-water microemulsion in the aqueous phase. This behavior is not favorable to achieve ultralow interfacial tension with surfactants. In a Winsor Type II system, the surfactant forms a water-in-oil emulsion in the oil phase. This behavior leads to surfactant retention in the oil phase and is unfavorable for an enhanced oil recovery (EOR) process. In a Winsor Type III system, the surfactant forms a microemulsion in a separate phase between the oil and aqueous phases. This phase is a continuous layer containing surfactant, water and dissolved hydrocarbons. This situation is ideal to achieve ultralow interfacial tension values and is favorable for EOR. |
| Enhanced Oil Recovery | wormhole | A large, empty channel that can penetrate several feet into the formation, caused by the nonuniform dissolution of limestone or dolomite by hydrochloric acid [HCl]. Wormholes are created during matrix stimulation or acid fracturing of carbonate formations. The purpose of matrix stimulation is to create highly conductive wormholes to bypass damage. However, in fracture acidizing, wormholing is a problem, since it is an unwanted diversion of the live acid from the hydraulic fracture system, which causes a reduction of the etched fracture length. |
| Enhanced Oil Recovery | Young-Laplace equation | A relationship describing the pressure difference across an interface between two fluids at a static, curved interface. This relationship defines the capillary pressure difference at such an interface. (1.) Pc = ? (1/R1 + 1/R2), (2.) Pc = 2?/r, where Pc = capillary pressure ? = interfacial tension between the fluids R1, R2 = principal radii of curvature of the interface r = capillary radius. |
| Enhanced Oil Recovery, Drilling Fluids | hydrophile-lipophile balance number | A number on the scale of one to 40 according to the HLB system, introduced by Griffin (1949 and 1954). The HLB system is a semi-empirical method to predict what type of surfactant properties a molecular structure will provide. The HLB system is based on the concept that some molecules have hydrophilic groups, other molecules have lipophilic groups, and some have both. Weight percentage of each type of group on a molecule or in a mixture predicts what behavior the molecular structure will exhibit. Water-in-oil emulsifiers have a low HLB numbers, typically around 4. Solubilizing agents have high HLB numbers. Oil-in-water emulsifiers have intermediate to high HLB numbers. Reference: Griffin WC: Classification of Surface-Active Agents by ‘HLB,’ Journal of the Society of Cosmetic Chemists 1 (1949): 311. |
| Enhanced Oil Recovery, Drilling Fluids | interfacial tension | A property of the interface between two immiscible phases. When the phases are both liquid, it is termed interfacial tension; when one of the phases is air, it is termed surface tension. Interfacial tension is the Gibbs free energy per unit area of interface at fixed temperature and pressure. Interfacial tension occurs because a molecule near an interface has different molecular interactions than an equivalent molecule within the bulk fluid. Surfactant molecules preferentially position themselves at the interface and thereby lower the interfacial tension. |
| Enhanced Oil Recovery, Drilling Fluids | lipophilic | Pertaining to an attraction for oil by a surface of a material or a molecule. This term is applied to the oil-wetting behavior of treatment chemicals for oil muds. Lipophilic oil-mud additives are required because most minerals drilled and additives such as barite are naturally hydrophilic and must be rendered lipophilic. |
| Enhanced Oil Recovery, Drilling Fluids | surfactant | A chemical that preferentially adsorbs at an interface, lowering the surface tension or interfacial tension between fluids or between a fluid and a solid. This term encompasses a multitude of materials that function as emulsifiers, dispersants, oil-wetters, water-wetters, foamers and defoamers. The type of surfactant behavior depends on the structural groups on the molecule (or mixture of molecules). Hydrophile-lipophile balance (HLB) number helps define the function that a molecular group will perform. |
| Enhanced Oil Recovery, Enhanced Oil Recovery | alkaline flooding | An enhanced oil recovery technique in which an alkaline chemical such as sodium hydroxide, sodium orthosilicate or sodium carbonate is injected during polymer flooding or waterflooding operations. The alkaline chemical reacts with certain types of oils, forming surfactants inside the reservoir. Eventually, the surfactants reduce the interfacial tension between oil and water and trigger an increase in oil production. Alkaline flooding is not recommended for carbonate reservoirs because of the abundance of calcium: the mixture between the alkaline chemical and the calcium ions can produce hydroxide precipitation that may damage the formation. Alkaline flooding is also known as caustic flooding. |
| Enhanced Oil Recovery, Enhanced Oil Recovery | chemical flooding | A general term for injection processes that use special chemical solutions. Micellar, alkaline and soap-like substances are used to reduce surface tension between oil and water in the reservoir, whereas polymers such as polyacrylamide or polysaccharide are employed to improve sweep efficiency. The chemical solutions are pumped through specially distributed injection wells to mobilize oil left behind after primary or secondary recovery. Chemical flooding is a major component of enhanced oil recovery processes and can be subdivided into micellar-polymer flooding and alkaline flooding. The general procedure of a chemical flooding includes a preflush (low-salinity water), a chemical solution (micellar or alkaline), a mobility buffer and, finally, a driving fluid (water), which displaces the chemicals and the resulting oil bank to production wells. The preflush and the mobility buffer are optional fluids. |
| Enhanced Oil Recovery, Enhanced Oil Recovery | chemical injection | A general term for injection processes that use special chemical solutions to improve oil recovery, remove formation damage, clean blocked perforations or formation layers, reduce or inhibit corrosion, upgrade crude oil, or address crude oil flow-assurance issues. Injection can be administered continuously, in batches, in injection wells, or at times in production wells. |
| Enhanced Oil Recovery, Enhanced Oil Recovery | mobility ratio | The mobility of an injectant divided by that of the fluid it is displacing, such as oil. The mobility of the oil is defined ahead of the displacement front while that of the injectant is defined behind the displacement front, so the respective effective permeability values are evaluated at different saturations. |
| Enhanced Oil Recovery, Enhanced Oil Recovery | secondary recovery | The second stage of hydrocarbon production during which an external fluid such as water or gas is injected into the reservoir through injection wells located in rock that has fluid communication with production wells. The purpose of secondary recovery is to maintain reservoir pressure and to displace hydrocarbons toward the wellbore. The most common secondary recovery techniques are gas injection and waterflooding. Normally, gas is injected into the gas cap and water is injected into the production zone to sweep oil from the reservoir. A pressure-maintenance program can begin during the primary recovery stage, but it is a form or enhanced recovery. The secondary recovery stage reaches its limit when the injected fluid (water or gas) is produced in considerable amounts from the production wells and the production is no longer economical. The successive use of primary recovery and secondary recovery in an oil reservoir produces about 15% to 40% of the original oil in place. |
| Enhanced Oil Recovery, Enhanced Oil Recovery | wettability change | A type of damage in which the formation wettability is modified, generating a change in relative permeability that eventually affects well productivity. Surfactants or other additives in drilling fluids, especially oil-base mud, or other injected fluids can change formation wettability. A naturally water-wet formation could be changed into an oil-wet formation with consequent production impairment caused by reduction of oil relative permeability. Wettability change is normally treated with mutual solvents to remove the rock-oil coating (asphaltene or paraffin precipitation), followed by a strong water-wet surfactant to reduce the tendency of further hydrocarbon precipitation. |
| Enhanced Oil Recovery, Formation Evaluation | wettability | The preference of a solid to contact one liquid or gas, known as the wetting phase, rather than another. The wetting phase will tend to spread on the solid surface and a porous solid will tend to imbibe the wetting phase, in both cases displacing the nonwetting phase. Rocks can be water-wet, oil-wet or intermediate-wet. The intermediate state between water-wet and oil-wet can be caused by a mixed-wet system, in which some surfaces or grains are water-wet and others are oil-wet, or a neutral-wet system, in which the surfaces are not strongly wet by either water or oil. Both water and oil wet most materials in preference to gas, but gas can wet sulfur, graphite and coal. Wettability affects relative permeability, electrical properties, nuclear magnetic resonance relaxation times and saturation profiles in the reservoir. The wetting state impacts waterflooding and aquifer encroachment into a reservoir. Reservoir wetting preference can be determined by measuring the contact angle of crude oil and formation water on silica or calcite crystals or by measuring the characteristics of core plugs in either an Amott imbibition test or a USBM test. |
| Enhanced Oil Recovery, Formation Evaluation, Geophysics | crosswell tomography | A technique for measuring a signal that is broadcast from a transmitter or source located in one well, to a receiver array placed in a neighboring well. This technique is used to create a display of formation properties such as acoustic velocity and attenuation, seismic reflectivity, or electromagnetic resistivity in the area between wells. The reservoir-scale data acquired with this technique can be used to bridge the gap between wellbore measurements and surface measurements. |
| Enhanced Oil Recovery, Formation Evaluation, Shale Gas | estimated ultimate recovery | The amount of oil and gas expected to be economically recovered from a reservoir or field by the end of its producing life. Estimated ultimate recovery can be referenced to a well, a field, or a basin. |
| Enhanced Oil Recovery, Geology | immiscible | Pertaining to a condition in which two fluids are incapable of forming molecularly distributed mixtures or attaining homogeneity at that scale. The fluids separate into two phases with an interface between them. For example, oil and water are immiscible. |
| Enhanced Oil Recovery, Geology | miscible | Pertaining to a condition in which two or more fluids can mix in all proportions and form a single homogeneous phase. |
| Enhanced Oil Recovery, Heavy Oil | API gravity | A specific gravity scale developed by the American Petroleum Institute (API) for measuring the relative density of various petroleum liquids, expressed in degrees. API gravity is gradated in degrees on a hydrometer instrument and was designed so that most values would fall between 10° and 70° API gravity. The arbitrary formula used to obtain this effect is: API gravity = (141.5/SG at 60°F) – 131.5, where SG is the specific gravity of the fluid. |
| Enhanced Oil Recovery, Heavy Oil | cold heavy oil production with sand | A non-thermal primary process for producing heavy oil, also called CHOPS. In this method, continuous production of sand improves the recovery of heavy oil from the reservoir. There is both a theoretical basis and physical evidence that, in many cases, wormholes are formed in the unconsolidated sand reservoir, thereby increasing oil productivity. In most cases, an artificial lift system is used to lift the oil with sand. |
| Enhanced Oil Recovery, Heavy Oil | dry forward combustion | A type of in situ combustion in which the burning front moves in the same direction as the injected air. As air is continuously supplied at the injection well, the fire ignited at this location moves toward the production wells. During forward combustion, the temperature behind the burning front is high, indicating a great amount of heat stored in the formation matrix. The injected gas heats on contact with the matrix and recovers only a small amount of the heat, with considerable losses to the surrounding formations. Another drawback of dry forward combustion is the presence of a highly viscous oil zone surrounding the production well. The fluid in this zone remains at the original reservoir temperature and its forward displacement by the heated oil is normally difficult. |
| Enhanced Oil Recovery, Heavy Oil | fire flooding | A method of thermal recovery in which a flame front is generated in the reservoir by igniting a fire at the sandface of an injection well. Continuous injection of air or other gas mixture with high oxygen content will maintain the flame front. As the fire burns, it moves through the reservoir toward production wells. Heat from the fire reduces oil viscosity and helps vaporize reservoir water to steam. The steam, hot water, combustion gas and a bank of distilled solvent all act to drive oil in front of the fire toward production wells. |
| Enhanced Oil Recovery, Heavy Oil | fireflooding | A method of thermal recovery in which a flame front is generated in the reservoir by igniting a fire at the sandface of an injection well. Continuous injection of air or other gas mixture with high oxygen content will maintain the flame front. As the fire burns, it moves through the reservoir toward production wells. Heat from the fire reduces oil viscosity and helps vaporize reservoir water to steam. The steam, hot water, combustion gas and a bank of distilled solvent all act to drive oil in front of the fire toward production wells. |
| Enhanced Oil Recovery, Heavy Oil | first-contact miscibility | A condition of two fluids that are miscible that is, they form a single phase when mixed in any proportion when first brought into contact at a given pressure and temperature. In reservoir gasflooding, the injected gas composition, oil composition, temperature and the injection pressure determine the condition of first-contact miscibility. In contrast, fluids that develop miscibility after exchanging components have multiple-contact miscibility. |
| Enhanced Oil Recovery, Heavy Oil | five spot | An injection pattern in which four input or injection wells are located at the corners of a square and the production well sits in the center. The injection fluid, which is normally water, steam or gas, is injected simultaneously through the four injection wells to displace the oil toward the central production well. |
| Enhanced Oil Recovery, Heavy Oil | five-spot | An injection pattern in which four input or injection wells are located at the corners of a square and the production well sits in the center. The injection fluid, which is normally water, steam or gas, is injected simultaneously through the four injection wells to displace the oil toward the central production well. |
| Enhanced Oil Recovery, Heavy Oil | huff and puff | Slang term for a cyclic process in which a well is injected with a recovery enhancement fluid and, after a soak period, the well is put back on production. Examples are cyclic steam injection and cyclic CO2 injection. |
| Enhanced Oil Recovery, Heavy Oil | in-situ combustion | A method of thermal recovery in which fire is generated inside the reservoir by injecting a gas containing oxygen, such as air. A special heater in the well ignites the oil in the reservoir and starts a fire. The heat generated by burning the heavy hydrocarbons in place produces hydrocarbon cracking, vaporization of light hydrocarbons and reservoir water in addition to the deposition of heavier hydrocarbons known as coke. As the fire moves, the burning front pushes ahead a mixture of hot combustion gases, steam and hot water, which in turn reduces oil viscosity and displaces oil toward production wells. Additionally, the light hydrocarbons and the steam move ahead of the burning front, condensing into liquids, which adds the advantages of miscible displacement and hot waterflooding. In situ combustion is also known as fire flooding or fireflood. |
| Enhanced Oil Recovery, Heavy Oil | inverted five spot | An injection pattern in which four production wells are located at the corners of a square and the injector well sits in the center. |
| Enhanced Oil Recovery, Heavy Oil | reverse combustion | A type of in-situ combustion in which the burning front moves in an opposite direction to the injected air. Initially, air is injected into a production well and the fire is ignited. After the burning front has advanced some distance from the production well, air is supplied only near the injection well. The burning front advances toward the injection well while the oil moves toward the production well. Reverse combustion actually refers to dry reverse combustion and can be used to recover extremely viscous oil or tar. In reverse combustion, the liquid blocking problem is solved because a hot zone is maintained near the production well. Despite this advantage, this process is not as efficient as dry forward combustion because lighter fractions of the oil are burned and heavier fractions are left behind the burning front. Another drawback is the possibility of a spontaneous ignition in the injector well, which will divert air for combustion near the injector well instead of near the producer. |
| Enhanced Oil Recovery, Heavy Oil | steam soak | Another term for soak phase, in cyclic steam injection, the second phase between the steam-injection phase and the production phase. During the soak phase, the well is shut in for several days to allow uniform heat distribution to thin the oil. |
| Enhanced Oil Recovery, Heavy Oil | toe to heel air injection | An in-situ combustion method for producing heavy oil. In this technique, the fireflooding starts from a vertical well, while the oil is produced from a horizontal well having its toe in close proximity to the vertical air-injection well. This production method is a modification of conventional fire flooding techniques in which the flame front from a vertical well pushes the oil to be produced from another vertical well. |
| Enhanced Oil Recovery, Heavy Oil | WAG | An enhanced oil recovery process whereby water injection and gas injection are carried out alternately for periods of time to provide better sweep efficiency and reduce gas channeling from injector to producer. This process is used mostly in CO2 floods to improve hydrocarbon contact time and sweep efficiency of the CO2. |
| Enhanced Oil Recovery, Heavy Oil | water alternating gas | An enhanced oil recovery process whereby water injection and gas injection are carried out alternately for periods of time to provide better sweep efficiency and reduce gas channeling from injector to producer. This process is used mostly in CO2 floods to improve hydrocarbon contact time and sweep efficiency of the CO2. |
| Enhanced Oil Recovery, Heavy Oil | wet combustion | An in situ combustion technique in which water is injected simultaneously or alternately with air into a formation. Wet combustion actually refers to wet forward combustion and was developed to use the great amount of heat that would otherwise be lost in the formation. The injected water recovers the heat from behind the burning front and transfers it to the oil bank ahead. Because of this additional energy, the oil displacement is more efficient and requires less air. In spite of these advantages, a wet combustion process cannot avoid liquid-blocking problems and use of wet combustion is limited by the oil viscosity. Wet combustion is also called in situ steam generation or a combination of forward combustion and waterflooding, which is abbreviated as COFCAW. |
| Enhanced Oil Recovery, Heavy Oil | steam oil ratio | Parameter used to monitor the efficiency of oil production processes based on steam injection. Commonly abbreviated as SOR, it measures the volume of steam required to produce one unit volume of oil. Typical values of SOR for cyclic steam stimulation are in the range of three to eight, while typical SOR values for steam assisted gravity drainage are in the range of two to five. The lower the SOR, the more efficiently the steam is utilized and the lower the associated fuel costs. |
| Enhanced Oil Recovery, Heavy Oil | thermal simulation | The finite-difference or finite-element reservoir simulation that includes energy equations and calculations used to describe heat conduction, heat and fluid convection, and latent heat exchanges occurring in the reservoir rock and fluids during a thermal recovery process such as steamflooding, steam assisted gravity drainage, or in-situ combustion. Combustion thermal simulation also requires equations for modeling combustion reaction kinetics. |
| Enhanced Oil Recovery, Production Testing | primary production | Also known as primary recovery, the first stage of hydrocarbon production, in which natural reservoir energy, such as gasdrive, waterdrive or gravity drainage, displaces hydrocarbons from the reservoir, into the wellbore and up to surface. Initially, the reservoir pressure is considerably higher than the bottomhole pressure inside the wellbore. This high natural differential pressure drives hydrocarbons toward the well and up to surface. However, as the reservoir pressure declines because of production, so does the differential pressure. To reduce the bottomhole pressure or increase the differential pressure to increase hydrocarbon production, it is necessary to implement an artificial lift system, such as a rod pump, an electrical submersible pump or a gas-lift installation. Production using artificial lift is considered primary recovery. The primary recovery stage reaches its limit either when the reservoir pressure is so low that the production rates are not economical, or when the proportions of gas or water in the production stream are too high. During primary recovery, only a small percentage of the initial hydrocarbons in place are produced, typically around 10% for oil reservoirs. |
| Enhanced Oil Recovery, Production Testing | reservoir-drive mechanisms | Natural forces in the reservoir that displace hydrocarbons out of the reservoir into the wellbore and up to surface. Reservoir-drive mechanisms include gasdrive (gas cap or solution gasdrive), waterdrive (bottomwater drive or edgewater drive), combination drive, and gravity drainage. Waterdrive is the most efficient drive mechanism, followed by gasdrive and gravity drainage. Reservoir-drive mechanisms are also called natural drives. |
| Enhanced Oil Recovery, Well Completions | water wet | Pertaining to the adhesion of a liquid to the surface of a solid. In water-wet conditions, a thin film of water coats the surface of the formation matrix, a condition that is desirable for efficient oil transport. Treatments that change the wettability of the formation from water-wet to oil-wet can significantly impair productivity. |
| Enhanced Oil Recovery, Well Completions | waterflooding | A method of secondary recovery in which water is injected into the reservoir formation to displace residual oil. The water from injection wells physically sweeps the displaced oil to adjacent production wells. Potential problems associated with waterflood techniques include inefficient recovery due to variable permeability, or similar conditions affecting fluid transport within the reservoir, and early water breakthrough that may cause production and surface processing problems. |
| Enhanced Oil Recovery, Well Completions, Well Workover and Intervention, Shale Gas | stimulation | A treatment performed to restore or enhance the productivity of a well. Stimulation treatments fall into two main groups, hydraulic fracturing treatments and matrix treatments. Fracturing treatments are performed above the fracture pressure of the reservoir formation and create a highly conductive flow path between the reservoir and the wellbore. Matrix treatments are performed below the reservoir fracture pressure and generally are designed to restore the natural permeability of the reservoir following damage to the near-wellbore area. Stimulation in shale gas reservoirs typically takes the form of hydraulic fracturing treatments. |
| Enhanced Oil Recovery, Well Testing | bubblepoint | The pressure and temperature conditions at which the first bubble of gas comes out of solution in oil. At discovery, all petroleum reservoir oils contain some natural gas in solution. Often the oil is saturated with gas when discovered, meaning that the oil is holding all the gas it can at the reservoir temperature and pressure, and that it is at its bubblepoint. Occasionally, the oil will be undersaturated. In this case, as the pressure is lowered, the pressure at which the first gas begins to evolve from the oil is defined as the bubblepoint. |
| Enhanced Oil Recovery, Well Testing | dewpoint | The pressure at which the first condensate liquid comes out of solution in a gas condensate. Many gas condensate reservoirs are saturated at initial conditions, meaning that the dewpoint is equal to the initial reservoir pressure. Condensate dissolution is called retrograde condensation because this is counter to the behavior of pure substances, which vaporize when the pressure drops below the saturation pressure under isothermal (constant temperature) conditions. |
| Enhanced Oil Recovery, Well Testing | equation of state | An equation that specifies fluid density as a function of pressure and temperature. A large body of scientific literature describes these functions for all kinds of hydrocarbons, and also for complex mixes of various hydrocarbons with other hydrocarbons and with other fluids. Once the components of a reservoir fluid are determined, the known hydrocarbon properties become a valuable tool in making further calculations of well-test results and predictions of future well and reservoir behavior. |
| Enhanced Oil Recovery, Well Workover and Intervention, Well Completions, Shale Gas | well stimulation | A treatment performed to restore or enhance the productivity of a well. Stimulation treatments fall into two main groups, hydraulic fracturing treatments and matrix treatments. Fracturing treatments are performed above the fracture pressure of the reservoir formation and create a highly conductive flow path between the reservoir and the wellbore. Matrix treatments are performed below the reservoir fracture pressure and generally are designed to restore the natural permeability of the reservoir following damage to the near-wellbore area. Stimulation in shale gas reservoirs typically takes the form of hydraulic fracturing treatments. |
| Enhanced Oil Recovery | sour corrosion | The corrosion caused by contact with hydrogen sulfide [H2S] dissolved in water. Sour corrosion takes the form of sulfide stress cracking or hydrogen embrittlement. |
| Formation Evaluation | a | The value a in the relation of formation factor (F) to porosity (phi): F = a / phim. The value a is derived empirically from best fits of measured values of F and phi on a group of rock samples. It has no clear physical significance, although it has been related to grain shape and tortuosity. In the saturation equation, it always occurs associated with the water resistivity as (a * Rw). It is sometimes claimed that a must be 1 since at phi = 1, F must be 1. However, a material with phi = 1 is not a rock: a is essentially an empirical factor for rocks and as such can take any value. A wide range of values has been found, from 0.5 to 5. |
| Formation Evaluation | accelerator source | A device for producing high-energy neutrons by using a charged particle accelerator. Neutron generators are used in various pulsed neutron devices and some neutron porosity measurements. In a typical device, deuterium (2D) and tritium (3T) ions are accelerated towards a target also containing the same isotopes. When 2D and 3T collide, they react to produce a neutron with an energy of about 14.1 MeV. The first neutron generators were built in the late 1950s and soon led to the first pulsed neutron capture log. |
| Formation Evaluation | accuracy | The closeness of the agreement between the result of the measurement and the conventional true value of the quantity. Accuracy should not be confused with precision. (ISO) Core measurements have well-defined calibration techniques and standards. Logging measurements are characterized during tool design and construction, and calibrated regularly to some standard. The quoted accuracy of a log then depends on the initial characterization, the reproducibility of the standard, and the stability of the measurement between calibrations and under downhole conditions. The actual accuracy also depends on the equipment performing and being operated to specification. |
| Formation Evaluation | acid effect | The change in a pulsed neutron capture measurement produced by acidizing a carbonate formation. Acidizing tends to increase the porosity as well as leave chlorides in the formation, thereby increasing the capture cross section. Both of these results affect the formation thermal decay time and must be taken into account in the interpretation. |
| Formation Evaluation | acoustic log | A record of some acoustic property of the formation or borehole. The term is sometimes used to refer specifically to the sonic log, in the sense of the formation compressional slowness. However, it may also refer to any other sonic measurement, for example shear, flexural and Stoneley slownesses or amplitudes, or to ultrasonic measurements such as the borehole televiewer and other pulse-echo devices, and even to noise logs. |
| Formation Evaluation | acoustic mode | A situation in which acoustic energy that propagates in one direction is confined in the other two directions as, for example, a mode confined to an interface between two different materials or within the borehole. The Stoneley wave, tube wave and flexural mode have important applications in formation evaluation, while most of the others, such as the Rayleigh wave and the various guided borehole modes (normal mode, leaky mode and hybrid mode), are considered interference that must be filtered out. In y slow formations, leaky modes can help determine formation compressional slowness. |
| Formation Evaluation | acoustic transducer | A device for transforming electrical energy into sound, or vice versa. In sonic logging applications, acoustic transducers are usually made of piezoelectric ceramic or magnetostrictive materials, and may be used as either receivers or transmitters in a frequency range between about 1 and 30 kHz. The transducers are excited as either monopoles, emitting or receiving sound in all directions, or dipoles, emitting or receiving in one plane. In ultrasonic logging applications, acoustic transducers are made of piezoelectric ceramic materials, and often are used in alternating transmitter/receiver (pulse-echo) mode, in a frequency range from a few hundred kilohertz to a few megahertz. |
| Formation Evaluation | acquisition log | The log that is actually recorded while taking the measurements. It is distinct from a playback, which is produced later on from digital data. |
| Formation Evaluation | activation log | A log of elemental concentrations derived from the characteristic energy levels of gamma rays emitted by a nucleus that has been activated by neutron bombardment. The carbon-oxygen log, elemental capture spectroscopy log, pulsed neutron spectroscopy log, aluminum activation log and oxygen activation log are all examples of activation logs. However, the term is most commonly used to refer to the aluminum and oxygen activation logs, the latter also being known as a water-flow log. |
| Formation Evaluation | adjacent bed | A formation layer above or below the layer being measured by a logging tool. The term “surrounding bed” is used in particular to describe the adjacent layers above or below a horizontal well. In a vertical well, the term “shoulder bed” is more common, and is used in particular in resistivity logging to describe the layers above and below a reservoir. The term “adjacent bed” is used in both cases. |
| Formation Evaluation | Alford rotation | A processing technique to project formation shear data recorded in any two orthogonal directions into the fast and slow shear directions in the presence of shear-wave anisotropy. In the sonic logging application, a dipole transmitter excites a flexural mode that is recorded at one set of receivers that is in-line with the dipole and other receivers that are 90o out of line (the cross-dipole component). A similar recording is made of the wave from a second dipole transmitter, mounted orthogonally to the first. The flexural-wave velocity is closely related to the formation shear velocity, particularly at low frequencies and in hard formations. Using all four waveforms, the Alford rotation is used to determine the speed and direction of the fast and the slow shear wave. Reference: Alford RM: “Shear Data in the Presence of Azimuthal Anisotropy: Dilley, Texas,” Expanded Abstracts, 56th SEG Annual International Meeting and Exposition, Houston, Texas, USA, November 2-6, 1986, Paper S9.6 |
| Formation Evaluation | alpha processing | A technique for combining a measurement that has a high accuracy but low precision with another measurement of the same quantity that has a high precision but low accuracy in order to produce a result that is better than either alone. Alpha processing is used to improve the vertical resolution of neutron porosity and other dual-detector nuclear logs. The detector near the source has better precision than the far detector in the sense that it responds more precisely to vertical changes. However, the near detector is less accurate because it is more affected by the borehole environment. Alpha processing mathematically superimposes the rapid changes of the near detector on the slowly changing but accurate far detector to produce an accurate log with high vertical resolution. The technique is also used to improve results from the carbon-oxygen log and other pulsed neutron spectroscopy measurements. Two methods are used to determine the carbon/oxygen ratio. The windows method counts the number of gamma rays within energy windows placed at the main peaks for carbon and oxygen. This method has good statistical precision but poor accuracy, as gamma rays from other elements contaminate these windows. The other method, spectral stripping, compares the total spectrum against standards for many elements, inverting the spectrum to obtain the yield for each element. This method is more accurate but has less statistical precision. Averaging over a number of measurements, alpha processing adjusts the windows result with the more accurate spectral stripping in order to obtain a precise and accurate result. |
| Formation Evaluation | altered zone | A near-wellbore formation zone, a few inches thick, whose acoustic velocity has been affected by impregnation with drilling fluids, stress relief, or both. The acoustic velocity of the rock in the immediate vicinity of the borehole wall can be much slower than that in the virgin formation. To measure the formation velocity, it may be necessary to use a sonic logging tool that has a greater spacing between transmitter and receiver array (about 10 to 15 ft [3 to 4.5 m]) than the standard sonic tool (about 3 to 5 ft [0.9 to 1.5 m]). The altered zone may also give rise to different acoustic modes, for example the hybrid mode or a second Stoneley wave. |
| Formation Evaluation | aluminum activation log | A wireline log of the concentration by weight of aluminum in the formation, based on the principle of neutron activation. Aluminum (27Al) can be activated by capturing relatively low-energy neutrons from a chemical source to produce the isotope 28Al, which decays with a half-life of 2.3 minutes and emits a relatively easily detected 1.78 MeV gamma ray. A natural gamma ray spectrometer will detect this gamma ray along with the other natural gamma rays. If the natural gamma spectrum has been measured before activation, it can be subtracted from the spectrum after activation to give an estimate of Al content. Al is a relatively direct indicator of the volume of clay, since clay minerals are alumino-silicates. |
| Formation Evaluation | antisqueeze | The effect on a laterolog whereby the current lines are no longer properly focused but spread out at a certain distance into the formation. The effect occurs opposite a high-resistivity bed with low-resistivity shoulders. The result is that laterolog devices, in particular deep devices, tend to read too low and have less depth of investigation. Shoulder bed correction charts correct for these effects in certain well-defined situations, such as no invasion in horizontal beds with vertical wells. |
| Formation Evaluation | API unit | The unit of radioactivity used for natural gamma ray logs. This unit is based on an artificially radioactive concrete block at the University of Houston, Texas, USA, that is defined to have a radioactivity of 200 American Petroleum Institute (API) units. This was chosen because it was considered to be twice the radioactivity of a typical shale. The formation is the primary standard for calibrating gamma ray logs. However, even when properly calibrated, different gamma ray tools will not necessarily have identical readings downhole because their detectors can have different spectral sensitivities. They will read the same only if the downhole formation contains the same proportions of thorium, potassium and uranium as the Houston standard. For example, logging while drilling (LWD) tools have thicker housings than wireline tools, causing a different spectral response to the three sources of radioactivity, and therefore a different total gamma ray reading in some formations. The nuclear well log calibration facility at the University of Houston, known as the API pits, was opened in 1959 for the calibration of natural gamma ray and neutron logs. A facility for calibrating natural gamma ray spectroscopy logs was added later. |
| Formation Evaluation | apparent matrix | A calculation of the properties of the solid fraction of a rock from the combination of two logs. For example, by combining the density and neutron porosity measurements, it is possible to compute an apparent matrix density; by combining neutron porosity and sonic measurement, it is possible to compute an apparent matrix traveltime. The computations assume a particular fluid, usually fresh water, and particular response equations. The results are often displayed as quicklook logs for lithology identification. The word matrix is used here in the formation evaluation sense of the term rather than the geological one. |
| Formation Evaluation | Archie equation | A particular relation proposed by G.E. Archie between the formation factor (F) and porosity (phi), in which F = 1 / phim, where the porosity exponent, m, is a constant for a particular formation or type of rock. In the original work, Archie proposed that m lay between 1.8 and 2.0 for consolidated sandstones, and close to 1.3 for loosely consolidated sandstones. m was named the cementation exponent shortly afterwards. This relation is also known as the Archie II equation. |
| Formation Evaluation | Archie rock | A rock whose petrophysical properties are well described by the Archie equation with constant values for the porosity exponent and the saturation exponent. Such rocks typically have very little clay, a regular pore structure and high-salinity water. The term often is used to describe a rock that is petrophysically simple. |
| Formation Evaluation | armor | The metal strands on the outside of a wireline logging cable. Typical cables have two layers of metal strands, one wound clockwise and the other counterclockwise. The armor gives the cable its strength. It is used as the current return in some electrical measurements. |
| Formation Evaluation | array induction | An induction tool or log that consists of several mutually balanced arrays whose signals are recorded separately and combined in software to produce the response desired. Typically, there is one transmitter and five to ten pairs of receivers and bucking coils that are balanced to remove direct coupling. The signals are combined in a wide variety of ways to produce the responses desired, as for example, deep-reading, high vertical resolution or some combination of both. There are trade-offs in any response. For example, a deep-reading log typically will not have high vertical resolution. If it does, it will be more sensitive to the invasion condition and cave effect. |
| Formation Evaluation | array laterolog | An electrode device with multiple current electrodes configured in several different ways to produce several different responses. A typical array consists of a central electrode emitting survey current, with multiple guard electrodes above and below it. Current is sent between different guard electrodes to achieve greater or less focusing. The greater the focusing, the greater the depth of investigation. About five basic measurements are obtained in this way. This hardware focusing may be further improved by software focusing, in which the signals from the basic measurements are superimposed mathematically to ensure proper focusing in a wide range of conditions. |
| Formation Evaluation | array propagation resistivity | A resistivity recorded by a measurements-while-drilling propagation tool consisting of an array of transmitters and receivers whose signals are recorded separately and combined by software to produce the response desired. In a typical design, five transmitters emit a signal, and the phase shift and attenuation between two receivers are recorded. The phase shifts and attenuations are combined in different ways to produce borehole-compensated logs with different depths of investigation and radial resolution. |
| Formation Evaluation | array sonic | A type of acoustic logging tool that uses a large number of receivers, typically 4 to 12. Modern acoustic logging tools are designed to measure not only the compressional wave but also the shear and other acoustic waves generated by the transmitter. The separation and identification of these waves are facilitated by the use of an array of receivers placed about 6 in. [15 cm] apart, which is close enough to avoid aliasing but far enough to sample a significant moveout in the wave. The waveforms at each receiver are recorded and processed by signal processing techniques, such as slowness-time coherence, to measure the velocities of the different waves. |
| Formation Evaluation | attenuation | The reduction in amplitude of an electromagnetic wave passing through the formation, usually measured in decibels/meter, dB/m. The term is used in particular with reference to the propagation resistivity log and the electromagnetic propagation log. |
| Formation Evaluation | attenuation resistivity | The ability of a formation to resist electrical conduction, as derived from the reduction in amplitude of the electromagnetic wave generated in a propagation resistivity measurement. At the frequencies used and within the range of measurement, the attenuation depends almost solely on the resistivity, so that the former can be transformed to the latter with a simple algorithm. The transform also depends on transmitter/receiver spacings and tool design. For a 2-MHz measurement, a typical measurement range is 0.2 to 50 ohm-m. Above 50 ohm-m, the dependence of attenuation on resistivity is too small to measure accurately. |
| Formation Evaluation | azimuthal | Pertaining to being focused in one direction. An azimuthal, or azimuthally focused, measurement has one or more directions perpendicular to the surface of a logging tool from which it receives most of its signal. Examples are the density, laterolog and microresistivity logs. A nonazimuthal, or azimuthally symmetric measurement is one which measures equally in all directions around the tool. Examples are the induction, propagation resistivity log and gamma ray. |
| Formation Evaluation | azimuthal density | A type of logging while drilling density log in which the density is measured at different azimuths around the drill collar. The density measurement is focused, so that when the collar rotates, the measurement sees different azimuths around the borehole. An average density can be calculated by summing all the azimuthal data. Alternatively, the data can be summed over different segments, for example in four quadrants, to give an azimuthal density in four directions. When the hole is overgauge, certain quadrants will be firmly pressed against the borehole wall, while others may have a significant standoff and too high a delta rho. The good quadrants can then be chosen for formation evaluation. |
| Formation Evaluation | azimuthal laterolog | A type of electrode device that is able to measure resistivity in different directions around the sonde. In most laterologs, the electrodes are cylinders that average the resistivity azimuthally around the sonde. In azimuthal laterologs, the electrode is segmented radially in several portions, each of which responds to the resistivity in the direction it is facing. |
| Formation Evaluation | azimuthal resolution | An angle that characterizes the ability of an azimuthal logging measurement to resolve changes in different directions around the tool; alternatively, the smallest angle for which a significant change can be detected. |
| Formation Evaluation | backup curve | An extra curve on a log, designed to appear when the standard curve goes off track. For example, if the standard gamma ray curve is presented on a scale of 0 to 200 gAPI units, the backup curve may be scaled from 200 to 400 gAPI units. Alternatively, if the standard resistivity log is presented on a scale of 0 to 50 ohm-m, the backup curve might be presented on a scale of 0 to 500 ohm-m in the same track but be blanked off for readings below 50 ohm-m. The backup curve usually has the same coding as the standard curve but a different line weight. |
| Formation Evaluation | bad hole | A borehole that is not to gauge or is rugose. The term usually refers to the detrimental effect that such a borehole has on the response of logging measurements, in particular pad tools like the density or micro-resistivity. The existence of bad hole is usually determined by a caliper log and on various secondary measurements such as delta rho. |
| Formation Evaluation | balanced array | An array induction whose transmitters and receivers are arranged to produce a null reading in free space, that is, which has the mutual signal balanced to zero. |
| Formation Evaluation | base exchange | The quantity of positively charged ions (cations) that a clay mineral or similar material can accommodate on its negatively charged surface, expressed as milli-ion equivalent per 100 g, or more commonly as milliequivalent (meq) per 100 g. Clays are aluminosilicates in which some of the aluminum and silicon ions have been replaced by elements with different valence, or charge. For example, aluminum (Al+++) may be replaced by iron (Fe++) or magnesium (Mg++), leading to a net negative charge. This charge attracts cations when the clay is immersed in an electrolyte such as salty water and causes an electrical double layer. The cation-exchange capacity (CEC) is often expressed in terms of its contribution per unit pore volume, Qv. In formation evaluation, it is the contribution of cation-exchange sites to the formation electrical properties that is important. Various techniques are used to measure CEC in the laboratory, such as wet chemistry, multiple salinity and membrane potential. Wet chemistry methods, such as conductometric titration, usually involve destruction or alteration of the rock. Although quicker and simpler to perform, they are less representative of electrical properties in situ. The multiple salinity and membrane potential methods are more direct measurements of the effect of CEC on formation resistivity and spontaneous potential. |
| Formation Evaluation | base log | The log used as the reference for depths in the well. Each log may record formation features at slightly different depths due to their different response and the difficulty of aligning depths. It is therefore important to select one log to which the other logs are depth matched, and which is used as the reference for well-to-well correlation and mechanical operations such as perforating. The gamma ray is most often used as the base log since it can be recorded in both open and cased holes. |
| Formation Evaluation | BHT | The temperature in the borehole at total depth at the time it is measured. In log interpretation, the bottom hole temperature (BHT) is taken as the maximum recorded temperature during a logging run, or preferably the last of series of runs during the same operation. BHT is the temperature used for the interpretation of logs at total depth. Farther up the hole, the correct temperature is calculated by assuming a certain temperature gradient. The BHT lies between the bottomhole circulating temperature (BHCT) and the bottomhole static temperature (BHST). |
| Formation Evaluation | bimetallic corrosion | The electromagnetic force created by two different metals in contact with each other. If two such metals are in contact in a logging tool, and also communicate along a conductive borehole, then a potential drop is generated in the borehole. This potential drop will appear on the spontaneous potential (SP) log, where it can be confused with the electrochemical potential. Since the magnitude of the drop depends on the formation resistivity, the effect of bimetallism is often seen as a resistivity log superimposed on the normal SP. Under usual circumstances, the effect of bimetallism on the SP is small, and care is taken to avoid it. |
| Formation Evaluation | Biot theory | A theory for acoustic propagation in a porous and elastic medium developed by M.A. Biot. Compressional and shear velocities can be calculated by standard elastic theory from the composite density, shear and bulk modulus of the total rock. The problem is how to determine these from the properties of the constituent parts. Biot showed that the composite properties could be determined from the porosity and the elastic properties (density and moduli) of the fluid, the solid material, and the empty rock skeleton, or framework. To account for different frequencies of propagation, it is also necessary to know the frequency, the permeability of the rock, the viscosity of the fluid and a coefficient for the inertial drag between skeleton and fluid. Unlike the Gassmann model, the Biot theory takes into account frequency variations, and allows for relative motion between fluid and rock framework. As a result, it predicts some of the observed changes in velocity with frequency, as well as the critical frequency below which the Gassmann model is valid. It also predicts the existence of a so-called slow wave in addition to the shear wave and the compressional, or fast wave. The slow wave arises when the fluid and the skeleton move 1800 out-of-phase with each other. Its velocity is related to fluid mobility, but unfortunately has been observed only in the laboratory, not on logs. At logging frequencies, it degenerates into a diffusion phenomenon rather than a wave, and is apparently too highly attenuated to be observed in real rocks. However, in permeable formations, the Stoneley wave couples into the slow wave, causing the attenuation and dispersion that allow the measurement of Stoneley permeability. The full Biot theory is used mainly to analyze laboratory data. For practical log interpretation, it is more common to use the simpler Gassmann model. |
| Formation Evaluation | bit resistivity | The resistivity measured at the drill bit by a measurements-while-drilling (MWD) tool. The bit resistivity measurement responds to resistivity changes as the bit penetrates the formation, or when the time after bit is zero. It is thus an early indication of formation change. The measurement is similar to a wireline electrode device except that toroids are used instead of electrodes. A transmitter toroid induces a low-frequency current in the drillstring, which flows out of the bit and returns farther up the string. The magnitude of the current depends on the resistivity near the bit, and is measured by another toroid. The vertical resolution and depth of investigation depend on the distances between the toroids and the bit, which, in turn, depend on the type of bottomhole assembly (BHA) used. The depth of investigation is sufficient that the effect of the borehole is normally small. The measurement is unfocused and usually not borehole-corrected. Since both bit and drillstring are in physical contact with the formation, it is possible to make the measurement in oil-base muds. |
| Formation Evaluation | borehole compensation | An upgoing and downgoing arrangement of transducers in a logging tool, largely to offset spurious changes in reading caused by variations in borehole size or sonde tilt. The technique is used for measurements that rely on the propagation of a wave, such as sonic, propagation resistivity and electromagnetic propagation measurement. Propagation logs rely on measuring the difference in properties of a wave at two receivers. The borehole influences this difference if the tool is tilted or if there is a cave opposite one of the receivers. The effect can be compensated for by using two transmitters that radiate sequentially in opposite directions. In ideal conditions, the effect of a tilt or a cave is exactly opposite for the two transmitters, so that an average gives the correct result. Borehole compensation is different from borehole correction. |
| Formation Evaluation | borehole correction | The amount by which a log measurement must be adjusted in order to remove the contribution of the borehole. Although most log measurements are designed to pick up a minimum of signal from the borehole, some contribution usually remains. This contribution may be removed by software or by manual entry into correction charts. In resistivity logging, the correction replaces the borehole with a resistivity equal to that of the formation. In nuclear logging, the correction adjusts the reading to that which would be found in a standard condition, such as an 8-in. [20-cm] borehole filled with fresh water. |
| Formation Evaluation | borehole gravity | Pertaining to the detection of the Earth’s gravitational field within a wellbore. Subtle vertical variations of the Earth’s gravity field may be detected over the length of a borehole. These depend on the variations in the formation density not only above and below the sensor, but also laterally away from the borehole. Thus, borehole gravity measurements may be used to detect the following phenomena: – overburden pressure – lateral formation density changes arising from porosity changes away from the borehole (fracture fields, vugs) – lateral proximity to lithology changes, such as major faulting or salt intrusions – time-lapse density measurements to monitor fluid saturation changes during the life of a reservoir. In borehole gravity measurements, highly accurate formation density measurements, averaged over a large volume, may be made by comparing changes of gravity between measurement stations. |
| Formation Evaluation | borehole gravity meter | A logging instrument capable of making relative gravity measurements at stations along the borehole with a sensitivity and repeatability in the microGal range (about 1 part in 10-9 of the Earth’s gravity field) The only commercial measurement device capable of this precision is the LaCoste & Romberg borehole gravimeter, although several research projects have been proposed to replace this classic technology. |
| Formation Evaluation | borehole televiewer | An ultrasonic logging device with a radially mounted rotating transducer that is used to scan the borehole wall. The transducer (in transmit mode) emits a high-frequency pulse that is reflected by the borehole wall back to the transducer (in receive mode). In openhole applications, it can be used to measure the borehole diameter (by measuring the acoustic transit time between transducer and borehole wall) and the amplitude of acoustic signals reflected by the borehole wall. The transducer is rotated to produce a cross section of the borehole size and images of the borehole wall. These are used to identify fractures, breakouts and other borehole features. In cased hole, they are used to identify internal corrosion. |
| Formation Evaluation | Born method | A method of analyzing the response of an induction logging tool that considers the contribution of each element of the formation as a perturbation from the average background conductivity. The development of the solution is similar to the Born approximation in quantum mechanics, since the latter also involves a single scattering. The Born response is valid for modest formation contrasts. The zero-conductivity Born response is identical to the geometrical factor. |
| Formation Evaluation | bottom log interval | The bottom of the interval recorded on the log, or the deepest point at which the log readings are valid. At the bottom of the well, each log will have a valid first reading at a different depth. The bottom log interval is then either the lowermost first reading or the first reading of the most important log. |
| Formation Evaluation | bottomhole temperature | The temperature in the borehole at total depth at the time it is measured. In log interpretation, the bottom hole temperature (BHT) is taken as the maximum recorded temperature during a logging run, or preferably the last of series of runs during the same operation. BHT is the temperature used for the interpretation of logs at total depth. Farther up the hole, the correct temperature is calculated by assuming a certain temperature gradient. The BHT lies between the bottomhole circulating temperature (BHCT) and the bottomhole static temperature (BHST). |
| Formation Evaluation | bound fluid | Fluid in the pore space that does not flow under normal reservoir conditions. This fluid may include water, oil or gas, but most often refers just to bound water. Bound fluid does not flow on primary or secondary production, injection or invasion unless the rock wettability is altered. When used in connection with a nuclear magnetic resonance measurement, the term refers to the signal that occurs below a certain cutoff, typically 33 ms in sandstones and 100 ms in carbonates. The source of this signal is bound water, but may also include oil with a viscosity above about 60 cp in sandstones or 30 cp in carbonates. Note that, contrary to the sense of “bound,” this oil may or may not be moveable under normal reservoir conditions. |
| Formation Evaluation | bound water | Water in the pore space that does not flow under normal reservoir conditions. Bound water does not flow on primary or secondary production, injection or invasion unless the rock wettability is altered. When used in connection with a nuclear magnetic resonance measurement, the term refers to all the water that is not free to move. This includes capillary-bound water and clay-bound water. However, water in mineral hydrates is not included as it relaxes too fast to be measured by nuclear magnetic resonance (NMR). In practice, bound water is defined as the water signal below a certain cutoff, typically 33 ms in sandstones and 100 ms in carbonates. When used in connection with the dual water model, the term refers to the clay-bound water only. In the Hill-Shirley-Klein model, the term is known as the hydration water. |
| Formation Evaluation | bound-fluid log | A type of nuclear magnetic resonance (NMR) log that is designed to record properly only the bound fluid. Bound fluid is characterized by a fast relaxation time, typically less than 33 ms in sandstones and 100 ms in carbonate rocks. Therefore, the wait time for a bound fluid log can be much shorter than for standard NMR logs, with the result that logging speeds are much faster |
| Formation Evaluation | Boyle’s Law | A principle of physics stating that the product of pressure and volume divided by the temperature is a constant for an ideal gas. It is a good approximation for many real gases, such as helium, over reasonable ranges of temperature and pressure. |
| Formation Evaluation | Boyle’s Law Double Cell | A technique for measuring the grain volume of a core sample by observing the change in pressure of helium introduced into a chamber containing the sample. The rock sample is placed in a chamber of known volume. Helium is held in a reference chamber at known volume and pressure, typically 100 to 200 psi [689 to 1379 kPa]. The two chambers are connected, causing the helium to drop in pressure as it fills the sample chamber and the pores in the sample. The only volume not filled is the grain volume and the isolated pores. Neglecting the latter, the grain volume can then be calculated from Boyle’s Law using the pressure before and after connecting the chambers and the chamber volumes. |
| Formation Evaluation | Boyle’s Law Single Cell | A technique for measuring the pore volume of a core sample by observing the change in pressure of helium introduced into the pore space. The rock sample is held in a core holder whose internal walls are elastomers, so that the only void space is the internal pore volume. With a suitable holder, the sample can be held under a confining stress. Helium is held in a reference cell at known volume and pressure, typically 100 to 200 psi [689 to 1379 kPa]. The helium is introduced to the core sample, dropping in pressure as it fills the connected pore space. The effective pore volume is obtained from Boyle’s Law using the pressure before and after introduction of helium, and the reference volume. |
| Formation Evaluation | bridle | A special section of cable that is placed between the logging cable and the head of the logging tool. Unlike the logging cable, the steel load-bearing element is in the center, surrounded by the conductors that are held in an insulating jacket. The bridle is needed for most conventional electrical logs and laterologs in which the cable armor is used as a current return. To be effective, this return must be at some distance from the logging tool and insulated from it. Typical bridles are 80 ft [24 m] long. Electrodes may be wound on the outside of the bridle and connected to the logging tool for use as measurement references or for spontaneous potential measurements. |
| Formation Evaluation | broadside array | A particular arrangement of transmitters and receivers used in the electromagnetic propagation measurement in which the dipoles used as sensors are oriented perpendicular to the axis of the tool. The orientation is combined with relatively short spacings to give a significant signal even in the most attenuative environments, such as salty muds. |
| Formation Evaluation | bucking coil | A coil in an induction logging tool designed to buck out, or reduce, the direct coupling between transmitter and receiver coils. The direct coupling signal is far larger than the formation signal. The bucking coil is wound with the opposite polarity to the main receiver coil, and placed in series with it at a location that minimizes the direct coupling. The combination of transmitter, main receiver and bucking coils is known as a mutually balanced array. |
| Formation Evaluation | bucking current | On a laterolog device, the current sent through a guard electrode (A1) with the purpose of focusing the current sent by the central current emitting electrode (A0). The bucking current maintains A1 and A0 at the same potential, thereby forcing the current from A0 to run approximately perpendicular to the sonde into the formation. |
| Formation Evaluation | bulk relaxation | In a nuclear magnetic resonance measurement, the loss of coherent energy by hydrogen atoms as they interact with each other in bulk fluids. Bulk relaxation in fluids is caused primarily by fluctuating local magnetic fields arising from the random tumbling motion of neighboring molecules. Local field fluctuations may be high, but the fast movement of molecules tends to average these out. Thus the bulk relaxation depends strongly on the rate of movement and is affected by temperature and viscosity. In water-wet rocks, hydrocarbons do not touch the pore walls and are not affected by surface relaxation. Thus the T1 and T2 of hydrocarbons are the result only of bulk and diffusion relaxation. This is an important feature of NMR logging. Based on this feature, direct hydrocarbon-typing techniques have been developed for the detection and characterization of hydrocarbons. |
| Formation Evaluation | buoyancy method | A technique for measuring the bulk volume of a core sample by submerging it in a bath of mercury and observing the increase in weight of the bath, following Archimedes principle. The bulk volume is calculated from the increase in weight divided by the density of mercury at the temperature of the bath. The sample must not touch the side of the bath and be only a few millimeters below the surface. Mercury is used because it is so strongly nonwetting and therefore does not enter the pore space. Other, less toxic, liquids may be used in the bath, such as brine, refined oil or toluene. In this case, the sample must be fully saturated with the liquid before immersion. In an alternative method, the saturated sample is weighed in air and then again once immersed. The bulk volume is then the difference in weight divided by the density of the liquid used. |
| Formation Evaluation | butterfly chart | A plot representing the effect of invasion on resistivity measurements that have different depths of investigation. The plot assumes a step profile model of invasion and determines true resistivity, flushed zone resistivity and diameter of invasion from ratios of deep-, medium- and shallow-resistivity measurements. Strictly speaking, when both resistive and conductive invasion are plotted, the chart is called a butterfly chart. When only one is plotted it is known as a tornado chart. |
| Formation Evaluation | button resistivity | The resistivity measured by the buttons of a measurements-while-drilling (MWD) toroid device. Typically three buttons, each with a different depth of investigation, are mounted on a sleeve attached to the drillstring, and by their nature are azimuthally focused. The measurement is similar to a wireline microresistivity log, except that toroids are used instead of electrodes for transmitting and monitoring. The button resistivities are focused measurements with vertical resolutions and depths of investigation of a few inches. With three button measurements, it is possible to correct for the presence of invasion, assuming a step profile. |
| Formation Evaluation | calibration | The process of adjusting a measurement to a standard, so that copies of the same type of logging tool or laboratory instrument will read the same. The tool or instrument is placed in the presence of a calibrator or calibrating environment, for example, a source of gamma rays for a gamma ray tool, or the air, far from the ground, for an induction tool. Calibration coefficients, typically a gain and an offset, are calculated so that the tool or instrument reads correctly in the calibrator. The coefficients are then applied during subsequent measurements. The term master calibration is used for the regular, as for example quarterly, calibration of a logging tool in the workshop. For most wireline tools, a secondary calibrator is adjusted during the master calibration and taken to the wellsite so that a wellsite calibration can be done just prior to the logging job. Some tools, such as the gamma ray, are calibrated only at the wellsite. For most measurements-while-drilling tools, the environment requires that the calibration be performed at the workshop and only a verification made at the wellsite. For some measurements, there is a primary worldwide standard against which calibrators are calibrated, as for example, the radioactive formations at the University of Houston used to define gamma ray API units. |
| Formation Evaluation | camera | The device used in early logging to record logging measurements on photographic film. The camera consisted of a light shining on galvanometers, which reflected the light to produce a trace on one or more films. The galvanometers deflected according to the log measurement to give the log reading. The films were turned by the depth wheel, which gave the depth axis of the log. |
| Formation Evaluation | capillary pressure curve | The relationship describing the capillary pressure required to obtain a given nonwetting phase saturation in a rock. Rocks have a distribution of pore throat sizes, so as more pressure is applied to the nonwetting phase, increasingly smaller pore openings are invaded. The capillary pressure curve is important for understanding saturation distribution in the reservoir and affects imbibition and multiphase fluid flow through the rock. |
| Formation Evaluation | carbon density | The density of carbon in oil. This density affects the interpretation of the carbon-oxygen log. The term may also be used for the density of carbon in other materials. |
| Formation Evaluation | carbonate gamma ray | A gamma ray log from which the uranium contribution has been subtracted. In some rocks, and in particular in carbonate rocks, the contribution from uranium can be large and erratic, and can cause the carbonate to be mistaken for a shale. The carbonate gamma ray is then a better indicator of shaliness. |
| Formation Evaluation | cartridge | The section of a wireline logging tool that contains the telemetry, the electronics and power supplies for the measurement, as distinct from the sonde that contains the measurement sensors. Strictly speaking, the term refers to the package of electronic hardware inside a steel housing, but it is also used to refer to the complete assembly including housing. |
| Formation Evaluation | cation exchange capacity | The quantity of positively charged ions (cations) that a clay mineral or similar material can accommodate on its negatively charged surface, expressed as milli-ion equivalent per 100 g, or more commonly as milliequivalent (meq) per 100 g. Clays are aluminosilicates in which some of the aluminum and silicon ions have been replaced by elements with different valence, or charge. For example, aluminum (Al+++) may be replaced by iron (Fe++) or magnesium (Mg++), leading to a net negative charge. This charge attracts cations when the clay is immersed in an electrolyte such as salty water and causes an electrical double layer. The cation-exchange capacity (CEC) is often expressed in terms of its contribution per unit pore volume, Qv. In formation evaluation, it is the contribution of cation-exchange sites to the formation electrical properties that is important. Various techniques are used to measure CEC in the laboratory, such as wet chemistry, multiple salinity and membrane potential. Wet chemistry methods, such as conductometric titration, usually involve destruction or alteration of the rock. Although quicker and simpler to perform, they are less representative of electrical properties in situ. The multiple salinity and membrane potential methods are more direct measurements of the effect of CEC on formation resistivity and spontaneous potential. |
| Formation Evaluation | cation-exchange capacity | The quantity of positively charged ions (cations) that a clay mineral or similar material can accommodate on its negatively charged surface, expressed as milli-ion equivalent per 100 g, or more commonly as milliequivalent (meq) per 100 g. Clays are aluminosilicates in which some of the aluminum and silicon ions have been replaced by elements with different valence, or charge. For example, aluminum (Al+++) may be replaced by iron (Fe++) or magnesium (Mg++), leading to a net negative charge. This charge attracts cations when the clay is immersed in an electrolyte such as salty water and causes an electrical double layer. The cation-exchange capacity (CEC) is often expressed in terms of its contribution per unit pore volume, Qv. In formation evaluation, it is the contribution of cation-exchange sites to the formation electrical properties that is important. Various techniques are used to measure CEC in the laboratory, such as wet chemistry, multiple salinity and membrane potential. Wet chemistry methods, such as conductometric titration, usually involve destruction or alteration of the rock. Although quicker and simpler to perform, they are less representative of electrical properties in situ. The multiple salinity and membrane potential methods are more direct measurements of the effect of CEC on formation resistivity and spontaneous potential. |
| Formation Evaluation | cave effect | The effect of a sharp change in the borehole diameter, such as that caused by a cave or rugose hole, on an induction log. In smooth boreholes of constant diameter, the effect of the borehole is well understood and can be corrected for. However, a sharp increase in diameter over a small depth interval can induce signals on one coil in the array and not in others. This signal is not handled by the normal borehole correction and may result in a spike on the log. The spike usually is significant only when the resistivity is high and the contrast between formation and borehole resistivity is very large. The spike also depends on the design of the array or the processing. |
| Formation Evaluation | cement bond log | A log that uses the variations in amplitude of an acoustic signal traveling down the casing wall between a transmitter and receiver to determine the quality of cement bond on the exterior casing wall. The fundamental principle is that the acoustic signal will be more attenuated in the presence of cement than if the casing were uncemented. The measurement is largely qualitative, as there is no indication of azimuthal cement variations such as channeling, and as it is sensitive to the effect of a microannulus. |
| Formation Evaluation | cementation exponent | The exponent of porosity, m, in the relation of formation factor, F, to porosity, phi. In the Archie equation, F = 1 / phim, H. Guyod termed m the cementation exponent because m was observed to be higher in cemented rock. The more general term is porosity exponent. |
| Formation Evaluation | channeling | The condition in which cement flows in a channel only on some sides of the casing or borehole annulus, and thus does not provide adequate hydraulic isolation in all radial azimuths. The channel frequently manifests itself as an intermediate amplitude signal on a cement bond log. Pulse-echo tools are able to detect a channel because they measure the cement bond at different azimuths. |
| Formation Evaluation | chemical neutron source | An encapsulated radioactive material that emits neutrons for use in neutron porosity measurements. The most common source relies on alpha-beryllium reactions in a 241Am-Be mixture. Beryllium releases a neutron of approximately 4 MeV when struck by an alpha particle. The americium is the source of alpha particles. 253Californium fission is an intense source of 2.3 MeV neutrons but is used only in special applications due to its short half-life of 2.65 years and special licensing requirements. |
| Formation Evaluation | clay bound water | Water within the clay lattice or near the surface within the electrical double layer. This water does not move when fluid is flowed through the rock. In the normal definition used by a log analyst, clay-bound water is not part of the effective porosity and is the difference between total porosity and effective porosity. Clay-bound water is understood to include the interlayer water, although the contribution of the latter to the electrical properties of the clay may be different from the water in the electrical double layer. In the dual-water and the Hill-Shirley-Klein models, the volume of clay-bound water is related to the cation-exchange capacity per unit volume, Qv, by expressions that depend on the salinity and temperature of the electrolyte in which the clay is immersed. Direct measurement of the clay-bound water volume in the laboratory is difficult. |
| Formation Evaluation | clay-bound water | Water within the clay lattice or near the surface within the electrical double layer. This water does not move when fluid is flowed through the rock. In the normal definition used by a log analyst, clay-bound water is not part of the effective porosity and is the difference between total porosity and effective porosity. Clay-bound water is understood to include the interlayer water, although the contribution of the latter to the electrical properties of the clay may be different from the water in the electrical double layer. In the dual-water and the Hill-Shirley-Klein models, the volume of clay-bound water is related to the cation-exchange capacity per unit volume, Qv, by expressions that depend on the salinity and temperature of the electrolyte in which the clay is immersed. Direct measurement of the clay-bound water volume in the laboratory is difficult. |
| Formation Evaluation | coding | The characteristics of the trace used to display a log. The most common codings are solid, long-dashed, short-dashed and dotted. The trace can also have a different line weight or thickness, from light to heavy. |
| Formation Evaluation | compatible scales | Scales for different logs that are chosen so that the logs will overlay in certain conditions. For example, a sandstone-compatible scale may have the neutron log scaled from 0.45 to -0.15 vol/vol and the density from 1.9 to 2.9 g/cm3. Then, in a pure quartz sandstone filled with fresh water, the two logs will overlay as the porosity varies. |
| Formation Evaluation | compensated density log | A density log that has been corrected for the effect of mud and mudcake by using two or more detectors at different spacings from the source. The shorter the spacing, the shallower the depth of investigation and the larger the effect of the mudcake. Thus, a short spaced detector, which is very sensitive to the mudcake, can be used to correct a long-spaced detector, which is only slightly sensitive to it. In a typical two-detector compensation scheme, the density measured by the longest spacing detector is corrected by an amount, delta rho, which is a function of the difference between long- and short-spacing densities. The correction is found to depend on the difference between formation and mudcake density multiplied by mudcake thickness. Although there are three unknowns, simple functions are reliable for moderate corrections. Experimental results are often presented in the form of a spine and ribs plot. There are other schemes using, for example, more detectors. Dual detector density logs were introduced in the mid 1960s. |
| Formation Evaluation | compensated neutron log | A neutron porosity log in which the effects of the borehole environment are minimized by using two detectors. In the most common technique, the two source-detector spacings are chosen so that the ratio of the two count rates is relatively independent of the borehole environment. This ratio is then calibrated in terms of porosity in a known formation and borehole environment typically with the tool placed against the side of an 8-in. [20-cm] borehole in a limestone block, both filled with fresh water at surface temperature and pressure. The response is also determined at different porosities and in sandstones, dolomites and other borehole environments. Correction factors are developed to convert the measured log to the standard conditions. The source and detectors are not azimuthally focused. Wireline tools are run eccentralized against the borehole wall. Since the neutrons emitted into the mud are strongly attenuated, the resulting log is effectively focused into the formation. Measurements-while-drilling tools will normally be unfocused since they are centralized unless the borehole is overgauge. The vertical resolution is about 2 ft [0.6 m], but can be improved by alpha processing. |
| Formation Evaluation | compensated-density log | A density log that has been corrected for the effect of mud and mudcake by using two or more detectors at different spacings from the source. The shorter the spacing, the shallower the depth of investigation and the larger the effect of the mudcake. Thus, a short spaced detector, which is very sensitive to the mudcake, can be used to correct a long-spaced detector, which is only slightly sensitive to it. In a typical two-detector compensation scheme, the density measured by the longest spacing detector is corrected by an amount, delta rho, which is a function of the difference between long- and short-spacing densities. The correction is found to depend on the difference between formation and mudcake density multiplied by mudcake thickness. Although there are three unknowns, simple functions are reliable for moderate corrections. Experimental results are often presented in the form of a spine and ribs plot. There are other schemes using, for example, more detectors. Dual detector density logs were introduced in the mid 1960s. |
| Formation Evaluation | composite log | A single log created by splicing together two logs of the same type run at different times in the well; or by splicing two different types of log run at the same time. For example, it is common practice to splice all the basic logs run over different depth intervals in a well to obtain a single composite record. |
| Formation Evaluation | compressibility | The ratio of the percent change in volume to the change in pressure applied to a fluid or rock. |
| Formation Evaluation | Compton scattering | A gamma ray interaction in which the gamma ray collides with an electron, transferring part of its energy to the electron, while itself being scattered at a reduced energy. Compton scattering occurs with high probability at intermediate gamma ray energies, between 75 keV and 10 MeV in sedimentary formations. When a beam of gamma rays traverses a material, the total reduction due to Compton scattering depends on the electron density of the material the higher the density, the larger the reduction. This is the basis for the density log. Compton scattering is also an important mechanism in gamma ray detectors. |
| Formation Evaluation | computed tomography | A technique for imaging a core by scanning it with a highly focused source of X-rays and recording the attenuated X-rays on the other side. The source and detector are rotated and moved along the core. The measurements are combined mathematically to give a full core image. |
| Formation Evaluation | conductive invasion | A situation in which the resistivity of the flushed zone is less than the resistivity of the undisturbed zone. Such a setting generally favors the use of electrode resistivity devices (laterologs, ring resistivity), which respond to resistivity, rather than induction and propagation resistivity devices, which respond to conductivity. |
| Formation Evaluation | conductive rock matrix model | A model, or set of equations, for the resistivity response of formations with conductive minerals, such as shaly sands. The model is used to analyze core data and to calculate water saturation from resistivity and other logs. The conductive rock matrix model (CRMM) was proposed by W. Givens. The model treats the rock as two components in parallel: a conductive pore network with fluid that is free to move, and the remainder of the rock, which may have conductive minerals or immobile but conductive water. The model is not concerned with the origin of this conductivity, but gives it a resistivity, Rm. The two components are in parallel as follows: 1 / Rt = 1 / Rp + 1 / Rm where Rp is the resistance of the free-fluid pore network and can be expressed in terms of the porosity and formation water resistivity by the Archie equation. The model was developed from core data, and can explain the observed variations of the porosity exponent with porosity and the saturation exponent with water saturation in shaly sands. For log analysis Rm needs to be related to parameters that can be measured by logs. Reference: Givens WW: Formation Factor, Resistivity Index and Related Equations Based upon a Conductive rock Matrix Model (CRMM), Transactions of the SPWLA 27th Annual Logging Symposium, Houston, Texas, USA, June 9-13, 1986, paper P. |
| Formation Evaluation | conductivity | The ability of a material to conduct electricity. It is the inverse of resistivity and is measured in siemens per meter (S/m) or mho/m. The conductivity is a property of the material, whereas the conductance also depends on the volume measured. The two are related by a system constant, which in simple cases is the length between the measurement electrodes divided by the area. In the most general case, the conductivity is the current density divided by the electric field and depends on the frequency of the applied signal. |
| Formation Evaluation | conductometric titration | A technique for estimating the cation-exchange capacity of a sample by measuring the conductivity of the sample during titration. The technique includes crushing a core sample and mixing it for some time in a solution like barium acetate, during which all the cation-exchange sites are replaced by barium (Ba++) ions. The solution is then titrated with another solution, such as MgSO4, while observing the change in conductivity as the magnesium (Mg++) ions replace the Ba++ ions. For several reasons, but mainly because the sample must be crushed, the measured cation-exchange capacity may differ from that which affects the in situ electrical properties of the rock. |
| Formation Evaluation | core gamma log | A log obtained in the laboratory by moving the core past a gamma ray detector. The log can be of the total gamma ray in API units, or of the spectral response in weight concentrations of thorium, uranium and potassium. The main purpose is to correlate the depth of each section of core with the depth of a log. |
| Formation Evaluation | core image | An image of the external or internal features of a core. External images are photographs taken under natural or ultraviolet light; natural light highlights lithology and sedimentary structures, while ultraviolet light causes hydrocarbon zones to fluoresce. Internal images are obtained using X-rays or nuclear magnetic resonance (NMR). X-ray techniques measure the attenuation of X-rays passing through the core. The attenuation depends mainly on the density. Hence the image reflects density and lithology changes, internal bedding planes, fractures and nodules. These techniques include, in increasing resolution, fluoroscopy, X-radiography and computed tomography. Most NMR images measure the quantity and relaxation time of hydrogen, and therefore give information on fluid distribution. Some NMR techniques examine carbon, sodium and phosphorous. |
| Formation Evaluation | core plug | A plug, or sample, taken from a conventional core for analysis. Core plugs are typically 1 in. to 1 1/2 in. [2.5 to 3.8 cm] in diameter and 1 in. to 2 in. [5 cm] long. Core plugs are ordinarily cut perpendicular to the axis of the core or parallel to the axis, called horizontal and vertical plugs, respectively, when cut from a vertical wellbore. The terms horizontal and vertical are often applied for cores cut from a deviated or horizontal wellbore, even though they are not strictly correct unless core orientation is measured and plugs are cut to the true in-situ orientation. |
| Formation Evaluation | corkscrew hole | The result of certain drilling conditions that cause the borehole to take the shape of a corkscrew. Most logging tools are much longer than the wavelength of the corkscrew, and therefore see it as a change in standoff or a change in hole size. For this reason, the corkscrew is often observed as a wave on the caliper log. A corkscrew hole affects measurements sensitive to standoff, such as induction and neutron porosity, and may affect pad tools, if they cannot follow the changes. |
| Formation Evaluation | corrected gamma ray | A gamma ray log from which the uranium contribution has been subtracted. In some rocks, and in particular in carbonate rocks, the contribution from uranium can be large and erratic, and can cause the carbonate to be mistaken for a shale. The carbonate gamma ray is then a better indicator of shaliness. |
| Formation Evaluation | correlation log | A log run for the purpose of correlating between wells. The most common logs used for this purpose are the gamma ray, the resistivity and the acoustic log; the most common depth scales are 1/500 and 1/1000, or 2 in./100 ft [5 cm/30 m] and 1 in./100 ft [2.5 cm/30 m]. |
| Formation Evaluation | CPMG | In a nuclear magnetic resonance (NMR) measurement, referring to the cycle of radio frequency pulses designed by Carr, Purcell, Meiboom and Gill to produce pulse echoes and counteract dephasing due to magnetic field inhomogeneities. In the CPMG sequence, an initial radio frequency pulse is applied long enough to tip the protons into a plane perpendicular to the static magnetic field (the 90o pulse). Initially the protons precess in unison, producing a large signal in the antenna, but then quickly dephase due to the inhomogeneities. Another pulse is applied, long enough to reverse their direction of precession (the 180o pulse), and causing them to come back in phase again after a short time. Being in phase, they produce another strong signal called an echo. They quickly dephase again but can be rephased by another 180o pulse. Rephasing is repeated many times, while measuring the magnitude of each echo. This magnitude decreases with time due to molecular relaxation mechanisms surface, bulk and diffusion. One measurement typically may comprise many hundreds of echoes, while the time between each echo (the echo spacing) is of the order of 1 ms or less. Carr HY and Purcell EM: ?Effects of Diffusion on Free Precession in Nuclear Magnetic Resonance Experiments,? Physical Review 94, no. 3 (1954): 630-638. Meiboom S and Gill D: ?Modified Spin-Echo Method for Measuring Nuclear Relaxation Times,? The Review of Scientific Instruments 29, no. 8 (1958): 688-691. |
| Formation Evaluation | cross dipole | Describing a waveform or a log that has been recorded by a set of dipole receivers oriented orthogonally (or 900 out of line) with a dipole transmitter. In sonic logging, cross-dipole flexural modes are used to determine shear anisotropy together with in-line flexural modes. The data are processed using the Alford rotation. |
| Formation Evaluation | crossplot porosity | The porosity obtained by plotting two porosity logs against each other, normally density and neutron porosity. The computation assumes a particular fluid, usually fresh water, and particular response equations. The result is largely independent of lithology and is often a more reliable estimate of porosity than a single porosity log. It is often displayed as a quicklook log. |
| Formation Evaluation | crosswell electromagnetic tomography | A technique for measuring formation resistivity between two or more wells. This technique measures the signal between an electromagnetic induction transmitter in one well, and a receiver array located in another well. The transmitting antenna broadcasts a continuous sinusoidal signal at programmable frequencies. Tomographic processing creates a map of resistivity of the area between the wells. Measurements acquired by this technique have a greater depth of investigation than conventional logging tools and are sensitive to fluid content. Crosswell electromagnetic induction surveys fill an intermediate role between high-resolution well logs and lower-resolution surface measurements. Asset managers utilize crosswell electromagnetic surveys for a variety of applications, such as monitoring sweep efficiency, identifying bypassed pay, planning infill drilling locations and improving the effectiveness of reservoir simulations. |
| Formation Evaluation | curve | The presentation on hard copy of log data from a single measurement versus depth. The term is also used to refer to the log data themselves, as a synonym for a single log. |
| Formation Evaluation | data rate | The rate at which measurements are transmitted between logging tool and surface. In measurement-while-drilling (MWD), if the data rate is low in comparison with the drilling or tripping speed, the sampling interval or the amount of data transmitted must be reduced or else information will be lost. In wireline logging, the data rate can limit the logging speed or the number of tools in the tool string. |
| Formation Evaluation | Dean-Stark extraction | A method for the measurement of fluid saturations in a core sample by distillation extraction. The water in the sample is vaporized by boiling solvent, then condensed and collected in a calibrated trap. This gives the volume of water in the sample. The solvent is also condensed, then flows back over the sample and extracts the oil. Extraction continues for a minimum of two days until the extracted solvent is clean or the sample shows no more fluorescence. The weight of the sample is measured before and after extraction. Then the volume of oil is calculated from the loss in weight of the sample minus the weight of the water removed from it. Saturations are calculated from the volumes. |
| Formation Evaluation | deconvolution | With reference to induction logging, a method of removing shoulder-bed effects from an induction log. The term refers to early 6FF40 and deep induction logs in which the standard method of deconvolution was based on three measurements separated by 78 in. [198 cm] in depth. The three measurements were weighted by an amount calculated to reduce the effect of shoulder beds on the readings in a high-resistivity bed. Originally, the resistivity of the shoulder beds could be input, but in later usage this resistivity became standardized at 1 ohm-m. The deconvolution was not effective in high-contrast formations. In modern tools, the shoulder effect is corrected by using an inverse filter or an automatic inversion. |
| Formation Evaluation | deep induction | A particular type of induction log that was designed to read deep into the formation while maintaining reasonable vertical resolution. The deep induction log (ID) is based on the measurement of a 6FF40 array and was combined with a medium induction array to form the dual induction tool. Versions built after 1968 had a small extra transmitter coil to reduce the borehole effect on the medium induction while changing the deep response very little. The midpoint of the ID integrated radial geometrical factor is at 62 in. [157 cm] radius for high resistivities, reducing to 45 in. [114 cm] at 1 ohm-m. ID receives very little signal from within 20 in. [50 cm] of the tool. The vertical resolution is about 8 ft [2.4 m] but varies with local conditions. |
| Formation Evaluation | delta rho | A log that shows the magnitude of the correction applied to the long-spacing detector of a density measurement. When delta rho is above a certain value, typically +/- 0.15 g/cm3, the correction may no longer be accurate, and needs to be examined in more detail. Delta rho is also used as a qualitative indicator of borehole rugosity. |
| Formation Evaluation | delta t stretch | A feature on a sonic log caused by low signal amplitude that results in erroneously long traveltimes. Sonic logs that do not record waveforms measure the acoustic traveltime between transmitter and receiver by detecting the first signal at the receiver above a certain threshold (first motion detection). The threshold is small so that the signal is detected just after it crosses the zero signal baseline. However, if the threshold is set too high, or the signal is too small, the system will not trigger at the sharp zero crossing but at some later point on the waveform. This increases the apparent transmitter-receiver time. Delta-t stretch is more likely at the far receiver, where signals are weaker, so that the apparent traveltime calculated between receivers is too large. In the extreme case, the system triggers on the next cycle of the waveform, known as cycle skipping. |
| Formation Evaluation | delta-t stretch | A feature on a sonic log caused by low signal amplitude that results in erroneously long traveltimes. Sonic logs that do not record waveforms measure the acoustic traveltime between transmitter and receiver by detecting the first signal at the receiver above a certain threshold (first motion detection). The threshold is small so that the signal is detected just after it crosses the zero signal baseline. However, if the threshold is set too high, or the signal is too small, the system will not trigger at the sharp zero crossing but at some later point on the waveform. This increases the apparent transmitter-receiver time. Delta-t stretch is more likely at the far receiver, where signals are weaker, so that the apparent traveltime calculated between receivers is too large. In the extreme case, the system triggers on the next cycle of the waveform, known as cycle skipping. |
| Formation Evaluation | density measurement | A measurement of the bulk density of the formation, based on the reduction in gamma ray flux between a source and a detector due to Compton scattering. The gamma ray source, usually 137Cs (cesium), is chosen so that gamma ray energies are high enough to interact by Compton scattering but not by pair production. The detectors discriminate against low gamma ray energies that may have been influenced by photoelectric absorption. Although Compton scattering depends on electron density and not bulk density, density logs are calibrated to give the correct bulk density in the majority of sedimentary rocks. Due to the Z/A effect there are small differences in some formations. The measurement responds to the average density of the material between source and detector. In the wireline measurement, care is taken to minimize the mud between the sensors and the formation by pressing a pad against the borehole wall, with source and detector focused into the formation. In the logging-while-drilling measurement, a sleeve may be mounted on the collar around the sensors to exclude the mud. The detectors measure the gamma rays scattered from the formation. Even then, mudcake or borehole rugosity can affect the measurement. It is common practice to compensate for the mudcake by using two or more detectors at different spacings. |
| Formation Evaluation | departure curve | A graph that shows the effects of environmental factors on the ideal response of a measurement. The name comes from the departure of the actual response from the ideal. The term is used most commonly in relation to the effect of hole size, mud resistivity, bed thickness, invasion and other factors on electrical logs. |
| Formation Evaluation | dephasing | In a nuclear magnetic resonance (NMR) measurement, the loss of synchronization of hydrogen atoms precessing at different speeds about the static magnetic field. When the signals from individual atoms are not synchronized, they are out of phase and the total signal is reduced. The dephasing occurs either because of inhomogeneities in the static magnetic field or through molecular processes. Dephasing due to inhomogeneities is known as the free-induction decay and is corrected by the CPMG sequence. Molecular dephasing is known as transverse relaxation. |
| Formation Evaluation | depth datum | Also known as depth reference, the point in a well from which depth is measured. Alternatively, the depth reference is the point at which the depth is defined as being zero. It is typically the top of the kelly bushing or the level of the drill floor on the rig that is used to drill the well. The depth measured from that point is the measured depth (MD) for the well. Even when the drilling rig has been removed, all subsequent measurements and operations in the well are still tied in to the same depth reference. However, for multiwell studies, the depths are normally shifted to the permanent datum. The depth reference and its elevation above the permanent datum are recorded on the log heading. In some contexts, the term may refer to any point from which depth is measured. |
| Formation Evaluation | depth derived | Referring to a borehole-compensation scheme for sonic logs that combines measurements taken when the logging tool is at two different depths in the borehole. In normal borehole-compensation schemes, the effects of caves and sonde tilt are minimized by combining measurements from a second transmitter (T2) above a pair of receivers with those from the first transmitter (T1) below the receivers. This arrangement makes the logging tool unacceptably long for the long-spacing sonic log. In the depth-derived system, T2 is located below T1, at a distance equal to the receiver spacing. T1 is fired and the transit time between the receivers at depth z (TT1z) is recorded as usual. Then when T1 and T2 are at depth z, both are fired sequentially and the difference in time for their signals to reach one of the receivers is recorded (TT2z). The average of TT1z and TT2z is borehole-compensated since the acoustic signals traveled in opposite directions for the two measurements. |
| Formation Evaluation | depth mark | A magnetic mark placed on a logging cable as a reference for depth measurements. The marks are placed on the cable at regular intervals, usually 100 ft [30 m] or 50 m [164 ft], under a certain tension in a workshop. The intervals may change slightly as a function of tension downhole, but this change can be corrected for. During logging operations, the marks are detected by a magnetic mark detector, and then used to check and correct the depth read by the depth wheel. |
| Formation Evaluation | depth matched | Pertaining to two or more logging curves that have been aligned in depth. Logs recorded on different runs will not be exactly aligned at all depths because of the difficulty of perfect depth control. If the two logs are offset by the same amount throughout the log, then only a simple depth shift is required. If the offset varies, then the logs need to be depth matched. Depth matching is simplest if both runs contain the same type of log, such as a gamma ray. The two gamma rays can then be aligned, either manually or with software, and the other logs shifted by the same amount. Otherwise the alignment is based on two logs that respond in a similar fashion, such as a neutron porosity and a shallow laterolog. Depth matching also may be needed for logs recorded on the same run. Although there is a fixed distance between the measure points and the depth reference, the apparent distance will vary if the tool moves unevenly up the hole, due to stick and slip or yo-yo effects. Depth matching is then necessary. |
| Formation Evaluation | depth of invasion | The distance from the borehole wall that the mud filtrate has penetrated into the formation. The depth of invasion affects whether a log measures the invaded zone, the undisturbed zone or part of each zone. The term is closely related to the diameter of invasion, the latter being twice the depth of invasion plus the borehole diameter. Depth of invasion is a more appropriate parameter for describing the response of pad and azimuthally focused measurements such as density and microresistivity logs. The term is well-defined in the case of a step profile of invasion. In the case of an annulus or a transition zone, two depths must be defined, corresponding to the inner and outer limits of the annulus or transition zone. When the invasion model is not specified, the term usually refers to the outer limit of invasion. |
| Formation Evaluation | depth of investigation | A distance that characterizes how far a logging tool measures into the formation from the face of the tool or the borehole wall. The depth of investigation summarizes the radial response of the measurement in one or more directions. For nuclear and resistivity measurements, the depth of investigation should be associated with the percentage of signal received from within that depth, typically either 50% or 90%. Most quoted depths of investigation assume a homogeneous formation with certain properties, such as a given resistivity or fluid content. The depths of investigation can vary considerably in inhomogeneous conditions, and at different values of the properties concerned. They should be considered only a qualitative guide to tool response. For other measurements, the depth of investigation is either well-defined by the tool physics (in the case of nuclear magnetic resonance), or else can be given only approximately, an accurate value being too dependent on formation properties (in the case of acoustic and electromagnetic propagation). The term is used for all measurements but is most appropriate for azimuthally focused devices such as nuclear logs. For azimuthally symmetric devices such as resistivity logs, the term radius of investigation is more appropriate. |
| Formation Evaluation | depth reference | The point in a well from which depth is measured. Alternatively, the depth reference is the point at which the depth is defined as being zero. It is typically the top of the kelly bushing or the level of the drill floor on the rig that is used to drill the well. The depth measured from that point is the measured depth (MD) for the well. Even when the drilling rig has been removed, all subsequent measurements and operations in the well are still tied in to the same depth reference. However, for multiwell studies, the depths are normally shifted to the permanent datum. The depth reference and its elevation above the permanent datum are recorded on the log heading. In some contexts, the term may refer to any point from which depth is measured. |
| Formation Evaluation | depth wheel | A calibrated wheel used to drive the depth recording system in wireline logging. The wheel is pressed against the logging cable as the cable is spooled onto the drum and therefore turns as the cable is run in and out of the borehole. After zeroing the depth on surface, the depth wheel provides the depth input to the recording system. Small errors in calibration and slippage can cause the wheel to introduce systematic errors in the depth recorded. For this reason, the depth is checked and corrected using depth marks. The depth wheel is also referred to as a depth encoder. Modern encoders have two wheels so that slippage can be detected by differences between the two measurements. |
| Formation Evaluation | depth-derived | Referring to a borehole-compensation scheme for sonic logs that combines measurements taken when the logging tool is at two different depths in the borehole. In normal borehole-compensation schemes, the effects of caves and sonde tilt are minimized by combining measurements from a second transmitter (T2) above a pair of receivers with those from the first transmitter (T1) below the receivers. This arrangement makes the logging tool unacceptably long for the long-spacing sonic log. In the depth-derived system, T2 is located below T1, at a distance equal to the receiver spacing. T1 is fired and the transit time between the receivers at depth z (TT1z) is recorded as usual. Then when T1 and T2 are at depth z, both are fired sequentially and the difference in time for their signals to reach one of the receivers is recorded (TT2z). The average of TT1z and TT2z is borehole-compensated since the acoustic signals traveled in opposite directions for the two measurements. |
| Formation Evaluation | detail log | A log with a depth scale chosen to show sufficient detail of the formation. The most common scales are 1/200 or 5 in./100 ft. |
| Formation Evaluation | diameter of invasion | The distance from the borehole wall into the formation that the mud filtrate has penetrated. The term assumes equal invasion on all sides of the borehole. It is the diameter of the circle thus formed, with the center being the center of the borehole. The diameter of invasion affects whether a log measures the invaded zone, the undisturbed zone, or part of each zone. The term is closely related to the depth of invasion, being twice the depth of invasion plus the borehole diameter. Diameter of invasion is a more appropriate parameter for describing the response of azimuthally symmetric measurements such as induction, laterolog and propagation resistivity. The term is well-defined in the case of a step profile of invasion. In the case of an annulus or a transition zone, two diameters must be defined, corresponding to the inner and outer limits of the annulus or transition zone. When the invasion model is not specified, the term usually refers to the outer limit of invasion. |
| Formation Evaluation | diameter of investigation | A distance that characterizes how far a logging tool measures into the formation from the axis of the tool or borehole. The term is similar to depth of investigation but is appropriate only for azimuthally symmetric measurements such as resistivity. |
| Formation Evaluation | dielectric constant | The degree to which a medium resists the flow of electric charge, defined as the ratio of the electric displacement to the electric field strength. It is more common to use the relative dielectric constant. |
| Formation Evaluation | dielectric permittivity | The degree to which a medium resists the flow of electric charge, defined as the ratio of the electric displacement to the electric field strength. It is more common to use the relative dielectric permittivity. |
| Formation Evaluation | dielectric propagation log | A log of the high-frequency (on the order of 25 MHz) dielectric properties of the formation. The log usually includes two curves the relative dielectric permittivity, symbolized by epsilon which is unitless, and the resistivity in ohm-m. At the frequency used, water molecules have a strong effect on the dielectric properties, so that both relative dielectric permittivity and conductivity increase with the volume of water present. Relative dielectric permittivity can be used to distinguish hydrocarbons from water of any salinity. However, the effect of salinity is more important than the salinity effect with the high-frequency electromagnetic propagation log, and the interpretation is more complex. The advantage of the dielectric propagation log is that the lower frequency permits a larger depth of investigation and therefore an analysis of the undisturbed zone. |
| Formation Evaluation | dielectric resistivity | The resistivity of the formation derived by combining the attenuation and phase shift of a propagation resistivity measurement. Common practice is to transform attenuation and phase shift independently to resistivity, assuming a certain transform between permittivity and resistivity. These relations lose accuracy at high resistivity. However, by combining the two measurements, both the dielectric permittivity and resistivity can be determined without need for a transform. The dielectric resistivity extends the range of measurement, typically up to 3000 ohm-m. |
| Formation Evaluation | differential SP | The spontaneous potential (SP) measured between two electrodes placed close together in the borehole, as opposed to the normal SP, which is measured with one electrode in the borehole and one at surface. |
| Formation Evaluation | differential spectrum | A technique in nuclear magnetic resonance (NMR) logging that is based on the difference between the T2 distributions, or spectra, acquired at different polarization times. The technique often is used to detect gas or light oil. These fluids have long T1 that exceed 1 s. A measurement made with a long polarization time will polarize much of these fluids and give significant signal at the appropriate T2. A measurement made with a short polarization time will polarize little of these fluids and will give a much smaller signal. Other fluids, with shorter T1, will be polarized in both cases, so that a difference in signal at the appropriate T2 identifies gas or light oil. |
| Formation Evaluation | diffusion relaxation | In a nuclear magnetic resonance (NMR) measurement, the loss of coherent energy by hydrogen atoms as they move within the pore space. Hydrogen atoms that move significantly within the pores during a NMR measurement will encounter different magnetic fields and hence will precess at different rates, or dephase. Dephasing contributes only to T2 and is most significant in gas or light oils. The magnitude depends on the field gradient, the echo spacing and the diffusion coefficient of the fluid. Diffusion relaxation can be induced in water by using long echo spacings. This is the basis of the enhanced diffusion technique. |
| Formation Evaluation | dip correction | An algorithm for correcting the effects of dip or borehole deviation on the response of a logging measurement. These effects are significant for deep-reading logs such as induction and electrode devices. The standard processing used to produce these logs assumes a vertical well with horizontal formation layers. In the presence of a relative dip between the borehole and formation layers, the logs may read incorrectly. For older logs such as the dual induction, a set of inverse filters can be designed to correct for dip effect up to about 60. For modern array logs, iterative forward modeling with a one-dimensional layered earth model can correct up to about 85. |
| Formation Evaluation | direct hydrocarbon typing | The application of nuclear magnetic resonance (NMR) logging to the determination of hydrocarbon type (gas, light oil, medium oil, heavy oil), using only NMR data. Three techniques are most commonly used: differential spectrum, shifted spectrum and enhanced diffusion. |
| Formation Evaluation | dispersed clay | Clay that is scattered throughout the pore space. There are three general types: pore lining, pore filling and pore bridging. The terms dispersed clay and dispersed shale tend to be used synonymously. |
| Formation Evaluation | distillation extraction | A technique for cleaning core samples in which the water fraction is removed by distillation and the oil fraction is extracted using solvents. Cleaning is done with either the Soxhlet or, when fluid saturation measurements are required, the Dean-Stark apparatus. Different solvents are used depending on the type of fluids and rock, the most common ones being toluene, methanol, xylene and chloroform. Several solvents may be used in sequence. |
| Formation Evaluation | drillpipe conveyed | Pertaining to the use of drillpipe to move wireline logging tools up and down a borehole. In difficult conditions–high well deviation, rough hole–wireline logging tools cannot reach the bottom of the hole under their own weight. In drillpipe-conveyed logging operations, the tools are moved mechanically by the drillpipe, while a wireline maintains the electrical connection. |
| Formation Evaluation | drillpipe conveyed | Describing sensors that are embedded in drill collars in order to record measurements-while-drilling. |
| Formation Evaluation | dual induction | The combination of a deep-induction and a medium-induction array on the same sonde. In a typical implementation, the two arrays share the same transmitters but have different receivers. If the dual-induction log is combined with a shallow laterolog or microresistivity log, it is possible to correct for the effect on invasion on the deep log, assuming a step profile. |
| Formation Evaluation | dual water | A model of shaly formations that considers there to be two waters in the pore space: far water, which is the normal formation water; and near water (or clay-bound water) in the electrical double layer near the clay surface. The clay-bound water consists of clay counter-ions and the associated water of hydration. The volume of this layer is determined by its thickness, which is constant at high salinities, and its area, which is proportional to the counter-ion concentration per unit pore volume (Qv). The volume of clay-bound water per unit pore volume, Swb, can therefore be written as: Swb = alpha * vq * Qv where vq = 0.28 cm3/meq at 25oC is the factor relating volume to counter-ion concentration at high salinity and is a function only of temperature, and alpha = 1 above a certain salinity, below which it increases with temperature and with decreasing salinity. The fractional volume of the far water is then (1 ? alpha?* vq * Qv). The dual-water concept was developed for the interpretation of resistivity in shaly sands, but is also useful in the interpretation of nuclear and nuclear magnetic resonance logs. In these cases, the parameter most used is the total volume of clay-bound water in the rock, equal to Swb multiplied by the total porosity. |
| Formation Evaluation | eccentralizer | A device that helps to keep a wireline logging tool away from the center of the borehole. Typical devices are a single bow spring mounted on the outside surface of the logging tool or a set of rubber fingers mounted at the bottom. Some measurements, such as induction logs, respond better when the tool is eccentralized, while others, including acoustic logs, are better when centralized. |
| Formation Evaluation | echo spacing | The time between each echo in a nuclear magnetic resonance (NMR) measurement. The time is also the time to the first echo and is therefore an important parameter in defining the fastest relaxation time that can be measured. In standard measurements, the echo spacing ranges from 0.2 to 1 ms. |
| Formation Evaluation | effective porosity | In the original definition of core analysts, the volume of connected pores in a unit volume of rock. Effective porosity in this sense is the total porosity less the isolated porosity. It is the porosity measured by most core analysis techniques that do not involve disaggregating the sample. In these techniques, the porosity is usually measured on totally dried core samples. Drying removes most of the clay-bound water. In log interpretation, effective porosity means the total porosity less the clay-bound water. The definition is based on the analysis of shaly formations, in which the clay-bound water is considered immobile and hence ineffective. Isolated porosity is rare in such formations and is ignored, being included in the effective porosity. Effective porosity on dried core samples is therefore greater than effective porosity from log analysis, and close to the total porosity from log analysis. In humidity-dried cores, part of the clay-bound water is not removed, and the difference is reduced. In some usage, the capillary-bound water is not considered part of the effective porosity. In this case effective porosity is synonymous with free fluid. Effective porosity is measured in volume/volume, percent or porosity units, p.u. |
| Formation Evaluation | effective water saturation | The fraction of water in the pore space corresponding to the effective porosity. It is expressed in volume/volume, percent or saturation units. Unless otherwise stated, water saturation is the fraction of formation water in the undisturbed zone. The saturation is known as the total water saturation if the pore space is the total porosity, but is called effective water saturation if the pore space is the effective porosity. If used without qualification, the term water saturation usually refers to the effective water saturation. |
| Formation Evaluation | elastic neutron scattering | A neutron interaction in which the kinetic energy lost by a neutron in a nuclear collision is transferred to the nucleus. The energy of a neutron is reduced more efficiently in collisions with nuclei of similar mass to the neutron, like hydrogen and other elements of low atomic mass. Elastic neutron scattering is one of the main principles behind the neutron porosity log, as well as the pulsed neutron capture log, which is mainly used to determine water saturation behind casing. |
| Formation Evaluation | electrical anisotropy | A difference in vertical and horizontal resistivity within a formation and at the scale of the resistivity measurement. Although there are several possible types of anisotropy, the term usually is used when the electrical properties are the same in all horizontal directions, but different in the vertical direction. For horizontal beds, this type of anisotropy is more fully known as transverse isotropy with a vertical axis of symmetry (TIV). The term may also refer to a difference in resistivities measured parallel and perpendicular to bedding. |
| Formation Evaluation | electrical log | A log recorded using an electrical wireline. In this sense, the term refers to any log recorded on a wireline, whether it measures an electrical quantity or not. The term dates from the early days of logging when the only logs were the spontaneous potential and resistivity from conventional electrode devices. |
| Formation Evaluation | electrical log | A wireline log of formation resistivity produced by a simple, unfocused arrangement of current emitting and measure electrodes. Conventional electrical devices have four electrodes-current emitting (A), current return (B), measure (M) and measure reference (N)-which may be placed in different configurations. The two most common configurations are the normal and lateral. The currents used are low frequency, typically less than 500 Hz. H.G. Doll recorded the first electrical log on September 5, 1927, in the Pechelbronn field, Alsace, France. |
| Formation Evaluation | electrical survey | A particular combination of a spontaneous potential log and three electrical logs consisting of a 16-in. [40-cm] short normal, a 64-in. [162-cm] long normal and an 18-ft, 8-in. [5.7-m] lateral. With this combination, it is possible to correct for the effects of invasion in many average logging environments. This combination is sometimes called a conventional electrical log or survey, or simply electrical log, and is also referred to as the ES. |
| Formation Evaluation | electrode device | A logging tool based on an arrangement of simple metallic electrodes working at low frequency (less than 500 Hz). The term includes conventional electrical logs, laterologs, micrologs and other microresistivity logs. Electrode devices are used for both wireline and measurements-while-drilling logs. In all electrode devices, a current (IO) and a voltage (VO) are measured on the appropriate electrodes or combinations of electrodes. The apparent formation resistivity is then determined by: Ra = K VO / IO, where K is a system constant for the device concerned. |
| Formation Evaluation | electrode resistivity | The resistivity measured by an electrode device. The term often is used to distinguish this resistivity from that measured by the measurements-while-drilling propagation resistivity, or the wireline induction resistivity. The term may also refer to measurements-while-drilling toroid devices, such as bit resistivity, ring resistivity and button resistivity. These devices do not use electrodes but have a similar response. Electrode resistivity measurements respond to resistivity, not conductivity. They are therefore best at measuring true formation resistivity at high resistivities, high contrasts in resistivity between formation and drilling mud, and for conductive invasion. These are conditions that are not well covered by propagation and induction resistivity. |
| Formation Evaluation | electrokinetic potential | The electromagnetic force, in millivolts, generated by an electrolyte flowing through a permeable medium. This potential is an unwanted contribution to the spontaneous potential (SP) log. In principle, there is no flow into a permeable reservoir at the time of logging, since the mudcake has isolated the reservoir from the borehole. However, it is possible for mudcakes and shales to produce an electrokinetic potential at the time of logging. In normal conditions, any potential is small and equal along the borehole, so that the effect on the SP is negligible. A significant electrokinetic potential can be generated in particular conditions, for example, high differential pressure or poor mudcakes. |
| Formation Evaluation | electromagnetic propagation | Pertaining to logs that measure the properties of electromagnetic waves as they move through a formation. Measurements-while-drilling (MWD) propagation resistivity logs work between about 100 kHz and 10 MHz. Dielectric propagation logs work between 20 and 200 MHz. Logs made above 200 MHz and into the GHz range are known as electromagnetic propagation logs. Below about 100 kHz, the measurements are based on the properties of standing waves, not of propagation. Induction and laterolog tools work in this range. |
| Formation Evaluation | electromagnetic propagation measurement | A measurement of the high frequency (about 1 GHz) dielectric properties of the formation. In a typical tool, a microwave transmitter is placed a few inches below two receivers separated by 4 cm [1.6 in.]. At this frequency, the response is best explained as the propagation of a wave. Thus the phase shift and attenuation of the wave between the receivers are measured and transformed to give the log measurements of propagation time and attenuation. Because of the short spacings, the measurement has excellent vertical resolution and reads within inches of the borehole wall except at high resistivity. Different transmitter and receiver spacings and orientations are used, leading to different arrays, such as the endfire array and the broadside array. An ideal measurement would give the plane wave properties of the formation. However, the geometry of the measurement precludes this, so that a correction, known as the spreading-loss correction, is needed for the attenuation and to a much smaller extent for the propagation time. The measurement is also affected by the dielectric properties and thickness of the mudcake. Borehole compensation is used to correct for sonde tilt or a rough borehole wall. |
| Formation Evaluation | elemental capture spectroscopy | Referring to a log of the yields of different elements in the formation, as measured by capture gamma ray spectroscopy using a pulsed neutron generator. The log is a type of pulsed neutron spectroscopy log that uses only the capture spectrum. The capture spectrum is formed by many elements, but since the main purpose of the log is to determine lithology, the principal outputs are the relative yields of silicon, calcium, iron, sulfur, titanium and gadolinium. The yields give information only on the relative concentration of these elements. To get absolute elemental concentrations, it is necessary to calibrate to cores, or, more often, use a model such as the oxide-closure model. The depth of investigation of the measurement is several inches into the formation. It can be run in open or cased hole. The absolute elemental concentrations are insensitive to fluids in the borehole and formation. |
| Formation Evaluation | endfire array | A particular arrangement of transmitters and receivers used in the electromagnetic propagation measurement in which the dipoles used as sensors are oriented along the axis of the tool. The orientation is combined with relatively long spacings to give deeper penetration, and hence less effect of mudcake or rugosity. |
| Formation Evaluation | enhanced diffusion | A technique in nuclear magnetic resonance (NMR) logging based on a long echo spacing, specially chosen to enhance the diffusion of formation water. Echo spacings in standard NMR logs are too short to allow any significant diffusion relaxation from water. Long echo spacings, for example 3 s, cause diffusion relaxation to limit the maximum T2 from water. They also limit the maximum T2 from light hydrocarbons. However, there is a certain range of viscosity of medium oil that is less affected. The enhanced diffusion technique therefore permits the identification of some medium oils. |
| Formation Evaluation | environmental corrections | The adjustments that must be made to log measurements to bring them back to the standard conditions for which the tool has been characterized. Different measurements require different corrections. For example resistivity measurements usually require correction for the borehole, invasion and shoulder beds, and may also be corrected for apparent dip, anisotropy and surrounding beds in horizontal wells. Density measurements require correction only for borehole size, while neutron porosity measurements require corrections for temperature, pressure and a large number of borehole and formation parameters. Not all corrections are significant in all cases. Corrections can be calculated manually, using charts, or applied through software. Conventionally, corrections are applied sequentially, as for example first borehole then invasion. In some situations, such as the combination of deep invasion and high apparent dip on a resistivity measurement, the corrections are too interdependent for sequential application to be accurate. The solution is iterative forward modeling. |
| Formation Evaluation | epithermal neutron porosity measurement | A measurement based on the slowing down of neutrons between a source and one or more detectors that measure neutrons at the epithermal level, where their energy is above that of the surrounding matter, between approximately 0.4 and 10 eV. The slowing-down process is dominated by hydrogen, and is characterized by a slowing-down length. By measuring the neutrons at the epithermal level, rather than the thermal level, the response is a purer estimate of hydrogen index, unaffected by thermal absorbers. On the other hand, the count rate is smaller for the same source and source-detector spacing. Epithermal measurements have been made with both the compensated neutron technique and by using a pad pressed against the borehole wall with detectors focused into the formation. |
| Formation Evaluation | equivalent conductance | With reference to the conductivity of brines, the ionic conductivity provided by a unit ion concentration. The conductivity of the brine (Cw, in siemens per meter or S/m) is determined by the equivalent conductance, (B, in S/m per meq/cm3) and the ion concentration (N, in milli-ion equivalent, or more commonly milliequivalent, meq per cm3) as Cw = B * N. |
| Formation Evaluation | equivalent water resistivity | The effective resistivity of the formation water or the mud filtrate, as it affects the electrochemical potential. The electrochemical potential is the main source of the spontaneous potential log. The equivalent water resistivity, or equivalent resistivity, is defined as the reciprocal of the activity of a solution, so that the formula for the electrochemical potential, Ec, can be written as: Ec = – K log10 (Rmfe / Rwe) where K is a coefficient, and Rmfe and Rwe are the equivalent water resistivities. The importance of Rmfe and Rwe is that they are equal to the actual water resistivities for NaCl solutions above about 0.1 ohm-m. In salty waters below 0.1ohm-m, the equivalent resistivity is lower by an amount that depends on temperature and salinity. In fresh waters, divalent ions such as Ca++ and Mg++ have a strong effect on Ec. Rwe is then related to Rw by an empirical transform that assumes average concentrations of these ions. For fresh mud filtrates, it has been common practice to set Rmfe = 0.85 * Rmf. |
| Formation Evaluation | excavation effect | That part of the effect of gas on the neutron porosity measurement that is not explained by differences in hydrogen index. By using the concept of hydrogen index, the only significant contributor to the neutron porosity in a gas zone is the liquid-filled porosity, since the hydrogen indices of gas and matrix are close to zero. However, the resultant liquid-filled porosity is found to be too low. The error comes from treating the gas-filled porosity as matrix. If this matrix is excavated and replaced with gas, the correct response can be predicted. |
| Formation Evaluation | far water | Water that is far from the clay surface, as distinct from clay-bound water (or “near” water). The term is used in the dual-water model. It includes the capillary-bound water and the free water. |
| Formation Evaluation | fast diffusion | The rapid rate of diffusion of molecules in pore fluids during a nuclear magnetic resonance (NMR) measurement. In fast diffusion, the hydrogen within a certain volume diffuses fast enough that only one T2 peak is observed for the whole volume. This is the case in a single pore, because the surface relaxation is not strong enough for observation of separate T2 peaks, for example, for water near the surface of a grain and water in the middle of the pore. Fast diffusion is also considered to occur between most clay- and capillary-bound water, between normal pores and micropores within some carbonates, and in some other systems. |
| Formation Evaluation | fast formation | A formation where the velocity of the compressional wave traveling through the borehole fluid is less than the velocity of the shear wave through the surrounding formation. In such conditions a shear head wave is generated, so that standard techniques based on monopole transducers can be used to measure formation shear velocity. In hard formations, several normal modes are excited in addition to the Stoneley and leaky modes. |
| Formation Evaluation | fast formation arrival | An early signal in a cement-bond log. In some formations, particularly carbonates of low porosity, it is possible that the first acoustic signal to arrive at the receiver passes through the formation rather than through the casing, and hence its amplitude is unrelated to the cement bond. This manifests itself by a shortening of the transmitter-to-receiver traveltime and by anomalous patterns on the variable-density log. In such cases, it may be assumed that the cement bond is good, as the signal would be unlikely to be transmitted through the formation with sufficient amplitude to be detected if cement bond were poor. |
| Formation Evaluation | fast neutron reaction | A neutron interaction in which the neutron is absorbed by the target nuclei, which then emit nuclear particles such as alpha or beta particles, gamma rays, protons or additional neutrons. Fast neutron reactions have a small probability of occurrence relative to the other principal interactions, except at high neutron energy. |
| Formation Evaluation | fast-formation arrival | An early signal in a cement-bond log. In some formations, particularly carbonates of low porosity, it is possible that the first acoustic signal to arrive at the receiver passes through the formation rather than through the casing, and hence its amplitude is unrelated to the cement bond. This manifests itself by a shortening of the transmitter-to-receiver traveltime and by anomalous patterns on the variable-density log. In such cases, it may be assumed that the cement bond is good, as the signal would be unlikely to be transmitted through the formation with sufficient amplitude to be detected if cement bond were poor. |
| Formation Evaluation | fast-neutron reaction | A neutron interaction in which the neutron is absorbed by the target nuclei, which then emit nuclear particles such as alpha or beta particles, gamma rays, protons or additional neutrons. Fast neutron reactions have a small probability of occurrence relative to the other principal interactions, except at high neutron energy. |
| Formation Evaluation | filtrate slump | The downward vertical movement of filtrate with time after invasion. In hydrocarbon zones, the filtrate is heavier than the formation fluid. Therefore, in a vertical well, gravity causes the filtrate to sink to the bottom of a permeable zone, while the hydrocarbons move back to the borehole at the top. In a horizontal well, the mud filtrate will sink below the well, leaving hydrocarbons above it. The amount of movement depends, among other factors, on the time since invasion, the fluid mobilities and the difference in fluid densities. In water zones, the direction of movement depends on the relative densities of filtrate and formation water. In the more usual case of fresh filtrate and salty formation water, the filtrate will move upwards. |
| Formation Evaluation | first reading | The depth of the first reliable reading of a curve on a log. For the typical bottom-to-top survey, the curve readings before the tool is picked up from the bottom of the hole are not reliable–they are straight lines that do not represent the formation at the depth indicated. With several logging tools in a tool string, the first reading of each curve will be at a different depth, depending on the measure point of each tool. |
| Formation Evaluation | fish | The surface electrode used as the reference electrode for the spontaneous potential (SP) measurement. The metal electrode is attached to the end of a long electric cable and typically placed in the mud pit, or, in the case of an offshore rig, in the sea. The SP is a measurement of the natural electrical potential between an electrode in the well and the fixed reference electrode on surface. |
| Formation Evaluation | fishing bell | Also known as the head, the device that connects the end of the logging cable or the bridle to the top of the logging tool. It contains the weak point, so that when the weak point is broken and the cable removed, the uppermost assembly left in the hole is the head. The top of the head is specially designed to ease fishing of the logging tool. |
| Formation Evaluation | flexural mode | A type of acoustic propagation along the borehole that is visualized as a shaking of the borehole across its diameter. The flexural mode is excited by a dipole source, and measured by dipole receivers oriented in the same direction. Its speed is chiefly a function of the formation shear velocity, the borehole size and fluid velocity, and the frequency. It is used to estimate formation shear velocity, and is the only technique available in slow formations where shear velocity is less than borehole-fluid velocity. In this situation, shear head waves are not generated by a monopole source, so that standard monopole techniques cannot be used. The flexural wave is sensitive to properties of the altered zone, as well as to formation anisotropy, whether intrinsic or stress-induced. |
| Formation Evaluation | fluoroscopy | A technique for imaging a core by moving a core between a source of X-rays and a fluorescent screen. The image on the screen is intensified and recorded by a video camera. |
| Formation Evaluation | flushed zone | The volume close to the borehole wall in which all of the moveable fluids have been displaced by mud filtrate. The flushed zone contains filtrate and the remaining hydrocarbons, the percentage of the former being the flushed-zone water saturation, Sxo. In simple models, the flushed zone and the invaded zone are synonymous. |
| Formation Evaluation | flushed zone water saturation | The fraction of water in a given pore space in the flushed zone. It is expressed in volume/volume, percent or saturation units and is given the symbol Sxo. Unless otherwise stated, the pore space concerned is usually the effective porosity. If the pore space concerned is the total porosity, the saturation is more correctly known as the total flushed-zone water saturation; or if it is the effective porosity, the effective flushed-zone water saturation. |
| Formation Evaluation | flushed-zone water saturation | The fraction of water in a given pore space in the flushed zone. It is expressed in volume/volume, percent or saturation units and is given the symbol Sxo. Unless otherwise stated, the pore space concerned is usually the effective porosity. If the pore space concerned is the total porosity, the saturation is more correctly known as the total flushed-zone water saturation; or if it is the effective porosity, the effective flushed-zone water saturation. |
| Formation Evaluation | formation | A general term for the rock around the borehole. In the context of formation evaluation, the term refers to the volume of rock seen by a measurement made in the borehole, as in a log or a well test. These measurements indicate the physical properties of this volume. Extrapolation of the properties beyond the measurement volume requires a geological model. |
| Formation Evaluation | formation exposure time | The time that has elapsed between the bit first penetrating a formation and a log being recorded opposite the formation. In logging-while-drilling operations, this time is different for each log, since it depends on the drilling rate and the distance between the bit and the particular logging sensor. |
| Formation Evaluation | formation factor | The ratio of the resistivity of a rock filled with water (Ro) to the resistivity of that water (Rw). G.E. Archie postulated that the formation factor (F) was a constant independent of Rw and solely a function of pore geometry (the Archie equation I). It has since been shown that F is independent of Rw only for a certain class of petrophysically simple rocks (Archie rocks). In rocks with conductive minerals, such as shaly sands, there is a more complex dependence. In such cases, the ratio Ro/Rw is known as the apparent formation factor and may vary with Rw , temperature and the type of ion in solution. The intrinsic formation factor is then defined as F corrected for the effect of shale, or else the value of Ro/Rw at the limit of high salinity (low Rw ). The correction for the effect of shale depends on the saturation equation used, for example Waxman-Smits, dual water, SGS or CRMM. Unless otherwise stated, the term formation factor usually refers to the apparent formation factor. F has been related to porosity (phi) by several formulae (Archie, Humble and others) that have the general expression F = a / phim, where a is a constant and m the porosity exponent. |
| Formation Evaluation | formation water | Water in the undisturbed zone around a borehole. The resistivity and other properties of this water are used in the interpretation of measurements made in the borehole or from the surface. Although formation water normally is the same as the geological formation water, or interstitial water, it may be different because of the influx of injection water. |
| Formation Evaluation | forward modeling | The technique of determining what a given sensor would measure in a given formation and environment by applying a set of theoretical equations for the sensor response. Forward modeling is used to determine the general response of most electromagnetic logging measurements, unlike nuclear measurements whose response is determined mainly in laboratory experiments. Forward modeling is also used for interpretation, particularly in horizontal wells and complex environments. In this case, iterative forward modeling is used. The set of theoretical equations (the forward models) can be 1D, 2D or 3D. The more complex the geometry, the more factors can be modeled but the slower the computing time. |
| Formation Evaluation | Fourier transform infrared spectroscopy | A technique for quantitative mineralogical analysis of a sample of rock by measuring the effect of midrange infrared radiation transmitted through the sample. This radiation excites vibrations in the chemical bonds within the mineral molecules at particular frequencies characteristic of each bond. The transmitted radiation is compared with the spectral standards for a wide variety of minerals to determine the abundance of each mineral in the sample. Typically, a core plug is ground finely and a small (approximately 1 g) representative sample selected and dispersed in a potassium bromide matrix for the measurement. |
| Formation Evaluation | free fluid | Fluid in the pore space that can flow under normal reservoir conditions. This fluid may include water, oil or gas, and will flow on production, injection or invasion. When the term is used in connection with nuclear magnetic resonance measurements, it refers to the signal that occurs above a certain cutoff, typically 33 ms in sandstones and 100 ms in carbonates. The source of this signal is free water and oil with a viscosity below about 60 cp in sandstones, and 30 cp in carbonates. Note that, contrary to the sense of “free,” this oil may or may not be residual under normal reservoir conditions. |
| Formation Evaluation | free induction decay | In a nuclear magnetic resonance measurement, the decay, or relaxation, caused by dephasing in an inhomogeneous magnetic field. Since this relaxation is not related to formation properties, it is unwanted and corrected by using the CPMG pulse sequence. |
| Formation Evaluation | free water | Water in the pore space that can flow under normal reservoir conditions. When used in connection with nuclear magnetic resonance (NMR) measurements, free water is all the water that is not clay bound, capillary bound or in mineral hydrates. The latter is in any case excluded as it relaxes too fast to be measured by NMR. When used in connection with the dual-water model, the term means the far water. |
| Formation Evaluation | free-induction decay | In a nuclear magnetic resonance measurement, the decay, or relaxation, caused by dephasing in an inhomogeneous magnetic field. Since this relaxation is not related to formation properties, it is unwanted and corrected by using the CPMG pulse sequence. |
| Formation Evaluation | fresh core | A core that is in the same state as when it was brought to the surface. A fresh core is sealed as soon as possible after retrieval from the well to minimize the loss of fluids and exposure to air. The term implies that the core is analyzed before being stored, after storage it is known as preserved core. Since the purpose is to minimize alteration, a fresh core has often been drilled with a bland mud, either water- or oil-base, but with a minimum of chemical additives and weighting material. |
| Formation Evaluation | fresh water | Formation water with low salinity. Water is considered fresh when its low conductivity makes the interpretation of resistivity logs difficult. The salinity at which this becomes important depends on temperature and clay content, among other factors, but is generally somewhere less than 10 ppk. |
| Formation Evaluation | FTIR | A method for obtaining quantitative mineralogical analysis of a rock sample by measuring the effect of midrange infrared radiation transmitted through the sample. |
| Formation Evaluation | full waveform | A log or a recording in which the complete signal received at an acoustic transducer is recorded. With full-waveform recording, it is possible to determine the slowness not only of the first arrival but also of later arrivals. In borehole sonic logging, these may be the shear, flexural and Stoneley waves. The waveforms are recorded by an array of receivers in an array-sonic tool, and processed with a suitable technique such as slowness-time coherence. |
| Formation Evaluation | Gal | The unit of acceleration commonly used in gravity surveying. 1 Gal (1000 milliGal or 106 microGal) = 1 cm/sec/sec. |
| Formation Evaluation | gamma ray interactions | Phenomena resulting from the transfer of energy from a gamma ray to matter, usually to an electron. The probability of a specific interaction occurring depends on the atomic number of the material and the energy of the gamma ray. In formation evaluation, there are two types of interactions of interest: the photoelectric effect, which indicates lithology, and Compton scattering, which depends on formation density. The third type of interaction, pair production, occurs at energies above those used for logging. |
| Formation Evaluation | gamma ray log | A log of the total natural radioactivity, measured in API units. The measurement can be made in both openhole and through casing. The depth of investigation is a few inches, so that the log normally measures the flushed zone. Shales and clays are responsible for most natural radioactivity, so the gamma ray log often is a good indicator of such rocks. However, other rocks are also radioactive, notably some carbonates and feldspar-rich rocks. The log is also used for correlation between wells, for depth correlation between open and cased hole, and for depth correlation between logging runs. The gamma ray log was the first nuclear well log and was introduced in the late 1930s. |
| Formation Evaluation | gamma-gamma log | A measurement that uses a source of gamma rays and a detector of gamma rays. The term is synonymous with density log, and is just an older name. |
| Formation Evaluation | geochemical log | A log of elemental concentrations from which the geochemistry of the formation may be derived. Several logs provide information on elemental weight concentrations: natural gamma ray spectroscopy, elemental capture spectroscopy or pulsed neutron spectroscopy and aluminum activation. The combination of some or all of their outputs is known as a geochemical log, since it provides information on most of the principal elements found in sedimentary rocks. Pulsed neutron spectroscopy provides relative elemental yields, whereas absolute concentrations are needed for quantitative results. Absolute concentrations can be derived by calibration to core or by using a model such as the oxide-closure model. The absolute elemental concentrations can then be converted into mineral concentrations using a model that defines what minerals are present. The first complete geochemical logs were run in the mid 1980s. |
| Formation Evaluation | geometrical factor | The response of a logging measurement as a function of distance from the tool. The geometrical factor can be radial, reflecting the response perpendicular to the tool; vertical, reflecting the response along the tool axis; or two-dimensional, reflecting both. It can also be a differential geometrical factor, which is the contribution to the signal at a particular distance; or integrated, which is the sum of all signals from the tool to a particular distance. The term geometrical factor was introduced for induction logging since it gave a convenient method for computing the reading in a heterogeneous environment. For example, in an invaded formation the log reading, Clog, can be written as: Clog = Gi * Cxo + (1 – Gi) * Ct where Gi is the geometrical factor for a diameter of invasion Di, and Cxo and Ct are the conductivities of the invaded zone and the undisturbed zone. A true geometrical factor depends only on the geometry of the volume concerned, which in practice is only true for induction logs at zero conductivity. However, the term has come to be used for other cases and for other measurements even though the geometrical factor depends significantly on formation properties. The correct term for these cases is the pseudogeometrical factor. |
| Formation Evaluation | GR | An abbreviation for gamma ray, usually with reference to the gamma ray log. |
| Formation Evaluation | grain density | The density of the grains in a formation or core sample. As used in log and core analysis, the term ‘grain’ refers to all the solid material in the rock, since, when interpreting the measurements, no effort is made to distinguish grains from other solid material. The grain density of core samples is calculated from the measured dry weight divided by the grain volume. In logs, grain density is calculated from the density log, using an estimate of porosity and knowledge of the fluid content. |
| Formation Evaluation | gravity units | One-tenth of a milliGal: (10 g.u. = 1.0 mGal). Gravity units are sometimes used in old gravity maps. |
| Formation Evaluation | guard electrode | An electrode on a laterolog sonde that focuses the current sent by the central current-emitting electrode (A0). The guard electrode is held at the same potential as A0, thereby forcing the current from A0 to run approximately perpendicular to the sonde into the formation, and preventing it from running up the borehole to a great extent. |
| Formation Evaluation | guard log | A measurement made by a type of electrode device in which the current flow and hence the measurement is focused in a disk that is concentric with and perpendicular to the sonde. The term usually refers to a Laterolog-3 device. Guard logs may be recorded by either wireline or measurements-while-drilling tools. |
| Formation Evaluation | head | The device that connects the end of the logging cable or the bridle to the top of the logging tool. It contains the weak point, so that when the weak point is broken and the cable removed, the uppermost assembly left in the hole is the head. The top of the head is specially designed to ease fishing of the logging tool, and is also known as the fishing bell. |
| Formation Evaluation | head wave | A pressure wave in the borehole fluid generated by the passage of either the acoustic compressional wave or the shear wave in the formation. These pressure waves are recorded by logging tools using hydrophones and are the basis for the sonic log. A head wave is generated only when the compressional or the shear speed is faster than the fluid speed. In slow formations, where the shear speed is less than the fluid speed, no shear head wave is created. |
| Formation Evaluation | heading | The first page or pages on a log print, which include information about the well, the survey, the mud properties and other relevant data. |
| Formation Evaluation | horizontal resistivity | The resistivity of a formation measured by flowing current in a horizontal plane. In anisotropic formations the horizontal and vertical resistivities are different. In a vertical well, wireline induction logs and measurements-while-drilling propagation logs measure the horizontal resistivity, whereas laterologs measure the horizontal resistivity with some component of the vertical. In deviated and horizontal wells, all these logs measure some mixture of both vertical and horizontal resistivity. |
| Formation Evaluation | housing | The outside steel case of a cartridge or a sonde in a wireline logging tool. The housing isolates the electronics, power supplies and sensors from the borehole and bears the pressure burden. |
| Formation Evaluation | Humble formula | A particular relation between the formation factor (F) and porosity (phi) proposed by the Humble Oil Company. The original formula was expressed as F = 0.62 / phi2.15. A nearly equivalent form, with a simpler porosity exponent, is F = 0.81 / phi2. These formulae are considered most suitable for relatively high-porosity, sucrosic, or granular, rocks. See Winsauer WO, Shearin HM, Masson PH and Williams M: Resistivity of Brine-Saturated Sands in Relation to Pore Geometry, AAPG Bulletin 36 (1952): 253-277. |
| Formation Evaluation | hybrid scale | An early scale used for the presentation of resistivity logs. The scale has two parts, equally divided about a midpoint. The left part is linear in resistivity, for example 0 on the left edge to 50 ohm-m at the midpoint. The right part is linear in conductivity, from 0 on the right to 1/50 = 20 mS/m at the midpoint. In this way, it was possible to display the complete range of resistivity in one track. It was subsequently replaced by the logarithmic scale. |
| Formation Evaluation | hydrogen index | The number of hydrogen atoms per unit volume divided by the number of hydrogen atoms per unit volume of pure water at surface conditions. The hydrogen index (HI) is thus the density of hydrogen relative to that of water. It is a key factor in the response of a neutron porosity log. |
| Formation Evaluation | ID | A particular type of induction log that was designed to read deep into the formation while maintaining reasonable vertical resolution. The deep induction log (ID) is based on the measurement of a 6FF40 array and was combined with a medium induction array to form the dual induction tool. Versions built after 1968 had a small extra transmitter coil to reduce the borehole effect on the medium induction while changing the deep response very little. The midpoint of the ID integrated radial geometrical factor is at 62 in. [157 cm] radius for high resistivities, reducing to 45 in. [114 cm] at 1 ohm-m. ID receives very little signal from within 20 in. [50 cm] of the tool. The vertical resolution is about 8 ft [2.4 m] but varies with local conditions. |
| Formation Evaluation | IM | A particular type of induction log designed to read an intermediate distance into the formation while maintaining good vertical resolution. The medium-induction array of eight coils (IM) is produced by three transmitters and five receivers running at 20 kHz. A small fourth transmitter coil was added in tools built since 1968. The midpoint of the integrated radial geometrical factor is 30 in. [76 cm] in radius. The vertical resolution is about 4 ft [1.2 m] but varies with conditions. The IM is combined with a deep-induction log on the same sonde to produce a dual induction log. |
| Formation Evaluation | in situ fluid analysis | Analysis performed by downhole tools to determine physical and chemical properties of fluids. Typical analyses that can be performed downhole include basic density and viscosity measurements at sampling pressure and temperature. |
| Formation Evaluation | in situ viscosity evaluation | Downhole measurement of fluid viscosity, typically performed either with logging tools based on nuclear magnetic resonance (NMR) or with sampling tools such as formation testers. |
| Formation Evaluation | induced gamma ray spectroscopy | The principle of an activation log, which is a log of elemental concentrations derived from the characteristic energy levels of gamma rays emitted by a nucleus that has been activated by neutron bombardment. The term is often used to refer specifically to the pulsed neutron spectroscopy measurement. |
| Formation Evaluation | induction | Related to a wireline log of formation resistivity based on the principle of inducing alternating current loops in the formation and measuring the resultant signal in a receiver. In the simplest device, an alternating current of medium frequency (10?s of kHz) is passed through a transmitter coil, thereby inducing an alternating magnetic field in the formation. This field creates current loops in the formation. The loops produce their own magnetic field, which induce a current when they cross the receiver coil. This signal is proportional to the conductivity of the formation, with contributions from different regions of the formation summing approximately in conductivity. As a result, the induction log is most accurate at high conductivities and with resistive invasion. However, below about 1 ohm-m skin effect becomes important. Practical induction-logging tools use arrays of several coils, designed to achieve a specific focusing and depth of investigation. These arrays are either hardwired, such as the 6FF40, 5FF40 and others, or consist of several simple arrays that are combined in software (an array induction). For many years, the most common induction log was the 6FF40. This was often combined with the medium induction and a shallow laterolog or microresistivity log so as to correct for the effect of invasion, assuming a step profile. Induction logs also need borehole correction and shoulder-bed correction. In older tools, this was accomplished through multiple correction charts, while modern tools include software for this purpose. H.G. Doll introduced the first practical induction-logging technique in 1949. See Doll HG: Introduction to Induction logging and Application to Logging of Wells Drilled with Oil Base Mud Journal of Petroleum Technology 1, no. 6 (June 1949): 148-162. |
| Formation Evaluation | induction electrical survey | A combination of a 6FF40 induction log with an electrode measurement such as a 16-in. [40-cm] short normal. In some cases other induction arrays, such as the 5FF27, may have been used. The use of both measurements in the induction electrical survey (IES) gave a qualitative indication of invasion. |
| Formation Evaluation | inelastic neutron scattering | A neutron interaction in which part of the kinetic energy lost by a neutron in a nuclear collision excites the nucleus. The excited nucleus will usually emit characteristic gamma rays upon de-excitation. Inelastic neutron scattering is possible only if the neutron energy exceeds a characteristic threshold for the element. Inelastic neutron scattering is the principle behind the carbon-oxygen log, which is used to determine water saturation behind casing. |
| Formation Evaluation | inertial resistance | The extra resistance of a porous medium to fluid flow, beyond that predicted by Darcy’s law, caused by local accelerations within the tortuous pore volume. The inertial resistance is proportional to the fluid density times the flow rate. The inertial resistance is significant with gas because flow rates can be high. In laboratory measurements of permeability, inertial resistance can be important for high-permeability samples where high flow rates are needed to have significant pressure gradients. The effect is corrected by using the Forchheimer equation and making measurements at several flow rates (often through an unsteady state test). |
| Formation Evaluation | insert | A section on a log print that gives the scales of the curves displayed and the depth scale. There usually is an insert at the beginning and end of each interval surveyed. |
| Formation Evaluation | in-situ fluid analysis | Analysis performed by downhole tools to determine physical and chemical properties of fluids. Typical analyses that can be performed downhole include basic density and viscosity measurements at sampling pressure and temperature. |
| Formation Evaluation | invaded zone | The volume close to the borehole wall in which some or all of the moveable fluids have been displaced by mud filtrate. It consists of the flushed zone and the transition zone or annulus. In simple models, the invaded zone and the flushed zone are considered synonymous. |
| Formation Evaluation | invasion | The process by which mud filtrate, and sometimes whole mud, enters a permeable formation. The mud filtrate displaces some or all of the moveable fluids in the formation, leaving an invaded zone. The invasion process is complex. It is generally considered to start with a short initial spurt loss when the bit penetrates the rock. During this period, invasion depends on formation permeability, among other factors. A mudcake is soon formed, after which invasion is either described as dynamic, when mud is being circulated, or static, when it is not. In both cases, the volume of invasion depends little on formation properties and strongly on other factors such as mudcake permeability and differential pressure. However, the profile of the invasion front within the formation, both vertically and radially, does depend on formation properties. With high permeability and with different densities of filtrate and formation fluid, gravity can cause vertical movement of the filtrate, leading to different depths of invasion at the top and bottom of a zone. With two moveable phases (oil and water) and differing permeabilities, an annulus can be formed. The radial profile from the wellbore out to the undisturbed zone depends on permeability, with lower permeabilities leading to sharper transitions. |
| Formation Evaluation | inverse | Referring to a type of conventional electrical log in which the current-emitting and the current-return electrodes (A and B) are placed close together on the sonde, with the measure electrode (M) several feet away and the measure return (N) far away. This arrangement is sensitive to the potential gradient between A and B. The spacing is defined by the distance from M to the midpoint between A and B. The most common spacing is 18 ft, 8 in. [5.7 m]. The lateral gives a sharper response to a bed boundary than a normal but also introduces several artifacts that can give misleading results. |
| Formation Evaluation | inverse filter | Generally, a finite impulse response (FIR) filter that has been designed to transform the usually irregular vertical response functions of raw measurements into a smooth, well-behaved response function such as a Gaussian response or a Kaiser window function. The criteria for designing inverse filters can include vertical response, depth of investigation and near-field (cave effect) response. Inverse filters have been used for many years to improve the response of induction arrays. |
| Formation Evaluation | irreducible water | The lowest water saturation, Swi, that can be achieved in a core plug by displacing the water by oil or gas. The state is usually achieved by flowing oil or gas through a water-saturated sample, or spinning it in a centrifuge to displace the water with oil or gas. The term is somewhat imprecise because the irreducible water saturation is dependent on the final drive pressure (when flowing oil or gas) or the maximum speed of rotation (in a centrifuge). The related term connate water saturation is the lowest water saturation found in situ. |
| Formation Evaluation | isolated porosity | The pore volume not connected to the pore network. Isolated porosity can be significant in volcanic rocks and some carbonates, for example as vugular, moldic and intraparticle porosity. |
| Formation Evaluation | iterative forward modeling | The use of repeated forward modeling of a logging tool response to produce modeled logs that very closely match the measured logs. The final model is then the log analyst?s best estimate of the formation properties. Iterative forward modeling is a hand-operated inversion. The technique is used mainly for laterologs and induction logs when the formation or the environment are complex, so that the environmental effects cannot be separated and treated individually by automatic inversion. Iterative forward modeling allows the log analyst to use local knowledge and petrophysics to select between the many possible solutions that are mathematically correct. These cases occur most often in horizontal wells, or vertical wells with the combined effects of invasion and large resistivity contrast between beds. |
| Formation Evaluation | J factor | Another term for pseudogeometrical factor, the response of a logging measurement as a function of distance from the tool. The pseudogeometrical factor is normally radial, reflecting the response perpendicular to the tool. It can be a differential factor, which is the contribution to the signal at a particular distance, but is more normally integrated, which is the sum of all signals from the tool to a particular distance. The pseudogeometrical factor developed from the concept of the geometrical factor, and is expressed in the same way. For example, for a radial distance x from the tool, the integrated radial pseudogeometrical factor, Jx, can be written as: Jx = (Ux – Ut) / (Uxo – Ut) where Ut is the log reading of the undisturbed zone (or, alternatively, the reading with no invasion), Uxo is the log reading of the flushed zone (or, alternatively, the reading with infinite invasion), and Ux is the log reading with a step profile invasion to depth x. Unlike the geometrical factor, Jx depends on the values of both Uxo and Ut. Pseudogeometrical factors are a useful way to express the radial response (or vertical response) in typical conditions. The physics of each measurement determines how much Jx varies with Uxo and Ut. Pseudogeometrical factors are often used to express the response of nuclear and resistivity logs, but are not appropriate for acoustic and electromagnetic propagation logs (where the response is too dependent on the contrast in properties), or nuclear magnetic resonance logs (where the response is too localized). |
| Formation Evaluation | K coefficient | With reference to the spontaneous potential log, the coefficient, K, in the equation relating electrochemical potential to the chemical activity of the mud filtrate and formation water. Ec = – K log10 (aw / amf). The coefficient is equal to kT/e in which k is the Boltzman’s constant, e is the electron charge and T is the absolute temperature. K is equal to 71 at 25oC [77oF], 12 from the liquid junction potential and 59 from the membrane potential for a perfect shale. |
| Formation Evaluation | lag | The distance between the static measure point and the dynamic measure point of a logging measurement. For nuclear logs and any others that must be recorded over a significant time period, there is a difference between the measure point with the tool stationary and moving. If the tool is moving during this period, the effective center of measurement will be a certain distance from the point at which the measurement started. This distance is the lag. The lag depends on the logging speed and the sampling interval. |
| Formation Evaluation | laminated sand | A particular model, or equation, for deriving the water saturation from resistivity and other logs. The model assumes a laminar shale distribution and considers the total resistivity to be the sum in parallel of the sand and shale laminae. |
| Formation Evaluation | laser diffraction | A technique for analyzing the grain-size distribution of a core sample. A cleaned, disaggregated sample is dispersed in a carrier fluid. The grains cause diffraction of a laser beam directed through the fluid. The angle of scattering is inversely proportional to the particle size, while the intensity of scattering is proportional to the number of particles. Laser diffraction also may be referred to as laser sieve analysis. |
| Formation Evaluation | last reading | The depth of the last reliable reading of a log. For the normal bottom-to-top survey, the last reliable reading often occurs just before the logging tool enters the casing. With several logging tools in a tool string, the last readings will be at different depths, depending on the measure point of each measurement. |
| Formation Evaluation | lateral | Referring to a type of conventional electrical log in which the current-emitting and the current-return electrodes (A and B) are placed close together on the sonde, with the measure electrode (M) several feet away and the measure return (N) far away. This arrangement is sensitive to the potential gradient between A and B. The spacing is defined by the distance from M to the midpoint between A and B. The most common spacing is 18 ft, 8 in. [5.7 m]. The lateral gives a sharper response to a bed boundary than a normal but also introduces several artifacts that can give misleading results. |
| Formation Evaluation | leaky mode | A type of acoustic energy that propagates in one direction while being confined in the other two directions, in this case by the borehole wall. Leaky modes can be considered as multiply reflected and constructively interfering waves propagating in the borehole. Each time a compressional wave hits the borehole wall, part of the energy is reflected into the borehole, while the rest is converted to compressional or shear energy that radiates into the formation, hence the term ‘leaky’. Leaky modes are dispersive, starting at a certain cutoff frequency with the formation compressional velocity and increasing towards the borehole fluid velocity at high frequency. In slow formations, where no head wave is generated because the borehole fluid is faster than the formation compressional wave, the low-frequency end of the leaky mode can be used to determine formation compressional velocity. The term ‘hybrid mode’ is used to describe a form of leaky mode that is associated with an altered zone. |
| Formation Evaluation | limestone compatible scale | Display ranges chosen for the density and neutron porosity logs such that the two curves will overlay at all porosity values providing the matrix is pure calcite and the pores are filled with fresh water. The most common overlay spans two tracks, with the density reading from 1.95 to 2.95 g/cm3, and the neutron in limestone porosity units from 0.45 to ?0.15 vol/vol. |
| Formation Evaluation | limestone porosity unit | A transform from raw log data chosen so that a log recorded in these units will give the correct porosity of the formation, providing the matrix is pure calcite and the pores are filled with fresh water. The unit, which may be in vol/vol or p.u., is most commonly used for neutron porosity logs but may also be used for density and acoustic logs. The definition is strictly true only if all borehole and other environmental corrections have been applied. |
| Formation Evaluation | limestone-compatible scale | Display ranges chosen for the density and neutron porosity logs such that the two curves will overlay at all porosity values providing the matrix is pure calcite and the pores are filled with fresh water. The most common overlay spans two tracks, with the density reading from 1.95 to 2.95 g/cm3, and the neutron in limestone porosity units from 0.45 to ?0.15 vol/vol. |
| Formation Evaluation | liquid saturation method | A technique for measuring the pore volume of a core sample from the difference in its weight when dry and when saturated with a liquid. A clean, dry sample is weighed and then evacuated for several hours in a vacuum chamber, flushing with CO2 to remove remaining air if necessary. A de-aerated liquid is introduced into the chamber and pressured to ensure complete saturation. The saturated sample is then weighed again. The difference in weight divided by the density of the liquid is the connected, or effective, pore volume. It is also common to measure the weight of the sample when immersed in the liquid. The grain and bulk volume can then be calculated as in the buoyancy method. |
| Formation Evaluation | liquid-junction potential | The electromagnetic force generated by a boundary between solutions of high salinity and low salinity. In a permeable formation, a liquid-junction potential is generated between the invaded zone and the undisturbed zone when the mud filtrate and the formation water have different salinities. This potential is one component of the electrochemical potential, from which the spontaneous potential log is derived. The other, much larger component is the membrane potential at a shale boundary. The liquid-junction potential is reduced if there is clay in the permeable formation, since this generates another, local membrane potential with the opposite polarity to the liquid-junction potential. |
| Formation Evaluation | log | The measurement versus depth or time, or both, of one or more physical quantities in or around a well. The term comes from the word “log” used in the sense of a record or a note. Wireline logs are taken downhole, transmitted through a wireline to surface and recorded there. Measurements-while-drilling (MWD) and logging while drilling (LWD) logs are also taken downhole. They are either transmitted to surface by mud pulses, or else recorded downhole and retrieved later when the instrument is brought to surface. Mud logs that describe samples of drilled cuttings are taken and recorded on surface. |
| Formation Evaluation | log | To record a measurement versus depth or time, or both, of one or more physical quantities in or around a well. |
| Formation Evaluation | log | Associated with the information from a log. For example, a log print is a paper print on which log data have been recorded. |
| Formation Evaluation | log | The display of one or more log measurements on a strip of paper or film (a hard copy) with depth in one axis. In this sense, the term refers to the display not only of the measurement but of other relevant information. A typical log is presented on folded paper of indeterminate length, but about 8.5-in. [21.5-cm] wide. It consists of a heading, well sketch, logging tool sketch, insert, main log, repeat section and tail. When the term is used in this sense, each log measurement is usually referred to as a curve. |
| Formation Evaluation | logarithmic mean | The average value of a set of measurements, calculated by taking the logarithms of the measurements, finding the arithmetic average of the logarithms and then taking the antilogarithm of the average. |
| Formation Evaluation | logging | Pertaining to a log. |
| Formation Evaluation | logging run | An operation in which a logging tool is lowered into a borehole and then retrieved from the hole while recording measurements. The term is used in three different ways. First, the term refers to logging operations performed at different times during the drilling of a well. For example, Run 3 would be the third time logs had been recorded in that well. Second, the term refers to the number of times a particular log has been run in the well. Third, the term refers to different runs performed during the same logging operation. For example, resistivity and nuclear logs may be combined in one tool string and recorded during the first run, while acoustic and nuclear magnetic resonance logs may be recorded during the second run. |
| Formation Evaluation | logging tool | The downhole hardware needed to make a log. The term is often shortened to simply “tool.” Measurements-while-drilling (MWD) logging tools, in some cases known as logging while drilling (LWD) tools, are drill collars into which the necessary sensors and electronics have been built. Wireline logging tools are typically cylinders from 1.5 to 5 in. [3.8 to 12.7 cm] in diameter. Since the total length is more than can be conveniently handled in one piece, the logging tool is divided into different sections that are assembled at the wellsite. These sections consist of cartridges and sondes. Different measurements can be combined to make up a tool string. The total length of a tool string may range from 10 to 100 ft [3 to 30 m] or more. Flexible joints are added in long tool strings to ease passage in the borehole, and to allow different sections to be centralized or eccentralized. If the total length is very long, it is often preferable to make two or more logging runs with shorter tool strings. |
| Formation Evaluation | logging unit | The cabin that contains the surface hardware needed to make wireline logging measurements. The logging unit contains at the minimum the surface instrumentation, a winch, a depth recording system and a data recorder. The surface instrumentation controls the logging tool, processes the data received and records the results digitally and on hard copy. The winch lowers and raises the cable in the well. A depth wheel drives the depth recording system. The data recorder includes a digital recorder and a printer. |
| Formation Evaluation | long spacing sonic log | Log recorded by a sonic tool with a longer transmitter-to-receiver spacing (generally 10 to 15 ft) than a standard sonic tool. The rock near the borehole is sometimes altered by drilling fluids, stress relief, or both, causing a thin zone whose velocity is lower than that of the true formation. With standard spacings, the wave traveling through the altered zone may arrive first at the receiver, since this zone is closer to both transmitter and receiver. The increased spacing permits the wave traveling through the true formation to arrive first and be measured. The depth of investigation varies with slowness and transmitter-receiver spacing but is of the order of 2 to 3 ft. An increased transmitter-to-receiver spacing also allows better separation of waveforms relating to different acoustic waves, such as compressional, shear and Stoneley arrivals. |
| Formation Evaluation | longitudinal relaxation | During a nuclear magnetic resonance measurement, the loss of energy by hydrogen atoms in a rock as they align themselves with the static magnetic field. The atoms behave like spinning bar magnets so that when a static magnetic field is applied, they initially precess about the field. Then, through interactions with nuclei and electrons, they lose energy, or relax, and align themselves with the magnetic field. The relaxation of the hydrogen atoms does not occur immediately but grows exponentially with a time constant T1. There are two mechanisms for longitudinal relaxation, surface relaxation and bulk relaxation. |
| Formation Evaluation | long-spacing sonic log | Log recorded by a sonic tool with a longer transmitter-to-receiver spacing (generally 10 to 15 ft) than a standard sonic tool. The rock near the borehole is sometimes altered by drilling fluids, stress relief, or both, causing a thin zone whose velocity is lower than that of the true formation. With standard spacings, the wave traveling through the altered zone may arrive first at the receiver, since this zone is closer to both transmitter and receiver. The increased spacing permits the wave traveling through the true formation to arrive first and be measured. The depth of investigation varies with slowness and transmitter-receiver spacing but is of the order of 2 to 3 ft. An increased transmitter-to-receiver spacing also allows better separation of waveforms relating to different acoustic waves, such as compressional, shear and Stoneley arrivals. |
| Formation Evaluation | m | The exponent of porosity, m, in the relation of formation factor, F, to porosity, phi. In the Archie equation, F = 1 / phim, H. Guyod termed m the cementation exponent because m was observed to be higher in cemented rock. The more general term is porosity exponent. |
| Formation Evaluation | magnetic mud | A drilling mud with a significant magnetic susceptibility. The magnetic susceptibility may affect the response of some logging measurements, mainly the induction X signal and nuclear magnetic resonance logs. The most common magnetic muds contain iron filings or magnetite. Other paramagnetic minerals such as hematite and ilmenite may contribute, although their magnetic susceptibility is considerably less. |
| Formation Evaluation | maximum recorded temperature | The highest temperature recorded on a logging run. It is usually taken to be the bottomhole temperature for use in log interpretation. However, on the first logging run or runs after circulation, the mud may be hottest some distance above the bottom of the hole. |
| Formation Evaluation | measure point | The position on a logging tool that best represents the center of the vertical response of the measurement. For a simple single transmitter-single receiver measurement, the measure point is half-way between the transmitter and receiver. |
| Formation Evaluation | measure point | The point on a logging tool at which it is considered the logging measurement is made. It is the center of the vertical response, or in some cases an alternative, more suitable point. For measurements that must be recorded over a significant time period, there is a difference between the static and dynamic measure point, known as the lag. |
| Formation Evaluation | measurement after drilling | Measurements made by measurements-while-drilling (MWD) tools subsequent to the initial bit run. MWD logs are recorded while drilling the well. However, these tools can also record logs at later times when the drillstring is in the hole. This may be while pulling out after drilling, or on a subsequent bit run or circulating trip. The latter is also known as logging while tripping. |
| Formation Evaluation | measurement error | The difference between the true value and that which is reported from a measurement. |
| Formation Evaluation | measurement range | The range of values for a quantity for which the error of a measuring instrument is intended to lie within specified limits. Within this range, the measurement has a well-defined accuracy or applicability. Outside the range, it does not. It is distinct from the operating range, within which the instrument will provide a measurement but the error is not well-defined. |
| Formation Evaluation | medium induction | A particular type of induction log designed to read an intermediate distance into the formation while maintaining good vertical resolution. The medium-induction array of eight coils (IM) is produced by three transmitters and five receivers running at 20 kHz. A small fourth transmitter coil was added in tools built since 1968. The midpoint of the integrated radial geometrical factor is 30 in. [76 cm] in radius. The vertical resolution is about 4 ft [1.2 m] but varies with conditions. The IM is combined with a deep-induction log on the same sonde to produce a dual induction log. |
| Formation Evaluation | membrane potential | The electromagnetic force generated across an ion-selective membrane when solutions on either side of the membrane have different salinities. Shales and clays are cationic membranes, since they allow the passage of cations, such as Na+, but not anions, such as Cl-. When the drilling mud in the borehole and the formation water have different salinities, a membrane potential is generated at the boundary between a shale and a permeable formation. This potential is one component of the electrochemical potential, from which the spontaneous potential (SP) log is derived. The other, much smaller component is the liquid-junction potential. The membrane potential is reduced if the shale is not a good cationic membrane, or in other words has a low cation-exchange capacity. A membrane potential may also be generated across the mudcake if there is no flushed zone; for example if the mud filtrate has moved vertically since invasion took place, and by clay within a shaly sand, but with the opposite polarity to the normal SP potentials. The membrane potential is also used in core analysis to determine the cation-exchange capacity of a sample. In this case, the clay within the sample is the ion-selective membrane, and the potential generated across it is related to the cation-exchange capacity per unit pore volume, Qv. As a method of measuring Qv, the technique is faster than the multiple salinity method, and more representative of the in-situ value than destructive methods such as conductometric titration. However, care is needed in making the measurement and deriving the appropriate Qv. |
| Formation Evaluation | mercury displacement method | A technique for measuring the bulk volume of a core sample by observing the displacement of mercury in a chamber. The chamber is first filled to a reference level and the volume recorded. The sample is introduced and the new volume recorded. The difference is the bulk volume of the sample. If the sample is weighed, its bulk density can also be calculated. Mercury is used because it is strongly nonwetting and therefore does not enter the pore space. |
| Formation Evaluation | microcylindrical log | An electrode device with small spacings from which the current flow, and hence the measurement, is focused a short distance into the formation. The microcylindrical log measures the resistivity of the flushed zone with minimum influence from the mudcake or the undisturbed zone. The electrodes are mounted on a pad that is pressed against the borehole wall. The current is focused both parallel and perpendicular to the tool axis. Three measurements are made, each with a different depth of investigation. These measurements are combined to solve for the mudcake and flushed-zone resistivity. |
| Formation Evaluation | microlaterolog | An electrode device with small spacings from which the current flow, and hence the measurement, is focused a short distance into the formation. Introduced in 1953, the microlaterolog measures the resistivity of the flushed zone with minimum influence from the mudcake or the undisturbed zone. The central current emitting electrode (A0) is surrounded by a guard electrode that emits sufficient current to focus the current from A0 a certain distance into the formation. The electrodes are mounted on a pad that is pressed against the borehole wall. In a typical tool design, 90% of the signal comes from within 3 in. [7.6 cm] of the pad, ensuring that the undisturbed zone rarely has an effect. |
| Formation Evaluation | microlog | An unfocused electrode device with small spacings, mounted on a pad and pressed against the borehole wall. The typical microlog has one current-emitting electrode and two measure electrodes in line above it, one at 1 in. [2.5 cm], the other at 2 in. [5 cm]. The potential at the 2-in. electrode gives a 2-in. micronormal log. The difference in potential between the two measure electrodes gives a 1-in. x 1-in. microinverse log. The micronormal reads deeper than the microinverse. Introduced in 1948, the microlog is used to detect permeable zones across which a mudcake has formed. Since the mudcake is usually less resistive than the invaded zone, the microinverse will read less than the micronormal opposite permeable zones. If the resistivity and thickness of the mudcake are known, it is possible to estimate the resistivity of the flushed zone. The log is usually presented on a linear scale, chosen to emphasize the lower readings often seen opposite permeable zones with mudcake. |
| Formation Evaluation | microporosity | That part of the pore space that has a characteristic dimension less than 1 micron. In general, this includes not only very small pores but also the porosity associated with surface roughness. The water in this pore space is part of the capillary-bound water and the small-pore water. Water in micropores is not expected to flow on production. The term is also defined as porosity that cannot be seen at magnifications less than 50x. |
| Formation Evaluation | microresistivity | Related to a log of the resistivity of the flushed zone recorded by a wireline electrode device. The device is mounted on a pad and pressed against the borehole wall. Several designs exist, for example microlog, microlaterolog, proximity log, microspherical log and microcylindrical log. The microlog, being unfocused, is a more qualitative measurement. The other measurements are focused. They try to minimize the effect of mudcake and rugose hole, while reading as short a distance as possible into the formation, to remain unaffected by the undisturbed zone. They are usually combined with a laterolog or induction log to correct the latter for the effects of invasion and for saturation determination in quick-look ratio methods. The logs are presented on a logarithmic scale from, for example 0.2 to 2000 ohm-m. |
| Formation Evaluation | microspherical log | An electrode device with small spacings from which the current flow, and hence the measurement, is focused a short distance into the formation. The microspherical log measures the resistivity of the flushed zone with minimum influence from the mudcake or the undisturbed zone. The principle of spherical focusing is used. The electrodes are mounted on a pad that is pressed against the borehole wall. In a typical tool design, 90% of the signal comes from within 3 in. [7.6 cm] of the pad, ensuring that the undisturbed zone rarely has an effect. |
| Formation Evaluation | minute mark | An annotation made on a log print once every minute. By reading the depth interval between each minute mark, it is possible to check the logging speed. Minute marks are typically made by blanking out the vertical grid line on the far left of the print for a short interval every minute. |
| Formation Evaluation | monopole | Describing a type of acoustic transducer that emits or receives energy in all directions. Monopole transducers are used in standard sonic logs, and also in array-sonic logs to record shear and Stoneley waves. |
| Formation Evaluation | moveable hydrocarbons | The volume of hydrocarbons per unit volume of rock that can be moved on production, measured in volume/volume or porosity units. Typically only primary and secondary production methods are considered when estimating moveable hydrocarbons. Moveable hydrocarbons are not necessarily the same as moved hydrocarbons, which are those hydrocarbons that have been moved by invasion. |
| Formation Evaluation | moved hydrocarbons | The volume of hydrocarbons per unit volume of rock that have been moved by invasion, measured in volume/volume or porosity units. Moved hydrocarbons are not necessarily the same as moveable hydrocarbons, which are those hydrocarbons that can be moved on primary and secondary production. |
| Formation Evaluation | multiple salinity | A technique used for the determination of the electrical properties of a shaly core sample. The sample is flushed with brines of different salinities, and the conductivity determined after each flush. A plot of the conductivity of the sample (C0) versus the conductivity of the brine (Cw) gives the excess conductivity caused by clays and other surface conductors. Then, using a suitable model (Waxman-Smits, dual water, SGS) it is possible to determine the intrinsic formation factor and porosity exponent, and the cation-exchange capacity. |
| Formation Evaluation | n | The exponent, n, in the relation of water saturation, Sw, to resistivity index, I (I = Sw-n) for a sample of rock. It expresses the effect on the resistivity of desaturating the sample, or replacing water with a non-conductive fluid. In petrophysically simple, water-wet rocks (Archie rocks), n is constant for different values of Sw, and a single average n can be found for a particular reservoir or formation. A typical value is 2. In more complex rocks, n changes with Sw, although often being about 2 near Sw = 1. In rocks with conductive minerals, such as shaly sands, n becomes increasingly lower as Sw is reduced. This change is negligible for high-salinity waters, but increases as the salinity is reduced. In shaly-sand saturation equations, such as Waxman-Smits, dual water, SGS and CRMM, n is the intrinsic n, determined with high-salinity water or with the clay effects removed. The variation of I with Sw is then predicted, with varying success, by the different equations. In carbonates with multiple pore types, such as fractures, vugs, interparticle porosity and microporosity, n may change as each pore type is desaturated. A different n may be used for a different range of Sw. In all cases, n increases if any pores are oil-wet. Values up to 8 have been reported in very oil-wet rocks. |
| Formation Evaluation | native state core | A core taken so as to preserve the in-situ water saturation of the rock. A native-state core is usually drilled with oil-base mud or crude oil from the same reservoir. |
| Formation Evaluation | natural gamma ray spectroscopy | The technique of measuring the spectrum, or number and energy, of gamma rays emitted as natural radioactivity by the formation. There are three sources of natural radioactivity in the Earth: 40K, 232Th and 238U, or potassium, thorium and uranium. These radioactive isotopes emit gamma rays that have characteristic energy levels. The quantity and energy of these gamma rays can be measured in a scintillation detector. A log of natural gamma ray spectroscopy is usually presented as a total gamma ray log and the weight fraction of potassium (%), thorium (ppm) and uranium (ppm). The primary standards for the weight fractions are formations with known quantities of the three isotopes. Natural gamma ray spectroscopy logs were introduced in the early 1970s, although they had been studied from the 1950s. |
| Formation Evaluation | neutron capture | A neutron interaction in which the neutron is absorbed by the target nucleus, producing an isotope in an excited state. The activated isotope de-excites instantly through the emission of characteristic gamma rays. Neutron capture, also called thermal capture, usually occurs at low thermal energies at which the neutrons have about the same energy as the surrounding matter, typically below 0.4 eV (0.025 eV at room temperature). Some elements are better thermal absorbers than others. Neutron capture is an important principle behind the pulsed neutron capture log, the elemental capture spectroscopy log, the pulsed neutron spectroscopy log and the thermal neutron porosity measurement. |
| Formation Evaluation | neutron generator | A device for producing high-energy neutrons by using a charged particle accelerator. Neutron generators are used in various pulsed neutron devices and some neutron porosity measurements. In a typical device, deuterium (2D) and tritium (3T) ions are accelerated towards a target also containing the same isotopes. When 2D and 3T collide, they react to produce a neutron with an energy of about 14.1 MeV. The first neutron generators were built in the late 1950s and soon led to the first pulsed neutron capture log. |
| Formation Evaluation | neutron interactions | Phenomena involving the transfer of energy from neutrons to nuclei. The reaction rate of neutrons with matter depends on the density of neutrons, their velocity, the nuclear density and the particular interaction cross section. There are four principal neutron interactions that affect formation evaluation: elastic neutron scattering, inelastic neutron scattering, fast-neutron reactions and neutron capture. |
| Formation Evaluation | neutron log | Normally synonymous with a neutron porosity log. However, the term is sometimes broadened to include an activation log. |
| Formation Evaluation | neutron porosity | Referring to a log of porosity based on the effect of the formation on fast neutrons emitted by a source. Hydrogen has by far the biggest effect in slowing down and capturing neutrons. Since hydrogen is found mainly in the pore fluids, the neutron porosity log responds principally to porosity. However, the matrix and the type of fluid also have an effect. The log is calibrated to read the correct porosity assuming that the pores are filled with fresh water and for a given matrix (limestone, sandstone or dolomite). It is presented in units of porosity (vol/vol or p.u.) for the matrix chosen. Older logs were presented in counts per second or API units. The depth of investigation is several inches, so that the log reads mainly in the flushed zone. The neutron porosity log is strongly affected by clay and gas. Hydrogen occurs in clays and hydrated minerals as well as pore fluids. Gas has a low hydrogen density, so that gas zones have a very low apparent porosity. The measurement is based on either thermal or epithermal neutron detection. Thermal neutrons have about the same energy as the surrounding matter, typically less than 0.4 eV, while epithermal neutrons have higher energy, between about 0.4 and 10 eV. Being a statistical measurement, the precision is greatest at high count rates, which in this case occurs at low porosity. Neutron porosity logs were introduced in the early 1940s. The first tools were known as neutron-gamma tools, since the detector measured the gamma rays emitted on capture. Neutron-neutron tools, using a thermal neutron detector were introduced in about 1950. |
| Formation Evaluation | neutron-activation log | A record of elemental concentrations derived from the characteristic energy levels of gamma rays emitted by a nucleus that has been activated by neutron bombardment. In the context of production logging, the term normally refers to the activation of silicon and aluminum to determine the quality of a gravel pack. Silicon and aluminum are activated by a neutron source to produce isotopes that decay with a half-life of 2.3 minutes emitting a 1.78 MeV gamma ray. These gamma rays are counted in a detector placed below the source, with a high count indicating a high quantity of silicon in a sand pack, or aluminum in a bauxite pack. The log is run slowly so that oxygen and other activated elements have decayed before the detector crosses the activated interval. The carbon-oxygen log, elemental-capture spectroscopy log, pulsed-neutron spectroscopy log, aluminum-activation log and the oxygen-activation log are also examples of neutron-activation logs. |
| Formation Evaluation | NMR | Pertaining to a measurement of the nuclear magnetic properties of formation hydrogen. The basic core and log measurement is the T2 decay, presented as a distribution of T2 amplitudes versus time at each sample depth, typically from 0.3 ms to 3 s. The T2 decay is further processed to give the total pore volume (the total porosity) and pore volumes within different ranges of T2. The most common volumes are the bound fluid and free fluid. A permeability estimate is made using a transform such as the Timur-Coates or SDR permeability transforms. By running the log with different acquisition parameters, direct hydrocarbon typing and enhanced diffusion are possible. |
| Formation Evaluation | normal | Referring to a type of conventional electrical log in which the current emitting electrode (A) and the measure electrode (M) are placed close together on the sonde, and the current return electrode (B) and the measure reference electrode (N) far away. The response is determined mainly by the distance between A and M. The larger AM, the deeper the measurement, but the poorer the bed boundary response. Although many distances have been used, the most common are 16 in. [40 cm], known as the short normal, and 64 in. [162 cm], known as the long normal. |
| Formation Evaluation | normal mode | A type of acoustic energy that propagates in one direction while being confined in the other two directions, in this case by the borehole wall. Normal modes are propagated as reflections off the borehole wall, and exist only in hard rock. They are highly dispersive, starting with the formation shear velocity at a certain cutoff frequency and decreasing at high frequencies to the borehole fluid velocity. Below the cutoff frequency, they do not exist. Normal mode #0 is often considered to be the tube wave and starts at zero frequency. Normal mode #1 is called the pseudoRayleigh, and starts at around 5 kHz. The other normal modes start at increasingly higher frequencies. |
| Formation Evaluation | nuclear magnetic resonance | Pertaining to a measurement of the nuclear magnetic properties of formation hydrogen. The basic core and log measurement is the T2 decay, presented as a distribution of T2 amplitudes versus time at each sample depth, typically from 0.3 ms to 3 s. The T2 decay is further processed to give the total pore volume (the total porosity) and pore volumes within different ranges of T2. The most common volumes are the bound fluid and free fluid. A permeability estimate is made using a transform such as the Timur-Coates or SDR permeability transforms. By running the log with different acquisition parameters, direct hydrocarbon typing and enhanced diffusion are possible. |
| Formation Evaluation | nuclear magnetic resonance measurement | A measurement of the nuclear magnetic resonance (NMR) properties of hydrogen in the formation. There are two phases to the measurement: polarization and acquisition. First, the hydrogen atoms are aligned in the direction of a static magnetic field (B0). This polarization takes a characteristic time T1. Second, the hydrogen atoms are tipped by a short burst from an oscillating magnetic field that is designed so that they precess in resonance in a plane perpendicular to B0. The frequency of oscillation is the Larmor frequency. The precession of the hydrogen atoms induces a signal in the antenna. The decay of this signal with time is caused by transverse relaxation and is measured by the CPMG pulse sequence. The decay is the sum of different decay times, called T2. The T2 distribution is the basic output of a NMR measurement. The NMR measurement made by both a laboratory instrument and a logging tool follow the same principles very closely. An important feature of the NMR measurement is the time needed to acquire it. In the laboratory, time presents no difficulty. In a log, there is a trade-off between the time needed for polarization and acquisition, logging speed and frequency of sampling. The longer the polarization and acquisition, the more complete the measurement. However, the longer times require either lower logging speed or less frequent samples. |
| Formation Evaluation | oilfield battery | An electrochemical source that provides electrical power to a downhole or surface tool used for determining the location or assisting in the extraction of hydrocarbons. Unlike a conventional battery, an oilfield battery must safely provide power sufficient to enable a tool to perform as specified, even on extended jobs. Designed with rugged external packaging to fit within a confined tool space, it must tolerate extreme temperatures while withstanding high shock and vibration. Its internal chemistry must also accommodate a long shelf life. |
| Formation Evaluation | oxide closure model | A model for converting relative elemental yields from a pulsed neutron spectroscopy log to absolute weight concentrations using the assumption that the sum of all oxides in the rock matrix is 1. The model is based on the observation that, with few exceptions, sedimentary minerals are oxides, so that the sum of the dry weight percent of all oxides must be 100%. The weight percent of an oxide can be calculated from the dry weight percent of the cation by knowing the chemical formula (for example, SiO2 from Si). The absolute dry weight percent, W, of element i is given by Wi = F * Yi / Si where F is the unknown normalization factor, Yi is the measured spectral gamma ray yield and Si is the tool sensitivity to that element, measured in the laboratory. The dry weight percent of the oxide is then Oi = F* Xi * Yi / Si where Xi is the oxide association factor, given by the chemical formula. Since the sum of all Oi equals 1, it is possible to calculate F and determine each Wi . |
| Formation Evaluation | oxide-closure model | A model for converting relative elemental yields from a pulsed neutron spectroscopy log to absolute weight concentrations using the assumption that the sum of all oxides in the rock matrix is 1. The model is based on the observation that, with few exceptions, sedimentary minerals are oxides, so that the sum of the dry weight percent of all oxides must be 100%. The weight percent of an oxide can be calculated from the dry weight percent of the cation by knowing the chemical formula (for example, SiO2 from Si). The absolute dry weight percent, W, of element i is given by Wi = F * Yi / Si where F is the unknown normalization factor, Yi is the measured spectral gamma ray yield and Si is the tool sensitivity to that element, measured in the laboratory. The dry weight percent of the oxide is then Oi = F* Xi * Yi / Si where Xi is the oxide association factor, given by the chemical formula. Since the sum of all Oi equals 1, it is possible to calculate F and determine each Wi . |
| Formation Evaluation | pad | That part of a wireline logging tool that is pressed firmly against the borehole wall. The pad holds sensors that are focused in one direction and must be as close as possible to the borehole wall. The density detectors and the microresistivity electrodes are examples of sensors that must be placed on pads. Some pads are a rigid part of the logging tool. Others have articulated joints attaching them to the logging tool, with a backup arm to press the pad against the borehole wall. |
| Formation Evaluation | pair production | A gamma ray interaction in which the gamma ray, or photon, is converted into an electron and a positron when the gamma ray enters the strong electric field near the nucleus of an atom. The gamma ray energy must equal at least the rest mass of an electron and a positron (1.022 MeV) for the interaction to be possible. Following pair production, the positron will annihilate with an electron, emitting two gamma rays of 0.511 MeV. The highest probability of occurrence is at high gamma ray energy, above 10 MeV, and in a material of high atomic number. |
| Formation Evaluation | parallel resistivity | The resistivity of a formation measured by flowing current parallel to the bedding planes. In anisotropic formations, the parallel and perpendicular resistivities are different. |
| Formation Evaluation | pass | With reference to logging, an operation in which a logging tool is lowered into a borehole and then retrieved from it while recording measurements. |
| Formation Evaluation | PEF | A log of photoelectric absorption properties. The log measures the photoelectric absorption factor, Pe, which is defined as (Z/10) 3.6 where Z is the average atomic number of the formation. Pe is unitless, but since it is proportional to the photoelectric cross section per electron, it is sometimes quoted in barns/electron. Since fluids have very low atomic numbers, they have very little influence, so that Pe is a measure of the rock matrix properties. Sandstones have low Pe, while dolomites and limestones have high Pe. Clays, heavy minerals and iron-bearing minerals have high Pe. Thus, the log is very useful for determining mineralogy. In interpretation, PEF is normally converted to the simpler volumetric cross section, U in barns/cm3, by taking the product of PEF and density. The log is recorded as part of the density measurement. The depth of investigation is of the order of one inch, which is normally in the flushed zone. PEF can be affected by heavy minerals such as barite in the mudcake or mud filtrate. PEF logs were introduced in the late 1970s. |
| Formation Evaluation | permanent datum | The level to which all subsurface depths in an area are referred, normally the mean sea level. In individual wells, the depth is measured from the depth reference. However, in order to compare data between wells it is important to have a valid, area wide reference for comparison. This is the permanent datum level. |
| Formation Evaluation | permeameter | An apparatus for measuring the permeability of a core sample. Measurements are made either by placing the sample in a chamber (also known as a core holder), or by placing a probe on the surface of the sample. Core-holder measurements are made either with gas or liquid, and either in steady state or unsteady-state conditions. Other variables include the confining pressure and the direction of measurement, which can be axial (along the axis of a cylindrical core sample), transverse (perpendicular to the axis), or radial (to the center of a hollow cylinder). In probe measurements, gas is injected into the sample under either steady- or unsteady-state conditions. Probe permeameters are also known as minipermeameters. |
| Formation Evaluation | permittivity | The degree to which a medium resists the flow of electric charge, defined as the ratio of the electric displacement to the electric field strength. It is more common to use the relative dielectric permittivity. |
| Formation Evaluation | perpendicular resistivity | The resistivity of a formation measured by flowing current perpendicular to the bedding planes. In anisotropic formations, the parallel and perpendicular resistivities are different. |
| Formation Evaluation | phase shift | The change in position of the peaks of a sinusoidal electromagnetic wave as it passes through the formation. If the sinusoidal wave picked up by two receivers a certain distance apart in a formation are compared, it is found that the wave has been attenuated and shifted in time. The shift is known as a phase shift. The term is used in particular with reference to the propagation resistivity log and the electromagnetic propagation log. |
| Formation Evaluation | phase shift resistivity | The ability of the formation to resist electrical conduction, as derived from the change in position of the peaks of an electromagnetic wave generated in a propagation resistivity measurement. At the frequencies used, the phase shift depends mainly on the resistivity of the material with a small dependence on dielectric permittivity, particularly at high resistivity. Common practice is to transform the phase shift to resistivity assuming that the dielectric permittivity is related to resistivity by a simple algorithm. The transform also depends on transmitter/receiver spacings and tool design. For a 2-MHz measurement, a typical measurement range is 0.2 to 200 ohm-m. Above 200 ohm-m, the dielectric effects become too variable and it is preferable to use the dielectric resistivity. |
| Formation Evaluation | phase-shift resistivity | The ability of the formation to resist electrical conduction, as derived from the change in position of the peaks of an electromagnetic wave generated in a propagation resistivity measurement. At the frequencies used, the phase shift depends mainly on the resistivity of the material with a small dependence on dielectric permittivity, particularly at high resistivity. Common practice is to transform the phase shift to resistivity assuming that the dielectric permittivity is related to resistivity by a simple algorithm. The transform also depends on transmitter/receiver spacings and tool design. For a 2-MHz measurement, a typical measurement range is 0.2 to 200 ohm-m. Above 200 ohm-m, the dielectric effects become too variable and it is preferable to use the dielectric resistivity. |
| Formation Evaluation | photoelectric effect | A gamma ray interaction in which the gamma ray is fully absorbed by a bound electron. If the energy transferred exceeds the binding energy to the atom, the electron will be ejected. Normally, the ejected electron will be replaced within the material and a characteristic X-ray will be emitted with an energy that is dependent on the atomic number of the material. The highest probability for this effect occurs at low gamma ray energy and in a material of high atomic number. The photoelectric effect is the principle behind the PEF log, which identifies lithology. |
| Formation Evaluation | Pickett plot | A double logarithmic plot of a resistivity measurement on the x-axis versus a porosity measurement on the y-axis. The plot is named after G.R. Pickett. The plot is based on taking the logarithm of the Archie equation. Points of constant water saturation (Sw) will plot on a straight line with negative slope of value m. Water zones define the lowermost line on the plot. Since Sw = 1, the water resistivity can be determined from a point on the line. Once the water line is established, other parallel lines can be drawn for different Sw, assuming a constant n (usually 2). Other data can then be plotted and interpreted in terms of Sw. The same technique can be applied to the flushed zone, using flushed-zone measurements. See Pickett GR: “A Review of Current Techniques for Determination of Water Saturation from Logs,” paper SPE 1446, presented at the SPE Rocky Mountain Regional Meeting, Denver, Colorado, USA, May 23-24, 1966; SPE Journal of Petroleum Technology (November 1966): 1425-1435. |
| Formation Evaluation | pickup | The depth at which the tool string is picked up off the bottom of the well during a wireline logging survey. Pick-up can be observed by an increase in cable tension and by the start of activity in the log curves. When the logging tool is lowered to the bottom of the well, it is common practice to spool in some extra cable. When the cable is pulled back out, the tool remains stationary before it is picked up off the bottom. During this time the log readings are static but the depth, which is recorded by the movement of the cable, is changing. |
| Formation Evaluation | pick-up | The depth at which the tool string is picked up off the bottom of the well during a wireline logging survey. Pick-up can be observed by an increase in cable tension and by the start of activity in the log curves. When the logging tool is lowered to the bottom of the well, it is common practice to spool in some extra cable. When the cable is pulled back out, the tool remains stationary before it is picked up off the bottom. During this time the log readings are static but the depth, which is recorded by the movement of the cable, is changing. |
| Formation Evaluation | play back | To generate a log from digital data some time after the actual acquisition of the data. This log is distinct from the acquisition log. Some of the parameters for processing the log may or may not be different from those of the acquisition log. |
| Formation Evaluation | playback | A log that has been generated from digital data some time after the actual acquisition of the data. It is distinct from the acquisition log. Some of the parameters for processing the log may or may not be different from those of the acquisition log. |
| Formation Evaluation | polarization horn | The effect on a propagation resistivity or induction log of charge buildup at the boundary between two formation layers with different dielectric properties. In a vertical well with horizontal layers, the current loops generated by the tool in the formation are parallel to the layers and do not cross bed boundaries. However, with an apparent dip between borehole and formation, the loops cross the bed boundaries and generate a charge buildup at the boundaries. The charge buildup acts like a secondary transmitter that increases the measured resistivity. The result is a spike to high resistivity as the tool crosses the bed boundary. In deviated or horizontal wells, polarization horns on measurements-while-drilling propagation logs often are used to detect a bed boundary. The spike increases with apparent dip and resistivity contrast between beds. The magnitude of polarization spikes varies with tool type and spacing, being larger for the propagation tools. |
| Formation Evaluation | polarization time | The time allotted for the alignment of hydrogen atoms with the static magnetic field during a nuclear magnetic resonance (NMR) measurement. The alignment of hydrogen atoms follows an exponential rule such that after a polarization time PT the percentage aligned is 100*(1 ? e-PT/T1) where T1 is their longitudinal relaxation time. An infinite polarization time is therefore needed to align every hydrogen atom, but 95% are aligned after a time of 3*T1. Typical polarization times for a standard NMR log are between 1 and 4 s. |
| Formation Evaluation | pore throat | In an intergranular rock, the small pore space at the point where two grains meet, which connects two larger pore volumes. The number, size and distribution of the pore throats control many of the resistivity, flow and capillary-pressure characteristics of the rock. |
| Formation Evaluation | porosimeter | An instrument for measuring the pore volume, and hence the porosity, of a core sample. The term is also used for some instruments that actually measure grain volume, such as the Boyle?s Law Double-Cell method. Pore volume is then obtained from the difference between bulk volume and grain volume. Pore volume is most commonly measured directly by Boyle’s Law Single-Cell method, summation of fluids or liquid saturation. Bulk volume is most commonly measured by buoyancy, mercury displacement or a physical measurement of size (calipering); grain volume by Boyle?s law Double-Cell method or disaggregation of the sample. Except for disaggregation, all techniques determine the effective porosity, in the sense of all but the isolated pores. |
| Formation Evaluation | porosity exponent | The exponent, m, in the relation of formation factor (F) to porosity (phi). For a single sample, F is related to phi using the Archie equation F = 1 / phim, with m being the only coefficient needed. In this case, m has been related to many physical parameters, but above all to the tortuosity of the pore space. In theory, it can range from 1 for a bundle of tubes to infinity for porosity that is completely unconnected. For a simple packing of equal spheres, m = 1.5. With a more tortuous pore space or more isolated pores, m increases, while with fractures or conductive solids, m decreases. As a general average for typical reservoir rocks, m is often taken as 2. For a group of rock samples, it is common practice to find a relationship between F and phi that uses two coefficients (F = a / phim). In this case m, like a, becomes an empirical constant of best fit between F and phi, and may take a wide range of values. In complex formations, such as shaly sands or carbonates with multiple pore types, a constant m does not give good results. One solution is to vary m, with the variability related to parameters such as porosity, shaliness, or rock texture, or else determined directly from logs in zones where the water saturation is known or can be computed from a nonresistivity measurement such as electromagnetic propagation. In shaly sands, the preferred solution is to use a saturation equation, such as Waxman-Smits, dual water, SGS or CRMM, in which m is defined as the intrinsic m, determined from the intrinsic formation factor at high salinities or after correction for the effect of shale. In carbonates with multiple pore types, such as fractures, vugs, interparticle porosity and microporosity, one solution is to use equations with different porosity exponents for each pore type. The volume of each pore type must then be determined from logs or borehole images. |
| Formation Evaluation | porosity unit | A unit equal to the percentage of pore space in a unit volume of rock. It is abbreviated to p.u. and lies between 0 and 100. |
| Formation Evaluation | porous plate technique | A method for desaturating a core sample by placing one end in capillary contact with a porous plate and applying gas or oil under pressure to the remaining surfaces. The liquid in the original fully saturated sample is expelled through the porous plate. At different pressure stages, the sample is weighed to determine the loss of liquid, and the gas or oil pressure increased. Desaturation continues until no more weight loss is observed, at which time the sample is at irreducible water saturation. Core samples are desaturated to measure, for example, capillary pressure, irreducible water saturation, resistivity index or nuclear magnetic resonance response. |
| Formation Evaluation | potassium | An element with an atomic number of 19. The 40K isotope is radioactive, decaying with the emission of a single gamma ray of 1.46 MeV with a half-life of 1.3 * 109 years to give a stable isotope of argon. Potassium is the largest source of natural radioactivity. It occurs in illite, alkali feldspars, micas and some evaporite minerals. It also occurs in some drilling mud systems. The 40K isotope is only a small fraction, about 0.012%, of the total potassium, the main isotope being 39K, which has an abundance of about 1.7% in the Earth’s crust. For the purposes of logging, the total potassium is calculated from the measured quantity of 40K and scaled in percent by weight. It is a valuable aid in determining the mineral content of a formation. |
| Formation Evaluation | precision | The closeness of agreement between the results obtained by applying a measurement procedure several times on identical materials and under prescribed measurement conditions. The smaller the random part of experimental error, the more precise the measurement procedure. (ISO) In logging, the term usually describes the repeatability of a statistical measurement, such as a nuclear log. The precision must then refer to a particular set of conditions, for example, the speed of logging and the formation properties. |
| Formation Evaluation | preserved core | A core that has been preserved in the same state as when it was brought to the surface. The term implies that the core has been stored for a period before analysis. If this has not been the case, it is known as fresh core. The goal of preservation is to maintain the original fluid content, fluid distribution, rock wettability and mechanical integrity. Preserved cores are typically sealed and protected from mechanical damage. Depending on the core and the objective, they may also be frozen or placed in humidity ovens. Preservation may be wet, in which the core is submerged in a suitably prepared brine, or dry, without any fluid. |
| Formation Evaluation | propagation resistivity | A measurements-while-drilling log of formation resistivity. The log normally contains at least one attenuation and one phase-shift resistivity reading. In many cases there will be multiple curves of both, the difference being the depth of investigation. For the same nominal depth of investigation, the attenuation resistivity reads deeper than the phase-shift resistivity and is less affected by invasion, but more affected by surrounding beds and apparent dip. The attenuation measurement has a poorer vertical resolution and is less affected by anisotropy. Depths of investigation and vertical resolution of both measurements vary with the average formation resistivity. Although depths of investigation are less than with wireline resistivity logs, the invasion at the time of measurement is usually small and it is possible to derive the resistivity of the undisturbed zone. |
| Formation Evaluation | propagation resistivity measurement | A measurement of the formation resistivity made on drillpipe at a frequency in the range of 100 kHz to 10 GHz, most commonly 2 MHz. The basic measurement is accomplished using a transmitter and two receivers. At these frequencies, the response is best explained as the propagation of a wave. Thus, the phase shift and attenuation of the wave between the receivers are measured and transformed to give the phase shift and the attenuation resistivity. In practice, multiple transmitters may be used to obtain different depths of investigation and achieve borehole compensation. The wavelength is such that the borehole has a minor effect, but one for which correction may be needed. |
| Formation Evaluation | propagation time | A measurement of the high frequency (about 1 GHz) dielectric properties of the formation. In a typical tool, a microwave transmitter is placed a few inches below two receivers separated by 4 cm [1.6 in.]. At this frequency, the response is best explained as the propagation of a wave. Thus the phase shift and attenuation of the wave between the receivers are measured and transformed to give the log measurements of propagation time and attenuation. Because of the short spacings, the measurement has excellent vertical resolution and reads within inches of the borehole wall except at high resistivity. Different transmitter and receiver spacings and orientations are used, leading to different arrays, such as the endfire array and the broadside array. An ideal measurement would give the plane wave properties of the formation. However, the geometry of the measurement precludes this, so that a correction, known as the spreading-loss correction, is needed for the attenuation and to a much smaller extent for the propagation time. The measurement is also affected by the dielectric properties and thickness of the mudcake. Borehole compensation is used to correct for sonde tilt or a rough borehole wall. |
| Formation Evaluation | proximity log | An electrode device with small spacings from which the current flow, and hence the measurement, is focused a short distance into the formation. The proximity log measures the resistivity of the flushed zone with minimum influence from the mudcake or the undisturbed zone. The central current-emitting electrode (A0) is surrounded by a guard electrode that emits sufficient current to focus the current from A0 a certain distance into the formation. The electrodes are mounted on a pad that is pressed against the borehole wall. In a typical tool design, 90% of the signal comes from within 5 to 10 in. [13 to 25 cm] of the pad. This is deeper than the microlaterolog, which ensures that the mudcake has less effect but means that the proximity log is more often affected by the undisturbed zone. |
| Formation Evaluation | pseudogeometrical factor | The response of a logging measurement as a function of distance from the tool. The pseudogeometrical factor is normally radial, reflecting the response perpendicular to the tool. It can be a differential factor, which is the contribution to the signal at a particular distance, but is more normally integrated, which is the sum of all signals from the tool to a particular distance. The pseudogeometrical factor developed from the concept of the geometrical factor, and is expressed in the same way. For example, for a radial distance x from the tool, the integrated radial pseudogeometrical factor, Jx, can be written as: Jx = (Ux – Ut) / (Uxo – Ut) where Ut is the log reading of the undisturbed zone (or, alternatively, the reading with no invasion), Uxo is the log reading of the flushed zone (or, alternatively, the reading with infinite invasion), and Ux is the log reading with a step profile invasion to depth x. Unlike the geometrical factor, Jx depends on the values of both Uxo and Ut. Pseudogeometrical factors are a useful way to express the radial response (or vertical response) in typical conditions. The physics of each measurement determines how much Jx varies with Uxo and Ut. Pseudogeometrical factors are often used to express the response of nuclear and resistivity logs, but are not appropriate for acoustic and electromagnetic propagation logs (where the response is too dependent on the contrast in properties), or nuclear magnetic resonance logs (where the response is too localized). |
| Formation Evaluation | pseudostatic spontaneous potential | The ideal spontaneous potential (SP) that would be observed opposite a shaly, permeable bed if the SP currents were prevented from flowing. In the middle of a thick, permeable bed whose resistivity is not too high, the SP reads close to the pseudostatic spontaneous potential (PSP). In other conditions, however, the SP may be significantly less than the PSP. The PSP ignores other potential sources and assumes that a surrounding shale is a perfect cationic membrane. The ratio of the PSP to the static spontaneous potential is known as the SP reduction factor, alpha. Alpha is less than 1 and is a function of the shaliness, or cation-exchange capacity, within the sand. The higher this cation-exchange capacity, the larger the internal membrane potential. The latter has the opposite polarity to the liquid-junction potential and reduces the SP. The PSP, and alpha, are reduced when hydrocarbons are introduced into shaly sands, because the cation-exchange capacity in the sands is forced into a smaller conductive pore volume and therefore has a larger relative effect. |
| Formation Evaluation | PSP | A mnemonic for the pseudostatic spontaneous potential. |
| Formation Evaluation | pulse echo | A technique in which an ultrasonic transducer, in transmit mode, emits a high-frequency acoustic pulse towards the borehole wall, where it is reflected back to the same transducer operating in receive mode. The measurement consists of the amplitude of the received signal, the time between emission and reception, and sometimes the full waveform received. Tools that use this technique either have multiple transducers, facing in different directions, or rotate the transducer while making measurements, thereby obtaining a full image of the borehole wall. Pulse-echo techniques are used in the borehole televiewer. In cased hole, the waveform is analyzed to give indications of cement-bond quality and casing corrosion. |
| Formation Evaluation | pulsed neutron spectroscopy log | A wireline log of the yields of different elements in the formation, measured using induced gamma ray spectroscopy with a pulsed neutron generator. The elemental yields are derived from two intermediate results: the inelastic and the capture spectrum. The inelastic spectrum is the basis for the carbon-oxygen log, and can also give information on other elements. The capture spectrum depends on many elements, mainly hydrogen, silicon, calcium, iron, sulfur and chlorine. Since the elemental yields give information only on the relative concentration of elements, they are normally given as ratios, such as C/O, Cl/H, Si/(Si + Ca), H/(Si + Ca) and Fe/(Si + Ca). These ratios are indicators of oil, salinity, lithology, porosity and clay, respectively. To get absolute concentrations, it is necessary to calibrate to cores or, more often, use a model such as the oxide-closure model. The depth of investigation of the log is several inches into the formation. It can be run in open or cased hole. Pulsed neutron spectroscopy logs were introduced in the mid 1970s after a decade or more of investigation. |
| Formation Evaluation | pulsed neutron spectroscopy measurement | A measurement of the spectrum of gamma rays emitted by a formation bombarded by high-energy neutrons. Neutrons are emitted by a high-energy neutron generator (14.1 MeV). The neutrons interact with different nuclei, which may emit characteristic gamma rays through inelastic neutron scattering, fast-neutron reactions and neutron capture. When pulses from a neutron generator are used, it is possible to separate the different interactions in time after each neutron pulse. Inelastic and fast-neutron interactions occur very soon after the neutron burst, while most of the capture events occur later. The two types can therefore be separated to give a so-called inelastic spectrum and a capture spectrum. The spectra are analyzed either by counting gamma rays in windows placed at the main peaks for the elements concerned, or by comparison with spectral standards, or by combining the two (alpha processing). The resultant logs are known as pulsed neutron spectroscopy logs, the most common of which are the carbon-oxygen log and the elemental capture spectroscopy log. |
| Formation Evaluation | pulse-echo | A technique in which an ultrasonic transducer, in transmit mode, emits a high-frequency acoustic pulse towards the borehole wall, where it is reflected back to the same transducer operating in receive mode. The measurement consists of the amplitude of the received signal, the time between emission and reception, and sometimes the full waveform received. Tools that use this technique either have multiple transducers, facing in different directions, or rotate the transducer while making measurements, thereby obtaining a full image of the borehole wall. Pulse-echo techniques are used in the borehole televiewer. In cased hole, the waveform is analyzed to give indications of cement-bond quality and casing corrosion. |
| Formation Evaluation | quadrant density | A log of formation density from one of the quadrants of an azimuthal density tool. |
| Formation Evaluation | quick look | A log, or a display of several logs, that has been generated by a simple computation of log data. The quicklook is intended to make it easy to identify particular features in a section of log. The term is used for single curves designed, for example, to identify hydrocarbon zones, estimate porosity or identify lithology. Examples are Rwa, crossplot porosity, ratio method and apparent matrix density. The term is also used for a complete log containing some combination of quicklook curves, original logs and lithology display. In all cases the computations are based on simple models. |
| Formation Evaluation | quicklook | A log, or a display of several logs, that has been generated by a simple computation of log data. The quicklook is intended to make it easy to identify particular features in a section of log. The term is used for single curves designed, for example, to identify hydrocarbon zones, estimate porosity or identify lithology. Examples are Rwa, crossplot porosity, ratio method and apparent matrix density. The term is also used for a complete log containing some combination of quicklook curves, original logs and lithology display. In all cases the computations are based on simple models. |
| Formation Evaluation | R signal | The resistive (R) signal, or that part of the alternating signal at the receiver of an induction-logging tool that is in phase with the transmitter current. This signal depends on the formation conductivity and is the main or, in older tools, the only source of the induction log. It must be separated from the out-of-phase, or reactive (X) signal, which depends in a different way on formation conductivity and may contain a large component from direct coupling between transmitter and receiver. |
| Formation Evaluation | Rv | Abbreviation for vertical resistivity. |
| Formation Evaluation | Rwa | The apparent resistivity of the formation water, calculated from log measurements of porosity (phi) and resistivity (Rt) and using the Archie equation with Sw = 1, so that Rwa = phim * Rt. Rwa is a quick-look method of determining if there are hydrocarbons. In a hydrocarbon zone, Rwa will be higher than the actual resistivity of the formation water (Rw), which must be known. A typical rule of thumb is that if Rwa > 3 * Rw, then there are producible hydrocarbons. Rwa is often calculated and output as a quicklook log. |
| Formation Evaluation | radial processing | The inversion of resistivity logs with differing depths of investigation into a model of the formation invasion profile. For dual induction and dual-laterolog tools, this was done graphically with a tornado chart and assuming a step profile. Array tools have built-in inversion algorithms, and several formation models into which the data can be inverted. |
| Formation Evaluation | radial resolution | A distance that characterizes the ability of a logging measurement to resolve changes in the formation perpendicular to the tool. Alternatively, the term refers to the smallest distance for which a significant change can be detected. The resolution is a feature of the radial response, which is often summarized by a geometrical or pseudogeometrical factor. A measurement with good or high radial resolution will have a sharp peak in the radial differential geometric factor at some distance from the tool. |
| Formation Evaluation | radial response | The response of a logging measurement as a function of the distance perpendicular to the tool. Radial responses are determined by computer simulation or laboratory measurement. For some measurements, mainly resistivity, the radial response can be shaped as desired through signal processing. In general, the radial response depends on the formation properties throughout the measurement volume. Most quoted radial responses have been determined in formations that are vertically homogeneous and have small radial changes. They can then be summarized by a geometrical factor or a pseudogeometrical factor. These factors are appropriate for volumetric measurements such as nuclear and resistivity measurements, but not for others such as acoustic propagation. |
| Formation Evaluation | radius of investigation | A distance that characterizes how far a logging tool measures into the formation from the axis of the tool or borehole. The term is similar to depth of investigation, but is more appropriate for certain azimuthally symmetric measurements, mainly resistivity. The radius of investigation summarizes the radial response of the measurement in one or more distances, and should be associated with the percentage of signal received from within that depth, typically either 50% or 90%. Most quoted depths of investigation assume a homogeneous formation with certain properties, such as a given resistivity or fluid content. The radius of investigation can vary considerably in inhomogeneous conditions, and at different values of the properties concerned. It should be considered only a qualitative guide to tool response. |
| Formation Evaluation | random error | A nonreproducible error that is generally imputable to the physics of the measurement. For example, the statistical errors in nuclear measurements are random errors. |
| Formation Evaluation | real time data | With reference to measurements-while-drilling (MWD) and logging while drilling, the data transmitted to surface shortly after being recorded. These are distinct from the data recorded into memory. Only a subset of the recorded data can be transmitted as real-time data because of the limited data rate of MWD telemetry systems. This reduces the number of channels, the sample interval, or both, in the real-time data. |
| Formation Evaluation | real-time data | With reference to measurements-while-drilling (MWD), the data transmitted to surface shortly after being recorded. These are distinct from the recorded data. Only a subset of the recorded data can be transmitted as real-time data because of the limited data rate of MWD telemetry systems. This reduces the number of channels, the sample interval, or both, in the real-time data. |
| Formation Evaluation | recorded data | With reference to measurements-while-drilling (MWD), the data recorded by the logging tools and stored in a downhole electronic memory. These data are subsequently retrieved when the tools are brought to surface. Recorded data are distinct from real-time data. Depending on the time between trips, the amount of data storage may affect the number of channels recorded, the sample interval, or both. |
| Formation Evaluation | recorder | Also called camera, the device used in early logging to record logging measurements on photographic film. The camera consisted of a light shining on galvanometers, which reflected the light to produce a trace on one or more films. The galvanometers deflected according to the log measurement to give the log reading. The films were turned by the depth wheel, which gave the depth axis of the log. |
| Formation Evaluation | reference point | The position on a logging tool string that is used as the reference for depth measurements. Each measurement has a different measure point. In normal practice, each measurement is shifted in depth by the distance between the measure point and the reference point. This ensures that all measurements are recorded at the same depth. For a wireline tool, the reference point is normally the bottom of the tool string. For measurements-while-drilling, the reference is the bit (the driller’s depth). The term is sometimes used to mean the depth reference. |
| Formation Evaluation | relative dielectric constant | The degree to which a medium resists the flow of electric charge divided by the degree to which free space resists such charge. The degree, or dielectric constant, is defined as the ratio of the electric displacement to the electric field strength. The term is also known as the relative dielectric permittivity. However, at high frequencies, it is no longer constant and decreases with frequency. Relative dielectric permittivities, which are unitless, vary from about 4 to 400 in rock, but in rare cases may reach several thousand. |
| Formation Evaluation | relative dielectric permittivity | The degree to which a medium resists the flow of electric charge divided by the degree to which free space resists such charge. The degree, or dielectric permittivity, is defined as the ratio of the electric displacement to the electric field strength. The term is also known as the relative dielectric constant. However, at high frequencies, it is no longer constant and decreases with frequency. Relative dielectric permittivities, which are unitless, vary from about 4 to 400 in rock, but in rare cases may reach several thousand. |
| Formation Evaluation | relaxation time | In a nuclear magnetic resonance (NMR) measurement, the characteristic time for a loss of coherent energy, or relaxation, by protons in rocks. There are two types of relaxation: longitudinal relaxation, which is the time (T1) needed to align protons in a static magnetic field; and transverse relaxation, which is the time (T2) needed for protons to lose their coherent energy in an NMR measurement. Relaxations are exponential decays, for which T1 and T2 are the time constants. Different mechanisms contribute to T1 and T2. Surface relaxation and bulk relaxation contribute to both T1 and T2. Surface, bulk and diffusion relaxation contribute to T2. |
| Formation Evaluation | repeat section | An interval of log that has been recorded for a second time. The repeat section is typically 200 ft [60 m] long. The purpose is to judge the repeatability of the measurement by recording it twice over the same interval with the same recording parameters. Strictly speaking, the repeatability can be judged properly only if the depth measurement is the same on both runs, if the tool takes the same path in the borehole and if there have been no changes in the borehole or formation. In practice, the repeat section gives a good overall picture of the repeatability of the log. There can be more than one repeat section. |
| Formation Evaluation | repeatability | The quantitative value that is equal to or below the absolute difference between two test results, within a probability of 95%. (ISO) In a test of repeatability, the results are obtained independently by the normal and correct operation of the same method on identical test material, in a short space of time, and under the same test conditions (such as the same operator, same apparatus, same laboratory). |
| Formation Evaluation | repeatability | The representative parameters of the dispersion of the population that may be associated with the results are qualified by the term repeatability, for example “repeatability standard deviation,” or “repeatability variance.” (ISO) |
| Formation Evaluation | repeatability | The closeness of agreement between independent results obtained in the normal and correct operation of the same method on identical test material, in a short space of time, and under the same test conditions (such as the same operator, same apparatus, same laboratory). (ISO) The repeatability of core and log measurements can be checked properly in the laboratory. The repeatability of a downhole log is checked by recording a repeat section (or repeat stations for stationary measurements). The term repeatability is used even though it is not possible to ensure that the same test material is used. On two separate runs, the tool may not take the same path in the borehole, and therefore may not measure the same volume of formation. |
| Formation Evaluation | reproducibility | The quantitative value that is equal to or below the absolute difference between two test results obtained by operators in different laboratories, using the standard test method, within a probability of 95%. (ISO) |
| Formation Evaluation | reproducibility | The representative parameters of the dispersion of the population which may be associated with the results are qualified by the term reproducibility, for example “reproducibility standard deviation,” “reproducibility variance.” (ISO) |
| Formation Evaluation | reproducibility | The closeness of agreement between individual results obtained in the normal and correct operation of the same method on identical test material, but under different test conditions (such as different operators, different apparatus, different laboratories). (ISO) The reproducibility of core and log measurements can be checked properly in the laboratory. However, it is difficult to check the reproducibility of downhole log measurements because of the problem of ensuring that the same test material is used, in other words that the same volume of formation is measured each time, or that the formation fluids have not changed. |
| Formation Evaluation | resistive invasion | A situation in which the resistivity of the flushed zone is greater than the resistivity of the undisturbed zone. Such a setting generally favors the use of induction devices, which respond to conductivity, rather than electrode resistivity devices (laterologs, ring resistivity), which respond to resistivity. |
| Formation Evaluation | resistivity | The ability of a material to resist electrical conduction. It is the inverse of conductivity and is measured in ohm-m. The resistivity is a property of the material, whereas the resistance also depends on the volume measured. The two are related by a system constant, which in simple cases is the length between the measurement electrodes divided by the area. In the general case, the resistivity is the electric field divided by the current density and depends on the frequency of the applied signal. |
| Formation Evaluation | resistivity index | The ratio of the true resistivity to the resistivity of the same rock filled with water. The resistivity index can be expressed as I = Rt / Ro, where Ro is the water filled resistivity and Rt is the true resistivity. It is related to the water saturation by the saturation exponent, n (I = Sw-n), and is a key component of the Archie equation. |
| Formation Evaluation | resistivity log | A log of the resistivity of the formation made by an electrode device such as a laterolog. In this sense the term is used to distinguish the log from an induction measurement, which responds more directly to conductivity. |
| Formation Evaluation | resistivity log | A log of the resistivity of the formation, expressed in ohm-m. The resistivity can take a wide range of values, and, therefore, for convenience is usually presented on a logarithmic scale from, for example, 0.2 to 2000 ohm-m. The resistivity log is fundamental in formation evaluation because hydrocarbons do not conduct electricity while all formation waters do. Therefore a large difference exists between the resistivity of rocks filled with hydrocarbons and those filled with formation water. Clay minerals and a few other minerals, such as pyrite, also conduct electricity, and reduce the difference. Some measurement devices, such as induction and propagation resistivity logs, may respond more directly to conductivity, but are presented in resistivity. |
| Formation Evaluation | resolution matched | Relating to two or more logging measurements that have the same resolution. The term normally refers to vertical resolution, but could also be used for azimuthal or radial resolution. |
| Formation Evaluation | response matched | Relating to two or more logging measurements that have the same response. The term normally refers to vertical resolution, but could also be used for azimuthal or radial resolution. The term implies that all the features of the vertical response are matched, ideally in all conditions. In practice, it is used to describe a more detailed matching of the vertical response than with resolution-matched curves. |
| Formation Evaluation | restored state core | A core that has been cleaned but then flushed with reservoir fluids to reestablish the in situ condition of the rock. The main purpose of a restored state core is to measure the wettability and related properties such as relative permeability. |
| Formation Evaluation | retort method | A technique for measuring the fluid saturations in a core sample by heating the sample and measuring the volumes of water and oil driven off. The sample is crushed and weighed before being placed in the retort. It is then heated in stages or directly to 650oC [1200oF] during which the fluids are vaporized, collected, condensed and separated. Plateaus in the rise of the cumulative water volume with temperature are sometimes analyzed to indicate when free water, surface clay-bound water and interlayer clay-bound water have been driven off. The volumes of water and oil are measured directly, but corrections are needed to account for alterations in the oil. The volume of gas also is needed for accurate results. This is measured on a separate, adjacent sample by injecting mercury under pressure and measuring the volume absorbed. Before injection, the sample is weighed and its bulk volume determined by mercury displacement. The total pore volume is then the sum of the volumes of gas, oil and water. The saturation of each component is the ratio of its volume to the total pore volume. |
| Formation Evaluation | ring resistivity | The resistivity measured by the ring of a measurements-while-drilling (MWD) toroid device. The ring resistivity is a focused measurement with a depth of investigation and a vertical resolution of a few inches. It is not azimuthal. The measurement is similar to a wireline laterolog except that toroids are used instead of electrodes for transmitting and monitoring. Like a laterolog, the signal is proportional to resistivity, and is thus most effective at high resistivities, high formation to mud-resistivity contrast and in the presence of conductive invasion. It is usually combined with the bit resistivity, and possibly also with the button resistivities. In the same way as a laterolog, two transmitter toroids are used to force current to flow approximately perpendicularly into the formation at the ring. Other toroids measure the current flow and to balance the currents emitted by the two transmitters. |
| Formation Evaluation | routine core analysis | The set of measurements normally carried out on core plugs or whole core. These generally include porosity, grain density, horizontal permeability, fluid saturation and a lithologic description. Routine core analyses often include a core gamma log and measurements of vertical permeability. Measurements are made at room temperature and at either atmospheric confining pressure, formation confining pressure, or both. Routine core analysis is distinct from special core analysis (SCAL). Recommended practices for routine core analysis are available in the API document RP40. |
| Formation Evaluation | R-signal | The resistive (R) signal, or that part of the alternating signal at the receiver of an induction-logging tool that is in phase with the transmitter current. This signal depends on the formation conductivity and is the main or, in older tools, the only source of the induction log. It must be separated from the out-of-phase, or reactive (X) signal, which depends in a different way on formation conductivity and may contain a large component from direct coupling between transmitter and receiver. |
| Formation Evaluation | rugose | Pertaining to a borehole wall that is rough. |
| Formation Evaluation | rugosity | A qualitative description of the roughness of a borehole wall. Alternatively, the term pertains to a borehole whose diameter changes rapidly with depth. The term usually refers to changes at the scale of logging measurements, a few inches to a few feet, and to the effect this has on logging tool responses. Rugosity can be observed on caliper logs, image logs and by its effect on measurements with a small depth of investigation. |
| Formation Evaluation | run | With reference to logging, an operation in which a logging tool is lowered into a borehole and then retrieved from it while recording measurements. |
| Formation Evaluation | sampling error | The error introduced by the sampling process caused by making measurements on only a limited portion of a formation. |
| Formation Evaluation | sampling interval | The depth or time between successive measurements by a sensor. For measurements-while-drilling (MWD) logs, the sampling interval is most commonly a time. For wireline measurements, it is most commonly a depth. |
| Formation Evaluation | sandstone compatible scale | Display ranges chosen for the density and neutron porosity logs such that the two curves will overlay at all porosity values providing the matrix is pure quartz and the pores are filled with fresh water. The most common overlay spans two tracks, with the density reading from 1.9 to 2.9 g/cm3, and the neutron in sandstone porosity units from 0.45 to ?0.15 vol/vol. |
| Formation Evaluation | sandstone porosity unit | A transform from raw log data chosen so that a log recorded in these units will give the correct porosity of the formation providing the matrix is pure quartz and the pores are filled with fresh water. The unit, which may be in vol/vol or p.u., is most commonly used for neutron porosity logs but may also be used for density and acoustic logs. The definition is strictly true only if all borehole and other environmental corrections have been applied. |
| Formation Evaluation | sandstone-compatible scale | Display ranges chosen for the density and neutron porosity logs such that the two curves will overlay at all porosity values providing the matrix is pure quartz and the pores are filled with fresh water. The most common overlay spans two tracks, with the density reading from 1.9 to 2.9 g/cm3, and the neutron in sandstone porosity units from 0.45 to ?0.15 vol/vol. |
| Formation Evaluation | saturation equation | An equation for calculating the water saturation from resistivity and other logs. There are many saturation equations. Practical equations are all extensions of the Archie equation, which is valid for a rock with very little clay, or very high salinity water, and with a regular pore structure. The majority deal with the problem of shaly sands, and can be divided into two main groups?those that treat the shale as a volume of conductive material (Vsh models), and those that analyze the effect of clay counter-ions. Vsh models take many forms. Typical examples are the Simandoux, laminated sand and Indonesian equations. The other group includes the Waxman-Smits, Dual Water and SGS equations. Most nonshaly sand equations deal with the problem of mixed pore types, for example combinations of fractures, isolated pores and intergranular pores. |
| Formation Evaluation | saturation exponent | The exponent, n, in the relation of water saturation, Sw, to resistivity index, I (I = Sw-n) for a sample of rock. It expresses the effect on the resistivity of desaturating the sample, or replacing water with a non-conductive fluid. In petrophysically simple, water-wet rocks (Archie rocks), n is constant for different values of Sw, and a single average n can be found for a particular reservoir or formation. A typical value is 2. In more complex rocks, n changes with Sw, although often being about 2 near Sw = 1. In rocks with conductive minerals, such as shaly sands, n becomes increasingly lower as Sw is reduced. This change is negligible for high-salinity waters, but increases as the salinity is reduced. In shaly-sand saturation equations, such as Waxman-Smits, dual water, SGS and CRMM, n is the intrinsic n, determined with high-salinity water or with the clay effects removed. The variation of I with Sw is then predicted, with varying success, by the different equations. In carbonates with multiple pore types, such as fractures, vugs, interparticle porosity and microporosity, n may change as each pore type is desaturated. A different n may be used for a different range of Sw. In all cases, n increases if any pores are oil-wet. Values up to 8 have been reported in very oil-wet rocks. |
| Formation Evaluation | saturation unit | A unit equal to the percentage of a given fluid in the total volume of a pore space. The term is abbreviated to s.u. and lies between 0 and 100. |
| Formation Evaluation | scintillation detector | A device for measuring the number and energy of gamma rays. The device consists of a crystal and a photomultiplier. In the crystal, an incident gamma ray imparts energy to electrons through Compton scattering, photoelectric absorption and pair production. The electrons excite the detector crystal lattice. Crystal de-excitation emits visible or near-visible light, the scintillation, which is detected by the photomultiplier and transformed into an electrical pulse. The frequency and amplitude of the electric pulse are related to the number of gamma rays and their respective energy levels, and are recorded in a log. Scintillation detectors are used in all natural gamma ray, induced gamma ray and density logging devices. |
| Formation Evaluation | secondary porosity index | An indicator of the porosity that does not contribute to a sonic measurement of interval transit time. The transit time is little affected by vugs, fractures and other irregular events because the sonic wave finds a faster path around them. Spherical pores such as oomolds also have less effect on traveltime than oblate pores. Thus, when the sonic porosity is less than some measurement of the total porosity, the difference can be attributed to the presence of post-depositional, or secondary, porosity. The sonic porosity is usually derived from the Wyllie time-average equation, or some other suitable transform, and the total porosity taken as the density-neutron crossplot porosity. |
| Formation Evaluation | secondary-porosity index | An indicator of the porosity that does not contribute to a sonic measurement of interval transit time. The transit time is little affected by vugs, fractures and other irregular events because the sonic wave finds a faster path around them. Spherical pores such as oomolds also have less effect on traveltime than oblate pores. Thus, when the sonic porosity is less than some measurement of the total porosity, the difference can be attributed to the presence of post-depositional, or secondary, porosity. The sonic porosity is usually derived from the Wyllie time-average equation, or some other suitable transform, and the total porosity taken as the density-neutron crossplot porosity. |
| Formation Evaluation | shale baseline | The average reading of the spontaneous potential (SP) log opposite the shale layers in a well. Opposite shales, the SP is relatively constant and changes only slowly with depth. This is the shale baseline. The log is normally adjusted by the logging engineer to read near zero at the baseline. Sharp shifts in the baseline can sometimes be observed, for example when two permeable beds with different formation water salinities are separated by a shale that is not a perfect cationic membrane, or when the formation water salinity changes within a permeable bed. |
| Formation Evaluation | shifted spectrum | A technique in nuclear magnetic resonance (NMR) logging based on the shift in the T2 distributions, or spectra, acquired with different echo spacings. The technique is usually used to detect gas or light oil. These fluids have a significant diffusion relaxation. A measurement made with a standard short echo spacing will give a signal from these fluids at a certain T2. A measurement made with a long echo spacing will cause more diffusion relaxation and a shorter T2. Other fluids, with minor contribution from diffusion, will not be changed. Gas and light oil can therefore be identified by the shift between the two T2 distributions. |
| Formation Evaluation | shoulder bed | A formation layer above or below the layer being measured by a logging tool. The term is used in particular in resistivity logging to describe the layers above and below a reservoir. Some resistivity tools, such as induction and laterolog devices, can sense beds located tens of feet from the measure point and can be significantly affected by shoulder beds even when the reservoir is thick. The term is more commonly used for vertical wells, and is derived from the typical picture of resistivity log response across a reservoir: a high resistivity reservoir (the head) with two low-resistivity shales above and below (the shoulders). The term also may be used in horizontal wells, although in that context the term surrounding bed is more common. The term adjacent bed is used in both cases. |
| Formation Evaluation | sidewall | Relating to being held against, or taken from, the side of the borehole. The term also describes a measurement that must be made by pressing the sonde against the side of the borehole in order to minimize borehole effects, as, for example, a sidewall epithermal neutron log. |
| Formation Evaluation | sidewall core | A core taken from the side of the borehole, usually by a wireline tool. Sidewall cores may be taken using percussion or mechanical drilling. Percussion cores are taken by firing hollow bullets into the formation. The bullets are attached to the tool by fasteners, and are retrieved, along with the core inside, by pulling up the tool and the fasteners. Percussion coring tools typically hold 20 to 30 bullets, but two or three tools can be combined on one run in the hole. Mechanical tools use hollow rotary drills to cut and then pull out core plugs. Up to 75 plugs can be recovered on one run. With full recovery, cores from typical percussion tools are 1 in. [2.5 cm] in diameter by 1 3/4 in. [4.4 cm] long, while those from mechanical tools are 0.91 in. [2.3 cm] in diameter by 2 in. [5 cm] long. The latter are also known as rotary sidewall cores. |
| Formation Evaluation | sieve analysis | A technique for analyzing the grain-size distribution of a core sample. A cleaned, weighed core sample is disaggregated and agitated through a series of stacked screens with progressively smaller openings. The material left on each screen is weighed in order to give a distribution of quantity versus sieve size. Sieve analysis may be done dry, wet or a combination of both. Wet analysis is necessary for analyzing any clay fraction. |
| Formation Evaluation | sigma | The macroscopic cross section for the absorption of thermal neutrons, or capture cross section, of a volume of matter, measured in capture units (c.u.). Sigma is also used as an adjective to refer to a log of this quantity. Sigma is the principal output of the pulsed neutron capture log, which is mainly used to determine water saturation behind casing. Thermal neutrons have about the same energy as that of the surrounding matter, typically less than 0.4 eV. |
| Formation Evaluation | skin effect | The loss in amplitude and change in phase of an electromagnetic field as it penetrates into a conductive medium. In an induction log, the skin effect causes a reduction of the R-signal (in-phase) and an increase in the X-signal (out-of-phase) at the receiver. It has a significant effect on the 6FF40 array, particularly below 1 ohm-m. Since the magnitude of the reduction depends on the conductivity, the skin effect can be corrected for by using a fixed function of the measured conductivity. A much improved method is to estimate the correction from the X-signal measured in balanced arrays. |
| Formation Evaluation | slow formation | A formation in which the velocity of the compressional wave traveling through the borehole fluid is greater than the velocity of the shear wave through the surrounding formation. In such conditions, there is no critical refraction of the shear wave and no shear head wave generated, so that standard techniques based on monopole transducers cannot be used to measure formation shear velocity. Instead, it is necessary to use dipole sources to excite the flexural mode. The velocity of the latter is closely related to that of the shear wave. In very slow formations, such as in high-porosity gas sands, the formation compressional velocity also may be less than the borehole fluid velocity, causing no compressional head wave. In such cases, it is possible to estimate the formation compressional velocity from the low-frequency end of a leaky mode. |
| Formation Evaluation | slowing-down length | A parameter used to characterize neutron interactions in bulk material above the thermal region. The slowing-down length (Ls) is proportional to the root-mean-square distance from the point of emission of a high-energy neutron to the point at which its energy has decreased to the lower edge of the epithermal energy region. The slowing-down length is the physical parameter that best describes the response of an epithermal neutron porosity measurement, and describes a large part of the response of a thermal neutron porosity measurement. Thermal neutrons have about the same energy as the surrounding matter, typically less than about 0.4 eV, while epithermal neutrons have higher energy, between about 0.4 and 10 eV. |
| Formation Evaluation | slowing-down time | With reference to pulsed neutron logging, the characteristic time for the decay of the epithermal neutron population. The slowing-down time of a formation is strongly dependent on the porosity. In openhole pulsed neutron logging, it is also dependent on the standoff between tool and borehole wall. Epithermal neutrons have energies above that of the surrounding matter, between about 0.4 eV and 10 eV. |
| Formation Evaluation | slowness time coherence | A technique used for identifying and measuring the slowness and time of arrival of coherent acoustic energy propagating across an array of receivers. The different packets of coherent energy can then be identified in terms of their origin, for example compressional, shear, Stoneley or other arrivals. In formation evaluation slowness-time coherence is used in conjunction with an array-sonic tool in which the full waveforms at each receiver have been recorded. The technique consists in passing a narrow window across the waveforms and measuring the coherence within the window for a wide range of slownesses and times of arrival. |
| Formation Evaluation | slowness-time coherence | A technique used for identifying and measuring the slowness and time of arrival of coherent acoustic energy propagating across an array of receivers. The different packets of coherent energy can then be identified in terms of their origin, for example compressional, shear, Stoneley or other arrivals. In formation evaluation slowness-time coherence is used in conjunction with an array-sonic tool in which the full waveforms at each receiver have been recorded. The technique consists in passing a narrow window across the waveforms and measuring the coherence within the window for a wide range of slownesses and times of arrival. |
| Formation Evaluation | small pore water | Water in microporosity or other small pores. The term usually refers to the nuclear magnetic resonance signal of such water, which occurs at very short times and overlaps the signal from clay-bound water. |
| Formation Evaluation | small-pore water | Water in microporosity or other small pores. The term usually refers to the nuclear magnetic resonance signal of such water, which occurs at very short times and overlaps the signal from clay-bound water. |
| Formation Evaluation | sonde | The section of a logging tool that contains the measurement sensors, as distinct from the cartridge, which contains the electronics and power supplies. |
| Formation Evaluation | sonde error | The measurement of an induction tool in a nonconducting medium before correction. Electronic offsets and coupling within the tool cause a signal in the receivers even in a nonconducting medium such as air. This signal is cancelled either electronically or in software. Sonde errors change with temperature and pressure downhole. This can be allowed for by characterizing the sonde’s response to temperature and pressure on the surface. Sonde error is measured by placing the tool far from the ground in air. Ideally measurements are made at two distances, since the ground signal can then be determined from the difference and eliminated. Originally, sonde error referred only to the R-signal, since this was the only signal being used. The term now refers to both R- and X-signals. |
| Formation Evaluation | sonic measurement | The technique for recording a borehole sonic log, in the sense of measurement of any of the acoustic properties in and around the borehole. The standard sonic measurement, based on first motion detection, normally can be used only to determine formation compressional slowness. For all other sonic measurements, such as shear, flexural and Stoneley slownesses and amplitudes, it is necessary to record the full waveform using an array-sonic tool and process with a technique such as slowness-time coherence. |
| Formation Evaluation | sonic measurement | A technique for recording the formation compressional slowness based on the transit time between transmitter and receiver. In the most basic wireline sonic measurement, an acoustic transducer emits a sonic signal, of between about 10 and 30 kHz, which is detected at two receivers farther up the hole. The time between emission and reception is measured for each receiver, and subtracted to give the traveltime in the interval between the two receivers. If the receivers are two feet apart, then this time is divided by two to give the interval transit time, or slowness, of the formation. This type of measurement is also known as first motion detection. This technique works because the first arrival at the receiver is a wave that has traveled from the transmitter to the borehole wall, where it has generated a compressional wave in the formation. Some of this wave is critically refracted up the borehole wall, generating head waves in the borehole fluid as it progresses. Some of these strike the receiver, arriving in most cases well before any signal traveling directly through the mud. Furthermore, if the logging tool is parallel to the borehole wall, the traveltime in the mud is cancelled by taking the difference between the traveltime to the two receivers. Problems of irregular hole or a tilted tool are avoided by using borehole compensation. The depth of investigation depends on the slowness, the transmitter-to-receiver spacing and the presence or absence of an altered zone. It is generally within the invaded zone, and of the order of several inches. |
| Formation Evaluation | Soxhlet extractor | An apparatus for cleaning core samples using the distillation extraction method. In the Soxhlet apparatus (also called extractor, or chamber), the sample soaks in hot solvent that is periodically siphoned off, distilled and returned to the sample. The process continues until the siphoned-off solvent becomes clear. In the Soxhlet apparatus, the sample soaks in the solvent, while in the Dean-Stark apparatus, the solvent flows through the sample from top to bottom. |
| Formation Evaluation | specific permeability | The permeability of a porous medium to a specific fluid, when that fluid is the only fluid present. Permeability is defined as a property of the porous medium. However, the permeability measured on samples often depends on the fluid used. For example, liquids can affect the permeability through fines movement and clay alteration; gas permeability depends on slippage and inertial resistance, unless fully corrected for these effects. It is therefore more correct to talk of specific permeability to a particular fluid, although, in practice, the shorter term, permeability, is common. |
| Formation Evaluation | spectrum | The distribution of gamma ray energies, or the number of gamma rays as a function of gamma ray energy. |
| Formation Evaluation | spherical focusing | A technique for focusing an electrode device based on maintaining a spherical equipotential surface centered at the main current electrode. Unlike the laterolog, which tries to maintain equipotential lines parallel to the sonde, spherical focusing tries to maintain the spherical equipotential lines that would exist in a homogeneous formation with no borehole. This is achieved with a particular arrangement of current-emitting electrode, current-return electrodes and monitor electrodes. This arrangement creates two spherical equipotential spheres with a constant voltage drop between them. The resistivity is determined from the current flowing between the spheres and the voltage drop. The depth of investigation is determined by the radii of the two spheres. Spherical focusing is used to produce shallow-reading resistivity logs and the pad-based microspherical log. |
| Formation Evaluation | spine and ribs plot | For a two-detector density tool, the plot of long-spacing versus short-spacing count rates for different formation densities, mudcake densities and mudcake thicknesses. The plot takes its name from the spine, which is the locus of points with no mudcake, and the ribs, which show the effect of mudcake at certain fixed formation densities. The plot illustrates graphically that for a given formation density there is only one rib for all normal mudcake densities and thicknesses. Thus, although there are three unknowns, it is possible to make a correction using two measurements. |
| Formation Evaluation | spiral borehole | Another term for corkscrew hole, the result of certain drilling conditions that cause the borehole to take the shape of a corkscrew. Most logging tools are much longer than the wavelength of the corkscrew, and therefore see it as a change in standoff or a change in hole size. For this reason, the corkscrew is often observed as a wave on the caliper log. A corkscrew hole affects measurements sensitive to standoff, such as induction and neutron porosity, and may affect pad tools, if they cannot follow the changes. |
| Formation Evaluation | spontaneous potential | A log of the natural difference in electrical potential, in millivolts, between an electrode in the borehole and a fixed reference electrode on the surface. The most useful component of this difference is the electrochemical potential since it can cause a significant deflection opposite permeable beds. The magnitude of the deflection depends mainly on the salinity contrast between drilling mud and formation water, and the clay content of the permeable bed. The spontaneous potential (SP) log is therefore used to detect permeable beds and to estimate formation water salinity and formation clay content. The SP log cannot be recorded in nonconductive mud. The SP can be affected by several factors that make interpretation difficult. First, there are other possible sources of electrical potential not related to the electrochemical effect, for example, the electrokinetic potential and bimetallism. Many of these are small and constant throughout the log, and can be lumped together in the shale baseline. Second, the SP can measure only the potential drop in the borehole, and not the full electrochemical potential. The ideal SP opposite a clean bed is known as the static spontaneous potential (SSP), and opposite a shaly bed as the pseudostatic spontaneous potential (PSP). The SP is always less than the SSP or the PSP and more rounded at the boundaries between shales and permeable beds. The SP was first recognized by C. Schlumberger, M. Schlumberger and E.G. Leonardon in 1931, and the first published examples were from Russian oil fields. |
| Formation Evaluation | spreading loss | The additional loss in amplitude of an electromagnetic wave emitted by an electromagnetic propagation or dielectric propagation measurement compared to that of a plane wave. The spreading loss depends on the geometry of the transmitter-receiver array and also on the dielectric properties of the formation. The same effect also causes a small correction to the propagation time. |
| Formation Evaluation | square log | A log in which the changes in reading with depth only occur abruptly, with no transition. A square log is often an approximation of a real log, in which the continuously varying input log has been approximated by constant values and abrupt changes. A square log contains less data than a real log but can be useful for further processing. |
| Formation Evaluation | SSP | A mnemonic for the static spontaneous potential. |
| Formation Evaluation | standoff | A piece of material designed to hold a logging tool a certain distance away from the borehole wall. It is usually made of hard rubber and consists of four to six fins of the desired length. |
| Formation Evaluation | standoff | The distance between the external surface of a logging tool and the borehole wall. This distance has an important effect on the response of some logging measurements, notably induction and neutron porosity logs. For resistivity tools, the effect of standoff is taken into account in the borehole correction. In the neutron porosity tool, it is usually corrected for separately. In a smooth, regular hole, the standoff is constant and determined by the geometry of the logging tool string and the borehole. In rugose or irregular holes, standoff varies with depth. |
| Formation Evaluation | static spontaneous potential | The ideal spontaneous potential (SP) that would be observed opposite a permeable bed if the SP currents were prevented from flowing and any shaliness in the bed were ignored. The static spontaneous potential (SSP) is equal to the electrochemical potential. When current is flowing, the SP measures only that fraction of the potential drop that occurs in the borehole. In normal conditions, this potential drop is much higher than the drop in the formation because the cross-sectional area of the borehole is much smaller, and hence its resistance much higher. It is for this reason that in the middle of a thick, clean bed whose resistivity is not too high, the SP reads close to the SSP. However, in other conditions the SP is significantly less than the SSP. As well as ignoring shaliness in the sand, the SSP ignores other sources of potential and assumes a surrounding shale that is a perfect cationic membrane. |
| Formation Evaluation | steady state | A system that has reached equilibrium for the measurement or phenomenon concerned. In the case of permeability measurements on core samples, a steady state is reached when the flow rate, the upstream and the downstream pressures no longer change with time. At this point the permeability can be calculated from the flow rate and pressures and applying Darcy’s equation. If gas is used, the inertial resistance and gas slippage (Klinkenberg effect) should be corrected for. |
| Formation Evaluation | step profile | With reference to invasion, an abrupt change from the flushed zone to the undisturbed zone, with no transition zone or annulus. This simple model is used most commonly in connection with older resistivity logs since it allows the invasion to be represented by three parameters: flushed-zone resistivity, undisturbed-zone resistivity and diameter of invasion. The model assumes equal invasion at all azimuths. Newer array logs allow more complex invasion models to be interpreted. |
| Formation Evaluation | stick and slip | The irregular movement of a logging tool up a well due to it being stuck at some point and then being released. In normal operation, the cable is pulled smoothly out of the well and the logging tool follows. However, the tool can become stuck by differential pressure or an irregular hole. The cable stretches, and its tension increases, until the tool is freed. At this point it moves, or slips, quickly up the hole until the normal movement is resumed. Since the depth measurement is driven by the cable, the log readings opposite a zone of stick and slip are displayed at incorrect depths. Furthermore, since each measurement has a different measure point, the zone of stick and slip shows up at a different depth on each measurement. |
| Formation Evaluation | Stoneley permeability | The ability of fluid to move through a rock, as measured by the reduction in amplitude or increase in slowness of the acoustic Stoneley wave generated in the borehole. The velocity and amplitude of the Stoneley wave are reduced by the presence of mobile fluids in the formation. Physically, the effect can be seen as a coupling of the Stoneley energy into a formation wave known as the slow wave, as predicted by the Biot theory. The amount of reduction is a complicated function of this mobility (or permeability divided by viscosity), the properties of the borehole fluid, the pore fluid and the mudcake, the elastic properties of the rock and the frequency. Since all these factors are measured or estimated from logs, it is possible to determine formation mobility. In practice, the mobility needs to be reasonably high for the method to be accurate. |
| Formation Evaluation | stress induced anisotropy | A situation in which the formation shear-wave velocity varies azimuthally around the borehole, because unequal stresses in the formation have caused azimuthal variations in the stress concentrations around the borehole. These stress concentrations change the shear-wave speeds in the region surrounding the well from those in the far field, such that a characteristic response is observed in the dispersion of the dipole flexural mode. |
| Formation Evaluation | stress-induced anisotropy | A situation in which the formation shear-wave velocity varies azimuthally around the borehole, because unequal stresses in the formation have caused azimuthal variations in the stress concentrations around the borehole. These stress concentrations change the shear-wave speeds in the region surrounding the well from those in the far field, such that a characteristic response is observed in the dispersion of the dipole flexural mode. |
| Formation Evaluation | structural shale | A particular type of shale distribution in which the shale exists as grains within a rock framework, in contrast to dispersed shale and laminar shale. The term also refers to a formation model or saturation equation based on this distribution. |
| Formation Evaluation | summation of fluids method | A technique for measuring the effective porosity of a core sample by summing the volumes of the fluids recovered from it. The volumes of the gas, oil and water in the sample usually are determined by the retort method, which also determines the bulk volume. The porosity is then the ratio of the total fluid volume to the bulk volume. |
| Formation Evaluation | surrounding bed | A formation layer above or below the layer being measured by a logging tool. The term is used in particular to describe the adjacent layers above or below a horizontal well. In a vertical well, the term shoulder bed is more common. The term adjacent bed is used in both cases. |
| Formation Evaluation | survey | The measurement versus depth or time, or both, of one or more physical quantities in or around a well. In early years, the term was used more often than log. |
| Formation Evaluation | survey | To record a measurement versus depth or time, or both, of one or more physical quantities in or around a well. In early years, the term was used more often than log. |
| Formation Evaluation | systematic error | A reproducible inaccuracy of measurement introduced by either faulty design, failing equipment, inadequate calibration, inferior procedure or a change in the measurement environment. |
| Formation Evaluation | T1 | In a nuclear magnetic resonance measurement, the characteristic time for longitudinal relaxation. In rocks, longitudinal relaxation is the inverse sum of the surface relaxation and bulk relaxation. T1 is not normally measured in NMR logging, but is an important parameter in deciding the polarization time and hence the logging speed. T1 is closely related to the transverse relaxation time, T2. The ratio T1/T2 in water-filled rocks is typically between 1.5 and 2.5. In light hydrocarbons and gas, the ratio increases up to 10 and more as the viscosity decreases. |
| Formation Evaluation | T2 | In nuclear magnetic resonance (NMR) logging, the characteristic time for transverse relaxation. In rocks, there are three components of the transverse relaxation: surface, Ts; bulk, Tb; and diffusion relaxation, Td. T2 is the inverse sum of each component for each fluid, as follows: 1/T2 = 1/Ts + 1/Tb + 1/Td. Because of the reciprocal sum, the smallest of the three types of relaxations is the most important in determining the final T2 for each fluid. There is not one single value of T2 for a rock but a wide distribution of values lying anywhere between fractions of a millisecond and several seconds. The distribution of T2 values is the principal output of an NMR log. |
| Formation Evaluation | tail | The last page or pages on a log print, which may contain data about the well, the recording parameters and the calibration of the measurements. |
| Formation Evaluation | tapered cutoff | In a nuclear magnetic resonance measurement, the use of a gradual rather than a sharp cutoff to distinguish between bound water and free water. A sharp cutoff at, for example T2 = 33 ms in sandstones, is normally used to distinguish free water (all T2s above 33 ms) from bound water (all T2s below 33ms). In a water-filled rock, in the fast diffusion limit, T2 is directly related to pore size. The distinction between bound and free water is based on the assumption that all free water resides in large pores, and all bound water in small pores. However, in rocks with large pores, a significant volume of bound water exists on the surface of the grains around a large pore. Being part of a large pore, it gives a long T2 and will be incorrectly counted as free water. One solution is the tapered cutoff, in which the bound water is the sum of all the T2 below a minimum, for example 5 ms, and is then a progressively smaller fraction of the volume at T2s up to a maximum, for example 500 ms. All signal above 500 ms represents free water. The form of the taper is usually empirical, but is based on some model of pore shape, such as a bundle of tubes. See Kleinberg RL and Boyd A: ‘Tapered Cutoffs for Magnetic Resonance Bound Water Volume’ paper SPE 38737, presented at the SPE Annual Technical Conference and Exhibition, San Antonio, Texas, USA, October 5-8, 1997. |
| Formation Evaluation | telemetry | A system for converting the measurements recorded by a wireline or measurements-while-drilling (MWD) tool into a suitable form for transmission to the surface. In the case of wireline logging, the measurements are converted into electronic pulses or analog signals that are sent up the cable. In the case of MWD, they are usually converted into an amplitude or frequency-modulated pattern of mud pulses. Some MWD tools use wirelines run inside the drillpipe. Others use wireless telemetry, in which signals are sent as electromagnetic waves through the Earth. Wireless telemetry is also used downhole to send signals from one part of an MWD tool to another. |
| Formation Evaluation | terrain correction | The positive gravity correction that accounts for the deviation of the topography from the horizontal slab of infinite extent assumed in the Bouguer correction. Local topographic features always decrease the gravity measurement because the attractive force of the topography above the station is away from the Earth, and the effect of topography below the station is negative because of the absence of attractive material. |
| Formation Evaluation | thermal capture | A neutron interaction in which the neutron is absorbed by the target nucleus, producing an isotope in an excited state. The activated isotope de-excites instantly through the emission of characteristic gamma rays. Thermal capture, also called neutron capture, usually occurs at low thermal energies at which the neutrons have about the same energy as the surrounding matter, typically below 0.4 eV (0.025 eV at room temperature). Some elements are better thermal absorbers than others. Neutron capture is an important principle behind the pulsed neutron capture log, the elemental capture spectroscopy log, the pulsed neutron spectroscopy log and the thermal neutron porosity measurement. |
| Formation Evaluation | thermal diffusion length | A parameter used to characterize thermal neutron interactions in bulk material. Thermal diffusion length (Ld) is the characteristic distance between the point at which a neutron becomes thermal and the point of its final capture. It is related to the quantity of thermal absorbers in the formation, and therefore is an important factor in the thermal neutron porosity measurement. Thermal neutrons have about the same energy as the surrounding matter, typically less than 0.4 eV (0.025 eV at room temperature). |
| Formation Evaluation | thermal neutron absorber | An element, or mineral, that is particularly effective in absorbing thermal neutrons (neutrons with about the same energy as the surrounding matter, typically less than 0.4 eV). The elements gadolinium, boron, chlorine, hydrogen and iron are thermal absorbers (in decreasing order of effectiveness). The effect of chlorine is used in a pulsed neutron capture log to distinguish salty water from hydrocarbons. In a thermal neutron porosity measurement, the effect of hydrogen is important, while boron and iron affect the response in shales. |
| Formation Evaluation | thermal neutron porosity measurement | A measurement of the slowing down and capture of neutrons between a source and one or more thermal neutron detectors. The neutron source emits high-energy neutrons that are slowed mainly by elastic scattering to near thermal levels. Thermal neutrons have about the same energy as the surrounding matter, typically less than 0.4 eV. The slowing-down process is dominated by hydrogen. At thermal levels, the neutrons diffuse through the material until they undergo thermal capture. Capture is dominated by chlorine, hydrogen and other thermal neutron absorbers. Typical thermal neutron measurements use a chemical neutron source and two thermal neutron detectors. An accelerator source (neutron generator) is sometimes used. Some, mainly earlier, devices measure the gamma rays emitted by thermal capture, rather than thermal neutrons. |
| Formation Evaluation | thorium | An element with an atomic number of 90. The 232Th isotope is radioactive and decays with a half-life of 1.4 * 1010 years through a series of intermediate isotopes to a stable isotope of lead. The intermediate isotopes emit a wide range of gamma rays, the most prominent being that of thallium, 208Tl. It is assumed that formations are in secular equilibrium; that is, the relative proportions of parent and daughter isotopes remain constant, and the measured spectrum is directly related to the amount of 232Th. The concentration in the Earth’s crust is about 12 parts per million, ppm, by weight. Thorium-bearing minerals are rare. Thorium is a trace element associated with clays and heavy minerals. It is very immobile so that quantity measured today probably was present at the time of deposition. A log of thorium is presented in parts per million. It is often a good measure of clay content. |
| Formation Evaluation | tick mark | In logging while drilling, a mark associated with each measurement indicating when a sample was taken. It is usually presented as a short bar in the depth track. Widely spaced tick marks indicate a low sampling rate. In wireline logging, a tick mark indicates the cumulative volume of some quantity, such as hole volume or traveltime. The term is sometimes spelled tic mark |
| Formation Evaluation | time after bit | The time that has elapsed between the bit first penetrating a formation and a log being recorded opposite it. In logging while drilling, this time is different for each log, since it depends on the drilling rate and the distance between the bit and the particular logging sensor. |
| Formation Evaluation | time lapse | Pertaining to techniques in which the same quantity is measured at different times in the life of a reservoir. Normally the only change in a time-lapse measurement or survey will be due to changes in water or gas saturation. Thus, a comparison of two logs run at different times, such as a year apart, should simply reflect the change in fluid saturations in the pore space. The most common time-lapse logs are made with pulsed neutron capture, pulsed neutron spectroscopy and borehole gravity measurements. |
| Formation Evaluation | time-lapse | Pertaining to techniques in which the same quantity is measured at different times in the life of a reservoir. Normally the only change in a time-lapse measurement or survey will be due to changes in water or gas saturation. Thus, a comparison of two logs run at different times, such as a year apart, should simply reflect the change in fluid saturations in the pore space. The most common time-lapse logs are made with pulsed neutron capture, pulsed neutron spectroscopy and borehole gravity measurements. |
| Formation Evaluation | tool string | The downhole hardware needed to make a log. Measurements-while-drilling (MWD) logging tools, in some cases known as logging while drilling (LWD) tools, are drill collars into which the necessary sensors and electronics have been built. Wireline logging tools are typically cylinders from 1.5 to 5 in. [3.8 to 12.7 cm] in diameter. Since the total length is more than can be conveniently handled in one piece, the logging tool is divided into different sections that are assembled at the wellsite. These sections consist of cartridges and sondes. Different measurements can be combined to make up a tool string. The total length of a tool string may range from 10 to 100 ft [3 to 30 m] or more. Flexible joints are added in long tool strings to ease passage in the borehole, and to allow different sections to be centralized or eccentralized. If the total length is very long, it is often preferable to make two or more logging runs with shorter tool strings. |
| Formation Evaluation | top log interval | The top of the interval recorded on the log, or the shallowest point at which the log readings are valid. If the top of the log is at the casing shoe, the last valid reading of many logs will be a short distance below. However, it is common to give the depth of the casing shoe as the top log interval. |
| Formation Evaluation | tornado chart | A plot representing the effect of invasion on resistivity measurements that have different depths of investigation. The plot assumes a step-profile model of invasion and determines true resistivity, flushed zone resistivity and diameter of invasion from ratios of deep-, medium- and shallow-resistivity measurements. Strictly speaking, when both resistive invasion and conductive invasion are plotted, the chart is called a butterfly chart. When only one is plotted, it is known as a tornado chart. |
| Formation Evaluation | torpedo | The connection between the wireline logging cable and the bridle. The torpedo consists of an outer mechanical connection enclosing electrical connections between the conductors. |
| Formation Evaluation | tortuosity | A measure of deviation from a straight line. It is the ratio of the actual distance traveled between two points, including any curves encountered, divided by the straight line distance. Tortuosity is used by drillers to describe wellbore trajectory, by log analysts to describe electrical current flow through rock and by geologists to describe pore systems in rock and the meander of rivers. |
| Formation Evaluation | total porosity | The total pore volume per unit volume of rock. It is measured in volume/volume, percent or porosity units. The total porosity is the total void space and as such includes isolated pores and the space occupied by clay-bound water. It is the porosity measured by core analysis techniques that involve disaggregating the sample. It is also the porosity measured by many log measurements, including density, neutron porosity and nuclear magnetic resonance logs. |
| Formation Evaluation | trace | The presentation on hard copy of log data from a single measurement versus depth. The term originated with the early optical recorders in which log data were recorded on film using an optical trace. Now the term curve is more common. |
| Formation Evaluation | track | A vertical section of a log presentation over which one particular set of data is displayed. The track divides the presentation into different sections, each with a certain set of log curves or other data, such as depth numbers. The section is vertical in the sense that it is along the depth or time axis of the log. The curves are usually blanked off when they run outside their allotted track. Tracks are typically numbered from left to right across the page (when viewed with depth or time increasing towards the bottom of the page). |
| Formation Evaluation | transverse relaxation | The loss of coherent energy by protons in a rock while precessing about a static magnetic field during a nuclear magnetic resonance measurement. The loss of coherent energy, or relaxation, due to the free induction decay is corrected by the CPMG pulse sequence. This leaves three mechanisms for relaxation: surface relaxation, bulk relaxation and diffusion relaxation, all of which depend on formation properties. Transverse relaxation is characterized by an exponential decay of time constant T2. |
| Formation Evaluation | true resistivity | With reference to core analysis, the resistivity of a sample only partially filled with water. Called Rt, it is used in contrast to the resistivity of a sample completely filled with water, Ro. The water may be replaced by any nonconductive fluid, usually air or dead oil. |
| Formation Evaluation | true resistivity | With reference to log analysis, the resistivity of the undisturbed formation. It is derived from a resistivity log that has been corrected as far as possible for all environmental effects, such as borehole, invasion and surrounding bed effects. Hence, it is taken as the true resistivity of the undisturbed formation in situ and is called Rt. |
| Formation Evaluation | ultra-long spaced electrical log | A conventional electrical log with very long electrode spacing. The ultralong-spaced electrical log (ULSEL) is a normal device with a distance between the current-emitting electrode and the measure electrode of between about 75 ft [23 m] and 1000 ft [305 m]. The long spacings are designed to locate objects within tens of feet of the borehole. These objects may be a salt dome or casing, in the case of a relief well. |
| Formation Evaluation | ultrasonic measurement | In the context of borehole logging, measurements of acoustic signals that are in the hundreds of kilohertz to the low-megahertz range. Such ultrasonic instruments are mostly of the pulse-echo type, and are used in the borehole televiewer, and in various cased-hole devices to determine corrosion and cement-bond quality. |
| Formation Evaluation | uncertainty | Result of the evaluation aimed at characterizing the range within which the true value of a quantity is estimated to lie, generally with a given likelihood. (ISO) The uncertainty is the amount of possible inaccuracy. It is often a statistical estimate of this range, such as the half-width of a gaussian distribution. The quantity may have been measured or derived from an equation. |
| Formation Evaluation | undisturbed zone | The part of the formation that has not been affected by invasion. |
| Formation Evaluation | unsteady state | A system that is in a transient state at the time of a measurement. In the case of permeability measurements on core samples, two transient techniques are used. In the pressure falloff method, the sample is at atmospheric pressure, either in a chamber or under a probe. Fluid at a higher pressure is released into one end of the sample. The decay of pressure with time at that end of the sample is recorded and analyzed by techniques similar to those used for transient well tests. In the pulse-decay method, the sample is held in a chamber and connected to two reservoirs, all of which are filled with a fluid at high pressure. The pressure in one of the reservoirs is increased a small amount and then re-connected to the sample. The change in pressure with time of this sample is recorded and analyzed using flow equations. |
| Formation Evaluation | uranium | An element with an atomic number of 92. The 238U isotope is radioactive and decays with a half-life of 4.4 * 109 years through a series of intermediate isotopes to a stable isotope of lead. The intermediate isotopes emit a wide range of gamma rays, the most prominent being that of bismuth, 214Bi. It is assumed that formations are in secular equilibrium; that is, the relative proportions of parent and daughter isotopes remain constant, and the measured spectrum is directly related to the amount of 238U. The concentration in the Earth?s crust is about 4 ppm by weight. Uranium-bearing minerals are rare. Uranium is a soluble trace element that is transported easily and can be precipitated far from its source. It is most frequently found in carbonates and organic materials. A log of uranium is presented in parts per million, ppm. It is useful for detecting organic material, but is otherwise considered not useful for quantitative evaluation. In the corrected gamma ray log, the uranium contribution is removed to differentiate carbonates from shales. |
| Formation Evaluation | variable-density log | A presentation of the acoustic waveform at a receiver of a sonic or ultrasonic measurement, in which the amplitude is presented in color or the shades of a gray scale. The variable-density log is commonly used as an adjunct to the cement-bond log, and offers better insights into its interpretation; in most cases microannulus and fast-formation-arrival effects can be identified using this additional display. In openhole, it may be displayed alongside the sonic log transit-time as a qualitative presentation of the acoustic wave train, and is sometimes used for fracture detection by examination of the chevron patterns given by Stoneley wave reflections (and other wave reflections) at fractures crossing the borehole. |
| Formation Evaluation | verification | A check performed at the wellsite to establish whether a logging measurement is functioning properly. Verification is also known as an operational check. The verification may be done before or after the survey and may be presented with the log. Verification is distinct from calibration. |
| Formation Evaluation | verification listing | A description of the contents of a digital record. |
| Formation Evaluation | vertical resistivity | The resistivity of a formation measured by flowing current in a vertical plane. In anisotropic formations, the horizontal and vertical resistivities are different. In a vertical well, wireline induction logs and measurements-while-drilling propagation logs measure the horizontal resistivity, whereas laterologs measure the horizontal resistivity with some component of the vertical. In deviated and horizontal wells, all these logs measure some mixture of both vertical and horizontal resistivity. |
| Formation Evaluation | vertical resolution | A distance that characterizes the ability of a logging tool to resolve changes parallel to the tool axis. The word vertical implies a vertical well, but the term is used at other wellbore deviations. The vertical resolution summarizes the vertical response of the measurement in one or more distances. Most quoted vertical resolutions assume a homogeneous formation with stated properties. Vertical resolutions can vary considerably in more complex conditions, and at different values of the properties concerned. They should be considered only a qualitative guide to tool response. There are several different definitions of the vertical resolution distance. First, and most commonly, it is the interval within which a large percentage, typically 90%, of the vertical response occurs. Second, it is the minimum bed thickness needed for the measurement to read within a small percentage, typically 10%, of the true value at the center of the bed. Third, it may refer to the smallest bed thickness for which a significant change can be detected by the measurement. For acoustic and electromagnetic propagation measurements, it is taken, with reasonable accuracy, as the span of the receiver array. For nuclear and nuclear magnetic resonance measurements, which must be acquired during a significant time interval, the vertical resolution also depends on the logging speed and the precision required. |
| Formation Evaluation | vertical response | The response of a logging measurement as a function of distance parallel to the tool axis. The word vertical implies a vertical well, but the term is used at other wellbore deviations. Vertical responses are determined by computer simulation or laboratory measurement. For some measurements, mainly resistivity, the vertical response can be shaped as desired through signal processing. In general, the vertical response depends on the formation properties throughout the measurement volume. Most quoted vertical responses have been determined in formations that are radially homogeneous and have small vertical changes. They can then be summarized by a geometrical factor or a pseudogeometrical factor. These factors are appropriate for volumetric measurements such as nuclear and resistivity, but not for others such as acoustic propagation. |
| Formation Evaluation | virgin zone | The part of the formation that has not been affected by invasion. |
| Formation Evaluation | volumetric cross section | The cross section of a material to photoelectric absorption, in barns/cm3. The volumetric cross section, U, is from the product of the photoelectric factor, PEF or Pe , and the electron density. In practice, U is usually calculated using the bulk density instead of the electron density. U is a volumetric quantity, whereas Pe is not. U is more useful in log interpretation since it can be used in a linear mixing law in terms of the volumes of the formation components. |
| Formation Evaluation | wait time | The time allotted for the alignment of protons with the static magnetic field during a nuclear magnetic resonance measurement. The term is used more generally with reference to logging tools, and is synonymous with the more general term polarization time. |
| Formation Evaluation | water filled resistivity | The resistivity of a sample completely filled with water. Called Ro, it is used in contrast to the resistivity of a sample only partially filled with water, Rt. The ratio Rt / Ro is called the resistivity index, I. |
| Formation Evaluation | water saturation | The fraction of water in a given pore space. It is expressed in volume/volume, percent or saturation units. Unless otherwise stated, water saturation is the fraction of formation water in the undisturbed zone. The saturation is known as the total water saturation if the pore space is the total porosity, but is known as effective water saturation if the pore space is the effective porosity. If used without qualification, the term usually refers to the effective water saturation. |
| Formation Evaluation | water wet | Describing the preference of a solid to be in contact with a water phase rather than an oil or gas phase. Water-wet rocks preferentially imbibe water. Generally, sandstones and carbonates are water-wet before contact with crude oil, but may be altered by components of the crude oil to become oil-wet. Certain minerals, as well as different crystallographic faces of the same mineral, may be variably prone to being oil- or water-wet. |
| Formation Evaluation | water-filled resistivity | The resistivity of a sample completely filled with water. Called Ro, it is used in contrast to the resistivity of a sample only partially filled with water, Rt. The ratio Rt / Ro is called the resistivity index, I. |
| Formation Evaluation | water-wet | Describing the preference of a solid to be in contact with a water phase rather than an oil or gas phase. Water-wet rocks preferentially imbibe water. Generally, sandstones and carbonates are water-wet before contact with crude oil, but may be altered by components of the crude oil to become oil-wet. Certain minerals, as well as different crystallographic faces of the same mineral, may be variably prone to being oil- or water-wet. |
| Formation Evaluation | weak point | A piece of steel cable placed inside a logging head that is designed to break at a predetermined tension. If the logging tool becomes stuck in the borehole, there is a danger that the logging cable will break at surface, since this is the place of maximum tension. It is difficult to fish a long length of tangled cable in the borehole. The weak point is designed to break before the cable, so that the latter can be retrieved, leaving only the logging tool and head in the borehole. |
| Formation Evaluation | well log | The measurement versus depth or time, or both, of one or more physical quantities in or around a well. The term comes from the word “log” used in the sense of a record or a note. Wireline logs are taken downhole, transmitted through a wireline to surface and recorded there. Measurements-while-drilling (MWD) and logging while drilling (LWD) logs are also taken downhole. They are either transmitted to surface by mud pulses, or else recorded downhole and retrieved later when the instrument is brought to surface. Mud logs that describe samples of drilled cuttings are taken and recorded on surface. |
| Formation Evaluation | wet clay porosity | The proportion of a wet clay that is clay-bound water. A formation that has 100% clay would have a porosity equal to the wet-clay porosity (WCLP), all of it being clay-bound water, and a volume of dry clay equal to (1 – WCLP). The concept is used to relate the volume of clay-bound water, CBW, to the volume of dry clay, Vdcl, in an actual rock, since the ratio of the two is the same and equal to: CBW / Vdcl = WCLP / (1 – WCLP). |
| Formation Evaluation | wet-clay porosity | The proportion of a wet clay that is clay-bound water. A formation that has 100% clay would have a porosity equal to the wet-clay porosity (WCLP), all of it being clay-bound water, and a volume of dry clay equal to (1 – WCLP). The concept is used to relate the volume of clay-bound water, CBW, to the volume of dry clay, Vdcl, in an actual rock, since the ratio of the two is the same and equal to: CBW / Vdcl = WCLP / (1 – WCLP). |
| Formation Evaluation | whole core | A complete section of a conventionally drilled core. The section may be up to about 2 feet [0.6 m] in length, with typical core diameters lying between 1.75 and 5.25 in. [4.4 and 13.3 cm]. The term full-diameter core is also used, but generally refers to shorter sections of about 6 in. [15 cm]. The advantage of whole core analysis is that it measures properties on a larger scale, closer to that of the reservoir. This is particularly important for heterogeneous formations such as many carbonates or fractured materials. |
| Formation Evaluation | wireline | Related to any aspect of logging that employs an electrical cable to lower tools into the borehole and to transmit data. Wireline logging is distinct from measurements-while-drilling (MWD) and mud logging. |
| Formation Evaluation | X signal | The reactive (X) signal, or that part of the alternating signal at the receiver of an induction logging tool that is out of phase with the transmitter current. This signal, also known as the quadrature signal, is less sensitive to formation conductivity and must be separated from the R-signal, which is the main or, in older tools, the only source of the induction log. In a simple two-coil array, a large component of the X-signal is caused by direct coupling between transmitter and receiver coils. This direct signal is largely eliminated by use of a bucking coil. The X-signal provides a first-order correction to the skin effect of the R-signal. |
| Formation Evaluation | X-radiography | A technique for imaging a core by moving a source of X-rays along a core and recording the attenuated X-rays on the other side on a suitable photographic film. |
| Formation Evaluation | X-ray diffraction | A technique for the semiquantitative mineralogical analysis of a sample of rock by measuring the diffraction peaks in X-rays diffracted by the sample. The position of the diffraction peaks is a measure of the distance between discrete crystallographic diffracting planes within minerals, while their intensity indicates the quantity of the mineral. The technique is only semiquantitative because the size and shape of the diffraction peak are strongly influenced by the geometry of the measurement, for example orientation of the minerals, and sample preparation. Fine particles such as clays must be separated from larger particles and measured separately if they are to be detected properly. To reduce errors associated with preferred orientation of minerals, samples are most commonly ground to a powder before analysis, a technique known as powder X-ray diffraction. |
| Formation Evaluation | X-ray fluorescence (XRF) | A technique for elemental analysis of samples based on the characteristic fluorescence given off by different elements subjected to X-rays. In core analysis, X-ray fluorescence often is used to help determine mineral content. The elemental volumes are inverted to mineral volumes by assuming certain standard formulae for mineral composition. |
| Formation Evaluation | X-signal | The reactive (X) signal, or that part of the alternating signal at the receiver of an induction logging tool that is out of phase with the transmitter current. This signal, also known as the quadrature signal, is less sensitive to formation conductivity and must be separated from the R-signal, which is the main or, in older tools, the only source of the induction log. In a simple two-coil array, a large component of the X-signal is caused by direct coupling between transmitter and receiver coils. This direct signal is largely eliminated by use of a bucking coil. The X-signal provides a first-order correction to the skin effect of the R-signal. |
| Formation Evaluation | Z/A effect | The ratio of the electron density to the bulk density. The electron density is equal to the bulk density multiplied by 2Z/A where Z is the average atomic number and A is the average atomic weight of the formation. The density log actually responds to electron density, whereas the desired measurement is the bulk density. Although for most rocks 2Z/A is close to 1, it is not practical to adjust 2Z/A for each formation. Instead the electron density is scaled to give the correct bulk density in limestone filled with fresh water. Then it is found that in the majority of sedimentary rocks, the log reads the bulk density within 1%. The main exceptions are halite, sylvite and low-pressure gas, because the 2Z/A of chlorine and hydrogen are not close to 1. |
| Formation Evaluation, Drilling Fluids | filtrate | The liquid that passes through a filter cake from a slurry held against the filter medium, driven by differential pressure. Dynamic or static filtration can produce a filtrate. |
| Formation Evaluation, Drilling Fluids | mud filtrate | The liquid that passes through a filter cake from a slurry held against the filter medium, driven by differential pressure. Dynamic or static filtration can produce a filtrate. |
| Formation Evaluation, Enhanced Oil Recovery | diffusion | The process by which particles move over time within a material due to their kinetic motion. The term is most commonly used in pulsed neutron capture logging and in nuclear magnetic resonance (NMR) logging. In a pulsed neutron capture log, the term refers to the spread of neutrons away from the neutron generator. In NMR logging, diffusion refers to the movement of gas, oil or water molecules within the pore space. |
| Formation Evaluation, Enhanced Oil Recovery | drainage | The process of forcing a nonwetting phase into a porous rock. Oil migrates into most reservoirs as the non-wetting phase, so initial charging of the reservoir is a drainage process. |
| Formation Evaluation, Enhanced Oil Recovery | electrical double layer | With reference to formation evaluation, the layer between a clay particle and the formation water that has a particular distribution of ions. Clays have an excess negative charge on their surface. When in contact with formation water, this charge attracts an excess of positive cations, normally Na+ together with their molecules of hydration water, into a region near the interface. The layer next to the clay surface, the Stern layer, has no anions, and is always present. Outside the Stern layer is the Gouy layer, through which the ion concentration gradually approaches that of free brine. The thickness of this layer increases as brine salinity decreases. |
| Formation Evaluation, Enhanced Oil Recovery | imbibition | The process of absorbing a wetting phase into a porous rock. Imbibition is important in a waterdrive reservoir because it can advance or hinder water movement, affecting areal sweep. Spontaneous imbibition refers to the process of absorption with no pressure driving the phase into the rock. It is possible for the same rock to imbibe both water and oil, with water imbibing at low in situ water saturation, displacing excess oil from the surface of the rock grains, and oil imbibing at low in-situ oil saturation, displacing excess water. An imbibition test is a comparison of the imbibition potential of water and oil into a rock. The wettability of the rock is determined by which phase imbibes more. |
| Formation Evaluation, Enhanced Oil Recovery | oil wet | Pertaining to the preference of a solid to be in contact with an oil phase rather than a water or gas phase. Oil-wet rocks preferentially imbibe oil. Generally, polar compounds or asphaltenes deposited from the crude oil onto mineral surfaces cause the oil-wet condition. Similar compounds in oil-base mud also can cause a previously water-wet rock to become partially or totally oil-wet. |
| Formation Evaluation, Enhanced Oil Recovery | oil-wet | Pertaining to the preference of a solid to be in contact with an oil phase rather than a water or gas phase. Oil-wet rocks preferentially imbibe oil. Generally, polar compounds or asphaltenes deposited from the crude oil onto mineral surfaces cause the oil-wet condition. Similar compounds in oil-base mud also can cause a previously water-wet rock to become partially or totally oil-wet. |
| Formation Evaluation, Enhanced Oil Recovery | residual oil | Oil that does not move when fluids are flowed through the rock in normal conditions, for example primary and secondary recovery, and invasion. |
| Formation Evaluation, Geophysics | crosswell reflection tomography | A crosswell seismic technique that incorporates reflection traveltimes and direct traveltimes into a tomographic inversion algorithm to produce images of seismic velocity between wells. |
| Formation Evaluation, Geophysics | crosswell seismic tomography | A survey technique that measures the seismic signal transmitted from a source, located in one well, to a receiver array in a neighboring well. The resulting data are processed to create a reflection image or to map the acoustic velocity or other properties (velocities of P- and S-waves, for example) of the area between wells. Placement of the source and receiver array in adjacent wells not only enables the formation between wells to be surveyed, it also avoids seismic signal propagation through attenuative near-surface formations. Another advantage is that it places the source and receiver near the reservoir zone of interest, thereby obtaining better resolution than is possible with conventional surface seismic surveys. This technique is often used for high-resolution reservoir characterization when surface seismic or vertical seismic profile (VSP) data lack resolution, or for time-lapse monitoring of fluid movements in the reservoir. |
| Formation Evaluation, Geophysics | slowness | Also called interval transit time, The amount of time for a wave to travel a certain distance, proportional to the reciprocal of velocity, typically measured in microseconds per foot by an acoustic log and symbolized by t or DT. P-wave interval transit times for common sedimentary rock types range from 43 (dolostone) to 160 (unconsolidated shales) microseconds per foot, and can be distinguished from measurements of steel casing, which has a consistent transit time of 57 microseconds per foot. |
| Formation Evaluation, Geophysics | transmission tomography | A technique used in crosswell seismic and electromagnetic tomography for recording the direct signal from the source or transmitter in one well to the receiver array in another well. This technique is used for mapping the distribution of acoustic velocity and attenuation or electromagnetic resistivity between wells. |
| Formation Evaluation, Reservoir Characterization | remaining oil saturation |
Fraction of pore volume occupied by oil at any location in a reservoir at any time during its life. Remaining oil saturation has no petrophysical significance beyond an assessment of the amount of oil remaining in the rock. Considered a snapshot of the |
| Formation Evaluation, Reservoir Characterization | residual oil saturation | Fraction of pore volume occupied by oil at the end of oil displacement that used a specific fluid. This reservoir engineering quantity signifies the ultimate recovery under a given displacement process and represents the endpoint of the relative permeability curves in reservoir simulation. The residual oil saturation quantity is the saturation achieved after an infinite number of pore volumes of the displacing fluid have flowed through a particular portion of reservoir rock. To define residual oil saturation, the displacement method and the type, volume, direction and velocity of the displacing fluid must be known. Residual oil saturation is the ratio of the immobile residual oil volume divided by the effective porosity. |
| Formation Evaluation, Reservoir Characterization | ROS | Abbreviation for remaining oil saturation. Fraction of pore volume occupied by oil at any location in a reservoir at any time during its life. Remaining oil saturation has no petrophysical significance beyond an assessment of the amount of oil remaining in the rock. Considered a snapshot of the reservoir, this assessment depends on the location in the reservoir and time of the measurement. This saturation is measured without regard to the displacement process or processes that produced it. |
| Formation Evaluation, Reservoir Characterization | SOR | Abbreviation for residual oil saturation. Fraction of pore volume occupied by oil at the end of oil displacement that used a specific fluid. This reservoir engineering quantity signifies the ultimate recovery under a given displacement process and represents the endpoint of the relative permeability curves in reservoir simulation. The residual oil saturation quantity is the saturation achieved after an infinite number of pore volumes of the displacing fluid have flowed through a particular portion of reservoir rock. To define residual oil saturation, the displacement method and the type, volume, direction and velocity of the displacing fluid must be known. Residual oil saturation is the ratio of the immobile residual oil volume divided by the effective porosity. |
| Formation Evaluation, Shale Gas | core analysis | Laboratory study of a sample of a geologic formation, usually reservoir rock, taken during or after drilling a well. Economic and efficient oil and gas production is highly dependent on understanding key properties of reservoir rock, such as porosity, permeability, and wettability. Geoscientists have developed a variety of approaches, including log and core analysis techniques, to measure these properties. Core analysis is especially important in shale reservoirs because of the vertical and lateral heterogeneity of the rocks. Core analysis can include evaluation of rock properties and anisotropy; organic matter content, maturity, and type; fluid content; fluid sensitivity; and geomechanical properties. This information can be used to calibrate log and seismic measurements and to help in well and completion design, well placement, and other aspects of reservoir production. |
| Formation Evaluation, Shale Gas | formation evaluation | The measurement and analysis of formation and fluid properties through examination of formation cuttings or through the use of tools integrated into the bottomhole assembly while drilling, or conveyed on wireline or drillpipe after a borehole has been drilled. Formation evaluation is performed to assess the quantity and producibility of fluids from a reservoir. Formation evaluation guides wellsite decisions, such as placement of perforations and hydraulic fracture stages, and reservoir development and production planning. |
| Formation Evaluation, Well Completions, Shale Gas | reservoir quality (RQ) | A prediction of the likelihood of a rock to yield commercial quantities of hydrocarbons. Reservoir quality (RQ) is a geologic, engineering and economic assessment of a resource, its reserves and their producibility. The term RQ includes the following factors: volume of oil or gas in place, organic content (TOC), thermal maturity, effective porosity, fluid saturations—oil, gas and water, reservoir thickness and intrinsic permeability. |
| General Terms | API | Abbreviation for American Petroleum Institute, a trade association founded in 1919 with offices in Washington, DC, USA. The API is sponsored by the oil and gas industry and is recognized worldwide. Among its long-term endeavors is the development of standardized testing procedures for drilling equipment, drilling fluids and cements, called API Recommended Practices (“RPs”). The API licenses the use of its monogram (logo), monitors supplier quality assurance methods and sets minimum standards for materials used in drilling and completion operations, called API Specifications (“Specs”). The API works in conjunction with the International Organization of Standards (ISO). |
| General Terms | international nautical mile | A unit of distance used for marine and aerial navigation. An international nautical mile is approximately equivalent to the angle of one minute of latitude at Earth’s surface. The conventional value for the nautical mile was established at the First International Extraordinary Hydrographic Conference held in the Principality of Monaco in 1929 and named the international nautical mile. In SI units, the international nautical mile is equal to exactly 1,852 m, which is approximately 6,076 ft. |
| General Terms | knot | A unit of speed used for marine and aerial navigation. A knot is one nautical mile per hour. In SI units, the knot is equivalent to exactly 1,852/3,600 m/s, which is approximately 0.5144 m/s [1.688 ft/s]. |
| General Terms | moment | The product of a physical quantity and its distance from an axis or a tendency to cause motion around an axis. |
| General Terms | NACE | NACE, or NACE International, is a worldwide professional organization committed to corrosion prevention and control. NACE, founded in 1943, is headquartered in Houston, Texas, USA. Areas of focus include oil and gas, water, transportation and infrastructure protection. The original name was National Association of Corrosion Engineers—the source of its NACE acronym—although the organization formally adopted NACE International—The Corrosion Society, in 1993. Among its activities, the organization offers technical training and certification programs, conducts regional and international conferences, publishes industry standards, reports, publications and technical journals and assists in government relations activities. The NACE Materials Requirements include the widely used MR0175, corrosion resistant materials for oil and gas applications, and MR0103, sulfide stress cracking in corrosive environments. Publications can be ordered from NACE at http://www.nace.org/. |
| General Terms | nautical mile | A unit of distance used for marine and aerial navigation. A nautical mile is approximately equivalent to the angle of one minute of latitude at Earth’s surface. The conventional value for the nautical mile was established at the First International Extraordinary Hydrographic Conference held in the Principality of Monaco in 1929 and named the international nautical mile. In SI units, the nautical mile is equal to exactly 1,852 m, which is approximately 6,076 ft. Reference: Bureau International des Poids et Mesures: The International System of Units (SI), 8th edition. Paris: Organisation Intergouvernementale de la Convention du Mètre, 2006. |
| General Terms | standard temperature and pressure | Organizations establish a standard temperature and pressure to enable the comparison of datasets and to meet their members’ and stakeholders’ laboratory, industrial and regulatory requirements. For example, the International Union of Pure and Applied Chemistry (IUPAC) defines standard temperature to be 0°C [273.15°K, 32°F] and standard pressure to be 100 kPa [1 bar, 0.9869 atm, 14.504 psi] and the Society of Petroleum Engineers (SPE) defines standard temperature to be 15°C [288.15°K, 59°F] and standard pressure to be 100 kPa. |
| General Terms | STP | Abbreviation for standard temperature and pressure. Organizations establish a standard temperature and pressure to enable the comparison of datasets and to meet their members’ and stakeholders’ laboratory, industrial and regulatory requirements. For example, the International Union of Pure and Applied Chemistry (IUPAC) defines standard temperature to be 0°C [273.15°K, 32°F] and standard pressure to be 100 kPa [1 bar, 0.9869 atm, 14.504 psi] and the Society of Petroleum Engineers (SPE) defines standard temperature to be 15°C [288.15°K, 59°F] and standard pressure to be 100 kPa. |
| General Terms, Shale Gas | GIIP | Abbreviation for gas initially in place, the volume of gas in a reservoir before production. |
| Geochemistry, Geology | geochemistry | The study of the chemistry of the Earth and within solid bodies of the solar system, including the distribution, circulation and abundance of elements (and their ions and isotopes), molecules, minerals, rocks and fluids. For geochemists in the petroleum industry, source rock geochemistry is a major focus. Geochemical techniques can determine whether a given source rock is rich enough in organic matter to generate hydrocarbons, whether the source rock has generated hydrocarbons, and whether a particular oil sample was generated by a given source rock. |
| Geology | abnormal pressure | A subsurface condition in which the pore pressure of a geologic formation exceeds or is less than the expected, or normal, formation pressure. When impermeable rocks such as shales are compacted rapidly, their pore fluids cannot always escape and must then support the total overlying rock column, leading to abnormally high formation pressures. Excess pressure, called overpressure or geopressure, can cause a well to blowout or become uncontrollable during drilling. Severe underpressure can cause the drillpipe to stick to the underpressured formation. |
| Geology | absolute age | The measurement of age in years. The determination of the absolute age of rocks, minerals and fossils, in years before the present, is the basis for the field of geochronology. The measurement of the decay of radioactive isotopes, especially uranium, strontium, rubidium, argon and carbon, has allowed geologists to more precisely determine the age of rock formations. Tree rings and seasonal sedimentary deposits called varves can be counted to determine absolute age. Although the term implies otherwise, “absolute” ages typically have some amount of potential error and are inexact. Relative age, in contrast, is the determination of whether a given material is younger or older than other surrounding material on the basis of stratigraphic and structural relationships, such as superposition, or by interpretation of fossil content. |
| Geology | absolute permeability | The measurement of the permeability, or ability to flow or transmit fluids through a rock, conducted when a single fluid, or phase, is present in the rock. The symbol most commonly used for permeability is k, which is measured in units of darcies or millidarcies. |
| Geology | absolute pressure | The measurement of pressure relative to the pressure in a vacuum, equal to the sum of the pressure shown on a pressure gauge and atmospheric pressure. |
| Geology | abyss | The deepest area of the ocean basins. The depositional energy is low and fine-grained sediments are deposited slowly by waning turbidity currents or from suspension in the water. The water is thousands of meters deep (>2,000 m) [>6,520 ft] so it is cold and sunlight is minimal. |
| Geology | abyssal | Pertaining to the depositional environment of the deepest area of the ocean basins, the abyss. The depositional energy is low, the abyssal plain is flat and nearly horizontal, and fine-grained sediments are deposited slowly by waning turbidity currents or from suspension in the water. The water is thousands of meters deep (> 2000 m) [6520 ft], so the water is cold and sunlight is minimal. |
| Geology | accommodation | Sequence stratigraphic term for the amount of space available for sediment accumulation. Dominant influences on the amount of accommodation, or accommodation space, include subsidence and eustasy. |
| Geology | accumulation | The phase in the development of a petroleum system during which hydrocarbons migrate into and remain trapped in a reservoir. |
| Geology | accumulation | An occurrence of trapped hydrocarbons, an oil field. |
| Geology | active margin | A boundary of colliding lithospheric plates. The present subduction zones of the Pacific Rim, the older mountains of the Alps, and the Himalayas represent active margins. |
| Geology | aeolian | Pertaining to the environment of deposition of sediments by wind, such as the sand dunes in a desert. Because fine-grained sediments such as clays are removed easily from wind-blown deposits, eolian sandstones are typically clean and well-sorted. |
| Geology | aggradation | The accumulation of stratigraphic sequences by deposition that stacks beds atop each other, building upwards during periods of balance between sediment supply and accommodation. |
| Geology | aggradational | Related to the accumulation of stratigraphic sequences by deposition that stacks beds atop each other, building upwards during periods of balance between sediment supply and accommodation. |
| Geology | alidade | A telescopic surveying device used to construct surface topographic and geologic maps in the field. The alidade is mounted on a plane table, which has a sheet of paper on which to draw the map, and an object or location is sighted through the alidade. The edge of the alidade is aligned in the azimuthal direction of the object or location. The vertical angle from which elevation of the location can be calculated is measured using the calibrated arc of the alidade. |
| Geology | allochthon | A rock mass formed somewhere other than its present location, which was transported by fault movements, large-scale gravity sliding, or similar processes. |
| Geology | allochthonous | Pertaining to materials, particularly rock masses, that formed somewhere other than their present location, and were transported by fault movements, large-scale gravity sliding, or similar processes. Autochthonous material, in contrast, formed in its present location. Landslides can result in large masses of allochthonous rock, which typically can be distinguished from autochthonous rocks on the basis of their difference in composition. Faults and folds can also separate allochthons from autochthons. |
| Geology | allogenic | Pertaining to minerals or rock fragments that formed in one location but were transported to another location and deposited. Clastic sediments in a rock such as sandstone are allogenic, or formed elsewhere. |
| Geology | alluvial | Pertaining to the subaerial (as opposed to submarine) environment, action and products of a stream or river on its floodplain, usually consisting of detrital clastic sediments, and distinct from subaqueous deposition such as in lakes or oceans and lower energy fluvial deposition. Sediments deposited in an alluvial environment can be subject to high depositional energy, such as fast-moving flood waters, and may be poorly sorted or chaotic. |
| Geology | alluvium | Material deposited in an alluvial environment, typically detrital sediments that are poorly sorted. |
| Geology | angular unconformity | A surface that separates younger strata from eroded, dipping, older strata and represents a gap in the geologic record. |
| Geology | anhydrite | [CaSO4] A member of the evaporite group of minerals and the soft rock comprising anhydrite formed by precipitation of calcium sulfate from evaporation of seawater. Anhydrite can also form through the dehydration of gypsum, another sulfate mineral found in evaporites. Anhydrite may occur as a cap rock above salt domes. |
| Geology | anomalous | Different from what is typical or expected, or different from what is predicted by a theoretical model. The difference or anomaly may refer to the measurement of the difference between an observed or measured value and the expected values of a physical property. Anomalies can be of great interest in hydrocarbon and mineral exploration because they often indicate hydrocarbon and mineral prospects and accumulations, such as geologic structures like folds and faults. Geochemical anomalies at the surface of the Earth can also indicate an accumulation of hydrocarbons at depth. Geophysical anomalies, such as amplitude anomalies in seismic data and magnetic anomalies in the Earth’s crust, can also be associated with hydrocarbon accumulations. |
| Geology | anomaly | An entity or property that differs from what is typical or expected, or which differs from that predicted by a theoretical model. May be the measurement of the difference between an observed or measured value and the expected values of a physical property. Anomalies can be of great interest in hydrocarbon and mineral exploration because they often indicate hydrocarbon and mineral prospects and accumulations, such as geologic structures like folds and faults. Geochemical anomalies at the surface of the Earth can also indicate an accumulation of hydrocarbons at depth. Geophysical anomalies, such as amplitude anomalies in seismic data and magnetic anomalies in the Earth’s crust, can also be associated with hydrocarbon accumulations. |
| Geology | anoxic | The condition of an environment in which free oxygen is lacking or absent. |
| Geology | anticlinal | Pertaining to an anticline, an arch-shaped fold in rock in which rock layers are upwardly convex. The oldest rock layers form the core of the fold, and outward from the core progressively younger rocks occur. Anticlines form many excellent hydrocarbon traps, particularly in folds with reservoir-quality rocks in their core and impermeable seals in the outer layers of the fold. A syncline is the opposite type of fold, having downwardly convex layers with young rocks in the core. |
| Geology | anticlinal trap | A type of structural hydrocarbon trap whose closure is controlled by the presence of an anticline. |
| Geology | anticline | An arch-shaped fold in rock in which rock layers are upwardly convex. The oldest rock layers form the core of the fold, and outward from the core progressively younger rocks occur. Anticlines form many excellent hydrocarbon traps, particularly in folds with reservoir-quality rocks in their core and impermeable seals in the outer layers of the fold. A syncline is the opposite type of fold, having downwardly convex layers with young rocks in the core. |
| Geology | antithetic fault | A minor, secondary fault, usually one of a set, whose sense of displacement is opposite to its associated major and synthetic faults. Antithetic-synthetic fault sets are typical in areas of normal faulting. |
| Geology | apparent dip | The maximum inclination of a bedding plane, fault plane or other geological surface measured from a vertical cross section that is not perpendicular to the strike of the feature. Apparent dip corrected for well drift, or geometry, is referred to as true dip. |
| Geology | appraisal | The phase of petroleum operations that immediately follows successful exploratory drilling. During appraisal, delineation wells might be drilled to determine the size of the oil or gas field and how to develop it most efficiently. |
| Geology | aquifer | A body of rock whose fluid saturation, porosity and permeability permit production of groundwater. |
| Geology | aquifer | A water-bearing portion of a petroleum reservoir with a waterdrive. |
| Geology | arenaceous | Describing sandy-textured rock or sediment. Arenaceous does not necessarily imply silica-rich, but rather particles of sand size, 0.625 to 2 mm, according to the Udden-Wentworth scale. |
| Geology | argillaceous | Describing rocks or sediments containing particles that are silt- or clay-sized, less than 0.625 mm in size. Most have a high clay-mineral content, and many contain a sufficient percentage of organic material to be considered a source rock for hydrocarbon. |
| Geology | asphalt | A solid or nearly solid form of bitumen that can melt upon heating and contains impurities such as nitrogen, oxygen and sulfur. Asphalt forms naturally when the light components or volatiles of petroleum have been removed or evaporated. |
| Geology | asthenosphere | The relatively plastic layer of the upper mantle of the Earth on which the tectonic plates of the lithosphere move. The asthenosphere is approximately 200 km [124 miles] thick and, owing to its depth below the Earth’s surface, warm (~ 1400 oC) [2640 oF] but not molten. Here the mantle deforms by plastic flow in response to applied pressures above 100 MPa [14,500 psi]. This zone is considered coincidental, at least below oceanic crust, with the low-velocity zone of the upper mantle. |
| Geology | attitude | The orientation of a planar or linear feature in three-dimensional space. Planar features that are not horizontal, such as tilted strata, are described by their strike, or the azimuth of the intersection of the plane with a horizontal surface, and the dip, or the magnitude of its inclination from a horizontal reference. The trend and plunge of linear features, such as the axis of a fold, describe the azimuth of the line and its deviation from horizontal. |
| Geology | aulacogen | In plate tectonics, a failed rift arm. At the junctions of tectonic plates, three intersecting lithospheric plates typically are separated by “arms.” Arms might be areas of rifting, convergence or transform faults (similar to a strike-slip fault). The arm along which the motion that spreads the plates apart ceases is termed the failed arm, or aulacogen. Spreading or rifting along the other arms of the triple junction can form new oceanic basins, whereas the aulacogen can become a sediment-filled graben. |
| Geology | authigenic | Pertaining to minerals or materials that grow in place with a rock, rather than having been transported and deposited. These include quartz, chlorite and other pore-filling minerals or cements that grow during diagenesis. Evaporite minerals are authigenic, or formed in situ. |
| Geology | autochthon | Materials, especially rock masses, that formed in their present location and have not been transported. Fault surfaces can separate indigenous rocks from allochthonous rocks, although some allochthonous rocks are clearly delineated by their differing composition. |
| Geology | autochthonous | Materials, especially rock masses, that formed in their present location and have not been transported. Fault surfaces can separate indigenous rocks from allochthonous rocks, although some allochthonous rocks are clearly delineated by their differing composition. |
| Geology | axial surface | In folded rocks, the imaginary surface bisecting the limbs of the fold. The axial surface is called the axial plane when the fold is symmetrical and the lines defined by the points of maximum curvature of each folded layer, or hinge lines, are coplanar. |
| Geology | azimuth | The angle between the vertical projection of a line of interest onto a horizontal surface and true north or magnetic north measured in a horizontal plane, typically measured clockwise from north. |
| Geology | azimuthal | Pertaining to the angle between the vertical projection of a line of interest onto a horizontal surface and true north or magnetic north measured in a horizontal plane, typically measured clockwise from north. |
| Geology | barite | [BaSO4] A dense sulfate mineral that can occur in a variety of rocks, including limestone and sandstone, with a range of accessory minerals, such as quartz, chert, dolomite, calcite, siderite and metal sulfides. Barite is commonly used to add weight to drilling fluid. Barite is of significance to petrophysicists because excess barite can require a correction factor in some well log measurements. |
| Geology | base map | A map on which primary data and interpretations can be plotted. A base map typically includes locations of lease or concession boundaries, wells, seismic survey points and other cultural data such as buildings and roads, with a geographic reference such as latitude and longitude or Universal Transverse Mercator (UTM) grid information. Geologists use topographic maps as base maps for construction of surface geologic maps. Geophysicists typically use shot point maps, which show the orientations of seismic lines and the specific points at which seismic data were acquired, to display interpretations of seismic data. In the field, geologists can use a plane table and alidade to construct a base map. |
| Geology | basement | The rock layer below which economic hydrocarbon reservoirs are not expected to be found, sometimes called economic basement. Basement is usually older, deformed igneous or metamorphic rocks, which seldom develops the porosity and permeability necessary to serve as a hydrocarbon reservoir, and below which sedimentary rocks are not common. Basement rocks typically have different density, acoustic velocity, and magnetic properties from overlying rocks. |
| Geology | bed | A layer of sediment or sedimentary rock, or stratum. A bed is the smallest stratigraphic unit, generally a centimeter or more in thickness. To be labeled a bed, the stratum must be distinguishable from adjacent beds. |
| Geology | bed thickness | The thickness of a layer or stratum of sedimentary rock measured perpendicular to its lateral extent, presuming deposition on a horizontal surface. Because sediment deposition can occur on inclined surfaces, apparent or measured bed thickness might differ from true bed thickness. The thickness of a given bed often varies along its extent. |
| Geology | bedrock | Solid rock either exposed at the surface or situated below surface soil, unconsolidated sediments and weathered rock. |
| Geology | Benioff zone | A zone of the upper mantle in which earthquakes occur when a lithospheric plate is subducted, named in honor of seismologists Kiyoo Wadati and Hugo Benioff. The dip of this zone, also referred to as the Wadati-Benioff zone, coincides with the dip of the subducting plate. The Wadati-Benioff zone extends to a depth of about 700 km [435 miles] from the Earth’s surface |
| Geology | benthic | Pertaining to the environment and conditions of organisms living at the water bottom, or benthos. Also called benthonic. |
| Geology | benthos | Organisms that live at the bottom of a body of water. |
| Geology | bentonite | A material composed of clay minerals, predominantly montmorillonite with minor amounts of other smectite group minerals, commonly used in drilling mud. Bentonite swells considerably when exposed to water, making it ideal for protecting formations from invasion by drilling fluids. Montmorillonite forms when basic rocks such as volcanic ash in marine basins are altered. |
| Geology | biostratigraphic | Pertaining to biostratigraphy, the application of plant and animal fossils to date and correlate strata in order to elucidate Earth history, combining the principles of paleontology and stratigraphy. In the petroleum industry, biostratigraphy often denotes the use of terrestrial (pollen and spores) and marine (diatoms, foraminifera, nannofossils) microfossils to determine the absolute or relative age and depositional environment of a particular formation, source rock or reservoir of interest. |
| Geology | biostratigraphy | The application of plant and animal fossils to date and correlate strata in order to elucidate Earth history, combining the principles of paleontology and stratigraphy. In the petroleum industry, biostratigraphy often denotes the use of terrestrial (pollen and spores) and marine (diatoms, foraminifera, nannofossils) microfossils to determine the absolute or relative age and depositional environment of a particular formation, source rock or reservoir of interest. |
| Geology | bitumen | Naturally-occurring, inflammable organic matter formed from kerogen in the process of petroleum generation that is soluble in carbon bisulfide. Bitumen includes hydrocarbons such as asphalt and mineral wax. Typically solid or nearly so, brown or black, bitumen has a distinctive petroliferous odor. Laboratory dissolution with organic solvents allows determination of the amount of bitumen in samples, an assessment of source rock richness. |
| Geology | Bouma sequence | A characteristic sequence of sedimentary structures occurring in sedimentary rocks deposited in areas of deep water sedimentation by turbidity currents, which form deposits called turbidites. In theory, a complete Bouma sequence comprises sediments that fine upwards, consisting of a lowermost layer of coarse, chaotic clastic sediments deposited under conditions of high depositional energy overlain by successively finer grained and better stratified sediments like sands and muds deposited under calmer conditions that are labeled as Units A though E. In practice, however, the chaotic, high-energy nature of turbidite deposition can alter or remove underlying sediments so that incomplete sequences of sediments typically remain preserved. |
| Geology | brine | Water containing more dissolved inorganic salt than typical seawater. |
| Geology | calcite | [CaCO3] The crystalline form of calcium carbonate and chief constituent of limestone and chalk. Calcite reacts readily with dilute hydrochloric acid [HCl], so the presence of calcite can be tested by simply placing a drop of acid on a rock specimen. |
| Geology | caliche | A crust of coarse sediments or weathered soil rich in calcium carbonate. It forms when lime-rich groundwater rises to the surface by capillary action and evaporates into a crumbly powder, forming a tough, indurated sheet called calcrete. Caliche typically occurs in desert or semi-arid areas. Of particular concern to geophysicists is the difficulty in acquiring good seismic data when shooting through a layer of caliche. |
| Geology | cap rock | A relatively impermeable rock, commonly shale, anhydrite or salt, that forms a barrier or seal above and around reservoir rock so that fluids cannot migrate beyond the reservoir. It is often found atop a salt dome. The permeability of a cap rock capable of retaining fluids through geologic time is ~ 10-6-10-8 darcies. |
| Geology | caprock | A relatively impermeable rock, commonly shale, anhydrite or salt, that forms a barrier or seal above and around reservoir rock so that fluids cannot migrate beyond the reservoir. It is often found atop a salt dome. The permeability of a caprock capable of retaining fluids through geologic time is ~ 10-6-10-8 darcies. |
| Geology | carbonate | A group of minerals found mostly in limestone and dolostone that includes aragonite, calcite and dolomite. Calcite is the most abundant and important of the carbonate minerals. |
| Geology | carbonate | A class of sedimentary rock whose chief mineral constituents (95% or more) are calcite and aragonite (both CaCo3) and dolomite [CaMg(CO3)2], a mineral that can replace calcite during the process of dolomitization. Limestone, dolostone or dolomite, and chalk are carbonate rocks. Although carbonate rocks can be clastic in origin, they are more commonly formed through processes of precipitation or the activity of organisms such as coral and algae. Carbonates form in shallow and deep marine settings, evaporitic basins, lakes and windy deserts. Carbonate rocks can serve as hydrocarbon reservoir rocks, particularly if their porosity has been enhanced through dissolution. They rely on fractures for permeability. |
| Geology | cataclasite | A type of metamorphic rock with shearing and granulation of minerals caused by high mechanical stress during faulting or dynamic metamorphism, typically during episodes of plate tectonic activity. |
| Geology | cataclastic | Pertaining to a type of metamorphic rock with shearing and granulation of minerals caused by high mechanical stress during faulting or dynamic metamorphism, typically during episodes of plate tectonic activity. |
| Geology | CBM | Abbreviation for coalbed methane. Natural gas, predominantly methane [CH4], generated during coal formation and adsorbed in coal. Natural gas adsorbs to the surfaces of matrix pores within the coal and natural fractures, or cleats, as reservoir pressure increases. Production of natural gas from coal requires decreasing the pore pressure below the coal’s desorption pressure so that methane will desorb from surfaces, diffuse through the coal matrix and become free gas. Because the diffusivity and permeability of the coal matrix are ultralow, coal must have an extensive cleat system to ensure adequate permeability and flow of methane to wellbores at economic production rates. Coal seams are typically saturated with water. Consequently, the coal must be dewatered for efficient gas production. Dewatering reduces the hydrostatic pressure and promotes gas desorption from coal. As dewatering progresses, gas production often increases at a rate governed by how quickly gas desorbs from coal, the permeability of the cleat and the relative permeability of the gas-water system in the cleat. Eventually, the rate and amount of gas desorption decreases as the coal seam is depleted of its gas, and production declines. Coal seams with no water (dry coal) have been discovered and commercially exploited. In these reservoirs, the adsorbed gas is held in place by free gas in the cleats. Consequently, gas production consists of both free gas from the cleat system and desorbed gas from the matrix. |
| Geology | cement | The binding material in sedimentary rocks that precipitates between grains from pore fluids. Calcite and quartz are common cement-forming minerals. |
| Geology | cementation | The process of precipitation of cement between mineral or rock grains and forming solid clastic sedimentary rock, one phase of lithification. |
| Geology | chalk | A porous marine limestone composed of fine-grained remains of microorganisms with calcite shells, coccolithophores, such as the White Cliffs of Dover (UK). The Austin Chalk of the US Gulf coast is a prolific, fractured oil reservoir that spurred widespread horizontal drilling activity. |
| Geology | channel | A linear, commonly concave-based depression through which water and sediment flow and into which sediment can be deposited in distinctive, often elongated bodies. Channels can occur in a variety of morphologies, e.g., straight, meandering or braided. In some areas, coarse sediments can fill channels of streams or rivers that cut through finer grained sediments or rocks. The close proximity of coarse-grained and fine-grained sediments can ultimately lead to the formation of stratigraphic hydrocarbon traps. |
| Geology | chert | A sedimentary rock and a variety of quartz made of extremely fine-grained, or cryptocrystalline, silica, also called chalcedony. The silica might be of organic origin, such as from the internal structures of sponges called spicules, or inorganic origin, such as precipitation from solution. The latter results in the formation of flint. Chert can form beds, but is more common as nodules in carbonate rocks. |
| Geology | cherty | Containing chert, a sedimentary rock and a variety of quartz made of extremely fine-grained, or cryptocrystalline, silica, also called chalcedony. The silica might be of organic origin, such as from the internal structures of sponges called spicules, or inorganic origin, such as precipitation from solution. The latter results in the formation of flint. Chert can form beds, but is more common as nodules in carbonate rocks. |
| Geology | chlorite | [(Mg,Al,Fe) 12(Si,Al) 8O20(OH) 16] A platy, pale green mineral of the mica group of sheet silicates, also considered to be a type of clay mineral, found in sedimentary and low-grade metamorphic rocks. Chlorite is a common authigenic mineral lining the pores of sandstones. In some cases, the presence of authigenic chlorite on sand grains can inhibit the growth of pore-filling cements during diagenesis and preserve pore space for occupation by hydrocarbons. |
| Geology | chronostratigraphic chart | A graphic display, with geologic time along the vertical axis and distance along the horizontal axis, to demonstrate the relative ages and geographic extent of strata or stratigraphic units in a given area, also known as a Wheeler diagram. In addition, information from seismic data, well logs and rock samples, and biostratigraphic and lithostratigraphic information can be shown within each chronostratigraphic unit. A chronostratigraphic chart can concisely illustrate sequence stratigraphic interpretations. |
| Geology | chronostratigraphy | The study of the ages of strata. The comparison, or correlation, of separated strata can include study of their relative or absolute ages. |
| Geology | CI | The value of the separation between two adjacent contours. A net pay isopach map might have a contour interval of 10 feet [3 m], whereas a structure contour map might have a contour interval of 1000 feet [300 m]. Contour intervals are chosen according to the map scale and the amount and distribution of control points. |
| Geology | clastic intrusion | Structures formed by sediment injection. Because they resemble intrusive and extrusive igneous features, much of the vocabulary for describing clastic intrusions, or injectites, comes from igneous geology. Sills are emplaced parallel to bedding, whereas dikes cut through bedding. The strata containing the intrusion are called host strata and the layers that feed the intrusion are the parent beds. Sand-injection features exhibit size scales from millimeters to kilometers, and have been seen in cores, borehole image logs, seismic sections, outcrops, aerial photographs and satellite images. |
| Geology | clastic sediment | Sediment consisting of broken fragments derived from preexisting rocks and transported elsewhere and redeposited before forming another rock. Examples of common clastic sedimentary rocks include siliciclastic rocks such as conglomerate, sandstone, siltstone and shale. Carbonate rocks can also be broken and reworked to form clastic sedimentary rocks. |
| Geology | clathrate | An unusual occurrence of hydrocarbon in which molecules of natural gas, typically methane, are trapped in ice molecules. More generally, hydrates are compounds in which gas molecules are trapped within a crystal structure. Hydrates form in cold climates, such as permafrost zones and in deep water. To date, economic liberation of hydrocarbon gases from hydrates has not occurred, but hydrates contain quantities of hydrocarbons that could be of great economic significance. Hydrates can affect seismic data by creating a reflection or multiple. |
| Geology | clay | Fine-grained sediments less than 0.0039 mm in size. |
| Geology | clean | Pertaining to a sedimentary rock, such as sandstone or limestone, that contains only minimal amounts of clay minerals. Clean reservoir rocks typically have better porosity and permeability than dirty rocks whose pores are clogged with fine clay particles. Clean and dirty are qualitative, descriptive terms. |
| Geology | clear brine | Water containing more dissolved inorganic salt than typical seawater. |
| Geology | closure | The vertical distance from the apex of a structure to the lowest structural contour that contains the structure. Measurements of both the areal closure and the distance from the apex to the lowest closing contour are typically incorporated in calculations of the estimated hydrocarbon content of a trap. |
| Geology | coal | A carbon-rich sedimentary rock that forms from the remains of plants deposited as peat in swampy environments. Burial and increase in temperature bring about physical and chemical changes called coalification. Because of the organic origin of coal, it cannot be classified as a mineral. The main types of coal, anthracite, bituminous coal and lignite, can be distinguished by their hardness and energy content, which are affected by their organic content as well as their conditions of formation. Natural gas associated with coal, called coal gas or coalbed methane, can be produced economically from coal beds in some areas. In some basins coals form source rocks. |
| Geology | coal bed methane | Natural gas, predominantly methane [CH4], generated during coal formation and adsorbed in coal. Natural gas adsorbs to the surfaces of matrix pores within the coal and natural fractures, or cleats, as reservoir pressure increases. Production of natural gas from coal requires decreasing the pore pressure below the coal’s desorption pressure so that methane will desorb from surfaces, diffuse through the coal matrix and become free gas. Because the diffusivity and permeability of the coal matrix are ultralow, coal must have an extensive cleat system to ensure adequate permeability and flow of methane to wellbores at economic production rates. Coal seams are typically saturated with water. Consequently, the coal must be dewatered for efficient gas production. Dewatering reduces the hydrostatic pressure and promotes gas desorption from coal. As dewatering progresses, gas production often increases at a rate governed by how quickly gas desorbs from coal, the permeability of the cleat and the relative permeability of the gas-water system in the cleat. Eventually, the rate and amount of gas desorption decreases as the coal seam is depleted of its gas, and production declines. Coal seams with no water (dry coal) have been discovered and commercially exploited. In these reservoirs, the adsorbed gas is held in place by free gas in the cleats. Consequently, gas production consists of both free gas from the cleat system and desorbed gas from the matrix. |
| Geology | coal seam gas | Natural gas, predominantly methane [CH4], generated during coal formation and adsorbed in coal. Natural gas adsorbs to the surfaces of matrix pores within the coal and natural fractures, or cleats, as reservoir pressure increases. Production of natural gas from coal requires decreasing the pore pressure below the coal’s desorption pressure so that methane will desorb from surfaces, diffuse through the coal matrix and become free gas. Because the diffusivity and permeability of the coal matrix are ultralow, coal must have an extensive cleat system to ensure adequate permeability and flow of methane to wellbores at economic production rates. Coal seams are typically saturated with water. Consequently, the coal must be dewatered for efficient gas production. Dewatering reduces the hydrostatic pressure and promotes gas desorption from coal. As dewatering progresses, gas production often increases at a rate governed by how quickly gas desorbs from coal, the permeability of the cleat and the relative permeability of the gas-water system in the cleat. Eventually, the rate and amount of gas desorption decreases as the coal seam is depleted of its gas, and production declines. Coal seams with no water (dry coal) have been discovered and commercially exploited. In these reservoirs, the adsorbed gas is held in place by free gas in the cleats. Consequently, gas production consists of both free gas from the cleat system and desorbed gas from the matrix. |
| Geology | coalbed methane | Natural gas, predominantly methane [CH4], generated during coal formation and adsorbed in coal. Natural gas adsorbs to the surfaces of matrix pores within the coal and natural fractures, or cleats, as reservoir pressure increases. Production of natural gas from coal requires decreasing the pore pressure below the coal’s desorption pressure so that methane will desorb from surfaces, diffuse through the coal matrix and become free gas. Because the diffusivity and permeability of the coal matrix are ultralow, coal must have an extensive cleat system to ensure adequate permeability and flow of methane to wellbores at economic production rates. Coal seams are typically saturated with water. Consequently, the coal must be dewatered for efficient gas production. Dewatering reduces the hydrostatic pressure and promotes gas desorption from coal. As dewatering progresses, gas production often increases at a rate governed by how quickly gas desorbs from coal, the permeability of the cleat and the relative permeability of the gas-water system in the cleat. Eventually, the rate and amount of gas desorption decreases as the coal seam is depleted of its gas, and production declines. Coal seams with no water (dry coal) have been discovered and commercially exploited. In these reservoirs, the adsorbed gas is held in place by free gas in the cleats. Consequently, gas production consists of both free gas from the cleat system and desorbed gas from the matrix. |
| Geology | coal-bed methane | Natural gas, predominantly methane [CH4], generated during coal formation and adsorbed in coal. Natural gas adsorbs to the surfaces of matrix pores within the coal and natural fractures, or cleats, as reservoir pressure increases. Production of natural gas from coal requires decreasing the pore pressure below the coal’s desorption pressure so that methane will desorb from surfaces, diffuse through the coal matrix and become free gas. Because the diffusivity and permeability of the coal matrix are ultralow, coal must have an extensive cleat system to ensure adequate permeability and flow of methane to wellbores at economic production rates. Coal seams are typically saturated with water. Consequently, the coal must be dewatered for efficient gas production. Dewatering reduces the hydrostatic pressure and promotes gas desorption from coal. As dewatering progresses, gas production often increases at a rate governed by how quickly gas desorbs from coal, the permeability of the cleat and the relative permeability of the gas-water system in the cleat. Eventually, the rate and amount of gas desorption decreases as the coal seam is depleted of its gas, and production declines. Coal seams with no water (dry coal) have been discovered and commercially exploited. In these reservoirs, the adsorbed gas is held in place by free gas in the cleats. Consequently, gas production consists of both free gas from the cleat system and desorbed gas from the matrix. |
| Geology | coal-seam gas | Natural gas, predominantly methane [CH4], generated during coal formation and adsorbed in coal. Natural gas adsorbs to the surfaces of matrix pores within the coal and natural fractures, or cleats, as reservoir pressure increases. Production of natural gas from coal requires decreasing the pore pressure below the coal’s desorption pressure so that methane will desorb from surfaces, diffuse through the coal matrix and become free gas. Because the diffusivity and permeability of the coal matrix are ultralow, coal must have an extensive cleat system to ensure adequate permeability and flow of methane to wellbores at economic production rates. Coal seams are typically saturated with water. Consequently, the coal must be dewatered for efficient gas production. Dewatering reduces the hydrostatic pressure and promotes gas desorption from coal. As dewatering progresses, gas production often increases at a rate governed by how quickly gas desorbs from coal, the permeability of the cleat and the relative permeability of the gas-water system in the cleat. Eventually, the rate and amount of gas desorption decreases as the coal seam is depleted of its gas, and production declines. Coal seams with no water (dry coal) have been discovered and commercially exploited. In these reservoirs, the adsorbed gas is held in place by free gas in the cleats. Consequently, gas production consists of both free gas from the cleat system and desorbed gas from the matrix. |
| Geology | collision | An interaction of lithospheric plates that can result in the formation of mountain belts and subduction zones. The collision of two plates of continental lithosphere, known as an A-type collision, can produce high mountains as rocks are folded, faulted and uplifted to accommodate the converging plates, as observed in the Alps and the Himalayas. B-type collisions, in which oceanic lithospheric plates collide with continental lithospheric plates, typically produce a subduction zone where the relatively denser oceanic plate descends below the relatively lighter continental plate, as seen on the Pacific coast of South America. |
| Geology | compaction | The physical process by which sediments are consolidated, resulting in the reduction of pore space as grains are packed closer together. As layers of sediment accumulate, the ever increasing overburden pressure during burial causes compaction of the sediments, loss of pore fluids and formation of rock as grains are welded or cemented together. |
| Geology | competent | Describes a bed that maintains its original thickness during deformation. Often pertains to relatively brittle, solid strata that deform by faulting, fracturing or folding, rather than flowing under stress. Incompetent beds are more ductile and tend to flow under stress, so their bed thickness changes more readily during deformation |
| Geology | concentric fold | The deformation of rock layers in which the thickness of each layer, measured perpendicular to initial undeformed layering, is maintained after the rock layers have been folded. |
| Geology | condensate | A low-density, high-API gravity liquid hydrocarbon phase that generally occurs in association with natural gas. Its presence as a liquid phase depends on temperature and pressure conditions in the reservoir allowing condensation of liquid from vapor. The production of condensate reservoirs can be complicated because of the pressure sensitivity of some condensates: During production, there is a risk of the condensate changing from gas to liquid if the reservoir pressure drops below the dew point during production. Reservoir pressure can be maintained by fluid injection if gas production is preferable to liquid production. Gas produced in association with condensate is called wet gas. The API gravity of condensate is typically 50 degrees to 120 degrees. |
| Geology | condensed section | In sequence stratigraphy, a section of fine-grained sedimentary rocks that accumulated slowly, thereby representing a considerable span of time by only a thin layer. In condensed sections, fossils and organic, phosphatic and glauconitic material tend to be concentrated compared with rapidly deposited sections that contain few fossils. Condensed sections are most commonly deposited during transgressions. In such cases they are associated with “maximum flooding surfaces” and form important sequence stratigraphic markers. |
| Geology | conformable | Parallel strata that have undergone a similar geologic history, deposited in succession without interruption. |
| Geology | conformity | A bedding surface separating younger from older strata, along which there is no evidence of subaerial or submarine erosion or of nondeposition, and along which there is no evidence of a significant hiatus. Unconformities (sequence boundaries) and flooding surfaces (parasequence boundaries) pass laterally into correlative conformities, or correlative surfaces. |
| Geology | connate water | Water trapped in the pores of a rock during formation of the rock. The chemistry of connate water can change in composition throughout the history of the rock. Connate water can be dense and saline compared with seawater. Formation water, or interstitial water, in contrast, is simply water found in the pore spaces of a rock, and might not have been present when the rock was formed. Connate water is also described as fossil water. |
| Geology | consolidated | Pertaining to sediments that have been compacted and cemented to the degree that they become coherent, relatively solid rock. Typical consequences of consolidation include an increase in density and acoustic velocity, and a decrease in porosity. |
| Geology | consolidation | Compaction and cementation of sediments to the degree that they become coherent, relatively solid rock. Typical consequences of consolidation include an increase in density and acoustic velocity, and a decrease in porosity. |
| Geology | contact | The interface, also called fluid contact, that separates fluids of different densities in a reservoir. Horizontal contacts are usually assumed, although tilted contacts occur in some reservoirs. The contact between fluids is usually gradual rather than sharp, forming a transition zone of mixed fluid. A mixed-fluid reservoir will stratify according to fluid density, with gas at the top, oil in the middle, and water below. Production of fluids often perturbs the fluid contacts in a reservoir. |
| Geology | continental shelf | Continental shelf, or the area at the edges of a continent from the shoreline to a depth of 200 m [660 ft], where the continental slope begins. The shelf is commonly a wide, flat area with a slight seaward slope. The term is sometimes used as a for platform. |
| Geology | contour | A line on a map that represents a constant value of the parameter being mapped. This line includes points of equal value and separates points of higher value from points of lower value. Contours are commonly drawn on maps to portray the structural configuration of the Earth’s surface or formations in the subsurface. For example, structure maps contain contours of constant elevation with respect to a datum (such as sea level). Contours are also used to interpret subsurface configurations of rock bodies in areas of limited control, such as drawing contours of the thickness of a common rock unit in several widely separated wells to extrapolate its thickness in a nearby undrilled location. |
| Geology | contour interval | The value of the separation between two adjacent contours. A net pay isopach map might have a contour interval of 10 feet [3 m], whereas a structure contour map might have a contour interval of 1000 feet [300 m]. Contour intervals are chosen according to the map scale and the amount and distribution of control points. |
| Geology | contour map | A map displaying lines that include points of equal value and separate points of higher value from points of lower value. Common types of contour maps include topographic contour maps, which show the elevation of the Earth’s surface; structure contour maps, which show the elevation or depth of a formation; and gross or net sand or pay maps, which show variations in the thickness of a stratigraphic unit, also called isopachs. |
| Geology | convection | The density- and heat-driven cycling, transfer or circulation of energy through which material initially warms up and becomes relatively less dense, then rises, cools and becomes relatively more dense, and finally sinks. As a consequence of convection, material can turn over repeatedly in a convection cell. Within the Earth, radiogenic heating results in convection appearing in the mantle and might drive plate tectonic motions. Convection also occurs in the ocean waters and in the Earth’s atmosphere. |
| Geology | convergence | The movement of tectonic plates toward each other, generating compressional forces and ultimately resulting in collision, and in some cases subduction, of tectonic plates. The boundary where tectonic plates converge is called a convergent margin. |
| Geology | convergent | Pertaining to the movement of tectonic plates toward each other, generating compressional forces and ultimately resulting in collision, and in some cases subduction, of tectonic plates. The boundary where tectonic plates converge is called a convergent margin. |
| Geology | core | Innermost layer of the Earth. Studies of compressional and shear waves indicate that the core makes up nearly 3500 km [2170 miles] of the Earth’s radius of 6370 km [3950 miles]. Such studies also demonstrate that because shear waves do not pass through the outer part of the core (2250 km [1400 miles] thick), it is liquid (only solids can shear). The inner core is solid and 1220 km [750 miles] thick. The core’s iron and nickel composition was inferred through studies of the Earth’s gravitational field and average density. The relatively low density of the outer layers of the Earth suggests a dense inner layer. |
| Geology | correlate | To seek a comparison or equivalence. Scientists attempt to compare or match up well log signatures, chemical signatures, seismic signatures, fossils and rock samples across wide areas to determine the equivalence, extent, thickness, quality, relative age or other properties of stratigraphic units and rock bodies. |
| Geology | correlation | A connection of points from well to well in which the data suggest that the points were deposited at the same time (chronostratigraphic) or have similar and related characteristics. |
| Geology | craton | A stable area of continental crust that has not undergone much plate tectonic or orogenic activity for a long period. A craton includes a crystalline basement of commonly Precambrian rock called a shield, and a platform in which flat-lying or nearly flat-lying sediments or sedimentary rock surround the shield. A commonly cited example of a craton is the Canadian Shield. |
| Geology | crest | The highest point of a wave, mountain or geologic structure. |
| Geology | critical moment | The time of maximum depth of burial of a hydrocarbon source rock. The critical moment is the time of highest probability of entrapment and preservation of hydrocarbons in a petroleum system-after traps form and hydrocarbons migrate into a reservoir and accumulate-and marks the beginning of preservation in a viable petroleum system. |
| Geology | crop out | In the case of a body of rock, to be exposed at the surface of the Earth. Construction of highways and other man-made facilities and resultant removal of soil and rock has created spectacular outcrops in some regions. |
| Geology | cross section | A diagram of a vertical section through a volume, as opposed to the surface, “bird’s eye,” or plan view of a map. Cross sections are useful for displaying the types and orientations of subsurface structures and formations. |
| Geology | cross-sectional | Pertaining to a diagram of a vertical section through a volume, as opposed to the surface, “bird’s eye,” or plan view of a map. Cross sections are useful for displaying the types and orientations of subsurface structures and formations. |
| Geology | crude oil | A general term for unrefined petroleum or liquid petroleum. |
| Geology | crust | The thin, outermost shell of the Earth that is typically 5 km to 75 km thick [3 to 46 miles]. The continental crust comprises rocks similar in composition to granite and basalt (i.e., quartz, feldspar, biotite, amphibole and pyroxene) whereas the composition of oceanic crust is basaltic (pyroxene and feldspar). The crust overlies the more dense rock of the mantle, which consists of rocks composed of minerals like pyroxene and olivine, and the iron and nickel core of the Earth. The Mohorovicic discontinuity abruptly separates the crust from the mantle; the velocity of compressional waves is significantly higher below the discontinuity. The crust, mantle and core of the Earth are distinguished from the lithosphere and asthenosphere on the basis of their composition and not their mechanical behavior. |
| Geology | CSG | Abbreviation for coal seam gas. Natural gas, predominantly methane [CH4], generated during coal formation and adsorbed in coal. Natural gas adsorbs to the surfaces of matrix pores within the coal and natural fractures, or cleats, as reservoir pressure increases. Production of natural gas from coal requires decreasing the pore pressure below the coal’s desorption pressure so that methane will desorb from surfaces, diffuse through the coal matrix and become free gas. Because the diffusivity and permeability of the coal matrix are ultralow, coal must have an extensive cleat system to ensure adequate permeability and flow of methane to wellbores at economic production rates. Coal seams are typically saturated with water. Consequently, the coal must be dewatered for efficient gas production. Dewatering reduces the hydrostatic pressure and promotes gas desorption from coal. As dewatering progresses, gas production often increases at a rate governed by how quickly gas desorbs from coal, the permeability of the cleat and the relative permeability of the gas-water system in the cleat. Eventually, the rate and amount of gas desorption decreases as the coal seam is depleted of its gas, and production declines. Coal seams with no water (dry coal) have been discovered and commercially exploited. In these reservoirs, the adsorbed gas is held in place by free gas in the cleats. Consequently, gas production consists of both free gas from the cleat system and desorbed gas from the matrix. |
| Geology | cubic packing | The arrangement in space of uniform spheres (atoms and molecules in mineral crystals, or grains in clastic sedimentary rocks) that results in a cubic material structure. Cubic packing is mechanically unstable, but it is the most porous packing arrangement, with about 47% porosity in the ideal situation. Most sediments are not uniform spheres of the same size, nor can they be arranged in a cubic structure naturally, so most sediments have much less than 47% porosity. |
| Geology | dead oil | Oil at sufficiently low pressure that it contains no dissolved gas or a relatively thick oil or residue that has lost its volatile components. |
| Geology | decollement | A fault surface parallel to a mechanically weak horizon or layer, or parallel to bedding, that detaches or separates deformed rocks above from undeformed or differently deformed rocks below. Decollements, or decollement surfaces, are typical of regions of thrust faulting such as the Alps. |
| Geology | deepwater play | Exploration activity located in offshore areas where water depths exceed approximately 600 feet [200 m], the approximate water depth at the edge of the continental shelf. While deep-water reservoir targets are geologically similar to reservoirs drilled both in shallower present-day water depths as well as onshore, the logistics of producing hydrocarbons from reservoirs located below such water depths presents a considerable technical challenge. |
| Geology | deep-water play | Exploration activity located in offshore areas where water depths exceed approximately 600 feet [200 m], the approximate water depth at the edge of the continental shelf. While deep-water reservoir targets are geologically similar to reservoirs drilled both in shallower present-day water depths as well as onshore, the logistics of producing hydrocarbons from reservoirs located below such water depths presents a considerable technical challenge. |
| Geology | delta | An area of deposition or the deposit formed by a flowing sediment-laden current as it enters an open or standing body of water, such as a river spilling into a gulf. As a river enters a body of water, its velocity drops and its ability to carry sediment diminishes, leading to deposition. The term has origins in Greek because the shape of deltas in map view can be similar to the Greek letter delta. The shapes of deltas are subsequently modified by rivers, tides and waves. There is a characteristic coarsening upward of sediments in a delta. The three main classes of deltas are river-dominated (Mississippi River), wave-dominated (Nile River), and tide-dominated (Ganges River). Ancient deltas contain some of the largest and most productive petroleum systems. |
| Geology | deltaic | Pertaining to an area of deposition or the deposit formed by a flowing sediment-laden current as it enters an open or standing body of water, such as a river spilling into a gulf. As a river enters a body of water, its velocity drops and its ability to carry sediment diminishes, leading to deposition. The term has origins in Greek because the shape of deltas in map view can be similar to the Greek letter delta. The shapes of deltas are subsequently modified by rivers, tides and waves. There is a characteristic coarsening upward of sediments in a delta. The three main classes of deltas are river-dominated (Mississippi River), wave-dominated (Nile River), and tide-dominated (Ganges River). Ancient deltas contain some of the largest and most productive petroleum systems. |
| Geology | density | Mass per unit of volume. Density is typically reported in g/cm3 (for example, rocks) or pounds per barrel (drilling mud) in the oil field. |
| Geology | density current | An influx of rapidly moving, sediment-laden water down a slope into a larger body of water; the suspended sediment causes the current to have a higher density than the clearer water into which it flows, hence the name. Such currents can occur in lakes and oceans, in some cases as by-products of earthquakes or mass movements such as slumps. The sedimentary deposits that form as the current loses energy are called turbidites and can be preserved as Bouma sequences. Density currents are characteristic of trench slopes of convergent plate margins and continental slopes of passive margins. |
| Geology | depocenter | The area of thickest deposition in a basin. |
| Geology | deposit | Sediments that have accumulated, usually after being moved by wind, water or ice. |
| Geology | depositional energy | The relative kinetic energy of the environment. A high-energy environment might consist of a rapidly flowing stream that is capable of carrying coarse-grained sediments, such as gravel and sand. Sedimentation in a low-energy environment, such as an abyssal plain, usually involves very fine-grained clay or mud. Depositional energy is not simply velocity. For example, although glaciers do not move quickly, they are capable of carrying large boulders. |
| Geology | depositional environment | The area in which and physical conditions under which sediments are deposited, including sediment source; depositional processes such as deposition by wind, water or ice; and location and climate, such as desert, swamp or river. |
| Geology | depositional system | The three-dimensional array of sediments or lithofacies that fills a basin. Depositional systems vary according to the types of sediments available for deposition as well as the depositional processes and environments in which they are deposited. The dominant depositional systems are alluvial, fluvial, deltaic, marine, lacustrine and eolian systems. |
| Geology | detrital | Pertaining to particles of rock derived from the mechanical breakdown of preexisting rocks by weathering and erosion. Detrital fragments can be transported to recombine and, through the process of lithification, become sedimentary rocks. Detrital is usually used synonymously with clastic, although a few authors differentiate between weathering of particles, which forms detrital sediments, and mechanical breakage, which produces clastic sediments. |
| Geology | detritus | Particles of rock derived from the mechanical breakdown of preexisting rocks by weathering and erosion. Detrital fragments can be transported to recombine and, through the process of lithification, become sedimentary rocks. Detrital is usually used synonymously with clastic, although a few authors differentiate between weathering of particles, which forms detrital sediments, and mechanical breakage, which produces clastic sediments. |
| Geology | development | The phase of petroleum operations that occurs after exploration has proven successful, and before full-scale production. The newly discovered oil or gas field is assessed during an appraisal phase, a plan to fully and efficiently exploit it is created, and additional wells are usually drilled. |
| Geology | dextral | Pertaining to a strike-slip fault or right-lateral fault in which the block across the fault moves to the right. If it moves left, the relative motion is described as sinistral. Clockwise rotation or spiraling is also described as dextral. |
| Geology | diagenesis | The physical, chemical or biological alteration of sediments into sedimentary rock at relatively low temperatures and pressures that can result in changes to the rock’s original mineralogy and texture. After deposition, sediments are compacted as they are buried beneath successive layers of sediment and cemented by minerals that precipitate from solution. Grains of sediment, rock fragments and fossils can be replaced by other minerals during diagenesis. Porosity usually decreases during diagenesis, except in rare cases such as dissolution of minerals and dolomitization. Diagenesis does not include weathering processes. Hydrocarbon generation begins during diagenesis. There is not a clear, accepted distinction between diagenesis and metamorphism, although metamorphism occurs at pressures and temperatures higher than those of the outer crust, where diagenesis occurs. |
| Geology | diagenetic | Pertaining to diagenesis, which is the physical, chemical or biological alteration of sediments into sedimentary rock at relatively low temperatures and pressures that can result in changes to the rock’s original mineralogy and texture. After deposition, sediments are compacted as they are buried beneath successive layers of sediment and cemented by minerals that precipitate from solution. Grains of sediment, rock fragments and fossils can be replaced by other minerals during diagenesis. Porosity usually decreases during diagenesis, except in rare cases such as dissolution of minerals and dolomitization. Diagenesis does not include weathering processes. Hydrocarbon generation begins during diagenesis. There is not a clear, accepted distinction between diagenesis and metamorphism, although metamorphism occurs at pressures and temperatures higher than those of the outer crust, where diagenesis occurs |
| Geology | diagenetic porosity | A type of secondary porosity created during diagenesis, commonly through dissolution or dolomitization or both. Diagenesis usually destroys porosity, so diagenetic porosity is rare. |
| Geology | diapir | A relatively mobile mass that intrudes into preexisting rocks. Diapirs commonly intrude vertically through more dense rocks because of buoyancy forces associated with relatively low-density rock types, such as salt, shale and hot magma, which form diapirs. The process is known as diapirism. By pushing upward and piercing overlying rock layers, diapirs can form anticlines, salt domes and other structures capable of trapping hydrocarbons. Igneous intrusions are typically too hot to allow the preservation of preexisting hydrocarbons. |
| Geology | diatom | A microscopic, single-celled, freshwater or saltwater algae that has a silica-rich cell wall called a frustule. Diatoms are so abundant that they can form thick layers of sediment composed of the frustules of the organisms that died and sank to the bottom. Frustules have been an important component of deep-sea deposits since Cretaceous time. Diatomite is the sedimentary rock that forms from diatom frustules. |
| Geology | diatomaceous | Pertaining to a diatom, which is a microscopic, single-celled, freshwater or saltwater algae that has a silica-rich cell wall called a frustule. Diatoms are so abundant that they can form thick layers of sediment composed of the frustules of the organisms that died and sank to the bottom. Frustules have been an important component of deep-sea deposits since Cretaceous time. Diatomite is the sedimentary rock that forms from diatom frustules. |
| Geology | diatomite | A soft, silica-rich sedimentary rock comprising diatom remains that forms most commonly in lakes and deep marine areas. Diatomite can form an excellent reservoir rock. The Belridge diatomite in the San Joaquin basin, California, USA, is a prolific oil-producing formation. |
| Geology | differential compaction | A phenomenon that occurs after the deposition of some sediments such that different parts of the sedimentary accumulation develop different degrees of porosity or settle unevenly during burial beneath successive layers of sediment. This can result from location on an uneven surface, such as near and over a reef structure, or near a growth fault, or from different susceptibility to compaction. The porosity in a formation that has experienced differential compaction can vary considerably from one area to another. |
| Geology | dike | An intrusive rock that invades preexisting rocks, commonly in a tabular shape that cuts vertically or nearly vertically across preexisting layers. Dikes form from igneous and sedimentary rocks. |
| Geology | dip | The magnitude of the inclination of a plane from horizontal. True, or maximum, dip is measured perpendicular to strike. Apparent dip is measured in a direction other than perpendicular to strike. |
| Geology | dipping bed | A layer of rock or sediment that is not horizontal. |
| Geology | dirty | Describing sedimentary rock that contains clay minerals. Even small amounts of clay minerals in pores can drastically reduce porosity and permeability. Dirty and clean are qualitative, descriptive terms to describe the relative amount of clay minerals in a rock |
| Geology | disconformity | A geologic surface that separates younger strata from older strata and represents a time of nondeposition, possibly combined with erosion. Some disconformities are highly irregular whereas others have no relief and can be difficult to distinguish within a series of parallel strata. |
| Geology | disharmonic | Pertaining to structures in which the shapes of adjacent layers differ or do not conform to one another. Folds of rock layers that have different mechanical properties or competence tend to be disharmonic, with a change in fold shape, symmetry or wavelength from one layer to the next. |
| Geology | displacement | The offset of segments or points that were once continuous or adjacent. Layers of rock that have been moved by the action of faults show displacement on either side of the fault surface. |
| Geology | dolomite | [CaMg(CO3)2] A widely-distributed carbonate mineral and chief constituent of dolostone. |
| Geology | dolomitization | The geochemical process in supratidal sabkha areas where magnesium [Mg] ions from the evaporation of seawater replace calcium [Ca] ions in calcite, forming the mineral dolomite. The volume of dolomite is less than that of calcite, so the replacement of calcite by dolomite in a rock increases the pore space in the rock by 13% and forms an important reservoir rock. Dolomitization can occur during deep burial diagenesis. |
| Geology | dolostone | A rock composed chiefly (> 90%) of dolomite. The rock is sometimes called dolomite, but dolostone is preferable to avoid ambiguity between the mineral and rock names. Replacement dolomite that forms soon after deposition is typically fine-grained and preserves original sedimentary structures. Recrystallization late in diagenesis produces coarser grained dolomite, destroys sedimentary structures and results in higher porosity. |
| Geology | dome | A type of anticline that is circular or elliptical rather than elongate. The upward migration of salt diapirs can form domes, called salt domes. |
| Geology | down dip | Located down the slope of a dipping plane or surface. In a dipping (not flat-lying) hydrocarbon reservoir that contains gas, oil and water, the gas is updip, the gas-oil contact is downdip from the gas, and the oil-water contact is still farther downdip. |
| Geology | down lap | The termination of more steeply dipping overlying strata against a surface or underlying strata that have lower apparent dips; a term used to describe a particular geometry of reflections in seismic data in sequence stratigraphy. |
| Geology | downdip | Located down the slope of a dipping plane or surface. In a dipping (not flat-lying) hydrocarbon reservoir that contains gas, oil and water, the gas is updip, the gas-oil contact is downdip from the gas, and the oil-water contact is still farther downdip. |
| Geology | downlap | The termination of more steeply dipping overlying strata against a surface or underlying strata that have lower apparent dips; a term used to describe a particular geometry of reflections in seismic data in sequence stratigraphy. |
| Geology | drape | A configuration of layers of rock that has the appearance of a fold, but might form simply through sagging or differential compaction of layers around a preexisting structure (such as a reef) or on an uneven surface. |
| Geology | dry gas | Natural gas that occurs in the absence of condensate or liquid hydrocarbons, or gas that has had condensable hydrocarbons removed. Dry gas typically has a gas-to-oil ratio exceeding 100,000 scf/STB. |
| Geology | dry rock | A subsurface rock that lacks contact with aquifers or meteoric water within the Earth. |
| Geology | dyke | An intrusive rock that invades preexisting rocks, commonly in a tabular shape that cuts vertically or nearly vertically across preexisting layers. Dikes form from igneous and sedimentary rocks. |
| Geology | earthquake | The sudden release of accumulated stress in the Earth by movement or shaking. Earthquakes are caused by tectonic activity, volcanoes and human activity (such as explosions). Earthquakes occur in the outer 720 km [445 miles] of the Earth, where rocks tend to break rather than flow under stress. The magnitude of earthquakes is determined according to the logarithmic Richter scale. An earthquake of magnitude 4.5 can cause damage, although humans can feel earthquakes as weak as magnitude 2.0. The San Francisco earthquake of 1906 measured 8.25 on the Richter scale, and the largest ever recorded were 8.9 magnitude earthquakes in Colombia and Ecuador (1906) and Japan (1933), and 9.5 in Chile (1960). |
| Geology | effective permeability | The ability to preferentially flow or transmit a particular fluid when other immiscible fluids are present in the reservoir (e.g., effective permeability of gas in a gas-water reservoir). The relative saturations of the fluids as well as the nature of the reservoir affect the effective permeability. In contrast, absolute permeability is the measurement of the permeability conducted when a single fluid or phase is present in the rock. |
| Geology | effective porosity | The interconnected pore volume or void space in a rock that contributes to fluid flow or permeability in a reservoir. Effective porosity excludes isolated pores and pore volume occupied by water adsorbed on clay minerals or other grains. Total porosity is the total void space in the rock whether or not it contributes to fluid flow. Effective porosity is typically less than total porosity. |
| Geology | elastic | Pertaining to a material that can undergo stress, deform, and then recover and return to its original shape after the stress ceases. Once stress exceeds the yield stress or elastic limit of a material, permanent deformation occurs and the material will not return to its original shape once the stress is removed. In some materials, including rocks, elastic behavior depends on the temperature and the duration of the stress as well as its intensity. |
| Geology | elastic deformation | The deformation that can be recovered when an applied stress has been removed. When the elastic limit of a material has been exceeded, nonrecoverable, permanent deformation occurs. |
| Geology | elastic limit | The yield point, or the point at which a material can no longer deform elastically. When the elastic limit is exceeded by an applied stress, permanent deformation occurs. |
| Geology | elasticity | Ability of a material to undergo stress, deform, and then recover and return to its original shape after the stress ceases. Once stress exceeds the yield stress or elastic limit of a material, permanent deformation occurs and the material will not return to its original shape when the stress is removed. In some materials, including rocks, elastic behavior depends on the temperature and the duration of the stress as well as its intensity. |
| Geology | en echelon | Describing parallel or subparallel, closely-spaced, overlapping or step-like minor structural features in rock, such as faults and tension fractures, that are oblique to the overall structural trend. |
| Geology | eolian | Pertaining to the environment of deposition of sediments by wind, such as the sand dunes in a desert. Because fine-grained sediments such as clays are removed easily from wind-blown deposits, eolian sandstones are typically clean and well-sorted. |
| Geology | erode | To cause or undergo erosion, the process of denudation of rocks, including physical, chemical and biological breakdown and transportation. The material from the rocks can be transported by wind, water, ice, or abrasive solid particles, or by mass-wasting, as in rock falls and landslides. |
| Geology | erosion | The process of denudation of rocks, including physical, chemical and biological breakdown and transportation. |
| Geology | estuarine | Pertaining to an estuary, a semi-enclosed coastal environment of deposition in which a river mouth permits freshwater to contact and mix with seawater. |
| Geology | estuary | A semi-enclosed coastal environment of deposition in which a river mouth permits freshwater to contact and mix with seawater. |
| Geology | eustasy | Global sea level variations. Changes in sea level can result from movement of tectonic plates altering the volume of ocean basins, or when changes in climate affect the volume of water stored in glaciers and in polar icecaps. Eustasy affects positions of shorelines and processes of sedimentation, so interpretation of eustasy is an important aspect of sequence stratigraphy. |
| Geology | eustatic | Pertaining to eustasy, a term for global sea level and its variations. Changes in sea level can result from movement of tectonic plates altering the volume of ocean basins, or when changes in climate affect the volume of water stored in glaciers and in polar icecaps. Eustasy affects positions of shorelines and processes of sedimentation, so interpretation of eustasy is an important aspect of sequence stratigraphy. |
| Geology | evaporite | A class of sedimentary minerals and sedimentary rocks that form by precipitation from evaporating aqueous fluid. Common evaporite minerals are halite, gypsum and anhydrite, which can form as seawater evaporates, and the rocks limestone and dolostone. Certain evaporite minerals, particularly halite, can form excellent cap rocks or seals for hydrocarbon traps because they have minimal porosity and they tend to deform plastically (as opposed to brittle fracturing that would facilitate leakage). |
| Geology | evaporitic | Pertaining to evaporite, a class of sedimentary minerals and sedimentary rocks that form by precipitation from evaporating aqueous fluid. Common evaporite minerals are halite, gypsum and anhydrite, which can form as seawater evaporates, and the rocks limestone and dolostone. Certain evaporite minerals, particularly halite, can form excellent cap rocks or seals for hydrocarbon traps because they have minimal porosity and they tend to deform plastically (as opposed to brittle fracturing that would facilitate leakage). |
| Geology | exploration | The initial phase in petroleum operations that includes generation of a prospect or play or both, and drilling of an exploration well. Appraisal, development and production phases follow successful exploration. |
| Geology | exploratory | Pertaining to exploration, the initial phase in petroleum operations that includes generation of a prospect or play or both, and drilling of an exploration well. Appraisal, development and production phases follow successful exploration. |
| Geology | extended reach drilling | Mobil Oil Company first used this term in the early 1980s for drilling directional wells in which the drilled horizontal reach (HR) attained at total depth (TD) exceeded the true vertical depth (TVD) by a factor greater than or equal to two. Extended-reach drilling (ERD) is particularly challenging for directional drilling and requires specialized planning to execute well construction. Since the term was coined, the scope of extended-reach drilling has broadened and the definition, which is now more flexible, includes deep wells with horizontal distance-to-depth, or H:V, ratios less than two. The drilling industry’s ERD database classifies wells, with increasing degree of well construction complexity, into low-, medium-, extended- and very extended-reach wells. Construction complexity depends on many factors, including water depth (for offshore wells), rig capability, geologic constraints and overall TVD. For example, a vertical well with TVD greater than 7,620 m [25,000 ft] is considered an extended-reach well. Also, depending on the conditions, drilling a well in deep water or through salt may be classified as ERD even if the well’s horizontal extent is not more than twice its TVD. |
| Geology | extended-reach drilling | Mobil Oil Company first used this term in the early 1980s for drilling directional wells in which the drilled horizontal reach (HR) attained at total depth (TD) exceeded the true vertical depth (TVD) by a factor greater than or equal to two. Extended-reach drilling (ERD) is particularly challenging for directional drilling and requires specialized planning to execute well construction. Since the term was coined, the scope of extended-reach drilling has broadened and the definition, which is now more flexible, includes deep wells with horizontal distance-to-depth, or H:V, ratios less than two. The drilling industry’s ERD database classifies wells, with increasing degree of well construction complexity, into low-, medium-, extended- and very extended-reach wells. Construction complexity depends on many factors, including water depth (for offshore wells), rig capability, geologic constraints and overall TVD. For example, a vertical well with TVD greater than 7,620 m [25,000 ft] is considered an extended-reach well. Also, depending on the conditions, drilling a well in deep water or through salt may be classified as ERD even if the well’s horizontal extent is not more than twice its TVD. |
| Geology | fabric | Rock fabric: Fabric refers to the spacing, arrangement, distribution, size, shape and orientation of the constituents of rocks such as minerals, grains, porosity, layering, bed boundaries, lithology contacts and fractures. |
| Geology | facies | The overall characteristics of a rock unit that reflect its origin and differentiate the unit from others around it. Mineralogy and sedimentary source, fossil content, sedimentary structures and texture distinguish one facies from another. |
| Geology | fairway | The trend along which a particular geological feature is likely, such as a sand fairway or a hydrocarbon fairway. Prediction of conceptual fairways helps explorationists develop prospects. Along a sand fairway, for example, sand was transported and, presumably, was deposited, allowing an interpretation of the presence of reservoir rock in the fairway. |
| Geology | fault | A break or planar surface in brittle rock across which there is observable displacement. Depending on the relative direction of displacement between the rocks, or fault blocks, on either side of the fault, its movement is described as normal, reverse or strike-slip. According to terminology derived from the mining industry, the fault block above the fault surface is called the hanging wall, while the fault block below the fault is the footwall. Given the geological complexity of some faulted rocks and rocks that have undergone more than one episode of deformation, it can be difficult to distinguish between the various types of faults. Also, areas deformed more than once or that have undergone continual deformation might have fault surfaces that are rotated from their original orientations, so interpretation is not straightforward. In a normal fault, the hanging wall moves down relative to the footwall along the dip of the fault surface, which is steep, from 45o to 90o. A growth fault is a type of normal fault that forms during sedimentation and typically has thicker strata on the downthrown hanging wall than the footwall. A reverse fault forms when the hanging wall moves up relative to the footwall parallel to the dip of the fault surface. A thrust fault, sometimes called an overthrust, is a reverse fault in which the fault plane has a shallow dip, typically much less than 45o. Movement of normal and reverse faults can also be oblique as opposed to purely parallel to the dip direction of the fault plane. The motion along a strike-slip fault, also known as a transcurrent or wrench fault, is parallel to the strike of the fault surface, and the fault blocks move sideways past each other. The fault surfaces of strike-slip faults are usually nearly vertical. A strike-slip fault in which the block across the fault moves to the right is described as a dextral strike-slip fault. If it moves left, the relative motion is described as sinistral. A transform fault is a particular type of strike-slip fault that is a boundary of an oceanic tectonic plate. The actual movement of a transform fault is opposite to its apparent displacement. The presence of a fault can be detected by observing characteristics of rocks such as changes in lithology from one fault block to the next, breaks and offsets between strata or seismic events, and changes in formation pressure in wells that penetrate both sides of a fault. Some fault surfaces contain relatively coarse rubble that can act as a conduit for migrating oil or gas, whereas the surfaces of other faults are smeared with impermeable clays or broken grains that can act as a fault seal. |
| Geology | fault trap | A type of structural hydrocarbon trap in which closure is controlled by the presence of at least one fault surface. |
| Geology | feldspar | [alkali feldspar (K,Na)AlSi3O8] [plagioclase feldspar NaAlSi3O8 – CaAl2Si2O8] A group of rock-forming silicate minerals that are essential constituents of igneous rocks and are common in sandstones. Feldspar can weather to form clay minerals. Feldspar can occur in all three major rock types and forms approximately 60% of the crust of the Earth. |
| Geology | felsic | Pertaining to minerals or igneous rocks composed of minerals such as quartz and feldspar that are relatively light in color and density. The word comes from the terms feldspar and silica. Granite is a felsic igneous rock. (Compare with mafic.) |
| Geology | fence diagram | A graphical display of three-dimensional data and interpretations in two-dimensional perspective view. Geologic cross sections can be displayed in a network to form a fence diagram. Stratigraphic changes can be displayed clearly in fence diagrams. |
| Geology | field | An accumulation, pool, or group of pools of hydrocarbons or other mineral resources in the subsurface. A hydrocarbon field consists of a reservoir in a shape that will trap hydrocarbons and that is covered by an impermeable, sealing rock. Typically, the term implies an economic size. |
| Geology | flower structure | Folded structures associated with strike-slip faults. In areas where strike-slip faults occur in converging crust, or transpression, rocks are faulted upward in a positive flower structure. In areas of strike-slip faulting in diverging crust, or transtension, rocks drop down to form a negative flower structure. Flower structures can form hydrocarbon traps. The term “flower structure” reflects the resemblance of the structure to the petals of a flower in cross section. |
| Geology | fluid contact | The interface that separates fluids of different densities in a reservoir. Horizontal contacts are usually assumed, although tilted contacts occur in some reservoirs. The contact between fluids is usually gradual rather than sharp, forming a transition zone of mixed fluid. A mixed-fluid reservoir will stratify according to fluid density, with gas at the top, oil in the middle, and water below. Production of fluids often perturbs the fluid contacts in a reservoir. |
| Geology | fluvial | Pertaining to an environment of deposition by a river or running water. Fluvial deposits tend to be well sorted, especially in comparison with alluvial deposits, because of the relatively steady transport provided by rivers. |
| Geology | fold | A wave-like geologic structure that forms when rocks deform by bending instead of breaking under compressional stress. Anticlines are arch-shaped folds in which rock layers are upwardly convex. The oldest rock layers form the core of the fold, and outward from the core progressively younger rocks occur. A syncline is the opposite type of fold, having downwardly convex layers with young rocks in the core. Folds typically occur in anticline-syncline pairs. The hinge is the point of maximum curvature in a fold. The limbs occur on either side of the fold hinge. The imaginary surface bisecting the limbs of the fold is called the axial surface. The axial surface is called the axial plane in cases where the fold is symmetrical and the lines containing the points of maximum curvature of the folded layers, or hinge lines, are coplanar. Concentric folding preserves the thickness of each bed as measured perpendicular to original bedding. Similar folds have the same wave shape, but bed thickness changes throughout each layer, with thicker hinges and thinner limbs. |
| Geology | formation | The fundamental unit of lithostratigraphy. A body of rock that is sufficiently distinctive and continuous that it can be mapped. In stratigraphy, a formation is a body of strata of predominantly one type or combination of types; multiple formations form groups, and subdivisions of formations are members. |
| Geology | formation | A surface land form. |
| Geology | formation pressure | The pressure of fluids within the pores of a reservoir, usually hydrostatic pressure, or the pressure exerted by a column of water from the formation’s depth to sea level. When impermeable rocks such as shales form as sediments are compacted, their pore fluids cannot always escape and must then support the total overlying rock column, leading to anomalously high formation pressures. Because reservoir pressure changes as fluids are produced from a reservoir, the pressure should be described as measured at a specific time, such as initial reservoir pressure. |
| Geology | formation water | Water that occurs naturally within the pores of rock. Water from fluids introduced to a formation through drilling or other interference, such as mud and seawater, does not constitute formation water. Formation water, or interstitial water, might not have been the water present when the rock originally formed. In contrast, connate water is the water trapped in the pores of a rock during its formation, and may be called fossil water. |
| Geology | fossil | Preserved remnants of plants or animals, such as skeletons, shells, casts or molds, tracks or borings, and feces. |
| Geology | fracture gradient | The pressure required to induce fractures in rock at a given depth. |
| Geology | fracture porosity | A type of secondary porosity produced by the tectonic fracturing of rock. Fractures themselves typically do not have much volume, but by joining preexisting pores, they enhance permeability significantly. In exceedingly rare cases, nonreservoir rocks such as granite can become reservoir rocks if sufficient fracturing occurs. |
| Geology | free water | Water that is mobile, available to flow, and not bound to surfaces of grains or minerals in rock. |
| Geology | fresh water | Water that is low in dissolved salt ( < 2000 ppm). |
| Geology | gas hydrate | An unusual occurrence of hydrocarbon in which molecules of natural gas, typically methane, are trapped in ice molecules. More generally, hydrates are compounds in which gas molecules are trapped within a crystal structure. Hydrates form in cold climates, such as permafrost zones and in deep water. To date, economic liberation of hydrocarbon gases from hydrates has not occurred, but hydrates contain quantities of hydrocarbons that could be of great economic significance. Hydrates can affect seismic data by creating a reflection or multiple. |
| Geology | gas in solution | Gas that is dissolved in a liquid, such as water or oil. |
| Geology | gas oil contact | A bounding surface in a reservoir above which predominantly gas occurs and below which predominantly oil occurs. Gas and oil are miscible, so the contact between gas and oil is transitional, forming a zone containing a mix of gas and oil. |
| Geology | gas prone | The quality of a source rock that makes it more likely to generate gas than oil. The nature of the organic matter or kerogen in source rocks varies from coaly, plant-like material commonly found in terrestrial source rocks to algal or other marine material that makes up marine source rocks. Terrestrial source rocks are commonly gas-prone. |
| Geology | gas sand | A porous sand layer or sand body charged with natural gas. |
| Geology | gas water contact | A bounding surface in a reservoir above which predominantly gas occurs and below which predominantly water occurs. Gas and water are somewhat miscible, so the contact between gas and water is not necessarily sharp and there is typically a transition zone between 100% gas and 100% water in reservoirs. |
| Geology | gas-oil contact | A bounding surface in a reservoir above which predominantly gas occurs and below which predominantly oil occurs. Gas and oil are miscible, so the contact between gas and oil is transitional, forming a zone containing a mix of gas and oil. |
| Geology | gas-prone | The quality of a source rock that makes it more likely to generate gas than oil. The nature of the organic matter or kerogen in source rocks varies from coaly, plant-like material commonly found in terrestrial source rocks to algal or other marine material that makes up marine source rocks. Terrestrial source rocks are commonly gas-prone. |
| Geology | gas-water contact | A bounding surface in a reservoir above which predominantly gas occurs and below which predominantly water occurs. Gas and water are somewhat miscible, so the contact between gas and water is not necessarily sharp and there is typically a transition zone between 100% gas and 100% water in reservoirs. |
| Geology | generation | The formation of hydrocarbons from a source rock as bitumen forms from kerogen and accumulates as oil or gas. Generation depends on three main factors: the presence of organic matter rich enough to yield hydrocarbons, adequate temperature, and sufficient time to bring the source rock to maturity. Pressure and the presence of bacteria and catalysts also affect generation. Generation is a critical phase in the development of a petroleum system. |
| Geology | geochronology | The study of the relative or absolute age of rocks, minerals and fossils. Absolute age is the measurement of age in years, but “absolute” ages typically have some amount of error and are inexact. Relative age, in contrast, is the approximate age of rocks, fossils or minerals made by determining the age of the material relative to other surrounding material. |
| Geology | geologic | Pertaining to geology, the study of the Earth-its history, structure, composition, life forms and the processes that continue to change it. |
| Geology | geologic map | A map showing the type and spatial distribution of rocks at the surface of the Earth. Rock formations are color-coded and symbols for geological structures are annotated, so age relationships are evident. Topographic contours and cultural features can also appear on geologic maps. |
| Geology | geologic time scale | A chronological chart of the stages and ages of events in the history of the Earth, from its initial formation to present, that has been constructed on the basis of the rock record. As is the typical natural position of rocks, the oldest event is at the bottom of the chart and the youngest is at the top. Both absolute and relative ages of rocks and fossils supplement interpretations from rocks. The vastness of geologic time and the slowness of geological processes are difficult to capture in a simple chart. |
| Geology | geological | Pertaining to geology, the study of the Earth-its history, structure, composition, life forms and the processes that continue to change it. |
| Geology | geologist | A scientist trained in the study of the Earth. In the petroleum industry, geologists perform a wide variety of functions, but typically generate prospects and interpret data such as maps, well logs, outcrops, cuttings, core samples and seismic data. |
| Geology | geology | The study of the Earth-its history, structure, composition, life forms and the processes that continue to change it. |
| Geology | geomagnetic polarity reversal | The periodic switching of the magnetic north and south poles of the Earth throughout time, probably as a result of movement of fluid within the Earth’s core. The onset and duration of the many episodes of reversed polarity have been documented by examining the polarity of magnetic minerals within rocks of different ages from around the world, particularly in basalts or igneous rocks of the oceanic crust. Oceanic basalts record the Earth’s magnetic field as they solidify from molten lava symmetrically on each side of the midoceanic ridges. These data have been compiled to create a time scale known as the geomagnetic polarity time scale (GPTS). In the oil field, borehole recordings allow direct correlation to GPTS and well-to-well correlations. |
| Geology | geomagnetic polarity time scale | A record of the onset and duration of the multitude of episodes of reversal of the Earth’s magnetic polarity, or geomagnetic polarity reversals. The GPTS was developed by thorough study of rocks from around the world, during which it was observed that rocks from specific time periods contained magnetic minerals whose orientation was opposite to that of the current magnetic field. By comparing the patterns of magnetic reversals with those of rocks of known age, the approximate ages of rocks can be established. This is particularly useful for basalts of the oceanic crust, which record the Earth’s magnetic field as they solidify from molten lava symmetrically about the midocean ridges. The time scale has been accurately extended back to the Upper Jurassic, the age of oldest existing oceanic crust. |
| Geology | geopressure | The pressure within the Earth, or formation pressure. The common oilfield usage, however, is to indicate anomalous subsurface pore pressure that is higher or lower than the normal, predicted hydrostatic pressure for a given depth, or the pressure exerted per unit area by a column of fresh water from sea level to a given depth. Abnormally low pore pressure might occur in areas where fluids have been drained, such as a depleted hydrocarbon reservoir. Abnormally high pore pressure might occur in areas where burial of water-filled sediments by an impermeable sediment such as clay was so rapid that fluids could not escape and the pore pressure increased with deeper burial. |
| Geology | geopressure gradient | The change in pore pressure per unit depth, typically in units of pounds per square inch per foot (psi/ft) or kilopascals per meter (kPa/m). The geopressure gradient might be described as high or low if it deviates from the normal hydrostatic pressure gradient of 0.433 psi/ft [9.8 kPa/m]. |
| Geology | geopressured | Subject to the pressure within the Earth, or formation pressure. The common oilfield usage, however, is to indicate anomalous subsurface pore pressure that is higher or lower than the normal, predicted hydrostatic pressure for a given depth, or the pressure exerted per unit area by a column of fresh water from sea level to a given depth. Abnormally low pore pressure might occur in areas where fluids have been drained, such as a depleted hydrocarbon reservoir. Abnormally high pore pressure might occur in areas where burial of water-filled sediments by an impermeable sediment such as clay was so rapid that fluids could not escape and the pore pressure increased with deeper burial. |
| Geology | geostatic pressure | The pressure of the weight of overburden, or overlying rock, on a formation; also called lithostatic pressure. |
| Geology | glacial | Pertaining to the environment of deposition by glaciers. |
| Geology | glauconite | [(K,Na,Ca) 1.2-2.0(Fe+3,Al,Fe+2,Mg)4(Si7-7.6Al1-0.4O20)(OH)47nH20] A green silicate mineral found in sedimentary rocks and formed on continental shelves characterized by slow sedimentation and organic matter, such as fecal pellets, present in an oxidizing environment. In sufficient quantity, it can form a sandy, green deposit such as the Cretaceous greensands of the US and UK. |
| Geology | Global Positioning System | A system of numerous Earth-orbiting satellites that can be used to determine the location (latitude, longitude and elevation) of a receiver or station on the Earth within about 2 m [6 ft]. Fixed receivers on Earth can be used to determine the relative motions of fault blocks and lithospheric plates. Hand-held receivers can be used for producing accurate geologic maps, acquiring navigation data for 3D seismic surveys, and positioning wells in the field. |
| Geology | GOC | Abbreviation for gas-oil contact, a bounding surface in a reservoir above which predominantly gas occurs and below which predominantly oil occurs. Gas and oil are miscible, so the contact between gas and oil is transitional, forming a zone containing a mix of gas and oil. |
| Geology | GPS | A system of numerous Earth-orbiting satellites that can be used to determine the location (latitude, longitude and elevation) of a receiver or station on the Earth within about 2 m [6 ft]. Fixed receivers on Earth can be used to determine the relative motions of fault blocks and lithospheric plates. Hand-held receivers can be used for producing accurate geologic maps, acquiring navigation data for 3D seismic surveys, and positioning wells in the field. |
| Geology | GPTS | A record of the onset and duration of the multitude of episodes of reversal of the Earth’s magnetic polarity, or geomagnetic polarity reversals. The GPTS was developed by thorough study of rocks from around the world, during which it was observed that rocks from specific time periods contained magnetic minerals whose orientation was opposite to that of the current magnetic field. By comparing the patterns of magnetic reversals with those of rocks of known age, the approximate ages of rocks can be established. This is particularly useful for basalts of the oceanic crust, which record the Earth’s magnetic field as they solidify from molten lava symmetrically about the midocean ridges. The time scale has been accurately extended back to the Upper Jurassic, the age of oldest existing oceanic crust. |
| Geology | graben | A relatively low-standing fault block bounded by opposing normal faults. Graben (used as both singular and plural) can form in areas of rifting or extension, where normal faults are the most common type of fault. Between graben are relatively high-standing blocks called horsts. A half-graben is a downdropped block bounded by a normal fault on only one side. |
| Geology | grain density | The density of a rock or mineral with no porosity, also known as matrix density, commonly in units of g/cm3. |
| Geology | granite | A coarse-grained, plutonic or intrusive igneous rock of felsic composition having large crystals of quartz, feldspar and mica. In the oil field, “granite” is sometimes used incorrectly to indicate any type of hard rock. |
| Geology | groundwater | Water in the subsurface below the water table. Groundwater is held in the pores of rocks, and can be connate, from meteoric sources, or associated with igneous intrusions. |
| Geology | growth fault | A type of normal fault that develops and continues to move during sedimentation and typically has thicker strata on the downthrown, hanging wall side of the fault than in the footwall. Growth faults are common in the Gulf of Mexico and in other areas where the crust is subsiding rapidly or being pulled apart. |
| Geology | GWC | Abbreviation for gas-water contact, a bounding surface in a reservoir above which predominantly gas occurs and below which predominantly water occurs. Gas and water are somewhat miscible, so the contact between gas and water is not necessarily sharp and there is typically a transition zone between 100% gas and 100% water in reservoirs. |
| Geology | gypsum | [CaSO472H2O] A highly insoluble sulfate mineral that is the first to precipitate from evaporating seawater. Dehydration of gypsum can produce anhydrite. Fine-grained gypsum is called alabaster. |
| Geology | hade | The angle that a fault plane, or other stratigraphic and structural surfaces, makes with the vertical, as measured perpendicular to the strike of that plane or surface. The hade angle is the complementary angle to the dip angle, that is, hade = 90°?dip. |
| Geology | halite | [NaCl] A soft, soluble evaporite mineral commonly known as salt or rock salt. Because salt is less dense than many sedimentary rocks, it is relatively buoyant and can form salt domes, pillars or curtains by flowing and breaking through or piercing overlying sediments, as seen in the Gulf of Mexico and the Zagros fold belt. Halite can be critical in forming hydrocarbon traps and seals because it tends to flow rather than fracture during deformation, thus preventing hydrocarbons from leaking out of a trap even during and after some types of deformation. |
| Geology | hard rock | A term applied to hard rocks, or igneous and metamorphic rocks that are distinguished from sedimentary rocks because they are typically more difficult to disaggregate. Well cemented sedimentary rocks are sometimes described as being hard, but are usually called soft rock. The term can be used to differentiate between rocks of interest to the petroleum industry (soft rocks) and rocks of interest to the mining industry (hard rocks). |
| Geology | hardground | A horizon cemented by precipitation of calcite just below the sea floor. Local concretions form first in a hardground and can be surrounded by burrows of organisms until the cement is well developed. |
| Geology | harmonic | Pertaining to structures in which the shapes of adjacent layers resemble or conform to one another. Folds of rock layers that have similar mechanical properties or competence tend to be harmonic, with little change in fold shape, symmetry or wavelength from one layer to the next. |
| Geology | hiatal | Pertaining to a hiatus, a cessation in deposition of sediments during which no strata form or an erosional surface forms on the underlying strata; a gap in the rock record. This period might be marked by development of a lithified sediment (hardground) or burrowed surface characteristic of periods when sea level was relatively low. A disconformity can result from a hiatus. |
| Geology | hiatus | A cessation in deposition of sediments during which no strata form or an erosional surface forms on the underlying strata; a gap in the rock record. This period might be marked by development of a lithified sediment (hardground) or burrowed surface characteristic of periods when sea level was relatively low. A disconformity can result from a hiatus. |
| Geology | homogeneity | The quality of uniformity of a material. If irregularities are distributed evenly in a mixture of material, the material is homogeneous. (Compare with heterogeneity.) |
| Geology | homogeneous | Possessing the quality of uniformity. If irregularities are distributed evenly in a material, the material is homogeneous. (Compare with heterogeneous.) |
| Geology | horizon | An informal term used to denote a surface in or of rock, or a distinctive layer of rock that might be represented by a reflection in seismic data. The term is often used incorrectly to describe a zone from which hydrocarbons are produced. |
| Geology | horst | A relatively high-standing area formed by the movement of normal faults that dip away from each other. Horsts occur between low-standing fault blocks called graben. Horsts can form in areas of rifting or extension, where normal faults are the most abundant variety of fault. |
| Geology | hydrate | To cause the incorporation of water into the atomic structure of a mineral. |
| Geology | hydrate | A chemical combination of water and another substance. Gypsum is a hydrate mineral. Its anhydrous equivalent is anhydrite. |
| Geology | hydrate | An unusual occurrence of hydrocarbon in which molecules of natural gas, typically methane, are trapped in ice molecules. More generally, hydrates are compounds in which gas molecules are trapped within a crystal structure. Hydrates form in cold climates, such as permafrost zones and in deep water. To date, economic liberation of hydrocarbon gases from hydrates has not occurred, but hydrates contain quantities of hydrocarbons that could be of great economic significance. Hydrates can affect seismic data by creating a reflection or multiple. |
| Geology | hydration | Incorporation of water into the atomic structure of a mineral, i.e., the chemical combination of water and another substance. Gypsum is a hydrate mineral. Its anhydrous equivalent is anhydrite. |
| Geology | hydraulic head | The force per unit area exerted by a column of liquid at a height above a depth (and pressure) of interest. Fluids flow down a hydraulic gradient, from points of higher to lower hydraulic head. The term is sometimes used synonymously with hydrostatic head. |
| Geology | hydrocarbon | A naturally occurring organic compound comprising hydrogen and carbon. Hydrocarbons can be as simple as methane [CH4], but many are highly complex molecules, and can occur as gases, liquids or solids. The molecules can have the shape of chains, branching chains, rings or other structures. Petroleum is a complex mixture of hydrocarbons. The most common hydrocarbons are natural gas, oil and coal. |
| Geology | hydrocarbon kitchen | An area of the subsurface where source rock has reached appropriate conditions of pressure and temperature to generate hydrocarbons; also known as source kitchen, oil kitchen or gas kitchen. |
| Geology | hydrostatic head | The height of a column of freshwater that exerts pressure at a given depth. Some authors use the term synonymously with hydrostatic pressure. |
| Geology | hydrostatic pressure | The normal, predicted pressure for a given depth, or the pressure exerted per unit area by a column of freshwater from sea level to a given depth. Abnormally low pressure might occur in areas where fluids have been drained, such as a depleted hydrocarbon reservoir. Abnormally high pressure might occur in areas where burial of water-filled sediments by an impermeable sediment such as clay was so rapid that fluids could not escape and the pore pressure increased with deeper burial. |
| Geology | hydrothermal | Pertaining to hot fluids, particularly hot water, or the activity of hot water, or precipitates thereof. Hydrothermal alteration can change the mineralogy of rock, producing different minerals, including quartz, calcite and chlorite. Hydrothermal activity is commonly associated with hot water that accompanies, or is heated by, magma. |
| Geology | hydrothermal alteration | A change of preexisting rocks or minerals caused by the activity of hot solutions, such as fluids accompanying or heated by magma. Quartz, serpentine and chlorite are minerals commonly associated with hydrothermal alteration. Ore deposits, such as lead (as the mineral galena), zinc (sphalerite), and copper (malachite), can occur in areas of hydrothermal alteration. |
| Geology | igneous | Pertaining to one of three main classes of rocks (igneous, metamorphic and sedimentary). Igneous rocks crystallize from molten rock, or magma, with interlocking mineral crystals. Igneous rocks that crystallize slowly, typically below the surface of the Earth, are plutonic igneous rocks and have large crystals (large enough to see with the naked eye). Volcanic igneous rocks crystallize quickly at the Earth’s surface and have small crystals (usually too small to see without magnification). Common examples include granite (plutonic) and rhyolite (volcanic), diorite (plutonic) and andesite (volcanic), and gabbro (plutonic) and basalt (volcanic). Igneous rocks typically comprise the minerals quartz, mica, feldspar, amphibole, pyroxene and olivine. |
| Geology | illite | [K1-1.5Al4(Si7-6.5Al1-1.5O20)(OH)4] A group of clay minerals formed during the alteration of silicate minerals such as mica and feldspar and commonly found in marine shales. |
| Geology | immature | Pertaining to a hydrocarbon source rock that has not fully entered optimal conditions for generation. |
| Geology | impermeable | Pertaining to a rock that is incapable of transmitting fluids because of low permeability. Shale has a high porosity, but its pores are small and disconnected, so it is relatively impermeable. Impermeable rocks are desirable sealing rocks or cap rocks for reservoirs because hydrocarbons cannot pass through them readily. |
| Geology | in situ | In the original location or position, such as a large outcrop that has not been disturbed by faults or landslides. Tests can be performed in situ in a reservoir to determine its pressure and temperature and fluid properties. |
| Geology | incompetent | Pertaining to strata that are relatively ductile and tend to flow under stress rather than deform by brittle faulting or fracturing. The bed thickness of incompetent beds tends to change during deformation. |
| Geology | injectite | Structures formed by sediment injection. Because they resemble intrusive and extrusive igneous features, much of the vocabulary for describing injectites, or clastic intrusions, comes from igneous geology. Sills are emplaced parallel to bedding, whereas dikes cut through bedding. The strata containing the intrusion are called host strata and the layers that feed the intrusion are the parent beds. Sand-injection features exhibit size scales from millimeters to kilometers, and have been seen in cores, borehole image logs, seismic sections, outcrops, aerial photographs and satellite images. |
| Geology | in-situ | In the original location or position, such as a large outcrop that has not been disturbed by faults or landslides. Tests can be performed in situ in a reservoir to determine its pressure and temperature and fluid properties. |
| Geology | interstitial water | Water that occurs naturally within the pores of rock. Water from fluids introduced to a formation through drilling or other interference, such as mud and seawater, does not constitute interstitial water. Interstitial water, or formation water, might not have been the water present when the rock originally formed. In contrast, connate water is the water trapped in the pores of a rock during its formation, also called fossil water. |
| Geology | inversion | The reversal of features, particularly structural features such as faults, by reactivation. For example, a normal fault might move in a direction opposite to its initial movement. |
| Geology | inversion | The atypical appearance of structural and topographic features, such as an anticline being exposed in a valley instead of as a hill; also called inverted relief. |
| Geology | isochore | A contour connecting points of equal true vertical thickness of strata, formations, reservoirs or other rock units. A map that displays isochores is an isochore map. The terms isopach and isopach map are incorrectly used interchangeably to describe isochores and isochore maps. Isopachs and isochores are equivalent only if the rock layer is horizontal. |
| Geology | isopach | A contour that connects points of equal thickness. Commonly, the isopachs, or contours that make up an isopach map, display the stratigraphic thickness of a rock unit as opposed to the true vertical thickness. Isopachs are true stratigraphic thicknesses; i.e., perpendicular to bedding surfaces. |
| Geology | isostasy | The state of gravitational equilibrium between the lithosphere and the asthenosphere of the Earth such that lithospheric plates “float” at a given elevation depending on their thickness. The balance between the elevation of the lithospheric plates and the asthenosphere is achieved by the flowage of the denser asthenosphere. Various hypotheses about isostasy take into account density (Pratt hypothesis), thickness (Airy hypothesis), and pressure variations to explain topographic variations among lithospheric plates. The current model consists of several layers of different density. |
| Geology | isostatic | Pertaining to isostasy, the state of gravitational equilibrium between the lithosphere and the asthenosphere of the Earth such that lithospheric plates “float” at a given elevation depending on their thickness. The balance between the elevation of the lithospheric plates and the asthenosphere is achieved by the flowage of the denser asthenosphere. Various hypotheses about isostasy take into account density (Pratt hypothesis), thickness (Airy hypothesis), and pressure variations to explain topographic variations among lithospheric plates. The current model consists of several layers of different density. |
| Geology | isotropic | Directionally uniform, such that the physical properties of the material do not vary in different directions. In rocks, changes in physical properties in different directions, such as the alignment of mineral grains or the seismic velocity measured parallel or perpendicular to bedding surfaces, are forms of anisotropy. (Compare with homogeneity). |
| Geology | isotropy | A quality of directional uniformity in material such that physical properties do not vary in different directions. In rocks, changes in physical properties in different directions, such as the alignment of mineral grains or the seismic velocity measured parallel or perpendicular to bedding surfaces, are forms of anisotropy. (Compare with homogeneity.) |
| Geology | joint | A surface of breakage, cracking or separation within a rock along which there has been no movement parallel to the defining plane. The usage by some authors can be more specific: When walls of a fracture have moved only normal to each other, the fracture is called a joint. |
| Geology | kaolinite | [Al4Si4O10(OH)8] A type of clay mineral from the kaolin group that forms through the weathering of feldspar and mica group minerals. Unlike some clay minerals like montmorillonite, kaolinite is not prone to shrinking or swelling with changes in water content. |
| Geology | karst | A type of topography formed in areas of widespread carbonate rocks through dissolution. Sink holes, caves and pock-marked surfaces are typical features of a karst topography. |
| Geology | lacustrine | Pertaining to an environment of deposition in lakes, or an area having lakes. Because deposition of sediment in lakes can occur slowly and in relatively calm conditions, organic-rich source rocks can form in lacustrine environments. |
| Geology | lamination | A fine layer (~ 1 mm thick) in strata, also called a lamina, common in fine-grained sedimentary rocks such as shale, siltstone and fine sandstone. A sedimentary bed comprises multiple laminations, or laminae. |
| Geology | lease | The act of acquiring acreage for exploration or production activity. |
| Geology | lease | An area of surface land on which exploration or production activity occurs. |
| Geology | limestone | A carbonate sedimentary rock predominantly composed of calcite of organic, chemical or detrital origin. Minor amounts of dolomite, chert and clay are common in limestones. Chalk is a form of fine-grained limestone. |
| Geology | lineament | A long linear or gently curving feature on the surface of a terrestrial planet or moon that is suggestive of an underlying geologic structure or contact. Most lineaments are identified through remote sensing, such as satellite imagery or topographic, gravimetric and magnetic data. |
| Geology | listric fault | A normal fault that flattens with depth and typically found in extensional regimes. This flattening manifests itself as a curving, concave-up fault plane whose dip decreases with depth. |
| Geology | lithification | The process by which unconsolidated sediments become sedimentary rock. Sediments typically are derived from preexisting rocks by weathering, transported and redeposited, and then buried and compacted by overlying sediments. Cementation causes the sediments to harden, or lithify, into rock. |
| Geology | lithofacies | A mappable subdivision of a stratigraphic unit that can be distinguished by its facies or lithology-the texture, mineralogy, grain size, and the depositional environment that produced it. |
| Geology | lithologic | Pertaining to lithology, the macroscopic nature of the mineral content, grain size, texture and color of rocks. |
| Geology | lithologic contact | The surface that separates rock bodies of different lithologies, or rock types. A contact can be conformable or unconformable depending upon the types of rock, their relative ages and their attitudes. A fault surface can also serve as a contact. |
| Geology | lithology | The macroscopic nature of the mineral content, grain size, texture and color of rocks. |
| Geology | lithosphere | The brittle outer layer of the Earth that includes the crust and uppermost mantle. It is made up of six major and several minor tectonic plates that move around on the softer asthenosphere. The lithosphere of the oceans tends to be thinner (in some oceanic areas, less than 50 km [30 miles] thick) and more dense than that of the continents (more than 120 km [70 miles] thick in places like the Himalayas) because of isostasy. The movement of the plates of the lithosphere results in convergence, or collisions, that can form mountain belts and subduction zones, and divergence of the plates and the creation of new crust as material wells up from below separating plates. The lithosphere and asthenosphere are distinguished from the crust, mantle and core of the Earth on the basis of their mechanical behavior and not their composition. |
| Geology | lithostatic pressure | The pressure of the weight of overburden, or overlying rock, on a formation; also called geostatic pressure. |
| Geology | lithostratigraphic | Pertaining to lithostratigraphy, the study and correlation of strata to elucidate Earth history on the basis of their lithology, or the nature of the well log response, mineral content, grain size, texture and color of rocks. |
| Geology | lithostratigraphy | The study and correlation of strata to elucidate Earth history on the basis of their lithology, or the nature of the well log response, mineral content, grain size, texture and color of rocks. |
| Geology | littoral | Pertaining to an environment of deposition affected by tides, the area between high tide and low tide. Given the variation of tides and land forms from place to place, geologists describe littoral zones locally according to the fauna capable of surviving periodic exposure and submersion. |
| Geology | Ma | Mega annum. The abbreviation for million years that is most commonly used in the geologic literature. |
| Geology | mafic | Pertaining to minerals or igneous rocks composed of minerals that are rich in iron and magnesium, dense, and typically dark in color. The term comes from the words magnesium and ferric. Common mafic minerals are olivine and pyroxene. Basalt is a mafic igneous rock. (Compare with felsic.) |
| Geology | magma | The molten rock in the Earth that can either rise to the surface as lava and form extrusive igneous rock or cool within the Earth to form plutonic igneous rock. |
| Geology | magnetic reversal sequence | The periodic switching of the magnetic north and south poles of the Earth throughout time, probably as a result of movement of fluid within the Earth’s core. The onset and duration of the many episodes of reversed polarity have been documented by examining the polarity of magnetic minerals within rocks of different ages from around the world, particularly in basalts or igneous rocks of the oceanic crust. Oceanic basalts record the Earth’s magnetic field as they solidify from molten lava symmetrically on each side of the midoceanic ridges. These data have been compiled to create a time scale known as the geomagnetic polarity time scale (GPTS). In the oil field, borehole recordings allow direct correlation to GPTS and well-to-well correlations. |
| Geology | mantle | The intermediate layer of the Earth beneath the crust that is about 2900 km thick [1820 miles] and overlies the core of the Earth. The mantle consists of dense igneous rocks like pyroxenite and dunite, composed of the minerals pyroxene and olivine. The crust, mantle and core of the Earth are distinguished from the lithosphere and asthenosphere on the basis of their composition and not their mechanical behavior. The Mohorovicic discontinuity abruptly separates the crust from the mantle, where the velocity of compressional waves is significantly higher. |
| Geology | marine | Pertaining to sediments or environments in seas or ocean waters, between the depth of low tide and the ocean bottom. |
| Geology | marl | A sedimentary rock containing a mix of clay and calcium carbonate. Compositionally, marls comprise 35% to 65% clay and 65% to 35% calcium carbonate. Thus, marl encompasses a spectrum that ranges from calcareous shale to muddy or shaly limestone. |
| Geology | marsh | An environment from which water rarely drains that supports primarily grassy vegetation and does not form peat. |
| Geology | massif | A block of rock that forms a structural or topographic feature, such as a block of igneous of metamorphic rock within an area of mountain building, or orogeny. A massif can be as large as a mountain and is typically more rigid than the rocks that surround it. |
| Geology | matrix | The finer grained, interstitial particles that lie between larger particles or in which larger particles are embedded in sedimentary rocks such as sandstones and conglomerates. |
| Geology | maturity | The state of a source rock with respect to its ability to generate oil or gas. As a source rock begins to mature, it generates gas. As an oil-prone source rock matures, the generation of heavy oils is succeeded by medium and light oils. Above a temperature of approximately 100 oC [212 oF], only dry gas is generated, and incipient metamorphism is imminent. The maturity of a source rock reflects the ambient pressure and temperature as well as the duration of conditions favorable for hydrocarbon generation. |
| Geology | metamorphic | One of three main classes of rock (igneous, metamorphic and sedimentary). Metamorphic rocks form from the alteration of preexisting rocks by changes in ambient temperature, pressure, volatile content, or all of these. Such changes can occur through the activity of fluids in the Earth and movement of igneous bodies or regional tectonic activity. The texture of metamorphic rocks can vary from almost homogeneous, or nonfoliated, to foliated rocks with a strong planar fabric or foliation produced by alignment of minerals during recrystallization or by reorientation. Common foliated metamorphic rocks include gneiss, schist and slate. Marble, or metamorphosed limestone, can be foliated or non-foliated. Hornfels is a nonfoliated metamorphic rock. Graphite, chlorite, talc, mica, garnet and staurolite are distinctive metamorphic minerals. |
| Geology | metamorphism | The process by which the characteristics of rocks are altered or the rock is recrystallized. Metamorphism of igneous, sedimentary, or preexisting metamorphic rock can produce new metamorphic rock. Such alteration occurs as rocks respond to changes in temperatures, pressures and fluids, commonly along the edges of colliding lithospheric plates. The pressures and temperatures at which metamorphism occurs are higher than those of diagenesis, but no clear boundary between the two has been established. |
| Geology | methane | [CH4] The lightest and most abundant of the hydrocarbon gases and the principal component of natural gas. Methane is a colorless, odorless gas that is stable under a wide range of pressure and temperature conditions in the absence of other compounds. |
| Geology | methane hydrate | An unusual occurrence of hydrocarbon in which molecules of methane are trapped in ice molecules. More generally, hydrates are compounds in which gas molecules are trapped within a crystal structure. Hydrates form in cold climates, such as permafrost zones and in deep water. To date, economic liberation of hydrocarbon gases from hydrates has not occurred, but hydrates contain quantities of hydrocarbons that could be of great economic significance. Hydrates can affect seismic data by creating a reflection or multiple. |
| Geology | micrite | Dense, fine-grained carbonate mud or rocks composed of mud that forms by erosion of larger carbonate grains, organic precipitation (such as from algae), or inorganic precipitation. The grains in micrite are generally less than 4 microns in size. |
| Geology | micropaleontology | The study of microfossils too small to be seen without the use of a microscope. Marine microfossils such as foraminifera are important for stratigraphic correlation. |
| Geology | midoceanic ridge | The mountainous, linear axis of ocean basins along which rifting occurs and new oceanic crust forms as magma wells up and solidifies. The most prominent midoceanic ridges are those of the Atlantic and Indian Oceans. The new crust is made of mafic igneous rock called basalt, commonly referred to as midocean ridge basalt, or MORB, whose composition reflects that of the deeper mantle of the Earth. The presence of the spreading plate boundaries of the midoceanic ridges; their symmetrically spreading, successively older crust outward from the ridge; and the lack of oceanic crust older than approximately 200 Ma support the theory of plate tectonics and the recycling of oceanic crust through the process of subduction. |
| Geology | migrate | For hydrocarbons to move from their source into reservoir rocks. The movement of newly generated hydrocarbons out of their source rock is primary migration, also called expulsion. The further movement of the hydrocarbons into reservoir rock in a hydrocarbon trap or other area of accumulation is secondary migration. Migration typically occurs from a structurally low area to a higher area because of the relative buoyancy of hydrocarbons in comparison to the surrounding rock. Migration can be local or can occur along distances of hundreds of kilometers in large sedimentary basins, and is critical to the formation of a viable petroleum system. |
| Geology | migration | The movement of hydrocarbons from their source into reservoir rocks. The movement of newly generated hydrocarbons out of their source rock is primary migration, also called expulsion. The further movement of the hydrocarbons into reservoir rock in a hydrocarbon trap or other area of accumulation is secondary migration. Migration typically occurs from a structurally low area to a higher area because of the relative buoyancy of hydrocarbons in comparison to the surrounding rock. Migration can be local or can occur along distances of hundreds of kilometers in large sedimentary basins, and is critical to the formation of a viable petroleum system. |
| Geology | Milankovitch cycles | The variation of the Earth’s exposure to the sun’s rays, or insolation, that results from variations in the orbit of the Earth and the tilt of its axis, and that might affect climate, sea level and sedimentation. Such variations are thought to occur in distinct time periods on the order of thousands of years. Ice ages might be a consequence of Milankovitch cycles. Milutin Milankovitch (1879 to 1958) was a Yugoslavian mathematician and physicist who specialized in studies of solar radiation and the orbit of the Earth. |
| Geology | mineral | A crystalline substance that is naturally occurring, inorganic, and has a unique or limited range of chemical compositions. Minerals are homogeneous, having a definite atomic structure. Rocks are composed of minerals, except for rare exceptions like coal, which is a rock but not a mineral because of its organic origin. Minerals are distinguished from one another by careful observation or measurement of physical properties such as density, crystal form, cleavage (tendency to break along specific surfaces because of atomic structure), fracture (appearance of broken surfaces), hardness, luster and color. Magnetism, taste and smell are useful ways to identify only a few minerals. |
| Geology | mineralogy | Mineralogy is the science and study of minerals with regard to their chemical composition, structure, formation and properties. |
| Geology | Moho | The boundary between the crust and the mantle of the Earth, which varies from approximately 5 km [3 miles] under the midoceanic ridges to 75 km [46 miles] deep under the continents. This boundary, commonly called “the Moho,” was recognized in 1909 by Croatian seismologist Andrija Mohorovicic on the basis of its abruptly higher compressional wave (P-wave) velocity. |
| Geology | Mohorovicic discontinuity | The boundary between the crust and the mantle of the Earth, which varies from approximately 5 km [3 miles] under the midoceanic ridges to 75 km [46 miles] deep under the continents. This boundary, commonly called “the Moho,” was recognized in 1909 by Croatian seismologist Andrija Mohorovicic on the basis of its abruptly higher compressional wave (P-wave) velocity. |
| Geology | moldic porosity | A type of secondary porosity created through the dissolution of a preexisting constituent of a rock, such as a shell, rock fragment or grain. The pore space preserves the shape, or mold, of the dissolved material. |
| Geology | montmorillonite | [(1/2Ca,Na)0.7(Al,Mg,Fe)4(Si,Al)8O20(OH)47nH20] A type of smectite clay mineral that tends to swell when exposed to water. Montmorillonite forms through the alteration of silicate minerals in alkaline conditions in basic igneous rocks, such as volcanic ash that can accumulate in the oceans. Montmorillonite is a component of bentonite commonly used in drilling fluids. |
| Geology | MRS | Abbreviation for magnetic reversal sequence, the periodic switching of the magnetic north and south poles of the Earth throughout time, probably as a result of movement of fluid within the Earth’s core. The onset and duration of the many episodes of reversed polarity have been documented by examining the polarity of magnetic minerals within rocks of different ages from around the world, particularly in basalts or igneous rocks of the oceanic crust. Oceanic basalts record the Earth’s magnetic field as they solidify from molten lava symmetrically on each side of the midoceanic ridges. These data have been compiled to create a time scale known as the geomagnetic polarity time scale (GPTS). In the oil field, borehole recordings allow direct correlation to GPTS and well-to-well correlations. |
| Geology | My | Abbreviation for million years. The preferred abbreviation is Ma. |
| Geology | MYBP | Abbreviation for millions of years before present. The preferred abbreviation is Ma. |
| Geology | natural gas | A naturally occurring mixture of hydrocarbon gases that is highly compressible and expansible. Methane [CH4] is the chief constituent of most natural gas (constituting as much as 85% of some natural gases), with lesser amounts of ethane [C2H6], propane [C3H8], butane [C4H10] and pentane [C5H12]. Impurities can also be present in large proportions, including carbon dioxide, helium, nitrogen and hydrogen sulfide. |
| Geology | neritic | Describing the environment and conditions of the marine zone between low tide and the edge of the continental shelf, a depth of roughly 200 m [656 ft]. A neritic environment supports marine organisms, also described as neritic, that are capable of surviving in shallow water with moderate exposure to sunlight. |
| Geology | nonconformity | A geological surface that separates younger overlying sedimentary strata from eroded igneous or metamorphic rocks and represents a large gap in the geologic record. |
| Geology | normal fault | A type of fault in which the hanging wall moves down relative to the footwall, and the fault surface dips steeply, commonly from 50o to 90o. Groups of normal faults can produce horst and graben topography, or a series of relatively high- and low-standing fault blocks, as seen in areas where the crust is rifting or being pulled apart by plate tectonic activity. A growth fault is a type of normal fault that forms during sedimentation and typically has thicker strata on the downthrown hanging wall than the footwall. |
| Geology | normal pressure | The pore pressure of rocks that is considered normal in areas in which the change in pressure per unit of depth is equivalent to hydrostatic pressure. The normal hydrostatic pressure gradient for freshwater is 0.433 pounds per square inch per foot (psi/ft), or 9.792 kilopascals per meter (kPa/m), and 0.465 psi/ft for water with 100,000 ppm total dissolved solids (a typical Gulf Coast water), or 10.516 kPa/m. |
| Geology | offset | The horizontal displacement between points on either side of a fault, which can range from millimeters to kilometers. Perhaps the most readily visible examples of offset are features such as fences or roads that have been displaced by strike-slip faults, such as the San Andreas fault of California, USA. |
| Geology | oil field | The surface area above a subsurface oil accumulation is called an oil field. |
| Geology | oil field | An accumulation, pool or group of pools of oil in the subsurface. An oil field consists of a reservoir in a shape that will trap hydrocarbons and that is covered by an impermeable or sealing rock. Typically, industry professionals use the term with an implied assumption of economic size. |
| Geology | oil kitchen | An area of the subsurface where source rock has reached appropriate conditions of pressure and temperature to generate liquid hydrocarbons as opposed to gas. |
| Geology | oil pool | A subsurface oil accumulation. An oil field can consist of one or more oil pools or distinct reservoirs within a single large trap. The term “pool” can create the erroneous impression that oil fields are immense caverns filled with oil, instead of rock filled with small oil-filled pores. |
| Geology | oil prone | The quality of a source rock that makes it more likely to generate oil than gas. The nature of the organic matter (kerogen) in source rocks varies from coaly, plant-like material commonly found in terrestrial source rocks to algal or other marine material that makes up marine source rocks. Marine source rocks are commonly oil-prone. |
| Geology | oil sand | A porous sand layer or sand body filled with oil. |
| Geology | oil water contact | A bounding surface in a reservoir above which predominantly oil occurs and below which predominantly water occurs. Although oil and water are immiscible, the contact between oil and water is commonly a transition zone and there is usually irreducible water adsorbed by the grains in the rock and immovable oil that cannot be produced. The oil-water contact is not always a flat horizontal surface, but instead might be tilted or irregular. |
| Geology | oilfield | Pertaining to the surface area above a subsurface oil accumulation. |
| Geology | oilfield | Pertaining to an oil field, an accumulation, pool or group of pools of oil in the subsurface. An oil field consists of a reservoir in a shape that will trap hydrocarbons and that is covered by an impermeable or sealing rock. Typically, industry professionals use the term with an implied assumption of economic size. |
| Geology | oil-prone | The quality of a source rock that makes it more likely to generate oil than gas. The nature of the organic matter (kerogen) in source rocks varies from coaly, plant-like material commonly found in terrestrial source rocks to algal or other marine material that makes up marine source rocks. Marine source rocks are commonly oil-prone. |
| Geology | oil-water contact | A bounding surface in a reservoir above which predominantly oil occurs and below which predominantly water occurs. Although oil and water are immiscible, the contact between oil and water is commonly a transition zone and there is usually irreducible water adsorbed by the grains in the rock and immovable oil that cannot be produced. The oil-water contact is not always a flat horizontal surface, but instead might be tilted or irregular. |
| Geology | onlap | The termination of shallowly dipping, younger strata against more steeply dipping, older strata, or the termination of low-angle reflections in seismic data against steeper reflections. Onlap is a particular pattern of reflections in seismic data that, according to principles of sequence stratigraphy, occurs during periods of transgression. |
| Geology | orogenic | Pertaining to a major episode of plate tectonic activity in which lithospheric plates collide and produce mountain belts, in some cases including the formation of subduction zones and igneous activity. Thrust faults and folds are typical geological structures seen in areas of orogeny |
| Geology | orogeny | A major episode of plate tectonic activity in which lithospheric plates collide and produce mountain belts, in some cases including the formation of subduction zones and igneous activity. Thrust faults and folds are typical geological structures seen in areas of orogeny. |
| Geology | outcrop | A body of rock exposed at the surface of the Earth. Construction of highways and other man-made facilities and resultant removal of soil and rock has created spectacular outcrops in some regions. |
| Geology | overburden | Rock overlying an area or point of interest in the subsurface. |
| Geology | overmature | Pertaining to a hydrocarbon source rock that has generated as much hydrocarbon as possible and is becoming thermally altered. |
| Geology | overpressure | Subsurface pressure that is abnormally high, exceeding hydrostatic pressure at a given depth. The term geopressure is commonly, and incorrectly, used synonymously. Abnormally high pore pressure can occur in areas where burial of fluid-filled sediments is so rapid that pore fluids cannot escape, so the pressure of the pore fluids increases as overburden increases. Drilling into overpressured strata can be hazardous because overpressured fluids escape rapidly, so careful preparation is made in areas of known overpressure. |
| Geology | overthrust | A thrust fault having a relatively large lateral displacement. |
| Geology | OWC | Abbreviation for oil-water contact, a bounding surface in a reservoir above which predominantly oil occurs and below which predominantly water occurs. Although oil and water are immiscible, the contact between oil and water is commonly a transition zone and there is usually irreducible water adsorbed by the grains in the rock and immovable oil that cannot be produced. The oil-water contact is not always a flat horizontal surface, but instead might be tilted or irregular. |
| Geology | paleontology | The study of fossilized, or preserved, remnants of plant and animal life. Changes in the Earth through time can be documented by observing changes in the fossils in successive strata and the environments in which they formed or were preserved. Fossils can also be compared with their extant relatives to assess evolutionary changes. Correlations of strata can be aided by studying their fossil content, a discipline called biostratigraphy. |
| Geology | paludal | Pertaining to a depositional environment or organisms from a marsh. It also refers to the type of environment in which palustrine sediments can accumulate. |
| Geology | palustrine | Describing material deposited in or growing in a marsh. |
| Geology | palynology | The study of fossilized remnants of microscopic entities having organic walls, such as pollen, spores and cysts from algae. Changes in the Earth through time can be documented by studying the distribution of spores and pollen. Well log and other correlations are enhanced by incorporating palynology. Palynology also has utility in forensics. |
| Geology | parallel fold | The deformation of rock layers in which the thickness of each layer, measured perpendicular to initial (undeformed) layering, is maintained after the rock layers have been folded. |
| Geology | parasequence | Relatively conformable depositional units bounded by surfaces of marine flooding, surfaces that separate older strata from younger and show an increase in water depth in successively younger strata. Parasequences are usually too thin to discern on seismic data, but when added together, they form sets called parasequence sets that are visible on seismic data. |
| Geology | passive margin | The margin of a continent and ocean that does not coincide with the boundary of a lithospheric plate and along which collision is not occurring. Passive margins are characterized by rifted, rotated fault blocks of thick sediment, such as the present-day Gulf of Mexico and Atlantic margins of North America. |
| Geology | pay | A reservoir or portion of a reservoir that contains economically producible hydrocarbons. The term derives from the fact that it is capable of “paying” an income. Pay is also called pay sand or pay zone. The overall interval in which pay sections occur is the gross pay; the smaller portions of the gross pay that meet local criteria for pay (such as minimum porosity, permeability and hydrocarbon saturation) are net pay. |
| Geology | permeability | The ability, or measurement of a rock’s ability, to transmit fluids, typically measured in darcies or millidarcies. The term was basically defined by Henry Darcy, who showed that the common mathematics of heat transfer could be modified to adequately describe fluid flow in porous media. Formations that transmit fluids readily, such as sandstones, are described as permeable and tend to have many large, well-connected pores. Impermeable formations, such as shales and siltstones, tend to be finer grained or of a mixed grain size, with smaller, fewer, or less interconnected pores. Absolute permeability is the measurement of the permeability conducted when a single fluid, or phase, is present in the rock. Effective permeability is the ability to preferentially flow or transmit a particular fluid through a rock when other immiscible fluids are present in the reservoir (for example, effective permeability of gas in a gas-water reservoir). The relative saturations of the fluids as well as the nature of the reservoir affect the effective permeability. Relative permeability is the ratio of effective permeability of a particular fluid at a particular saturation to absolute permeability of that fluid at total saturation. If a single fluid is present in a rock, its relative permeability is 1.0. Calculation of relative permeability allows for comparison of the different abilities of fluids to flow in the presence of each other, since the presence of more than one fluid generally inhibits flow. |
| Geology | permeable | Pertaining to a rock’s ability to transmit fluids. Permeability is typically measured in darcies or millidarcies. The term was defined by Henry Darcy, who showed that the common mathematics of heat transfer could be modified to adequately describe fluid flow in porous media. Formations that transmit fluids readily, such as sandstones, are described as permeable and tend to have many large, well-connected pores. Impermeable formations, such as shales and siltstones, tend to be finer grained or of a mixed grain size, with smaller, fewer, or less interconnected pores. Absolute permeability is the measurement of the permeability conducted when a single fluid, or phase, is present in the rock. Effective permeability is the ability to preferentially flow or transmit a particular fluid through a rock when other immiscible fluids are present in the reservoir (for example, effective permeability of gas in a gas-water reservoir). The relative saturations of the fluids as well as the nature of the reservoir affect the effective permeability. Relative permeability is the ratio of effective permeability of a particular fluid at a particular saturation to absolute permeability of that fluid at total saturation. If a single fluid is present in a rock, its relative permeability is 1.0. Calculation of relative permeability allows for comparison of the different abilities of fluids to flow in the presence of each other, since the presence of more than one fluid generally inhibits flow. |
| Geology | petrographic | Pertaining to petrography, The examination of rocks in thin section. Rock samples can be glued to a glass slide and the rock ground to 0.03-mm thickness in order to observe mineralogy and texture using a microscope. (A petrographic microscope is a transmitted-light polarizing microscope.) Samples of sedimentary rock can be impregnated with blue epoxy to highlight porosity. |
| Geology | petrography | The examination of rocks in thin section. Rock samples can be glued to a glass slide and the rock ground to 0.03-mm thickness in order to observe mineralogy and texture using a microscope. (A petrographic microscope is a transmitted-light polarizing microscope.) Samples of sedimentary rock can be impregnated with blue epoxy to highlight porosity. |
| Geology | petroleum | A complex mixture of naturally occurring hydrocarbon compounds found in rock. Petroleum can range from solid to gas, but the term is generally used to refer to liquid crude oil. Impurities such as sulfur, oxygen and nitrogen are common in petroleum. There is considerable variation in color, gravity, odor, sulfur content and viscosity in petroleum from different areas. |
| Geology | petroleum system | Geologic components and processes necessary to generate and store hydrocarbons, including a mature source rock, migration pathway, reservoir rock, trap and seal. Appropriate relative timing of formation of these elements and the processes of generation, migration and accumulation are necessary for hydrocarbons to accumulate and be preserved. The components and critical timing relationships of a petroleum system can be displayed in a chart that shows geologic time along the horizontal axis and the petroleum system elements along the vertical axis. Exploration plays and prospects are typically developed in basins or regions in which a complete petroleum system has some likelihood of existing. |
| Geology | petrologic | Pertaining to the macroscopic features of rocks, such as their occurrence, origin and history, structure (usually by examining outcrops in the field), and their texture and composition (by studying smaller samples more closely). |
| Geology | petrology | The study of macroscopic features of rocks, such as their occurrence, origin and history, and structure (usually by examining outcrops in the field) and their texture and composition (by studying smaller samples more closely). |
| Geology | pinch out | To taper to a zero edge. |
| Geology | pinch-out | A reduction in bed thickness resulting from onlapping stratigraphic sequences. |
| Geology | pinch-out | A type of stratigraphic trap. The termination by thinning or tapering out (“pinching out”) of a reservoir against a nonporous sealing rock creates a favorable geometry to trap hydrocarbons, particularly if the adjacent sealing rock is a source rock such as a shale. |
| Geology | plane table | A flat drawing board mounted on a tripod used in combination with an alidade to construct topographic or geologic maps in the field. A sheet of paper or mylar covering the plane table is annotated during map construction. |
| Geology | plankton | Minute organisms that float or drift passively near the surface of oceans and seas. Plant-like plankton, or phytoplankton, include diatoms. Zooplankton are animals that have a limited ability to move themselves. The changes in plankton over time are useful for estimation of relative ages of rocks that contain the fossilized remains of plankton. |
| Geology | plastic | Pertaining to a material that can deform permanently without rupturing. |
| Geology | plastic deformation | Permanent mechanical or physical alteration that does not include rupture. Plastic deformation of rocks typically occurs at high temperatures and pressures, conditions under which rocks become relatively viscous. |
| Geology | plate tectonics | The unifying geologic theory developed to explain observations that interactions of the brittle plates of the lithosphere with each other and with the softer underlying asthenosphere result in large-scale changes in the Earth. The theory of plate tectonics initially stemmed from observations of the shapes of the continents, particularly South America and Africa, which fit together like pieces in a jigsaw puzzle and have similar rocks and fossils despite being separated by a modern ocean. As lithospheric plates heat up or cool down depending on their position, or their tectonic environment, relative to each other and to warmer areas deeper within the Earth, they become relatively more or less dense than the asthenosphere and thus tend to rise as molten magma or sink in cold, brittle slabs or slide past each other. Mountain belts can form during plate collisions or an orogeny; diverging plates or rifts can create new midoceanic ridges; plates that slide past one another create transform fault zones (such as the San Andreas fault); and zones of subduction occur where one lithospheric plate moves beneath another. Plate tectonic theory can explain such phenomena as earthquakes, volcanic or other igneous activity, midoceanic ridges and the relative youth of the oceanic crust, and the formation of sedimentary basins on the basis of their relationships to lithospheric plate boundaries. Convection of the mantle is postulated to be the driving mechanism for the movement of lithospheric plates. Measurements of the continents using the Global Positioning System confirm the relative motions of plates. Age determinations of the oceanic crust confirm that such crust is much younger than that of the continents and has been recycled by the process of subduction and regenerated at midoceanic ridges. |
| Geology | plateau | A topographic feature consisting of a large flat area at a relatively high elevation with steep sides. |
| Geology | platform | A relatively flat, nearly level area of sedimentary rocks in a continent that overlies or abuts the basement rocks of a craton. |
| Geology | plunge | The angle between a linear feature and a horizontal line in a vertical plane containing both lines. |
| Geology | point bar | An arcuate deposit of sediment, usually sand, that occurs along the convex inner edges of the meanders of channels and builds outward as the stream channel migrates. |
| Geology | pore | A discrete void within a rock, which can contain air, water, hydrocarbons or other fluids. In a body of rock, the percentage of pore space is the porosity. |
| Geology | pore pressure | The pressure of fluids within the pores of a reservoir, usually hydrostatic pressure, or the pressure exerted by a column of water from the formation’s depth to sea level. When impermeable rocks such as shales form as sediments are compacted, their pore fluids cannot always escape and must then support the total overlying rock column, leading to anomalously high formation pressures. Because reservoir pressure changes as fluids are produced from a reservoir, the pressure should be described as measured at a specific time, such as initial reservoir pressure. |
| Geology | pore pressure gradient | The change in pore pressure per unit of depth, typically in units of psi/ft or kPa/m. Pressure increases predictably with depth in areas of normal pressure. The normal hydrostatic pressure gradient for freshwater is 0.433 psi/ft, or 9.792 kPa/m, and 0.465 psi/ft for water with 100,000 ppm total dissolved solids (a typical Gulf Coast water), or 10.516 kPa/m. Deviations from normal pressure are described as high or low pressure. |
| Geology | pore-pressure gradient | The change in pore pressure per unit of depth, typically in units of psi/ft or kPa/m. Pressure increases predictably with depth in areas of normal pressure. The normal hydrostatic pressure gradient for freshwater is 0.433 psi/ft, or 9.792 kPa/m, and 0.465 psi/ft for water with 100,000 ppm total dissolved solids (a typical Gulf Coast water), or 10.516 kPa/m. Deviations from normal pressure are described as high or low pressure. |
| Geology | porosity | The percentage of pore volume or void space, or that volume within rock that can contain fluids. Porosity can be a relic of deposition (primary porosity, such as space between grains that were not compacted together completely) or can develop through alteration of the rock (secondary porosity, such as when feldspar grains or fossils are preferentially dissolved from sandstones). Porosity can be generated by the development of fractures, in which case it is called fracture porosity. Effective porosity is the interconnected pore volume in a rock that contributes to fluid flow in a reservoir. It excludes isolated pores. Total porosity is the total void space in the rock whether or not it contributes to fluid flow. Thus, effective porosity is typically less than total porosity. Shale gas reservoirs tend to have relatively high porosity, but the alignment of platy grains such as clays makes their permeability very low. |
| Geology | porous | Pertaining to rocks that incorporate pores or void spaces, which can contain air, water, hydrocarbons or other fluids. In a body of rock, the percentage of pore space is the porosity. Porosity can be a relic of deposition (primary porosity, such as space between grains that were not compacted together completely) or can develop through alteration of the rock (secondary porosity, such as when feldspar grains or fossils are preferentially dissolved from sandstones). Porosity can be generated by the development of fractures, in which case it is called fracture porosity. Effective porosity is the interconnected pore volume in a rock that contributes to fluid flow in a reservoir. It excludes isolated pores. Total porosity is the total void space in the rock whether or not it contributes to fluid flow. Thus, effective porosity is typically less than total porosity. Shale gas reservoirs tend to have relatively high porosity, but the alignment of platy grains such as clays makes their permeability very low. |
| Geology | post mature | Pertaining to a hydrocarbon source rock that has generated as much hydrocarbon as possible and is becoming thermally altered. |
| Geology | post-mature | Pertaining to a hydrocarbon source rock that has generated as much hydrocarbon as possible and is becoming thermally altered. |
| Geology | preservation | The phase of a petroleum system after hydrocarbons accumulate in a trap and are subject to degradation, remigration, tectonism or other unfavorable or destructive processes. |
| Geology | pressure gradient | The change in pressure per unit of depth, typically in units of psi/ft or kPa/m. Pressure increases predictably with depth in areas of normal pressure. The normal hydrostatic pressure gradient for freshwater is 0.433 psi/ft, or 9.792 kPa/m, and 0.465 psi/ft for water with 100,000 ppm total dissolved solids (a typical Gulf Coast water), or 10.516 kPa/m. Deviations from normal pressure are described as high or low pressure. |
| Geology | primary migration | The expulsion of newly generated hydrocarbons from a source rock. The further movement of the hydrocarbons into reservoir rock in a hydrocarbon trap or other area of accumulation is secondary migration. |
| Geology | primary porosity | The porosity preserved from deposition through lithification. |
| Geology | production | The phase that occurs after successful exploration and development and during which hydrocarbons are drained from an oil or gas field. |
| Geology | progradation | The accumulation of sequences by deposition in which beds are deposited successively basinward because sediment supply exceeds accommodation. Thus, the position of the shoreline migrates into the basin during episodes of progradation, a process called regression. |
| Geology | prospect | An area of exploration in which hydrocarbons have been predicted to exist in economic quantity. A prospect is commonly an anomaly, such as a geologic structure or a seismic amplitude anomaly, that is recommended by explorationists for drilling a well. Justification for drilling a prospect is made by assembling evidence for an active petroleum system, or reasonable probability of encountering reservoir-quality rock, a trap of sufficient size, adequate sealing rock, and appropriate conditions for generation and migration of hydrocarbons to fill the trap. A single drilling location is also called a prospect, but the term is more properly used in the context of exploration. A group of prospects of a similar nature constitutes a play. |
| Geology | quartz | [SiO2] An abundant rock-forming mineral composed of silicon and oxygen, also called silica. Quartz sand grains are a major constituent of sandstone and other clastic sedimentary rocks. |
| Geology | radial faulting | Multiple faults whose fault planes strike outward from a common center. Such faults typically are associated with salt domes, impact craters or volcanoes. |
| Geology | reef | A mound, ridge, or buildup of sediment or sedimentary rock, most commonly produced by organisms that secrete shells such as corals. Reefs are typically taller than the sediment that surrounds them, resistant to weathering and wave action, and preserved within sediment of a different composition. Carbonate reefs form in a limited range of temperatures, water depths, salinities and wave activities, so their occurrence can be used to interpret past environmental conditions. Because the rocks that surround reefs can differ in composition and permeability, porous reefs can form stratigraphic traps for hydrocarbons. Porosity of reefal limestones depends on post-depositional diagenetic changes. |
| Geology | reefal | Pertaining to reef, a mound, ridge, or buildup of sediment or sedimentary rock, most commonly produced by organisms that secrete shells such as corals. Reefs are typically taller than the sediment that surrounds them, resistant to weathering and wave action, and preserved within sediment of a different composition. Carbonate reefs form in a limited range of temperatures, water depths, salinities and wave activities, so their occurrence can be used to interpret past environmental conditions. Because the rocks that surround reefs can differ in composition and permeability, porous reefs can form stratigraphic traps for hydrocarbons. Porosity of reefal limestones depends on post-depositional diagenetic changes. |
| Geology | regression | The migration of shoreline into a basin during progradation due to a fall in relative sea level. Deposition during a regression can juxtapose shallow-water sediments atop deep-water sediments. |
| Geology | regressive | Pertaining to regression, the migration of shoreline into a basin during progradation due to a fall in relative sea level. Deposition during a regression can juxtapose shallow-water sediments atop deep-water sediments. |
| Geology | relative age | The approximate age determination of rocks, fossils or minerals made by comparing whether the material is younger or older than other surrounding material. Relative age is estimated according to stratigraphic and structural relationships, such as superposition, and by fossil content, since the relative ages and successions of fossils have been established by paleontologists. The measurement of the decay of radioactive isotopes, especially uranium, rubidium, argon and carbon, has allowed geologists to more precisely determine the age in years of rock formations, known as the absolute age. Tree rings and seasonal sedimentary deposits called varves can be counted to determine absolute age. Although the term implies otherwise, “absolute” ages typically have some amount of potential error and are inexact. |
| Geology | relative permeability | A dimensionless term devised to adapt the Darcy equation to multiphase flow conditions. Relative permeability is the ratio of effective permeability of a particular fluid at a particular saturation to absolute permeability of that fluid at total saturation. If a single fluid is present in a rock, its relative permeability is 1.0. Calculation of relative permeability allows comparison of the different abilities of fluids to flow in the presence of each other, since the presence of more than one fluid generally inhibits flow. |
| Geology | reservoir | A subsurface body of rock having sufficient porosity and permeability to store and transmit fluids. Sedimentary rocks are the most common reservoir rocks because they have more porosity than most igneous and metamorphic rocks and form under temperature conditions at which hydrocarbons can be preserved. A reservoir is a critical component of a complete petroleum system. |
| Geology | reservoir pressure | The pressure of fluids within the pores of a reservoir, usually hydrostatic pressure, or the pressure exerted by a column of water from the formation’s depth to sea level. When impermeable rocks such as shales form as sediments are compacted, their pore fluids cannot always escape and must then support the total overlying rock column, leading to anomalously high formation pressures. Because reservoir pressure changes as fluids are produced from a reservoir, the pressure should be described as measured at a specific time, such as initial reservoir pressure. |
| Geology | retrogradation | The accumulation of sequences by deposition in which beds are deposited successively landward because sediment supply is limited and cannot fill the available accommodation. Thus, the position of the shoreline migrates backward onto land, a process called transgression, during episodes of retrogradation. |
| Geology | reverse fault | A type of fault formed when the hanging wall fault block moves up along a fault surface relative to the footwall. Such movement can occur in areas where the Earth’s crust is compressed. A thrust fault, sometimes called an overthrust if the displacement is particularly great, is a reverse fault in which the fault plane has a shallow dip, typically much less than 45o. |
| Geology | rheologic | Pertaining to rheology, generally, the study of how matter deforms and flows, including its elasticity, plasticity and viscosity. In geology, rheology is particularly important in studies of moving ice, water, salt and magma, as well as in studies of deforming rocks. |
| Geology | rheological | Pertaining to rheology, generally, the study of how matter deforms and flows, including its elasticity, plasticity and viscosity. In geology, rheology is particularly important in studies of moving ice, water, salt and magma, as well as in studies of deforming rocks. |
| Geology | rheology | Generally, the study of how matter deforms and flows, including its elasticity, plasticity and viscosity. In geology, rheology is particularly important in studies of moving ice, water, salt and magma, as well as in studies of deforming rocks. |
| Geology | rhombohedral packing | The most compact arrangement in space of uniform spheres (atoms and molecules in mineral crystals, or grains in sedimentary rocks) that results in a structure having no more than 26% porosity. Rhombohedral packing is more stable mechanically than cubic packing. Cubic packing is the most porous packing arrangement, with about 47% porosity in the ideal situation. Most sediments, however, are not uniform spheres of the same size, nor can they be arranged in a cubic structure naturally, so most sediments have much less than 47% porosity of ideal cubic packing and commonly less than the 26% porosity of ideal rhombohedral packing. |
| Geology | rift | Region in which the Earth’s crust is pulling apart and creating normal faults and down-dropped areas or subsidence. |
| Geology | rock | An aggregate of minerals or organic matter (in the case of coal, which is not composed of minerals because of its organic origin), or volcanic glass (obsidian, which forms a rock but is not considered a mineral because of its amorphous, noncrystalline nature). Rocks can contain a single mineral, such as rock salt (halite) and certain limestones (calcite), or many minerals, such as granite (quartz, feldspar, mica and other minerals). There are three main types of rocks. Sedimentary rocks like sandstone and limestone form at the Earth’s surface through deposition of sediments derived from weathered rocks, biogenic activity or precipitation from solution. Igneous rocks originate deeper within the Earth, where the temperature is high enough to melt rocks, to form magma that can crystallize within the Earth or at the surface by volcanic activity. Metamorphic rocks form from other preexisting rocks during episodes of deformation of the Earth at temperatures and pressures high enough to alter minerals but inadequate to melt them. Such changes can occur by the activity of fluids in the Earth and movement of igneous bodies or regional tectonic activity. Rocks are recycled from one type to another by the constant changes in the Earth. |
| Geology | s.g. | The dimensionless ratio of the density of a material to that of the same volume of water. Most common minerals have specific gravities between 2 and 7. |
| Geology | sabkha | An environment of coastal sedimentation characterized by arid or semiarid conditions above the level of high tide and by the absence of vegetation. Evaporites, eolian deposits and tidal-flood deposits are common in sabkhas. |
| Geology | salt | [NaCl] A soft, soluble evaporite mineral also known as halite or rock salt. Because salt is less dense than many sedimentary rocks, it is relatively buoyant and can form salt domes, pillars or curtains by flowing and breaking through or piercing overlying sediments, as seen in the Gulf of Mexico and the Zagros fold belt. Halite can be critical in forming hydrocarbon traps and seals because it tends to flow rather than fracture during deformation, thus preventing hydrocarbons from leaking out of a trap even during and after some types of deformation. |
| Geology | salt dome | A mushroom-shaped or plug-shaped diapir made of salt, commonly having an overlying cap rock. Salt domes form as a consequence of the relative buoyancy of salt when buried beneath other types of sediment. The salt flows upward to form salt domes, sheets, pillars and other structures. Hydrocarbons are commonly found around salt domes because of the abundance and variety of traps created by salt movement and the association with evaporite minerals that can provide excellent sealing capabilities. |
| Geology | sand | A detrital grain between 0.0625 mm and 2 mm in diameter. Sand is larger than silt but smaller than a granule according to the Udden-Wentworth scale. Sand is also a term used for quartz grains or for sandstone. |
| Geology | sandstone | A clastic sedimentary rock whose grains are predominantly sand-sized. The term is commonly used to imply consolidated sand or a rock made of predominantly quartz sand, although sandstones often contain feldspar, rock fragments, mica and numerous additional mineral grains held together with silica or another type of cement. The relatively high porosity and permeability of sandstones make them good reservoir rocks. |
| Geology | saturation | The relative amount of water, oil and gas in the pores of a rock, usually as a percentage of volume. |
| Geology | scout | A petroleum industry worker who tracks competitive exploration and production activity, either for a company or on a free-lance basis. Scouts can facilitate trading of technical data such as well logs among companies before such data enter the public domain unless the operations or data are held “tight.” |
| Geology | scout | To inspect an area or to monitor activity. |
| Geology | scout ticket | A brief report about a well from the time it is permitted through drilling and completion. A scout ticket typically includes the location, total depth, logs run, production status and formation tops. |
| Geology | seal | A relatively impermeable rock, commonly shale, anhydrite or salt, that forms a barrier or cap above and around reservoir rock such that fluids cannot migrate beyond the reservoir. A seal is a critical component of a complete petroleum system. The permeability of a seal capable of retaining fluids through geologic time is ~ 10-6 to 10-8 darcies. |
| Geology | secondary migration | The movement of generated hydrocarbons into a reservoir after their expulsion, or primary migration, from a source rock. |
| Geology | secondary porosity | The porosity created through alteration of rock, commonly by processes such as dolomitization, dissolution and fracturing. |
| Geology | sediment | The unconsolidated grains of minerals, organic matter or preexisting rocks, that can be transported by water, ice or wind, and deposited. The processes by which sediment forms and is transported occur at or near the surface of the Earth and at relatively low pressures and temperatures. Sedimentary rocks form from the accumulation and lithification of sediment. Sediments are classified according to size by the Udden-Wentworth scale. |
| Geology | sedimentary | One of the three main classes of rock (igneous, metamorphic and sedimentary). Sedimentary rocks are formed at the Earth’s surface through deposition of sediments derived from weathered rocks, biogenic activity or precipitation from solution. Clastic sedimentary rocks such as conglomerates, sandstones, siltstones and shales form as older rocks weather and erode, and their particles accumulate and lithify, or harden, as they are compacted and cemented. Biogenic sedimentary rocks form as a result of activity by organisms, including coral reefs that become limestone. Precipitates, such as the evaporite minerals halite (salt) and gypsum can form vast thicknesses of rock as seawater evaporates. Sedimentary rocks can include a wide variety of minerals, but quartz, feldspar, calcite, dolomite and evaporite group and clay group minerals are most common because of their greater stability at the Earth’s surface than many minerals that comprise igneous and metamorphic rocks. Sedimentary rocks, unlike most igneous and metamorphic rocks, can contain fossils because they form at temperatures and pressures that do not obliterate fossil remnants. |
| Geology | sedimentary basin | A depression in the crust of the Earth formed by plate tectonic activity in which sediments accumulate. Continued deposition can cause further depression or subsidence. Sedimentary basins, or simply basins, vary from bowl-shaped to elongated troughs. If rich hydrocarbon source rocks occur in combination with appropriate depth and duration of burial, hydrocarbon generation can occur within the basin. |
| Geology | sedimentation | The process of creation, transportation and deposition of sediments. |
| Geology | seep | A naturally occurring, typically slow leakage of fluid—water, oil or gas—at the Earth’s surface. A seep results from migration of the fluid from its source or reservoir formation because the formation pressure exceeds the formation’s seal capacity such as during rapid loading of the overburden by sedimentation or during fluid expansion or from damage to the seal such as by faulting or tectonism. |
| Geology | seismite | An injectite attributable to earthquake or seismic shaking. |
| Geology | sequence | A group of relatively conformable strata that represents a cycle of deposition and is bounded by unconformities or correlative conformities. Sequences are the fundamental unit of interpretation in sequence stratigraphy. Sequences comprise systems tracts. |
| Geology | sequence boundary | A surface that separates older sequences from younger ones, commonly an unconformity (indicating subaerial exposure), but in limited cases a correlative conformable surface. A sequence boundary is an erosional surface that separates cycles of deposition. |
| Geology | sequence stratigraphy | A field of study in which basin-filling sedimentary deposits, called sequences, are interpreted in a framework of eustasy, sedimentation and subsidence through time in order to correlate strata and predict the stratigraphy of relatively unknown areas. Sequences tend to show cyclicity of changes in relative sea level and widespread unconformities, processes of sedimentation and sources of sediments, climate and tectonic activity over time. Sequence stratigraphic study promotes thorough understanding of the evolution of basins, but also allows for interpretations of potential source rocks and reservoir rocks in both frontier areas (having seismic data but little well data) and in more mature hydrocarbon provinces. Prediction of reservoir continuity is currently a key question in mature hydrocarbon provinces where sequence stratigraphy is being applied. The field originated during the 1960s with the study of the stratigraphy of the continental USA, where numerous unconformities could be correlated widely, and led to the proposal that major unconformities might mark synchronous global-scale events. Through sequence stratigraphy, widely-separated sediments that occur between correlatable unconformities could be compared with each other. Studies of outcrops and seismic lines bore out these concepts, which initially were called Seismic Stratigraphy and first published widely in 1977. Further study of seismic lines led to the interpretation of the geometry or architecture of seismic events as representing particular styles of sedimentation and depositional environments, and the integration of such interpretations with well log and core data. Because of the simultaneous, competitive nature of the research, numerous oil companies and academic groups use the terminology of sequence stratigraphy differently, and new terms are added continually. |
| Geology | shale oil | Oil obtained by artificial maturation of oil shale. The process of artificial maturation uses controlled heating, or pyrolysis, of kerogen to release the shale oil. |
| Geology | shear strain | The amount of deformation by shearing, in which parallel lines slide past each other in differing amounts. The measurement is expressed as the tangent of the change in angle between lines that were initially perpendicular. |
| Geology | shelf | Continental shelf, or the area at the edges of a continent from the shoreline to a depth of 200 m [660 ft], where the continental slope begins. The shelf is commonly a wide, flat area with a slight seaward slope. The term is sometimes used as a for platform. |
| Geology | silica | A chemically resistant dioxide of silicon that occurs in crystalline (quartz), amorphous (opal) and cryptocrystalline (chert) forms. |
| Geology | silicate mineral | A group of rock-forming minerals in which SiO4 tetrahedra combine with cations. Silicate minerals are the most abundant type of mineral. Olivine, pyroxene, amphibole, mica, quartz and feldspar are types of silicate minerals. |
| Geology | siliciclastic sediment | Silica-based, noncarbonaceous sediments that are broken from preexisting rocks, transported elsewhere, and redeposited before forming another rock. Examples of common siliciclastic sedimentary rocks include conglomerate, sandstone, siltstone and shale. Carbonate rocks can also be broken and reworked to form other types of clastic sedimentary rocks. |
| Geology | similar fold | A type of fold in which the thickness of the layers remains constant when measured parallel to the axial surface and the layers have the same wave shape, but the thickness along each layer varies. The folded layers tend to be thicker in the hinge of the fold and thinner along the limbs of the fold. |
| Geology | sinistral | Pertaining to a strike-slip or left-lateral fault in which the block across the fault moves to the left; also called a sinistral strike-slip fault. If it moves to the right, the relative motion is described as dextral. Counterclockwise rotation or spiraling is also described as sinistral. |
| Geology | slip | Relative displacement of two formerly adjacent points that have been separated by faulting. Slip is used to describe motion along a fault with respect to the distance and direction that one side of the fault has moved relative to the other. Slip is a vector, expressed in terms of distance and direction. |
| Geology | smectite | A group of clay minerals that includes montmorillonite. This type of mineral tends to swell when exposed to water. Bentonite includes minerals of the smectite group. |
| Geology | soft rock | A general term for sedimentary rocks, although it can imply a distinction between rocks of interest to the petroleum industry and rocks of interest to the mining industry. |
| Geology | sorting | The range of sedimentary grain sizes that occurs in sediment or sedimentary rock. The term also refers to the process by which sediments of similar size are naturally segregated during transport and deposition according to the velocity and transporting medium. Well-sorted sediments are of similar size (such as desert sand), while poorly-sorted sediments have a wide range of grain sizes (as in a glacial till). A well-sorted sandstone tends to have greater porosity than a poorly sorted sandstone because of the lack of grains small enough to fill its pores. Conglomerates tend to be poorly sorted rocks, with particles ranging from boulder size to clay size. |
| Geology | sour | Contaminated with sulfur or sulfur compounds, especially hydrogen sulfide. Crude oil and gas that are sour typically have an odor of rotten eggs if the concentration of sulfur is low. At high concentrations, sulfur is odorless and deadly. |
| Geology | specific gravity | The dimensionless ratio of the density of a material to that of the same volume of water. Most common minerals have specific gravities between 2 and 7. |
| Geology | spill point | The structurally lowest point in a hydrocarbon trap that can retain hydrocarbons. Once a trap has been filled to its spill point, further storage or retention of hydrocarbons will not occur for lack of reservoir space within that trap. The hydrocarbons spill or leak out, and they continue to migrate until they are trapped elsewhere. |
| Geology | strain | The permanent deformation evident in rocks and other solid bodies that have experienced a sufficiently high applied stress. A change in shape, such as folding, faulting, fracturing, or change, generally a reduction, in volume are common examples of strain seen in rocks. Strain can be described in terms of normal and shear components, and is the ratio of the change in length or volume to the initial length or volume. For more on strain: Means WD: Stress and Strain. New York, New York, Springer-Verlag, 1976. |
| Geology | strata | Layers of sedimentary rock. The singular form is stratum. |
| Geology | stratigraphic trap | A variety of sealed geologic container capable of retaining hydrocarbons, formed by changes in rock type or pinch-outs, unconformities, or sedimentary features such as reefs. Structural traps, in contrast, consist of geologic structures in deformed strata such as faults and folds whose geometries permit retention of hydrocarbons. |
| Geology | stratum | A layer of sedimentary rock. The plural form is strata. |
| Geology | stress | The force applied to a body that can result in deformation, or strain, usually described in terms of magnitude per unit of area, or intensity. |
| Geology | strike | The azimuth of the intersection of a plane, such as a dipping bed, with a horizontal surface. |
| Geology | strike-slip fault | A type of fault whose surface is typically vertical or nearly so. The motion along a strike-slip fault is parallel to the strike of the fault surface, and the fault blocks move sideways past each other. A strike-slip fault in which the block across the fault moves to the right is described as a dextral strike-slip fault. If it moves left, the relative motion is described as sinistral. Local deformation near bends in strike-slip faults can produce pull-apart basins and grabens. Flower structures are another by-product of strike-slip faults. A wrench fault is a type of strike-slip fault in which the fault surface is nearly vertical. |
| Geology | structural | Pertaining to structure, the geometry and spatial arrangement of rocks. The structure or deformation can include many mechanisms, such as folding, faulting and fracturing. Structure can usually be interpreted in terms of the deformation of the crust of the Earth as continents and tectonic plates move and collide. |
| Geology | structural trap | A variety of sealed geologic structure capable of retaining hydrocarbons, such as a fault or a fold. Stratigraphic traps form where changes in rock type can retain hydrocarbons. |
| Geology | structure | A geological feature produced by deformation of the Earth’s crust, such as a fold or a fault; a feature within a rock, such as a fracture or bedding surface; or, more generally, the spatial arrangement of rocks. |
| Geology | structure map | A type of subsurface map whose contours represent the elevation of a particular formation, reservoir or geologic marker in space, such that folds, faults and other geologic structures are clearly displayed. Its appearance is similar to that of a topographic map, but a topographic map displays elevations of the Earth’s surface and a structure map displays the elevation of a particular rock layer, generally beneath the surface. |
| Geology | stylolite | Wave-like or tooth-like, serrated, interlocking surfaces most commonly seen in carbonate and quartz-rich rocks that contain concentrated insoluble residue such as clay minerals and iron oxides. Stylolites are thought to form by pressure solution, a dissolution process that reduces pore space under pressure during diagenesis. |
| Geology | subduction | A plate tectonic process in which one lithospheric plate descends beneath another into the asthenosphere during a collision at a convergent plate margin. Because of the relatively higher density of oceanic lithosphere, it will typically descend beneath the lighter continental lithosphere during a collision. In a collision of plates of continental lithosphere, the density of the two plates is so similar that neither tends to be subducted and mountains form. As a subducted plate descends into the asthenosphere, Earthquakes can occur, especially in the Wadati-Benioff zone, but, if the plate descends deeply into the mantle, it will eventually be heated to the point of melting. Volcanoes can form above a descending plate. |
| Geology | subsalt | An exploration and production play type in which prospects exist below salt layers. Until relatively recently, many explorationists did not seek prospects below salt because seismic data had been of poor quality below salt (i.e., it was not possible to map traps accurately) or because they believed that reservoir-quality rock or hydrocarbons did not exist below salt layers. Advances in seismic processing and compelling drilling results from exploration wells encouraged companies to generate and drill prospects below salt layers, salt sheets and other previously disregarded potential traps. The offshore Gulf of Mexico contains numerous subsalt-producing fields, and similar areas are being explored internationally. |
| Geology | subsidence | The relative sinking of the Earth’s surface. Plate tectonic activity (particularly extension of the crust, which promotes thinning and sinking), sediment loading and removal of fluid from reservoirs are processes by which the crust can be depressed. Subsidence can produce areas in which sediments accumulate and, ultimately, form sedimentary basins |
| Geology | superposition | The stratigraphic principle that, in the case of undeformed, flat-lying strata, younger layers are deposited atop older ones, such that the top layer is youngest and underlying layers increase in age with depth. Nicolaus Steno articulated the law of superposition of strata in the 17th century. |
| Geology | swamp | A wetland depositional environment in which water is present either permanently or intermittently and in which trees and large woody plants can grow but peat does not form. Swamps can contain considerable quantities of organic matter. |
| Geology | sweet | Pertaining to crude oil or natural gas lacking appreciable amounts of sulfur or sulfur compounds. |
| Geology | syncline | Basin- or trough-shaped fold in rock in which rock layers are downwardly convex. The youngest rock layers form the core of the fold and outward from the core progressively older rocks occur. Synclines typically do not trap hydrocarbons because fluids tend to leak up the limbs of the fold. An anticline is the opposite type of fold, having upwardly-convex layers with old rocks in the core. |
| Geology | synthetic fault | A type of minor fault whose sense of displacement is similar to its associated major fault. Antithetic-synthetic fault sets are typical in areas of normal faulting. |
| Geology | systems tract | Subdivisions of sequences that consist of discrete depositional units that differ in geometry from other systems tracts and have distinct boundaries on seismic data. Different systems tracts are considered to represent different phases of eustatic changes. A lowstand systems tract develops during times of relatively low sea level; a highstand systems tract at times of high sea level; and a transgressive systems tract at times of changing sea level. |
| Geology | tadpole | A symbol plotted on a dipmeter log used to indicate dip angle and direction. The symbol, named for its resemblance to a tadpole (a toad in its larval stage of development) consists of a circle with a short line (or tail) extending from it. A solid black dot shows good data, while an open circle indicates a less reliable measurement. The circular part of the tadpole is plotted against depth on the y-axis and dip magnitude is on the x-axis. The tail extends in the azimuthal direction of dip; the azimuth measurement is displayed with north or 0° being up, east or 90° at right, south or 180°is down and west or 270° is left. |
| Geology | tectonic environment | Location relative to the boundary of a tectonic plate, particularly a boundary along which plate tectonic activity is occurring or has occurred. |
| Geology | tectonics | Also known as plate tectonics, the unifying geologic theory developed to explain observations that interactions of the brittle plates of the lithosphere with each other and with the softer underlying asthenosphere result in large-scale changes in the Earth. The theory of plate tectonics initially stemmed from observations of the shapes of the continents, particularly South America and Africa, which fit together like pieces in a jigsaw puzzle and have similar rocks and fossils despite being separated by a modern ocean. As lithospheric plates heat up or cool down depending on their position, or their tectonic environment, relative to each other and to warmer areas deeper within the Earth, they become relatively more or less dense than the asthenosphere and thus tend to rise as molten magma or sink in cold, brittle slabs or slide past each other. Mountain belts can form during plate collisions or an orogeny; diverging plates or rifts can create new midoceanic ridges; plates that slide past one another create transform fault zones (such as the San Andreas fault); and zones of subduction occur where one lithospheric plate moves beneath another. Plate tectonic theory can explain such phenomena as earthquakes, volcanic or other igneous activity, midoceanic ridges and the relative youth of the oceanic crust, and the formation of sedimentary basins on the basis of their relationships to lithospheric plate boundaries. Convection of the mantle is postulated to be the driving mechanism for the movement of lithospheric plates. Measurements of the continents using the Global Positioning System confirm the relative motions of plates. Age determinations of the oceanic crust confirm that such crust is much younger than that of the continents and has been recycled by the process of subduction and regenerated at midoceanic ridges. |
| Geology | tectonism | Plate tectonic activity. |
| Geology | terrestrial | Pertaining to sediments or depositional environments on land or above the level of high tide. |
| Geology | thrust fault | A type of reverse fault in which the fault plane has a very shallow dip, typically much less than 45o. The hanging wall fault block moves up the fault surface relative to the footwall. In cases of considerable lateral movement, the fault is described as an overthrust fault. Thrust faults can occur in areas of compression of the Earth’s crust. |
| Geology | tight | Secrecy or confidentiality of information. Operators typically try to prevent disclosure of results from exploration wells and will hold any such information “tight”. A tight hole is a well whose status and data are not widely disseminated by the operator. |
| Geology | tight oil | Oil found in relatively impermeable reservoir rock. Production of tight oil comes from very low permeability rock that must be stimulated using hydraulic fracturing to create sufficient permeability to allow the mature oil and/or natural gas liquids to flow at economic rates. |
| Geology | topographic map | A contour map that displays the elevation of the Earth’s surface. A topographic map is commonly used as the base map for surface geological mapping. |
| Geology | transform fault | A particular type of strike-slip fault that is a boundary of an oceanic tectonic plate. The actual movement of a transform fault is opposite to its apparent displacement because of the interplay of spreading and faulting between tectonic plates. |
| Geology | transgression | The migration of shoreline out of a basin and onto land during retrogradation. A transgression can result in sediments characteristic of shallow water being overlain by deeper water sediments. |
| Geology | transgressive | Pertaining to transgression, the migration of shoreline out of a basin and onto land during retrogradation. A transgression can result in sediments characteristic of shallow water being overlain by deeper water sediments. |
| Geology | transpression | The simultaneous occurrence of strike-slip faulting and compression, or convergence, of the Earth’s crust. In areas of transpression, rocks can be faulted upward to form a positive flower structure. Areas of strike-slip faulting in rifting or diverging crust are experiencing transtension, in which rocks can drop down to form a negative flower structure. |
| Geology | transtension | The simultaneous occurrence of strike-slip faulting and extension, rifting, or divergence of the Earth’s crust. In areas of transtension, rocks can be faulted downward to form a negative flower structure. Areas of strike-slip faulting in converging crust are experiencing transpression, in which rocks can be faulted upwards to form a positive flower structure. |
| Geology | trap | A configuration of rocks suitable for containing hydrocarbons and sealed by a relatively impermeable formation through which hydrocarbons will not migrate. Traps are described as structural traps (in deformed strata such as folds and faults) or stratigraphic traps (in areas where rock types change, such as unconformities, pinch-outs and reefs). A trap is an essential component of a petroleum system. |
| Geology | trend | Used synonymously with the term play to describe an area in which hydrocarbons occur, such as the Wilcox trend of the Gulf Coast |
| Geology | trend | The azimuth or orientation of a linear feature, such as the axis of a fold, normally expressed as a compass bearing. |
| Geology | true dip | The maximum angle that a bedding plane, fault plane or other geological surface declines away from a horizontal plane measured in a vertical plane that is perpendicular to the strike of the structure. Any measurement obtained at an angle not perpendicular to strike will yield an apparent dip, which is always less than the maximum angle of declination that is obtained when measuring perpendicular to formation strike. |
| Geology | true stratigraphic thickness | The thickness of a bed or rock body after correcting for the dip of the bed or body and the deviation of the well that penetrates it. The values of true stratigraphic thickness in an area can be plotted and contours drawn to create an isopach map. |
| Geology | true vertical thickness | The thickness of a bed or rock body measured vertically at a point. The values of true vertical thickness in an area can be plotted and contours drawn to create an isochore map. |
| Geology | tuff | Lithified volcanic ash. |
| Geology | tuffaceaous | Containing tuff, which is lithified volcanic ash. |
| Geology | turbidite | Sedimentary deposits formed by turbidity currents in deep water at the base of the continental slope and on the abyssal plain. Turbidites commonly show predictable changes in bedding from coarse layers at the bottom to finer laminations at the top, known as Bouma sequences, that result from different settling velocities of the particle sizes present. The high energy associated with turbidite deposition can result in destruction of earlier deposited layers by subsequent turbidity currents. |
| Geology | turbidity current | An influx of rapidly moving, sediment-laden water down a slope into a larger body of water; also called a density current because the suspended sediment results in the current having a higher density than the clearer water into which it flows. Such currents can occur in lakes and oceans, in some cases as by-products of earthquakes or mass movements such as slumps. The sedimentary deposits that form as the current loses energy are called turbidites and can be preserved as Bouma sequences. Turbidity currents are characteristic of trench slopes of convergent plate margins and continental slopes of passive margins. |
| Geology | Udden-Wentworth scale | A grade scale for classifying the diameters of sediments. Particles larger than 64 mm in diameter are classified as cobbles. Smaller particles are pebbles, granules, sand and silt. Those smaller than 0.0039 mm are clay. Several other grain size scales are in use, but the Udden-Wentworth scale (commonly called the Wentworth scale) is the one that is most frequently used in geology. |
| Geology | unconformity | A geological surface separating older from younger rocks and representing a gap in the geologic record. Such a surface might result from a hiatus in deposition of sediments, possibly in combination with erosion, or deformation such as faulting. An angular unconformity separates younger strata from eroded, dipping older strata. A disconformity represents a time of nondeposition, possibly combined with erosion, and can be difficult to distinguish within a series of parallel strata. A nonconformity separates overlying strata from eroded, older igneous or metamorphic rocks. The study and interpretation of unconformities locally, regionally and globally is the basis of sequence stratigraphy. |
| Geology | unconformity trap | A type of hydrocarbon trap whose closure is controlled by the presence of an unconformity. There is disagreement about whether unconformity traps are structural or stratigraphic traps. |
| Geology | underpressure | Pore pressure less than normal or hydrostatic pressure. Underpressure, or a zone of underpressure, is common in areas or formations that have had hydrocarbon production. |
| Geology | underpressured | Referring to pore pressure less than normal or hydrostatic pressure. Underpressure, or a zone of underpressure, is common in areas or formations that have had hydrocarbon production. |
| Geology | uniformitarianism | The geological principle formulated by James Hutton in 1795 and publicized by Charles Lyell in 1830 that geological processes occurring today have occurred similarly in the past, often articulated as, “The present is the key to the past. |
| Geology | Universal Transverse Mercator grid (UTM) | A worldwide grid system of rectangular map coordinates that uses metric (SI) units. A location is specified on the basis of its location within one of 60 zones worldwide of 6o of longitude and 8o of latitude each that are subdivided into subzones that are 100,000 m [330,000 ft] on each side. Locations consist of a series of numbers and letters that can be accurate to within an area of one square meter. The headquarters of the Geological Society of America are at 13TDQ8743172 (Merrill, 1986). Information about the UTM grid, including grid ticks on quadrangle maps, can be found on most maps produced by the US Geological Survey. Latitude and longitude coordinates, or geographic coordinates, are another means of locating a point at the Earth’s surface, but the accuracy, computer compatibility and uniqueness of UTM have resulted in its finding acceptance within the scientific community. |
| Geology | updip | Located up the slope of a dipping plane or surface. In a dipping (not flat-lying) hydrocarbon reservoir that contains gas, oil and water, the gas is updip, the gas-oil contact is downdip from the gas, and the oil-water contact is still farther downdip. |
| Geology | varve | A rhythmic sequence of sediments deposited in annual cycles in glacial lakes. Light-colored, coarse summer grains are deposited by rapid melting of the glacier. The summer layers grade upward to layers of finer, dark winter grains of clay minerals or organic material that are deposited slowly from suspension in quiet water while streams and lakes are icebound. Varves are useful to the study of geochronology because they can be counted to determine the absolute age of some Pleistocene rocks of glacial origin. |
| Geology | vesicle | Bubble-shaped cavities in volcanic rock formed by expansion of gas dissolved in the precursor magma. |
| Geology | vesicular | Pertaining to vesicles, bubble-shaped cavities in volcanic rock formed by expansion of gas dissolved in the precursor magma. |
| Geology | vesicular porosity | A type of porosity resulting from the presence of vesicles, or gas bubbles, in igneous rock. |
| Geology | virgin pressure | The original, undisturbed pressure of a reservoir prior to fluid production. |
| Geology | vitrinite | A type of woody kerogen that is relatively uniform in composition. Since vitrinite changes predictably and consistently upon heating, its reflectance is a useful measurement of source rock maturity. Strictly speaking, the plant material that forms vitrinite did not occur prior to Ordovician time. Also, because vitrinite originated in wood, its occurrence in marine rocks might be limited by the depositional processes that act in a given depositional environment. |
| Geology | vitrinite reflectance | A measurement of the maturity of organic matter with respect to whether it has generated hydrocarbons or could be an effective source rock. |
| Geology | volcanic | Pertaining to one or more volcanoes, surface features of the Earth that allow magma, ash and gas to erupt. The vent can be a fissure or a conical structure. |
| Geology | volcano | A surface feature of the Earth that allows magma, ash and gas to erupt. The vent can be a fissure or a conical structure. |
| Geology | vug | A cavity, void or large pore in a rock that is commonly lined with mineral precipitates. |
| Geology | vuggy | Containing vugs, which are cavities, voids or large pores in a rock that are commonly lined with mineral precipitates. |
| Geology | vugular | Referring to vugs, which are cavities, voids or large pores in a rock that are commonly lined with mineral precipitates. |
| Geology | vugular porosity | Pore space consisting of cavities or vugs. Vugular porosity can occur in rocks prone to dissolution, such as limestone, in which case it is secondary porosity. |
| Geology | Wadati-Benioff zone | A zone of the upper mantle in which earthquakes occur when a lithospheric plate is subducted, named in honor of seismologists Kiyoo Wadati and Hugo Benioff. The dip of the Wadati-Benioff zone coincides with the dip of the subducting plate. The Wadati-Benioff zone extends to a depth of about 700 km [435 miles] from the Earth’s surface. |
| Geology | weathering | The physical, chemical and biological processes that decompose rock at and below the surface of the Earth through low pressures and temperatures and the presence of air and water. Weathering includes processes such as dissolution, chemical weathering, disintegration and hydration. |
| Geology | Wentworth scale | Another name for the Udden-Wentworth scale, a grade scale for classifying the diameters of sediments. Particles larger than 64 mm in diameter are classified as cobbles. Smaller particles are pebbles, granules, sand and silt. Those smaller than 0.0039 mm are clay. Several other grain size scales are in use, but the Udden-Wentworth scale (commonly called the Wentworth scale) is the one that is most frequently used in geology. |
| Geology | wet gas | Natural gas that contains less methane (typically less than 85% methane) and more ethane and other more complex hydrocarbons. |
| Geology | wrench fault | A type of strike-slip fault in which the fault surface is vertical, and the fault blocks move sideways past each other. Given the geological complexity of some deformed rocks, including rocks that have experienced more than one episode of deformation, it can be difficult to distinguish a wrench fault from a strike-slip fault. Also, areas can be deformed more than once or experience ongoing structuring such that fault surfaces can be rotated from their original orientations. |
| Geology | yield point | The elastic limit, or the point at which a material can no longer deform elastically. When the elastic limit is exceeded by an applied stress, permanent deformation occurs. |
| Geology | zonal | Pertaining to a zone, an interval or unit of rock differentiated from surrounding rocks on the basis of its fossil content or other features, such as faults or fractures. For example, a fracture zone contains numerous fractures. A biostratigraphic zone contains a particular fossil or fossils. |
| Geology | zone | An interval or unit of rock differentiated from surrounding rocks on the basis of its fossil content or other features, such as faults or fractures. For example, a fracture zone contains numerous fractures. A biostratigraphic zone contains a particular fossil or fossils. |
| Geology, Drilling Fluids | geothermal gradient | The rate of increase in temperature per unit depth in the Earth. Although the geothermal gradient varies from place to place, it averages 25 to 30 °C/km [15 °F/1000 ft]. Temperature gradients sometimes increase dramatically around volcanic areas. It is particularly important for drilling fluids engineers to know the geothermal gradient in an area when they are designing a deep well. The downhole temperature can be calculated by adding the surface temperature to the product of the depth and the geothermal gradient. |
| Geology, Drilling Fluids | temperature gradient | Also known as geothermal gradient, the rate of increase in temperature per unit depth in the Earth. Although the geothermal gradient varies from place to place, it averages 25 to 30 °C/km [15 °F/1000 ft]. Temperature gradients sometimes increase dramatically around volcanic areas. It is particularly important for drilling fluids engineers to know the geothermal gradient in an area when they are designing a deep well. The downhole temperature can be calculated by adding the surface temperature to the product of the depth and the geothermal gradient. |
| Geology, Drilling, Reservoir Characterization | tortuosity | A measure of deviation from a straight line. It is the ratio of the actual distance traveled between two points, including any curves encountered, divided by the straight line distance. Tortuosity is used by drillers to describe wellbore trajectory, by log analysts to describe electrical current flow through rock and by geologists to describe pore systems in rock and the meander of rivers. |
| Geology, Geochemistry, Shale Gas | kerogen | The naturally occurring, solid, insoluble organic matter that occurs in source rocks and can yield oil upon heating. Kerogen is the portion of naturally occurring organic matter that is nonextractable using organic solvents. Typical organic constituents of kerogen are algae and woody plant material. Kerogens have a high molecular weight relative to bitumen, or soluble organic matter. Bitumen forms from kerogen during petroleum generation. Kerogens are described as Type I, consisting of mainly algal and amorphous (but presumably algal) kerogen and highly likely to generate oil; Type II, mixed terrestrial and marine source material that can generate waxy oil; and Type III, woody terrestrial source material that typically generates gas. |
| Geology, Geochemistry, Shale Gas | pyrolysis | A type of geochemical analysis in which a rock sample is subject to controlled heating in an inert gas to or past the point of generating hydrocarbons in order to assess its quality as a source rock, the abundance of organic material in it, its thermal maturity, and the quality of hydrocarbons it might generate or have generated. Pyrolysis breaks large hydrocarbon molecules into smaller molecules. This process is used to determine the quality of shale as a source rock and is instrumental in evaluating shale gas plays. |
| Geology, Geochemistry, Shale Gas | source rock | A rock rich in organic matter which, if heated sufficiently, will generate oil or gas. Typical source rocks, usually shales or limestones, contain about 1% organic matter and at least 0.5% total organic carbon (TOC), although a rich source rock might have as much as 10% organic matter. Rocks of marine origin tend to be oil-prone, whereas terrestrial source rocks (such as coal) tend to be gas-prone. Preservation of organic matter without degradation is critical to creating a good source rock, and necessary for a complete petroleum system. Under the right conditions, source rocks may also be reservoir rocks, as in the case of shale gas reservoirs. |
| Geology, Geophysics | aerated layer | The surface or near-surface, unconsolidated sedimentary layer that has been subject to weathering and whose pores are air-filled instead of liquid-filled. An aerated layer typically has a low seismic velocity. |
| Geology, Geophysics | low-velocity layer | Also known as weathered layer, a near-surface, possibly unconsolidated layer of low seismic velocity. The base of the weathered layer commonly coincides with the water table and a sharp increase in seismic velocity. The weathered layer typically has air-filled pores. |
| Geology, Geophysics | weathered layer | A near-surface, possibly unconsolidated layer of low seismic velocity. The base of the weathered layer commonly coincides with the water table and a sharp increase in seismic velocity. The weathered layer typically has air-filled pores. |
| Geology, Geophysics, Formation Evaluation | drained test | A drained test is one in which the pore fluid in the sample is able to flow and equilibrate to imposed pore pressure conditions; the fluid mass and volume will vary but its pressure will be constant. A drained test could be on a dry sample. |
| Geology, Geophysics, Formation Evaluation | UCS | Abbreviation for unconfined compressive strength, or uniaxial compressive strength. |
| Geology, Geophysics, Formation Evaluation | unconfined compressive strength | A measure of a material’s strength. The unconfined compressive strength (UCS) is the maximum axial compressive stress that a right-cylindrical sample of material can withstand under unconfined conditions—the confining stress is zero. It is also known as the uniaxial compressive strength of a material because the application of compressive stress is only along one axis—the longitudinal axis—of the sample. |
| Geology, Geophysics, Formation Evaluation | undrained test | An undrained test is one in which the fluid in the sample is not able to flow and equilibrate to imposed pore pressure conditions; the fluid mass remains the same while the fluid volume and pressure will vary. |
| Geology, Geophysics, Formation Evaluation | uniaxial compressive strength | A measure of a material’s strength. The uniaxial compressive strength (UCS) is the maximum axial compressive stress that a right-cylindrical sample of material can withstand before failing. It is also known as the unconfined compressive strength of a material because confining stress is set to zero. |
| Geology, Geophysics, Shale Gas | aeolotropy | Predictable variation of a property of a material with the direction in which it is measured, which can occur at all scales. For a crystal of a mineral, variation in physical properties observed in different directions is aeolotropy (also known as anisotropy). In rocks, variation in seismic velocity measured parallel or perpendicular to bedding surfaces is a form of aeolotropy. Often found where platy minerals such as micas and clays align parallel to depositional bedding as sediments are compacted, aeolotropy is common in shales. |
| Geology, Reservoir Characterization | stratigraphic analysis | An analysis of the history, composition, relative ages and distribution of strata, and the interpretation of strata to elucidate Earth history. The comparison, or correlation, of separated strata can include study of their lithology, fossil content, and relative or absolute age, or lithostratigraphy, biostratigraphy, and chronostratigraphy. |
| Geology, Reservoir Characterization | stratigraphy | The study of the history, composition, relative ages and distribution of strata, and the interpretation of strata to elucidate Earth history. The comparison, or correlation, of separated strata can include study of their lithology, fossil content, and relative or absolute age, or lithostratigraphy, biostratigraphy, and chronostratigraphy. |
| Geology, Shale Gas | basin | A depression in the crust of the Earth, caused by plate tectonic activity and subsidence, in which sediments accumulate. Sedimentary basins vary from bowl-shaped to elongated troughs. Basins can be bounded by faults. Rift basins are commonly symmetrical; basins along continental margins tend to be asymmetrical. If rich hydrocarbon source rocks occur in combination with appropriate depth and duration of burial, then a petroleum system can develop within the basin. Most basins contain some amount of shale, thus providing opportunities for shale gas exploration and production. |
| Geology, Shale Gas | catagenesis | The physical and chemical alteration of sediments and pore fluids at temperatures and pressures higher than those of diagenesis. Catagenesis involves heating in the range of 50° to 150°C [122° to 302°F]. At these temperatures, chemical bonds break down in kerogen and clays within shale, generating liquid hydrocarbons. At the high end of this temperature range, secondary cracking of oil molecules can generate gas molecules. |
| Geology, Shale Gas | exploration play | An area in which hydrocarbon accumulations or prospects of a given type occur. For example the shale gas plays in North America include the Barnett, Eagle Ford, Fayetteville, Haynesville, Marcellus, and Woodford, among many others. Outside North America, shale gas potential is being pursued in many parts of Europe, Africa, Asia, and South America. |
| Geology, Shale Gas | fracture permeability | That portion of a dual-porosity reservoirs permeability that is associated with the secondary porosity created by open, natural fractures. In many of these reservoirs, fracture permeability can be the major controlling factor of the flow of fluids. |
| Geology, Shale Gas | free gas | The gaseous phase present in a reservoir or other contained area. Gas may be found either dissolved in reservoir fluids or as free gas that tends to form a gas cap beneath the top seal on the reservoir trap. Both free gas and dissolved gas play important roles in the reservoir-drive mechanism. |
| Geology, Shale Gas | heterogeneity | The quality of variation in rock properties with location in a reservoir or formation. Shale gas reservoirs are heterogeneous formations whose mineralogy, organic content, natural fractures, and other properties vary from place to place. This heterogeneity makes petroleum system modeling, formation evaluation, and reservoir simulation critical to maximizing production from shale reservoirs. |
| Geology, Shale Gas | heterogeneous | Possessing the quality of variation in rock properties with location in a reservoir or formation. Shale gas reservoirs are heterogeneous formations whose mineralogy, organic content, natural fractures, and other properties vary from place to place. This heterogeneity makes petroleum system modeling, formation evaluation, and reservoir simulation critical to maximizing production from shale reservoirs. |
| Geology, Shale Gas | shale | A fine-grained, fissile, detrital sedimentary rock formed by consolidation of clay- and silt-sized particles into thin, relatively impermeable layers. It is the most abundant sedimentary rock. Shale can include relatively large amounts of organic material compared with other rock types and thus has potential to become a rich hydrocarbon source rock, even though a typical shale contains just 1% organic matter. Its typical fine grain size and lack of permeability, a consequence of the alignment of its platy or flaky grains, allow shale to form a good cap rock for hydrocarbon traps. Gas shows from shales during drilling have led some shales to be targeted as potential gas reservoirs. Various clay types and volumes influence the quality of the reservoir from a petrophysical and geomechanical perspective. The quality of shale reservoirs depends on their thickness and extent, organic content, thermal maturity, depth and pressure, fluid saturations, and permeability, among other factors. |
| Geology, Shale Gas | shaly | Containing shale, a fine-grained, fissile, detrital sedimentary rock formed by consolidation of clay- and silt-sized particles into thin, relatively impermeable layers. It is the most abundant sedimentary rock. Shale can include relatively large amounts of organic material compared with other rock types and thus has potential to become a rich hydrocarbon source rock, even though a typical shale contains just 1% organic matter. Its typical fine grain size and lack of permeability, a consequence of the alignment of its platy or flaky grains, allow shale to form a good cap rock for hydrocarbon traps. Gas shows from shales during drilling have led some shales to be targeted as potential gas reservoirs. Various clay types and volumes influence the quality of the reservoir from a petrophysical and geomechanical perspective. The quality of shale reservoirs depends on their thickness and extent, organic content, thermal maturity, depth and pressure, fluid saturations, and permeability, among other factors. |
| Geology, Shale Gas | tight | Describing a relatively impermeable reservoir rock from which hydrocarbon production is difficult. Reservoirs can be tight because of smaller grains or matrix between larger grains, or they might be tight because they consist predominantly of silt- or clay-sized grains, as is the case for shale reservoirs. Stimulation of tight formations can result in increased production from formations that previously would have been abandoned or produced uneconomically. |
| Geology, Shale Gas | TOC | The concentration of organic material in source rocks as represented by the weight percent of organic carbon. A value of approximately 0.5% total organic carbon by weight percent is considered the minimum for an effective source rock, although values of 2% are considered the minimum for shale gas reservoirs; values exceeding 10% exist, although some geoscientists assert that high total organic carbon values indicate the possibility of kerogen filling pore space rather than other forms of hydrocarbons. Total organic carbon is measured from 1-g samples of pulverized rock that are combusted and converted to CO or CO2. If a sample appears to contain sufficient total organic carbon to generate hydrocarbons, it may be subjected to pyrolysis. |
| Geology, Shale Gas | unconventional resource | An umbrella term for oil and natural gas that is produced by means that do not meet the criteria for conventional production. What has qualified as unconventional at any particular time is a complex function of resource characteristics, the available exploration and production technologies, the economic environment, and the scale, frequency and duration of production from the resource. Perceptions of these factors inevitably change over time and often differ among users of the term. At present, the term is used in reference to oil and gas resources whose porosity, permeability, fluid trapping mechanism, or other characteristics differ from conventional sandstone and carbonate reservoirs. Coalbed methane, gas hydrates, shale gas, fractured reservoirs, and tight gas sands are considered unconventional resources. |
| Geology | bathyal | Pertaining to the environment of deposition and the organisms of the ocean between depths of 200 m [656 ft], the edge of the continental shelf, and 2000 m [6560 ft]. The bathyal environment is intermediate between the neritic environment and the abyss. |
| Geophysics | polarity | The nature of the positive and negative portions of the seismic wavelet, the positive and negative aspects of electrical equipment, or the north and south orientations of magnets and the Earth’s magnetic field. |
| Geophysics | abnormal events | A term to indicate features in seismic data other than reflections, including events such as diffractions, multiples, refractions and surface waves. Although the term suggests that such events are not common, they often occur in seismic data. |
| Geophysics | absorbing boundary conditions | An algorithm used in numerical simulation along the boundary of a computational domain to absorb all energy incident upon that boundary and to suppress reflection artifacts. |
| Geophysics | absorptance | The ratio of absorbed incident energy to the total energy to which a body is exposed. |
| Geophysics | absorption | The conversion of one form of energy into another as the energy passes through a medium. For example, seismic waves are partially converted to heat as they pass through rock. |
| Geophysics | absorption band | The range of wavelengths of energy that can be absorbed by a given substance. |
| Geophysics | accelerometer | A device used during surveying to measure the acceleration of a ship or aircraft, or to detect ground acceleration in boreholes or on the Earth’s surface produced by acoustic vibrations. |
| Geophysics | acoustic | Pertaining to sound. Generally, acoustic describes sound or vibrational events, regardless of frequency. The term sonic is limited to frequencies and tools operated in the frequency range of 1 to 25 kilohertz. |
| Geophysics | acoustic basement | The portion of the Earth below which strata cannot be imaged with seismic data, or the deepest relatively continuous reflector. Acoustic basement, in some regions, coincides with economic basement and geologic basement, or that portion of the Earth that does not comprise sedimentary rocks. |
| Geophysics | acoustic coupler | An obsolete piece of equipment that converts acoustic signals from analog to electrical form and back. A common use of an acoustic coupler was to provide an interface between a telephone and an early type of computer modem. |
| Geophysics | acoustic emission | A type of elastic wave produced by deformation or brittle failure of material and characterized by relatively high frequency. |
| Geophysics | acoustic impedance | The product of density and seismic velocity, which varies among different rock layers, commonly symbolized by Z. The difference in acoustic impedance between rock layers affects the reflection coefficient. |
| Geophysics | acoustic impedance section | A seismic reflectivity section, or a 2D or 3D seismic section, that has been inverted for acoustic impedance. Sonic and density logs can be used to calibrate acoustic impedance sections. |
| Geophysics | acoustic log | A display of traveltime of acoustic waves versus depth in a well. The term is commonly used as a synonym for a sonic log. Some acoustic logs display velocity. |
| Geophysics | acoustic positioning | A method of calculating the position of marine seismic equipment. Range measurements are made whereby distance is equal to acoustic signal traveltime from transmitter to hydrophone multiplied by the speed of sound in water. When sufficient acoustic ranges with a proper geometric distribution are collected, location coordinates x, y and z of the marine seismic equipment can be computed by the method of trilateration (measuring the lengths of the sides of overlapping triangles). Acoustic positioning is commonly used in towed streamer and ocean-bottom cable seismic acquisition modes. |
| Geophysics | acoustic transparency | The quality of a medium whose acoustic impedance is constant throughout, such that it contains no seismic reflections. An example of an acoustically transparent medium is water. |
| Geophysics | acoustic traveltime | The duration of the passage of a signal from the source through the Earth and back to the receiver. A time seismic section typically shows the two-way traveltime of the wave. |
| Geophysics | acoustic velocity | The rate at which a sound wave travels through a medium. Unlike the physicist’s definition of velocity as a vector, its usage in geophysics is as a property of a medium: distance divided by traveltime. Velocity can be determined from laboratory measurements, acoustic logs, vertical seismic profiles or from velocity analysis of seismic data. It can vary vertically, laterally and azimuthally in anisotropic media such as rocks, and tends to increase with depth in the Earth because compaction reduces porosity. Velocity also varies as a function of how it is derived from the data. For example, the stacking velocity derived from normal moveout measurements of common depth point gathers differs from the average velocity measured vertically from a check-shot or vertical seismic profile (VSP). Velocity would be the same only in a constant-velocity (homogeneous) medium. |
| Geophysics | acoustic velocity log | A display of traveltime of acoustic waves versus depth in a well. The term is commonly used as a synonym for a sonic log. Some acoustic logs display velocity. |
| Geophysics | acoustic wave | An elastic body wave or sound wave in which particles oscillate in the direction the wave propagates. P-waves are the waves studied in conventional seismic data. P-waves incident on an interface at other than normal incidence can produce reflected and transmitted S-waves, in that case known as converted waves. |
| Geophysics | acquisition | The generation and recording of seismic data. Acquisition involves many different receiver configurations, including laying geophones or seismometers on the surface of the Earth or seafloor, towing hydrophones behind a marine seismic vessel, suspending hydrophones vertically in the sea or placing geophones in a wellbore (as in a vertical seismic profile) to record the seismic signal. A source, such as a vibrator unit, dynamite shot, or an air gun, generates acoustic or elastic vibrations that travel into the Earth, pass through strata with different seismic responses and filtering effects, and return to the surface to be recorded as seismic data. Optimal acquisition varies according to local conditions and involves employing the appropriate source (both type and intensity), optimal configuration of receivers, and orientation of receiver lines with respect to geological features. This ensures that the highest signal-to-noise ratio can be recorded, resolution is appropriate, and extraneous effects such as air waves, ground roll, multiples and diffractions can be minimized or distinguished, and removed through processing. |
| Geophysics | aeromagnetic survey | Measurements of the Earth’s magnetic field gathered from aircraft. Magnetometers towed by an airplane or helicopter can measure the intensity of the Earth’s magnetic field. The differences between actual measurements and theoretical values indicate anomalies in the magnetic field, which in turn represent changes in rock type or in thickness of rock units. |
| Geophysics | AGC | Abbreviation for automatic gain control. A system to automatically control the gain, or the increase in the amplitude of an electrical signal from the original input to the amplified output. AGC is commonly used in seismic processing to improve visibility of late-arriving events in which attenuation or wavefront divergence has caused amplitude decay. |
| Geophysics | AGC time constant | The exponential rate constant (?) that determines how quickly the output amplitude of an electrical signal that is under automatic gain control (AGC) responds to a sudden increase or decrease in input signal amplitude. Mathematically, Af(t) = Ai(t) + ?Ai (1 ? e?t/?) where Af is the output signal amplitude, Ai is the input signal amplitude (Ai), ?Ai is the change in input signal amplitude and t is time. When t equals ?, the function (1 ? e?t/?) equals (1 ? 1/e) equals 0.63. Therefore, the AGC time constant (?) is the amount of time that elapses for the output signal of AGC to reflect 63% of the change in the input signal amplitude. |
| Geophysics | air gun | A source of seismic energy used in acquisition of marine seismic data. This gun releases highly compressed air into water. Air guns are also used in water-filled pits on land as an energy source during acquisition of vertical seismic profiles. |
| Geophysics | air shooting | A method of seismic acquisition using charges detonated in the air or on poles above the ground as the source. Air shooting is also called the Poulter method after American geophysicist Thomas Poulter. |
| Geophysics | air wave | A sound wave that travels through the air at approximately 330 m/s and can be generated and recorded during seismic surveying. Air waves are a type of coherent noise. |
| Geophysics | alias filter | A filter, or a set of limits used to eliminate unwanted portions of the spectra of the seismic data, to remove frequencies that might cause aliasing during the process of sampling an analog signal during acquisition or when the sample rate of digital data is being decreased during seismic processing. |
| Geophysics | aliasing | The distortion of frequency introduced by inadequately sampling a signal, which results in ambiguity between signal and noise. Aliasing can be avoided by sampling at least twice the highest frequency of the waveform or by filtering frequencies above the Nyquist frequency, the highest frequency that can be defined accurately by that sampling interval. |
| Geophysics | amplitude | The difference between the maximum displacement of a wave and the point of no displacement, or the null point. The common symbol for amplitude is a. |
| Geophysics | amplitude anomaly | An abrupt increase in seismic amplitude that can indicate the presence of hydrocarbons, although such anomalies can also result from processing problems, geometric or velocity focusing or changes in lithology. Amplitude anomalies that indicate the presence of hydrocarbons can result from sudden changes in acoustic impedance, such as when a gas sand underlies a shale, and in that case, the term is used synonymously with hydrocarbon indicator. |
| Geophysics | amplitude distortion | The inability of a system to exactly match input and output amplitude, a general example being an electronic amplifier and the classic example being a home stereophonic amplifier. |
| Geophysics | amplitude distortion | A change in the amplitude of a waveform that is generally undesirable, such as in seismic waves. |
| Geophysics | amplitude variation with offset | Variation in seismic reflection amplitude with change in distance between shotpoint and receiver that indicates differences in lithology and fluid content in rocks above and below the reflector. AVO analysis is a technique by which geophysicists attempt to determine thickness, porosity, density, velocity, lithology and fluid content of rocks. Successful AVO analysis requires special processing of seismic data and seismic modeling to determine rock properties with a known fluid content. With that knowledge, it is possible to model other types of fluid content. A gas-filled sandstone might show increasing amplitude with offset, whereas a coal might show decreasing amplitude with offset. A limitation of AVO analysis using only P-energy is its failure to yield a unique solution, so AVO results are prone to misinterpretation. One common misinterpretation is the failure to distinguish a gas-filled reservoir from a reservoir having only partial gas saturation (“fizz water”). However, AVO analysis using source-generated or mode-converted shear wave energy allows differentiation of degrees of gas saturation. AVO analysis is more successful in young, poorly consolidated rocks, such as those in the Gulf of Mexico, than in older, well-cemented sediments. |
| Geophysics | amplitude variation with offset and azimuth | The azimuthal variation of the AVO response. |
| Geophysics | anaerobic | The condition of an environment in which free oxygen is lacking or absent. |
| Geophysics | anaerobic | A description of organisms that can survive in the absence of oxygen, particularly bacteria. |
| Geophysics | angle of approach | The acute angle at which a wavefront impinges upon an interface, such as a seismic wave impinging upon strata. Normal incidence is the case in which the angle of incidence is zero, the wavefront is parallel to the surface and its raypath is perpendicular, or normal, to the interface. Snell’s law describes the relationship between the angle of incidence and the angle of refraction of a wave. |
| Geophysics | angle of incidence | The acute angle at which a raypath impinges upon a line normal to an interface, such as a seismic wave impinging upon strata. Normal incidence is the case in which the angle of incidence is zero, the wavefront is parallel to the surface and its raypath is perpendicular, or normal, to the interface. Snell’s law describes the relationship between the angle of incidence and the angle of refraction of a wave. |
| Geophysics | angular dispersion | The variation of seismic velocity in different directions. |
| Geophysics | antialias filter | A filter, or a set of limits used to eliminate unwanted portions of the spectra of the seismic data, to remove frequencies that might cause aliasing during the process of sampling an analog signal during acquisition or when the sample rate of digital data is being decreased during seismic processing. |
| Geophysics | aperture | A portion of a data set, such as seismic data, to which functions or filters are applied. Aperture time, for example, can be specified, such as a window from 1.2 to 2.8 seconds. |
| Geophysics | aperture | A mechanism to limit the affects of measurements on a device or system. In seismic data acquisition, the length of the spread has the effect of an aperture. |
| Geophysics | apparent anisotropy | In seismic data, the ratio of the velocity determined from normal moveout (i.e., primarily a horizontal measurement) to velocity measured vertically in a vertical seismic profile or similar survey. Apparent anisotropy is of particular importance when migrating long-offset seismic data and analyzing AVO data accurately. The normal moveout velocity involves the horizontal component of the velocity field, which affects sources and receivers that are offset, but the horizontal velocity field is not involved in velocity calculations from vertically measured time-depth pairs. |
| Geophysics | apparent velocity | In geophysics, the velocity of a wavefront in a certain direction, typically measured along a line of receivers and symbolized by va. Apparent velocity and velocity are related by the cosine of the angle at which the wavefront approaches the receivers: va = v cos ?, where va = apparent velocity v = velocity of wavefront ? = angle at which a wavefront approaches the geophone array. |
| Geophysics | apparent wavelength | The wavelength measured by receivers when a wave approaches at an angle. The relationship between true and apparent wavelength can be shown mathematically as follows: ? = ?a sin ?, where ? = wavelength ?a = apparent wavelength ? = angle at which a wavefront approaches the geophone array. |
| Geophysics | applied-potential method | A technique to map a potential field generated by stationary electrodes by moving an electrode around the survey area. |
| Geophysics | array | Generally, a geometrical configuration of transducers (sources or receivers) used to generate or record a physical field, such as an acoustic or electromagnetic wavefield or the Earth’s gravity field. |
| Geophysics | array | A geometrical arrangement of seismic sources (a source array, with each individual source being activated in some fixed sequence in time) or receivers (a hydrophone or geophone array) that is recorded by one channel. |
| Geophysics | arrival | An event or appearance of seismic data as a reflection, refraction, diffraction or other similar feature, or the time at which seismic data appear. An event in a seismic section can represent a geologic interface. |
| Geophysics | arrival time | The elapsed time between the release of seismic energy from a source and its arrival at the receiver. |
| Geophysics | attenuate | The removal of undesirable features, such as multiple events, from seismic data. |
| Geophysics | attenuation | The loss of energy or amplitude of waves as they pass through media. Seismic waves lose energy through absorption, reflection and refraction at interfaces, mode conversion and spherical divergence, or spreading of the wave. |
| Geophysics | attribute | A measurable property of seismic data, such as amplitude, dip, frequency, phase and polarity. Attributes can be measured at one instant in time or over a time window, and may be measured on a single trace, on a set of traces or on a surface interpreted from seismic data. Attribute analysis includes assessment of various reservoir parameters, including ahydrocarbon indicator, by techniques such as amplitude variation with offset (AVO) analysis. |
| Geophysics | autocorrelation | The comparison of a waveform to itself. Autocorrelation is useful in the identification of multiples or other regularly repeating signals, and in designing deconvolution filters to suppress them. |
| Geophysics | automatic gain control | A system to control the gain, or the increase in the amplitude of an electrical signal from the original input to the amplified output, automatically. AGC is commonly used in seismic processing to improve visibility of late-arriving events in which attenuation or wavefront divergence has caused amplitude decay. |
| Geophysics | autotrack | To use computer software to pick a particular reflection or attribute in seismic data automatically. Autotracking can speed interpretation of three-dimensional seismic data, but must be checked for errors, especially in areas of faulting and stratigraphic changes. |
| Geophysics | autotracking | Use of computer software to pick a particular reflection or attribute in seismic data automatically. Autotracking can speed interpretation of three-dimensional seismic data, but must be checked for errors, especially in areas of faulting and stratigraphic changes. |
| Geophysics | AVA | Abbreviation for amplitude variation with angle of incidence. |
| Geophysics | AVAZ | Abbreviation for amplitude variation with azimuth. |
| Geophysics | average velocity | In geophysics, the depth divided by the traveltime of a wave to that depth. Average velocity is commonly calculated by assuming a vertical path, parallel layers and straight raypaths, conditions that are quite idealized compared to those actually found in the Earth. |
| Geophysics | AVO | Abbreviation for amplitude variation with offset. Variation in seismic reflection amplitude with change in distance between shotpoint and receiver that indicates differences in lithology and fluid content in rocks above and below the reflector. AVO analysis is a technique by which geophysicists attempt to determine thickness, porosity, density, velocity, lithology and fluid content of rocks. Successful AVO analysis requires special processing of seismic data and seismic modeling to determine rock properties with a known fluid content. With that knowledge, it is possible to model other types of fluid content. A gas-filled sandstone might show increasing amplitude with offset, whereas a coal might show decreasing amplitude with offset. A limitation of AVO analysis using only P-energy is its failure to yield a unique solution, so AVO results are prone to misinterpretation. One common misinterpretation is the failure to distinguish a gas-filled reservoir from a reservoir having only partial gas saturation (“fizz water”). However, AVO analysis using source-generated or mode-converted shear wave energy allows differentiation of degrees of gas saturation. AVO analysis is more successful in young, poorly consolidated rocks, such as those in the Gulf of Mexico, than in older, well-cemented sediments. |
| Geophysics | AVOAZ | Abbreviation for amplitude variation with offset and azimuth. The azimuthal variation of the AVO response. |
| Geophysics | axis of rotational symmetry | An axis of rotational invariance. A material whose properties exhibit cylindrical, or invariant rotational, symmetry may be rotated about this axis by any amount and its properties will be indistinguishable from what they were before the rotation. |
| Geophysics | back propagation | A method for reconstructing the location and shape of the wave at an earlier time using the wave equation. |
| Geophysics | back stripping | A modeling technique to assess the geologic history of rock layers through the use of geologic cross sections or seismic sections. Removal of the youngest layers of rock at the top of the section allows restoration of the underlying layers to their initial, undisturbed configurations. Successively older layers can be removed sequentially to further assess the effects of compaction, development of geologic structures and other processes on an area. |
| Geophysics | back-propagation | A method for reconstructing the location and shape of the wave at an earlier time using the wave equation. |
| Geophysics | backscatter | A reflection phenomenon of energy in which a nonreflective surface, which is a surface that does not reflect energy coherently, randomly scatters energy. No coherent reflected energy can be identified and the energy is scattered in all directions, including back in the direction from which it came. For example, light can be scattered or redistributed by rough, nonreflective surfaces. |
| Geophysics | back-stripping | A modeling technique to assess the geologic history of rock layers through the use of geologic cross sections or seismic sections. Removal of the youngest layers of rock at the top of the section allows restoration of the underlying layers to their initial, undisturbed configurations. Successively older layers can be removed sequentially to further assess the effects of compaction, development of geologic structures and other processes on an area. |
| Geophysics | band | A range of frequencies or wavelengths. Audible frequencies of sound and visible wavelengths of light are examples of bands. In seismic data, band-pass frequencies are within the limits of a specific filter, while band-reject frequencies exceed the acceptable range of frequencies. |
| Geophysics | band limited function | A function or time series whose Fourier transform is restricted to a finite range of frequencies or wavelengths. |
| Geophysics | band pass | Frequencies within the acceptable limits of a filter. The term is commonly used as an adjective, as in “band-pass filter,” to denote a filter that passes a range of frequencies unaltered while rejecting frequencies outside the range. |
| Geophysics | band reject | Frequencies beyond the limits of a filter. |
| Geophysics | band-limited function | A function or time series whose Fourier transform is restricted to a finite range of frequencies or wavelengths. |
| Geophysics | band-pass | Frequencies within the acceptable limits of a filter. The term is commonly used as an adjective, as in “band-pass filter,” to denote a filter that passes a range of frequencies unaltered while rejecting frequencies outside the range. |
| Geophysics | band-reject | Frequencies beyond the limits of a filter. |
| Geophysics | base of weathering | The lower boundary of the near-surface, low-velocity zone in which rocks are physically, chemically or biologically broken down, in some cases coincident with a water table. Static corrections to seismic data can compensate for the low velocity of the weathered layer in comparison with the higher-velocity strata below. |
| Geophysics | base station | A reference location for a survey, or a survey point whose measured values of a given parameter of interest are understood and can be used to normalize other survey points. Accurate knowledge of base stations is critical in gravity and magnetic surveying. |
| Geophysics | baseline | A line joining base stations whose transmissions are synchronized during surveying. |
| Geophysics | baseline | A reference line, such as a “shale baseline,” a line representing the typical value of a given measurement for a shale on a well log, or the zero-amplitude line of a seismic trace. |
| Geophysics | basic wavelet | The shape of a wavelet produced by reflection of an actual wave train at one interface with a positive reflection coefficient. The embedded wavelet is useful for generating a convolutional model, or the convolution of an embedded wavelet with a reflectivity function and random noise, during seismic processing or interpretation. |
| Geophysics | Beaufort scale | The 0 to 12 scale for measurement of wind strength according to its effect on objects such as trees, flags and water established by Admiral Francis Beaufort (1774 to 1857). According to the Beaufort scale, at wind speeds below 1 knot or 1 km/hr, seas are calm. Whitecaps on water and blowing dust and leaves correspond to a Beaufort number of 4, with winds of 11 to 16 knots [20 to 28 km/hr]. Hurricane-force winds, greater than 64 knots [> 118 km/hr], have a Beaufort number of 12. |
| Geophysics | bel | The unit of measurement to describe or compare the intensity of acoustic or electrical signal, named for American inventor Alexander Graham Bell (1847 to 1922). Measurements are typically given in tenths of a bel, or decibels. The logarithm of the ratio of the sound or signal to a standard provides the decibel measurement. Sounds on the order of one decibel are barely audible to humans but can cause pain when on the order of 1012 decibels. The symbol for the unit is B, but dB is the standard unit. |
| Geophysics | benchmark | A standard against which the performance of processes are measured. |
| Geophysics | benchmark | A permanently fixed marker cited in surveying, such as a concrete block or steel plate, with an inscription of location and elevation. |
| Geophysics | bias | An adjustment of the relative positive and negative excursions of reflections during seismic processing by bulk shifting the null point, or baseline, of the data to emphasize peaks at the expense of troughs or vice versa. Some authors describe bias as a systematic distortion of seismic data to achieve greater continuity. |
| Geophysics | bimetallism | A subdivision of a seismic survey. The area of a three-dimensional survey is divided into bins, which are commonly on the order of 25 m [82 ft] long and 25 m wide; traces are assigned to specific bins according to the midpoint between the source and the receiver, reflection point or conversion point. Bins are commonly assigned according to common midpoint (CMP), but more sophisticated seismic processing allows for other types of binning. Traces within a bin are stacked to generate the output trace for that bin. Data quality depends in part on the number of traces per bin, or the fold. |
| Geophysics | bimetallism | To sort seismic data into small areas according to the midpoint between the source and the receiver, reflection point or conversion point prior to stacking. |
| Geophysics | bin | To sort seismic data into small areas according to the midpoint between the source and the receiver, reflection point or conversion point prior to stacking. |
| Geophysics | bin | A subdivision of a seismic survey. The area of a three-dimensional survey is divided into bins, which are commonly on the order of 25 m [82 ft] long and 25 m wide; traces are assigned to specific bins according to the midpoint between the source and the receiver, reflection point or conversion point. Bins are commonly assigned according to common midpoint (CMP), but more sophisticated seismic processing allows for other types of binning. Traces within a bin are stacked to generate the output trace for that bin. Data quality depends in part on the number of traces per bin, or the fold. |
| Geophysics | bird | A device containing a magnetometer and possibly other instruments that can be towed by an aircraft during aeromagnetic surveying or in a marine seismic streamer to provide dynamic information about the streamer position. |
| Geophysics | birefringence | The splitting of an incident wave into two waves of different velocities and orthogonal polarizations. Birefringence occurs in optical mineralogy (see petrography) when plane-polarized light passes through an anisotropic mineral and emerges as two rays traveling at different speeds, the difference between which is characteristic of a mineral. In seismology, incident S-waves can exhibit birefringence as they split into a quasi-shear and a pure-shear wave. Although birefringence was first described by Danish physician Erasmus Bartholin (1625 to 1698) in crystals in 1669, the phenomenon was not fully understood until French physicist Etienne-Louis Malus (1775 to 1812) described polarized light in 1808. |
| Geophysics | blind zone | A shadow zone, or a zone through which waves do not pass, or cannot be recorded, or in which reflections do not occur. |
| Geophysics | blind zone | A layer or body of rock that cannot be detected by seismic refraction, typically because its velocity is lower than that of the overlying rocks; also known as a hidden layer. |
| Geophysics | BM | A permanently fixed marker cited in surveying, such as a concrete block or steel plate, with an inscription of location and elevation. |
| Geophysics | body wave | A wave that propagates through a medium rather than along an interface. P-waves and S-waves are examples of body waves. |
| Geophysics | borehole seismic data | Seismic data measured with receivers, sources or both in a well, such as a check-shot survey, vertical seismic profile (VSP), crosswell seismic data or single-well imaging. By directly measuring the acoustic velocity of each formation encountered in a well, the well logs and borehole seismic data can be correlated to surface seismic data more easily. Borehole seismic data, including both S- and P-waves, can be gathered in a cased or openhole. This term is commonly used to distinguish between borehole sonic data (with frequencies typically greater than 1000 Hz) and borehole seismic data (with frequencies typically less than 1000 Hz). |
| Geophysics | Bouguer anomaly | The remaining value of gravitational attraction after accounting for the theoretical gravitational attraction at the point of measurement, latitude, elevation, the Bouguer correction and the free-air correction (which compensates for height above sea level assuming there is only air between the measurement station and sea level). This anomaly is named for Pierre Bouguer, a French mathematician (1698 to 1758) who demonstrated that gravitational attraction decreases with altitude. |
| Geophysics | Bouguer correction |
The adjustment to a measurement of gravitational acceleration to account for elevation and the density of rock between the measurement station and a reference level. It can be expressed mathematically as the product of the density of the rock, the height |
| Geophysics | bow tie | A concave-upward event in seismic data produced by a buried focus and corrected by proper migration of seismic data. The focusing of the seismic wave produces three reflection points on the event per surface location. The name was coined for the appearance of the event in unmigrated seismic data. Synclines, or sags, commonly generate bow ties. |
| Geophysics | brachistochrone | The fastest route that a seismic ray can travel between two points, generally dictated by Fermat’s principle. |
| Geophysics | break | An arrival of energy propagated from the energy source at the surface to the geophone in the wellbore in vertical seismic profiles and check-shot surveys, or an indication of seismic energy on a trace. |
| Geophysics | bright spot | A seismic amplitude anomaly or high amplitude that can indicate the presence of hydrocarbons. Bright spots result from large changes in acoustic impedance and tuning effect, such as when a gas sand underlies a shale, but can also be caused by phenomena other than the presence of hydrocarbons, such as a change in lithology. The term is often used synonymously with hydrocarbon indicator. |
| Geophysics | brute stack | A processed seismic record that contains traces from a common midpoint that have been added together but has undergone only cursory velocity analysis, so the normal-moveout correction is a first attempt. Typically, no static corrections are made before the brute stack. |
| Geophysics | bubble effect | Bubble pulses or bubble noise that affect data quality. In marine seismic acquisition, the gas bubble produced by an air gun oscillates and generates subsequent pulses that cause source-generated noise. Careful use of multiple air guns can cause destructive interference of bubble pulses and alleviate the bubble effect. A cage, or a steel enclosure surrounding a seismic source, can be used to dissipate energy and reduce the bubble effect. |
| Geophysics | buggy vibro | A vibrator truck equipped with wide tires to allow access to rugged or soggy terrain while causing less damage to the environment. |
| Geophysics | bulk modulus | The ratio of stress to strain, abbreviated as K. The bulk modulus is an elastic constant equal to the change in applied pressure (?P) divided by the ratio of the change in volume to the original volume of a body (?V/V). K = ?V (?P / ?V), where K = bulk modulus V = volume ?P = partial derivative of pressure ?V = partial derivative of volume. |
| Geophysics | cable | A bundle of electrical wires that connects geophones, or the entire carrier system for marine hydrophones, which includes the hydrophones, the electrical wires, the stress member, spacers, the outer skin of the cable, and the streamer filler, which is typically kerosene or a buoyant plastic. The cable relays data to the seismic recording truck or seismic vessel. |
| Geophysics | calibration | A method of adjusting a data set against a control that has properties to which the data set should conform. |
| Geophysics | cap | A small, electrically activated explosive charge that detonates a larger charge. Caps, also called seismic caps or blasting caps, are used for seismic acquisition with an explosive source to achieve consistent timing of detonation. |
| Geophysics | caprock effect | A type of positive gravity anomaly that results from the presence of a dense cap rock overlying a relatively low-density salt dome. |
| Geophysics | cavitation | An implosion produced by locally low pressure, such as the collapse of a gas bubble in liquid (the energy of which is used as the source of seismic energy from air guns). |
| Geophysics | CDP | In multichannel seismic acquisition where beds do not dip, the common reflection point at depth on a reflector, or the halfway point when a wave travels from a source to a reflector to a receiver. In the case of flat layers, the common depth point is vertically below the common midpoint. In the case of dipping beds, there is no common depth point shared by multiple sources and receivers, so dip moveout processing is necessary to reduce smearing, or inappropriate mixing, of the data. |
| Geophysics | channel wave | A type of elastic wave propagated and confined in a layer whose velocity is lower than that of the surrounding layers, such as a layer of coal. |
| Geophysics | character | A distinguishing feature of a waveform in a seismic event, such as shape, frequency, phase or continuity. |
| Geophysics | check shot survey | A type of borehole seismic data designed to measure the seismic traveltime from the surface to a known depth. P-wave velocity of the formations encountered in a wellbore can be measured directly by lowering a geophone to each formation of interest, sending out a source of energy from the surface of the Earth, and recording the resultant signal. The data can then be correlated to surface seismic data by correcting the sonic log and generating a synthetic seismogram to confirm or modify seismic interpretations. It differs from a vertical seismic profile in the number and density of receiver depths recorded; geophone positions may be widely and irregularly located in the wellbore, whereas a vertical seismic profile usually has numerous geophones positioned at closely and regularly spaced intervals in the wellbore. |
| Geophysics | check-shot survey | A type of borehole seismic data designed to measure the seismic traveltime from the surface to a known depth. P-wave velocity of the formations encountered in a wellbore can be measured directly by lowering a geophone to each formation of interest, sending out a source of energy from the surface of the Earth, and recording the resultant signal. The data can then be correlated to surface seismic data by correcting the sonic log and generating a synthetic seismogram to confirm or modify seismic interpretations. It differs from a vertical seismic profile in the number and density of receiver depths recorded; geophone positions may be widely and irregularly located in the wellbore, whereas a vertical seismic profile usually has numerous geophones positioned at closely and regularly spaced intervals in the wellbore. |
| Geophysics | circle shooting | A technique for acquiring full-azimuth marine seismic data. This technique uses a vessel equipped with source arrays and streamers to shoot and record seismic data; however, unlike conventional surveys acquired in a series of parallel straight lines, circle shooting surveys are acquired as the vessel steams in a series of overlapping, continuously linked circles, or coils. The circular shooting geometry acquires a full range of offset data across every azimuth to sample the subsurface geology in all directions. The resulting full azimuth (FAZ) data are used to image complex geology, such as highly faulted strata, basalt, carbonate reefs and subsalt formations. |
| Geophysics | CMP | In multichannel seismic acquisition, the point on the surface halfway between the source and receiver that is shared by numerous source-receiver pairs. Such redundancy among source-receiver pairs enhances the quality of seismic data when the data are stacked. The common midpoint is vertically above the common depth point, or common reflection point. Common midpoint is not the same as common depth point, but the terms are often incorrectly used as synonyms. |
| Geophysics | coherence | A measure of the similarity of two seismic traces. |
| Geophysics | coherence | The quality of two wave trains, or waves consisting of several cycles, being in phase. |
| Geophysics | coherence | The similarity of two mathematical functions as evaluated in the frequency domain. |
| Geophysics | coherence | A quantitative assessment of the similarity of three or more functions, also called semblance. |
| Geophysics | coherence filtering | A technique for removing noise and emphasizing coherent events from multiple channels of seismic data. |
| Geophysics | coherent | Pertaining to seismic events that show continuity from trace to trace. Seismic processing to enhance recognition of coherent events and emphasize discontinuities such as faults and stratigraphic changes has gained popularity since the mid-1990s. |
| Geophysics | coherent detection | A technique for removing noise and emphasizing coherent events from multiple channels of seismic data. |
| Geophysics | coherent noise | Undesirable seismic energy that shows consistent phase from trace to trace, such as ground roll and multiples. |
| Geophysics | common depth point | In multichannel seismic acquisition where beds do not dip, the common reflection point at depth on a reflector, or the halfway point when a wave travels from a source to a reflector to a receiver. In the case of flat layers, the common depth point is vertically below the common midpoint. In the case of dipping beds, there is no common depth point shared by multiple sources and receivers, so dip moveout processing is necessary to reduce smearing, or inappropriate mixing, of the data. |
| Geophysics | common midpoint | In multichannel seismic acquisition, the point on the surface halfway between the source and receiver that is shared by numerous source-receiver pairs. Such redundancy among source-receiver pairs enhances the quality of seismic data when the data are stacked. The common midpoint is vertically above the common depth point, or common reflection point. Common midpoint is not the same as common depth point, but the terms are often incorrectly used as synonyms. |
| Geophysics | common midpoint method | Method of seismic reflection surveying and processing that exploits the redundancy of multiple fold to enhance data quality by reducing noise. During acquisition, an energy source is supplied to a number of shotpoints simultaneously. Once data have been recorded, the energy source is moved farther down the line of acquisition, but enough overlap is left that some of the reflection points are re-recorded with a different source-to-receiver offset. Multiple shotpoints that share a source-receiver midpoint are stacked. The number of times that a common midpoint is recorded is the fold of the data. |
| Geophysics | common offset | Pertaining to traces that have the same offset, or distance between source and receiver. |
| Geophysics | common receiver | Pertaining to traces that have a different source but share a receiver. |
| Geophysics | common receiver gather | A display of seismic traces that share a receiver. |
| Geophysics | common reflection point | In multichannel seismic acquisition, the common midpoint on a reflector, or the halfway point when a wave travels from a source to a reflector to a receiver that is shared by numerous locations if the reflector is flat-lying. Like common depth point, this term is commonly misused, because in the case of dipping layers, common reflection points do not exist. |
| Geophysics | common source | Pertaining to traces that have a different receiver but share a source. |
| Geophysics | common source gather | A display of seismic traces that share a source. |
| Geophysics | common-offset | Pertaining to traces that have the same offset, or distance between source and receiver. |
| Geophysics | common-receiver | Pertaining to traces that have a different source but share a receiver. |
| Geophysics | common-receiver gather | A display of seismic traces that share a receiver. |
| Geophysics | common-source | Pertaining to traces that have a different receiver but share a source. |
| Geophysics | common-source gather | A display of seismic traces that share a source. |
| Geophysics | compaction correction | A change made to porosity measurements, such as those from sonic logs, to compensate for the lack of compaction, or the predicted loss of pore space as sediments are buried by overburden. Compaction corrections are commonly performed in uncompacted sediments. |
| Geophysics | complex trace analysis | A mathematical method to determine seismic attributes, including reflection strength and instantaneous frequency, by using the Hilbert transform, a special form of the Fourier transform, and the quadrature trace, or the component of the signal that is 90 degrees out of phase. The Fourier transform a(t) = h(t) + jx(t), where h(t) = seismic trace x(t) = quadrature trace can be used to determine reflection strength by combining h and x: r(t) = [h(t)2 + x(t)2]1/2, and to determine instantaneous phase: ?(t) = tan?1[x(t)/h(t)]. |
| Geophysics | complex-trace analysis | A mathematical method to determine seismic attributes, including reflection strength and instantaneous frequency, by using the Hilbert transform, a special form of the Fourier transform, and the quadrature trace, or the component of the signal that is 90 degrees out of phase. The Fourier transform a(t) = h(t) + jx(t), where h(t) = seismic trace x(t) = quadrature trace can be used to determine reflection strength by combining h and x: r(t) = [h(t)2 + x(t)2]1/2, and to determine instantaneous phase: ?(t) = tan?1[x(t)/h(t)]. |
| Geophysics | compressional wave | An elastic body wave or sound wave in which particles oscillate in the direction the wave propagates. P-waves are the waves studied in conventional seismic data. P-waves incident on an interface at other than normal incidence can produce reflected and transmitted S-waves, in that case known as converted waves. |
| Geophysics | conductance | The product of conductivity and thickness, typically measured in units of siemens (S). In the inversion of electrical and electromagnetic measurements, the conductance of a layer or zone is usually much better determined than either the conductivity or thickness individually. |
| Geophysics | conductance | The reciprocal of resistance in a direct current circuit, measured in siemens (formerly mhos). In an alternating current circuit, conductance is the resistance divided by the square of impedance, also measured in siemens. |
| Geophysics | convergence | In mathematics, the process in which a sequence of numbers approaches a fixed value called the “limit” of the sequence. This term is often used in modeling or inversion to describe the situation in which a sequence of calculated values approach, or converge with, measured values. |
| Geophysics | convergent | In mathematics, pertaining to the process in which a sequence of numbers approaches a fixed value called the “limit” of the sequence. This term is often used in modeling or inversion to describe the situation in which a sequence of calculated values approach, or converge with, measured values. |
| Geophysics | converted wave | A seismic wave that changes from a P-wave to an S-wave, or vice versa, when it encounters an interface. |
| Geophysics | convolution | A mathematical operation on two functions that is the most general representation of the process of linear (invariant) filtering. Convolution can be applied to any two functions of time or space (or other variables) to yield a third function, the output of the convolution. Although the mathematical definition is symmetric with respect to the two input functions, it is common in signal processing to say that one of the functions is a filter acting on the other function. The response of many physical systems can be represented mathematically by a convolution. For example, a convolution can be used to model the filtering of seismic energy by the various rock layers in the Earth; deconvolution is used extensively in seismic processing to counteract that filtering. The mathematical form of the convolution of two functions, a filter f(t) and a time-series x(t), is y(t) = ? f(t??)x(?)d?, where y(t) is the output of the convolution. In the frequency domain, convolution is simply the product of the Fourier transforms (FT) of the two functions: Y(?) = F(?)*X(?), where X(?) = FT of the time series x(t) F(?) = FT of the filter f(t) Y(?) = FT of the output y(t) ? = angular frequency. |
| Geophysics | convolve | To perform a convolution, which is a mathematical operation on two functions that is the most general representation of the process of linear (invariant) filtering. Convolution can be applied to any two functions of time or space (or other variables) to yield a third function, the output of the convolution. Although the mathematical definition is symmetric with respect to the two input functions, it is common in signal processing to say that one of the functions is a filter acting on the other function. The response of many physical systems can be represented mathematically by a convolution. For example, a convolution can be used to model the filtering of seismic energy by the various rock layers in the Earth; deconvolution is used extensively in seismic processing to counteract that filtering. |
| Geophysics | correlation | The comparison of seismic waveforms in the time domain, similar to coherence in the frequency domain. |
| Geophysics | coupling | The state of being attached to another entity: A well-planted geophone has a coupling to the Earth’s surface or to a borehole wall that allows it to record ground motion during acquisition of seismic data. |
| Geophysics | coupling | An electrical or mechanical device that joins parts of systems and can affect the interaction of, or energy transfer between, parts of systems. Electrical couplings promote the passage of certain signals but prevent the passage of others, such as an alternating current coupling that excludes direct current. |
| Geophysics | CRG | Abbreviation for common receiver gather. A display of seismic traces that share a receiver. |
| Geophysics | critical angle | The angle of incidence according to Snell’s law at which a refracted wave travels along the interface between two media. It can be quantified mathematically as follows: sin ?c = V1 / V2, where ?c = the critical angle V1 = velocity of the first medium V2 = velocity of the first medium, which is greater than V1. |
| Geophysics | critical damping | The minimum damping that will prevent or stop oscillation in the shortest amount of time, typically associated with oscillatory systems like geophones. Critical damping is symbolized by ?c. |
| Geophysics | critical reflection | A reflection, typically at a large angle, that occurs when the angle of incidence and the angle of reflection of a wave are equal to the critical angle. |
| Geophysics | crosscorrelate | The comparison of different waveforms in digital form to quantify their similarity. A normalized crosscorrelation, or a correlation coefficient, equal to unity indicates a perfect match, whereas a poor match will yield a value close to zero. |
| Geophysics | crosscorrelation | The comparison of different waveforms in digital form to quantify their similarity. A normalized crosscorrelation, or a correlation coefficient, equal to unity indicates a perfect match, whereas a poor match will yield a value close to zero. |
| Geophysics | crossline | A seismic line within a 3D survey perpendicular to the direction in which the data were acquired. |
| Geophysics | crustal magnetic field | The magnetic field associated with the Earth’s crust arises from induced and remanent magnetism. The crustal field—also referred to as the anomaly field—varies in direction and strength when measured over the Earth’s surface. It is relatively strong in the vicinity of ferrous and magnetic materials, such as in the oceanic crust and near concentrations of metal ores, and is a focus of geophysical mineral exploration. |
| Geophysics | cultural anomaly | A local geophysical anomaly generated by a man-made feature, such as electrical and communications wires, steel beams and tanks and railroad tracks. |
| Geophysics | cultural noise | Undesirable energy, or noise, generated by human activity, such as automobile traffic that interferes with seismic surveying, or electrical power lines or the steel in pipelines that can adversely affect electromagnetic methods. |
| Geophysics | curve fitting | The generation of a theoretical equation to define a given data set. In contrast, curve matching involves the comparison of well-understood data to a data set of interest. |
| Geophysics | curve matching | The graphical comparison of well-understood data sets, called type curves, to another data set. If a certain type curve closely corresponds to a data set, then an interpretation of similarity can be made, although, as Sheriff (1991) points out, there might be other type curves that also match the data of interest. Curve matching differs from curve fitting in that curve fitting involves theoretical models rather than actual examples. |
| Geophysics | cycle skip | An anomalously high transit time in a log, such as a continuous velocity log, observable as a spike on the log, commonly caused by the presence of fractures, gas, unconsolidated formations, aerated drilling mud and enlarged boreholes. |
| Geophysics | damping | The opposition, slowing or prevention of oscillation, or decreasing vibration amplitude, as kinetic energy dissipates. Frictional damping can be important in the use of geophones for seismic surveys, since a vibrating instrument is difficult to read. Eddy currents can produce electromagnetic damping. The classic example of damping from physics is the slowing of a swinging pendulum unless it has a steady supply of energy. |
| Geophysics | datum | An agreed and known value, such as the elevation of a benchmark or sea level, to which other measurements are corrected. In seismic data, the term refers to an arbitrary planar surface to which corrections are made and on which sources and receivers are assumed to lie to minimize the effects of topography and near-surface zones of low velocity. |
| Geophysics | datum correction | A value added to reflection times of seismic data to compensate for the location of the geophone and source relative to the seismic datum. |
| Geophysics | dB | The unit of measurement to compare the relative intensity of acoustic or electrical signal, equal to one-tenth of a bel, named for American inventor Alexander Graham Bell (1847 to 1922). The logarithm of the ratio of the sound or signal to a standard provides the decibel measurement. The symbol for the unit is dB. Humans typically hear sounds in the range of 20 to 50 dB in conversation, and upwards of 90 dB when exposed to heavy machinery or aircraft. |
| Geophysics | decibel | The unit of measurement to compare the relative intensity of acoustic or electrical signal, equal to one-tenth of a bel, named for American inventor Alexander Graham Bell (1847 to 1922). The logarithm of the ratio of the sound or signal to a standard provides the decibel measurement. The symbol for the unit is dB. Humans typically hear sounds in the range of 20 to 50 dB in conversation, and upwards of 90 dB when exposed to heavy machinery or aircraft. |
| Geophysics | deconvolution | A step in seismic signal processing to recover high frequencies, attenuate multiples, equalize amplitudes, produce a zero-phase wavelet or for other purposes that generally affect the waveshape. Deconvolution, or inverse filtering, can improve seismic data that were adversely affected by filtering, or convolution that occurs naturally as seismic energy is filtered by the Earth. Deconvolution can also be performed on other types of data, such as gravity, magnetic or well log data. |
| Geophysics | deep seismic sounding | A seismic profile recorded specifically to study the lower crust, the Mohorovicic discontinuity and the mantle of the Earth, typically using refraction methods. Most standard seismic reflection profiles record only a small fraction (typically, on the order of 10 km [6 miles]) of the Earth’s crust, which is 5 to 75 km [3 to 45 miles] thick. |
| Geophysics | deep tow | A method of marine seismic acquisition in which a boat tows a receiver well below the surface of the water to get closer to features of interest or to reduce noise due to conditions of the sea. Deep tow devices are used for some side-scan sonar, gravity and magnetic surveys. |
| Geophysics | density contrast | The variation in the mass per unit volume of rocks, which affects the local gravitational field of the Earth. A density contrast also contributes to an acoustic impedance contrast, which affects the reflection coefficient. |
| Geophysics | density profile | A series of gravity measurements made along a line or over an area of a locally high topographic feature to remove or compensate for the effect of topography on deeper density readings. |
| Geophysics | depth controller | A device used in acquisition of marine seismic data that keeps streamers at a certain depth in the water. |
| Geophysics | depth conversion | The process of transforming seismic data from a scale of time (the domain in which they are acquired) to a scale of depth to provide a picture of the structure of the subsurface independent of velocity. Depth conversion, ideally, is an iterative process that begins with proper seismic processing, seismic velocity analysis and study of well data to refine the conversion. Acoustic logs, check-shot surveys and vertical seismic profiles can aid depth conversion efforts and improve correlation of well logs and drilling data with surface seismic data. |
| Geophysics | depth map | A two-dimensional representation of subsurface structure with contours in depth that have been converted from seismic traveltimes. |
| Geophysics | depth migration | A step in seismic processing in which reflections in seismic data are moved to their correct locations in space, including position relative to shotpoints, in areas where there are significant and rapid lateral or vertical changes in velocity that distort the time image. This requires an accurate knowledge of vertical and horizontal seismic velocity variations. |
| Geophysics | depth point | A point on the surface for which the depth to a horizon has been calculated in a refraction seismic survey. The term is commonly misused as a synonym for common depth point. |
| Geophysics | depth section | A display of seismic data with a scale of units of depth rather than time along the vertical axis. Careful migration and depth conversion are essential for creating depth sections. |
| Geophysics | detectable limit | The minimum thickness necessary for a layer of rock to be visible or distinct in reflection seismic data. Generally, the detectable limit is at least 1/30 of the wavelength. Acquisition of higher frequency seismic data generally results in better detection or vertical resolution of thinner layers. |
| Geophysics | detector | A sensor or receiver, such as a geophone or hydrophone, gravimeter or magnetometer. |
| Geophysics | deterministic deconvolution | A type of inverse filtering, or deconvolution, in which the effects of the filter are known by observation or assumed, as opposed to statistical deconvolution. |
| Geophysics | detonator | A small, electrically activated explosive charge that explodes a larger charge. Detonators, also called caps, seismic caps or blasting caps, are used for seismic acquisition with an explosive source to achieve consistent timing of detonation. |
| Geophysics | dielectric | A material used in a capacitor to store a charge from an applied electrical field. A pure dielectric does not conduct electricity. |
| Geophysics | difference map | A map that represents the change from one map to another, such as a reservoir map of an area made from two different seismic surveys separated in production history (one possible product of 4D seismic data), or an isochron map that displays the variation in time between two seismic events or reflections. |
| Geophysics | differential weathering correction | A type of static correction that compensates for delays in seismic reflection or refraction times from one point to another, such as among geophone groups in a survey. These delays can be induced by low-velocity layers such as the weathered layer near the Earth’s surface. |
| Geophysics | diffraction | A type of event produced by the radial scattering of a wave into new wavefronts after the wave meets a discontinuity such as a fault surface, an unconformity or an abrupt change in rock type. Diffractions appear as hyperbolic or umbrella-shaped events on a seismic profile. Proper migration of seismic data makes use of diffracted energy to properly position reflections. |
| Geophysics | diffraction stack | Also known as Kirchhoff migration, a method of seismic migration that uses the integral form (Kirchhoff equation) of the wave equation. All methods of seismic migration involve the backpropagation (or continuation) of the seismic wavefield from the region where it was measured (Earth’s surface or along a borehole) into the region to be imaged. In Kirchhoff migration, this is done by using the Kirchhoff integral representation of a field at a given point as a (weighted) superposition of waves propagating from adjacent points and times. Continuation of the wavefield requires a background model of seismic velocity, which is usually a model of constant or smoothly varying velocity. Because of the integral form of Kirchhoff migration, its implementation reduces to stacking the data along curves that trace the arrival time of energy scattered by image points in the earth. |
| Geophysics | diffusion | The movement of ions or molecules from regions of high concentration to low concentration within a solution. |
| Geophysics | diffusion | The conduction of heat by the movement of molecules. |
| Geophysics | diffusion equation | A partial differential equation describing the variation in space and time of a physical quantity that is governed by diffusion. The diffusion equation provides a good mathematical model for the variation of temperature through conduction of heat and the propagation of electromagnetic waves in a highly conducting medium. The diffusion equation is a parabolic partial differential equation whose characteristic form relates the first partial derivative of a field with respect to time to its second partial derivatives with respect to spatial coordinates. It is closely related to the wave equation. ?2E = j ? ? ? E, where E = electrical field ? = angular frequency ? = magnetic permeability ? = electrical conductivity ? = vector differential operator. |
| Geophysics | dilatancy | The increase in the volume of rocks as a result of deformation, such as when fractures develop. |
| Geophysics | dilatancy theory | A possible explanation for volume changes in rocks due to strain, such as microfracturing or cracking, and the accompanying change in the ratio of P- to S-wave velocity. Support for dilatancy theory comes in the form of porosity increases from 20 to 40% that have been measured in laboratory experiments using rock samples. |
| Geophysics | dilatation | The process of changing volume as stress is applied to a body. |
| Geophysics | dilatation | The volumetric strain produced by applying stress to a body. |
| Geophysics | dilatation | A rarefaction, or decrease in pressure and density of a medium as molecules are displaced by a P-wave. As P-waves pass through the Earth, the Earth undergoes compression and expansion. These changes in volume contribute to the positive and negative amplitudes of a seismic trace. |
| Geophysics | dilatational wave | An elastic body wave or sound wave in which particles oscillate in the direction the wave propagates. P-waves are the waves studied in conventional seismic data. P-waves incident on an interface at other than normal incidence can produce reflected and transmitted S-waves, in that case known as converted waves. |
| Geophysics | dim spot | A type of local seismic event that, in contrast to a bright spot, shows weak rather than strong amplitude. The weak amplitude might correlate with hydrocarbons that reduce the contrast in acoustic impedance between the reservoir and the overlying rock, or might be related to a stratigraphic change that reduces acoustic impedance. |
| Geophysics | dip moveout | The difference in the arrival times or traveltimes of a reflected wave, measured by receivers at two different offset locations, that is produced when reflectors dip. Seismic processing compensates for DMO. |
| Geophysics | dip moveout | The procedure in seismic processing that compensates for the effects of a dipping reflector. DMO processing was developed in the early 1980s. |
| Geophysics | dipole | A pair of opposite (and equal) electrical charges. The strength of the dipole is a vector quantity whose direction points from the positive to the negative charge and whose magnitude is the product of the absolute value of the charge times the separation. A point dipole is an idealized mathematical representation of a dipole in which the separation of the charges goes to zero while their charge increases so that the product (dipole strength) remains constant. |
| Geophysics | dipole | Two poles of opposite polarity that can generate a field, such as an electric or magnetic field or a dipole source and dipole receiver used in sonic logging for excitation and detection of shear waves. |
| Geophysics | dipole | A small antenna used in electromagnetic surveying that can be represented mathematically as a dipole. |
| Geophysics | dipole field | The primary contribution to Earth’s main magnetic field. |
| Geophysics | directivity | The property of some seismic sources whereby the amplitude, frequency, velocity or other property of the resulting seismic waves varies with direction. A directional charge, such as a length of primer cord or a linear array of charges, can be used when directivity is desirable. Directivity is also a property of geophone arrays, air guns, explosives or vibrators, which can be positioned to reduce horizontal traveling noise such as ground roll. Receivers in the form of groups in which the individual geophones or hydrophones are separated from each other in linear (1D) or areal (2D) arrays are directional, and are designed to suppress signal arriving nearly horizontally and to pass nearly vertical arrivals with minimum attenuation or distortion. Directivity is often present, but the difficulty in accounting for it during seismic processing makes it undesirable in most cases. |
| Geophysics | discontinuity | A subsurface boundary or interface at which a physical quantity, such as the velocity of transmission of seismic waves, changes abruptly. The velocity of P-waves increases dramatically (from about 6.5 to 8.0 km/s) at the Mohorovicic discontinuity between the Earth’s crust and mantle. |
| Geophysics | dispersion | A type of distortion of a wave train in which the velocity of the wave varies with frequency. Surface waves and electromagnetic body waves typically exhibit dispersion, whereas P-waves in most rocks show little change in velocity with frequency. |
| Geophysics | dispersion | The phenomenon of a wave separating into its frequency constituents as it passes through a medium. Each frequency component travels at its phase velocity (vp), which is the product of the frequency (f) and wavelength (?) of that component. Angular dispersion results from anisotropy, which causes velocity to vary with direction. Dispersion affects all types of waves, including light, electromagnetic, sound, elastic, gravity and water waves. |
| Geophysics | distortion | The inability of a system to exactly match input and output, a general example being an electronic amplifier and the classic example being a home stereophonic amplifier. |
| Geophysics | diurnal variation | The daily variation in properties of the Earth, such as the temperature or the local geomagnetic field, or the daily change in sunlight. Such variations depend in part on latitude, proximity to the ocean, the effects of solar radiation and tides and other factors. |
| Geophysics | divergence | The loss of energy from a wavefront as a consequence of geometrical spreading, observable as a decrease in wave amplitude. Spherical divergence decreases energy with the square of the distance. Cylindrical divergence decreases energy with the distance. |
| Geophysics | divergence | In mathematics, a process in which a sequence of numbers does not tend to a fixed limit (the opposite of convergence). Divergence is a mathematical property of a vector field that is a local measure of its rate of spreading. |
| Geophysics | divergence | In Cartesian coordinates, divergence is the sum of the partial derivatives of each component of the vector field with respect to the corresponding spatial coordinate: div V = ?·V = ?Vx/?x + ?Vy/?y + ?Vz/?z |
| Geophysics | Dix formula | An equation used to calculate the interval velocity within a series of flat, parallel layers, named for American geophysicist C. Hewitt Dix (1905 to 1984). Sheriff (1991) cautions that the equation is misused in situations that do not match Dix’s assumptions. The equation is as follows: Vint = [(t2 VRMS22 ? t1 VRMS12) / (t2 ? t1)]1/2, where Vint = interval velocity t1 = traveltime to the first reflector t2 = traveltime to the second reflector VRMS1 = root-mean-square velocity to the first reflector VRMS2 = root-mean-square velocity to the second reflector. |
| Geophysics | DMO | The difference in the arrival times or traveltimes of a reflected wave, measured by receivers at two different offset locations, that is produced when reflectors dip. Seismic processing compensates for DMO. |
| Geophysics | domain | The set of values an independent variable can take. For example, the independent variable of the time domain is time; and for the frequency domain, it is frequency. |
| Geophysics | doodlebugger | Slang term to describe a seismologist performing seismic field work. |
| Geophysics | double refraction | The splitting of an incident wave into two waves of different velocities and orthogonal polarizations. Double refraction, or birefringence, occurs in optical mineralogy (see petrography) when plane-polarized light passes through an anisotropic mineral and emerges as two rays traveling at different speeds, the difference between which is characteristic of a mineral. In seismology, incident S-waves can exhibit birefringence as they split into a quasi-shear and a pure-shear wave. Although birefringence was first described by Danish physician Erasmus Bartholin (1625 to 1698) in crystals in 1669, the phenomenon was not fully understood until French physicist Etienne-Louis Malus (1775 to 1812) described polarized light in 1808. |
| Geophysics | downhole receiver | A receiver located in a wellbore, as opposed to a location on the Earth’s surface. |
| Geophysics | downhole source | A seismic source located in a wellbore rather than at the Earth’s surface. |
| Geophysics | downward continuation | A technique used to estimate the value of a potential field or seismic data at a surface beneath a measured surface. The method is risky because it assumes continuity of the field, so anomalies affect predictions, especially if they occur beneath the measured surface. Noise can be exaggerated and affect calculations adversely. |
| Geophysics | drift | In calibration of a check-shot survey, the difference between geometrically corrected transit time and integrated sonic time. |
| Geophysics | drift | A gradual change in a measurement or recording device during surveying. Reference to or repetition of a measurement at a base station can indicate whether drift is a problem. |
| Geophysics | drill noise vertical seismic profile | A technique for acquiring a vertical seismic profile that uses the noise of the drill bit as a source and receivers laid out along the ground or seabed. In deep water, the receiver arrays can be deployed vertically. Acquisition and processing are typically more challenging than in the more conventional types of VSPs, but the technique can yield time-depth information and, less frequently, reflection information, while the well is being drilled. The information from a drill-noise VSP can be used to improve time-depth conversions while drilling, decide where to set casing in a well and evaluate drilling hazards, such as anomalous pore pressure |
| Geophysics | drill-noise vertical seismic profile | A technique for acquiring a vertical seismic profile that uses the noise of the drill bit as a source and receivers laid out along the ground or seabed. In deep water, the receiver arrays can be deployed vertically. Acquisition and processing are typically more challenging than in the more conventional types of VSPs, but the technique can yield time-depth information and, less frequently, reflection information, while the well is being drilled. The information from a drill-noise VSP can be used to improve time-depth conversions while drilling, decide where to set casing in a well and evaluate drilling hazards, such as anomalous pore pressure. |
| Geophysics | dropout | The loss of information from a magnetic tape that occurs if the tape is damaged or exposed to dirt. |
| Geophysics | dropout | The failure of a channel or geophone to record a shot or shots in a seismic survey, which results in a loss of data. |
| Geophysics | DSS | A seismic profile recorded specifically to study the lower crust, the Mohorovicic discontinuity and the mantle of the Earth, typically using refraction methods. Most standard seismic reflection profiles record only a small fraction (typically, on the order of 10 km [6 miles]) of the Earth’s crust, which is 5 to 75 km [3 to 45 miles] thick. |
| Geophysics | dynamic correction | A time-variant operation performed on seismic data. Normal moveout (NMO) is a dynamic correction. |
| Geophysics | dynamic range | The ratio of or difference between the highest and the lowest reading, or strongest and weakest signal, that can be recorded or reproduced by an instrument without distortion. |
| Geophysics | dynamite | A type of explosive used as a source for seismic energy during data acquisition. Originally, dynamite referred specifically to a nitroglycerin-based explosive formulated in 1866 by Alfred Bernhard Nobel (1833 to 1896), the Swedish inventor who endowed the Nobel prizes. The term is incorrectly used to mean any explosive rather than the original formulation. |
| Geophysics | EDA | A form of azimuthal anisotropy that occurs when fractures or microcracks are not horizontal. Waves that travel parallel to the fractures have a higher velocity than waves traveling perpendicular to fractures. |
| Geophysics | eddy current | An alternating or transient electrical current in a conductive medium in the presence of a time-varying magnetic field. The eddy current generates its own electromagnetic field. |
| Geophysics | eel | A hydrophone array in a cable that can be attached to a streamer for acquisition of marine seismic data. The eel can be suspended from the streamer so that the eel is close to the seafloor but the streamer remains high enough to avoid obstacles on the seafloor such as reefs or debris from human activity. |
| Geophysics | elastic anisotropy | The variation of elastic properties with direction. For example, elastic anisotropy occurs when seismic, or elastic, waves travel through rock at differing velocities in various directions. Elastic anisotropy occurs if there is a preferred alignment of a material’s fabric elements—crystals, grains, cracks, bedding planes, joints or fractures—on a scale smaller than the length of the wave. This alignment causes waves to propagate fastest in its direction. Elastic anisotropy is sometimes called seismic anisotropy, velocity anisotropy, traveltime anisotropy, acoustic anisotropy or slowness anisotropy. |
| Geophysics | elastic constants | A set of constants, also known as elastic moduli, that defines the properties of material that undergoes stress, deforms, and then recovers and returns to its original shape after the stress ceases. The elastic constants include the bulk modulus, Lame constant, Poisson’s ratio, shear modulus, and Young’s modulus. Elastic constants are important in seismology because the velocity of waves depends on the elastic constants and density of the rock. |
| Geophysics | elastic deformation | A temporary change in shape caused by applied stress. The change in shape is not permanent and the initial shape is completely recovered once the stress is removed. |
| Geophysics | elastic impedance | The product of the density of a medium and its shear wave velocity. |
| Geophysics | elastic moduli | A set of constants that defines the properties of material that undergoes stress, deforms, and then recovers and returns to its original shape after the stress ceases. The elastic constants include the bulk modulus, Lame constant, Poisson’s ratio, shear modulus, and Young’s modulus. Elastic constants are important in seismology because the velocity of waves depends on the elastic constants and density of the rock. |
| Geophysics | elastic wave | A seismic or acoustic wave, such as a P-wave. |
| Geophysics | electrical conductivity | The ability of a material to support the flow of an electrical current. In linear isotropic materials, the electric current density at any point in space is proportional to the electric field; the constant of proportionality is the electrical conductivity. Conductivity is the inverse of resistivity in isotropic materials, and is measured in siemens per meter or the archaic units of mhos per meter. The electrical conductivity of the Earth can be measured by electromagnetic methods. |
| Geophysics | electrical permittivity | The ability of a material to store a charge from an applied electrical field without conducting electricity. |
| Geophysics | electrical resistivity | The ability of a material to resist or inhibit the flow of an electrical current, measured in ohm-meters. Resistivity is the reciprocal of conductivity. |
| Geophysics | electromagnetic method | A group of techniques in which natural or artificially generated electric or magnetic fields are measured at the Earth’s surface or in boreholes in order to map variations in the Earth’s electrical properties (resistivity, permeability or permittivity). Most applications of surface electromagnetic methods today are for mineral and groundwater exploration or for shallow environmental mapping. Electromagnetic or electrical logging is, however, the main technique used in oil exploration to measure the amount of hydrocarbons in the pores of underground reservoirs. Inductive electromagnetic (EM) methods include a variety of low frequency (a few Hz to several kHz) techniques deploying large or small wire coils at or near the surface. In older usage, “electromagnetic method” tended to refer only to inductive methods. This term is now commonly used for any method employing electromagnetic fields, including methods that use direct current (electrical or resistivity methods) and induced polarization (IP), methods that use microwave frequencies (ground-penetrating radar), and methods that use natural electromagnetic fields (magnetotelluric methods). |
| Geophysics | elevation correction | Any compensating factor used to bring measurements to a common datum or reference plane. In gravity surveying, elevation corrections include the Bouguer and free-air corrections. Seismic data undergo a static correction to reduce the effects of topography and low-velocity zones near the Earth’s surface. Well log headers include the elevation of the drilling rig’s kelly bushing and, for onshore locations, the height of the location above sea level, so that well log depths can be corrected to sea level. |
| Geophysics | embedded wavelet | The shape of a wavelet produced by reflection of an actual wave train at one interface with a positive reflection coefficient. The embedded wavelet is useful for generating a convolutional model, or the convolution of an embedded wavelet with a reflectivity function and random noise, during seismic processing or interpretation. |
| Geophysics | epsilon (?) | A P-wave parameter for a medium in which the elastic properties exhibit vertical transverse isotropy. Epsilon (?) is the P-wave anisotropy parameter and equal to half the ratio of the difference between the horizontal and vertical P-wave velocities squared divided by the vertical P-wave velocity squared. ? ? ½ [(C11 ? C33) / C33] = ½ [(VP?2 ? VP?2) / VP?2] P-wave parameter (?) for a medium in which the elastic properties exhibit vertical transverse isotropy, where C11 is the horizontal P-wave modulus (perpendicular to the symmetry axis), C33 is the vertical P-wave modulus (parallel to the symmetry axis), VP? is the horizontal P-wave velocity and VP? is the vertical P-wave velocity. Reference: Thomsen L: “Weak Elastic Anisotropy,” Geophysics 51, no. 10 (October 1986): 1954–1966. |
| Geophysics | equipotential method | A technique to map a potential field generated by stationary electrodes by moving an electrode around the survey area. |
| Geophysics | eta (?) | A measure of the anellipticity of the P-wave phase slowness—the inverse of P-wave phase velocity—in rock exhibiting vertical transverse isotropy. ? = (? ? ?) / (1 + 2?) Anellipticity of P-wave phase slowness for a medium in which the elastic properties exhibit vertical transverse isotropy. Eta (?) is the anellipticity and ? and ? are the P-wave anisotropy parameters. When ? and ? are equal, ? = 0 and the P-wave phase slowness is an ellipse. When ? = ? = 0, the P-wave phase slowness is isotropic. Reference: Alkhalifah T and Tsvankin I: “Velocity Analysis for Transversely Isotropic Media,” Geophysics 60, no. 5 (September–October 1995): 1550–1566. |
| Geophysics | event | An appearance of seismic data as a diffraction, reflection, refraction or other similar feature produced by an arrival of seismic energy. An event can be a single wiggle within a trace, or a consistent lining up of several wiggles over several traces. An event in a seismic section can represent a geologic interface, such as a fault, unconformity or change in lithology. |
| Geophysics | explosive seismic data | Surface seismic data acquired using an explosive energy source, such as dynamite. |
| Geophysics | extended spread | An in-line offset spread. |
| Geophysics | extensive dilatancy anisotropy | A form of azimuthal anisotropy that occurs when fractures or microcracks are not horizontal. Waves that travel parallel to the fractures have a higher velocity than waves traveling perpendicular to fractures. |
| Geophysics | external disturbance field | A magnetic disturbance field generated by electric currents flowing in the ionosphere and magnetosphere and “mirror-currents” induced in the Earth and oceans by the external magnetic field time variations. The disturbance field, which is associated with diurnal field variations and magnetic storms, is affected by solar activity (solar wind), the interplanetary magnetic field and the Earth’s magnetic field. The external magnetic field exhibits variations on several time scales, which may affect the applicability of magnetic reference models. Very long-period variations are related to the solar cycle of about 11 years. Short-term variations result from daily changes in solar radiation, atmospheric tides and conductivity. Irregular time variations are influenced by the solar wind. Perturbed magnetic states, called magnetic storms, occur and show impulsive and unpredictable rapid time variations. |
| Geophysics | fan shooting | A technique for acquiring seismic refraction data around local, high-velocity features such as salt domes by using a fan or arc-shaped geophone array around a central shotpoint. The data from the fan-shaped array are calibrated against a control profile acquired some distance from the anomalous feature. |
| Geophysics | fast Fourier transform | An iterative computer algorithm to perform the Fourier transform of digitized waveforms rapidly. |
| Geophysics | Faye correction | In gravity surveying, a correction of 0.3086 mGal/m [0.09406 mGal/ft] added to a measurement to compensate for the change in the gravitational field with height above sea level, assuming there is only air between the measurement station and sea level. |
| Geophysics | FD | In seismic surveying or processing, the use of a function of frequency rather than time to express an independent variable or measurement. In contrast, in the time domain, variables are expressed as a function of time instead of frequency. |
| Geophysics | Fermat’s principle | The principle that the path taken by a ray of light from one point to another is that which takes the minimum time (or the maximum time in select cases), named for its discoverer, French mathematician Pierre de Fermat (1601 to 1665). Snell’s law and the laws of reflection and refraction follow from Fermat’s principle. Fermat’s principle also applies to seismic waves. |
| Geophysics | FFT | An iterative computer algorithm to perform the Fourier transform of digitized waveforms rapidly. |
| Geophysics | field tape | A magnetic tape containing data recorded in the field, abbreviated FT. |
| Geophysics | filter | A process or algorithm using a set of limits used to eliminate unwanted portions of seismic data, commonly on the basis of frequency or amplitude, to enhance the signal-to-noise ratio of the data or to achieve deconvolution. |
| Geophysics | filter | To remove undesirable portions of data during seismic processing to increase the signal-to-noise ratio of seismic data. Filtering can eliminate certain frequencies, amplitudes or other information. |
| Geophysics | first arrival | The earliest arrival of energy propagated from the energy source at the surface to the geophone in the wellbore in vertical seismic profiles and check-shot surveys, or the first indication of seismic energy on a trace. On land, first breaks commonly represent the base of weathering and are useful in making static corrections. |
| Geophysics | first break | The earliest arrival of energy propagated from the energy source at the surface to the geophone in the wellbore in vertical seismic profiles and check-shot surveys, or the first indication of seismic energy on a trace. On land, first breaks commonly represent the base of weathering and are useful in making static corrections. |
| Geophysics | fixed source method | An acquisition technique commonly used in electromagnetic methods whereby the energy source or transmitter is kept in the same position, and detectors or receivers are moved to different spots to compile a profile or map. |
| Geophysics | fixed-source method | An acquisition technique commonly used in electromagnetic methods whereby the energy source or transmitter is kept in the same position, and detectors or receivers are moved to different spots to compile a profile or map. |
| Geophysics | f-k domain | The use of frequency (abbreviated as f) and wavenumber (k, the reciprocal of wavelength) as the reference framework, obtained by using the Fourier transform over time and space. |
| Geophysics | f-k plot | A graphical technique to distinguish subsets of data according to their direction and velocity by plotting and contouring frequency and wavenumber. |
| Geophysics | flattened section | A seismic section that has been redisplayed such that a reflection of interest not horizontal in the original display appears horizontal and flat. Such displays can shed light on geological conditions at the time a given sedimentary layer accumulate |
| Geophysics | footprint | The area covered by an array of towed streamers in marine seismic acquisition. |
| Geophysics | forward problem | The practice of taking a model and calculating what the observed values should be, such as predicting the gravity anomaly around a salt dome using a gravity model or predicting the traveltime of a seismic wave from a source to a receiver using a velocity model. |
| Geophysics | four component seismic data | Four-component (4C) borehole or marine seismic data are typically acquired using three orthogonally-oriented geophones and a hydrophone within an ocean-bottom sensor (deployed in node-type systems as well as cables). Provided the system is in contact with the seabed or the borehole wall, the addition of geophones allows measurement of shear (S) waves, whereas the hydrophone measures compressional (P) waves. |
| Geophysics | four dimensional seismic data | Three-dimensional (3D) seismic data acquired at different times over the same area to assess changes in a producing hydrocarbon reservoir with time. Changes may be observed in fluid location and saturation, pressure and temperature. 4D seismic data is one of several forms of time-lapse seismic data. Such data can be acquired on the surface or in a borehole. |
| Geophysics | four-component seismic data | Four-component (4C) borehole or marine seismic data are typically acquired using three orthogonally-oriented geophones and a hydrophone within an ocean-bottom sensor (deployed in node-type systems as well as cables). Provided the system is in contact with the seabed or the borehole wall, the addition of geophones allows measurement of shear (S) waves, whereas the hydrophone measures compressional (P) waves. |
| Geophysics | four-dimensional seismic data | Three-dimensional (3D) seismic data acquired at different times over the same area to assess changes in a producing hydrocarbon reservoir with time. Changes may be observed in fluid location and saturation, pressure and temperature. 4D seismic data is one of several forms of time-lapse seismic data. Such data can be acquired on the surface or in a borehole. |
| Geophysics | Fourier analysis | The process of decomposing a function of time or space into a sum (or integral) of sinusoidal functions (sines or cosines) with specific amplitudes and phases. |
| Geophysics | Fourier synthesis | The process of reconstructing a function of time or space from its sinusoidal components determined in Fourier analysis. |
| Geophysics | Fourier transform | A set of mathematical formulas used to convert a time function, such as a seismic trace, to a function in the frequency domain (Fourier analysis) and back (Fourier synthesis). The function is expressed as a convergent trigonometric series, similar to that first formulated by French mathematician Jean-Baptiste-Joseph, Baron Fourier (1768 to 1830). The Fourier transform is used extensively in signal processing to design filters and remove coherent noise. Many filtering operations are performed in the frequency domain. The Fourier transform has applications in image analysis and in pattern recognition in geological systems. |
| Geophysics | free air correction | In gravity surveying, a correction of 0.3086 mGal/m [0.09406 mGal/ft] added to a measurement to compensate for the change in the gravitational field with height above sea level, assuming there is only air between the measurement station and sea level. |
| Geophysics | free-air correction | In gravity surveying, a correction of 0.3086 mGal/m [0.09406 mGal/ft] added to a measurement to compensate for the change in the gravitational field with height above sea level, assuming there is only air between the measurement station and sea level. |
| Geophysics | frequency | The rate of repetition of complete wavelengths of electrical signals, light, sound and seismic waves measured in cycles per second, or hertz, and symbolized by f. Typical recorded seismic frequencies are in the range of 5 to 100 hertz. |
| Geophysics | frequency domain | In seismic surveying or processing, the use of a function of frequency rather than time to express an independent variable or measurement. In contrast, in the time domain, variables are expressed as a function of time instead of frequency. |
| Geophysics | Fresnel zone | A frequency- and range-dependent area of a reflector from which most of the energy of a reflection is returned and arrival times differ by less than half a period from the first break, named for French physicist Augustin-Jean Fresnel (1788 to 1827). Waves with such arrival times will interfere constructively and so be detected as a single arrival. Subsurface features smaller than the Fresnel zone usually cannot be detected using seismic waves. |
| Geophysics | FT | A magnetic tape containing data recorded in the field, abbreviated FT |
| Geophysics | FT | A set of mathematical formulas used to convert a time function, such as a seismic trace, to a function in the frequency domain (Fourier analysis) and back (Fourier synthesis). The function is expressed as a convergent trigonometric series, similar to that first formulated by French mathematician Jean-Baptiste-Joseph, Baron Fourier (1768 to 1830). The Fourier transform is used extensively in signal processing to design filters and remove coherent noise. Many filtering operations are performed in the frequency domain. The Fourier transform has applications in image analysis and in pattern recognition in geological systems. |
| Geophysics | Full-azimuth towed-streamer acquisition | A single-vessel technique of acquiring marine seismic data at a complete range of azimuths by towing streamers in a circular path. |
| Geophysics | gain | The change in the amplitude of an electrical signal from the original input to the amplified output. |
| Geophysics | gamma (?) | An S-wave parameter for a medium in which the elastic properties exhibit vertical transverse isotropy. Gamma (?) is the S-wave anisotropy parameter and is equal to half the ratio of the difference between the horizontally and vertically traveling SH-wave velocities squared divided by the vertically traveling SH-wave velocity squared; an SH-wave is a shear wave that is horizontally polarized. ? ? ½ [(C66 ? C44) / C44] = ½ [(VSH?2 ? VSH?2) / VSH?2] S-wave parameter (?) for a medium in which the elastic properties exhibit vertical transverse isotropy, where C66 is the modulus for a horizontally polarized and horizontally traveling S-wave (perpendicular to the symmetry axis), C44 is the modulus for a horizontally polarized and vertically traveling S-wave (parallel to the symmetry axis), VSH? is the velocity for a horizontally polarized and horizontally traveling S-wave and VSH? is the velocity for a horizontally polarized and vertically traveling S-wave. |
| Geophysics | gas chimney | A subsurface leakage of gas from a poorly sealed hydrocarbon accumulation. The gas can cause overlying rocks to have a low velocity. Gas chimneys are visible in seismic data as areas of poor data quality or push-downs. |
| Geophysics | gather | A display of seismic traces that share an acquisition parameter, such as a common midpoint gather, which contains traces having a common midpoint. |
| Geophysics | geomagnetic secular variation | How Earth’s magnetic field varies with time. These time variations, called secular variations, necessitate periodic updating of magnetic field maps and models. Two types of processes in the Earth’s core produce these variations. One process is related to variations in Earth’s main dipole field, which operate on time scales of hundreds or thousands of years. The other process is related to variations in Earth’s nondipole field, which operate on time scales on the order of tens of years. |
| Geophysics | geometric | Pertaining to variation of the survey geometry while maintaining the frequency of electromagnetic surveying. In contrast, parametric pertains to keeping frequency the same while varying the geometry. |
| Geophysics | geophone | A device used in surface seismic acquisition, both onshore and on the seabed offshore, that detects ground velocity produced by seismic waves and transforms the motion into electrical impulses. Geophones detect motion in only one direction. Conventional seismic surveys on land use one geophone per receiver location to detect motion in the vertical direction. Three mutually orthogonal geophones are typically used in combination to collect 3C seismic data. Hydrophones, unlike geophones, detect changes in pressure rather than motion. |
| Geophysics | geophone array | A geometrical arrangement of seismic receivers (geophones) with signals recorded by one channel. The array can contain numerous closely spaced geophones. |
| Geophysics | geophone cable | A bundle of electrical wires that connects geophones and relays data to the seismic recording truck or seismic vessel. |
| Geophysics | geophone interval | The distance between geophones or the centers of groups of geophones. |
| Geophysics | geophone offset | In surface seismic acquisition, the horizontal distance from source to geophone. In a vertical seismic profile, geophone offset is the horizontal distance between the source and the wellhead or the surface projection of the geophone in the case of a deviated well. Offset between seismic source and receiver creates a delay, or moveout, in the arrival time of a reflection that can be corrected before stacking and can be used to determine velocity. |
| Geophysics | geophone pattern | A geometrical arrangement of seismic receivers (geophones) with signals recorded by one channel. The array can contain numerous closely spaced geophones. |
| Geophysics | geophysicist | A scientist trained in the study of the physics of the Earth, particularly its electrical, gravitational and magnetic fields and propagation of elastic (seismic) waves within it. In the petroleum industry, geophysicists perform a variety of functions, chiefly the processing and interpretation of seismic data and generation of subsurface maps on the basis of seismic data. Such interpretations enhance understanding of subsurface geology. |
| Geophysics | geophysics | The study of the physics of the Earth, especially its electrical, gravitational and magnetic fields and propagation of elastic (seismic) waves within it. Geophysics plays a critical role in the petroleum industry because geophysical data are used by exploration and development personnel to make predictions about the presence, nature and size of subsurface hydrocarbon accumulations. |
| Geophysics | ghost | A short-path multiple, or a spurious reflection that occurs when seismic energy initially reverberates upward from the shallow subsurface and then is reflected downward, such as at the base of weathering or between sources and receivers and the sea surface. |
| Geophysics | Gibbs’ phenomenon | The ringing near a discontinuity in a signal that is caused by incomplete Fourier synthesis, or missing frequencies. |
| Geophysics | gravimeter | A device used to measure the acceleration due to gravity, or, more specifically, variations in the gravitational field between two or more points. |
| Geophysics | gravimetry | The measurement of gravity or the study of its variations. |
| Geophysics | gravity | The Earth’s gravitational field, or the attractive force produced by the mass of the Earth. Variations in the gravitational field can be used to map changes in the density of formations in the Earth. Gravity surveys can be used to map the extent or depth of sedimentary basins or even individual hydrocarbon prospects. |
| Geophysics | gravity anomaly | The difference between the actual value of gravity measured at a location and the value predicted by a particular Earth model. Gravity anomalies are usually determined by adjusting the known value of (absolute) gravity at a reference station by Bouguer, free-air or other corrections and subtracting the final predicted value from the measurement. (A different description is that the various corrections are subtracted from the data to reduce it to the reference level. Both interpretations are valid provided it is remembered that the resulting gravity anomaly can be caused by density anomalies-i.e., differences in density between Earth and the theoretical model-that can lie anywhere either above or below the reference level.) |
| Geophysics | gravity survey | The measurement of gravitational acceleration over an area, usually presented as a map or profile of Bouguer or free-air anomalies. |
| Geophysics | grid | A regular spatial arrangement of points, such as x-y coordinates. |
| Geophysics | grid | To convert irregularly spaced points to a regular spacing by interpolation. |
| Geophysics | ground roll | A type of coherent noise generated by a surface wave, typically a low-velocity, low-frequency, high-amplitude Rayleigh wave. Ground roll can obscure signal and degrade overall data quality, but can be alleviated through careful selection of source and geophone arrays, filters and stacking parameters. |
| Geophysics | group | A set of seismometers whose output is sent to a common data channel to record a seismic trace. A large group is known as a patch. |
| Geophysics | group interval | The distance between geophones or groups of geophones. |
| Geophysics | group velocity | The velocity that wave energy—comprised of a wave group, train or packet of individual wave phases or components—travels through a medium. The wave energy may be grouped into an envelope that is shrink-wrapped around it. The shape of the envelope around the wave-energy group changes with distance because the individual wave phases move apart from one another. vg = vp ? ? (?vp/??) = vp + f (?vp/?f) Relation of group velocity to phase velocity. As a wave travels through a medium, its energy moves at the group velocity (vg) and its individual phases, or components, move at their phase velocity (vp). The wave changes shape with distance as each frequency (f), or wavelength (?), component moves at its separate phase velocity through the phenomenon of dispersion. Relative to the group velocity, each component moves with faster or slower phase velocity, depending on how phase velocity changes with wavelength or frequency. |
| Geophysics | guided wave | A type of elastic wave propagated and confined in a layer whose velocity is lower than that of the surrounding layers, such as a layer of coal. |
| Geophysics | gun | Abbreviation for air gun or water gun. An air gun is a source of seismic energy used in acquisition of marine seismic data. This gun releases highly compressed air into water. Air guns are also used in water-filled pits on land as an energy source during acquisition of vertical seismic profiles. A water gun is a source of energy for acquisition of marine seismic data that shoots water from a chamber in the tool into a larger body of water, creating cavitation. The cavity is a vacuum and implodes without creating secondary bubbles. This provides a short time signature and higher resolution than an air-gun source. |
| Geophysics | halo effect | An anomaly that occurs as a ring around a feature, such as electrical or geochemical rings around hydrocarbon accumulations. |
| Geophysics | harmonic distortion | A nonlinear change in waveform in which simple multiples of (1,2, … n times) the input frequencies, or harmonics, are generated. |
| Geophysics | head wave | A wave entering a relatively high-velocity medium whose incident and refracted angle is the critical angle. |
| Geophysics | header | The location, acquisition and processing parameters, and other pertinent information attached to a well log, seismic record and traces. |
| Geophysics | hertz | The unit of measurement of frequency, equivalent to one cycle per second and symbolized by Hz. The unit is named after German physicist Heinrich Hertz (1857 to 1894), who discovered electromagnetic waves. |
| Geophysics | hodogram | A graph or curve that displays time versus distance of motion. |
| Geophysics | hodogram | A crossplot of two components of particle motion over a time window. Hodograms are used in borehole seismology to determine arrival directions of waves and to detect shear-wave splitting. Data recorded along two geophone axes are displayed as a function of time. |
| Geophysics | horizon | An interface that might be represented by a seismic reflection, such as the contact between two bodies of rock having different seismic velocity, density, porosity, fluid content or all of those. |
| Geophysics | horizon slice | A map view of a particular reflection in a 3D seismic survey, as opposed to a horizontal (depth) slice or at a given time (a time slice). Slices are convenient displays for visual inspection of seismic attributes, especially amplitude. |
| Geophysics | horizontal transverse isotropy | Transverse isotropy that has a horizontal axis of rotational symmetry. In vertically fractured rocks, properties are uniform in vertical planes parallel to the fractures, but vary in the direction perpendicular to the fractures and across the fractures. |
| Geophysics | HTI | Abbreviation for horizontal transverse isotropy. Transverse isotropy that has a horizontal axis of rotational symmetry. In vertically fractured rocks, properties are uniform in vertical planes parallel to the fractures, but vary in the direction perpendicular to the fractures and across the fractures. |
| Geophysics | hydrocarbon indicator | A type of seismic amplitude anomaly, seismic event, or characteristic of seismic data that can occur in a hydrocarbon-bearing reservoir. Although bright spots, as hydrocarbon indicators are loosely called, can originate in numerous ways, they are not all indicative of the presence of hydrocarbons. Criteria to distinguish true hydrocarbon indicators (sometimes called HCIs) from other types of amplitude anomalies include: amplitude variation with offset bright or dim spot(s) in amplitude as a result of variations in lithology and pore fluids, sometimes occurring in groups of stacked reservoirs change or reversal in polarity because of velocity changes, also called phasing conformity with local structures diffractions that emanate from fluid contacts flat spot that represents a fluid (gas-oil or gas-water) contact, which can also show the downdip limit of the reservoir in some cases gas chimneys above leaking reservoirs shadow zones below the accumulation velocity push-down because of lower velocities of hydrocarbons than rocks difference in response between reflected pressure and shear energy. Hydrocarbon indicators are most common in relatively young, unconsolidated siliciclastic sediments with large impedance contrasts across lithologic boundaries, such as those in the Gulf of Mexico and offshore western Africa. An ongoing issue in exploration for hydrocarbon indicators is the difficulty in distinguishing between gas accumulations and water with a low degree of gas saturation (fizz water). |
| Geophysics | hydrophone | A device designed for use in detecting seismic energy in the form of pressure changes under water during marine seismic acquisition. Hydrophones are combined to form streamers that are towed by seismic vessels or deployed in a borehole. Geophones, unlike hydrophones, detect motion rather than pressure. |
| Geophysics | IIP | Inductive-source induced polarization. |
| Geophysics | image | The apparent source of a received wave. The image is the point in the subsurface that the rays would appear to have come from if they were not reflected, but were shot up from below. A ray that travels from a source and is reflected to a geophone has the same appearance as a ray that travels straight from the image and up to the geophone. |
| Geophysics | impedance | In acoustics, the product of velocity times density, also called acoustic impedance and symbolized by Z. The reflection coefficient of an interface depends on the contrast in acoustic impedance of the rock on either side of the interface. |
| Geophysics | impedance | In electromagnetics or electrical circuit theory, the ratio of voltage to current when these are represented by phasor quantities in alternating current circuits. (A phasor is a complex number that represents the amplitude and phase of a quantity that varies sinusoidally in time.) Electrical impedance, also symbolized by Z, is a complex number that has the same units (ohms) as resistivity. |
| Geophysics | impulsive seismic data | Seismic data whose energy source is impulsive and of short duration, as with an air gun, rather than vibratory, as with a vibrator. |
| Geophysics | in line | A seismic line within a 3D survey parallel to the direction in which the data were acquired. In marine seismic data, the in-line direction is that in which the recording vessel tows the streamers. |
| Geophysics | incident angle | The acute angle at which a raypath impinges upon a line normal to an interface, such as a seismic wave impinging upon strata. Normal incidence is the case in which the angle of incidence is zero, the wavefront is parallel to the surface and its raypath is perpendicular, or normal, to the interface. Snell’s law describes the relationship between the angle of incidence and the angle of refraction of a wave. |
| Geophysics | inclinometer | An instrument used to measure the dip of the Earth’s magnetic field. |
| Geophysics | induced polarization | An electromagnetic method that uses electrodes with time-varying currents and voltages to map the variation of electrical permittivity (dielectric constant) in the Earth at low frequencies. Induced polarization is observed when a steady current through two electrodes in the Earth is shut off: the voltage does not return to zero instantaneously, but rather decays slowly, indicating that charge has been stored in the rocks. This charge, which accumulates mainly at interfaces between clay minerals, is responsible for the IP effect. This effect can be measured in either the time domain by observing the rate of decay of voltage or in the frequency domain by measuring phase shifts between sinusoidal currents and voltages. It is often used in exploration for minerals and can sometimes distinguish different types of mineralization. The IP method can probe to subsurface depths of thousands of meters. |
| Geophysics | in-line | A seismic line within a 3D survey parallel to the direction in which the data were acquired. In marine seismic data, the in-line direction is that in which the recording vessel tows the streamers. |
| Geophysics | interpretation | In geophysics, analysis of data to generate reasonable models and predictions about the properties and structures of the subsurface. Interpretation of seismic data is the primary concern of geophysicists. |
| Geophysics | interval time | The elapsed time between two seismic events. |
| Geophysics | interval transit time | The amount of time for a wave to travel a certain distance, proportional to the reciprocal of velocity, typically measured in microseconds per foot by an acoustic log and symbolized by t or DT. P-wave interval transit times for common sedimentary rock types range from 43 (dolostone) to 160 (unconsolidated shales) microseconds per foot, and can be distinguished from measurements of steel casing, which has a consistent transit time of 57 microseconds per foot. |
| Geophysics | interval velocity | The velocity, typically P-wave velocity, of a specific layer or layers of rock, symbolized by vint and commonly calculated from acoustic logs or from the change in stacking velocity between seismic events on a common midpoint gather. |
| Geophysics | inverse problem | The problem of determining the value or spatial variation of a physical property or feature by comparing measurements to the predictions of a model. For example, seismic traveltimes from a source to a receiver can be used to build a model of seismic velocity in the Earth, or earthquake arrival times can be used to determine the timing and focus (location) of an earthquake. A typical inverse problem in electromagnetics is to determine the variation of electrical conductivity in the Earth from measurements of induced electric and magnetic fields. A forward problem, in contrast, involves taking an assumed model and calculating what the observed values should be, such as the predicting seismic traveltimes between a source and a receiver given a velocity model. |
| Geophysics | inversion | A mathematical process by which data are used to generate a model that is consistent with the data, the process of solving the inverse problem. In seismology, surface seismic data, vertical seismic profiles and well log data can be used to perform inversion, the result of which is a model of Earth layers and their thickness, density and P- and S-wave velocities. Successful seismic inversion usually requires a high signal-to-noise ratio and a large bandwidth. |
| Geophysics | IP | An electromagnetic method that uses electrodes with time-varying currents and voltages to map the variation of electrical permittivity (dielectric constant) in the Earth at low frequencies. Induced polarization is observed when a steady current through two electrodes in the Earth is shut off: the voltage does not return to zero instantaneously, but rather decays slowly, indicating that charge has been stored in the rocks. This charge, which accumulates mainly at interfaces between clay minerals, is responsible for the IP effect. This effect can be measured in either the time domain by observing the rate of decay of voltage or in the frequency domain by measuring phase shifts between sinusoidal currents and voltages. It is often used in exploration for minerals and can sometimes distinguish different types of mineralization. The IP method can probe to subsurface depths of thousands of meters |
| Geophysics | isochron | A line joining points of equal time or age, such as a reflection in a seismic profile or contours in an isochron map. |
| Geophysics | isochron map | A contour map that displays the variation in time between two seismic events or reflections. |
| Geophysics | isostatic correction | A correction for variations in the density or thickness of the Earth’s crust. Isostatic corrections are commonly applied to gravity data and are made according to a specific model for isostasy. |
| Geophysics | jug | Archaic slang for a geophone. |
| Geophysics | jug hustler | Slang term for a member of a seismic acquisition crew or party who lays out cables and plants geophones for seismic acquisition and collects them after surveying. |
| Geophysics | Kirchhoff equation | A mathematical representation of the principle that a wavefield at a given point in space and time can be considered as the superposition of waves propagating from adjacent points and earlier times. It is an integral form of the wave equation in which the wave function at a point is represented as the sum (integral) of contributions from a surface enclosing the given point. The Kirchhoff equation (also called the Kirchhoff integral) is the basis for Kirchhoff migration. |
| Geophysics | Kirchhoff migration | A method of seismic migration that uses the integral form (Kirchhoff equation) of the wave equation. All methods of seismic migration involve the backpropagation (or continuation) of the seismic wavefield from the region where it was measured (Earth’s surface or along a borehole) into the region to be imaged. In Kirchhoff migration, this is done by using the Kirchhoff integral representation of a field at a given point as a (weighted) superposition of waves propagating from adjacent points and times. Continuation of the wavefield requires a background model of seismic velocity, which is usually a model of constant or smoothly varying velocity. Because of the integral form of Kirchhoff migration, its implementation reduces to stacking the data along curves that trace the arrival time of energy scattered by image points in the earth. |
| Geophysics | kriging | A statistical technique used with variograms, or two-point statistical functions that describe the increasing difference or decreasing correlation between sample values as separation between them increases, to determine the value of a point in a heterogeneous grid from known values nearby. |
| Geophysics | lag | The delay or difference in the arrival time of seismic events that can result from weathering of the rocks or variations in geologic structures in the subsurface. |
| Geophysics | lag | A term used in seismic processing to describe the interval between the zero-time of a crosscorrelation between two traces and the point of maximum correlation. |
| Geophysics | lag | The time delay of the onset of one sinusoidal oscillation, or frequency component of a trace, relative to another. Also known as a “phase-lag. |
| Geophysics | Lamé constant | One of two elastic constants named for French mathematician Gabriel Lamé (1795 to 1870). The first Lamé constant is ?, the bulk modulus (K) less two-thirds of the shear modulus (?): ? = K ? (2/3)? The second Lamé constant is the shear modulus (?): ? = ? / ? = (?F/A) / (?L/L), where ? = Shear modulus ? = Shear stress = ?F/A ?F = Increment of shear force A = Area acted on by the shear force ? = Shear strain = ?L/L ?L = Increment of transverse displacement parallel to A L = Original length. Lamé constants derived from elastic-wave velocities: ? = ?(VP2 ? 2VS2) ? = ?VS2 ?/? = (VP/VS)2 ? 2, where ? = Lamé’s first constant ? = Lamé’s second constant, the shear modulus VP = Compressional-wave (P-wave) velocity VS = Shear-wave (S-wave) velocity ? = Density. |
| Geophysics | Laplace equation | A partial differential equation that governs potential fields (in regions where there are no sources) and is equivalent, in three dimensions, to the inverse square law of gravitational or electrical attraction. In Cartesian coordinates, the Laplace equation equates the sum of the second partial (spatial) derivatives of the field to zero. (When a source is present, this sum is equal to the strength of the source and the resulting equation is called Poisson’s equation). The differential equation is named for French mathematician Pierre-Simon de Laplace (1749 to 1827), and applies to electrical, gravity and magnetic fields. ?2u = ?2u/?x2 + ?2u/?y2 + ?2u/?z2 = 0, where u(x,y,z) is a potential function. |
| Geophysics | layer | Archaic slang for a geophone. |
| Geophysics | layer cake geometry | Slang term for a member of a seismic acquisition crew or party who lays out cables and plants geophones for seismic acquisition and collects them after surveying. |
| Geophysics | least time path | The fastest route that a seismic ray can travel between two points, generally dictated by Fermat’s principle. |
| Geophysics | least-time path | The fastest route that a seismic ray can travel between two points, generally dictated by Fermat’s principle. |
| Geophysics | lithostratigraphic inversion | A seismic inversion technique that attempts to describe lithology of individual rock layers and evaluate properties and distribution of pore fluids through analysis of variation of reflected seismic amplitude with offset. |
| Geophysics | local magnetic interference | Magnetic interference caused by nearby structures such as metallic rigs and wells. The magnetic permeability of drillstrings and the remanent magnetization in drillstrings contribute to perturbations of the measured magnetic field. Operators may use nonmagnetic drill collars to reduce these effects along with software techniques to compensate for them. |
| Geophysics | long path multiple | A type of multiply-reflected seismic energy that appears as an event. Long-path multiples generate distinct events because their travel path is much longer than primary reflections giving rise to them. They typically can be removed by seismic processing. |
| Geophysics | long-path multiple | A type of multiply-reflected seismic energy that appears as an event. Long-path multiples generate distinct events because their travel path is much longer than primary reflections giving rise to them. They typically can be removed by seismic processing. |
| Geophysics | magnetic constant | Another term for magnetic permeability, the ratio of the density of the magnetic flux, B (in units of teslas), to the strength of the magnetic field, H (in units of amperes/meter), typically expressed in units of H/m. |
| Geophysics | magnetic field | The magnetic field measured near the Earth’s surface is the superposition of magnetic fields arising from various time-varying physical processes that are grouped into four general components: the main magnetic field, the crustal field, the external disturbance field and local magnetic interference. The significance of these contributions to direction, strength and stability of the magnetic field varies with geographic region and with magnetic survey direction. |
| Geophysics | magnetic permeability | The ratio of the density of the magnetic flux, B (in units of teslas), to the strength of the magnetic field, H (in units of amperes/meter), typically expressed in units of henries per meter (H/m). |
| Geophysics | magnetic total field | The modulus of the magnetic field vector. The magnetic total field is the magnitude, or absolute value, of the magnetic field vector. The magnetic total field describes the strength, or intensity, of the magnetic field, which is measured in units of nanoTesla (nT). The symbol for the magnetic total field is often F or Btotal. |
| Geophysics | magnetics | The study of the Earth’s magnetic field, a branch of geophysics that began with the observation by British scientist William Gilbert (1544 to 1603) that the Earth is a magnet. Variations in the magnetic field can be used to determine the extent of sedimentary basins and the depth to basement rocks, as well as to differentiate between igneous rocks and certain sedimentary rocks such as salt. High-resolution magnetic surveys can also be used to determine the locations of oil pipelines and production equipment. |
| Geophysics | magnetometer | An instrument used to measure the strength or direction of the Earth’s magnetic field. |
| Geophysics | magnetotelluric method | An electromagnetic method used to map the spatial variation of the Earth’s resistivity by measuring naturally occurring electric and magnetic fields at the Earth’s surface. These natural EM fields are generated (at all frequencies) in the Earth’s atmosphere mainly by lightning strokes and by interactions between the solar wind and the ionosphere. In the most general MT method, the horizontal components of the electric field and all three components of the magnetic field are measured at the surface. The measurements are used to determine specific ratios of electric to magnetic field components called tensor impedances. The technique was introduced the French geophysicist Louis Cagniard in the 1950s and has been popular for mineral exploration and regional geophysical mapping. It is used in oil exploration for low-cost reconnaissance of sedimentary basins and for exploration in areas where seismic surveys are difficult because of severe topography or the presence high-impedance volcanic rocks near the surface. The resolution of MT surveys is limited by the diffusive nature of EM propagation in the earth; it is usually on the order of hundreds of meters to kilometers. But the MT method can probe the Earth to depths of several tens of kilometers. |
| Geophysics | main magnetic field | Earth’s main magnetic field generated in the Earth’s fluid outer core by a self-exciting dynamo process. Approximately 95% of the total magnetic field measured at Earth’s surface comes from this main field, a significant portion of which may be described as the field of a dipole placed at the Earth’s center and tilted approximately 11° from the Earth’s rotational axis. The magnitude of the main magnetic field is nearly 60,000 nT near the Earth’s poles and about 30,000 nT near the equator. However, there are significant nondipole contributions to the main magnetic field that complicate its mathematical and graphical representation, including that the relative strengths of nondipole components change. As additional complications, the main field varies slowly because of changes within the Earth’s core and the magnetic dipole axis pole position itself wanders over time. |
| Geophysics | marker bed | A widespread distinctive rock unit that can be correlated readily over a large area. The most useful marker beds tend to form rapidly, such as during volcanic or geologically instantaneous depositional events, and have unusual seismic, magnetic, electrical or other physical properties that aid geological or geophysical interpretation. Coal beds and volcanic ash falls are examples of marker beds. |
| Geophysics | Maxwell’s equations |
(1.) ?·D = ? (2.) ?×H = J + (?D/?t) (3.) ?·B = 0 (4.) ?×E = ?(?B/?t), where D = electric displacement ? = electric charge density H = magnetic field strength J = electric current density B = magnetic flux density E = electric field strength. Equation (1) is equivalent to Coulomb’s law, the inverse square attraction of static electric charges. Equation (2) is Ampere’s law relating magnetic fields and currents, which was extended by Maxwell to include induction of a magnetic field by a time-varying electric displacement. Equation (3) is Coulomb’s law for magnetic flux, expressing the absence of isolated magnetic charges. Equation (4) is Faraday’s law of induction, relating an electric field to a time-varying magnetic flux. Maxwell’s equations are the starting point for all calculations involving surface or borehole EM methods. |
| Geophysics | midpoint | The halfway point between a seismic source and a receiver at the Earth’s surface. |
| Geophysics | migrate | To execute a step in seismic processing in which reflections in seismic data are moved to their correct locations in x-y-time space of seismic data. Migration improves seismic interpretation and mapping because the locations of geological structures, especially faults, are more accurate in migrated seismic data. Proper migration collapses diffractions from secondary sources such as reflector terminations against faults and corrects bow ties to form synclines. There are numerous methods of migration, such as dip moveout (DMO), frequency domain, ray-trace and wave-equation migration. |
| Geophysics | migration | A step in seismic processing in which reflections in seismic data are moved to their correct locations in the x-y-time space of seismic data, including two-way traveltime and position relative to shotpoints. Migration improves seismic interpretation and mapping because the locations of geological structures, especially faults, are more accurate in migrated seismic data. Proper migration collapses diffractions from secondary sources such as reflector terminations against faults and corrects bow ties to form synclines. There are numerous methods of migration, such as dip moveout (DMO), frequency domain, ray-trace and wave-equation migration. |
| Geophysics | minimum time path | The fastest route that a seismic ray can travel between two points, generally dictated by Fermat’s principle. |
| Geophysics | minimum-time path | The fastest route that a seismic ray can travel between two points, generally dictated by Fermat’s principle. |
| Geophysics | mis tie | A situation in interpretation of seismic data in which predicted and actual values differ, or when an interpreted reflection does not close, or tie, when interpreting intersecting lines; or when interpreted seismic data do not match results of drilling a well. Mis-ties commonly occur when data of different phases, rather than uniformly zero-phase data, are interpreted together, or data that have different datum corrections are tied. Mis-ties are described as static if they involve a bulk shift of data (as in the case of tying seismic sections with different datum corrections) or dynamic if the magnitude of the mis-tie varies with time (as in the case of data that have been migrated differently). |
| Geophysics | mis-tie | A situation in interpretation of seismic data in which predicted and actual values differ, or when an interpreted reflection does not close, or tie, when interpreting intersecting lines; or when interpreted seismic data do not match results of drilling a well. Mis-ties commonly occur when data of different phases, rather than uniformly zero-phase data, are interpreted together, or data that have different datum corrections are tied. Mis-ties are described as static if they involve a bulk shift of data (as in the case of tying seismic sections with different datum corrections) or dynamic if the magnitude of the mis-tie varies with time (as in the case of data that have been migrated differently). |
| Geophysics | model | A representation of a physical property or entity that can be used to make predictions or compare observations with assumptions. Mathematical velocity models are commonly used to predict the depth to a formation of interest. Physical models, such as layers of clay or putty, can be used to simulate rock layers. As Sheriff (1991) points out, agreement between data and a model does not prove that the model is correct, since there can be numerous models that agree with a given data set. |
| Geophysics | modulus of compression | Another term for bulk modulus, the ratio of stress to strain, abbreviated as k. The bulk modulus is an elastic constant equal to the applied stress divided by the ratio of the change in volume to the original volume of a body. |
| Geophysics | modulus of elasticity | Any one of a set of constants, also known as elastic moduli, that defines the properties of material that undergoes stress, deforms, and then recovers and returns to its original shape after the stress ceases. The elastic constants include the bulk modulus, Lame constant, Poisson’s ratio, shear modulus, and Young’s modulus. Elastic constants are important in seismology because the velocity of waves depends on the elastic constants and density of the rock. |
| Geophysics | modulus of rigidity | Another term for shear modulus, an elastic constant for the ratio of shear stress to shear strain. The shear modulus is one of the Lamé constants. It can be expressed mathematically as follows: ? = ? / ? = (?F/A) / (?L/L), where ? = Shear modulus ? = Shear stress = ?F/A ?F = Increment of shear force A = Area acted on by the shear force ? = Shear strain = ?L/L ?L = Increment of transverse displacement parallel to A L = Original length. |
| Geophysics | monument | A relatively permanent, fixed marker used in surveying, such as a concrete block or steel plate, with an inscription of location and elevation. |
| Geophysics | moveout | The difference in the arrival times or traveltimes of a reflected wave measured by receivers at two different offset locations. Normal moveout (NMO) is moveout caused by the separation between a source and a receiver in the case of a flat reflector. Dip moveout (DMO) occurs as an effect in addition to NMO when reflectors dip. Problems that require static corrections can also produce moveout. |
| Geophysics | moving-source method | An acquisition technique most commonly used in electromagnetic methods whereby the energy source or transmitter and detectors or receivers are kept in the same relative position and moved together to different spots to compile a profile or map. |
| Geophysics | MT | An electromagnetic method used to map the spatial variation of the Earth’s resistivity by measuring naturally occurring electric and magnetic fields at the Earth’s surface. These natural EM fields are generated (at all frequencies) in the Earth’s atmosphere mainly by lightning strokes and by interactions between the solar wind and the ionosphere. In the most general MT method, the horizontal components of the electric field and all three components of the magnetic field are measured at the surface. The measurements are used to determine specific ratios of electric to magnetic field components called tensor impedances. The technique was introduced the French geophysicist Louis Cagniard in the 1950s and has been popular for mineral exploration and regional geophysical mapping. It is used in oil exploration for low-cost reconnaissance of sedimentary basins and for exploration in areas where seismic surveys are difficult because of severe topography or the presence high-impedance volcanic rocks near the surface. The resolution of MT surveys is limited by the diffusive nature of EM propagation in the earth; it is usually on the order of hundreds of meters to kilometers. But the MT method can probe the Earth to depths of several tens of kilometers. |
| Geophysics | Multiazimuth towed-streamer acquisition | A marine seismic data acquisition method in which a conventional narrow-azimuth towed-streamer configuration is used to acquire data over a survey area in more than one direction. The number of directions is typically three or more. The azimuthal range for a multiazimuth survey is not continuous in azimuth, but is well sampled along the shooting directions. |
| Geophysics | multicomponent seismic data | Seismic data acquired in a land, marine, or borehole environment by using more than one geophone or accelerometer. 3C seismic data, a type of multicomponent seismic data, uses three orthogonally oriented geophones or accelerometers. 4C seismic data, another type of multicomponent seismic data, involves the addition of a hydrophone to three orthogonally oriented geophones or accelerometers. 3C multicomponent seismic data is particularly appropriate when the addition of a hydrophone (the basis for 4C seismic data) adds no value to the measurement, for example, on land. This technique allows determination of both the type of wave and its direction of propagation. |
| Geophysics | multiple reflection | Multiply reflected seismic energy, or any event in seismic data that has incurred more than one reflection in its travel path. Depending on their time delay from the primary events with which they are associated, multiples are characterized as short-path or peg-leg, implying that they interfere with the primary reflection, or long-path, where they appear as separate events. Multiples from the water bottom (the interface of the base of water and the rock or sediment beneath it) and the air-water interface are common in marine seismic data, and are suppressed by seismic processing. |
| Geophysics | mute | To remove the contribution of selected seismic traces in a stack to minimize air waves, ground roll and other early-arriving noise. Low-frequency traces and long-offset traces are typical targets for muting. |
| Geophysics | Narrow-azimuth seismic data | Conventional marine seismic data acquired using a single vessel to tow one or two seismic source arrays in front of a receiver spread. The resulting angle between the source and receivers, is about 20°. |
| Geophysics | natural frequency | The frequency of the normal, free oscillation or vibration of an entity or a system, such as the vibration of a tuning fork when struck or the open string of a musical instrument when plucked. A system oscillating at its natural frequency is said to resonate. |
| Geophysics | natural remanent magnetism | The magnetization retained by rocks from previous magnetic fields,abbreviated NRM. NRM is a record of the Earth’s magnetic field as it existed at the time that the rock formed, such as when magnetic crystals in igneous rocks solidified (also known as chemical remanent magnetism, CRM) or at the time of deposition of sedimentary rocks (known as depositional remanent magnetism, DRM). During deposition of sediments that become sedimentary rock, magnetized particles can settle with their magnetic pole aligned with that of the Earth at that time. |
| Geophysics | near surface correction | Another term for static correction, a bulk shift of a seismic trace in time during seismic processing. A common static correction is the weathering correction, which compensates for a layer of low seismic velocity material near the surface of the Earth. Other corrections compensate for differences in topography and differences in the elevations of sources and receivers. |
| Geophysics | near-surface correction | Another term for static correction, a bulk shift of a seismic trace in time during seismic processing. A common static correction is the weathering correction, which compensates for a layer of low seismic velocity material near the surface of the Earth. Other corrections compensate for differences in topography and differences in the elevations of sources and receivers. |
| Geophysics | nest | A geometrical arrangement of seismic receivers (geophones) with signals recorded by one channel. The array can contain numerous closely spaced geophones. |
| Geophysics | NMO | The effect of the separation between receiver and source on the arrival time of a reflection that does not dip, abbreviated NMO. A reflection typically arrives first at the receiver nearest the source. The offset between the source and other receivers induces a delay in the arrival time of a reflection from a horizontal surface at depth. A plot of arrival times versus offset has a hyperbolic shape. |
| Geophysics | NMR | Pertaining to a measurement of the nuclear magnetic properties of formation hydrogen. The basic core and log measurement is the T2 decay, presented as a distribution of T2 amplitudes versus time at each sample depth, typically from 0.3 ms to 3 s. The T2 decay is further processed to give the total pore volume (the total porosity) and pore volumes within different ranges of T2. The most common volumes are the bound fluid and free fluid. A permeability estimate is made using a transform such as the Timur-Coates or SDR permeability transforms. By running the log with different acquisition parameters, direct hydrocarbon typing and enhanced diffusion are possible. |
| Geophysics | noise | Anything other than desired signal. Noise includes disturbances in seismic data caused by any unwanted seismic energy, such as shot generation ground roll, surface waves, multiples, effects of weather and human activity, or random occurrences in the Earth. Noise can be minimized by using source and receiver arrays, generating minimal noise during acquisition and by filtering and stacking data during processing. |
| Geophysics | nondipole field | Contribution to Earth’s main magnetic field that is not represented by the dipole field. |
| Geophysics | normal incidence | The case in which a wavefront is parallel to an interface and its raypath is perpendicular, or normal, to the interface as the wave impinges upon the interface. |
| Geophysics | normal moveout | The effect of the separation between receiver and source on the arrival time of a reflection that does not dip, abbreviated NMO. A reflection typically arrives first at the receiver nearest the source. The offset between the source and other receivers induces a delay in the arrival time of a reflection from a horizontal surface at depth. A plot of arrival times versus offset has a hyperbolic shape. |
| Geophysics | normal moveout correction | A function of time and offset that can be used in seismic processing to compensate for the effects of normal moveout, or the delay in reflection arrival times when geophones and shotpoints are offset from each other. |
| Geophysics | normal-moveout correction | A function of time and offset that can be used in seismic processing to compensate for the effects of normal moveout, or the delay in reflection arrival times when geophones and shotpoints are offset from each other. |
| Geophysics | NRM | The magnetization retained by rocks from previous magnetic fields,abbreviated NRM. NRM is a record of the Earth’s magnetic field as it existed at the time that the rock formed, such as when magnetic crystals in igneous rocks solidified (also known as chemical remanent magnetism, CRM) or at the time of deposition of sedimentary rocks (known as depositional remanent magnetism, DRM). During deposition of sediments that become sedimentary rock, magnetized particles can settle with their magnetic pole aligned with that of the Earth at that time. |
| Geophysics | OBC | Typically an assembly of vertically oriented geophones and hydrophones connected by electrical wires and deployed on the seafloor to record and relay data to a seismic recording vessel. Such systems were originally introduced to enable surveying in areas of obstructions (such as production platforms) or shallow water inaccessible to ships towing seismic streamers (floating cables). Recent developments provide four component (4C) seabed systems to record shear wave (S-wave) as well as P-wave energy. |
| Geophysics | observer | The director of a seismic acquisition field crew who operates the recording equipment. |
| Geophysics | Occam’s inversion | A technique for inversion, or generating a model that is consistent with the data, of electromagnetic data, including resistivity and magnetotelluric data. The algorithm is named for William of Occam (1300 to 1349), who asserted that scientific hypotheses and reasoning should be as simple as possible. The use of Occam’s inversion produces a smooth model that fits a data set within certain tolerances, although a smooth model might not be the best fit to the data. |
| Geophysics | ocean-bottom cable | Typically an assembly of vertically oriented geophones and hydrophones connected by electrical wires and deployed on the seafloor to record and relay data to a seismic recording vessel. Such systems were originally introduced to enable surveying in areas of obstructions (such as production platforms) or shallow water inaccessible to ships towing seismic streamers (floating cables). Recent developments provide four component (4C) seabed systems to record shear wave (S-wave) as well as P-wave energy. |
| Geophysics | offset vertical seismic profile | A type of vertical seismic profile in which the source is located at an offset from the drilling rig during acquisition. This allows imaging to some distance away from the wellbore. |
| Geophysics | offset VSP | Abbreviation for offset vertical seismic profile, a type of vertical seismic profile in which the source is located at an offset from the drilling rig during acquisition. This allows imaging to some distance away from the wellbore. |
| Geophysics | Ohm’s law | The relationship between voltage (V), electric current (I) and resistance (R), named for German physicist Georg Simon Ohm (1789 to 1854), commonly expressed as the formula below: V/I = R. |
| Geophysics | one dimensional seismic data | A single seismic trace |
| Geophysics | one dimensional seismic data | A check-shot survey of a well, which can be used to correct the sonic log and generate a synthetic seismogram that displays changes in amplitude versus traveltime. |
| Geophysics | one way time | The time measured from a check-shot survey or vertical seismic profile (VSP), which is the time energy takes to travel from an energy source at the surface of the Earth to a receiver at a depth of interest. |
| Geophysics | one-dimensional seismic data | A single seismic trace. |
| Geophysics | one-dimensional seismic data | A check-shot survey of a well, which can be used to correct the sonic log and generate a synthetic seismogram that displays changes in amplitude versus traveltime. |
| Geophysics | one-way time | The time measured from a check-shot survey or vertical seismic profile (VSP), which is the time energy takes to travel from an energy source at the surface of the Earth to a receiver at a depth of interest. |
| Geophysics | p wave | An elastic body wave or sound wave in which particles oscillate in the direction the wave propagates. P-waves are the waves studied in conventional seismic data. P-waves incident on an interface at other than normal incidence can produce reflected and transmitted S-waves, in that case known as converted waves. |
| Geophysics | parametric | Pertaining to a method of seismic inversion to separate wavefields by iteratively developing a model of the data that conforms to the recorded data. Parametric inversion is used in processing vertical seismic profile (VSP) data. |
| Geophysics | parametric | Pertaining to variation of the frequency while maintaining the geometry of electromagnetic surveying. In contrast, geometric pertains to keeping the same geometry while varying the frequency. |
| Geophysics | party | A crew that acquires a survey or geophysical data. |
| Geophysics | party chief | The ultimate leader of a survey crew. |
| Geophysics | party manager | The actual leader of a survey crew. The party manager reports to the party chief. |
| Geophysics | patch | A large set of seismometers whose output is sent to a common data channel to record a seismic trace. |
| Geophysics | peak | The maximum positive or upward deflection, also known as the crest, of the seismic wavelet. The trough is the maximum negative amplitude or downward deflection of the wave. Seismic interpreters commonly pick or interpret seismic data on paper sections along the trough of a wavelet rather than the normally solid-filled peak for ease of viewing. |
| Geophysics | peg leg multiple | A type of short-path multiple, or multiply-reflected seismic energy, having an asymmetric path. Short-path multiples are added to primary reflections, tend to come from shallow subsurface phenomena and highly cyclical deposition, and can be suppressed by seismic processing. In some cases, the period of the peg-leg multiple is so brief that it interferes with primary reflections, and its interference causes a loss of high frequencies in the wavelet. |
| Geophysics | peg-leg multiple | A type of short-path multiple, or multiply-reflected seismic energy, having an asymmetric path. Short-path multiples are added to primary reflections, tend to come from shallow subsurface phenomena and highly cyclical deposition, and can be suppressed by seismic processing. In some cases, the period of the peg-leg multiple is so brief that it interferes with primary reflections, and its interference causes a loss of high frequencies in the wavelet. |
| Geophysics | permeability |
In magnetics, the ratio of the density of the magnetic flux, B (in units of teslas), to the strength of the magnetic field, H (in units of amperes/meter), typically in units of H/m. ? = B / H, where ? = magnetic permeability B = magnetic flux or magnetic |
| Geophysics | permittivity | The ability of a material to store a charge from an applied electrical field without conducting electricity. |
| Geophysics | perpendicular offset | Generally, the distance between a receiver and a source in a survey, such as an electromagnetic survey. In seismic surveys, perpendicular or normal offset is the component of the distance between the source and geophones at a right angle to the spread. |
| Geophysics | phantom | An interpretation of the presumed continuation of an event. In areas of discontinuous, divergent reflectors or incoherent data, drawing phantoms allows the interpreter to generate a map on a discontinuous event. |
| Geophysics | phase | A description of the motion of, or means of comparison of, periodic waves such as seismic waves. Waves that have the same shape, symmetry and frequency and that reach maximum and minimum values simultaneously are in phase. Waves that are not in phase are typically described by the angular difference between them, such as, “180 degrees out of phase.” Zero-phase wavelets are symmetrical in shape about zero time whereas non-zero-phase wavelets are asymmetrical. Non-zero-phase wavelets are converted to zero-phase wavelets to achieve the best resolution of the seismic data. Known (zero) phase well synthetics and vertical seismic profiles (VSPs) can be compared with local surface seismic data to determine the relative phase of the surface seismic wavelets. Such knowledge allows the surface seismic data to be “corrected” to zero phase. The units of phase are degrees. |
| Geophysics | phase velocity | The velocity at which a single frequency of a wave group or a phase—or part such as the crest or trough—of a wave group travels through a medium. The phase velocity (vp) is defined by a wavelength (?) and frequency (f) and given by vp = ? × f. vp = vg + ? (?vp/??) = vg ? f (?vp/?f) Relation of phase velocity to group velocity. As a wave travels through a medium, its energy moves at the group velocity (vg) and its individual phases, or components, move at their phase velocity (vp). The wave changes shape with distance as each frequency (f), or wavelength (?), component moves at its separate phase velocity through the phenomenon of dispersion. Relative to the group velocity, each component moves with faster or slower phase velocity, depending on how phase velocity changes with wavelength or frequency. |
| Geophysics | pick | A feature interpreted or selected from data, such as a seismic event. Correlation of seismic picks to geologic picks, such as formation tops interpreted from well logs, can improve interpretations. |
| Geophysics | pick | To interpret data, such as seismic sections, by selecting and tracking marker beds or other events. |
| Geophysics | plane wave | A wave that is far enough from its source that its wavefront has no effective curvature, or is planar, over a short distance. Seismic and electromagnetic waves are treated as plane waves even though that assumption is not strictly correct. |
| Geophysics | plant | To place seismometers on the ground. The seismometer should be firmly stuck or planted in the ground in the proper location and orientation for optimal seismic acquisition. |
| Geophysics | Poisson’s ratio | An elastic constant that is a measure of the compressibility of material perpendicular to applied stress, or the ratio of latitudinal to longitudinal strain. This elastic constant is named for Simeon Poisson (1781 to 1840), a French mathematician. Poisson’s ratio (?) can be expressed in terms of properties that can be measured in the field, including velocities of P-waves (VP) and S-waves (VS) as shown below. ? = ½ (VP2 ? 2VS2) / (VP2 ? VS2) Note that if VS = 0, then Poisson’s ratio equals 0.5, indicating either a fluid, because shear waves do not pass through fluids, or a material that maintains constant volume regardless of stress, also known as an ideal incompressible material. Poisson’s ratio for carbonate rocks is ~0.3, for sandstones ~0.2, and greater than 0.3 for shale. The Poisson’s ratio of coal is ~0.4. |
| Geophysics | polarity standard | The convention adopted by the Society of Exploration Geophysicists (SEG) for the display of zero-phase seismic data. If the signal arises from a reflection that indicates an increase in acoustic impedance, the polarity is, by convention, positive and is displayed as a peak. If the signal arises from a reflection that indicates a decrease in acoustic impedance, the polarity is negative and is displayed as a trough. There is another standard for minimum-phase data. In order to interpret seismic data acquired at different times within a region, to model data, or to assess bright or dim spots, some knowledge of the polarity of the data is essential to correlate or tie data properly. |
| Geophysics | post | To annotate a map or other display with data at the appropriate location. For example, geologists post formation tops on well logs, isopach maps and seismic profiles. Geophysicists post velocity values and traveltimes on maps before contouring. Engineers contour maps posted with pressure or production data. Posting can become an iterative process as new data become available and interpretations are updated. |
| Geophysics | potential field | A field that satisfies the Laplace equation. The Laplace equation is equivalent in three dimensions to the inverse square law of gravitational or electrical attraction (in source-free regions; in regions with sources, it becomes Poisson’s equation). Examples of potential fields include the field of the gravity potential and static electric and magnetic fields. |
| Geophysics | primary reflection | Seismic events whose energy has been reflected once. Multiples, in contrast, are events whose energy has been reflected more than once. A goal of seismic data processing is to enhance primary reflections, which are then interpreted as subsurface interfaces. |
| Geophysics | probe | In electromagnetic methods, to measure the variation of a property versus depth, including electrical, electromagnetic and magnetotelluric properties. Probing differs from profiling in that the goal of probing is to provide a record of vertical changes, whereas profiling documents lateral variations. |
| Geophysics | processing | Alteration of seismic data to suppress noise, enhance signal and migrate seismic events to the appropriate location in space. Processing steps typically include analysis of velocities and frequencies, static corrections, deconvolution, normal moveout, dip moveout, stacking, and migration, which can be performed before or after stacking. Seismic processing facilitates better interpretation because subsurface structures and reflection geometries are more apparent. |
| Geophysics | production | A measure of the efficiency of seismic acquisition. Production can be expressed in terms of the number of lines, shots or lengths (km or miles) of data acquired in a given time. |
| Geophysics | profile | To measure the lateral variation of a property, such as gravity or magnetic fields. Probing, in contrast, is the term used to describe the measurement of vertical variations of a property in electromagnetic and other nonseismic geophysical methods. |
| Geophysics | profiling | Measuring the lateral variation of a property, such as gravity or magnetic fields. Probing, in contrast, is the term used to describe the measurement of vertical variations of a property in electromagnetic and other nonseismic geophysical methods. |
| Geophysics | propagation constant | A property of a sinusoidal plane wave equal to twice pi divided by the wavelength. Also known as the wavenumber, the propagation constant is fundamental to the mathematical representation of wavefields. It is the spatial equivalent of angular frequency and expresses the increase in the cycle of the wave (measured in radians) per unit of distance. In nondispersive media, the wavespeed is the ratio of the angular frequency to the propagation constant. The propagation vector has magnitude equal to the propagation constant and points in the direction the wave is traveling. |
| Geophysics | pull up | A phenomenon of relative seismic velocities of strata whereby a shallow layer or feature with a high seismic velocity (e.g., a salt layer or salt dome, or a carbonate reef) surrounded by rock with a lower seismic velocity causes what appears to be a structural high beneath it. After such features are correctly converted from time to depth, the apparent structural high is generally reduced in magnitude. |
| Geophysics | pull-up | A phenomenon of relative seismic velocities of strata whereby a shallow layer or feature with a high seismic velocity (e.g., a salt layer or salt dome, or a carbonate reef) surrounded by rock with a lower seismic velocity causes what appears to be a structural high beneath it. After such features are correctly converted from time to depth, the apparent structural high is generally reduced in magnitude. |
| Geophysics | push down | A phenomenon of relative seismic velocities of strata whereby a shallow layer or feature with a low seismic velocity (e.g., a shale diapir or a gas chimney) surrounded by rock with a higher seismic velocity causes what appears to be a structural low beneath it. After such features are converted from time to depth, the apparent structural low is generally reduced in magnitude. Hydrocarbon indicators can display velocity push-downs because the velocity of hydrocarbon is slower than that of rock. |
| Geophysics | push-down | A phenomenon of relative seismic velocities of strata whereby a shallow layer or feature with a low seismic velocity (e.g., a shale diapir or a gas chimney) surrounded by rock with a higher seismic velocity causes what appears to be a structural low beneath it. After such features are converted from time to depth, the apparent structural low is generally reduced in magnitude. Hydrocarbon indicators can display velocity push-downs because the velocity of hydrocarbon is slower than that of rock. |
| Geophysics | P-wave | An elastic body wave or sound wave in which particles oscillate in the direction the wave propagates. P-waves are the waves studied in conventional seismic data. P-waves incident on an interface at other than normal incidence can produce reflected and transmitted S-waves, in that case known as converted waves. |
| Geophysics | Q | The dimensionless quality factor. It is the ratio of the peak energy of a wave to the dissipated energy. As waves travel, they lose energy with distance and time due to spherical divergence and absorption. Such energy loss must be accounted for when restoring seismic amplitudes to perform fluid and lithologic interpretations, such as amplitude versus offset (AVO) analysis. Q is also described as the reciprocal of attenuation, but that is not strictly correct because the attenuation coefficient has units of inverse length. |
| Geophysics | Q wave | A type of surface wave in which particles oscillate horizontally and perpendicularly to the direction of wave propagation. |
| Geophysics | quick look | A subset of a 3D seismic survey comprising low fold or simplified processing (such as omitting dip moveout processing) that can be evaluated soon after acquisition. |
| Geophysics | quick look | Borehole seismic data processed on site in the field |
| Geophysics | quicklook | A subset of a 3D seismic survey comprising low fold or simplified processing (such as omitting dip moveout processing) that can be evaluated soon after acquisition. |
| Geophysics | quicklook | Borehole seismic data processed on site in the field |
| Geophysics | Q-wave | A type of surface wave in which particles oscillate horizontally and perpendicularly to the direction of wave propagation. |
| Geophysics | radial array | An array of sources or receivers radiating outward from a central point, usually a borehole. |
| Geophysics | radial refraction | A borehole seismic method in which a surface source transmits seismic energy from various locations to a receiver in a wellbore to locate high-velocity features such as salt domes |
| Geophysics | radial refraction | A surveying technique used to identify local, high-velocity features such as salt domes, also called fan shooting. |
| Geophysics | random noise | Disturbances in seismic data that are not coherent (they lack a phase relationship between adjacent traces, unlike air waves and ground roll) and cannot be correlated to the seismic energy source. Random noise can be reduced or removed from data by stacking traces, filtering during processing or using arrays of geophones during acquisition. |
| Geophysics | rarefaction | A dilatation, or decrease in pressure and density of a medium as molecules are displaced by a P-wave. As P-waves pass through the Earth, the Earth undergoes compression and expansion. These changes in volume contribute to the positive and negative amplitudes of a seismic trace. |
| Geophysics | ray | A representation of the direction of travel of a seismic wave. |
| Geophysics | ray path | The path or direction along which wave energy propagates through the Earth. In isotropic media, the raypath is perpendicular to the local wavefront. The raypath can be calculated using ray tracing. Seismic energy travels through media of variable anisotropy and can propagate by diffraction, factors that complicate determination of raypaths. |
| Geophysics | ray tracing | A technique for predicting or determining arrival times of waves at detectors using raypaths. Ray tracing requires a velocity model and the assumption that rays behave according to Snell’s law. Ray tracing provides the traveltimes that are required for Kirchhoff migration. |
| Geophysics | Rayleigh wave | A type of surface wave in which particles move in an elliptical path within the vertical plane containing the direction of wave propagation. At the top of the elliptical path, particles travel opposite to the direction of propagation, and at the bottom of the path they travel in the direction of propagation. Because Rayleigh waves are dispersive, with different wavelengths traveling at different velocities, they are useful in evaluation of velocity variation with depth. Rayleigh waves make up most of the energy recorded as ground roll. |
| Geophysics | raypath | The path or direction along which wave energy propagates through the Earth. In isotropic media, the raypath is perpendicular to the local wavefront. The raypath can be calculated using ray tracing. Seismic energy travels through media of variable anisotropy and can propagate by diffraction, factors that complicate determination of raypaths. |
| Geophysics | receiver | A device that detects seismic energy in the form of ground motion or a pressure wave in fluid and transforms it to an electrical impulse. |
| Geophysics | record | In seismic data, the energy detected and measured by a receiver. Normally, most of the energy is provided by a seismic source. Noise records are obtained in the absence of a seismic source to measure background or ambient noise levels. |
| Geophysics | record | To detect and measure energy. |
| Geophysics | reflection | Generally, the return or rebound of particles or energy from the interface between two media. There are two laws of reflection, which state (1) that incident rays, reflected rays and the normal to the reflecting interface at the point of incidence are coplanar, and (2) that the angle of incidence is equal to the angle of reflection. In geophysics, reflection refers to the seismic energy or signal that returns from an interface of contrasting acoustic impedance, known as a reflector, according to Snell’s law. Reflection seismic surveys are useful for mapping geologic structures in the subsurface, interpreting sedimentary environments and evaluating hydrocarbon accumulations that might occur as amplitude anomalies. Reflection surveys are complicated by the variation of velocity as well as the various types of wave energy that are propagated within the Earth. In electromagnetics, variation in electrical properties produces reflections. |
| Geophysics | reflection coefficient |
The ratio of amplitude of the reflected wave to the incident wave, or how much energy is reflected. If the wave has normal incidence, then its reflection coefficient can be expressed as: R = (?2V2 ? ?1V1) / (?2V2 + ?1V1), where R = reflection coefficient, |
| Geophysics | reflection tomography | A technique to measure and display the three-dimensional distribution of velocity or reflectivity of a volume of the Earth by using numerous sources and receivers at the Earth’s surface. In reflection tomography, space is divided into cells, each having a certain velocity and reflectivity. The final model is the one whose velocities and reflectivities best describe the data. |
| Geophysics | reflectivity |
Another term for reflection coefficient, the ratio of amplitude of the reflected wave to the incident wave, or how much energy is reflected. If the wave has normal incidence, then its reflection coefficient can be expressed as: R = (?2V2 ? ?1V1) / (?2V2 |
| Geophysics | reflector | An interface between layers of contrasting acoustic, optical or electromagnetic properties. Waves of electromagnetism, heat, light and sound can be reflected at such an interface. In seismic data, a reflector might represent a change in lithology, a fault or an unconformity. A reflector is expressed as a reflection in seismic data. |
| Geophysics | refraction | The change in the direction of travel of a wavefront, or the bending of a ray, as it passes from one medium to another, expressed mathematically by Snell’s law. Refraction is a consequence of changes in wavelength and velocity of propagation of a wave produced by differences in refractive indices of the media. Refraction surveys where the incident and reflected angles are critical can be useful for evaluating increasing velocity gradients and locating features that have anomalously high velocities, such as a salt dome within surrounding rocks of lower velocities. |
| Geophysics | refractive index | The ratio of the speed of light in a vacuum to the speed of light in a given material, commonly symbolized by n. According to Snell’s law, the refractive index is also the ratio of sine of the angle of incidence to the sine of the angle of refraction. |
| Geophysics | refractor | A layer of rock that is sufficiently thick, areally extensive, and has a distinctly higher velocity than the rocks immediately above it such that it can transmit a head wave, or a wave transmitted at its critical incident angle. |
| Geophysics | remote sensing | The process of measuring, observing or analyzing features of the Earth from a distance. Satellite photography and radar are techniques commonly used for remote sensing. Many geophysicists do not consider seismic methods to be remote sensing because although seismic methods sense the subsurface remotely, the sources and receivers are in contact with the Earth |
| Geophysics | replacement velocity | An acoustic velocity value used during processing to produce static, vertical shifts in seismic and other time domain data in order to bring a specific point into alignment with some common elevation feature. Most often, the point in question is the 0.0 s time point, while the elevation feature is ground level. In other cases, the elevation feature may be arbitrary, such as 300 m above mean sea level. |
| Geophysics | resolution | The ability to distinguish between separate points or objects, such as sedimentary sequences in a seismic section. High frequency and short wavelengths provide better vertical and lateral resolution. Seismic processing can greatly affect resolution: deconvolution can improve vertical resolution by producing a broad bandwidth with high frequencies and a relatively compressed wavelet. Migration can improve lateral resolution by reducing the size of the Fresnel zone. |
| Geophysics | Rich-azimuth towed-streamer acquisition | A marine seismic data acquisition method using one or more seismic vessels to obtain a combination of multiazimuth and wide-azimuth geometries. A rich-azimuth seismic dataset can be formed by combining the data where multiple wide-azimuth surveys intersect. |
| Geophysics | Ricker wavelet | A zero-phase wavelet commonly convolved with a reflectivity trace to generate a synthetic seismogram. |
| Geophysics | rock mechanics | The study of the physical characteristics and behavior of rock. Rock mechanics can include analysis of and relationships between properties such as velocity, density, porosity, permeability, shear strength, and bending and crushing behavior, as well as the greater geological context of forces that deform strata and produce geological structures. |
| Geophysics | root mean square velocity | The value of the square root of the sum of the squares of the velocity values divided by the number of values, symbolized by vrms. The root-mean-square velocity is that of a wave through subsurface layers of different interval velocity along a specific raypath, and is typically several percent higher than the average velocity. The stacking velocity and the root-mean-square velocity approach equality when source-receiver offset approaches zero and layers are horizontal and isotropic. |
| Geophysics | root-mean-square velocity | The value of the square root of the sum of the squares of the velocity values divided by the number of values, symbolized by vrms. The root-mean-square velocity is that of a wave through subsurface layers of different interval velocity along a specific raypath, and is typically several percent higher than the average velocity. The stacking velocity and the root-mean-square velocity approach equality when source-receiver offset approaches zero and layers are horizontal and isotropic. |
| Geophysics | S wave | An elastic body wave in which particles oscillate perpendicular to the direction in which the wave propagates. S-waves are generated by most land seismic sources, but not by air guns. P-waves that impinge on an interface at non-normal incidence can produce S-waves, which in that case are known as converted waves. S-waves can likewise be converted to P-waves. S-waves, or shear waves, travel more slowly than P-waves and cannot travel through fluids because fluids do not support shear. Recording of S-waves requires receivers coupled to the solid Earth. Interpretation of S-waves can allow determination of rock properties such as fracture density and orientation, Poisson’s ratio and rock type by crossplotting P-wave and S-wave velocities, and by other techniques. |
| Geophysics | salt proximity survey | A type of refraction survey to help define a salt-sediment interface near a wellbore. The source is typically placed directly above the top of a salt dome and the receivers are placed at a number of locations within the borehole. This technique takes advantage of the fact that sound travels faster through the salt than the surrounding soft sediments, such as in the US Gulf Coast. This survey measures the fastest travel path, with part of its path through the salt. The resultant traveltimes are then inverted via a model to obtain a profile of the salt flanks relative to the borehole. |
| Geophysics | salt proximity vertical seismic profile | A type of reflection survey to help define a salt-sediment interface near a wellbore. |
| Geophysics | salt proximity VSP | A type of reflection survey to help define a salt-sediment interface near a wellbore |
| Geophysics | salt-proximity vertical seismic profile | A type of reflection survey to help define a salt-sediment interface near a wellbore. |
| Geophysics | salt-proximity VSP | A type of reflection survey to help define a salt-sediment interface near a wellbore. |
| Geophysics | sample frequency | The number of data points or measurements per unit of time or distance. |
| Geophysics | sample interval | The distance or time between data points or measurements. |
| Geophysics | sample rate | The number of measurements per unit of time, or the inverse of the sample interval. |
| Geophysics | seabed geophone | A type of receiver that can be positioned on the seafloor to acquire seismic data |
| Geophysics | secondary reflection | Multiply reflected seismic energy, or any event in seismic data that has incurred more than one reflection in its travel path. Depending on their time delay from the primary events with which they are associated, multiples are characterized as short-path or peg-leg, implying that they interfere with the primary reflection, or long-path, where they appear as separate events. Multiples from the water bottom (the interface of the base of water and the rock or sediment beneath it) and the air-water interface are common in marine seismic data, and are suppressed by seismic processing. |
| Geophysics | seismic | Pertaining to waves of elastic energy, such as that transmitted by P-waves and S-waves, in the frequency range of approximately 1 to 100 Hz. Seismic energy is studied by scientists to interpret the composition, fluid content, extent and geometry of rocks in the subsurface. “Seismic,” used as an adjective, is preferable to “seismics,” although “seismics” is used commonly as a noun. |
| Geophysics | seismic acquisition | The generation and recording of seismic data. Acquisition involves many different receiver configurations, including laying geophones or seismometers on the surface of the Earth or seafloor, towing hydrophones behind a marine seismic vessel, suspending hydrophones vertically in the sea or placing geophones in a wellbore (as in a vertical seismic profile) to record the seismic signal. A source, such as a vibrator unit, dynamite shot, or an air gun, generates acoustic or elastic vibrations that travel into the Earth, pass through strata with different seismic responses and filtering effects, and return to the surface to be recorded as seismic data. Optimal acquisition varies according to local conditions and involves employing the appropriate source (both type and intensity), optimal configuration of receivers, and orientation of receiver lines with respect to geological features. This ensures that the highest signal-to-noise ratio can be recorded, resolution is appropriate, and extraneous effects such as air waves, ground roll, multiples and diffractions can be minimized or distinguished, and removed through processing. |
| Geophysics | seismic impedance | Another term for acoustic impedance, the product of density and seismic velocity, which varies among different rock layers, commonly symbolized by Z. The difference in acoustic impedance between rock layers affects the reflection coefficient. |
| Geophysics | seismic interpretation | Analysis of seismic data to generate reasonable models and predictions about the properties and structures of the subsurface. Interpretation of seismic data is the primary concern of geophysicists. |
| Geophysics | seismic line | A display of seismic data along a line, such as a 2D seismic profile or a profile extracted from a volume of 3D seismic data. A seismic section consists of numerous traces with location given along the x-axis and two-way traveltime or depth along the y-axis. The section is called a depth section if the section has been converted from time to depth and a time section if this has not been done. |
| Geophysics | seismic modeling | The comparison, simulation or representation of seismic data to define the limits of seismic resolution, assess the ambiguity of interpretation or make predictions. Generation of a synthetic seismogram from a well log and comparing the synthetic, or modeled trace, with seismic data is a common direct modeling procedure. Generating a set of pseudologs from seismic data is the process known as seismic inversion, a type of indirect modeling. Models can be developed to address problems of structure and stratigraphy prior to acquisition of seismic data and during the interpretation of the data. As Sheriff (1991) points out, agreement between data and a model does not prove that the model is correct, since there can be numerous models that agree with a given data set. |
| Geophysics | seismic processing | Alteration of seismic data to suppress noise, enhance signal and migrate seismic events to the appropriate location in space. Processing steps typically include analysis of velocities and frequencies, static corrections, deconvolution, normal moveout, dip moveout, stacking, and migration, which can be performed before or after stacking. Seismic processing facilitates better interpretation because subsurface structures and reflection geometries are more apparent. |
| Geophysics | seismic record | Traces recorded from a single shotpoint. Numerous seismic records are displayed together in a single seismic section. |
| Geophysics | seismic reflection tomography | A technique to measure and display the three-dimensional distribution of velocity or reflectivity of a volume of the Earth by using numerous sources and receivers at the Earth’s surface. In reflection tomography, space is divided into cells, each having a certain velocity and reflectivity. The final model is the one whose velocities and reflectivities best describe the data |
| Geophysics | seismic refraction method | A seismic aquisition method in which the incident and reflected angles are critical. These refraction surveys can be useful for evaluating increasing velocity gradients and locating features that have anomalously high velocities, such as a salt dome within surrounding rocks of lower velocities. |
| Geophysics | seismic section | A display of seismic data along a line, such a 2D seismic profile or a profile extracted from a volume of 3D seismic data. A seismic section consists of numerous traces with location given along the x-axis and two-way traveltime or depth along the y-axis. The section is called a depth section if the section has been converted from time to depth and a time section if this has not been done. |
| Geophysics | seismic stratigraphy | Another term for sequence stratigraphy, a field of study in which basin-filling sedimentary deposits, called sequences, are interpreted in a framework of eustasy, sedimentation and subsidence through time in order to correlate strata and predict the stratigraphy of relatively unknown areas. Sequences tend to show cyclicity of changes in relative sea level and widespread unconformities, processes of sedimentation and sources of sediments, climate and tectonic activity over time. Sequence stratigraphic study promotes thorough understanding of the evolution of basins, but also allows for interpretations of potential source rocks and reservoir rocks in both frontier areas (having seismic data but little well data) and in more mature hydrocarbon provinces. Prediction of reservoir continuity is currently a key question in mature hydrocarbon provinces where sequence stratigraphy is being applied. The field originated during the 1960s with the study of the stratigraphy of the continental USA, where numerous unconformities could be correlated widely, and led to the proposal that major unconformities might mark synchronous global-scale events. Through sequence stratigraphy, widely-separated sediments that occur between correlatable unconformities could be compared with each other. Studies of outcrops and seismic lines bore out these concepts, which initially were called “Seismic Stratigraphy” and first published widely in 1977. Further study of seismic lines led to the interpretation of the geometry or architecture of seismic events as representing particular styles of sedimentation and depositional environments, and the integration of such interpretations with well log and core data. Because of the simultaneous, competitive nature of the research, numerous oil companies and academic groups use the terminology of sequence stratigraphy differently, and new terms are added continually. |
| Geophysics | seismic survey | A seismic data set measured and recorded with reference to a particular area of the Earth’s surface, to evaluate the subsurface. |
| Geophysics | seismic trace | The seismic data recorded for one channel. A seismic trace represents the response of the elastic wavefield to velocity and density contrasts across interfaces of layers of rock or sediments as energy travels from a source through the subsurface to a receiver or receiver array. |
| Geophysics | seismic velocity | The rate at which a seismic wave travels through a medium, that is, distance divided by traveltime. Seismic velocity can be determined from vertical seismic profiles or from velocity analysis of seismic data. It can vary vertically, laterally and azimuthally in anisotropic media and tends to increase with depth in the Earth because compaction reduces porosity. Velocity also varies as a function of how it is derived from the data. For example, the stacking velocity derived from normal moveout measurements of common depth point gathers differs from the average velocity measured vertically from a check-shot or vertical seismic profile (VSP). Velocity would be the same only in a constant-velocity (homogeneous) medium. |
| Geophysics | seismic wave |
A periodic vibrational disturbance in which energy is propagated through or on the surface of a medium without translation of the material. Waves can be differentiated by their frequency, amplitude, wavelength and speed of propagation. Seismic waves are |
| Geophysics | seismic while drilling vertical seismic profile | A technique using a seismic source on the surface and receivers in the borehole to acquire a vertical seismic profile (VSP) during pipe connections. Waveforms are transmitted to the surface during drilling operations and can be processed in time to yield reliable time-depth information and sometimes reflection information. Because the survey and analysis can be performed while a well is being drilled, the data can be considered in decisions during drilling operations. |
| Geophysics | seismic-while-drilling vertical seismic profile | A technique using a seismic source on the surface and receivers in the borehole to acquire a vertical seismic profile (VSP) during pipe connections. Waveforms are transmitted to the surface during drilling operations and can be processed in time to yield reliable time-depth information and sometimes reflection information. Because the survey and analysis can be performed while a well is being drilled, the data can be considered in decisions during drilling operations. |
| Geophysics | seismogram | Traces recorded from a single shotpoint. Numerous seismograms are displayed together in a single seismic section. |
| Geophysics | seismograph | A device or system that records the ground oscillations that make up exploration seismic data or earthquakes, sometimes used incorrectly as a synonym for geophone. A seismograph can include amplifiers, receivers and a recording device (such as a computer disk or magnetic tape) to record seismograms. A crude seismograph was built in 1855 by Italian physicist Luigi Palmieri (1807 to 1896). The modern seismograph, which used a pendulum, was invented in 1880 by James Ewing, Thomas Gray and Sir John Milne. |
| Geophysics | seismology | The study of seismic or elastic waves, such as from earthquakes, explosions or other causes. Interpretation of the structure and composition of the Earth from artificially created seismic waves is a chief concern of seismologists exploring for hydrocarbons and other resources. English physicist John Mitchell (1724 to 1793) is known as the founder of seismology in part because of his observation that one can determine an earthquake’s epicenter, or point of origin in the subsurface, by measuring the arrival time of earthquake waves at different locations. The invention of the modern seismograph in 1880 promoted further studies of earthquakes. |
| Geophysics | seismometer | A device that records seismic energy in the form of ground motion and transforms it to an electrical impulse. |
| Geophysics | semblance | A quantitative measure of the coherence of seismic data from multiple channels that is equal to the energy of a stacked trace divided by the energy of all the traces that make up the stack. If data from all channels are perfectly coherent, or show continuity from trace to trace, the semblance has a value of unity. |
| Geophysics | sensitivity | The smallest change in a measurement that can be recorded by an instrument. |
| Geophysics | SH wave | A shear wave that is polarized so that its particle motion and direction of propagation are contained in a horizontal plane. |
| Geophysics | shadow zone | Generally, an area of the Earth from which waves do not emerge or cannot be recorded. In seismology, the term is used to more specifically describe regions of the subsurface where P-waves and S-waves are difficult to detect, such as regions of the core at certain distances from the epicenter of an earthquake, or the point on the Earth’s surface directly above an earthquake. Such zones were first observed in 1914 by Beno Gutenberg (1889 to 1960), an American geologist born in Germany. Because of the molten nature of the outer core, S-waves are especially difficult to detect at 103 to 142 degrees from the epicenter of an earthquake and not observable from 142 to 180 degrees from the epicenter. Areas below salt features are also called shadow zones because the high velocity of salt bends and traps energy, so seismic data quality beneath salt is generally poor unless special seismic processing is performed. |
| Geophysics | shaped charge | Explosives designed to affect a certain direction preferentially. Shaped charges are most commonly used to perforate wells, but can be an energy source for seismic acquisition. |
| Geophysics | shear | A type of vertical seismic profile in which the source is a shear-wave source rather than a compressional-wave source. Shear waves travel through the Earth at about half the speed of compressional waves and respond differently to fluid-filled rock, and so can provide different additional information about lithology and fluid content of hydrocarbon-bearing reservoirs. |
| Geophysics | shear modulus | expressed mathematically as follows: ? = ? / ? = (?F/A) / (?L/L), where ? = Shear modulus ? = Shear stress = ?F/A ?F = Increment of shear force A = Area acted on by the shear force ? = Shear strain = ?L/L ?L = Increment of transverse displacement parallel to A L = Original length. |
| Geophysics | shear wave | Also known as S-wave, an elastic body wave in which particles oscillate perpendicular to the direction in which the wave propagates. S-waves are generated by most land seismic sources, but not by air guns. P-waves that impinge on an interface at non-normal incidence can produce S-waves, which in that case are known as converted waves. S-waves can likewise be converted to P-waves. S-waves, or shear waves, travel more slowly than P-waves and cannot travel through fluids because fluids do not support shear. Recording of S-waves requires receivers coupled to the solid Earth. Interpretation of S-waves can allow determination of rock properties such as fracture density and orientation, Poisson’s ratio and rock type by crossplotting P-wave and S-wave velocities, and by other techniques. |
| Geophysics | shoot a well | To acquire a type of borehole seismic data designed to measure the seismic traveltime from the surface to a known depth. P-wave velocity of the formations encountered in a wellbore can be measured directly by lowering a geophone to each formation of interest, sending out a source of energy from the surface of the Earth, and recording the resultant signal. The data can then be correlated to surface seismic data by correcting the sonic log and generating a synthetic seismogram to confirm or modify seismic interpretations. It differs from a vertical seismic profile in the number and density of receiver depths recorded; geophone positions may be widely and irregularly located in the wellbore, whereas a vertical seismic profile usually has numerous geophones positioned at closely and regularly spaced intervals in the wellbore. |
| Geophysics | short-path multiple | Multiply-reflected seismic energy with a shorter travel path than long-path multiples. Short-path multiples tend to come from shallow subsurface phenomena or highly cyclical sedimentation and arrive soon after, and sometimes very near, the primary reflections. Short-path multiples are less obvious than most long-path multiples and are less easily removed by seismic processing. |
| Geophysics | shot depth | The location of an explosive seismic source below the surface. Before acquisition of surface seismic data onshore using explosive sources such as dynamite, holes are drilled at shotpoints and dynamite is placed in the holes. The shotholes can be more than 50 m [164 ft] deep, although depths of 6 to 30 m [20 to 98 ft] are most common and depth is selected according to local conditions. With other “surface” sources, such as vibrators and shots from air shooting, the shots occur at the Earth’s surface. |
| Geophysics | shot point | One of a number of locations or stations at the surface of the Earth at which a seismic source is activated. |
| Geophysics | shotpoint | One of a number of locations or stations at the surface of the Earth at which a seismic source is activated. |
| Geophysics | SH-wave | A shear wave that is polarized so that its particle motion and direction of propagation are contained in a horizontal plane. |
| Geophysics | side scan sonar | A system for acoustic surveying most commonly deployed in marine environments and towed by a ship. The side-scan sonar generates a pulse on the order of 30 to 120 kHz that is reflected from the seafloor. Side-scan sonar records yield an image of the seafloor and shallow sediments. |
| Geophysics | side-scan sonar | A system for acoustic surveying most commonly deployed in marine environments and towed by a ship. The side-scan sonar generates a pulse on the order of 30 to 120 kHz that is reflected from the seafloor. Side-scan sonar records yield an image of the seafloor and shallow sediments. |
| Geophysics | sideswipe | A type of event in 2D seismic data in which a feature out of the plane of a seismic section is apparent, such as an anticline, fault or other geologic structure. A properly migrated 3D survey will not contain sideswipes. |
| Geophysics | signal | The portion of the seismic wave that contains desirable information. Noise is the undesirable information that typically accompanies the signal and can, to some extent, be filtered out of the data. |
| Geophysics | signal to noise ratio | The ratio of desirable to undesirable (or total) energy. The signal-to-noise ratio can be expressed mathematically as S/N or S/(S+N), although S/N is more commonly used. The signal-to-noise ratio is difficult to quantify accurately because it is difficult to completely separate signal from noise. It also depends on how noise is defined. |
| Geophysics | signal-to-noise ratio | The ratio of desirable to undesirable (or total) energy. The signal-to-noise ratio can be expressed mathematically as S/N or S/(S+N), although S/N is more commonly used. The signal-to-noise ratio is difficult to quantify accurately because it is difficult to completely separate signal from noise. It also depends on how noise is defined. |
| Geophysics | signature | A distinguishing feature of a waveform in a seismic event, such as shape, polarity, amplitude, frequency or phase. The signature of the seismic source waveform is of particular interest to geophysicists. |
| Geophysics | signature deconvolution | A step in seismic processing by which the signature of the seismic source in the seismic trace is changed to a known, shorter waveform by using knowledge of the source waveform. If the source waveform is known for each shot, then the process also minimizes variations between seismic records that result from changes in the source output. |
| Geophysics | simple multiple | An event in which one deeper and one near-surface reflector, such as the base of weathering or the ocean floor, are involved. The seismic energy bounces twice from the deep reflector and only once from the shallow reflector, causing the multiple to appear at roughly twice the traveltime of the primary reflection. |
| Geophysics | sinc x | A function commonly used in seismic processing. Sinc x is the Fourier transform of a boxcar function, which is a function with a rectangular-shaped aperture. |
| Geophysics | Single-azimuth towed-streamer acquisition | Conventional marine seismic data acquisition method using a single vessel to tow one or more seismic source arrays and streamers in a straight line as the vessel records seismic data. With this method, the angle between the source and receivers is narrow. |
| Geophysics | skid | A conveyance, such as a sled with runners or pontoons, used to transport geophysical gear to a location. Skids are commonly deployed in acquisition of seismic data in marshes or other areas of soft, soggy terrain. |
| Geophysics | skin depth |
The effective depth of penetration of an electromagnetic wave in a conductive medium. The skin depth is the distance in which the wave decays to 1/e (about 37%) of its value; it can be expressed as: ?s = (2/???)1/2 = (2/?)(?/?)1/2, where ?s = skin depth |
| Geophysics | slant stack | A process used in seismic processing to stack, or sum, traces by shifting traces in time in proportion to their offset. This technique is useful in areas of dipping reflectors. |
| Geophysics | smile | A concave-upward, semicircular event in seismic data that has the appearance of a smile and can be caused by poor data migration or migration of noise. |
| Geophysics | Snell’s law |
The mathematical description of refraction, or the physical change in the direction of a wavefront as it travels from one medium to another with a change in velocity and partial conversion and reflection of a P-wave to an S-wave at the interface of the |
| Geophysics | sonic | Some authors use the term to describe P-waves in fluids, or as a synonym for seismic or elastic. |
| Geophysics | sonic | Pertaining to sound waves in the frequency range of 1 to 25 kilohertz. |
| Geophysics | sonic log | A type of acoustic log that displays traveltime of P-waves versus depth. Sonic logs are typically recorded by pulling a tool on a wireline up the wellbore. The tool emits a sound wave that travels from the source to the formation and back to a receiver. |
| Geophysics | sound | Another term for probe, in electromagnetic methods, to measure the variation of a property versus depth, including electrical, electromagnetic and magnetotelluric properties. Probing differs from profiling in that the goal of probing is to provide a record of vertical changes, whereas profiling documents lateral variations. |
| Geophysics | source | A device that provides energy for acquisition of seismic data, such as an air gun, explosive charge or vibrator. |
| Geophysics | source pattern | A geometrical arrangement of seismic sources (a source array), with each individual source being activated in some fixed sequence in time. |
| Geophysics | source point | One of a number of locations or stations at the surface of the Earth at which a seismic source is activated. |
| Geophysics | sourcepoint | One of a number of locations or stations at the surface of the Earth at which a seismic source is activated. |
| Geophysics | SP | One of a number of locations or stations at the surface of the Earth at which a seismic source is activated. |
| Geophysics | space frequency domain | A display, also known as the f-k domain, of seismic data by wavenumber versus frequency rather than the intuitive display of location versus time for convenience during seismic processing. Working in the space-frequency domain provides the seismic processor with an alternative measure of the content of seismic data in which operations such as filtering of certain unwanted events can be accomplished more effectively. |
| Geophysics | space-frequency domain | A display, also known as the f-k domain, of seismic data by wavenumber versus frequency rather than the intuitive display of location versus time for convenience during seismic processing. Working in the space-frequency domain provides the seismic processor with an alternative measure of the content of seismic data in which operations such as filtering of certain unwanted events can be accomplished more effectively. |
| Geophysics | spacing | The distance between sources and receivers, particularly in logging tools. |
| Geophysics | spectral | Pertaining to a spectrum. The spectral content of a wavetrain or wavelet usually refers to its amplitude and phase as a function of frequency. |
| Geophysics | spectrum | Generally, a display of entities or properties according to magnitude. In geophysics, spectrum refers to a display of characteristics of a wavetrain or trace as a function of frequency, wavenumber, or arrival time. A common display of spectrum is amplitude as a function of frequency. |
| Geophysics | spherical divergence | The apparent loss of intensity of a gravitational or magnetic field with distance. Spherical divergence decreases energy with the square of the distance. |
| Geophysics | spherical divergence | The apparent loss of energy from a wave as it spreads during travel. Spherical divergence decreases energy with the square of the distance. |
| Geophysics | spherical harmonic | The solution to the Laplace equation expressed as spherical coordinates. The normal modes of the Earth, or the reverberations that follow earthquakes, have the form of spherical harmonics. Love waves and Rayleigh waves can also be expressed as spherical harmonics. |
| Geophysics | spherical wave | A wave generated from a point source, such as that generated by an underground explosion. Typical seismic sources such as vibrators and air-gun arrays emit elastic waves that are assumed to be spherical waves. |
| Geophysics | spontaneous potential | Naturally occurring (static) electrical potential in the Earth. Spontaneous potentials are usually caused by charge separation in clay or other minerals, by the presence of a semipermeable interface impeding the diffusion of ions through the pore space of rocks, or by natural flow of a conducting fluid (salty water) through the rocks. Variations in SP can be measured in the field and in wellbores to determine variations of ionic concentration in pore fluids of rocks. |
| Geophysics | spread | The geometrical pattern of groups of geophones relative to the seismic source. The output from a single shot is recorded simultaneously by the spread during seismic acquisition. Common spread geometries include in-line offset, L-spread, split-spread and T-spread. |
| Geophysics | stack | To sum traces to improve the signal-to-noise ratio, reduce noise and improve seismic data quality. Traces from different shot records with a common reflection point, such as common midpoint (CMP) data, are stacked to form a single trace during seismic processing. Stacking reduces the amount of data by a factor called the fold. |
| Geophysics | stack | A processed seismic record that contains traces that have been added together from different records to reduce noise and improve overall data quality. The number of traces that have been added together during stacking is called the fold. |
| Geophysics | stacking velocity | The distance-time relationship determined from analysis of normal moveout (NMO) measurements from common depth point gathers of seismic data. The stacking velocity is used to correct the arrival times of events in the traces for their varying offsets prior to summing, or stacking, the traces to improve the signal-to-noise ratio of the data. |
| Geophysics | static correction | Often called statics, a bulk shift of a seismic trace in time during seismic processing. A common static correction is the weathering correction, which compensates for a layer of low seismic velocity material near the surface of the Earth. Other corrections compensate for differences in topography and differences in the elevations of sources and receivers. |
| Geophysics | statics | Another term for static correction, a bulk shift of a seismic trace in time during seismic processing. A common static correction is the weathering correction, which compensates for a layer of low seismic velocity material near the surface of the Earth. Other corrections compensate for differences in topography and differences in the elevations of sources and receivers. |
| Geophysics | Stoneley wave | A type of large-amplitude interface, or surface, wave generated by a sonic tool in a borehole. Stoneley waves can propagate along a solid-fluid interface, such as along the walls of a fluid-filled borehole and are the main low-frequency component of signal generated by sonic sources in boreholes. Analysis of Stoneley waves can allow estimation of the locations of fractures and permeability of the formation. Stoneley waves are a major source of noise in vertical seismic profiles. |
| Geophysics | streamer | A surface marine cable, usually a buoyant assembly of electrical wires that connects hydrophones and relays seismic data to the recording seismic vessel. Multistreamer vessels tow more than one streamer cable to increase the amount of data acquired in one pass. |
| Geophysics | streamer feathering | In marine seismic acquisition, the lateral deviation of a streamer away from the towing direction because of a water current. |
| Geophysics | suppression | In seismic acquisition and processing, the attenuation of amplitudes to reduce the effects of noise or to prevent overload from the high energy of first breaks. |
| Geophysics | surface wave | A wave that propagates at the interface between two media as opposed to through a medium. A surface wave can travel at the interface between the Earth and air, or the Earth and water. Love waves and Rayleigh waves are surface waves. |
| Geophysics | survey | To measure and record data according to location on the Earth’s surface. In geophysics, the term is used in the context of acquiring seismic, electrical, gravity or magnetic data to evaluate the subsurface. |
| Geophysics | survey | A data set measured and recorded with reference to a particular area of the Earth’s surface, such as a seismic survey. |
| Geophysics | SV wave | A shear wave that is polarized so that its particle motion and direction of propagation occur in a vertical plane. |
| Geophysics | SV-wave | A shear wave that is polarized so that its particle motion and direction of propagation occur in a vertical plane. |
| Geophysics | S-wave | An elastic body wave in which particles oscillate perpendicular to the direction in which the wave propagates. S-waves are generated by most land seismic sources, but not by air guns. P-waves that impinge on an interface at non-normal incidence can produce S-waves, which in that case are known as converted waves. S-waves can likewise be converted to P-waves. S-waves, or shear waves, travel more slowly than P-waves and cannot travel through fluids because fluids do not support shear. Recording of S-waves requires receivers coupled to the solid Earth. Interpretation of S-waves can allow determination of rock properties such as fracture density and orientation, Poisson’s ratio and rock type by crossplotting P-wave and S-wave velocities, and by other techniques. |
| Geophysics | synthetic seismogram | The result of one of many forms of forward modeling to predict the seismic response of the Earth. A more narrow definition used by seismic interpreters is that a synthetic seismogram, commonly called a synthetic, is a direct one-dimensional model of acoustic energy traveling through the layers of the Earth. The synthetic seismogram is generated by convolving the reflectivity derived from digitized acoustic and density logs with the wavelet derived from seismic data. By comparing marker beds or other correlation points picked on well logs with major reflections on the seismic section, interpretations of the data can be improved. The quality of the match between a synthetic seismogram depends on well log quality, seismic data processing quality, and the ability to extract a representative wavelet from seismic data, among other factors. The acoustic log is generally calibrated with check-shot or vertical seismic profile (VSP) first-arrival information before combining with the density log to produce acoustic impedance. |
| Geophysics | tail buoy | A floating device used in marine seismic acquisition to identify the end of a streamer. Tail buoys allow the seismic acquisition crew to monitor the location and direction of streamers. They are commonly brightly colored, reflect radar signals, and are fitted with Global Positioning System (GPS) receivers. |
| Geophysics | tail mute | A cutoff in time, offset or both that has the effect of eliminating some types of noise from seismic data. A tail mute can be used to exclude slow surface waves such as ground roll. |
| Geophysics | tangential wave | Also known as S-wave, an elastic body wave in which particles oscillate perpendicular to the direction in which the wave propagates. S-waves are generated by most land seismic sources, but not by air guns. P-waves that impinge on an interface at non-normal incidence can produce S-waves, which in that case are known as converted waves. S-waves can likewise be converted to P-waves. S-waves, or shear waves, travel more slowly than P-waves and cannot travel through fluids because fluids do not support shear. Recording of S-waves requires receivers coupled to the solid Earth. Interpretation of S-waves can allow determination of rock properties such as fracture density and orientation, Poisson’s ratio and rock type by crossplotting P-wave and S-wave velocities, and by other techniques. |
| Geophysics | TAR | Steps in seismic processing to compensate for attenuation, spherical divergence and other effects by adjusting the amplitude of the data. The goal is to get the data to a state where the reflection amplitudes relate directly to the change in rock properties giving rise to them. |
| Geophysics | TDEM | A variation of the electromagnetic method in which electric and magnetic fields are induced by transient pulses of electric current in coils or antennas instead of by continuous (sinusoidal) current. These surveys have become a popular surface EM technique used in exploration for minerals and groundwater and for environmental mapping. |
| Geophysics | TE | A mode of the electromagnetic field that involves only one component of the electric field and the two components of the magnetic field perpendicular to it; e.g., the x-component of the electric field and y- and z-components of the magnetic field. The TE mode is useful in describing 2D models in which the electric field is perpendicular to the 2D plane of the model. For this case, Maxwell’s equations can be reduced to a single scalar equation for the electric field component, which simplifies calculations tremendously. There is an analogous mode for the magnetic field called the TM mode. A general EM field in a region without sources can be expressed as a sum of TE and TM modes. |
| Geophysics | telluric current | A low-frequency electrical current that occurs naturally over large areas at or near the surface of the Earth. Telluric currents are induced by changes in Earth’s magnetic field which are usually caused by interactions between the solar wind and the ionosphere (part of the upper atmosphere). |
| Geophysics | telluric current method | An electromagnetic method in which naturally occurring, low-frequency electric currents (telluric currents), are measured at a base station and compared with values measured at other stations. The normalized measurements of telluric current provide information about the direction of current flow and the conductance (conductivity times thickness) of sediments in the surveyed area. Extremely low-frequency telluric currents (with periods of days or months) provide information about conductivity in the deep interior of the Earth. |
| Geophysics | telluric-current method | An electromagnetic method in which naturally occurring, low-frequency electric currents (telluric currents), are measured at a base station and compared with values measured at other stations. The normalized measurements of telluric current provide information about the direction of current flow and the conductance (conductivity times thickness) of sediments in the surveyed area. Extremely low-frequency telluric currents (with periods of days or months) provide information about conductivity in the deep interior of the Earth. |
| Geophysics | TEM | A variation of the electromagnetic method in which electric and magnetic fields are induced by transient pulses of electric current in coils or antennas instead of by continuous (sinusoidal) current. These surveys have become a popular surface EM technique used in exploration for minerals and groundwater and for environmental mapping. |
| Geophysics | three dimensional seismic data | A set of numerous closely-spaced seismic lines that provide a high spatially sampled measure of subsurface reflectivity. Typical receiver line spacing can range from 300 m [1000 ft] to over 600 m [2000 ft], and typical distances between shotpoints and receiver groups is 25 m [82 ft] (offshore and internationally) and 110 ft or 220 ft [34 to 67 m] (onshore USA, using values that are even factors of the 5280 feet in a mile). Bin sizes are commonly 25 m, 110 ft or 220 ft. The resultant data set can be “cut” in any direction but still display a well sampled seismic section. The original seismic lines are called in-lines. Lines displayed perpendicular to in-lines are called crosslines. In a properly migrated 3D seismic data set, events are placed in their proper vertical and horizontal positions, providing more accurate subsurface maps than can be constructed on the basis of more widely spaced 2D seismic lines, between which significant interpolation might be necessary. In particular, 3D seismic data provide detailed information about fault distribution and subsurface structures. Computer-based interpretation and display of 3D seismic data allow for more thorough analysis than 2D seismic data. |
| Geophysics | three dimensional survey | The acquisition of seismic data as closely spaced receiver and shot lines such that there typically are no significant gaps in the subsurface coverage. A 2D survey commonly contains numerous widely spaced lines acquired orthogonally to the strike of geological structures and a minimum of lines acquired parallel to geological structures to allow line-to-line correlation of the seismic data and interpretation and mapping of structures. |
| Geophysics | three-component seismic data | A type of multicomponent seismic data acquired in a land, marine, or borehole environment by using three orthogonally oriented geophones or accelerometers. 3C is particularly appropriate when the addition of a hydrophone (the basis for 4C seismic data) adds no value to the measurement, as for example, on land. This technique allows determination of both the type of wave and its direction of propagation. |
| Geophysics | three-dimensional seismic data | A set of numerous closely-spaced seismic lines that provide a high spatially sampled measure of subsurface reflectivity. Typical receiver line spacing can range from 300 m [1000 ft] to over 600 m [2000 ft], and typical distances between shotpoints and receiver groups is 25 m [82 ft] (offshore and internationally) and 110 ft or 220 ft [34 to 67 m] (onshore USA, using values that are even factors of the 5280 feet in a mile). Bin sizes are commonly 25 m, 110 ft or 220 ft. The resultant data set can be “cut” in any direction but still display a well sampled seismic section. The original seismic lines are called in-lines. Lines displayed perpendicular to in-lines are called crosslines. In a properly migrated 3D seismic data set, events are placed in their proper vertical and horizontal positions, providing more accurate subsurface maps than can be constructed on the basis of more widely spaced 2D seismic lines, between which significant interpolation might be necessary. In particular, 3D seismic data provide detailed information about fault distribution and subsurface structures. Computer-based interpretation and display of 3D seismic data allow for more thorough analysis than 2D seismic data. |
| Geophysics | three-dimensional survey | The acquisition of seismic data as closely spaced receiver and shot lines such that there typically are no significant gaps in the subsurface coverage. A 2D survey commonly contains numerous widely spaced lines acquired orthogonally to the strike of geological structures and a minimum of lines acquired parallel to geological structures to allow line-to-line correlation of the seismic data and interpretation and mapping of structures. |
| Geophysics | TI | Abbreviation for transverse isotropy. Transverse isotropy, polar anisotropy, axial anisotropy and cross anisotropy are synonymous terms referring to the particular directional character of materials in which properties have the same values in all directions parallel to planes of isotropy and different values perpendicular to or crossing the planes of isotropy; this perpendicular direction is an axis of rotational symmetry. |
| Geophysics | tie | To correlate data in order to formulate or verify an interpretation or to demonstrate the relationship between data sets. Long, regional-scale 2D seismic lines are commonly tied to 3D surveys that cover a limited area, and 3D surveys of different vintages are tied to each other. Well logs are tied into seismic data routinely to determine the relationship between lithologic boundaries in the logs and seismic reflections. Properly tying all available data, including seismic data, well logs, check-shot surveys, synthetic seismograms and vertical seismic profiles, can reduce or, if there are sufficient data, eliminate ambiguity in interpretations. |
| Geophysics | tie | A comparison, or the location of a comparison, of data. Properly processed and interpreted seismic lines can show good ties, or correlations, at intersection points. |
| Geophysics | TIH | Abbreviation for horizontal transverse isotropy. Transverse isotropy that has a horizontal axis of rotational symmetry. In vertically fractured rocks, properties are uniform in vertical planes parallel to the fractures, but vary in the direction perpendicular to the fractures and across the fractures. |
| Geophysics | time domain | The use of a function of time rather than frequency to express an independent variable or measurement. In contrast, in the frequency domain, variables are expressed as a function of frequency instead of time. |
| Geophysics | time domain electromagnetic method | A variation of the electromagnetic method in which electric and magnetic fields are induced by transient pulses of electric current in coils or antennas instead of by continuous (sinusoidal) current. These surveys have become a popular surface EM technique used in exploration for minerals and groundwater and for environmental mapping. |
| Geophysics | time lapse seismic data | Seismic data from the surface or a borehole acquired at different times over the same area to assess changes in the subsurface with time, such as fluid movement or effects of secondary recovery. The data are examined for changes in attributes related to expressions of fluid content. Time-lapse seismic data can repeat 2D, 3D (which is known as 4D seismic data), crosswell and VSP data. |
| Geophysics | time migration | A migration technique for processing seismic data in areas where lateral velocity changes are not too severe, but structures are complex. Time migration has the effect of moving dipping events on a surface seismic line from apparent locations to their true locations in time. The resulting image is shown in terms of traveltime rather than depth, and must then be converted to depth with an accurate velocity model to be compared to well logs. |
| Geophysics | time slice | A horizontal display or map view of 3D seismic data having a certain arrival time, as opposed to a horizon slice that shows a particular reflection. A time slice is a quick, convenient way to evaluate changes in amplitude of seismic data. |
| Geophysics | time-domain electromagnetic method | A variation of the electromagnetic method in which electric and magnetic fields are induced by transient pulses of electric current in coils or antennas instead of by continuous (sinusoidal) current. These surveys have become a popular surface EM technique used in exploration for minerals and groundwater and for environmental mapping. |
| Geophysics | time-lapse seismic data | Seismic data from the surface or a borehole acquired at different times over the same area to assess changes in the subsurface with time, such as fluid movement or effects of secondary recovery. The data are examined for changes in attributes related to expressions of fluid content. Time-lapse seismic data can repeat 2D, 3D (which is known as 4D seismic data), crosswell and VSP data. |
| Geophysics | TIV | Abbreviation for vertical transverse isotropy. Transverse isotropy that has a vertical axis of rotational symmetry. In layered rocks, properties are uniform horizontally within a layer, but vary vertically and from layer to layer. |
| Geophysics | TM | A mode of the electromagnetic field that involves only one component of the magnetic field and the two components of the electric field perpendicular to it; e.g., the x-component of the magnetic field and y- and z-components of the electric field. The TM mode is useful in describing 2D models in which the magnetic field is perpendicular to the 2D plane of the model. For this case, Maxwell’s equations can be reduced to a single scalar equation for the magnetic field component, which simplifies calculations tremendously. |
| Geophysics | tomography | A technique to measure and display the three-dimensional distribution of velocity or reflectivity of a volume of the Earth by using numerous sources and receivers. There are several types of tomography used by geophysicists, including transmission tomography (which uses measurements between boreholes, surface-to-surface, or between a borehole and the surface), reflection or seismic tomography (based on standard reflection seismology), and diffraction tomography (using Fermat’s principle for computations instead of Snell’s law). Variations in velocity can be attributed to changes in density and elastic properties of rocks, which in turn are affected by the increasing temperature with depth in the Earth. Tomographic techniques have been used to construct maps of the Earth’s interior, deep in the mantle, as well as for mapping the shallow subsurface by borehole tomography. |
| Geophysics | trace | The seismic data recorded for one channel. A trace is a recording of the Earth’s response to seismic energy passing from the source, through subsurface layers, and back to the receiver. |
| Geophysics | transient electromagnetic method | A variation of the electromagnetic method in which electric and magnetic fields are induced by transient pulses of electric current in coils or antennas instead of by continuous (sinusoidal) current. These surveys have become a popular surface EM technique used in exploration for minerals and groundwater and for environmental mapping. |
| Geophysics | transit time | The duration of time for a P-wave to travel one foot, typically displayed on an acoustic log. The unit of microseconds per foot (or meter) is called the slowness, which is the inverse of velocity. Transit time is measured in microseconds per foot (?s/ft) or in microseconds per meter (?s/m). |
| Geophysics | transition zone | An area in which water is too shallow for acquisition of marine seismic data with towed streamers, such as near the shoreline, marshes and lagoons. In some cases, source explosives can be rammed into the unconsolidated sediments of transition zone environments rather than drilling more costly shot holes. Likewise, hydrophones can be placed by ramming to couple the receiver to the Earth better and to save time and money during survey acquisition. |
| Geophysics | transverse electric mode | A mode of the electromagnetic field that involves only one component of the electric field and the two components of the magnetic field perpendicular to it; e.g., the x-component of the electric field and y- and z-components of the magnetic field. The TE mode is useful in describing 2D models in which the electric field is perpendicular to the 2D plane of the model. For this case, Maxwell’s equations can be reduced to a single scalar equation for the electric field component, which simplifies calculations tremendously. There is an analogous mode for the magnetic field called the TM mode. A general EM field in a region without sources can be expressed as a sum of TE and TM modes. |
| Geophysics | transverse isotropy | Transverse isotropy, polar anisotropy, axial anisotropy and cross anisotropy are synonymous terms referring to the particular directional character of materials in which properties have the same values in all directions parallel to planes of isotropy and different values perpendicular to or crossing the planes of isotropy; this perpendicular direction is an axis of rotational symmetry. |
| Geophysics | transverse magnetic mode | A mode of the electromagnetic field that involves only one component of the magnetic field and the two components of the electric field perpendicular to it; e.g., the x-component of the magnetic field and y- and z-components of the electric field. The TM mode is useful in describing 2D models in which the magnetic field is perpendicular to the 2D plane of the model. For this case, Maxwell’s equations can be reduced to a single scalar equation for the magnetic field component, which simplifies calculations tremendously. |
| Geophysics | traveltime | The duration of the passage of a signal from the source through the Earth and back to the receiver. A time seismic section typically shows the two-way traveltime of the wave. |
| Geophysics | trough | The minimum (negative) deflection of the seismic wavelet. Seismic interpreters commonly pick or track seismic data on paper sections along the trough of a wavelet rather than the solid-colored peak. With the advent of workstations, this is no longer necessary because of automatic picking techniques and the ability to reverse the polarity of the data in real time. |
| Geophysics | true amplitude recovery | Steps in seismic processing to compensate for attenuation, spherical divergence and other effects by adjusting the amplitude of the data. The goal is to get the data to a state where the reflection amplitudes relate directly to the change in rock properties giving rise to them. |
| Geophysics | true-amplitude recovery | Steps in seismic processing to compensate for attenuation, spherical divergence and other effects by adjusting the amplitude of the data. The goal is to get the data to a state where the reflection amplitudes relate directly to the change in rock properties giving rise to them. |
| Geophysics | tube wave | An interface wave that occurs in cased wellbores when a Rayleigh wave encounters a wellbore and perturbs the fluid in the wellbore. The tube wave travels down the wellbore along the interface between the fluid in the wellbore and the wall of the wellbore. A tube wave suffers little energy loss and typically retains a very high amplitude which interferes with reflected arrivals occurring later in time on vertical seismic profile (VSP) data. Because the tube wave is coupled to the formation through which it is traveling, it can perturb the formation across open fractures intersecting the borehole. This squeezing effect can generate secondary tube waves which travel both up and down from the fracture location. Such events can be diagnostic of the presence of open fractures and their amplitude related qualitatively to the length and width, e.g., volume of the fluid-filled fracture space. This effect is generally seen only in shallow formations where the overburden pressure is lower. |
| Geophysics | tube wave | A Stoneley wave that occurs at the low frequencies of seismic data. |
| Geophysics | tuning effect | A phenomenon of constructive or destructive interference of waves from closely spaced events or reflections. At a spacing of less than one-quarter of the wavelength, reflections undergo constructive interference and produce a single event of high amplitude. At spacing greater than that, the event begins to be resolvable as two separate events. The tuning thickness is the bed thickness at which two events become indistinguishable in time, and knowing this thickness is important to seismic interpreters who wish to study thin reservoirs. The tuning thickness can be expressed by the following formula: Z = VI/2.8 fmax, where Z = tuning thickness of a bed, equal to 1/4 of the wavelength VI = interval velocity of the target fmax = maximum frequency in the seismic section. The equation assumes that the interfering wavelets are identical in frequency content and are zero-phase and is useful when planning a survey to determine the maximum frequency needed to resolve a given thickness. Spatial and temporal sampling requirements can then be established for the survey. |
| Geophysics | two dimensional seismic data | A group of 2D seismic lines acquired individually, as opposed to the multiple closely spaced lines acquired together that constitute 3D seismic data. |
| Geophysics | two dimensional seismic data | A vertical section of seismic data consisting of numerous adjacent traces acquired sequentially. |
| Geophysics | two dimensional survey | Seismic data or a group of seismic lines acquired individually such that there typically are significant gaps (commonly 1 km or more) between adjacent lines. A 2D survey typically contains numerous lines acquired orthogonally to the strike of geological structures (such as faults and folds) with a minimum of lines acquired parallel to geological structures to allow line-to-line tying of the seismic data and interpretation and mapping of structures. |
| Geophysics | two way traveltime | The elapsed time for a seismic wave to travel from its source to a given reflector and return to a receiver at the Earth’s surface. Minimum two-way traveltime is that of a normal-incidence wave with zero offset. |
| Geophysics | two-dimensional seismic data | A group of 2D seismic lines acquired individually, as opposed to the multiple closely spaced lines acquired together that constitute 3D seismic data. |
| Geophysics | two-dimensional seismic data | A vertical section of seismic data consisting of numerous adjacent traces acquired sequentially. |
| Geophysics | two-dimensional survey | Seismic data or a group of seismic lines acquired individually such that there typically are significant gaps (commonly 1 km or more) between adjacent lines. A 2D survey typically contains numerous lines acquired orthogonally to the strike of geological structures (such as faults and folds) with a minimum of lines acquired parallel to geological structures to allow line-to-line tying of the seismic data and interpretation and mapping of structures. |
| Geophysics | two-way traveltime | The elapsed time for a seismic wave to travel from its source to a given reflector and return to a receiver at the Earth’s surface. Minimum two-way traveltime is that of a normal-incidence wave with zero offset. |
| Geophysics | TWT | The elapsed time for a seismic wave to travel from its source to a given reflector and return to a receiver at the Earth’s surface. Minimum two-way traveltime is that of a normal-incidence wave with zero offset. |
| Geophysics | undershooting | A technique for acquisition of seismic data beneath areas that are difficult to access at the surface of the Earth, such as near rivers, drilling rigs, production platforms, environmentally sensitive areas or around seismically problematic features such as salt domes, which introduce uncertainty because of their high velocity. The sources and receivers are located on opposite sides of the feature. |
| Geophysics | upward continuation | The use of measurements of a field at one elevation, level or surface to determine the values of the field at a higher level. The technique is most often used on potential fields, such as gravity or magnetic fields, to reduce scattered measurements to a common level for a simpler interpretation. |
| Geophysics | variogram | A two-point statistical function that describes the increasing difference or decreasing correlation, or continuity, between sample values as separation between them increases. |
| Geophysics | velocity | The rate at which a wave travels through a medium (a scalar) or the rate at which a body is displaced in a given direction (a vector), commonly symbolized by v. Unlike the physicist’s definition of velocity as a vector, its usage in geophysics is as a property of a medium-distance divided by traveltime. Velocity can be determined from laboratory measurements, acoustic logs, vertical seismic profiles or from velocity analysis of seismic data. Velocity can vary vertically, laterally and azimuthally in anisotropic media such as rocks, and tends to increase with depth in the Earth because compaction reduces porosity. Velocity also varies as a function of how it is derived from the data. For example, the stacking velocity derived from normal moveout measurements of common depth point gathers differs from the average velocity measured vertically from a check-shot or vertical seismic profile (VSP). Velocity would be the same only in a constant velocity (homogeneous) medium. |
| Geophysics | velocity analysis | The process of calculating seismic velocity, typically by using common midpoint data, in order to better process seismic data. Successful stacking, time migration and depth migration all require proper velocity inputs. Velocity or stacking velocity can be calculated from normal moveout, or the change in arrival time produced by source-receiver offset. |
| Geophysics | velocity anomaly | A feature in seismic data that results from changes in velocity, both laterally and vertically. Pull-up and push-down are examples of velocity anomalies. |
| Geophysics | velocity correction | A change made in seismic data to present reflectors realistically. Velocity corrections typically require that assumptions be made about the seismic velocities of the rocks or sediments through which seismic waves pass. |
| Geophysics | velocity layering | Those thicknesses of rock or sediment that have a common velocity, as opposed to the sedimentary layering or bedding of the rock or sediments. |
| Geophysics | velocity survey | Measurements used to determine average velocity versus depth, such as from an acoustic log or check-shot survey. Acquiring a velocity survey is also known as “shooting a well. |
| Geophysics | vertical seismic profile | A class of borehole seismic measurements used for correlation with surface seismic data, for obtaining images of higher resolution than surface seismic images and for looking ahead of the drill bit; also called a VSP. Purely defined, VSP refers to measurements made in a vertical wellbore using geophones inside the wellbore and a source at the surface near the well. In the more general context, VSPs vary in the well configuration, the number and location of sources and geophones, and how they are deployed. Most VSPs use a surface seismic source, which is commonly a vibrator on land and an air gun in offshore or marine environments. VSPs include the zero-offset VSP, offset VSP, walkaway VSP, walk-above VSP, salt-proximity VSP, shear-wave VSP, and drill-noise or seismic-while-drilling VSP. A VSP is a much more detailed survey than a check-shot survey because the geophones are more closely spaced, typically on the order of 25 m [82 ft], whereas a check-shot survey might include measurements of intervals hundreds of meters apart. Also, a VSP uses the reflected energy contained in the recorded trace at each receiver position as well as the first direct path from source to receiver. The check-shot survey uses only the direct path traveltime. In addition to tying well data to seismic data, the vertical seismic profile also enables converting seismic data to zero-phase data and distinguishing primary reflections from multiples. |
| Geophysics | vertical transverse isotropy | Transverse isotropy that has a vertical axis of rotational symmetry. In layered rocks, properties are uniform horizontally within a layer, but vary vertically and from layer to layer. |
| Geophysics | vibrator | An adjustable mechanical source that delivers vibratory seismic energy to the Earth for acquisition of seismic data. Mounted on large trucks, vibrators are commonly used for acquisition of onshore seismic data. |
| Geophysics | vibratory seismic data | Seismic data whose energy source is a truck-mounted device called a vibrator that uses a vibrating plate to generate waves of seismic energy; also known as Vibroseis data (Vibroseis is a mark of Conoco). The frequency and duration of the energy can be controlled and varied according to the terrain and type of seismic data desired. The vibrator typically emits a linear “sweep” of at least seven seconds, beginning with high frequencies and decreasing with time (“downsweeping”) or going from low to high frequency (“upsweeping”). The frequency can also be changed in a nonlinear manner, such that certain frequencies are emitted longer than others. The resulting source wavelet is not impulsive. Vibrators are employed in land acquisition in areas where explosive sources cannot be used, and more than one vibrator can be used simultaneously to improve data quality. |
| Geophysics | VSP | A class of borehole seismic measurements used for correlation with surface seismic data, for obtaining images of higher resolution than surface seismic images and for looking ahead of the drill bit; also called a VSP. Purely defined, VSP refers to measurements made in a vertical wellbore using geophones inside the wellbore and a source at the surface near the well. In the more general context, VSPs vary in the well configuration, the number and location of sources and geophones, and how they are deployed. Most VSPs use a surface seismic source, which is commonly a vibrator on land and an air gun in offshore or marine environments. VSPs include the zero-offset VSP, offset VSP, walkaway VSP, walk-above VSP, salt-proximity VSP, shear-wave VSP, and drill-noise or seismic-while-drilling VSP. A VSP is a much more detailed survey than a check-shot survey because the geophones are more closely spaced, typically on the order of 25 m [82 ft], whereas a check-shot survey might include measurements of intervals hundreds of meters apart. Also, a VSP uses the reflected energy contained in the recorded trace at each receiver position as well as the first direct path from source to receiver. The check-shot survey uses only the direct path traveltime. In addition to tying well data to seismic data, the vertical seismic profile also enables converting seismic data to zero-phase data and distinguishing primary reflections from multiples. |
| Geophysics | VTI | Abbreviation for vertical transverse isotropy. Transverse isotropy that has a vertical axis of rotational symmetry. In layered rocks, properties are uniform horizontally within a layer, but vary vertically and from layer to layer. |
| Geophysics | walk above vertical seismic profile | A type of vertical seismic profile to accommodate the geometry of a deviated well; sometimes called a vertical incidence VSP. Each receiver is in a different lateral position with the source directly above the receiver for all cases. Such data provide a high-resolution seismic image of the subsurface below the trajectory of the well. |
| Geophysics | walk above VSP | A type of vertical seismic profile to accommodate the geometry of a deviated well; sometimes called a vertical incidence VSP. Each receiver is in a different lateral position with the source directly above the receiver for all cases. Such data provide a high-resolution seismic image of the subsurface below the trajectory of the well. |
| Geophysics | walk-above vertical seismic profile | A type of vertical seismic profile to accommodate the geometry of a deviated well; sometimes called a vertical incidence VSP. Each receiver is in a different lateral position with the source directly above the receiver for all cases. Such data provide a high-resolution seismic image of the subsurface below the trajectory of the well. |
| Geophysics | walk-above VSP | A type of vertical seismic profile to accommodate the geometry of a deviated well; sometimes called a vertical incidence VSP. Each receiver is in a different lateral position with the source directly above the receiver for all cases. Such data provide a high-resolution seismic image of the subsurface below the trajectory of the well. |
| Geophysics | walkaway vertical seismic profile | A type of vertical seismic profile in which the source is moved to progressively farther offset at the surface and receivers are held in a fixed location, effectively providing a mini 2D seismic line that can be of higher resolution than surface seismic data and provides more continuous coverage than an offset VSP. 3D walkaways, using a surface grid of source positions, provide 3D images in areas where the surface seismic data do not provide an adequate image due to near-surface effects or surface obstructions. Walkaway VSPs in which the receivers are placed just above the reservoir are gaining acceptance as a tool to quantify seismic attributes and calibrate surface seismic data. |
| Geophysics | walkaway VSP | A type of vertical seismic profile in which the source is moved to progressively farther offset at the surface and receivers are held in a fixed location, effectively providing a mini 2D seismic line that can be of higher resolution than surface seismic data and provides more continuous coverage than an offset VSP. 3D walkaways, using a surface grid of source positions, provide 3D images in areas where the surface seismic data do not provide an adequateimage due to near-surface effects or surface obstructions. Walkaway VSPs in which the receivers are placed just above the reservoir are gaining acceptance as a tool to quantify seismic attributes and calibrate surface seismic data. |
| Geophysics | Walsh-Hadamard transform | In digital signal processing, a nonsinusoidal transform by addition and subtraction. The Walsh-Hadamard transform is similar to Fourier series analysis, but uses square waves instead of sinusoidal waves. It is used predominantly in communication theory and, to a lesser extent, in filtering logs with a blocky character. |
| Geophysics | water bottom roll | The marine equivalent of ground roll. Water-bottom roll consists of a pseudo-Rayleigh wave traveling along the interface of the water and the seafloor. As the use of seabed receiver systems increases,noise from water-bottom roll has become more of a concern. |
| Geophysics | water gun | A source of energy for acquisition of marine seismic data that shoots water from a chamber in the tool into a larger body of water, creating cavitation. The cavity is a vacuum and implodes without creating secondary bubbles. This provides a short time signature and higher resolution than an air-gun source. |
| Geophysics | water-bottom roll | The marine equivalent of ground roll. Water-bottom roll consists of a pseudo-Rayleigh wave traveling along the interface of the water and the seafloor. As the use of seabed receiver systems increases, noise from water-bottom roll has become more of a concern. |
| Geophysics | wave | A periodic vibrational disturbance in which energy is propagated through or on the surface of a medium without translation of the material. Waves can be differentiated by their frequency, amplitude, wavelength and speed of propagation. Wavelength is defined as: ? = v / f, where ? = wavelength v = speed of propagation f = frequency. |
| Geophysics | wave equation | A mathematical expression to represent wave displacement and wave velocity (V) as functions of space (x, y, z) and time (t). ?2? = ?2?/?x2 + ?2?/?y2 + ?2?/?z2 = (1/V2) ?2?/?t2, where ? = wave displacement V = wave velocity x, y and z = space coordinates t = time. |
| Geophysics | waveform | The shape of a wave, typically shown as a graph of amplitude (or other quantity of interest) versus time. |
| Geophysics | wavefront | The edge of an advancing wave, which includes adjacent points that have the same phase. |
| Geophysics | wavelength | The distance between analogous points in a wave train, measured perpendicular to the wavefront. In seismic data, the wavelength is the seismic velocity divided by frequency. Wavelength is defined as: ? = v / f, where ? = wavelength v = velocity of propagation f = frequency. |
| Geophysics | wavelet | A one-dimensional pulse, usually the basic response from a single reflector. Its key attributes are its amplitude, frequency and phase. The wavelet originates as a packet of energy from the source point, having a specific origin in time, and is returned to the receivers as a series of events distributed in time and energy. The distribution is a function of velocity and density changes in the subsurface and the relative position of the source and receiver. The energy that returns cannot exceed what was input, so the energy in any received wavelet decays with time as more partitioning takes place at interfaces. Wavelets also decay due to the loss of energy as heat during propagation. This is more extensive at high frequency, so wavelets tend to contain less high-frequency energy relative to low frequencies at longer traveltimes. Some wavelets are known by their shape and spectral content, such as the Ricker wavelet. |
| Geophysics | wavelet extraction | A step in seismic processing to determine the shape of the wavelet, also known as the embedded wavelet, that would be produced by a wave train impinging upon an interface with a positive reflection coefficient. Wavelets may also be extracted by using a model for the reflections in a seismic trace, such as a synthetic seismogram. A wavelet is generated by deconvolving the trace with the set of reflection coefficients of the synthetic seismogram, a process also known as deterministic wavelet extraction. Wavelets may be extracted without a model for the reflections by generating a power spectrum of the data. By making certain assumptions, such as that the power spectrum contains information about the wavelet (and not the geology) and that the wavelet is of a certain phase (minimum, zero), a wavelet may be generated. This is also called statistical wavelet extraction. A particular processing approach to establishing the embedded wavelet is to compare the processed seismic response with the response measured by a vertical seismic profile (VSP) or generated synthetically through a synthetic seismogram in which the embedded wavelet is known. The wavelet can also be extracted through the autocorrelation of the seismic trace, in which case the phase of the wavelet has to be assumed. |
| Geophysics | wavenumber | The reciprocal of wavelength, so the number of wave cycles per unit of distance, abbreviated as k. |
| Geophysics | weathering correction | A method of compensating for delays in seismic reflection or refraction times induced by low-velocity layers such as the weathered layer near the Earth’s surface. It is a type of static correction. |
| Geophysics | well shoot | A type of borehole seismic data designed to measure the seismic traveltime from the surface to a known depth. P-wave velocity of the formations encountered in a wellbore can be measured directly by lowering a geophone to each formation of interest, sending out a source of energy from the surface of the Earth, and recording the resultant signal. The data can then be correlated to surface seismic data by correcting the sonic log and generating a synthetic seismogram to confirm or modify seismic interpretations. It differs from a vertical seismic profile in the number and density ofreceiver depths recorded; geophone positions may be widely and irregularly located in the wellbore, whereas a vertical seismic profile usually has numerous geophones positioned at closely and regularly spaced intervals in the wellbore. |
| Geophysics | Wide-azimuth towed-streamer acquisition | A marine seismic data acquisition method that uses one or more vessels to tow source arrays and streamers to record seismic signals, along with one or more source-only vessels sailing parallel to, but at some specified distance from, the recording vessel(s). The source-only vessels provide offset sources that generate reflections from a wide range of azimuths; these reflections are received by streamers towed by the recording vessel(s). |
| Geophysics | wiggle trace | A common seismic display that shows trace amplitude versus time as an oscillating line about a null point. |
| Geophysics | window | Another term for aperture, a portion of a data set, such as seismic data, to which functions or filters are applied. Aperture time, for example, can be specified, such as a window from 1.2 to 2.8 seconds. |
| Geophysics | window | Another term for aperture, a mechanism to limit the effects of measurements on a device or system. In seismic data acquisition, the length of the spread has the effect of an aperture. |
| Geophysics | work station | An interactive computer suitable for seismic data processing, interpretation and modeling that is particularly useful for studies of large quantities of seismic data, particularly 3D seismic data. |
| Geophysics | Young’s modulus | An elastic constant named after British physicist Thomas Young (1773 to 1829) that is the ratio of longitudinal stress to longitudinal strain and is symbolized by E. It can be expressed mathematically as follows: E = (F/A) / (?L/L), where E = Young’s modulus F = longitudinal force A = area F/A = longitudinal stress ?L = change in length L = original length ?L/L = longitudinal strain. |
| Geophysics | zero crossing | The null point of a seismic trace. At zero deflection, the phase of a periodic signal is zero or pi. |
| Geophysics | zero offset data | Seismic data acquired with no horizontal distance between the source and receiver. Stacking seismic data acquired with separated sources and receivers gives the data the appearance of zero-offset data. |
| Geophysics | zero offset vertical seismic profile | A conventional vertical seismic profile in which the energy source is positioned directly above the receivers, typically very close to the wellbore. |
| Geophysics | zero phase | Pertaining to seismic data whose wavelet is symmetrical about zero time. Deconvolution during seismic processing can convert data of mixed phase to zero-phase data, but is not always successful. Zero-phase data tend to provide sharper definition and less distortion between stratigraphic features in the subsurface, such as sand and shale layers. |
| Geophysics | zero-offset data | Seismic data acquired with no horizontal distance between the source and receiver. Stacking seismic data acquired with separated sources and receivers gives the data the appearance of zero-offset data. |
| Geophysics | zero-offset vertical seismic profile | A conventional vertical seismic profile in which the energy source is positioned directly above the receivers, typically very close to the wellbore. |
| Geophysics | zero-phase | Pertaining to seismic data whose wavelet is symmetrical about zero time. Deconvolution during seismic processing can convert data of mixed phase to zero-phase data, but is not always successful. Zero-phase data tend to provide sharper definition and less distortion between stratigraphic features in the subsurface, such as sand and shale layers. |
| Geophysics | Zoeppritz equations | A set of equations that describes the partitioning of energy in a wavefield relative to its angle of incidence at a boundary across which the properties of the rock and fluid content changes. |
| Geophysics, Formation Evaluation | delta t | Also called interval transit time, the amount of time for a wave to travel a certain distance, proportional to the reciprocal of velocity, typically measured in microseconds per foot by an acoustic log and symbolized by t or DT. P-wave interval transit times for common sedimentary rock types range from 43 (dolostone) to 160 (unconsolidated shales) microseconds per foot, and can be distinguished from measurements of steel casing, which has a consistent transit time of 57 microseconds per foot. |
| Geophysics, Formation Evaluation, Enhanced Oil Recovery | interwell tomography | A technique for measuring a signal that is broadcast from a transmitter or source located in one well, to a receiver array placed in a neighboring well. This technique is used to create a display of formation properties such as acoustic velocity and attenuation, seismic reflectivity, or electromagnetic resistivity in the area between wells. The reservoir-scale data acquired with this technique can be used to bridge the gap between wellbore measurements and surface measurements. |
| Geophysics, Geology | low velocity layer | Also known as weathered layer, a near-surface, possibly unconsolidated layer of low seismic velocity. The base of the weathered layer commonly coincides with the water table and a sharp increase in seismic velocity. The weathered layer typically has air-filled pores. |
| Geophysics, Geology, Shale Gas | anisotropic | Having directionally dependent properties. For a crystal of a mineral, variation in physical properties observed in different directions is anisotropy. In rocks, variation in seismic velocity measured parallel or perpendicular to bedding surfaces is a form of anisotropy. Often found where platy minerals such as micas and clays align parallel to depositional bedding as sediments are compacted, anisotropy is common in shales. |
| Geophysics, Shale Gas, Geology | anisotropy | Predictable variation of a property of a material with the direction in which it is measured, which can occur at all scales. For a crystal of a mineral, variation in physical properties observed in different directions is anisotropy. In rocks, variation in seismic velocity measured parallel or perpendicular to bedding surfaces is a form of anisotropy. Often found where platy minerals such as micas and clays align parallel to depositional bedding as sediments are compacted, anisotropy is common in shales. |
| Geophysics | baseline | The original survey of a set of surveys covering the same area but acquired over a period of time. In four-dimensional seismic data, it is the first seismic survey, which is then compared to subsequent surveys. |
| Heavy Oil | biodegradation | The breakdown of medium-weight crude oil by microbial organisms into heavy and light components. When the light components, typically methane, escape to the surface, the heavy ends are left behind. Biodegradation gradually raises oil viscosity, reduces API gravity, increases asphaltene content and increases concentration of certain metals and sulfur. |
| Heavy Oil | bitumen | A designation for a hydrocarbon fluid with a gravity of 10° API or lower, based upon the classification of the US Department of Energy. |
| Heavy Oil | blended crude | A mixture of crude oils, blended in the pipeline to create a crude with specific physical properties. Because heavy and extra-heavy crudes or bitumens cannot flow from the field to the refinery in their original state and at normal surface temperatures, they are blended with lighter crude oils primarily to reduce viscosity, thereby enabling transportation to a refinery. A secondary objective may be to provide a blended crude oil that has significantly higher value than the raw heavy crude. The blend is usually constructed so that the value of the overall blended volume is greater than the summed value of the initial volumes of individual heavy and light crudes. |
| Heavy Oil | calcium naphthenate | A calcium soap of naphthenic acids in crude oil. Naphthenates are formed through interaction of naphthenic acids in crude oil with metal ions such as calcium and sodium. Insoluble in either the oil or water phase, and with a density between that of oil and water, naphthenates tend to accumulate at the oil/water interface and act as surfactants to help stabilize emulsions. Naphthenates can also be deposited as solids in pipelines, and can cause flow-assurance problems. |
| Heavy Oil | CHOPS | The acronym for cold heavy oil production with sand. |
| Heavy Oil | cogeneration | The process of generating two or more forms of energy from a single energy source. For example, in a heavy oil field, turbines are often used to generate electricity while their waste heat is removed to generate steam. Other alternatives exist, with turbines being run by burning gas or crude oil. Alternatively, the primary heat source can be used to generate steam directly at extremely high pressure and temperature, with the steam being run through a turbine to generate electricity before the steam is distributed to injection wells. |
| Heavy Oil | cold production | Nonthermal primary methods of heavy oil production, which include technologies such as production with horizontal wells, multilaterals, CHOPS, water or gas injection. |
| Heavy Oil | cracking | The process of splitting a large heavy hydrocarbon molecule into smaller, lighter components. The process involves very high temperature and pressure and can involve a chemical catalyst to improve the process efficiency. |
| Heavy Oil | CSS | Abbreviation for cyclic steam stimulation. Better known as cyclic steam injection. |
| Heavy Oil | cyclic steam injection | A method of thermal recovery in which a well is injected with steam and then subsequently put back on production. A cyclic steam-injection process includes three stages. The first stage is injection, during which a slug of steam is introduced into the reservoir. The second stage, or soak phase, requires that the well be shut in for several days to allow uniform heat distribution to thin the oil. Finally, during the third stage, the thinned oil is produced through the same well. The cycle is repeated as long as oil production is profitable. Cyclic steam injection is used extensively in heavy-oil reservoirs, tar sands, and in some cases to improve injectivity prior to steamflood or in situ combustion operations. Cyclic steam injection is also called steam soak or the huff `n puff (slang) method. |
| Heavy Oil | diluent | A hydrocarbon fluid that is used to dilute heavy oil and reduce its viscosity for easier transportation. Generally a distillation tower cut such as naphtha is used as for heavy oil dilution and transportation. The added diluent may be recovered at the destination using distillation and the diluent may be subsequently pumped back for blending. |
| Heavy Oil | ESP | An electric downhole pump used in heavy oil production that is designed with vane and fin configurations to accommodate frictional losses and pump efficiencies caused by heavy oil viscosity. |
| Heavy Oil | fishbone wells | A series of multilateral well segments that trunk off a main horizontal well. The appearance closely resembles the ribs of a fish skeleton trunking off the main backbone. |
| Heavy Oil | flocculate | The term used to describe what small particles do when they aggregate into larger particles. In the context of heavy oil, asphaltenes are known to flocculate at the molecular level (before precipitation) and in the precipitated state. The extent of asphaltene flocculation changes with fluid composition, temperature and pressure. For precipitated asphaltenes, flocculation is also affected by the shear environment. |
| Heavy Oil | flocculation | The aggregation of small particles into larger particles. In the context of heavy oil, asphaltenes are known to flocculate at the molecular level (before precipitation) and in the precipitated state. The extent of asphaltene flocculation changes with fluid composition, temperature and pressure. For precipitated asphaltenes, flocculation is also affected by the shear environment. |
| Heavy Oil | foamy oil | An oil-continuous foam that contains dispersed gas bubbles produced at the wellhead from heavy oil reservoirs under solution gas drive. The nature of the gas dispersions in oil distinguishes foamy oil behavior from conventional heavy oil. The gas that comes out of solution in the reservoir does not coalesce into large gas bubbles nor into a continuous flowing gas phase. Instead it remains as small bubbles entrained in the crude oil, keeping the effective oil viscosity low while providing expansive energy that helps drive the oil toward the producing well. Foamy oil accounts for unusually high production in heavy oil reservoirs under solution-gas drive. |
| Heavy Oil | high temperature completions | Equipment or systems used for completion of wells in thermal production of heavy oil. |
| Heavy Oil | in-situ viscosity evaluation | Downhole measurement of fluid viscosity, typically performed either with logging tools based on nuclear magnetic resonance (NMR) or with sampling tools such as formation testers. |
| Heavy Oil | oil sand | In the context of heavy oil, an oil sand is a porous rock layer, often considered to be a mixture of sand, clay, water, and bitumen. The term is predominantly used in Canada, where over 170 billion barrels of bitumen are estimated to be held by large oil sand deposits in the Athabasca, Cold Lake and Peace River regions of Alberta. Oils from such sands may have a gravity of less than 10° API. Both in-situ recovery and mining methods are used to extract the resource, with surface mining used for extracting deposits of extra heavy oil at shallow depths of less than 100 m [328 ft]. |
| Heavy Oil | pour point | The temperature at which a fluid ceases to pour. The pour point for oil can be determined under protocols set forth in the ASTM D-97 pour point test, in which the pour point is established as that temperature at which oil ceases to flow when the sample is held at 90 degrees to the upright for five seconds. High pour points usually occur in crude oils that have significant paraffin content. Paraffins (or waxes) will start to precipitate as temperature decreases. At some point the precipitates accumulate to the point where the fluid can no longer flow. This phenomenon can occur with light oils as well as heavy oils. |
| Heavy Oil | resin | One of the four main components of petroleum, along with asphaltenes, aromatics and saturates (which include waxes). Resins, aromatics and saturates are also known as maltenes. Resin adds to the stickiness and viscosity of heavy oil. |
| Heavy Oil | SAGD | A thermal production method for heavy oil that pairs a high-angle injection well with a nearby production well drilled along a parallel trajectory. The pair of high-angle wells is drilled with a vertical separation of about 5 m [16 ft]. Steam is injected into the reservoir through the upper well. As the steam rises and expands, it heats up the heavy oil, reducing its viscosity. Gravity forces the oil to drain into the lower well where it is produced. |
| Heavy Oil | SARA analysis | A method for characterization of heavy oils based on fractionation, whereby a heavy oil sample is separated into smaller quantities or fractions, with each fraction having a different composition. Fractionation is based on the solubility of hydrocarbon components in various solvents used in this test. Each fraction consists of a solubility class containing a range of different molecular-weight species. In this method, the crude oil is fractionated to four solubility classes, referred to collectively as SARA: saturates, aromatics, resins, and asphaltenes. Saturates are generally iso- and cyclo-paraffins, while aromatics, resins, and asphaltenes form a continuum of molecules with increasing molecular weight, aromaticity, and heteroatom contents. Asphaltenes may also contain metals such as nickel and vanadium. This method is sometimes referred to as Asphaltene/Wax/Hydrate Deposition analysis. |
| Heavy Oil | steam management | The overall heat and fuel management for a steam injection process. It includes economically efficient use of fuel consumed to generate steam, minimization of heat losses in surface steam-distribution lines, proper splitting of steam flow and quality and intersections in steam-distribution lines, and effective management of steam and heat distribution in the reservoir. |
| Heavy Oil | tar sand | A sand body that contains heavy hydrocarbon residues such as tar or asphalt, or degraded oil that has lost its volatile components. Hydrocarbons can be liberated from tar sands by heating and other processes, but tar sands, such as the Athabasca tar sands of Canada, are not commonly commercial because of high costs of production. Among some workers in the field of heavy oil, this term is falling out of use, in favor of the term “oil sand. |
| Heavy Oil | THAI | Abbreviation for toe to heel air injection. |
| Heavy Oil | ultra heavy oil | A designation for a hydrocarbon fluid with a gravity of 10° API or lower, based upon the classification of the US Department of Energy. |
| Heavy Oil | upgrader | A refinery unit used to improve or upgrade heavy oil to produce higher-quality hydrocarbon liquids or upgraded synthetic crudes. The refining unit may include any combination of the following: hydrogen addition processes, carbon rejection processes or carbon concentration and removal processes. |
| Heavy Oil | VAPEX | A non-thermal heavy oil production method. Similar in concept to SAGD, in vapor extraction a solvent vapor is used to reduce viscosity of the heavy oil. The injected solvent vapor expands and dilutes the heavy oil by contact. The diluted heavy oil will drain by gravity to the lower horizontal well, to be produced. |
| Heavy Oil | wormhole | A high-porosity, high-permeability channel that develops when heavy oil is produced simultaneously with sand (during cold heavy oil production with sand, or CHOPS). Wormholes develop in a radial pattern away from the borehole and can extend 150 m [492 ft] from the borehole. The development of wormholes can cause reservoir pressure to fall below the bubblepoint, resulting in dissolved gas coming out of solution and forming foamy oil. |
| Heavy Oil, Enhanced Oil Recovery | CO2 injection | An enhanced oil recovery method in which carbon dioxide (CO2) is injected into a reservoir to increase production by reducing oil viscosity and providing miscible or partially miscible displacement of the oil. |
| Heavy Oil, Enhanced Oil Recovery | dry combustion | An in situ combustion technique in which only air or oxygen-enriched air mixtures are injected into a formation. A drawback related to dry combustion is the highly corrosive and noxious combustion products that are produced. |
| Heavy Oil, Enhanced Oil Recovery | emulsion | A dispersion of droplets of one liquid in another liquid with which it is incompletely miscible. Emulsions can form in heavy oils that contain a significant amount of asphaltenes. The asphaltenes act as surfactants with treatment or formation water. The resulting emulsion droplets have high-energy bonds creating a very tight dispersion of droplets that is not easily separated. These surface-acting forces can create both oil-in-water and/or water-in-oil emulsions. Such emulsions require temperature and chemical treating in surface equipment in order to separate. |
| Heavy Oil, Enhanced Oil Recovery | heavy oil | Crude oil with high viscosity (typically above 10 cp), and high specific gravity. The API classifies heavy oil as crudes with a gravity below 22.3° API. In addition to high viscosity and high specific gravity, heavy oils typically have low hydrogen-to-carbon ratios, high asphaltene, sulfur, nitrogen, and heavy-metal content, as well as higher acid numbers. |
| Heavy Oil, Enhanced Oil Recovery | hot waterflooding | A method of thermal recovery in which hot water is injected into a reservoir through specially distributed injection wells. Hot waterflooding reduces the viscosity of the crude oil, allowing it to move more easily toward production wells. Hot waterflooding, also known as hot water injection, is typically less effective than a steam-injection process because water has lower heat content than steam. Nevertheless, it is preferable under certain conditions such as formation sensitivity to fresh water. |
| Heavy Oil, Enhanced Oil Recovery | in situ combustion | A method of thermal recovery in which fire is generated inside the reservoir by injecting a gas containing oxygen, such as air. A special heater in the well ignites the oil in the reservoir and starts a fire. The heat generated by burning the heavy hydrocarbons in place produces hydrocarbon cracking, vaporization of light hydrocarbons and reservoir water in addition to the deposition of heavier hydrocarbons known as coke. As the fire moves, the burning front pushes ahead a mixture of hot combustion gases, steam and hot water, which in turn reduces oil viscosity and displaces oil toward production wells. Additionally, the light hydrocarbons and the steam move ahead of the burning front, condensing into liquids, which adds the advantages of miscible displacement and hot waterflooding. In situ combustion is also known as fire flooding or fireflood. |
| Heavy Oil, Enhanced Oil Recovery | inverted five-spot | An injection pattern in which four production wells are located at the corners of a square and the injector well sits in the center. |
| Heavy Oil, Enhanced Oil Recovery | liquid blocking | A phenomenon encountered during dry forward combustion in which an oil zone around the production well cannot be pushed forward by the heated oil. The fluid located in this zone is still at the original reservoir temperature. Therefore, the fluid is still highly viscous and normally not mobile. |
| Heavy Oil, Enhanced Oil Recovery | soak phase | In cyclic steam injection, the second phase between the steam-injection phase and the production phase. During the soak phase, the well is shut in for several days to allow uniform heat distribution to thin the oil. |
| Heavy Oil, Enhanced Oil Recovery | steam | A two-phase mixture of liquid water and steam produced from a generator. The latent heat of vaporization for steam is very high, and when the steam condenses in the reservoir a significant amount of heat is transferred from the steam to the formation rock and fluids. Since steam is lighter and more mobile than oil, gravity differences and channeling of the steam through the most permeable parts of the reservoir can create sweep efficiency problems during steam-injection processes. To increase sweep efficiency, there are two categories of improvements. The first is operational changes such as selective completion of injector wells, fracturing operations and constructing horizontal wells, and the second is the use of additives in the steam. For example, water-soluble surfactants modify interfacial properties of the oil-water system, and foams reduce steam mobility. |
| Heavy Oil, Enhanced Oil Recovery | steam chamber | The volume of reservoir in which mobile steam exists for an extended period of time. Within the steam chamber, rock temperature rises to the point where steam vapor can be sustained at reservoir pressure conditions. The steam chamber is normally found in the upper portion of a reservoir sand between a steam injector and a producer, where steam has broken through to the producer. With time, the steam chamber can expand to cover an entire area of a five-spot pattern steamflood. For a steam assisted gravity drainage (SAGD) system, the steam chamber in a mature field project can extend from a broad area across the top of the sand to a narrow finger down to the producing horizontal well near the bottom of the sand. Also referred to as a steam chest. |
| Heavy Oil, Enhanced Oil Recovery | steam-assisted gravity drainage | A thermal production method for heavy oil that pairs a high-angle injection well with a nearby production well drilled along a parallel trajectory. The pair of high-angle wells is drilled with a vertical separation of about 5 m [16 ft]. Steam is injected into the reservoir through the upper well. As the steam rises and expands, it heats up the heavy oil, reducing its viscosity. Gravity forces the oil to drain into the lower well where it is produced. |
| Heavy Oil, Enhanced Oil Recovery | TEOR | Abbreviation for thermal enhanced oil recovery, also known as thermal recovery, a general term for injection processes that introduce heat into a reservoir. Thermal recovery is used to produce viscous, thick oils with API gravities less than 20. These oils cannot flow unless they are heated and their viscosity is reduced enough to allow flow toward producing wells. During thermal recovery, crude oil undergoes physical and chemical changes because of the effects of the heat supplied. Physical properties such as viscosity, specific gravity and interfacial tension are altered. The chemical changes involve different reactions such as cracking, which is the destruction of carbon-carbon bonds to generate lower molecular weight compounds, and dehydrogenation, which is the rupture of carbon-hydrogen bonds. Thermal recovery is a major branch of enhanced oil recovery processes and can be subdivided in two types: hot fluid injection such as steam injection (steamflood or cyclic steam injection) and hot waterflooding and in-situ combustion processes. |
| Heavy Oil, Enhanced Oil Recovery | thermal recovery | A general term for injection processes that introduce heat into a reservoir. Thermal recovery is used to produce viscous, thick oils with API gravities less than 20. These oils cannot flow unless they are heated and their viscosity is reduced enough to allow flow toward producing wells. During thermal recovery, crude oil undergoes physical and chemical changes because of the effects of the heat supplied. Physical properties such as viscosity, specific gravity and interfacial tension are altered. The chemical changes involve different reactions such as cracking, which is the destruction of carbon-carbon bonds to generate lower molecular weight compounds, and dehydrogenation, which is the rupture of carbon-hydrogen bonds. Thermal recovery is a major branch of enhanced oil recovery processes and can be subdivided in two types: hot fluid injection such as steam injection (steamflood or cyclic steam injection) and hot waterflooding and in-situ combustion processes. |
| Heavy Oil, Enhanced Oil Recovery | vapor extraction | A non-thermal heavy oil production method. Similar in concept to SAGD, in vapor extraction a solvent vapor is used to reduce viscosity of the heavy oil. The injected solvent vapor expands and dilutes the heavy oil by contact. The diluted heavy oil will drain by gravity to the lower horizontal well, to be produced. |
| Heavy Oil, Enhanced Oil Recovery | viscous oil | Crude oil with high viscosity (typically above 10 cp), and high specific gravity. The API classifies heavy oil as crudes with a gravity below 22.3° API. In addition to high viscosity and high specific gravity, heavy oils typically have low hydrogen-to-carbon ratios, high asphaltene, sulfur, nitrogen, and heavy-metal content, as well as higher acid numbers. |
| Heavy Oil, Enhanced Oil Recovery | water-alternating gas | An enhanced oil recovery process whereby water injection and gas injection are carried out alternately for periods of time to provide better sweep efficiency and reduce gas channeling from injector to producer. This process is used mostly in CO2 floods to improve hydrocarbon contact time and sweep efficiency of the CO2. |
| Heavy Oil, Enhanced Oil Recovery, Well Completions | steam flood | A method of thermal recovery in which steam generated at surface is injected into the reservoir through specially distributed injection wells. When steam enters the reservoir, it heats up the crude oil and reduces its viscosity. The heat also distills light components of the crude oil, which condense in the oil bank ahead of the steam front, further reducing the oil viscosity. The hot water that condenses from the steam and the steam itself generate an artificial drive that sweeps oil toward producing wells. Another contributing factor that enhances oil production during steam injection is related to near-wellbore cleanup. In this case, steam reduces the interfacial tension that ties paraffins and asphaltenes to the rock surfaces while steam distillation of crude oil light ends creates a small solvent bank that can miscibly remove trapped oil. Steamflooding is also called continuous steam injection or steam drive. |
| Heavy Oil, Enhanced Oil Recovery, Well Completions | steamflood | A method of thermal recovery in which steam generated at surface is injected into the reservoir through specially distributed injection wells. When steam enters the reservoir, it heats up the crude oil and reduces its viscosity. The heat also distills light components of the crude oil, which condense in the oil bank ahead of the steam front, further reducing the oil viscosity. The hot water that condenses from the steam and the steam itself generate an artificial drive that sweeps oil toward producing wells. Another contributing factor that enhances oil production during steam injection is related to near-wellbore cleanup. In this case, steam reduces the interfacial tension that ties paraffins and asphaltenes to the rock surfaces while steam distillation of crude oil light ends creates a small solvent bank that can miscibly remove trapped oil. Steamflooding is also called continuous steam injection or steam drive. |
| Heavy Oil, Enhanced Oil Recovery, Well Completions | steam-oil ratio | Parameter used to monitor the efficiency of oil production processes based on steam injection. Commonly abbreviated as SOR, it measures the volume of steam required to produce one unit volume of oil. Typical values of SOR for cyclic steam stimulation are in the range of three to eight, while typical SOR values for steam assisted gravity drainage are in the range of two to five. The lower the SOR, the more efficiently the steam is utilized and the lower the associated fuel costs. |
| Heavy Oil | compositional fluid analysis | Any of a variety of analytical techniques carried out to determine the composition of a crude oil by breaking it down into basic chemical components. The hydrocarbon components are usually identified by carbon number fractions: C1, C2, C3, etc. up to Cn, where the limiting carbon number, n, is defined by the particular analytical technique. These analytical techniques include, but are not limited to, gas or liquid chromatography, cryogenic and flash distillations, true boiling-point distillations, structural fluid characterizations such as polynuclear aromatic hydrocarbon analysis, SARA analysis, sonic testing and other crude oil assay methods. Other nonhydrocarbon components can also be identified, such as nitrogen, heavy metals, sulfur and salts. |
| Oil and Gas Business | abandonment costs | The costs associated with abandoning a well or production facility. Such costs are specified in the authority for expenditure (AFE), and typically cover the plugging of wells; removal of well equipment, production tanks and associated installations; and surface remediation. |
| Oil and Gas Business | AFE | A budgetary document, usually prepared by the operator, to list estimated expenses of drilling a well to a specified depth, casing point or geological objective, and then either completing or abandoning the well. Such expenses may include excavation and surface site preparation, the daily rental rate of a drilling rig, costs of fuel, drillpipe, bits, casing, cement and logging, and coring and testing of the well, among others. This estimate of expenses is provided to partners for approval prior to commencement of drilling or subsequent operations. Failure to approve an authority for expenditure (AFE) may result in delay or cancellation of the proposed drilling project or subsequent operation. |
| Oil and Gas Business | assignment | The sale, transfer or conveyance of all or a fraction of ownership interest or rights owned in real estate or other such property. The term is commonly used in the oil and gas business to convey working interest, leases, royalty, overriding royalty interest and net profits interest. |
| Oil and Gas Business | authority for expenditure | A budgetary document, usually prepared by the operator, to list estimated expenses of drilling a well to a specified depth, casing point or geological objective, and then either completing or abandoning the well. Such expenses may include excavation and surface site preparation, the daily rental rate of a drilling rig, costs of fuel, drillpipe, bits, casing, cement and logging, and coring and testing of the well, among others. This estimate of expenses is provided to partners for approval prior to commencement of drilling or subsequent operations. Failure to approve an authority for expenditure (AFE) may result in delay or cancellation of the proposed drilling project or subsequent operation. |
| Oil and Gas Business | back in | The right to receive a reversionary interest at some future time, upon fulfillment of contractually specified conditions. This clause allows a lease-owner, lessee or a nonparticipating partner to reserve the option to participate in a well after it has produced enough to pay the operators expenses of drilling and completing that well. This clause is typically used in farmout agreements to convert the overriding royalty interest of a lease-owner, lessee or nonparticipating partner into a working interest upon payout of the well. When the election to convert the overriding royalty to working interest takes place, it is known as a back-in after payout (BIAPO). |
| Oil and Gas Business | back-in | The right to receive a reversionary interest at some future time, upon fulfillment of contractually specified conditions. This clause allows a lease-owner, lessee or a nonparticipating partner to reserve the option to participate in a well after it has produced enough to pay the operators expenses of drilling and completing that well. This clause is typically used in farmout agreements to convert the overriding royalty interest of a lease-owner, lessee or nonparticipating partner into a working interest upon payout of the well. When the election to convert the overriding royalty to working interest takes place, it is known as a back-in after payout (BIAPO). |
| Oil and Gas Business | bichromate salt | An agreement between two or more parties to review technical data prior to deciding whether to bid on a concession. The agreement also specifies the interests and the procedure for bidding between the parties in the event that the parties decide to bid on the concession. |
| Oil and Gas Business | bonus consideration | A monetary incentive given by the lessee (either an individual or company) to the lessor (mineral owner) for executing or ratifying an oil, gas and mineral lease. |
| Oil and Gas Business | carried working interest | A working interest generally paid in consideration for work related to the prospect. This interest is paid, or carried, for the drilling and or completion costs as specified in the contract between the parties, by another working interest owner typically until casing point is reached, or through the tanks, meaning through completion of the well, as agreed upon contractually. |
| Oil and Gas Business | communitization | The combining of smaller federal tracts of land to total the acreage required by the US Bureau of Land Management and/or state regulations to form a legal spacing and proration unit. |
| Oil and Gas Business | concession | A grant extended by a government to permit a company to explore for and produce oil, gas or mineral resources within a strictly defined geographic area, typically beneath government-owned lands or lands in which the government owns the rights to produce oil, gas or minerals. The grant is usually awarded to a company in consideration for some type of bonus or license fee and royalty or production sharing provided to the host government for a specified period of time. |
| Oil and Gas Business | conveyance | A written contract between a grantor and grantee, used to transfer title or rights to real estate or property. Typical conveyances include oil, gas and mineral leases; assignments; deeds and rights of way. |
| Oil and Gas Business | cost oil | A portion of produced oil that the operator applies on an annual basis to recover defined costs specified by a production sharing contract. |
| Oil and Gas Business | delay rental | Consideration paid to the lessor by a lessee to extend the terms of an oil and gas lease in the absence of operations and/or production that is contractually required to hold the lease. This consideration is usually required to be paid on or before the anniversary date of the oil and gas lease during its primary term, and typically extends the lease for an additional year. Nonpayment of the delay rental in the absence of production or commencement of operations will result in abandonment of the lease after its primary term has expired. |
| Oil and Gas Business | division order | An agreement between the operator and net revenue interest (NRI) owner in which the parties specify the fractional type of interest attributed to the NRI owner by the operator after an examination of title. |
| Oil and Gas Business | due diligence | The process of examining a seller’s property and records before a prospective buyer commits to its purchase. In the oil and gas business, due diligence may entail scrutiny of financial records, geological maps, well logs, surveyor notes, geophysical records, well tests, production statistics, legal records and other data pertaining to a well or lease. |
| Oil and Gas Business | farmee | The party that acquires the rights to drill and earn an assignment of the leasehold interest, receiving a farm-in. |
| Oil and Gas Business | farmor | The party that originally owns the leasehold interest and assigns the farmout. |
| Oil and Gas Business | farmout | A contractual agreement with an owner who holds a working interest in an oil and gas lease to assign all or part of that interest to another party in exchange for fulfilling contractually specified conditions. The farmout agreement often stipulates that the other party must drill a well to a certain depth, at a specified location, within a certain time frame; furthermore, the well typically must be completed as a commercial producer to earn an assignment. The assignor of the interest usually reserves a specified overriding royalty interest, with the option to convert the overriding royalty interest to a specified working interest upon payout of drilling and production expenses, otherwise known as a back-in after payout (BIAPO). |
| Oil and Gas Business | fee simple interest | Ownership of the entire and absolute right or interest to use or exploit a tract of land from the center of the earth to the stars, including the air, surface and minerals. |
| Oil and Gas Business | fee-simple interest | Ownership of the entire and absolute right or interest to use or exploit a tract of land from the center of the earth to the stars, including the air, surface and minerals. |
| Oil and Gas Business | foreign content | The amount of foreign personnel, material and services that working interest owners are permitted to employ, as defined under the terms of a concession when drilling and operating a well. |
| Oil and Gas Business | government take | The total amount of revenue that a host government receives from production. This amount can include taxes, royalties and government participation. |
| Oil and Gas Business | HBP | Abbreviation for held by production. |
| Oil and Gas Business | held by production | A provision in an oil, gas and mineral lease that perpetuates a companys right to operate a property or concession as long as the property or concession produces a minimum paying quantity of oil or gas. Also abbreviated as HBP. |
| Oil and Gas Business | horizontal severance | A method to convey or reserve oil, gas or mineral rights at specific depths or geologic horizons. |
| Oil and Gas Business | lessor royalty | A percentage share of production, or the value derived from production, which is granted to the lessor in the oil and gas lease, and which is free of the costs of drilling and producing. |
| Oil and Gas Business | licensing round | An occasion when a governmental body offers exploration acreage for leasing by exploration and production companies, typically in return for a fee and a performance or work obligation, such as acquisition of seismic data or drilling a well. Exploration licenses are initially of limited duration (about 5 years) after which there might be a requirement to return half or more of the licensed acreage to the state. If hydrocarbons are discovered, a separate production license or production-sharing agreement is usually drawn up before development can proceed. |
| Oil and Gas Business | local content | The amount of local personnel, material and services that working interest owners are required to employ when drilling and operating a well, as specified under the terms of a concession agreement. |
| Oil and Gas Business | marketing agreement | An agreement by which a party sells production on behalf of a producing company and then remits the proceeds, minus agreed-upon costs and expenses, to the producing company. |
| Oil and Gas Business | mineral interest | Ownership of the right to exploit, mine or produce all minerals lying beneath the surface of a property. In this case, minerals include all hydrocarbons. Mineral interests include: 1. the right to use as much of the surface as is reasonably necessary to access the minerals, 2. the right to execute any conveyances of mineral rights, 3. the right to receive bonus consideration, 4. the right to receive delay rentals and 5. the right to receive royalty. Any or all of the above five rights of mineral ownership may be conveyed by the mineral owner. |
| Oil and Gas Business | multiple service contract | A contract between a host country and an operator that specifies the services and costs of services that the operator must use in the development of a concession. |
| Oil and Gas Business | net profits interest | A share of net proceeds from production paid solely from the working interest owners share. It is sometimes granted in lieu of a royalty interest. |
| Oil and Gas Business | net revenue interest | A share of production after all burdens, such as royalty and overriding royalty, have been deducted from the working interest. It is the percentage of production that each party actually receives. |
| Oil and Gas Business | nonparticipating royalty | A percentage share of production, or the value derived from production, which is free of the costs of drilling and producing, created by the lessor or royalty owner and borne by the lessor or royalty owner out of the lessor royalty. This royalty is paid to nonparticipating interest holders who do not share or participate in bonus or rentals, or a right to explore, or a right to execute oil and gas leases. |
| Oil and Gas Business | nonparticipating royalty interest | Ownership in a share of production, paid to an owner who does not share in the right to explore or develop a lease, or receive bonus or rental payments. It is free of the cost of production, and is deducted from the royalty interest. |
| Oil and Gas Business | oil and gas lease | A contract between mineral owner, otherwise known as the lessor and a company or working interest owner, otherwise known as the lessee in which the lessor grants the lessee the right to explore, drill and produce oil, gas and other minerals for a specified primary term and as long thereafter as oil, gas or other minerals are being produced in paying quantities. This lease gives the lessee a working interest. The oil and gas lease is granted in exchange for royalty payments to the lessor. |
| Oil and Gas Business | operating agreement | An agreement between parties who own a working interest in a well that sets out responsibilities and duties of the operator and nonoperators, including drilling the test well and subsequent wells, and sharing of expenses and accounting methods. |
| Oil and Gas Business | operating interest | Ownership by a lessee, company or working interest owner, which is burdened with the costs of leasing the acreage and drilling and operating a well. |
| Oil and Gas Business | operator | The owner of the right to drill or produce a well, or the entity contractually charged with drilling of a test well and production of subsequent wells. |
| Oil and Gas Business | overriding royalty | A percentage share of production, or the value derived from production, which is free of all costs of drilling and producing, and is created by the lessee or working interest owner and paid by the lessee or working interest owner. |
| Oil and Gas Business | overriding royalty interest | Ownership in a percentage of production or production revenues, free of the cost of production, created by the lessee, company and/or working interest owner and paid by the lessee, company and/or working interest owner out of revenue from the well. |
| Oil and Gas Business | paid up lease | An oil and gas lease in which delay rentals for the entire primary term are paid in advance with the bonus consideration. |
| Oil and Gas Business | paid-up lease | An oil and gas lease in which delay rentals for the entire primary term are paid in advance with the bonus consideration. |
| Oil and Gas Business | participating interest | The proportion of exploration and production costs each party will bear and the proportion of production each party will receive, as set out in an operating agreement. |
| Oil and Gas Business | payout | The point at which all costs of leasing, exploring, drilling and operating have been recovered from production of a well or wells as defined by contractual agreement. |
| Oil and Gas Business | pooling | The accumulation of smaller tracts of land, the sum total acreage of which are required for a governmental agency to grant a well permit or assign a production quota or allowable to an operator. |
| Oil and Gas Business | preferential right to operate | The right that a party has reserved or acquired to operate a lease, well, unit and/or concession. |
| Oil and Gas Business | preferential right to purchase | The right that nonselling participating parties have in a lease, well or unit to proportionately acquire the interest that a participating party proposes to sell to a third party. |
| Oil and Gas Business | preflush | In chemical flooding, a fluid stage, normally low-salinity water, pumped ahead of the micellar or alkaline chemical solution. One of the purposes of the preflush is to displace reservoir brine containing potassium, sodium, calcium and magnesium ions from the near-wellbore area, avoiding adverse interactions with the chemical solution. The other purposes are to adjust reservoir salinity to favorable conditions for the surfactant (chemical solution) and to obtain information about reservoir flow patterns. Sometimes a preflush stage is not necessary, especially when brine-tolerant chemical systems are used. |
| Oil and Gas Business | primary term | The period of time during which an oil and gas lease will be in effect, in the absence of production, drilling or other operations specified by the lease. The oil and gas lease can be perpetuated past the primary term by production in paying quantities, drilling, operations and/or the payment of shut-in royalties specified by the lease. |
| Oil and Gas Business | production bonus | Payment by a well operator to a host country upon achievement of certain levels of production. |
| Oil and Gas Business | production payment | A portion of proceeds from production, specified by contract, and payable to the lessor or farmor, or host country until total payment has reached a predetermined limit specified by contract. |
| Oil and Gas Business | production penalty | A fine paid to the host country for failure to attain specified production rates over a defined period of time. |
| Oil and Gas Business | production service contract | An agreement between the parties to a well or wells and a host country to utilize specified goods and services from that country. |
| Oil and Gas Business | production sharing contract | An agreement between the parties to a well and a host country regarding the percentage of production each party will receive after the participating parties have recovered a specified amount of costs and expenses. |
| Oil and Gas Business | profit oil | The amount of production, after deducting cost oil production allocated to costs and expenses, that will be divided between the participating parties and the host government under the production sharing contract. |
| Oil and Gas Business | proration unit | The amount of acreage, determined by governmental authority that can be efficiently and economically drained by a well at a particular depth or horizon. |
| Oil and Gas Business | quitclaim | To convey whatever interests a grantor has at the time this particular instrument is executed. The grantor does this without warranty of title, either express or implied. |
| Oil and Gas Business | quitclaim | A legal instrument of conveyance that is usually used in title curative work to allow an owner or claimant to quit or give up their claim to a title. |
| Oil and Gas Business | regulatory body | An organization appointed by the government or industry to establish standards and ensure their compliance. |
| Oil and Gas Business | relinquishment | The return of part or all of a lease or concession to a lessor, farmor or host government. The return may be voluntary or compelled contractually. |
| Oil and Gas Business | right of first refusal | The right that other parties to a lease, well, unit and/or concession have to acquire the interest that a selling party owns prior to selling to any third party. |
| Oil and Gas Business | royalty | A percentage share of production, or the value derived from production, paid from a producing well. |
| Oil and Gas Business | royalty interest | Ownership of a percentage of production or production revenues, produced from leased acreage. The owner of this share of production does not bear any of the cost of exploration, drilling, producing, operating, marketing or any other expense associated with drilling and producing an oil and gas well. |
| Oil and Gas Business | secondary term | The term of an oil and gas lease in which the lease is held in force after expiration of the primary term. Production, operations, continuous drilling and/or shut-in royalty payments are often used to extend an oil and gas lease into its secondary term. |
| Oil and Gas Business | severance | The separation of mineral and/or royalty interest from fee-simple title. Severance of interests is usually accomplished by reservation in a deed or assignment or by conveyance in mineral or royalty deed, assignment or lease. |
| Oil and Gas Business | shut in royalty | A payment stipulated in the oil and gas lease, which royalty owners receive in lieu of actual production, when a gas well is shut-in due to lack of a suitable market, a lack of facilities to produce the product, or other cases defined within the shut-in provisions contained in the oil and gas lease. |
| Oil and Gas Business | shut-in royalty | A payment stipulated in the oil and gas lease, which royalty owners receive in lieu of actual production, when a gas well is shut-in due to lack of a suitable market, a lack of facilities to produce the product, or other cases defined within the shut-in provisions contained in the oil and gas lease. |
| Oil and Gas Business | spacing unit | An area allotted to a well by regulations or field rules issued by a governmental authority having jurisdiction for the drilling and production of a well. |
| Oil and Gas Business | surface interest | Ownership of the right or interest to exploit the surface of the land. Some landowners only have rights to the surface of their tract, while the government or other entity owns rights to any production obtained beneath that tract. |
| Oil and Gas Business | TEA | Abbreviaton for technical evaluation agreement. |
| Oil and Gas Business | term lease | An oil and gas lease that expires after a specified period of time, regardless of whether oil, gas and/or other minerals are being produced. |
| Oil and Gas Business | top lease | An oil and gas lease wherein the bonus consideration is paid at the signing of the lease. However, this lease becomes effective only after the expiration or termination of an existing lease on the tract of land. |
| Oil and Gas Business | unitization | The combining of multiple wells to produce from a specified reservoir. |
| Oil and Gas Business | vertical severance | A method to convey or reserve oil, gas, or mineral rights in a defined portion of land such as the Northwest Quarter of a tract. |
| Oil and Gas Business | working interest | A percentage of ownership in an oil and gas lease granting its owner the right to explore, drill and produce oil and gas from a tract of property. Working interest owners are obligated to pay a corresponding percentage of the cost of leasing, drilling, producing and operating a well or unit. After royalties are paid, the working interest also entitles its owner to share in production revenues with other working interest owners, based on the percentage of working interest owned. |
| Oil and Gas Business | technical evaluation agreement | An agreement between a host country and operator to allow the operator to evaluate geological, geophysical, engineering and transportation issues involving a concession. Also known as a TEA. |
| Perforating | jet perforating | The use of shaped explosive charges to create perforation tunnels. The explosive charge produces an extremely high-pressure jet that penetrates the casing or liner to shoot into the reservoir formation. The shaped charges are contained in a perforating gun assembly that can be conveyed on wireline, tubing or coiled tubing, depending on the application and the wellbore conditions. |
| Perforating | bank firing | A technique in which several perforating guns are run on a single trip into the well, and then all are fired simultaneously. |
| Perforating | bid and study agreement | A perforating charge designed to create perforations with a large-diameter entrance hole. These charges typically are used in sand control completions, in which efficient placement of the gravel pack treatment within the perforation tunnel is crucial. Altering the explosive charge design and materials creates a larger diameter entrance hole on the perforation while reducing the depth of penetration. However, gravel-pack treatments generally are applied in high-permeability formations where perforation tunnel length is less important. Wells that are to be hydraulically fractured also can benefit from larger perforations since the effective penetration is significantly increased by a high-conductivity fracture. |
| Perforating | big hole charge | A perforating charge designed to create perforations with a large-diameter entrance hole. These charges typically are used in sand control completions, in which efficient placement of the gravel pack treatment within the perforation tunnel is crucial. Altering the explosive charge design and materials creates a larger diameter entrance hole on the perforation while reducing the depth of penetration. However, gravel-pack treatments generally are applied in high-permeability formations where perforation tunnel length is less important. Wells that are to be hydraulically fractured also can benefit from larger perforations since the effective penetration is significantly increased by a high-conductivity fracture. |
| Perforating | booster | A small metal tube containing secondary high explosive that is crimped onto the end of the detonating cord. This explosive component is designed to provide reliable detonation transfer between perforating guns or other explosive devices, and often serves as an auxiliary explosive charge to ensure detonation. |
| Perforating | capsule gun | An exposed gun system used primarily in wireline operations. This gun system has shaped charges that are housed in individual pressure-tight capsules mounted on a metal strip, which is lowered into the well. Each pressure-tight capsule, along with the entire string, is thus exposed to well fluids. |
| Perforating | carrier gun | A perforating gun, consisting of a loading tube and shaped charges. The shaped charges are housed inside a metal tube or pipe known as a carrier. The carrier protects the charges against well fluids. |
| Perforating | casing gun | A perforating gun assembly designed to be used in a wellbore before the production tubulars or completion equipment have been installed, thus allowing access for a larger diameter gun assembly. Casing guns are typically 3- to 5-in. In diameter and carry up to four perforating charges per foot. |
| Perforating | chemical cutter | A downhole tool run on wireline to sever tubing at a predetermined point when the tubing string becomes stuck. When activated, the chemical cutter use a small explosive charge to forcefully direct high-pressure jets of highly corrosive material in a circumferential pattern against the tubular wall. The nearly instantaneous massive corrosion of the surrounding tubing wall creates a relatively even cut with minimal distortion of the tubing, aiding subsequent fishing operations. |
| Perforating | deep penetrating charge | A perforating charge designed to provide a long perforation tunnel, such as may be required to bypass any near-wellbore damage. Specially designed deep-penetrating charges achieve this additional length while retaining a medium-sized entrance hole, an important consideration in high-shot density applications. |
| Perforating | deep-penetrating charge | A perforating charge designed to provide a long perforation tunnel, such as may be required to bypass any near-wellbore damage. Specially designed deep-penetrating charges achieve this additional length while retaining a medium-sized entrance hole, an important consideration in high-shot density applications. |
| Perforating | detonating cord | A cord containing high-explosive material sheathed in a flexible outer case, which is used to connect the detonator to the main high explosive. This provides an extremely rapid initiation sequence that can be used to fire several charges simultaneously. |
| Perforating | detonator | A device containing primary high-explosive material that is used to initiate an explosive sequence. The two common types of detonators are electrical detonators (also known as blasting caps) and percussion detonators. Electrical detonators have a fuse material that burns when high voltage is applied to initiate the primary high explosive. Percussion detonators contain abrasive grit and primary high explosive in a sealed container that is activated by a firing pin. The impact force of the firing pin is sufficient to initiate the ballistic sequence that is then transmitted to the detonating cord. Several safety systems are used in conjunction with detonators to avoid accidental firing during rig-up or rig-down. Safety systems also are used to disarm the gun or ballistic assembly if downhole conditions are unsafe for firing. |
| Perforating | dog collar | A safety device used when running and retrieving tools or drill collars with a flush external surface that may easily pass through the rotary table slips. The dog collar is temporarily attached to the assembly between the tool joint and the slips. If the slips fail to hold the tool assembly, the dog collar will prevent the entire assembly from dropping through and being lost in the wellbore. |
| Perforating | effective shot density | A value that reflects the number of perforations per unit of length (usually feet) that are producing, or injecting, efficiently. Perforation efficiency may be compromised by gun failure or charge misfire, perforation debris, excessive standoff or poor orientation, or by combinations of these. The effective shot density may be used in treatment design models or to calculate likely productivity response. |
| Perforating | entrance hole | The hole created in the internal surface of the casing or liner by the perforating charge or bullet. The entrance hole should be clean, free from burrs and round to create an efficient flow path between the reservoir and wellbore. Depending on gun size and standoff, the entrance hole is typically between 3/8″ and 1/2″ in diameter. The perforation charge design generally is optimized to provide maximum penetration while achieving a medium-size entrance hole. Special charge designs can be used when optimizing the entrance hole size is important, as in sand-control applications. |
| Perforating | expendable gun | A perforating gun assembly that disintegrates upon firing, thereby reducing the volume and dimensions of retrieved components. Expendable guns are typically used where wellbore restrictions allow only limited access, as in through-tubing applications. The distortion caused to the gun assembly during firing would typically prevent recovery of a conventional gun design through the limited clearances. The expendable gun assembly breaks into small pieces that drop to the bottom of the well, leaving only a relatively small subassembly that is easily recovered to surface. |
| Perforating | fill sub | A pipe-shaped housing that protects the firing head of a tubing-conveyed perforating gun. It is used to accommodate or deflect debris that might fall toward the firing head while running into the hole or while on depth before shooting. |
| Perforating | firing head | A mechanical or electronic device used to detonate perforating charges conveyed by tubing, drillpipe, coiled tubing or slickline. This term thus connotes any such device that is not initiated electrically from surface by wireline. A mechanical firing head consists of a percussion detonator that is struck by a firing pin. An electronic firing head is battery powered, to initiate an electric detonator. Electronic firing head systems are used with slickline, coiled tubing and TCP. |
| Perforating | frac gun | A specialized perforating-gun system that contains shaped charges loaded at 0°, 60°, 120°, or 180° phase angles to provide a casing entrance hole of approximately 0.5 in. [1.3 cm], which is intended to be large enough for hydraulic fracturing operations. |
| Perforating | gravel pack gun | A perforating-gun system containing big-hole or gravel-pack charges. A gravel-pack gun usually has a large outside diameter to minimize standoff between the gun and casing. It is loaded to achieve a high shot density while producing large holes in the casing |
| Perforating | gravel-pack gun | A perforating-gun system containing big-hole or gravel-pack charges. A gravel-pack gun usually has a large outside diameter to minimize standoff between the gun and casing. It is loaded to achieve a high shot density while producing large holes in the casing. |
| Perforating | gun | A device used to perforate oil and gas wells in preparation for production. Containing several shaped explosive charges, perforating guns are available in a range of sizes and configurations. The diameter of the gun used is typically determined by the presence of wellbore restrictions or limitations imposed by the surface equipment. |
| Perforating | gun clearance | The distance between the external surface of the gun assembly and the internal surface of the casing or liner. The gun clearance depends on the position of the gun within the tubular and will vary between phases of shots on any gun unless the gun assembly is centralized. This variation in clearance contributes to variable performance of the perforations. The greater the gun clearance, the smaller the entrance hole-with the effect increasing with depth (pressure). |
| Perforating | gun zero | The point on a gun assembly or perforating string that is used when correlating depth. With use of a surface depth reference or datum point, any convenient point may be used, providing it is applied accurately and consistently. For downhole correlation on wireline applications, the gun zero point will relate to the correlating device, such as the casing collar locator (CCL) or gamma ray log tool. For nonwireline applications, the gun zero point will relate to the locating device, such as the tubing-end locator. The correlation process is crucial to a successful perforating operation; and therefore the importance of ensuring correct placement of the gun assembly prior to firing cannot be overstated. |
| Perforating | high explosive | Chemical explosive material having an extremely high reaction rate that creates very high combustion pressures, unlike low explosives that have a much lower reaction rate and are commonly used as propellants. High explosives are further categorized as primary- and secondary-high explosive. Primary-high explosives are very sensitive, can be detonated easily and are generally used only in percussion and electrical detonators. Secondary-high explosives are less sensitive, require a high-energy shock wave to achieve detonation and are safer to handle. Secondary-high explosives are used in almost all elements of a ballistic chain, other than the detonator, such as in detonating cord and shaped charges. |
| Perforating | high shot density gun | A perforating gun having more than four shots per foot. In addition to providing a greater number of perforations, a high-shot density gun also improves the phasing, or distribution of perforations, around the wellbore. |
| Perforating | high-shot density gun | A perforating gun having more than four shots per foot. In addition to providing a greater number of perforations, a high-shot density gun also improves the phasing, or distribution of perforations, around the wellbore. |
| Perforating | hollow carrier gun | A perforating gun, consisting of a loading tube and shaped charges. The shaped charges are housed inside a metal tube or pipe known as a carrier. The carrier protects the charges against well fluids. |
| Perforating | last-shot detection | A surface detection system used to ensure that all tubing-conveyed perforating guns have fired, from the top shot to the bottom shot. |
| Perforating | perforate | To create holes in the casing or liner to achieve efficient communication between the reservoir and the wellbore. The characteristics and placement of the communication paths (perforations) can have significant influence on the productivity of the well. Therefore, a robust design and execution process should be followed to ensure efficient creation of the appropriate number, size and orientation of perforations. A perforating gun assembly with the appropriate configuration of shaped explosive charges and the means to verify or correlate the correct perforating depth can be deployed on wireline, tubing or coiled tubing. |
| Perforating | perforate overbalanced | To create holes in the liner or casing under conditions in which the hydrostatic pressure inside the casing or liner is greater than the reservoir pressure. When the perforation is made, there will be a tendency for the wellbore fluid to flow into the reservoir formation. |
| Perforating | perforate underbalanced | To create holes in the liner or casing under conditions in which the hydrostatic pressure inside the casing or liner is less than the reservoir pressure. When the perforation is made, there will be a tendency for the reservoir fluid to flow into the wellbore. |
| Perforating | perforated interval | The section of wellbore that has been prepared for production by creating channels between the reservoir formation and the wellbore. In many cases, long reservoir sections will be perforated in several intervals, with short sections of unperforated casing between each interval to enable isolation devices, like packers, to be set for subsequent treatments or remedial operations. |
| Perforating | perforated liner | A wellbore tubular in which slots or holes have been made before the string is assembled and run into the wellbore. Perforated liners typically are used in small-diameter wellbores or in sidetracks within the reservoir where there is no need for the liner to be cemented in place, as is required for zonal isolation. |
| Perforating | perforating charge | An explosive device that utilizes a cavity-effect explosive reaction to generate a high-pressure, high-velocity jet that creates a perforation tunnel. The shape of the explosive material and powdered metal lining determine the shape of the jet and performance characteristics of the charge. The extremely high pressure and velocity of the jet cause materials, such as steel, cement and rock formations, to flow plastically around the jet path, thereby creating the perforation tunnel. |
| Perforating | perforating depth control log | A wireline log run to provide a means of depth correlation by comparing the position of casing collars to the reference log (gamma ray log). A short casing joint generally is run near the area to be perforated to assist in the correlation process. |
| Perforating | perforating depth-control log | A wireline log run to provide a means of depth correlation by comparing the position of casing collars to the reference log (gamma ray log). A short casing joint generally is run near the area to be perforated to assist in the correlation process. |
| Perforating | perforating gun | A device used to perforate oil and gas wells in preparation for production. Containing several shaped explosive charges, perforating guns are available in a range of sizes and configurations. The diameter of the gun used is typically determined by the presence of wellbore restrictions or limitations imposed by the surface equipment. |
| Perforating | perforation | The communication tunnel created from the casing or liner into the reservoir formation, through which oil or gas is produced. The most common method uses jet perforating guns equipped with shaped explosive charges. However, other perforating methods include bullet perforating, abrasive jetting or high-pressure fluid jetting. |
| Perforating | perforation density | The number of perforations per linear foot. This term is used to describe the configuration of perforating guns or the placement of perforations, and is often abbreviated to spf (shots per foot). An example would be an 8 spf casing gun. |
| Perforating | perforation penetration | A measure, or indicator, of the length that a useable perforation tunnel extends beyond the casing or liner into the reservoir formation. In most cases, a high penetration is desirable to enable access to that part of the formation that has not been damaged by the drilling or completion processes. |
| Perforating | perforation phasing | The radial distribution of successive perforating charges around the gun axis. Perforating gun assemblies are commonly available in 0-, 180-, 120-, 90- and 60-degree phasing. The 0-degree phasing is generally used only in small outside-diameter guns, while 60, 90 and 120 degree phase guns are generally larger but provide more efficient flow characteristics near the wellbore |
| Perforating | primer cord | A cord containing high-explosive material sheathed in a flexible outer case, which is used to connect the detonator to the main high explosive. This provides an extremely rapid initiation sequence that can be used to fire several charges simultaneously. |
| Perforating | radio safe detonator | Electric detonators used in wireline and electronic firing-head perforating operations, which are immune to radio interference and thus cannot be accidentally triggered by radio transmissions. |
| Perforating | radio silence | A procedure imposed during perforating operations in which radios at or near the wellsite are switched off to prevent accidental detonation of perforating guns. Radio silence is required for wireline operations when using a non-radio-safe detonator. It is typically imposed when rigging up perforating guns and until the guns have been run in the hole to 200 ft [61 m] below ground level or mud line. Radio silence is imposed again when retrieving the gun system as it passes the 200-ft mark. Radio silence must be maintained until it has been confirmed that all charges have been shot. This practice is not required when radio-safe detonators are used, but is common practice at many wellsites. |
| Perforating | radio-safe detonator | Electric detonators used in wireline and electronic firing-head perforating operations, which are immune to radio interference and thus cannot be accidentally triggered by radio transmissions. |
| Perforating | retrievable gun | A perforating gun designed to be retrieved from the wellbore after firing. Retrievable guns are generally configured for minimal debris and distortion of the gun body to help ensure easy retrieval. |
| Perforating | safety spacer | A blank gun section or spacer installed between the top perforating-gun assembly and firing head in a TCP operation. The safety spacer serves to position the gun assembly a safe distance below the rig floor during arming and disarming operations. The spacer should be a minimum of 10 feet [3 m] in length. In some cases, a longer safety spacer will be required to ensure that the gun assembly is positioned safely below the living quarters or other occupied areas of the drilling rig. |
| Perforating | scallop gun | A perforating gun with a recess profile in the perforating gun body adjacent to the shaped charge. The scallop profile reduces the external burrs created as the perforating jet exits the gun body, thereby reducing the risk of hang-up or damage as the gun assembly is retrieved. |
| Perforating | selective firing | The technique of selectively firing successive perforating guns arranged in a multiple gun assembly. This method is used when several intervals are to be perforated in one run and when the gun assembly must be relocated before the guns are fired. The resulting perforation pattern is known as selective perforating. |
| Perforating | selective perforating | A technique used to fire individual perforating guns when multiple guns have been run together in a single trip into the well. Selective firing is used to improve operational efficiency when several intervals are to be perforated. |
| Perforating | shaped charge | An explosive device that utilizes a cavity-effect explosive reaction to generate a high-pressure, high-velocity jet that creates a perforation tunnel. The shape of the explosive material and powdered metal lining determine the shape of the jet and performance characteristics of the charge. The extremely high pressure and velocity of the jet cause materials, such as steel, cement and rock formations, to flow plastically around the jet path, thereby creating the perforation tunnel. |
| Perforating | shoot a well | To perforate a wellbore in preparation for production. |
| Perforating | shot detection | A surface detection technique to verify that perforating guns have fired. This technique typically employs sensors that detect vibration or hydraulic shock at surface, and is used with TCP operations. |
| Perforating | stable arch | The geometric profile around a correctly placed perforation. With the removal of perforating debris and the crushed zone by flushing or stimulation treatment, the exposed formation forms an arch that is capable of withstanding the differential pressure and the forces created by fluid flow during production. An unstable formation interface, such as shattered formation surrounding the perforation tunnel, may result in plugging or collapse of the perforation tunnel. |
| Perforating | standoff | The space between the shaped charge and the internal surface of the perforating gun body. The standoff is generally sufficient to allow the shaped charge jet to form before exiting the gun body. |
| Perforating | strip gun | An exposed gun system used primarily in wireline operations. This gun system has shaped charges that are housed in individual pressure-tight capsules mounted on a metal strip, which is lowered into the well. Each pressure-tight capsule, along with the entire string, is thus exposed to well fluids. |
| Perforating | TCP | Abbreviation for tubing-conveyed perforating, the use of tubing, drillpipe or coiled tubing to convey perforating guns to the required depth. Initially, the technique was developed as a means for conveying the gun string on the production tubing, with the guns remaining in the well until they are removed during the first workover. The subsequent popularity of highly deviated and horizontal wells increased the requirement for tubing-conveyed perforating as the only means of gaining access to the perforating depth. |
| Perforating | through tubing gun | A perforating gun assembly designed to run through the restricted clearance of production tubing, then operate effectively within the larger diameter of the casing or liner below. A range of small-diameter guns has been developed for this purpose, although small-diameter casing guns also may be used when larger production tubing sizes permit. |
| Perforating | through-tubing gun | A perforating gun assembly designed to run through the restricted clearance of production tubing, then operate effectively within the larger diameter of the casing or liner below. A range of small-diameter guns has been developed for this purpose, although small-diameter casing guns also may be used when larger production tubing sizes permit. |
| Perforating | tubing conveyed perforating | The use of tubing, drillpipe or coiled tubing to convey perforating guns to the required depth. Initially, the technique was developed as a means for conveying the gun string on the production tubing, with the guns remaining in the well until they are removed during the first workover. The subsequent popularity of highly deviated and horizontal wells increased the requirement for tubing-conveyed perforating as the only means of gaining access to the perforating depth. The term is often abbreviated as TCP. |
| Perforating | tubing-conveyed perforating | The use of tubing, drillpipe or coiled tubing to convey perforating guns to the required depth. Initially, the technique was developed as a means for conveying the gun string on the production tubing, with the guns remaining in the well until they are removed during the first workover. The subsequent popularity of highly deviated and horizontal wells increased the requirement for tubing-conveyed perforating as the only means of gaining access to the perforating depth. The term is often abbreviated as TCP. |
| Perforating, Geophysics | detonate | To set off an explosive material. Explosive sources are used in seismic acquisition and explosive charges are used to perforate wells in preparation for production or injection. |
| Perforating, Well Completions | area open to flow | The calculated flow area provided by perforations across a specific zone of interest. The resulting value is used to calculate pressure drops and fluid-flow performance. |
| Perforating, Well Completions | blast joint | A section of heavy walled tubing that is placed across any perforated interval through which the production tubing must pass, such as may be required in multiple zone completions. In addition to being heavier than normal completion components, the wall of a blast joint is often treated to resist the jetting action that may result in the proximity of the perforations. |
| Perforating, Well Completions | bridge plug | A downhole tool that is located and set to isolate the lower part of the wellbore. Bridge plugs may be permanent or retrievable, enabling the lower wellbore to be permanently sealed from production or temporarily isolated from a treatment conducted on an upper zone. |
| Perforating, Well Completions | casing collar locator (CCL) | A downhole tool used to confirm or correlate treatment depth using known reference points on the casing string. The casing collar locator is an electric logging tool that detects the magnetic anomaly caused by the relatively high mass of the casing collar. A signal is transmitted to surface equipment that provides a screen display and printed log enabling the output to be correlated with previous logs and known casing features such as pup joints installed for correlation purposes. |
| Perforating, Well Completions | casing collar log | A log provided by a casing collar locator tool that generally incorporates a gamma ray log to correlate the relative position of casing string features, such as the location of a pup joint, with the reservoir or formation of interest. |
| Perforating, Well Completions | crushed zone | The rubblized or damaged zone surrounding a perforation tunnel where the action of the perforating charge or bullet has altered the formation structure and permeability. Although it is generally damaging to production, the severity or extent of the crushed zone depend greatly on the characteristics of the formation, the perforating charge and the underbalance or overbalance conditions at time of perforating. Measures to reduce the effect of the crushed zone include underbalanced perforating in which the crushed zone and perforating debris are flushed from the perforating tunnel by the reservoir fluid as soon as the perforation is created. Where overbalanced perforating techniques are used, it may be necessary to acidize the crushed zone to achieve maximum productivity from the perforated interval. |
| Perforating, Well Completions | damaged zone | The area surrounding the wellbore that has been harmed by the drilling process, generally as a result of mud or cement-filtrate invasion. Near-wellbore damage can significantly affect productivity and is typically easier to prevent than it is to cure. Although almost always present, a lightly damaged zone around the wellbore can be bypassed by perforation tunnels to create connecting conduits from the wellbore to the undamaged reservoir formation. More severe cases of damage may require a matrix-acidizing treatment to restore the natural permeability, or a hydraulic fracturing treatment to create a new high-conductivity flow path to the reservoir. |
| Perforating, Well Completions | drop ball | A ball that is dropped or pumped through the wellbore tubulars to activate a downhole tool or device. When the ball is located on a landing seat, hydraulic pressure generally is applied to operate the tool mechanism. |
| Perforating, Well Completions | mechanical skin | The reduction in permeability in the near-wellbore area resulting from mechanical factors such as the displacement of debris that plugs the perforations or formation matrix. Such damage in the near-wellbore area can have a significant effect on the productivity of a well. |
| Perforating, Well Completions | nitrogen cushion | A column of high-pressure nitrogen typically applied to a tubing string in preparation for drillstem testing or perforating operations in which the reservoir formation is to be opened to the tubing string. The nitrogen cushion allows a precise pressure differential to be applied before opening flow from the reservoir. Once flow begins, the nitrogen cushion pressure can be easily and safely bled down to flow formation fluids under a high degree of control. |
| Perforating, Well Completions | perforating acid | An acid treatment placed in the wellbore over the interval to be perforated. Because of the overbalance conditions at the time of perforating, the perforating acid is forced into the newly formed perforation tunnel to stimulate the crushed zone. Formulation of the perforating acid depends on the characteristics of the formation and the downhole equipment used. |
| Perforating, Well Completions | perforating fluid | A specially prepared fluid placed in the wellbore over the interval to be perforated. The ideal fluid is clean and solids-free (filtered), and will not react to cause damaging by-products on contact with the reservoir formation. Perforating in a dirty fluid may result in significant permeability damage that is difficult to treat and remove. |
| Perforating, Well Completions | shear pin | A short piece of brass or steel that is used to retain sliding components in a fixed position until sufficient force is applied to break the pin. Once the pin is sheared, the components can then move to operate or function the tool. |
| Perforating, Well Completions | tubing puncher | A special perforating gun, or charge, that is designed for limited penetration to allow an inner tubing or casing string to be perforated without damaging a surrounding outer string. These guns often are used in remedial or workover operations in which downhole communication devices, such as sliding sleeves, cannot be opened to allow circulation of well-kill fluids. |
| Perforating | bullet perforating | An early perforating method that used a hardened steel bullet or projectile, propelled by an explosive charge, to create a perforation tunnel. This method creates a low-permeability crushed zone and leaves the bullet and associated debris jammed at the end of the tunnel. Jet perforating is now the preferred method. |
| Production | emergency shutdown valve (ESDV) | A valve or a system of valves that, when activated, initiate a shutdown of the plant, process or platform they are tied to. |
| Production | adapter spool | An extension added to a short face-to-face valve to conform to standard API 6D (or ISO 14313: 1999) face-to-face dimensions. API 6D specifies requirements and gives recommendations for the design, manufacturing, testing and documentation of ball, check, gate and plug valves for application in pipeline systems. |
| Production | all-welded construction | As it pertains to a valve construction, a valve body that is completely welded and cannot be disassembled and repaired in the field. |
| Production | anchor pin | A pin welded to the body of a ball valve. This pin aligns the adapter plate and keeps the plate and gear operator from moving while the valve is being operated. |
| Production | angle valve | A variation of the globe valve in which the end connections are at right angles to each other, rather than being in line. |
| Production | API 6D: Specification for Pipeline Valves | API 6D specifies requirements and gives recommendations for the design, manufacturing, testing and documentation of ball, check, gate and plug valves for application in pipeline systems. |
| Production | ASME | Abbreviation for the American Society of Mechanical Engineers, a professional association that was founded in 1880. It “promotes the art, science, and practice of multidisciplinary engineering and allied sciences around the globe” via “continuing education, training and professional development, codes and standards, research, conferences and publications, government relations, and other forms of outreach.” The ASME develops codes and standards associated with the art, science, and practice of mechanical engineering that are accepted in more than 100 countries. |
| Production | back pressure | Pressure opposing the desired flow of a fluid in a pipe. Usually results from obstructions and tight bends in confinement vessels. |
| Production | ball valve | A valve using a spherical closure element (ball) which is rotated through 90° to open and close the valve. |
| Production | bevel gear operated (BGO) valve | A gate valve actuated by means of a set of bevel gears having the axis of the pinion gear at right angles to that of the larger ring gear. The reduction ratio of this gear set determines the multiplication of torque achieved |
| Production | block valve | A valve that isolates or blocks possible leaks in case of an emergency situation. |
| Production | block-and-bleed | The capability of obtaining a seal across the upstream and downstream seat rings of a valve when the body pressure is bled off to the atmosphere through blowdown valves or vent plugs. Useful in testing the integrity of seat seals and performing minor repairs under pressure. |
| Production | blowdown valve (BDV) | A valve or system of valves that, when activated, initiates a blowdown of a pipeline, plant, process or platform; similar to an emergency shutdown valve (ESDV) that shuts in a pipeline, the BDV opens a pipeline. |
| Production | body relief valve (BRV) | An optional relief valve installed on ball valves used in liquid service to provide for the relief of excess body pressure caused by thermal expansion. |
| Production | bonnet | The top part of a valve, attached to the body that guides the stem and adapts to extensions or operators. |
| Production | bubble-tight shutoff | A phrase describing the sealing ability of a valve. During air pressure testing of a new valve in the closed position, leakage past the seats is collected and bubbled through water. To qualify as bubble-tight, no bubbles should be observed in a prescribed time span. |
| Production | butterfly valve | A short face-to-face valve that has a movable vane in the center of the flow stream, which rotates 90° as the butterfly valve opens and closes. |
| Production | capacity factor (of a valve), Cv, Kv | The number of gallons of water per minute that will flow through a valve with a pressure drop of 1 psi. Also expressed as Kv in m3/hr?bar. |
| Production | Charpy test | A mechanical impact test conducted on a precisely machined coupon of the steel to be tested. The coupon is clamped in a special machine and subjected to a lateral hammer blow. This test provides a relative measure of the toughness of the material and its resistance to shock and impact loads. Often required for low temperature applications where testing is done at the expected minimum service temperature. |
| Production | check valve | A one-directional valve that is opened by the fluid flow in one direction and closes automatically when the flow stops or reverses direction. |
| Production | city gate | The metering and pressure-reducing station where gas is transferred from a high-pressure cross-country transmission line to a low-pressure distribution piping system, usually within a city. |
| Production | clevis | A U-shaped connector used to loosely join parts by means of a bolt or pin passing through the ends of the connector. |
| Production | coal gasification | The process of converting coal to a form of synthetic natural gas. |
| Production | control valve | A valve that controls a process variable, such as pressure, flow or temperature, by modulating its opening in response to a signal from a controller. |
| Production | controller | A device that measures a controlled variable, compares it with a predetermined setting and signals an actuator to readjust the opening of a valve in order to re-establish the original control setting. |
| Production | cylinder operator | A power-piston valve operator that converts hydraulic or pneumatic pressure into linear motion of a valve stem (piston rod). |
| Production | dip tube | Extending a blowdown valve (BDV) on large gate valves requires a tube that is located inside of the valve. This dip tube extends through the bonnet to the bottom of the body cavity. |
| Production | double block-and-bleed | A valve arrangement that ensures no flow in a line, although the valve may leak. It consists of two block valves in the main line with a small bleeder valve draining the line between the block valves. |
| Production | droop | A drop in outlet pressure of a regulator or control valve due to the travel of its valve or poppet as the required flow increases from low to maximum. A slight change in a control spring length due to valve travel will result in spring force variations and a change of outlet pressure. |
| Production | drop bar | A heavy steel bar that is dropped through the tubing or running string to fire the percussion detonator on a tubing-conveyed perforating (TCP) gun assembly. The drop bar must be capable of falling through the string with sufficient speed to impart the necessary force for detonation. Therefore, this method of firing is best suited to vertical or slightly deviated wellbores where there will be minimal drag or friction effect. |
| Production | drop sub | A device, shaped like a short length of pipe, which is used to drop TCP guns in the rathole or sump. It is commonly used to drop guns that are connected to the completion into the sump, thus providing access to the reservoir for subsequent intervention work. It may also be used to break the tool string into fishable sections. |
| Production | dropoff gun | A perforating gun assembly designed to be detached from the tubing or running string after firing. The detached assembly can then drop, or be pushed, to the bottom of the well depending on deviation and production requirements. Drop-off gun assemblies often are used in underbalanced perforating applications, eliminating the need to kill the well to recover the spent gun assembly. In such cases, the wellbore will be designed to accommodate the spent gun assembly without compromising productivity, while recovery of the gun assembly may be planned during subsequent workover operations. The drop-off mechanism may be automatic and actuated at time of firing, or be actuated after firing. |
| Production | drop-off gun | A perforating gun assembly designed to be detached from the tubing or running string after firing. The detached assembly can then drop, or be pushed, to the bottom of the well depending on deviation and production requirements. Drop-off gun assemblies often are used in underbalanced perforating applications, eliminating the need to kill the well to recover the spent gun assembly. In such cases, the wellbore will be designed to accommodate the spent gun assembly without compromising productivity, while recovery of the gun assembly may be planned during subsequent workover operations. The drop-off mechanism may be automatic and actuated at time of firing, or be actuated after firing. |
| Production | dye penetrant inspection | A nondestructive examination (NDE) method for detecting the presence of surface cracks and surface imperfections in welds or castings through use of a special red dye. |
| Production | dynamic seal | A sealing element used between parts that have relative motion, such as stem seals and seat seal O-rings. |
| Production | expanding gate valve | A gate valve comprising a separate gate and segment that move without touching the seats as the valve operates the gate and segment. It permits the valve to be opened and closed without wear. In the closed position, the gate and segment are forced against the seats. Continued downward movement of the gate causes the gate and segment to expand against the seats. When the valve reaches its full open position, the gate and segment seal off against the seats while the flow is isolated from the valve body. |
| Production | fitting | Any device used for connecting elements in fluid lines, including elbows, tees, nipples, unions and flanges. |
| Production | flow coefficient, Cv | The number of gallons of water per minute that will flow through a valve with a pressure drop of 1 psi, abbreviated Cv. Kv is the cubic meters of water per hour that cause a pressure drop of 1 bar. |
| Production | gas volume fraction (GVF) | The ratio of the gas volumetric flow rate to the total volumetric flow rate of all fluids. |
| Production | gas/oil ratio (GOR) | The ratio of the volume of gas that comes out of solution to the volume of oil at standard conditions. |
| Production | gate valve | A straight-through pattern valve whose closure element is a wedge or parallel-sided slab, situated between two fixed seating surfaces with means to move it in or out of the flow stream in a direction perpendicular to the pipeline axis. |
| Production | gland bushing | A valve part that retains or compresses the stem packing in a stuffing box (where used) or retains a stem O-ring, lip seal or stem O-ring bushing. |
| Production | globe valve | A valve whose closure element is a flat disc or conical plug sealing on a seat that is usually parallel to the flow axis. The tortuous flow path produces a relatively high pressure loss. |
| Production | greenfield | A new oil and gas field development. |
| Production | hangoff | A means of supporting the weight of a cable or other connection. |
| Production | hardfacing | A metalworking process where harder or tougher material is applied to a weaker base metal. A surface preparation, such as detonation gun or high-velocity oxygen flow, in which an alloy is deposited on a metal surface, usually by weld overlay, to increase abrasion or corrosion resistance. |
| Production | inner seat ring | The inner part of a two-piece valve seat assembly. |
| Production | inside-out air seat test | A pressure test that can be performed only on a trunnion mounted ball valve with double piston effect seats. By closing the valves and pressurizing the body cavity, all of the seals in an independent seating ball valve can be pressure tested. |
| Production | key stop | A method of restricting the travel of a ball valve from fully open to fully closed. The stem key bears against the ends of an arc machined in the adapter plate. |
| Production | laminar flow | The flow of a viscous fluid in which the fluid moves in parallel layers with a fixed velocity gradient from the centerline to the containing walls of the conduit. Sometimes referred to as streamline flow. It occurs at a low Reynolds number, a dimensionless term related to fluid viscosity and flow rates. |
| Production | lip seal | A circular seal ring of U-shaped cross section encompassing an elastomeric O-ring, which provides resiliency and ensures a seal at the inner and outer lips of the U. |
| Production | magnetic particle inspection (MPI) | A nondestructive inspection procedure for detecting surface cracks in welded areas through the use of fine iron particles in an electrical field. |
| Production | manifold | A common pipe or chamber having several lateral outlets. |
| Production | maximum working pressure (MWP) | The maximum pressure at which a valve can be operated. The maximum working pressures for various pressure classes are defined by ASME B16.34 or API 6A. |
| Production | metal-to-metal seal | The seal produced by metal-to-metal contact between the sealing face of the seat ring and the closure elements, without benefit of a synthetic seal. |
| Production | needle valve | A type of small valve used for flow metering, having a tapered needlepoint plug or closure element and a seat having a small orifice. |
| Production | nondestructive examination (NDE) | Inspection tests that are not destructive to the valve structure or function. |
| Production | nonrising stem | A gate valve having its stem threaded into the gate. As the stem turns, the gate moves but the stem does not rise. Stem threads are exposed to line fluids. |
| Production | packing | The deformable sealing material inserted into a valve stem stuffing box, which provides a tight seal about the stem when compressed by a gland. |
| Production | peak shaving | When daily usage of natural gas is charted on graphs, high peaks of usage during the winter and summer months can be detected. These peaks can be averaged out (shaved) when the daily consumption is augmented with standby supplies of synthetic natural gas, propane, or methane. |
| Production | pilot-operated regulator |
A regulator that is controlled by a second small-volume, high-accuracy regulator or pilot. This arrangement has the advantage of improving performance by reducing the effects of unbalanced pressure and droop. |
| Production | pinion shaft | The external input shaft of certain gear operators, which drives the internal reduction gearing. The pinion shaft can accept a hand wheel or power operator. |
| Production | piston effect | The sealing principle involved in utilizing line pressure to effect a seal across the floating seats of some valves. |
| Production | pitch-and-lead | Pertaining to screw threads, the pitch refers to the measurement between adjacent threads. The lead refers to the distance the screw advances in one complete revolution. Worm gears of gear operators also are identified by pitch and lead. Speed of operation and torque required are related to pitch and lead. |
| Production | plug valve | A quarter-turn valve whose closure element is usually a tapered plug having a rectangular port. |
| Production | radiographic inspection | A nondestructive evaluation (NDE) procedure that uses X-rays for locating flaws in welds, casting and fabricated parts. |
| Production | relief valve | A quick-acting, spring-loaded valve that opens to relieve pressure when the pressure exceeds the spring setting. Often installed on the body cavity of ball and gate valves to relieve thermal overpressure in liquid services. |
| Production | rising stem | A valve stem that rises as the valve is opened. |
| Production | road box | A concrete or metal box with a removable cover, enclosing and providing access to valves installed in buried lines alongside roads or streets. The valves are operated by removing the box cover and inserting a long-handled T-wrench which engages the valve stem or the pinion shaft of geared valves. |
| Production | Rockwell hardness number |
A numerical expression of the hardness of a metal as determined with a Rockwell Hardness Tester. There are several hardness scales. The most commonly used are the Rockwell B scale for soft metals and the Rockwell C scale for hard materials. |
| Production | Rockwell hardness testing | Rockwell hardness testing is a testing method that uses a conical diamond or a steel ball to indent the surface of metals or polymers. The hardness number is determined from the depth of deformation created by the indenter into the sample. |
| Production | safety valve | A quick-opening, pop-action valve used for fast relief of excessive pressure. |
| Production | schedule | A system for indicating the wall thickness of pipe. The higher the schedule number, the thicker the wall for a certain pipe size. |
| Production | Scotch yoke | A slotted link mechanism that works as the quarter-turn operator for use on quarter-turn valves in place of gears. The Scotch yoke has a torque output at the beginning and end of its stroke that is generally twice the magnitude of the torque output in the center of its stroke. |
| Production | seat | That part of a valve against which the closure element (gate, ball) affects a tight shutoff. In many ball valves and gate valves, it is a floating member usually containing a soft seating element. |
| Production | shutoff valve |
A valve designed only for on and off service. Not a throttling valve. Sometimes referred to as a block valve. |
| Production | slam retarder | A device to prevent the clapper of a check valve from slamming as it closes upon flow reversal. Hydraulic damping cylinders, rotary vanes and torsional springs are all used for this purpose. |
| Production | slurry service | An application involving a flowing medium consisting of small solid particles suspended in a liquid. Coal slurry consisting of about equal parts of coal and water is transported by pipeline from coal mines to plants where the coal is dewatered and burned. |
| Production | static seal | A sealing element used as a gasket between two nonmoving parts, for example, valve bonnet O-rings, ball valve body O-rings and flange gaskets. |
| Production | stem | A rod or shaft transmitting motion from an operator to the closure element of a valve. |
| Production | stem indicator | A position-indicating rod supplied with gate valves. It extends from the top of the valve stem and serves to indicate the relative position of the gate. |
| Production | stopple | A procedure used in the repair of a pipeline to isolate a section of line in the absence of a shutoff valve. After welding a flanged saddle to the pipe, the line is hot tapped—a method of making a connection to existing piping or pressure vessels without the interruption of emptying that section of pipe or vessel—and an expanding resilient plug is inserted into the pipe bore. When the repair is completed, the plug is withdrawn and a valve, installed on the saddle flange, is closed. |
| Production | stuffing box | The annular chamber provided around a valve stem in a sealing system into which deformable packing is introduced. |
| Production | subsea isolation valve | A valve used underwater, generally in a manifold that will close and isolate a particular pipeline or process in an emergency. |
| Production | surge | A surge is the transient sudden rise or fall of pressure in a pipeline. Pipeline surges can be positive or negative and are caused most frequently by the sudden closure of a block valve or emergency shutdown of a pump. Surge pressure in excess of the rated capacity of a pipeline can cause ruptures of the piping system. |
| Production | surge reliever | A valve designed to relieve pressure surges in pipelines carrying liquids, thus preventing line rupture due to transient pressures exceeding design limits of the pipe. A special flexible tube valve can function as a fast-acting surge reliever. |
| Production | swage | A tool for bending or forming cold metal to a required shape. |
| Production | swing check valve | A check valve in which the closure element is a hinged clapper that swings or rotates about a supporting shaft. |
| Production | throttling | The intentional restriction of flow by partially closing or opening a valve. A wide range of throttling is accomplished automatically in regulators and control valves. |
| Production | through-conduit | An expression characterizing valves when in the open position, wherein the bore presents a smooth uninterrupted interior surface across seat rings and through the valve port, thus affording minimum pressure drop. There are no cavities or large gaps in the bore between seat rings and body closures or between seat rings, balls and gates. Consequently, there are no areas where debris can accumulate and impede pipeline cleaning equipment or restrict the valve’s motion. |
| Production | top entry | The design of a particular valve or regulator where the unit can be serviced or repaired by leaving its body in the line and accessing its internals by removing a top portion of the unit. |
| Production | trunnion |
The part of a ball valve that holds the ball on a fixed vertical axis and about which the ball turns. The torque requirements of a trunnion-mounted ball valve are significantly less than for a floating ball design. |
| Production | turbulent flow | The random flow of a fluid in which the velocity at a certain point in the fluid varies irregularly. It occurs at a high Reynolds number, a dimensionless term related to fluid viscosity and flow rates. |
| Production | ultrasonic inspection | A nondestructive evaluation (NDE) inspection procedure that uses high-frequency sound waves to detect voids and imperfections of metal parts. |
| Production | variable orifice | A small variable profile valve put in a flowline and used with a pilot to restrict the flow into the pilot and make the pilot more or less sensitive to changing conditions. |
| Production | vent plug | A special pipe plug having a small Allen-wrench-operated vent valve. These special plugs are located at the bottom of most ball valves. With the line valve closed (and under pressure), the body cavity pressure can be vented through this small valve to check the tightness of seat seals or to make minor repairs. Having vented the body pressure, the vent plug can be removed to blow out debris and foreign material or to flush the body cavity. On some gate valves, the vent plug is installed on the bonnet for the purpose of venting the body. Such valves have separate drain valves. |
| Production | venturi valve | A venturi valve is a reduced-bore valve having a bore smaller in diameter than the inlet or outlet. For example, an 8-in. x 6-in. x 8-in. ball valve has 8-in. inlet and outlet connections, while the ball and seats are 6 in. The flow through a venturi valve will be reduced because of the smaller port. Venturi valves can often be economically substituted for plug valves. |
| Production | visible position indicator | A position-indicating rod supplied with gate valves. It extends from the top of the valve stem and serves to indicate the relative position of the gate. |
| Production | water hammer | The physical effect, often accompanied by loud banging, produced by pressure waves generated by a rapid change of velocity in a liquid system within piping. |
| Production | wedge gate | A gate whose seating surfaces are inclined to the direction of closing thrust so that mechanical force on the stem produces tight contact with the inclined seat rings. |
| Production | yoke | The part of a gate valve that serves as a spacer between the bonnet and the operator or actuator. |
| Production Facilities | absorption | The property of some liquids or solids to soak up water or other fluids. The natural gas dehydration process uses glycols (liquids) that absorb the water vapor to finally obtain dehydrated gas. In the same way, light oil, also called absorption oil, is used to remove the heavier liquid hydrocarbons from a wet gas stream to obtain dry gas. |
| Production Facilities | absorption oil | A light liquid hydrocarbon used to absorb or remove the heavier liquid hydrocarbons from a wet gas stream. Absorption oil is also called wash oil. |
| Production Facilities | acid gas | A gas that can form acidic solutions when mixed with water. The most common acid gases are hydrogen sulfide [H2S] and carbon dioxide [CO2] gases. Both gases cause corrosion; hydrogen sulfide is extremely poisonous. Hydrogen sulfide and carbon dioxide gases are obtained after a sweetening process applied to a sour gas. |
| Production Facilities | as delivered BTU | The number of BTUs in a cubic foot of natural gas. The natural gas heat energy (BTU) will depend mainly on its water content at the delivered pressure and temperature conditions. |
| Production Facilities | as-delivered BTU | The number of BTUs in a cubic foot of natural gas. The natural gas heat energy (BTU) will depend mainly on its water content at the delivered pressure and temperature conditions. |
| Production Facilities | battery | The installation of similar or identical units of equipment in a group, such as a separator battery, header battery, filter battery or tank battery. |
| Production Facilities | battery site | A portion of land that contains separators, treaters, dehydrators, storage tanks, pumps, compressors and other surface equipment in which fluids coming from a well are separated, measured or stored. |
| Production Facilities | blanket gas | A gas phase maintained above a liquid in a vessel to protect the liquid against air contamination, to reduce the hazard of detonation or to pressurize the liquid. The gas source is located outside the vessel. |
| Production Facilities | blow-by | A phenomenon in which free gas leaves with the liquid phase at the bottom of the separator. This condition may indicate a low liquid level or improper level control inside the separator. |
| Production Facilities | blowing the drip | Opening the valve on a drip to allow natural gas to blow or clear the pipe of all liquids. |
| Production Facilities | bottomhole heater | A device installed at the bottom of a well to increase the temperature of the fluid coming from the reservoir. Bottomhole heaters are used in low API gravity crude oils to reduce the fluid viscosity, thus reducing the high friction forces normally associated with these types of fluids |
| Production Facilities | brine | Water containing salts in solution, such as sodium, calcium or bromides. Brine is commonly produced along with oil. The disposal of oilfield brine is usually accomplished by underground injection into salt-water saturated formations or by evaporation in surface pits. |
| Production Facilities | British thermal unit | A measure of heat energy required to raise the temperature of one pound of water by one degree Fahrenheit. British thermal unit is abbreviated as BTU. |
| Production Facilities | BTU | Abbreviation for British thermal unit. |
| Production Facilities | carry over | A phenomenon in which free liquid leaves with the gas phase at the top of a separator. Carryover can indicate high liquid level, damage of the separator or plugged liquid valves at the bottom of the separator. |
| Production Facilities | carryover | A phenomenon in which free liquid leaves with the gas phase at the top of a separator. Carryover can indicate high liquid level, damage of the separator or plugged liquid valves at the bottom of the separator. |
| Production Facilities | compression ratio | The ratio of the volume of an engines cylinder at the beginning of the compression to its volume at the end of the compression process. For example, a cylinder with a volume of 20 cubic inches before compression and 1 cubic inch as its final volume after compression has a compression ratio of 20:1. |
| Production Facilities | compressor | A device that raises the pressure of air or natural gas. A compressor normally uses positive displacement to compress the gas to higher pressures so that the gas can flow into pipelines and other facilities. |
| Production Facilities | compressor plant | A facility consisting of many compressors, auxiliary treatment equipment and pipeline installations to pump natural gas under pressure over long distances. A compressor plant is also called a compressor station. Several compressor stations can be used to repressurize gas in large interstate gas pipelines or to link offshore gas fields to their final terminals. |
| Production Facilities | cut oil | A crude oil that contains water, normally in the form of an emulsion. The emulsion must be treated inside heaters using chemicals, which will break the mixture into its individual components (water and crude oil). |
| Production Facilities | cycle condensate | A condensate (liquid hydrocarbon) produced at surface from cycle gas. |
| Production Facilities | cycle gas | A gas that is compressed and injected back to the reservoir. In gas-condensate reservoirs, after the liquids or condensate are recovered at the surface, the residue gas (dry gas) is returned to the reservoir to maintain pressure. This prevents retrograde condensation, which will form unrecoverable liquid hydrocarbons in the reservoir. |
| Production Facilities | cycling plant | An oilfield installation used when producing from a gas-condensate reservoir. In a cycling plant, the liquids are extracted from the natural gas and then the remaining dry gas is compressed and returned to the producing formation to maintain reservoir pressure. This process increases the ultimate recovery of liquids. |
| Production Facilities | DEA unit | A treating system used to remove hydrogen sulfide [H2S], carbon dioxide [CO2] and carbonyl sulfide from a gas stream. The acid gases are absorbed by the diethanolamine (DEA), and sweet gas leaves at the top of the absorber. |
| Production Facilities | defoaming plates | In a separator, a series of inclined parallel plates or tubes to promote coalescence, or merging, of the foam bubbles liberated from the liquid. |
| Production Facilities | dehydrate | To remove water from a substance. The substance may be crude oil, natural gas or natural gas liquids (NGL). Fluid dehydration is needed to prevent corrosion and free-water accumulation in the low points of a pipeline. In the case of gas, it is especially important to avoid hydrate formation and also to meet pipeline requirements. Typical maximum allowable water vapor content is 7 pounds of water per million standard cubic feet. In colder climates, this threshold value could be 3 to 5 pounds per million standard cubic feet. Water vapor can also affect the sweetening and refining processes of a natural gas. Dehydration of crude oil is normally achieved using emulsion breakers, while gas dehydration is accomplished using various liquid desiccants such as glycols (ethylene, diethylene, triethylene and tetraethylene) or solid desiccants such as silica gel or calcium chloride [CaCl2]. |
| Production Facilities | dehydrator | A device used to remove water and water vapors from gas. Gas dehydration can be accomplished through a glycol dehydrator or a dry-bed dehydrator, which use a liquid desiccant and a solid desiccant, respectively. Gas dehydrators are designed to handle only water and gas vapors. If liquid water or oil enters the dehydrator, the device cannot work properly. |
| Production Facilities | desiccant | A substance used in a gas-dehydration unit to remove water and moisture. The desiccant can be liquid, such as methanol, glycol (ethylene, diethylene, triethylene, and tetraethylene). Desiccants also can be solid, such as silica gel or calcium chloride [CaCl2]. The most common gas-dehydration system (glycol dehydrator) uses liquid desiccants such as diethylene, triethylene and tetraethylene, which are substances that can be regenerated. Regeneration means that the water absorbed by these substances can be separated from them. Some liquid desiccants such as methanol or ethylene cannot be regenerated. Solid desiccants are also used for gas dehydration. They are placed as beds through which wet gas is passed. The main limitation of the use of solid desiccants is that they absorb only limited quantities of water. When the desiccant saturation point is reached, the solid desiccant must be replaced. Another limitation is that sometimes water cannot be removed from it. |
| Production Facilities | desulfurize | To remove sulfur or sulfur compounds from an oil or gas stream. |
| Production Facilities | downstream pipeline | A pipeline that receives natural gas or oil from another pipeline at some specific connection point |
| Production Facilities | drip | A small vessel in a pipeline to receive water and heavy hydrocarbons that drop out of a gas stream. Drips are normally installed in the lower points of flow lines and must be blown periodically to remove liquids. |
| Production Facilities | drip accumulator | A device used to collect water and heavy hydrocarbons that drop out of a gas stream in a pipeline. |
| Production Facilities | dry bed | A hygroscopic solid such as silica gel, calcium chloride [CaCl2] or other materials used in dry-bed dehydrators to absorb water and water vapor from a gas stream. |
| Production Facilities | dry bed dehydrator | A device that removes water and water vapor from a gas stream using two or more beds of solid desiccants, such as silica gel or calcium chloride [CaCl2]. Wet gas is passed through the solid material, which absorbs the water, and then dry gas is collected at the top of the device. The main limitation of this device is that the solid desiccant absorbs only limited quantities of water. When the desiccant saturation point is reached, it must be replaced and sometimes water cannot be removed from it. |
| Production Facilities | dry oil | A treated oil that contains small amounts of basic sediments and water (BS&W). Dry oil is also called clean oil. |
| Production Facilities | dry-bed dehydrator | A device that removes water and water vapor from a gas stream using two or more beds of solid desiccants, such as silica gel or calcium chloride [CaCl2]. Wet gas is passed through the solid material, which absorbs the water, and then dry gas is collected at the top of the device. The main limitation of this device is that the solid desiccant absorbs only limited quantities of water. When the desiccant saturation point is reached, it must be replaced and sometimes water cannot be removed from it. |
| Production Facilities | evaporation pit | A hole dug to contain brine for disposal by evaporation. Some evaporation pits are lined with plastic or asphalt to keep water from filtering through and contaminating nearby free-water aquifers. |
| Production Facilities | flare | The burning of unwanted gas through a pipe (also called a flare). Flaring is a means of disposal used when there is no way to transport the gas to market and the operator cannot use the gas for another purpose. Flaring generally is not allowed because of the high value of gas and environmental concerns. |
| Production Facilities | flare | An arrangement consisting of a vertical tower and burners used to burn combustible vapors. A flare is usually situated near a producing well or at a gas plant or refinery. A flare is also called a flare stack. |
| Production Facilities | flare gas | A vapor or gas that is burned through a pipe or burners. |
| Production Facilities | free water knockout | A vertical or horizontal separator used mainly to remove any free water that can cause problems such as corrosion and formation of hydrates or tight emulsions, which are difficult to break. A free-water knockout is commonly called a three-phase separator because it can separate gas, oil and free water. The liquids that are discharged from the free-water knockout are further treated in vessels called treaters. Free-water knockout is abbreviated as FWKO. |
| Production Facilities | free-water knockout | A vertical or horizontal separator used mainly to remove any free water that can cause problems such as corrosion and formation of hydrates or tight emulsions, which are difficult to break. A free-water knockout is commonly called a three-phase separator because it can separate gas, oil and free water. The liquids that are discharged from the free-water knockout are further treated in vessels called treaters. Free-water knockout is abbreviated as FWKO. |
| Production Facilities | FWKO | A vertical or horizontal separator used mainly to remove any free water that can cause problems such as corrosion and formation of hydrates or tight emulsions, which are difficult to break. A free-water knockout is commonly called a three-phase separator because it can separate gas, oil and free water. The liquids that are discharged from the free-water knockout are further treated in vessels called treaters. Free-water knockout is abbreviated as FWKO. |
| Production Facilities | gas processing plant | An installation that processes natural gas to recover natural gas liquids (condensate, natural gasoline and liquefied petroleum gas) and sometimes other substances such as sulfur. A gas processing plant is also known as a natural gas processing plant. |
| Production Facilities | gaswell gas | The gas produced or separated at surface conditions from the full well stream produced from a natural gas reservoir. |
| Production Facilities | gaswell liquids | The liquids separated at surface conditions from the full well stream produced from a natural gas reservoir. |
| Production Facilities | gathering lines | The pipes used to transport oil and gas from a field to the main pipeline in the area. |
| Production Facilities | gathering system | The flowline network and process facilities that transport and control the flow of oil or gas from the wells to a main storage facility, processing plant or shipping point. A gathering system includes pumps, headers, separators, emulsion treaters, tanks, regulators, compressors, dehydrators, valves and associated equipment. There are two types of gathering systems, radial and trunk line. The radial type brings all the flowlines to a central header, while the trunk-line type uses several remote headers to collect fluid. The latter is mainly used in large fields. The gathering system is also called the collecting system or gathering facility. |
| Production Facilities | glycol absorber | In a glycol dehydrator unit, the cylinder composed of various perforated trays in which wet gas and glycol are put in contact. |
| Production Facilities | glycol dehydrator | A unit used to remove minute water particles from natural gas if dehydration was not attained using separators. A glycol dehydrator unit is usually composed of an absorber and a reboiler. The wet gas enters at the bottom of the absorber. As the wet gas percolates upward, it releases its water into the glycol solution and dry gas is obtained at the top of the absorber. When the glycol solution becomes saturated with water, the glycol solution is pumped through a reboiler, also called a reconcentrator, which boils the glycol-water mixture and separates the glycol from the water. After separation, the glycol can return to the absorber to contact additional wet gas. |
| Production Facilities | gravity segregation | The tendency of fluids to stratify into different layers because of gravity forces. In gravity segregation, the heaviest fluid settles near the bottom and the lightest fluid rises to the top. Gravity segregation occurs inside reservoirs as well as in separator facilities. |
| Production Facilities | gun barrel | A settling tank used for treating oil. Oil and brine are separated only by gravity segregation forces. The clean oil floats to the top and brine is removed from the bottom of the tank. Gun barrels are found predominantly in older or marginal fields. A gun barrel is also called a wash tank. |
| Production Facilities | hatch | An opening in the top of a tank through which samples are taken or inspection is made. |
| Production Facilities | header | In a gathering system, a pipe arrangement that connects flowlines from several wellheads into a single gathering line. A header has production and testing valves to control the flow of each well, thus directing the produced fluids to production or testing vessels. Individual gas/oil ratios and well production rates of oil, gas and water can be assigned by opening and closing selected valves in a header and using individual metering equipment or separators. |
| Production Facilities | heater | Equipment that transfers heat to the produced gas stream. Heaters are especially used when producing natural gas or condensate to avoid the formation of ice and gas hydrates. These solids can plug the wellhead, chokes and flowlines. The production of natural gas is usually accompanied by water vapor. As this mixture is produced, it cools down on its way to the surface and also when the mixture passes through a surface production choke. This reduction of fluid temperature can favor the formation of gas hydrates if heaters are not used. Heaters may also be used to heat emulsions before further treating procedures or when producing crude oil in cold weather to prevent freezing of oil or formation of paraffin accumulations. |
| Production Facilities | heater treater | A vessel that uses heat to break oil-water emulsions so the oil can be accepted by the pipeline or transport. There are vertical and horizontal treaters. The main difference between them is the residence time, which is shorter in the vertical configuration compared with the horizontal one. |
| Production Facilities | horizontal separator | A vessel, with its cylindrical axes parallel to the ground, that is used to separate oil, gas and water from the produced stream. The horizontal separator can be a two-phase or three-phase separator. |
| Production Facilities | lean glycol | In a glycol dehydrator, glycol that has been boiled and no longer contains any water. When the glycol is lean, it can be pumped back to the absorber for reuse. |
| Production Facilities | liquefied natural gas carrier | A sea vessel used to transport liquefied petroleum gas (LPG). The term is commonly abbreviated as LNGC |
| Production Facilities | liquid desiccant | A hygroscopic liquid used to remove water and water vapor from a gas stream. Some liquid desiccants are glycols (diethylene, triethylene and tetraethylene), which are substances that can be regenerated. Regeneration means that the water absorbed by these substances can be separated from them. Some liquid desiccants, such as methanol or ethylene, cannot be regenerated. |
| Production Facilities | LNGC | Abbreviation for liquefied natural gas carrier, which is a sea vessel used to transport liquefied petroleum gas (LPG). |
| Production Facilities | loose emulsion | An emulsion with large and widely distributed droplets. A loose emulsion can be easy to break. |
| Production Facilities | mist | Small liquid droplets (moisture or liquid hydrocarbons) in a gas stream. In separators, mist extractors are used to collect mist. |
| Production Facilities | mist extractor | A device used to collect small liquid droplets (moisture or hydrocarbons) from the gas stream before it leaves the separator. The two most common types of mist extractors are wire-mesh pads and vanes. Once the small droplets of liquid are collected, they are removed along with the other liquids from the separator. |
| Production Facilities | multiphase pump | A pump that can handle the complete production from a well (oil, natural gas, water and sand, for example) without needing to separate or process the production stream near or at the wellhead. This reduces the cost associated with the surface facilities. Using multiphase pumps allows development of remote locations or previously uneconomical fields. Additionally, since the surface equipment, including separators, heater-treaters, dehydrators and pipes, is reduced, the impact on the environment is also reduced. Multiphase pumps can handle high gas volumes as well as the slugging and different flow regimes associated with multiphase production. Multiphase pumps include twin-screw pumps, piston pumps and helicoaxial pumps. |
| Production Facilities | oil and gas separator | A vessel that separates the well fluids into gas and total liquid. A two-phase separator can be horizontal, vertical or spherical. The liquid (oil, emulsion) leaves the vessel at the bottom through a level-control or dump valve. The gas leaves the vessel at the top, passing through a mist extractor to remove the small liquid droplets in the gas. Separators can be categorized according to their operating pressure. Low-pressure units handle pressures of 10 to 180 psi [69 to 1,241 kPa]. Medium-pressure separators operate from 230 to 700 psi [1,586 to 4,826 kPa]. High-pressure units handle pressures of 975 to 1,500 psi [6,722 to 10,342 kPa]. Gravity segregation is the main force that accomplishes the separation, which means the heaviest fluid settles to the bottom and the lightest fluid rises to the top. Additionally, inside the vessel, the degree of separation between gas and liquid will depend on the separator operating pressure, the residence time of the fluid mixture and the type of flow of the fluid. Turbulent flow allows more bubbles to escape than laminar flow. |
| Production Facilities | pipeline | A tube or system of tubes used for transporting crude oil and natural gas from the field or gathering system to the refinery. |
| Production Facilities | pipeline capacity | The quantity (volume) of oil and gas required to maintain a full pipeline. The static capacity of a pipeline is usually expressed as a volume per unit length (for example, bbl/ft). Nevertheless, the fluid volume passing through a pipeline in a specific time period will depend on initial pressure, flow characteristics, ground elevation, density and delivery pressure. |
| Production Facilities | pipeline gas | A sufficiently dry gas that will not drop out natural gas liquids (NGL) when entering the gas pipeline; also, gas with enough pressure to enter high-pressure gas pipelines. |
| Production Facilities | pipeline oil | Oil whose free water, sediment and emulsion content (BS&W) is sufficiently low to be acceptable for pipeline shipment. |
| Production Facilities | pipeline patrol | An inspection of a pipeline to check for leaks, washouts or other abnormal conditions. A pipeline patrol is commonly performed using airplanes. |
| Production Facilities | pressure storage tank | A tank designed for storing volatile liquids such as gasoline and liquefied petroleum gases (LPG), which generate high internal pressures. A pressure storage tank is commonly spherical. Other types include spheroidal or hemispherical vessels. Some pressure storage tanks can support several hundred pounds per square inch of internal pressure. A pressure storage tank is also called a pressure-type tank. |
| Production Facilities | raw natural gas | Gas produced from the well, unprocessed natural gas or the inlet natural gas to a plant. The raw gas still contains natural gas liquids (condensate, natural gasoline and liquefied petroleum gas), water and some other impurities such as nitrogen, carbon dioxide, hydrogen sulfide and helium. The raw gas must be processed in a gas processing plant to make the gas commercial. |
| Production Facilities | residence time | Another term for retention time, the amount of time a liquid stays in a vessel. The retention time assures that equilibrium between the liquid and gas has been reached at separator pressure. The retention time in a separator is determined by dividing the liquid volume inside the vessel by the liquid flow rate. The retention time usually varies between 30 seconds and 3 minutes. If a foaming crude is present, the retention time could be increased by four times its normal values. |
| Production Facilities | retention time | The amount of time a liquid stays in a vessel. The retention time assures that equilibrium between the liquid and gas has been reached at separator pressure. The retention time in a separator is determined by dividing the liquid volume inside the vessel by the liquid flow rate. The retention time usually varies between 30 seconds and 3 minutes. If a foaming crude is present, the retention time could be increased by four times its normal values. |
| Production Facilities | rich glycol | In a glycol dehydrator, glycol that contains water released by wet gas while percolating upward in the absorber. |
| Production Facilities | roll a tank | To agitate a tanks contents with gas or air injected through a roll line. This procedure is performed to settle out impurities or obtain a more homogeneous mixture of the chemicals added to oil, such as when chemicals used to break emulsions. The procedure is also used to mix chemicals before a stimulation treatment of an oil or gas well. |
| Production Facilities | roll line | A thin, perforated pipe placed around the internal circumference of a tank. The purpose of the roll line is to agitate the contents of a tank. |
| Production Facilities | separator | A cylindrical or spherical vessel used to separate oil, gas and water from the total fluid stream produced by a well. Separators can be either horizontal or vertical. Separators can be classified into two-phase and three-phase separators (commonly called free-water knockout). The two-phase type deals only with oil and gas, while the three-phase type handles oil, water and gas. Additionally, separators can be categorized according to their operating pressure. Low-pressure units handle pressures of 10 to 180 psi [69 to 1241 kPa]. Medium-pressure separators operate from 230 to 700 psi [1586 to 4826 kPa]. High-pressure units handle pressures of 975 to 1500 psi [6722 to 10,342 kPa]. Gravity segregation is the main force that accomplishes the separation, which means the heaviest fluid settles to the bottom and the lightest fluid rises to the top. Additionally, inside the vessel, the degree of separation between gas and liquid will depend on the separator operating pressure, the residence time of the fluid mixture and the type of flow of the fluid. Turbulent flow allows more bubbles to escape than laminar flow. |
| Production Facilities | SNG | Abbreviation for synthetic natural gas |
| Production Facilities | solid desiccant | A solid, such as silica gel or calcium chloride [CaCl2], used in a gas-dehydration unit to remove water and moisture. The desiccants are placed as beds through which wet gas is passed. The main limitation of the use of solid desiccants is that they absorb only limited quantities of water. When the desiccant saturation point is reached, the solid desiccant must be replaced. Another limitation is that sometimes water cannot be removed from it. |
| Production Facilities | spherical separator | A ball-shaped vessel used for fluid separation. A spherical separator can be used for two-phase or three-phase separation purposes. Spherical separators are less efficient than either horizontal or vertical cylindrical separators and are seldom used. Nevertheless, their compact size and ease of transportation have made them suitable for crowded processing areas. |
| Production Facilities | spot sample | A sample of liquid or sediments obtained at a specific depth inside a tank using a thief or a bottle. Spot samples are analyzed to determine the gravity of the oil and BS&W content of the fluid in the tank. |
| Production Facilities | STB | Abbreviation for stock tank barrel. |
| Production Facilities | stock tank | A storage tank for oil production after the oil has been treated |
| Production Facilities | stock tank barrel | A measure of the volume of treated oil stored in stock tanks. A stock tank barrel is commonly abbreviated as STB. |
| Production Facilities | strap | To measure the dimensions of an oil tank, such as external diameter and height, using a steel tape. Once the measurements are recorded, they may be used to prepare tank tables, which describe tank capacity. |
| Production Facilities | strapping tape | A graduated tape use to measure, or strap, producing tanks. The measurements are used to generate a tank table, which describes tank capacity. |
| Production Facilities | surge tank | A vessel placed in a flowline through which liquids or gases are flowed to neutralize sudden pressure surges. |
| Production Facilities | sweetening | A process used to remove hydrogen sulfide [H2S] and carbon dioxide [CO2] from a gas stream. These components are removed because they can form acidic solutions when they contact water, which will cause corrosion problems in gas pipelines. In a sweetening process, different types of ethanolamine can be used, including monoethanolamine (MEA), diethanolamine (DEA), diglycolamine (DGA) and methyldiethanolamine (MDEA). Hydrogen sulfide and carbon dioxide are absorbed by the ethanolamine and sweet gas leaves at the top of the absorber. The ethanolamine is heated and acid gas (hydrogen sulfide and carbon dioxide gases) and water vapor are obtained. The water is removed while the acid gas can be flared or further treated in a sulfur recovery unit to separate out elemental sulfur. Finally, the lean ethanolamine is returned to the absorber. |
| Production Facilities | synthetic natural gas | A gas obtained by heating coal or refining heavy hydrocarbons. Synthetic natural gas is abbreviated SNG. |
| Production Facilities | tank | A metal or plastic vessel used to store or measure a liquid. The three types of tanks in an oil field are drilling, production and storage tanks. |
| Production Facilities | tank battery | A group of tanks that are connected to receive crude oil production from a well or a producing lease. A tank battery is also called a battery. In the tank battery, the oil volume is measured and tested before pumping the oil into the pipeline system. |
| Production Facilities | tank bottoms | The settlings — sediment, dirt, oil emulsified with water and free water — that accumulate in the bottom of storage tanks. The tank bottoms are periodically cleaned up and settlings can be disposed of or treated by chemicals to recover additional hydrocarbons. Tank bottoms are also called tank settlings or tank sludge. |
| Production Facilities | tank calibration | Measurement of the dimensions of an oil tank, such as external diameter and height, using a steel tape. Once the measurements are recorded, they may be used to prepare tank tables, which describe tank capacity. |
| Production Facilities | tank dike | A structure constructed around an oil tank to contain the oil in case the tank collapses. The volume or space inside the tank dike should be greater than the volume of the tank. A tank dike is also called a fire wall. |
| Production Facilities | tank table | A table that shows the tank capacity in barrels as a function of the liquid level inside the tank. A tank table is also called a tank capacity table or gauge table. |
| Production Facilities | tankage | The capacity of all the tanks in a field |
| Production Facilities | tanker | A ship designed to transport crude oil, liquefied petroleum gas (LPG), liquefied natural gas (LNG), synthetic natural gas (SNG) or refined products. Tankers with 100,000 deadweight tons of capacity or more are called supertankers (very large crude carriers or ultralarge crude carriers). A tanker is also called a tank ship. |
| Production Facilities | TAPS | Abbreviation for Trans-Alaska Pipeline System |
| Production Facilities | test separator | A vessel used to separate and meter relatively small quantities of oil and gas. Test separators can be two-phase or three-phase, or horizontal, vertical or spherical. They can also be permanent or portable. Test separators sometimes are equipped with different meters to determine oil, water and gas rates, which are important to diagnose well problems, evaluate production performance of individual wells and manage reserves properly. Test separators can also be called well testers or a well checkers. |
| Production Facilities | thief | A device that can be lowered into a tank to obtain samples (liquid or sediments) at different depths. The samples are analyzed to determine the gravity and BS&W content of the fluid into the tank. |
| Production Facilities | thief hatch | An opening in the top of the stock tank. The thief hatch allows tank access for a thief or other level measuring devices. |
| Production Facilities | three phase separator | A vessel that separates the well fluids into gas and two types of liquids: oil and water. A three-phase separator can be horizontal, vertical or spherical. This type of separator is commonly called a free-water knockout because its main use is to remove any free water that can cause problems such as corrosion and formation of hydrates or tight emulsions, which are difficult to break. A free-water knockout is commonly called a three-phase separator because it can separate gas, oil and free water. The liquids that are discharged from the free-water knockout are further treated in vessels called treaters. Free-water knockout is abbreviated as FWKO. |
| Production Facilities | three-phase separator | A vessel that separates the well fluids into gas and two types of liquids: oil and water. A three-phase separator can be horizontal, vertical or spherical. This type of separator is commonly called a free-water knockout because its main use is to remove any free water that can cause problems such as corrosion and formation of hydrates or tight emulsions, which are difficult to break. A free-water knockout is commonly called a three-phase separator because it can separate gas, oil and free water. The liquids that are discharged from the free-water knockout are further treated in vessels called treaters. Free-water knockout is abbreviated as FWKO. |
| Production Facilities | tight emulsion | An emulsion with small and closely distributed droplets. A tight emulsion can be difficult to break. |
| Production Facilities | Trans-Alaska Pipeline System | An 800-mile [1287-km], 48-in. [122-cm] pipeline that transports more than 1 million barrels of oil from Deadhorse (near Prudhoe Bay) to Valdez, Alaska, USA. The Trans-Alaska Pipeline System was completed in 1977 and it is often abbreviated as TAPS. |
| Production Facilities | treater | A vessel used to treat oil-water emulsions so the oil can be accepted by the pipeline or transport. A treater can use several mechanisms. These include heat, gravity segregation, chemical additives and electric current to break emulsions. There are vertical and horizontal treaters. The main difference between them is the residence time, which is shorter in the vertical configuration compared with the horizontal one. A treater can be called a heater treater or an emulsion treater. |
| Production Facilities | two phase separator | A vessel that separates the well fluids into gas and total liquid. A two-phase separator can be horizontal, vertical or spherical. The liquid (oil, emulsion) leaves the vessel at the bottom through a level-control or dump valve. The gas leaves the vessel at the top, passing through a mist extractor to remove the small liquid droplets in the gas. |
| Production Facilities | two-phase separator | A vessel that separates the well fluids into gas and total liquid. A two-phase separator can be horizontal, vertical or spherical. The liquid (oil, emulsion) leaves the vessel at the bottom through a level-control or dump valve. The gas leaves the vessel at the top, passing through a mist extractor to remove the small liquid droplets in the gas. |
| Production Facilities | ULCC | Abbreviation for ultralarge crude carrier. |
| Production Facilities | ultralarge crude carrier | A supertanker with 500,000 deadweight tons of capacity or more. The term is commonly abbreviated as ULCC. |
| Production Facilities | vapor recovery unit | A system composed of a scrubber, a compressor and a switch. Its main purpose is to recover vapors formed inside completely sealed crude oil or condensate tanks. The switch detects pressure variations inside the tanks and turns the compressor on and off. The vapors are sucked through a scrubber, where the liquid trapped is returned to the liquid pipeline system or to the tanks, and the vapor recovered is pumped into gas lines. |
| Production Facilities | vertical separator | A vessel with its cylindrical axes perpendicular to the ground that is used to separate oil, gas and water from the production stream. The vessel can be a two-phase or three-phase separator. |
| Production Facilities | very large crude carrier | A supertanker with a capacity between 100,000 and 500,000 deadweight tons. The term is commonly abbreviated as VLCC. |
| Production Facilities | VLCC | Abbreviation for very large crude carrier. |
| Production Facilities | wet gas | Natural gas that contains water. |
| Production Facilities, Enhanced Oil Recovery | adsorption | The property of some solids and liquids to attract a liquid or a gas to their surfaces. Some solids, such as activated charcoal or silica gel, are used as surfaces of adhesion to gather liquid hydrocarbons from a natural gas stream. To complete the process, the solids are treated with steam to recover the liquid hydrocarbons. |
| Production Facilities, Production Testing | FLNG | Abbreviation for floating liquefied natural gas vessel. FLNGs are deployed offshore in locations that have limited access to pipelines to carry natural gas to the mainland. They serve to recover, liquefy, store, and transfer LNG produced from subsea wells. Processed using equipment on the ship’s deck, the natural gas may then be stored in the ship’s hull before it is offloaded to carriers and sent directly to LNG markets. These vessels reduce the need for flaring of natural gas to the atmosphere during production or testing. |
| Production Facilities | stage separation | An operation in which the well stream is passed through two or more separators that are arranged in series. The first separator is called first-stage separator, the second separator is called second-stage separator and additional separators are named according to their position in the series. The operating pressures are sequentially reduced, so the highest pressure is found at the first separator and the lowest pressure at the final separator. The objective of stage separation is to maximize the hydrocarbon liquid recovery and to provide maximum stabilization to the resultant phases (liquid and gas) leaving the final separator. Stabilization means that considerable amounts of gas or liquid will not evolve from the final liquid and gas phases, respectively, in places such as stock tanks or gas pipelines. Additionally, stage separation reduces the horsepower required by a compressor, since the gas is fed at higher pressures. |
| Production Logging | annular flow | A multiphase flow regime in which the lighter fluid flows in the center of the pipe, and the heavier fluid is contained in a thin film on the pipe wall. The lighter fluid may be a mist or an emulsion. Annular flow occurs at high velocities of the lighter fluid, and is observed in both vertical and horizontal wells. As the velocity increases, the film may disappear, leading to mist flow or emulsion flow. When the interface between the fluids is irregular, the term wavy annular flow may be used. |
| Production Logging | audio measurement | A technique for recording sound at different positions in the borehole to generate a noise log. The measurement technique uses a microphone to record signals in the audible range approximately 20 to 20,000 Hz . In some circumstances, the frequency of the signal can be related to the source of noise and the flow regime, while the amplitude of the signal can be related to the flow rate. The useful signal lies approximately between 100 and 5000 Hz, with lower frequencies generally representing background and mechanical noise. The measurement may record the total signal over all frequencies, the signal at a single frequency, or consist of a set of measurements over different frequency ranges. |
| Production Logging | basket flowmeter | A device for measuring in situ the velocity of fluid flow in a production or injection well in which the flow is diverted through the spinner by a set of metal vanes, or petals. The vanes are closed while running in the hole, and then opened with the tool stationary at the measurement depth. The petals do not seal completely against each other or against the side of the hole, so that not all the fluid is diverted. A type of diverter flowmeter, the petal basket design has generally been replaced since the late 1980s by the inflatable diverter and other designs. |
| Production Logging | bead tracer | A small, radioactive plastic sphere that is insoluble and used to make a tracer-loss measurement. The bead is designed to have the same density as the injection fluid so that it travels with the fluid when it is placed in the flow stream of an injection well. However, the bead does not enter the formation. It remains on the rock face in openhole, or within the perforation channel in cased hole, where it can be detected by a gamma ray log. A high radioactivity opposite a perforation indicates a large number of beads and hence a high injectivity. The technique was used mainly in the 1960s and 1970s. |
| Production Logging | biphasic | Referring to the flow of two immiscible fluids, oil and water, oil and gas, or gas and water. |
| Production Logging | bubble count | The frequency with which a local probe detects a change from one type of fluid to another. For example, if water is the continuous phase, the probe will respond digitally each time a bubble of oil or gas passes it. The average frequency of change is the bubble count rate, or bubble count. In this example, an increasing bubble count means an increasing oil or gas velocity. Bubble velocity can be calculated from bubble count and bubble size, the latter being estimated from an empirical correlation with water holdup. The depth at which the first bubbles are counted is a sensitive indicator of the lowest hydrocarbon entry. Since the bubble count is based on local probe measurements, both bubble count and bubble velocity can be presented as images, similar to the holdup image. |
| Production Logging | bypass velocity | In a spinner flowmeter, the theoretical minimum fluid velocity required to initiate spinner rotation, assuming the spinner response is linear. In this sense, it is synonymous with threshold velocity. However, it is sometimes taken to mean the fluid velocity at which a significant amount of flow begins to leak past a basket flowmeter, sufficient to cause the response to be nonlinear. |
| Production Logging | capacitance log | An in situ record of the capability of the fluid passing through a sensor to store electrical charge. Since water has a high dielectric constant, and hence capacitance, it can be distinguished from oil or gas. The capacitance, or fluid capacitance log, can therefore identify water and be scaled in terms of water holdup. However, the relation between capacitance and holdup depends strongly on whether the water is the continuous phase, complicating quantitative evaluation. The log was introduced in the 1960s with the so-called holdup meter. It was mainly used in three-phase flow, or when fluid-density measurements were insufficiently sensitive to water at low holdup, or with heavy oils. Since the late 1980s, other holdup measurements have been preferred. |
| Production Logging | capacitance meter | Another term for holdup meter, a device for determining the water holdup in a producing well by measuring the capacitance or impedance of the fluid. The holdup meter is used to produce a capacitance log. Since water has a high dielectric constant, and hence capacitance, it can be distinguished from oil or gas. The meter is a coaxial capacitor, with fluid flowing between a central probe and an external cage that act as electrodes. The meter has often been combined with a packer flowmeter or a diverter flowmeter, so that all the fluids in the well pass through the meter. |
| Production Logging | casing inspection log | An in situ record of casing thickness and integrity, to determine whether and to what extent the casing has undergone corrosion. The term refers to an individual measurement, or a combination of measurements using acoustic, electrical and mechanical techniques, to evaluate the casing thickness and other parameters. The log is usually presented with the basic measurements and an estimate of metal loss. It was first introduced in the early 1960s. Today the terms casing-evaluation log and pipe-inspection log are used synonymously. |
| Production Logging | casing potential profile | An in situ log of the electrical potential on the inner wall of a casing. The log is used to identify intervals that are susceptible to corrosion. A negative slope in the profile indicates a zone in which current is leaving the casing and therefore acting as an anode. Such zones are susceptible to corrosion. The log was first introduced in the early 1960s. Modern logs are recorded with the tool stationary, and measure the potential difference and casing resistance between several pairs of sensors pushed against the casing wall, and between sensors and surface. The log is usually represented with casing resistance and casing axial current. Sharp increases in casing resistance can indicate corroded zones or even holes in the casing. Decreasing axial current with depth indicates a corroding region. |
| Production Logging | casing-inspection log | An in situ record of casing thickness and integrity, to determine whether and to what extent the casing has undergone corrosion. The term refers to an individual measurement, or a combination of measurements using acoustic, electrical and mechanical techniques, to evaluate the casing thickness and other parameters. The log is usually presented with the basic measurements and an estimate of metal loss. It was first introduced in the early 1960s. Today the terms casing-evaluation log and pipe-inspection log are used synonymously. |
| Production Logging | casing-potential profile | An in situ log of the electrical potential on the inner wall of a casing. The log is used to identify intervals that are susceptible to corrosion. A negative slope in the profile indicates a zone in which current is leaving the casing and therefore acting as an anode. Such zones are susceptible to corrosion. The log was first introduced in the early 1960s. Modern logs are recorded with the tool stationary, and measure the potential difference and casing resistance between several pairs of sensors pushed against the casing wall, and between sensors and surface. The log is usually represented with casing resistance and casing axial current. Sharp increases in casing resistance can indicate corroded zones or even holes in the casing. Decreasing axial current with depth indicates a corroding region. |
| Production Logging | chemical marker injection | A technique in which a slug of material is introduced into the flowstream of a producing well to determine the flow rate of one or more of the fluids. The marker has specific properties, such as high neutron capture cross section, that allow it to be detected by sensors of a production logging tool. Some markers are specifically designed to be soluble in only one fluid phase, so that they can be used to produce a phase-velocity log. The term refers to nonradioactive markers, in contrast to the more traditional radioactive markers, or tracers. |
| Production Logging | chemical-marker injection | A technique in which a slug of material is introduced into the flowstream of a producing well to determine the flow rate of one or more of the fluids. The marker has specific properties, such as high neutron capture cross section, that allow it to be detected by sensors of a production logging tool. Some markers are specifically designed to be soluble in only one fluid phase, so that they can be used to produce a phase-velocity log. The term refers to nonradioactive markers, in contrast to the more traditional radioactive markers, or tracers. |
| Production Logging | controlled time survey | A method of determining injection-flow profiles by monitoring the reduction in tracer material as it moves down the well. A slug of radioactive tracer is added to the injection fluid. As the slug moves down the well, several gamma ray logs are recorded at well-defined time intervals. The position of the slug is seen as a large gamma ray peak whose size is proportional to the flow rate. A reduction in the size of the peak indicates a loss of fluid into the formation. Fluid velocity can be calculated from the time interval and the distance the peak has moved using timed-slug analysis. Tracer-loss measurements produce a type of radioactive-tracer log, used mainly to give a general idea of fluid flow in low flow-rate wells. In very low flow-rate wells, an alternative technique has been used in which the gamma ray detector is held stationary at some depth until the slug has passed. The detector is then moved down to another depth to observe the slug again. With these data, it is possible to make quantitative estimates of fluid flow. |
| Production Logging | crosscorrelation flowmeter | A device for measuring fluid velocity in a production well. The device measures the transit time of a disturbance between two sensors separated by a fixed distance. The technology applies to multiphase flow, in which the disturbance is caused, for example, by the passage of a bubble of gas over each sensor. In practice, there will be many bubbles of gas, so it is necessary to record both sensor signals over a time window and compare, or correlate, them. The two signals will correlate best after shifting one of them by a time corresponding to the average transit time of the bubbles. Different sensors may be used, for example a measure of electrical capacitance as in a holdup meter. The crosscorrelation flowmeter gives the velocity of the disturbance. Since this is caused by just one of the phases, it produces a type of phase velocity log. |
| Production Logging | cut | The fraction of the total flow rate produced from a well that is due to a particular fluid, for example the water cut in the case of water. The cut is normally quoted at standard surface conditions. |
| Production Logging | deflector flowmeter | A device for measuring in-situ the velocity of fluid flow in a production or injection well in which the total fluid flow is diverted to pass over an impeller, or spinner. Various techniques have been used to achieve this, one of the earliest being the packer flowmeter. In a typical modern device, the diverter consists of a fabric in a metal cage that is collapsed to pass through the tubing and other restrictions. Below the tubing, the cage is opened until an inflatable ring seals against the casing wall. At this point, the up-going production fluids are forced through the diverter and over an impeller. This ensures that the total casing flow is measured, but may also create an extra pressure drop and hence a change in multiphase flow structure. The diverter flowmeter is particularly suitable for low flow rates in vertical or moderately deviated wells. Readings are made with the tool stationary. |
| Production Logging | differential temperature log | A record of the difference in temperature between two vertical points in a well. Most differential-temperature logs are obtained by differentiating a normal temperature log with respect to depth. Some are obtained by recording the difference in temperature between two vertically displaced sensors. Note that the differential-temperature log and the radial differential-temperature log are not the same. |
| Production Logging | differential-temperature log | A record of the difference in temperature between two vertical points in a well. Most differential-temperature logs are obtained by differentiating a normal temperature log with respect to depth. Some are obtained by recording the difference in temperature between two vertically displaced sensors. Note that the differential-temperature log and the radial differential-temperature log are not the same. |
| Production Logging | dispersed bubble flow | A multiphase flow regime in pipes in which one fluid moves as small dispersed bubbles through a continuous fluid. The relative velocity of the bubbles depends mainly on the difference in density between the two fluids. Bubble flow normally occurs at low flow rate and low holdup of the bubbly fluid. As the velocity of the continuous fluid increases, the bubbles are dispersed into smaller, more widely separated bubbles. This is known as a dispersed or finely dispersed bubble flow, or sometimes dispersed flow. |
| Production Logging | distributed temperature log | A record of the change in temperature along a well, normally recorded by a fiber-optic cable. The distributed temperature is measured by sending a pulse of laser light down the optical fiber. Molecular vibration, which is directly related to temperature, creates weak reflected signals. These signals are detected at the surface and converted to a log of temperature along the well, sampled approximately every 1 m [3.28 ft] with a resolution of 0.1oC. The fiber-optic cable is normally installed at the time of well completion, so that the distributed-temperature log can be recorded at any later time without well intervention. Introduced in the mid-1990s, the technique can also be used to measure flow rates by creating a temperature transient and observing its movement along the well. |
| Production Logging | distributed-temperature log | A record of the change in temperature along a well, normally recorded by a fiber-optic cable. The distributed temperature is measured by sending a pulse of laser light down the optical fiber. Molecular vibration, which is directly related to temperature, creates weak reflected signals. These signals are detected at the surface and converted to a log of temperature along the well, sampled approximately every 1 m [3.28 ft] with a resolution of 0.1oC. The fiber-optic cable is normally installed at the time of well completion, so that the distributed-temperature log can be recorded at any later time without well intervention. Introduced in the mid-1990s, the technique can also be used to measure flow rates by creating a temperature transient and observing its movement along the well. |
| Production Logging | diverter flowmeter | A device for measuring in-situ the velocity of fluid flow in a production or injection well in which the total fluid flow is diverted to pass over an impeller, or spinner. Various techniques have been used to achieve this, one of the earliest being the packer flowmeter. In a typical modern device, the diverter consists of a fabric in a metal cage that is collapsed to pass through the tubing and other restrictions. Below the tubing, the cage is opened until an inflatable ring seals against the casing wall. At this point, the up-going production fluids are forced through the diverter and over an impeller. This ensures that the total casing flow is measured, but may also create an extra pressure drop and hence a change in multiphase flow structure. The diverter flowmeter is particularly suitable for low flow rates in vertical or moderately deviated wells. Readings are made with the tool stationary. |
| Production Logging | drag bag | Pertaining to a technique in which a packer flowmeter is partially inflated and dragged up the hole to give a continuous flow log. This obsolete technique was introduced in the 1960s because the packer flowmeter could make only stationary measurements. |
| Production Logging | drag-bag | Pertaining to a technique in which a packer flowmeter is partially inflated and dragged up the hole to give a continuous flow log. This obsolete technique was introduced in the 1960s because the packer flowmeter could make only stationary measurements. |
| Production Logging | eddy current measurement | A technique for measuring the effect of pits and holes in the inner wall of a casing on a high-frequency electrical signal induced in the casing. The eddy-current measurement is used in conjunction with a flux-leakage measurement to determine casing corrosion, the latter being sensitive to the defects on both the inner and outer walls. The principle of measurement is similar to the openhole induction log, but at higher frequencies. A transmitter coil produces a magnetic field that induces eddy currents in the casing wall. These currents generate their own magnetic field that induces a signal in two closely spaced receiver coils. In smooth casing, these signals are the same, but if the inner wall is pitted, the signals are different. Transmitter-receiver combinations are placed on multiple pads applied against the casing at several azimuths to fully cover the casing wall. |
| Production Logging | eddy-current measurement | A technique for measuring the effect of pits and holes in the inner wall of a casing on a high-frequency electrical signal induced in the casing. The eddy-current measurement is used in conjunction with a flux-leakage measurement to determine casing corrosion, the latter being sensitive to the defects on both the inner and outer walls. The principle of measurement is similar to the openhole induction log, but at higher frequencies. A transmitter coil produces a magnetic field that induces eddy currents in the casing wall. These currents generate their own magnetic field that induces a signal in two closely spaced receiver coils. In smooth casing, these signals are the same, but if the inner wall is pitted, the signals are different. Transmitter-receiver combinations are placed on multiple pads applied against the casing at several azimuths to fully cover the casing wall. |
| Production Logging | effective velocity | In the context of spinner flowmeters, the apparent fluid speed measured by the spinner as the tool is moved up and down the well. The effective velocity is the algebraic sum of the actual fluid velocity and the velocity with which the flowmeter is moving, as determined by cable speed. If the tool moves against the flow, the two velocities are added; if moved with the flow, they are subtracted. |
| Production Logging | electric probe | A small sensor in a production logging tool that distinguishes between hydrocarbon and water in its vicinity as it is moved up and down a production well. The electric probe was the first type of local probe to be introduced, appearing initially in the early 1990s. Electric probes measure the local and average holdup of water and give an image of the flow structure across the well. Most electric probes emit a high-frequency current and measure the amplitude of the signal, and hence the impedance, of the fluid in a small sphere near the probe. The output is designed to be digital, indicating conductance when the probe is in front of water, and no conductance when it is in front of hydrocarbon. The water must have a certain minimum salinity, below which the device will not work. |
| Production Logging | electrical impedance probe | Another term for electric probe, a small sensor in a production logging tool that distinguishes between hydrocarbon and water in its vicinity as it is moved up and down a production well. The electric probe was the first type of local probe to be introduced, appearing initially in the early 1990s. Electric probes measure the local and average holdup of water and give an image of the flow structure across the well. Most electric probes emit a high-frequency current and measure the amplitude of the signal, and hence the impedance, of the fluid in a small sphere near the probe. The output is designed to be digital, indicating conductance when the probe is in front of water, and no conductance when it is in front of hydrocarbon. The water must have a certain minimum salinity, below which the device will not work. |
| Production Logging | electromagnetic caliper | An in-situ measurement of the inside diameter of a casing or tubing using an electromagnetic technique. As with the electromagnetic thickness measurement, and usually measured at the same time, a coil centered inside the casing generates an alternating magnetic field. Another coil farther up the tool measures the phase shift introduced by the casing. At high frequency, the signal penetrates less than a tenth of a millimeter into the casing, and the phase shift can be related to the casing internal diameter. Unlike a mechanical or ultrasonic caliper, the measurement does not respond to nonmagnetic scale. For the purpose of determining the true internal diameter, this is a disadvantage, but for the purpose of determining corrosion, it is an advantage. |
| Production Logging | electromagnetic thickness | An in-situ measurement of the thickness of a casing or tubing string using an electromagnetic technique. The result is presented as a type of casing-inspection log, giving an estimate of metal loss and detecting corrosion. In the usual method, a coil centered inside the casing generates an alternating magnetic field. Another coil farther up the tool measures the phase shift introduced by the casing. This phase shift depends on the casing-wall thickness and internal diameter, as well as the casing conductivity and magnetic permeability. The effects change at different frequencies, so that by varying the frequency, the thickness and internal diameter can be uniquely determined. Electromagnetic thickness can also be measured using other techniques, for example from a casing-potential profile or a flux-leakage measurement. |
| Production Logging | emulsion flow | A multiphase-flow regime with oil as the continuous phase, in which water exists as small, approximately homogeneously distributed droplets. There may also be a thin film of water on the pipe wall. |
| Production Logging | flow concentrating | Referring to a type of spinner flowmeter in which most or all of the fluid flow in the well is diverted over the spinner by a device such as a basket or a packer. |
| Production Logging | flow loop | A laboratory instrument for investigating the characteristics of fluid flow in pipes and for studying the response of production logging instruments to this flow. The fluids are circulated continuously in a loop, passing through one main measurement section that can be placed at different deviations from vertical through horizontal. Fluid properties, holdups and velocities can all be varied. Flow loops are essential for the study of multiphase flow and the development of new production logging measurements. |
| Production Logging | flow meter | A device for measuring in-situ the velocity of fluid flow in a well, usually one completed for production or injection. The most common device is the spinner flowmeter, but torque flowmeters and crosscorrelation flowmeters also are used. In the 1940s and 1950s, various other surface-metering techniques were tried, but spinner flowmeters emerged as the most suitable for measuring downhole velocities. Spinner and torque flowmeters measure the average velocity of the fluids crossing the device, while crosscorrelation flowmeters measure the velocity of a particular phase. Although not normally called flowmeters, various other techniques measure flow velocity, for example water-flow logs, phase-velocity logs, distributed-temperature logs and even audio measurements. |
| Production Logging | flow profile | A recording of the in-situ rate of fluid flow at different depths in a well, normally one completed for production or injection. The flow profile is a log recorded in a unit such as barrels per day, or as a percentage of the total flow from the reservoir in a production well or into the reservoir in an injection well. In single-phase flow, the profile can be determined from a flowmeter. In multiphase flow, it is desirable to show the flow rates of each of the phases, in which case a holdup log and either a flowmeter or a phase velocity log are needed. |
| Production Logging | flow regime | A description of the geometrical distribution of a multiphase fluid moving through a pipe. Many different terms are used to describe this distribution, the distinction between each one being qualitative and somewhat arbitrary. In vertical or moderately deviated pipes, the most common flow regimes for gas-liquid mixtures are bubble flow, dispersed bubble flow, plug flow, slug flow, froth flow, mist flow, churn flow and annular flow. For oil-water mixtures, the most common regimes are bubble flow, slug flow and emulsion flow. In horizontal wells, there may be stratified or wavy stratified flow in addition to many of the regimes observed in vertical wells. Two-phase flow regimes have often been presented as plots, or maps, with the phase velocities or functions of them on each axis. Earlier maps were named after their authors, for example Griffith-Wallis, Duns-Ros and Taitel-Dukler. |
| Production Logging | flow structure | A description of the geometrical distribution of a fluid moving through a pipe. The term is similar to the term flow regime, but is used to describe larger scale features in which there may be more than one flow regime. For example, in a deviated well there may be bubble flow of gas in oil in the uppermost part of the pipe, and water only in the lowest part. |
| Production Logging | flow-concentrating | Referring to a type of spinner flowmeter in which most or all of the fluid flow in the well is diverted over the spinner by a device such as a basket or a packer. |
| Production Logging | flowing neutron log | A neutron porosity log recorded while the well is flowing to determine the gas-oil contact in the borehole. The log is often compared with a log run while the well is shut-in. The term was used in the1950s and 1960s but is now obsolete. |
| Production Logging | flowmeter | A device for measuring in-situ the velocity of fluid flow in a well, usually one completed for production or injection. The most common device is the spinner flowmeter, but torque flowmeters and crosscorrelation flowmeters also are used. In the 1940s and 1950s, various other surface-metering techniques were tried, but spinner flowmeters emerged as the most suitable for measuring downhole velocities. Spinner and torque flowmeters measure the average velocity of the fluids crossing the device, while crosscorrelation flowmeters measure the velocity of a particular phase. Although not normally called flowmeters, various other techniques measure flow velocity, for example water-flow logs, phase-velocity logs, distributed-temperature logs and even audio measurements. |
| Production Logging | fluid density log | A record of the density, or changes in density, of fluids in a production or injection well. Since gas, oil and water all have different densities, the log can determine the percentage, or holdup, of the different fluids, directly in the case of biphasic flow, and in combination with other measurements for triphasic flow. Fluid density is measured by a gradiomanometer or a nuclear fluid densimeter, and can also be derived from the depth derivative of a pressure sensor. |
| Production Logging | fluid interface log | An in-situ measurement of the flow profile made by pumping different fluids down the tubing and casing and observing the interface between them. The fluids are normally both water, but one may be fresh and the other salty, or else one may contain some tracer, so that the interface can be detected by a production-logging tool. After the tubing is run to the bottom of the well, an interface is introduced by one of two methods. In the static method, the total flow rate is held constant and the relative flow rate of the two streams is changed. The location of the interface after each change is used to determine the flow profile. In the dynamic method, one fluid is pumped at different rates. The log was used in the 1950s and 1960s but is now used rarely, having been replaced by fluid-density logs and others. |
| Production Logging | fluid-density log | A record of the density, or changes in density, of fluids in a production or injection well. Since gas, oil and water all have different densities, the log can determine the percentage, or holdup, of the different fluids, directly in the case of biphasic flow, and in combination with other measurements for triphasic flow. Fluid density is measured by a gradiomanometer or a nuclear fluid densimeter, and can also be derived from the depth derivative of a pressure sensor. |
| Production Logging | fluid-interface log | An in-situ measurement of the flow profile made by pumping different fluids down the tubing and casing and observing the interface between them. The fluids are normally both water, but one may be fresh and the other salty, or else one may contain some tracer, so that the interface can be detected by a production-logging tool. After the tubing is run to the bottom of the well, an interface is introduced by one of two methods. In the static method, the total flow rate is held constant and the relative flow rate of the two streams is changed. The location of the interface after each change is used to determine the flow profile. In the dynamic method, one fluid is pumped at different rates. The log was used in the 1950s and 1960s but is now used rarely, having been replaced by fluid-density logs and others. |
| Production Logging | flux leakage | A distortion of the magnetic flux that has been introduced into a casing by a low-frequency electromagnet or permanent magnet. The principle of flux leakage is used to detect casing corrosion, since flux leakage is caused by rapid changes in the thickness of the casing and by pits and holes in either the internal or external wall. Flux leakage distorts the magnetic-flux lines and induces a signal into an electric coil moving past it. In-situ flux-leakage measurements make use of this effect by placing coils on or close to the casing wall, azimuthally distributed to cover the entire wall. The results are often combined with a high-frequency, eddy-current measurement, designed to detect flaws only on the inner wall. |
| Production Logging | friction effect | In a gradiomanometer tool or pressure derivative calculation, the apparent increased fluid density observed due to frictional pressure losses along the tool and casing in a fast-flowing fluid. The magnitude of the correction depends on the flow rate, tool geometry and the casing size, and is negligible in most casings below about 2000 B/D [318 m3/d]. The fluid density will appear erroneously high unless this effect is corrected for. |
| Production Logging | fullbore spinner | A type of flowmeter in which the spinner blades are collapsed to pass through the tubing and other restrictions, and then opened up below to sense the full cross section of the casing or openhole. In this way, a much larger fraction of the flow is measured. Introduced in the 1970s, the fullbore spinner gives a better average flow velocity than a conventional flowmeter, particularly at low flow rates and with simple biphasic-flow regimes. However, when the flow structure is complex, such as with multiphase flow in highly deviated wells, the average flow velocity may not be meaningful. |
| Production Logging | gamma ray densitometer | A device for measuring the density of fluids in a completed well, using a radioactive source of gamma rays and a detector. In most instruments, a 137Cs (cesium) or 241Am (americium) source is used to induce Compton scattering, as in the openhole density measurement, except that the device is unfocused. The count rate at the detector then depends primarily on the density of the fluids in the well. In some devices, the fluids pass through an open space in the body of the tool within which the measurement is made. The results then reflect the density of the fluids passing through the tool. In other devices, the source and detector are isolated so that the gamma rays pass outside the tool. The results then reflect some average density of all the fluids within the well. In smaller casings, some formation signal may contaminate the measurement. Compared with a gradiomanometer, the nuclear fluid densimeter is a less direct measurement of density, and has a statistical uncertainty and less resolution. On the other hand, it is not affected by well deviation, friction or kinetic effects. |
| Production Logging | gas holdup log | A record of the fraction of gas present at different depths in the borehole. Although several techniques may be used for this purpose, the term usually refers to logs based on one of two principles. In the first, four or more optical probes are used to detect the passage of gas bubbles at different points across the borehole. As with other local probes, holdup is determined by the fraction of time the probe detects gas. In the second technique, a 57Co (cobalt) source emits low-energy gamma rays that undergo backscattering and photoelectric absorption in the borehole fluid before being counted in a detector. The number of counts is related to the fluid density, and can be calibrated in terms of gas holdup. The first technique produces an image of gas holdup along and around the borehole, while the second technique produces a log of the average holdup along the well. |
| Production Logging | gas-holdup log | A record of the fraction of gas present at different depths in the borehole. Although several techniques may be used for this purpose, the term usually refers to logs based on one of two principles. In the first, four or more optical probes are used to detect the passage of gas bubbles at different points across the borehole. As with other local probes, holdup is determined by the fraction of time the probe detects gas. In the second technique, a 57Co (cobalt) source emits low-energy gamma rays that undergo backscattering and photoelectric absorption in the borehole fluid before being counted in a detector. The number of counts is related to the fluid density, and can be calibrated in terms of gas holdup. The first technique produces an image of gas holdup along and around the borehole, while the second technique produces a log of the average holdup along the well. |
| Production Logging | gradiomanometer | A device for measuring the average density of the fluid at different depths in a completed production or injection well to produce a fluid-density log. Knowing the density of the individual phases allows their holdups to be determined, directly in the case of biphasic flow, and in combination with other measurements for triphasic flow. Introduced in the late 1950s, the gradiomanometer measures the pressure difference between two pressure sensors, placed approximately 2 ft [0.6 m] apart. The pressure difference reflects the average fluid density across the well within that depth interval. The resolution is high, around 0.005 g/cm3, but the accuracy can be affected by a friction effect, a kinetic effect and well deviation. The effect of deviation can be corrected, but the sensitivity to holdup is reduced as the deviation increases until it is zero in a horizontal well. Note: Gradiomanometer is a mark of Schlumberger. It is now a commonly accepted term for a certain tool that measures differential pressure. |
| Production Logging | gravel pack log | A record of the quality of a gravel pack and the quantity of solid particles it contains. The traditional logging technique uses a type of nuclear fluid densimeter, with a gamma ray source and a single detector. The number of gamma rays reaching the detector is inversely proportional to the gravel-pack density. The count rate is used qualitatively, being scaled in each well between zones with 100% pack and zero pack, or else compared before and after a repair to the pack. The density measurement is not entirely independent of the formation, and is not effective when the density of the particles and the completion fluid are similar. In these cases, a neutron activation or neutron-porosity measurement may be used. Other techniques include a neutron porosity log and tracer measurements. Radioactive tracers may be coated on the outside of the particles or else included within the particles. They can be detected by gamma ray logs, or if a variety of tracers is used they can be tracked with a multiple-isotope log. |
| Production Logging | gravel-pack log | A record of the quality of a gravel pack and the quantity of solid particles it contains. The traditional logging technique uses a type of nuclear fluid densimeter, with a gamma ray source and a single detector. The number of gamma rays reaching the detector is inversely proportional to the gravel-pack density. The count rate is used qualitatively, being scaled in each well between zones with 100% pack and zero pack, or else compared before and after a repair to the pack. The density measurement is not entirely independent of the formation, and is not effective when the density of the particles and the completion fluid are similar. In these cases, a neutron activation or neutron-porosity measurement may be used. Other techniques include a neutron porosity log and tracer measurements. Radioactive tracers may be coated on the outside of the particles or else included within the particles. They can be detected by gamma ray logs, or if a variety of tracers is used they can be tracked with a multiple-isotope log. |
| Production Logging | holdup | With reference to multiphase flow in pipes, the fraction of a particular fluid present in an interval of pipe. In multiphase flow, each fluid moves at a different speed due to different gravitational forces and other factors, with the heavier phase moving slower, or being more held up, than the lighter phase. The holdup of a particular fluid is not the same as the proportion of the total flow rate due to that fluid, also known as its cut. To determine in-situ flow rates, it is necessary to measure the holdup and velocity of each fluid. Holdup is usually given the symbol y, with the suffixes g, o or w for gas, oil or water. The sum of the holdups of the fluids present is unity. The holdup ratio is the ratio of the holdups of two fluids, and is sometimes used as a parameter to express the phenomenon. |
| Production Logging | holdup image | A two-dimensional display, using colors or different grey scales, of the holdup around the borehole versus depth. The x-axis of the image shows different segments of the borehole, normally inside a casing, displayed from the top of the hole clockwise around through the bottom and back to the top again. Depth is in the z-axis, while the values of holdup are represented by different colors or changes from black to white. The holdup image is constructed from between four and eight local probe measurements using interpolation within constraints. Images, sometimes called maps, are also made for bubble count and bubble velocity. |
| Production Logging | holdup log | A record of the fractions of different fluids present at different depths in the borehole. Various techniques are used to measure these fractions. The earliest techniques measured the fluid density, using a gradiomanometer or a nuclear fluid densimeter, or the dielectric properties, as in the capacitance or water-cut meter. While these techniques were satisfactory in near-vertical wells with two-phase flow, they were often found to be inadequate in highly deviated and horizontal wells, where flow structures are complex. More recent developments are based on the use of multiple local probes to detect bubbles of gas, oil or water, and on a combination of nuclear techniques usually known as three-phase holdup. |
| Production Logging | holdup map | A two-dimensional display, using colors or different grey scales, of the holdup around the borehole versus depth. The x-axis of the image shows different segments of the borehole, normally inside a casing, displayed from the top of the hole clockwise around through the bottom and back to the top again. Depth is in the z-axis, while the values of holdup are represented by different colors or changes from black to white. The holdup image is constructed from between four and eight local probe measurements using interpolation within constraints. Images, sometimes called maps, are also made for bubble count and bubble velocity. |
| Production Logging | holdup meter | A device for determining the water holdup in a producing well by measuring the capacitance or impedance of the fluid. The holdup meter is used to produce a capacitance log. Since water has a high dielectric constant, and hence capacitance, it can be distinguished from oil or gas. The meter is a coaxial capacitor, with fluid flowing between a central probe and an external cage that act as electrodes. The meter has often been combined with a packer flowmeter or a diverter flowmeter, so that all the fluids in the well pass through the meter. |
| Production Logging | impeller | A device that responds to fluid flow and is used as the sensor in a flowmeter. In a spinner flowmeter, the term refers to the spinner, or in some cases to one of the blades of the spinner. An impeller is also used in a torque flowmeter. |
| Production Logging | impulse activation | A type of oxygen activation technique for measuring water flow in which a short neutron burst is followed by a long observation period, during which the activated flowing oxygen is recognized at the detector by its signature. Stationary oxygen gives a gradually decaying signal, whereas flowing oxygen can be distinguished by a peak at a time after the neutron pulse that is related to its velocity. To cover a wide range in water velocity, several detectors at different spacings are needed. The flow volume can be estimated from the area under the peak. While recordings are typically made with the tool stationary, continuous logs are also possible. In an alternative type of impulse method, the neutrons are emitted in regular bursts until the count rate from the activated oxygen reaches a constant level. Then, after terminating the bursts, the time for the count rate to decrease by one half is measured. This time can be related to the water velocity. |
| Production Logging | injection pulsed neutron log | An in-situ recording in which a material with high neutron-capture cross section is injected into the flowstream of a production or injection well to determine fluid paths and velocities. The material used is normally borax or water with high salinity, both of which cause a significant increase in the capture cross section measured by a pulsed-neutron log. In the most common application, the material is injected across the producing intervals of a production well. By comparing pulsed neutron logs recorded before and after injection, the injectivity, and hence productivity, of each interval can be estimated. Any cement channels or leaks will also be observed. |
| Production Logging | injection/pulsed neutron log | An in-situ recording in which a material with high neutron-capture cross section is injected into the flowstream of a production or injection well to determine fluid paths and velocities. The material used is normally borax or water with high salinity, both of which cause a significant increase in the capture cross section measured by a pulsed-neutron log. In the most common application, the material is injected across the producing intervals of a production well. By comparing pulsed neutron logs recorded before and after injection, the injectivity, and hence productivity, of each interval can be estimated. Any cement channels or leaks will also be observed. |
| Production Logging | interval method | A technique for improving the accuracy of injection profiles measured using the velocity-shot method. In the normal velocity-shot method, the flow velocity is determined by the time of flight of a radioactive slug between two detectors or between ejector and detector. Since the distance involved may be several feet, the vertical resolution is low. The interval method consists of making overlapping measurements from which a higher resolution injection profile may be determined. |
| Production Logging | Joule Thomson | Referring to the change in temperature observed when a gas expands while flowing through a restriction without any heat entering or leaving the system. The change may be positive or negative. For each gas, there is an inversion point that depends on temperature and pressure, below which it is cooled and above which it is heated. For example, for methane at 100oC [212oF], the inversion point occurs at about 500 atmospheres [7350 psi]. The magnitude of the change of temperature with pressure depends on the Joule-Thomson coefficient for a particular gas. The Joule-Thomson effect often causes a temperature decrease as gas flows through pores of a reservoir to the wellbore. |
| Production Logging | Joule-Thomson | Referring to the change in temperature observed when a gas expands while flowing through a restriction without any heat entering or leaving the system. The change may be positive or negative. For each gas, there is an inversion point that depends on temperature and pressure, below which it is cooled and above which it is heated. For example, for methane at 100oC [212oF], the inversion point occurs at about 500 atmospheres [7350 psi]. The magnitude of the change of temperature with pressure depends on the Joule-Thomson coefficient for a particular gas. The Joule-Thomson effect often causes a temperature decrease as gas flows through pores of a reservoir to the wellbore. |
| Production Logging | kinetic effect | In a gradiomanometer tool, the pressure difference observed when the fluid velocity opposite the upper pressure sensor differs from that across the lower pressure sensor. This difference usually occurs opposite points of fluid entry or exit, and at sudden changes in diameter, such as at the tubing shoe. The result is a sharp deflection on the log that may be misinterpreted as a local change in fluid density. |
| Production Logging | laminar flow | A type of streamlined flow for single-phase fluids in which the fluid moves in parallel layers, or laminae. The layers flow smoothly over each other with instabilities being dampened by the viscosity. Laminar flow occurs in straight pipes when the Reynolds number is below a critical value, corresponding to a low production rate. Above this value, the flow is turbulent. For laminar flow in straight pipes, the velocity profile across the pipe is parabolic, increasing from zero at the wall of the pipe to a maximum at the center equal to twice the mean velocity. |
| Production Logging | local holdup | The fraction of a particular fluid measured in the vicinity of a small probe in a production well. The small, or local, probes respond digitally to the type of fluid in front of them, indicating gas, oil or water depending on the type of probe. The local holdup of oil, for example, is determined by the percentage of time the probe spends in front of oil. |
| Production Logging | local probe | A small sensor, part of a production logging tool, which determines the type of fluid in its vicinity as it moves up and down a production well. Typically there are four or more sensors, or probes, held on arms to measure the four quadrants of the well cross-section. The probes may be electrical, to distinguish hydrocarbon from water; optical, mainly to distinguish gas from liquid, but also oil from water; or dielectric, mainly to distinguish water from hydrocarbon, but also, with less resolution, oil from gas. They can detect bubbles that are larger than about 1 mm diameter. Their response is essentially digital, indicating either one fluid or the other, so that the percentage of time that they see a fluid is a direct measure of its holdup. The rate of change between the two fluids is known as the bubble count. The results can be averaged to give the mean holdup and bubble count, or converted into an image, showing the holdup or bubble count at different locations across the well at different depths. The image is particularly useful in highly deviated or horizontal wells where different flow regimes may be found in different quadrants. |
| Production Logging | magnetic flux leakage | A distortion of the magnetic flux that has been introduced into a casing by a low-frequency electromagnet or permanent magnet. The principle of flux leakage is used to detect casing corrosion, since flux leakage is caused by rapid changes in the thickness of the casing and by pits and holes in either the internal or external wall. Flux leakage distorts the magnetic-flux lines and induces a signal into an electric coil moving past it. In-situ flux-leakage measurements make use of this effect by placing coils on or close to the casing wall, azimuthally distributed to cover the entire wall. The results are often combined with a high-frequency, eddy-current measurement, designed to detect flaws only on the inner wall. |
| Production Logging | magnetic-flux leakage | A distortion of the magnetic flux that has been introduced into a casing by a low-frequency electromagnet or permanent magnet. The principle of flux leakage is used to detect casing corrosion, since flux leakage is caused by rapid changes in the thickness of the casing and by pits and holes in either the internal or external wall. Flux leakage distorts the magnetic-flux lines and induces a signal into an electric coil moving past it. In-situ flux-leakage measurements make use of this effect by placing coils on or close to the casing wall, azimuthally distributed to cover the entire wall. The results are often combined with a high-frequency, eddy-current measurement, designed to detect flaws only on the inner wall. |
| Production Logging | metal gain | The apparent increase in thickness of a casing or tubing string compared to the assumed value. Metal gain is determined by electromagnetic thickness, acoustic resonance or mechanical methods. The apparent increase is usually due a change of hardware, such as a casing coupling, a heavier joint, a pup joint, a mandrel or a valve. The term is used in contrast to metal loss caused by corrosion. |
| Production Logging | metal loss | The loss of material on the inside or outside of a casing or tubing due to corrosion. Monitoring metal loss in situ helps determine when the pipe may be at risk for leaking or failure. Metal loss is determined by comparing casing or tubing thickness measured by electromagnetic, acoustic resonance or mechanical methods with either an earlier measurement or an assumed value. |
| Production Logging | multi-capacitance flowmeter | A device for measuring in situ the velocity of fluid flow in a production or injection well by measuring the transit time of a disturbance between two dielectric sensors a fixed distance apart. The device is a type of crosscorrelation flowmeter that uses several pairs of capacitance, or dielectric, sensors held on an arm to span the borehole. |
| Production Logging | multifinger caliper | A device for measuring the diameter of the internal wall of a casing or tubing using multiple arms. By using a large number of arms, or fingers, the caliper can detect small changes in the wall of the pipe. The main purpose of the measurement is to detect deformations, the buildup of scale or metal loss due to corrosion. Typical multifinger calipers have between about 20 and 80 fingers, the larger numbers being necessary in larger pipes. |
| Production Logging | multipass method | A technique for interpreting the results from a spinner flowmeter using several logging runs of the flowmeter over the zone of interest at different speeds, both up and down. Spinner speed is a nearly linear function of the effective velocity of the fluid. Although this function can be measured on surface, it varies with the fluid and is most reliably determined in situ. After several passes are made, the function can be calibrated and the spinner speed converted into flow rate. The technique is applicable when the flow is single phase, or else multiphase with a sufficiently homogeneous flow regime such as with emulsion or dispersed bubble flow. |
| Production Logging | multiphase | Referring to a fluid with several different immiscible fluids (oil, water or gas). |
| Production Logging | multiphase holdup log | A record of the fractions, or holdups, of gas, oil and water present at different depths in a producing well using a combination of nuclear measurements recorded by a pulsed-neutron spectroscopy device. The technique is used mainly in deviated and horizontal wells, where the complex flow regimes cause conventional holdup measurements to be inaccurate. The pulsed-neutron spectroscopy measurement is processed to obtain the volume of oil in the borehole rather than the oil in the formation as in a conventional carbon-oxygen measurement. The gas holdup is determined from the ratio of counts received by the near and far detectors in either the inelastic or capture mode, a technique similar to the compensated-neutron log. The water holdup is the remaining fraction. Alternatively, the capture cross section of the borehole can be determined from the pulsed-neutron capture measurement. If the water is saline, and its salinity is known, the water holdup can be determined directly. |
| Production Logging | multiple isotope log | A record of the quantity of different radioactive isotopes near the borehole. The technique used is the same as for natural gamma ray spectroscopy, but measures the quantities of various short half-life radioactive tracers in addition to natural gamma rays. The log is run to monitor the results of processes that can be tagged, for example, hydraulic fracturing, gravel-pack placement, squeeze cementing, acid treatment and lost-circulation detection. Different radioactive tracers are added at different stages of the process so that by measuring the different tracers, it is possible to track the development, for example, of the fracture. The most common radioactive tracers are 110Ag (silver), 195Au (gold), 135I (iodine), 192Ir (iridium), 124Sb (antimony), and 46Sc (scandium). |
| Production Logging | multiple isotope spectroscopy | The technique used to produce a multiple-isotope log. |
| Production Logging | multiple-isotope log | A record of the quantity of different radioactive isotopes near the borehole. The technique used is the same as for natural gamma ray spectroscopy, but measures the quantities of various short half-life radioactive tracers in addition to natural gamma rays. The log is run to monitor the results of processes that can be tagged, for example, hydraulic fracturing, gravel-pack placement, squeeze cementing, acid treatment and lost-circulation detection. Different radioactive tracers are added at different stages of the process so that by measuring the different tracers, it is possible to track the development, for example, of the fracture. The most common radioactive tracers are 110Ag (silver), 195Au (gold), 135I (iodine), 192Ir (iridium), 124Sb (antimony), and 46Sc (scandium). |
| Production Logging | multiple-isotope spectroscopy | The technique used to produce a multiple-isotope log. |
| Production Logging | neutron activation log | A record of elemental concentrations derived from the characteristic energy levels of gamma rays emitted by a nucleus that has been activated by neutron bombardment. In the context of production logging, the term normally refers to the activation of silicon and aluminum to determine the quality of a gravel pack. Silicon and aluminum are activated by a neutron source to produce isotopes that decay with a half-life of 2.3 minutes emitting a 1.78 MeV gamma ray. These gamma rays are counted in a detector placed below the source, with a high count indicating a high quantity of silicon in a sand pack, or aluminum in a bauxite pack. The log is run slowly so that oxygen and other activated elements have decayed before the detector crosses the activated interval. The carbon-oxygen log, elemental-capture spectroscopy log, pulsed-neutron spectroscopy log, aluminum-activation log and the oxygen-activation log are also examples of neutron-activation logs. |
| Production Logging | noise log | A record of the sound measured at different positions in the borehole. Since fluid turbulence generates sound, high noise amplitudes indicate locations of greater turbulence such as leaks, channels and perforations. Noise logging is used primarily for channel detection, but has also been used to measure flow rates, identify open perforations, detect sand production and locate gas-liquid interfaces. The log may be either a continuous record against depth or a series of stationary readings. The log may indicate the total signal over all frequencies, the signal at a single frequency, or consist of a set of logs for different frequency ranges. Different frequency ranges can be tied to different sources of noise or different flow regimes. Although first introduced around 1955, the technique was not used commercially until after laboratory studies in the early 1970s. |
| Production Logging | nuclear fluid densimeter | A device for measuring the density of fluids in a completed well, using a radioactive source of gamma rays and a detector. In most instruments, a 137Cs (cesium) or 241Am (americium) source is used to induce Compton scattering, as in the openhole density measurement, except that the device is unfocused. The count rate at the detector then depends primarily on the density of the fluids in the well. In some devices, the fluids pass through an open space in the body of the tool within which the measurement is made. The results then reflect the density of the fluids passing through the tool. In other devices, the source and detector are isolated so that the gamma rays pass outside the tool. The results then reflect some average density of all the fluids within the well. In smaller casings, some formation signal may contaminate the measurement. Compared with a gradiomanometer, the nuclear fluid densimeter is a less direct measurement of density, and has a statistical uncertainty and less resolution. On the other hand, it is not affected by well deviation, friction or kinetic effects. |
| Production Logging | optical index | A measure of the amount of light reflected by a fluid from an optical probe. It is the same as the relative refractive index of light between the probe and the fluid, being close to 1 in gas, 1.35 in water and 1.5 in oil. |
| Production Logging | optical probe | A small sensor in a production logging tool that distinguishes the type of fluid in its vicinity as it is moved up and down a production well. The optical probe is sensitive to the optical index of the fluid in front of the probe. It can distinguish easily between gas and liquid, and less easily between oil and water. Light is sent from a source down a fiber-optic cable to the tip of the probe, and reflected back to a sensor, which converts it into an electrical signal. Gas, with a low optical index, reflects a significant amount of light and gives a large signal, while liquids give low signals. As with other local probes, the output is set to be nearly digital, so that the percentage of the time that the probe sees gas is a direct measure of gas holdup in front of the probe. By using several probes, it is possible to obtain the average holdup and an image of the flow structure across the well. |
| Production Logging | packer flowmeter | A device for measuring in situ the velocity of fluid flow in a production or injection well in which a packer is inflated between the tool housing and the casing wall, causing the total fluid flow to pass inside the tool and over a spinner. The measurement is made with the tool stationary, after borehole fluids have been pumped to inflate the packer. The packer flowmeter was introduced in the mid-1950s. It is a type of diverter flowmeter, but has generally been replaced by petal basket and inflatable diverter flowmeters. |
| Production Logging | petal basket flowmeter | A device for measuring in situ the velocity of fluid flow in a production or injection well in which the flow is diverted through the spinner by a set of metal vanes, or petals. The vanes are closed while running in the hole, and then opened with the tool stationary at the measurement depth. The petals do not seal completely against each other or against the side of the hole, so that not all the fluid is diverted. A type of diverter flowmeter, the petal basket design has generally been replaced since the late 1980s by the inflatable diverter and other designs. |
| Production Logging | phase velocity log | A record of the velocity with which a particular phase (gas, oil or water) moves in a producing well. While most flowmeters measure some average of all the fluids, the phase-velocity log identifies one particular phase. This is particularly important in highly deviated and horizontal wells with multiphase flow, where the flow structure is complicated. Phase-velocity measurements are made with either the crosscorrelation flowmeter, the water-flow log, or with chemical markers designed to mix specifically with one particular phase. Velocity-shot measurements, using radioactive tracers, have also been used. In a typical chemical marker technique, a gadolinium-rich marker is injected into the flow stream, dissolving in either oil or water. Gadolinium has a high capture cross section, or sigma, so that a slug of fluid with high sigma moves with the appropriate phase up the borehole. This slug can be detected by a standard pulsed-neutron capture tool, and the velocity of the phase computed from the time of flight between ejector and detector. |
| Production Logging | phase-velocity log | A record of the velocity with which a particular phase (gas, oil or water) moves in a producing well. While most flowmeters measure some average of all the fluids, the phase-velocity log identifies one particular phase. This is particularly important in highly deviated and horizontal wells with multiphase flow, where the flow structure is complicated. Phase-velocity measurements are made with either the crosscorrelation flowmeter, the water-flow log, or with chemical markers designed to mix specifically with one particular phase. Velocity-shot measurements, using radioactive tracers, have also been used. In a typical chemical marker technique, a gadolinium-rich marker is injected into the flow stream, dissolving in either oil or water. Gadolinium has a high capture cross section, or sigma, so that a slug of fluid with high sigma moves with the appropriate phase up the borehole. This slug can be detected by a standard pulsed-neutron capture tool, and the velocity of the phase computed from the time of flight between ejector and detector. |
| Production Logging | photon log | A record of the density in and around a completed well using a radioactive source of gamma rays and a detector. The log is recorded with a nuclear fluid densimeter. Originally, photon logs were run to determine the size of salt caverns. More recently, they have been run to evaluate the quality of gravel packs and sand cavities, and are then synonymous with gravel-pack logs. |
| Production Logging | plate out | To stay on the surface of the formation or a perforation tunnel. When, for example, bead tracers are injected into a well, they will be carried by the injection fluid. Instead of entering the formation with the fluid, the bead tracers will be held, like plates, on the surface. |
| Production Logging | plug flow | A multiphase flow regime in pipes in which most of the gas moves as large bubbles dispersed within a continuous liquid. The bubbles may span much of the pipe. There are also small bubbles within the liquid, but many of these have coalesced to form the larger bubbles, or plugs. In near-horizontal wells, the plugs are also known as elongated bubbles. Plug flow is similar to slug flow, but the bubbles are generally smaller and move more slowly. |
| Production Logging | production log | A record of one or more in-situ measurements that describe the nature and behavior of fluids in or around the borehole during production or injection. Production logs are run for the purpose of analyzing dynamic well performance and the productivity or injectivity of different zones, diagnosing problem wells, or monitoring the results of a stimulation or completion. The term is sometimes extended to include logs run to measure the physical condition of the well, for example cement bond and corrosion logs. The earliest production logs consisted of temperature logs (1930s) and flowmeters (1940s), to which were soon added fluid-density and capacitance logs (1950s). Flow-rate measurements were gradually improved by the development of tracer logs and improvement to the basic spinner flowmeter. These techniques were adequate for near-vertical wells with single or biphasic flow, but could be misleading in highly deviated, and especially horizontal, wells. New techniques were developed starting in the 1980s. These techniques focused on local probes to measure holdup at different points in the borehole, nuclear techniques to analyze the total holdup of all three phases, and phase-velocity logs for the analysis of individual fluids. At the same time, complex flow structures and flow regimes have been studied more extensively using flow loops. |
| Production Logging | pulsed neutron holdup log | A record of the fractions, or holdups, of gas, oil and water present at different depths in a producing well using a combination of nuclear measurements recorded by a pulsed-neutron spectroscopy device. The technique is used mainly in deviated and horizontal wells, where the complex flow regimes cause conventional holdup measurements to be inaccurate. The pulsed-neutron spectroscopy measurement is processed to obtain the volume of oil in the borehole rather than the oil in the formation as in a conventional carbon-oxygen measurement. The gas holdup is determined from the ratio of counts received by the near and far detectors in either the inelastic or capture mode, a technique similar to the compensated-neutron log. The water holdup is the remaining fraction. Alternatively, the capture cross section of the borehole can be determined from the pulsed-neutron capture measurement. If the water is saline, and its salinity is known, the water holdup can be determined directly. |
| Production Logging | pulsed-neutron holdup log | A record of the fractions, or holdups, of gas, oil and water present at different depths in a producing well using a combination of nuclear measurements recorded by a pulsed-neutron spectroscopy device. The technique is used mainly in deviated and horizontal wells, where the complex flow regimes cause conventional holdup measurements to be inaccurate. The pulsed-neutron spectroscopy measurement is processed to obtain the volume of oil in the borehole rather than the oil in the formation as in a conventional carbon-oxygen measurement. The gas holdup is determined from the ratio of counts received by the near and far detectors in either the inelastic or capture mode, a technique similar to the compensated-neutron log. The water holdup is the remaining fraction. Alternatively, the capture cross section of the borehole can be determined from the pulsed-neutron capture measurement. If the water is saline, and its salinity is known, the water holdup can be determined directly. |
| Production Logging | radial differential temperature log | A record of the difference in temperature between the opposite sides of the internal wall of a casing. The log is mainly used to detect a channel in the cement, since the fluid moving in the channel is likely to be cooler or warmer than its surroundings. The two temperature probes are held on arms that are extended to touch the casing wall at depths where a channel is suspected. The assembly is then rotated through 360o to give the radial differential-temperature log. A sinusoid indicates a channel. Temperature differences are small, typically 0.005 to 0.05oF [0.003 to 0.03oC], but can be enhanced by injecting cooler fluids from surface. |
| Production Logging | radial differential-temperature log | A record of the difference in temperature between the opposite sides of the internal wall of a casing. The log is mainly used to detect a channel in the cement, since the fluid moving in the channel is likely to be cooler or warmer than its surroundings. The two temperature probes are held on arms that are extended to touch the casing wall at depths where a channel is suspected. The assembly is then rotated through 360o to give the radial differential-temperature log. A sinusoid indicates a channel. Temperature differences are small, typically 0.005 to 0.05oF [0.003 to 0.03oC], but can be enhanced by injecting cooler fluids from surface. |
| Production Logging | radioactive tracer log | A record of the presence of tracer material placed in or around the borehole to measure fluid movement in injection wells. There are two traditional techniques for recording radioactive-tracer logs: the tracer-loss measurement, in which a tracer material is added to the completion fluid and its progress monitored with a gamma ray tool; and the velocity-shot measurement, in which the tracer is ejected from one part of a production logging tool and its progress monitored by one or more gamma ray detectors farther down the tool. Radioactive-tracer logs are used to determine injection-flow profiles and detect channels or leaks. They may also be used in production wells, but care must be taken to isolate the fluids on surface until the tracer has decayed to safe levels. The tracer is a radioactive isotope that is soluble in water, oil or gas, or else insoluble, as in the bead tracer. Different radioactive elements with distinct energies and lifetimes may be used. Today, for water injection, the most common is a water-soluble iodine tracer that has a half-life of 8.1 days, while for steam injection a gas with krypton is used. |
| Production Logging | radioactive-tracer log | A record of the presence of tracer material placed in or around the borehole to measure fluid movement in injection wells. There are two traditional techniques for recording radioactive-tracer logs: the tracer-loss measurement, in which a tracer material is added to the completion fluid and its progress monitored with a gamma ray tool; and the velocity-shot measurement, in which the tracer is ejected from one part of a production logging tool and its progress monitored by one or more gamma ray detectors farther down the tool. Radioactive-tracer logs are used to determine injection-flow profiles and detect channels or leaks. They may also be used in production wells, but care must be taken to isolate the fluids on surface until the tracer has decayed to safe levels. The tracer is a radioactive isotope that is soluble in water, oil or gas, or else insoluble, as in the bead tracer. Different radioactive elements with distinct energies and lifetimes may be used. Today, for water injection, the most common is a water-soluble iodine tracer that has a half-life of 8.1 days, while for steam injection a gas with krypton is used. |
| Production Logging | single pass method | A technique for interpreting the results from a spinner flowmeter using only one logging run over the zone of interest. Spinner speed is related to fluid velocity using laboratory-determined values for threshold velocity and spinner response. The single-pass method is generally considered inferior to the in-situ multipass or two-pass method. However, in highly deviated and horizontal wells, where the logging tools must be deployed using coiled tubing or a tractor, the cost of an additional pass is high. Single-pass interpretation, with improved spinner characterization, has therefore become more common in recent years. |
| Production Logging | single phase | The only component of a flow or other phenomenon, normally oil, water or gas. |
| Production Logging | single-pass method | A technique for interpreting the results from a spinner flowmeter using only one logging run over the zone of interest. Spinner speed is related to fluid velocity using laboratory-determined values for threshold velocity and spinner response. The single-pass method is generally considered inferior to the in-situ multipass or two-pass method. However, in highly deviated and horizontal wells, where the logging tools must be deployed using coiled tubing or a tractor, the cost of an additional pass is high. Single-pass interpretation, with improved spinner characterization, has therefore become more common in recent years. |
| Production Logging | single-phase | Referring to a flow or other phenomenon with only one component, normally oil, water or gas. |
| Production Logging | slip | The phenomenon in multiphase flow when one phase flows faster than another phase, in other words slips past it. Because of this phenomenon, there is a difference between the holdups and cuts of the phases. |
| Production Logging | slip velocity | The difference between the average velocities of two different fluids flowing together in a pipe. In vertical ascending flow, the lighter fluid flows faster than the heavier fluid. The slip velocity depends mainly on the difference in density between the two fluids, and their holdups. |
| Production Logging | slug flow | A multiphase-flow regime in pipes in which most of the lighter fluid is contained in large bubbles dispersed within, and pushing along, the heavier fluid. The word slug normally refers to the heavier, slower moving fluid, but sometimes to the bubbles of lighter fluid. There are also small bubbles within the liquid, but many of these have coalesced to form the large bubbles until they span much of the pipe. In gas-liquid mixtures, slug flow is similar to plug flow, but the bubbles are generally larger and move faster. As flow rates increase, slug flow becomes churn flow. |
| Production Logging | spinner flowmeter | A device for measuring in situ the velocity of fluid flow in a production or injection well based on the speed of rotation of an impeller, or spinner. The spinner can be helical, that is, longer than it is wide, or like a vane, which is similar to a fan blade. In both cases, the speed of rotation is measured and related to the effective velocity of the fluid. Friction and fluid viscosity cause the relationship to be slightly nonlinear at low effective velocities and introduce a threshold velocity below which the spinner does not turn. Results are interpreted using the multipass, two-pass or single-pass methods. There are several types of spinner flowmeter. The most common device uses a small vane-like spinner, about 1.5 in. [3.8 cm] in diameter, allowing the logging tool to pass through the tubing and other restrictions before reaching the reservoir interval. The small spinner captures only part of the fluid flow in the casing, too little to make it turn in some low flow-rate wells and possibly unrepresentative in multiphase flow settings. Other devices have been designed to capture more of the flow, for example the fullbore spinner and various types of flow-concentrating or diverter spinners, such as the packer flowmeter and the basket flowmeter. |
| Production Logging | stratified flow | A multiphase-flow regime in horizontal or near-horizontal wells in which the fluids are separated into different layers, with lighter fluids flowing above heavier fluids. Stratified flow is more likely to occur at low flow rates and in flat or downhill sections of horizontal wells. In uphill sections, and as the flow rate increases, the interface between the fluids becomes mixed and irregular, hence the term wavy stratified flow is often used. |
| Production Logging | superficial velocity | The velocity of fluid moving through a pipe, defined as the volumetric flow rate of that fluid divided by the cross-sectional area. In monophasic flow, it is equal to the mean velocity of the fluid. In multiphase flow, it is not a physically real velocity but is a convenient parameter for analysis. |
| Production Logging | Taitel Dukler | Referring to the description of different regimes for the simultaneous flow of gas and liquid in vertical pipes introduced by Y. Taitel and A. Dukler in 1980. The results are shown in the form of a crossplot or map with the superficial gas velocity, vgs, on the x-axis and the superficial liquid velocity, vls, on the y-axis. Different maps are constructed for different pipe sizes and fluid properties. The Taitel-Dukler map defines the transition between different flow regimes more closely than other models. Taitel and Dukler also described flow transitions in horizontal pipes. |
| Production Logging | Taitel-Dukler | Referring to the description of different regimes for the simultaneous flow of gas and liquid in vertical pipes introduced by Y. Taitel and A. Dukler in 1980. The results are shown in the form of a crossplot or map with the superficial gas velocity, vgs, on the x-axis and the superficial liquid velocity, vls, on the y-axis. Different maps are constructed for different pipe sizes and fluid properties. The Taitel-Dukler map defines the transition between different flow regimes more closely than other models. Taitel and Dukler also described flow transitions in horizontal pipes. |
| Production Logging | Taylor bubbles | In multiphase flow, large bubbles of the lighter phase that form by coalescence of small bubbles under certain conditions of fluid flow. The large bubbles occur during slug flow and plug flow. The term is named after G.I. Taylor. Reference: Davies RM and Taylor G: The Mechanics of Large Bubbles Rising Through Liquids and Through Liquids in Tubes, Proceedings of the Royal Society of London, Series A. 200 (February 22, 1950): 375-390. |
| Production Logging | temperature log | A record of the temperature gradient in a well. The temperature log is interpreted by looking for anomalies, or departures, from the reference gradient. This reference might be the geothermal gradient, a log recorded before production started or a log recorded with the well shut-in. Most anomalies are related to the entry of fluids into the borehole or fluid exit into the formation. Since the temperature is affected by material outside the casing, a temperature log is sensitive to not only the borehole but also the formation and the casing-formation annulus. Temperature logs have many applications, with the most common being to identify zones producing or taking fluid, to evaluate a cement or hydraulic fracture treatment, and to locate lost circulation zones and casing leaks. Since temperature takes time to dissipate, a temperature log tends to reflect the behavior of a well over a longer time period than other measurements. |
| Production Logging | threshold velocity | In a spinner flowmeter, the theoretical minimum fluid velocity required to initiate spinner rotation, assuming the spinner response is linear. The actual fluid velocity required to start spinner rotation is slightly higher because of additional viscous and mechanical effects. The threshold velocity is determined by extrapolating the spinner response at higher fluid velocities, where it is known to be nearly linear, back to the value that exists when spinner rotation is zero. |
| Production Logging | timed slug analysis | A technique for determining the velocity of fluid flow in an injection well based on measuring the time a slug of radioactive tracer takes to move down the well. The analysis is usually performed as part of a tracer-loss measurement. The depth of the slug is measured by running repeated gamma ray logs at well-defined time intervals. From the differences in depth and time, the velocity can be determined. |
| Production Logging | timed-slug analysis | A technique for determining the velocity of fluid flow in an injection well based on measuring the time a slug of radioactive tracer takes to move down the well. The analysis is usually performed as part of a tracer-loss measurement. The depth of the slug is measured by running repeated gamma ray logs at well-defined time intervals. From the differences in depth and time, the velocity can be determined. |
| Production Logging | torque flowmeter | A device for measuring in situ the velocity of fluid flow in a production or injection well based on the torque, or force, produced by the fluid on a stationary impeller. This torque can be related to the effective velocity of flow across the impeller. The torque flowmeter is sometimes used as an alternative to the spinner flowmeter. |
| Production Logging | tracer | A chemical or other substance placed in or around the borehole to measure fluid movement in injection wells. The two main types of tracers used during production logging are the bead tracer and the radioactive tracer. |
| Production Logging | tracer ejector measurement | Another term for velocity-shot measurement, a method of producing a radioactive-tracer log, in which a slug of radioactive material is injected into the flow stream of a production or injection well from one section of a logging tool and observed as it passes one or more gamma ray detectors in another section. The slug, or shot, causes a peak in the gamma ray reading as it passes a detector. The flow velocity is determined from the difference in the time of arrival of the slug at the two detectors, or between ejector and detector. This technique has been applied for many years using radioactive tracers such as iodine. Water-soluble tracers are the most common, but oil- and gas-soluble tracers are also used. Velocity-shot measurements are recorded with the tool stationary. They are more accurate than flowmeters at low flow rates, below approximately 100 B/D [16m3/d]. They are not usually run in production wells because of problems of tracer disposal. In multiphase flow, the tracer most often travels with the continuous phase, thereby giving a type of phase-velocity log. |
| Production Logging | tracer loss measurement | A method of determining injection-flow profiles by monitoring the reduction in tracer material as it moves down the well. A slug of radioactive tracer is added to the injection fluid. As the slug moves down the well, several gamma ray logs are recorded at well-defined time intervals. The position of the slug is seen as a large gamma ray peak whose size is proportional to the flow rate. A reduction in the size of the peak indicates a loss of fluid into the formation. Fluid velocity can be calculated from the time interval and the distance the peak has moved using timed-slug analysis. Tracer-loss measurements produce a type ofradioactive-tracer log, used mainly to give a general idea of fluid flow in low flow-rate wells. In very low flow-rate wells, an alternative technique has been used in which the gamma ray detector is held stationary at some depth until the slug has passed. The detector is then moved down to another depth to observe the slug again. With these data, it is possible to make quantitative estimates of fluid flow. |
| Production Logging | tracer measurement | A technique in which a tracer is injected into the flow stream of a production or injection well to determine fluid paths and velocities. Radioactive tracers have been used from the 1940s and are still common for determining flow profiles in injection wells. Tracers with high neutron-capture cross section, such as borax or high-salinity water, were introduced in the 1970s to record injection/pulsed neutron logs. In multiphase production wells, special tracers were introduced in the 1990s to move with only one phase, so as to give a phase-velocity log. Radioactive tracers with different energies are used to track the development of fractures, or other processes, in the multiple-isotope log. Tracer measurements are used qualitatively to determine the movement of fluids behind pipe, or quantitatively to determine fluid-flow velocity within the pipe. |
| Production Logging | tracer-ejector measurement | Another term for velocity-shot measurement, a method of producing a radioactive-tracer log, in which a slug of radioactive material is injected into the flow stream of a production or injection well from one section of a logging tool and observed as it passes one or more gamma ray detectors in another section. The slug, or shot, causes a peak in the gamma ray reading as it passes a detector. The flow velocity is determined from the difference in the time of arrival of the slug at the two detectors, or between ejector and detector. This technique has been applied for many years using radioactive tracers such as iodine. Water-soluble tracers are the most common, but oil- and gas-soluble tracers are also used. Velocity-shot measurements are recorded with the tool stationary. They are more accurate than flowmeters at low flow rates, below approximately 100 B/D [16m3/d]. They are not usually run in production wells because of problems of tracer disposal. In multiphase flow, the tracer most often travels with the continuous phase, thereby giving a type of phase-velocity log. |
| Production Logging | tracer-loss measurement | A method of determining injection-flow profiles by monitoring the reduction in tracer material as it moves down the well. A slug of radioactive tracer is added to the injection fluid. As the slug moves down the well, several gamma ray logs are recorded at well-defined time intervals. The position of the slug is seen as a large gamma ray peak whose size is proportional to the flow rate. A reduction in the size of the peak indicates a loss of fluid into the formation. Fluid velocity can be calculated from the time interval and the distance the peak has moved using timed-slug analysis. Tracer-loss measurements produce a type of radioactive-tracer log, used mainly to give a general idea of fluid flow in low flow-rate wells. In very low flow-rate wells, an alternative technique has been used in which the gamma ray detector is held stationary at some depth until the slug has passed. The detector is then moved down to another depth to observe the slug again. With these data, it is possible to make quantitative estimates of fluid flow. |
| Production Logging | turbulent flow | A type of flow for single-phase fluids in which the velocity at any point may vary in both direction and magnitude with time. Turbulent flow is characterized by random, irregular, locally circular currents, or vortices. It occurs in straight pipes when the Reynolds number is above a critical value, corresponding to a higher production rate. Below this value, the flow is laminar. For turbulent flow in straight pipes, the velocity increases from zero at the wall of the pipe, passes through a thin layer of laminar flow to reach a near constant value over most of the pipe. |
| Production Logging | two pass method | A technique for interpreting the results from a spinner flowmeter using two logging runs over the zone of interest, one up and one down. If the two passes are run at the same cable speed, they will overlay below the perforations, where there is no flow. If they were not run at the same speed, the curves are shifted to overlay. Elsewhere, the separation between the curves gives the relative contribution of each zone. Viscosity changes should have a small effect, since they will have the same influence on both passes. The technique is applicable when the flow is single-phase, or else multiphase with a sufficiently homogeneous-flow regime, such as with emulsion or dispersed bubble flow. |
| Production Logging | two-pass method | A technique for interpreting the results from a spinner flowmeter using two logging runs over the zone of interest, one up and one down. If the two passes are run at the same cable speed, they will overlay below the perforations, where there is no flow. If they were not run at the same speed, the curves are shifted to overlay. Elsewhere, the separation between the curves gives the relative contribution of each zone. Viscosity changes should have a small effect, since they will have the same influence on both passes. The technique is applicable when the flow is single-phase, or else multiphase with a sufficiently homogeneous-flow regime, such as with emulsion or dispersed bubble flow. |
| Production Logging | ultrasonic caliper | A device for measuring the internal diameter of a casing, tubing or open borehole using high-frequency acoustic signals. A transducer (in transmit mode) emits a high-frequency pulse that is reflected by the pipe or borehole wall back to the transducer (in receive mode). The diameter is determined from the time of flight of this echo and the fluid acoustic velocity. The transducer is rotated to produce a cross section of the borehole size and full-coverage images of the borehole wall. The measurement has high resolution and is used to detect deformations, the buildup of scale, or metal loss due to corrosion. The amplitude of the echo from the inner casing surface provides qualitative information on the state of the surface, such as rugosity or corrosion. Casing thickness may also be measured simultaneously, either by analysis of the casing resonance signal, or by detecting separately the echoes from the inner and outer casing surfaces. |
| Production Logging | velocity correction factor | The factor linking the velocity of single-phase liquid flow measured in the center of a pipe with the average velocity across the pipe. For vertical pipes with turbulent flow measured by standard flowmeters, the velocity-correction factor varies within a range of 0.75 to 0.95, but is often taken as 0.83. For laminar flow, it is theoretically 0.5. |
| Production Logging | velocity image | A two-dimensional display, using colors or different gray scales, of the bubble velocity around the borehole against depth. The x-axis of the image shows different segments of the borehole, normally inside a casing, displayed from the top of the hole clockwise around through the bottom and back to the top again. Depth is in the z-axis, while the values of bubble velocity are represented by different colors or changes from black to white. The velocity image is constructed from between four and eight local probe measurements using interpolation within constraints. Images, sometimes called maps, are also made for bubble count and holdup. |
| Production Logging | velocity map | Another term for velocity image, a two-dimensional display, using colors or different gray scales, of the bubble velocity around the borehole against depth. The x-axis of the image shows different segments of the borehole, normally inside a casing, displayed from the top of the hole clockwise around through the bottom and back to the top again. Depth is in the z-axis, while the values of bubble velocity are represented by different colors or changes from black to white. The velocity image is constructed from between four and eight local probe measurements using interpolation within constraints. Images, sometimes called maps, are also made for bubble count and holdup. |
| Production Logging | velocity shot measurement | A method of producing a radioactive-tracer log, in which a slug of radioactive material is injected into the flow stream of a production or injection well from one section of a logging tool and observed as it passes one or more gamma ray detectors in another section. The slug, or shot, causes a peak in the gamma ray reading as it passes a detector. The flow velocity is determined from the difference in the time of arrival of the slug at the two detectors, or between ejector and detector. This technique has been applied for many years using radioactive tracers such as iodine. Water-soluble tracers are the most common, but oil- and gas-soluble tracers are also used. Velocity-shot measurements are recorded with the tool stationary. They are more accurate than flowmeters at low flow rates, below approximately 100 B/D [16m3/d]. They are not usually run in production wells because of problems of tracer disposal. In multiphase flow, the tracer most often travels with the continuous phase, thereby giving a type of phase-velocity log. |
| Production Logging | velocity-correction factor | The factor linking the velocity of single-phase liquid flow measured in the center of a pipe with the average velocity across the pipe. For vertical pipes with turbulent flow measured by standard flowmeters, the velocity-correction factor varies within a range of 0.75 to 0.95, but is often taken as 0.83. For laminar flow, it is theoretically 0.5. |
| Production Logging | velocity-shot measurement | A method of producing a radioactive-tracer log, in which a slug of radioactive material is injected into the flow stream of a production or injection well from one section of a logging tool and observed as it passes one or more gamma ray detectors in another section. The slug, or shot, causes a peak in the gamma ray reading as it passes a detector. The flow velocity is determined from the difference in the time of arrival of the slug at the two detectors, or between ejector and detector. This technique has been applied for many years using radioactive tracers such as iodine. Water-soluble tracers are the most common, but oil- and gas-soluble tracers are also used. Velocity-shot measurements are recorded with the tool stationary. They are more accurate than flowmeters at low flow rates, below approximately 100 B/D [16m3/d]. They are not usually run in production wells because of problems of tracer disposal. In multiphase flow, the tracer most often travels with the continuous phase, thereby giving a type of phase-velocity log. |
| Production Logging | wall loss | The loss of material on the inside or outside of a casing or tubing due to corrosion. Monitoring wall loss in situ helps determine when the pipe may be at risk for leaking or failure. Wall loss is determined by comparing casing or tubing thickness measured by electromagnetic, acoustic resonance or mechanical methods with either an earlier measurement or an assumed value. |
| Production Logging | water cut meter | A device for determining the water holdup in a producing well by measuring the capacitance or impedance of the fluid. The term is a misnomer because water cut is not the same as water holdup except in the unlikely case where all phases flow at the same velocity. Since hydrocarbons travel faster than water in a production well, the water holdup is larger than the water cut. However, the water-cut meter was often combined with a flowmeter so that the water cut could be estimated by combining the two measurements. |
| Production Logging | water flow log | A record of the velocity and direction of water flowing in and around a borehole based on oxygen activation. The log may also include estimates of the flow volume and the distance from tool to flowing water. Water, and occasionally carbon dioxide, is the only source of moving oxygen in and around the borehole. Hence, water flow can be detected by oxygen activation, which, being a nuclear technique, is sensitive to flow inside and outside the casing. The measurement is sensitive to small flows, and can be configured to measure upward or downward flow. It is particularly useful as a leak and channel detector, to identify locations of water entry or exit and as a measurement of water velocity in multiphase flow. Logs may be continuous, but the most accurate measurements are made with the tool stationary. Although first tried in the 1960s, the log was not fully studied and implemented until the late 1970s with a purpose-built experimental tool. Standard pulsed-neutron spectroscopy tools were modified to record the log in the 1980s. |
| Production Logging | water-cut meter | A device for determining the water holdup in a producing well by measuring the capacitance or impedance of the fluid. The term is a misnomer because water cut is not the same as water holdup except in the unlikely case where all phases flow at the same velocity. Since hydrocarbons travel faster than water in a production well, the water holdup is larger than the water cut. However, the water-cut meter was often combined with a flowmeter so that the water cut could be estimated by combining the two measurements. |
| Production Logging | water-flow log | A record of the velocity and direction of water flowing in and around a borehole based on oxygen activation. The log may also include estimates of the flow volume and the distance from tool to flowing water. Water, and occasionally carbon dioxide, is the only source of moving oxygen in and around the borehole. Hence, water flow can be detected by oxygen activation, which, being a nuclear technique, is sensitive to flow inside and outside the casing. The measurement is sensitive to small flows, and can be configured to measure upward or downward flow. It is particularly useful as a leak and channel detector, to identify locations of water entry or exit and as a measurement of water velocity in multiphase flow. Logs may be continuous, but the most accurate measurements are made with the tool stationary. Although first tried in the 1960s, the log was not fully studied and implemented until the late 1970s with a purpose-built experimental tool. Standard pulsed-neutron spectroscopy tools were modified to record the log in the 1980s. |
| Production Logging, Production Facilities | FPSO | Abbreviation for floating production, storage, and offloading vessel. FPSOs are commonly deployed for offshore production in locations that have little infrastructure. They serve as gathering, processing and storage facilities for fluids produced from subsea wells. Processed using equipment on the ship’s deck, the fluids are then stored in the ship’s hull before they are offloaded to pipelines, barges, or ships. Produced natural gas may be sent to shore via pipeline or flared if no pipeline is readily available. |
| Production Logging, Well Completions, Formation Evaluation, Reservoir Characterization | correlate | To compare and fix measured depths with known features on baseline logs of the wellbore tubulars and the surrounding formation. |
| Production Logging, Well Workover and Intervention, Reservoir Characterization, Formation Evaluation | depth correlation | The process of comparing and fixing measured depths with known features on baseline logs of the wellbore tubulars and the surrounding formation. |
| Production Logging | oxygen activation | A phenomenon exploited for the purpose of detecting and quantifying the flow of water in or around a borehole based on oxygen activation. Oxygen (16O) can be activated by high-energy neutrons to produce an isotope of nitrogen (16N), which decays back to oxygen with a half-life of 7.1 seconds and emits an easily detected gamma ray of 6.13 MeV. The neutrons are supplied by the generator of a pulsed-neutron spectroscopy tool. The gamma rays are counted in detectors placed above the tool, for upward flow, or below it for downward flow. Various techniques have been developed to analyze the count rates in terms of water velocity, flow rate and distance from tool. The earliest methods were based of the ratio of the counts from two detectors. Stationary oxygen and other background signals are eliminated by calibration in a known zone of zero flow, by counting only in a window near 6.13 MeV, or by optimizing the detector spacings. More recently, impulse-activation techniques have been introduced. While they require the tool to be stationary, they are generally more accurate. |
| Production Logging | spinner reversal | The change in direction of rotation that occurs when a spinner flowmeter tool is moving in the same direction, but faster, than the fluid. When the tool is stationary, moving against the fluid flow, or moving in the same direction but slower than the flow, it will rotate in one direction, perhaps clockwise. However, if it moves in the same direction, but faster than the fluid, it will rotate counterclockwise. This may happen at the bottom of a producing well, and, unless identified, can lead to a false interpretation. |
| Production Testing | LNG | Abbreviation for liquefied natural gas. |
| Production Testing | productivity test | Tests in an oil or gas well to determine its flow capacity at specific conditions of reservoir and flowing pressures. The absolute open flow potential (AOFP) can be obtained from these tests, and then the inflow performance relationship (IPR) can be generated. |
| Production Testing | liquefied natural gas | Natural gas, mainly methane and ethane, which has been liquefied at cryogenic temperatures. This process occurs at an extremely low temperature and a pressure near the atmospheric pressure. When a gas pipeline is not available to transport gas to a marketplace, such as in a jungle or certain remote regions offshore, the gas may be chilled and converted to liquefied natural gas (a liquid) to transport and sell it. The term is commonly abbreviated as LNG. |
| Production Testing | absolute open flow potential | The maximum flow rate a well could theoretically deliver with zero pressure at the middle of the perforations. The term is commonly abbreviated as AOFP or OFP. |
| Production Testing | annubar | A device that uses Pitot tubes to measure the gas flow rate within a pipeline. The gas volume is calculated from the difference between the flowing pressure and the static pressure of the gas. |
| Production Testing | annular pressure | Fluid pressure in the annulus between tubing and casing or between two strings of casing. |
| Production Testing | annular production | Production of formation fluid through the casing-tubing annulus. |
| Production Testing | AOF | Abbreviation for absolute open flow. |
| Production Testing | AOFP | Abbreviation for absolute open flow potential. |
| Production Testing | asphaltic crude | Petroleum with a high content of naphthenic compounds, such as asphaltenes. Asphaltic crude is also known as naphthene-based crude oil when the paraffin wax content is low. |
| Production Testing | average reservoir pressure | A volumetric average of the pressure exerted by the fluids inside the reservoir at a specific depletion stage. Average reservoir pressure can be measured only when the well is shut in. |
| Production Testing | backpressure | Pressure registered on equipment or devices when fluid flows through. |
| Production Testing | barrels of liquid per day | A volume of fluid that refers to the daily total production of oil and water from a well. The volume of a barrel is equivalent to 42 US gallons, abbreviated BLPD. |
| Production Testing | barrels of oil per day | A common unit of measurement for the daily volume of crude oil produced by a well or from a field. The volume of a barrel is equivalent to 42 US gallons, abbreviated BOPD. |
| Production Testing | barrels of water per day | A common unit of measurement for the volume of water produced each day by a well or in a field. The volume of a barrel is equivalent to 42 US gallons, abbreviated BWPD. |
| Production Testing | Bcf | Abbreviation for billion cubic feet, a unit of measurement for large volumes of natural gas. |
| Production Testing | Bcf/D | Abbreviation for billion cubic feet per day, a unit of measurement for large production rates of natural gas. |
| Production Testing | beam | A fixed choke or a choke with an adjustable needle, sleeve or plate that can be changed to adjust the flow rate. The flow rate from a well is limited to conserve reservoir energy, decrease friction forces and improve production efficiency and prevent development of conditions that can reduce ultimate recovery. A high rate of fluid can generate a drastic cooling effect near the wellbore with the precipitation of scales and paraffins as well as a reduction of the oil relative permeability because of an increase in gas saturation. |
| Production Testing | bean | A fixed choke or a choke with an adjustable needle, sleeve or plate that can be changed to adjust the flow rate. |
| Production Testing | bilinear flow | A unit of measurement for large volumes of natural gas, abbreviated Bcf. |
| Production Testing | billion cubic feet | A common unit of measurement for large production rates of natural gas, abbreviated Bcf/D. |
| Production Testing | blockage | An obstruction in the pipeline, composed of asphaltenes, hydrates, waxes, scale and sand deposited on the internal wall of the pipeline forming a barrier to the normal flow of fluids. The conditions for blockage formation are mainly encountered in deepwater operations (low temperature and high pressure). |
| Production Testing | BLPD | Abbreviation for barrels of liquid per day, usually used in reference to total production of oil and water from a well. The volume of a barrel is equivalent to 42 US gallons. |
| Production Testing | BOPD | Abbreviation for barrels of oil per day, a common unit of measurement for volume of crude oil. The volume of a barrel is equivalent to 42 US gallons. |
| Production Testing | bottle test | A procedure in which different chemicals are added to bottle samples of an emulsion to determine which chemical is the most effective at breaking, or separating, the emulsion into oil and water. Once an effective chemical is determined, varying amounts of it are added to bottle samples of the emulsion to determine the minimum amount required to break the emulsion effectively. |
| Production Testing | bottom sample | A specimen obtained from the bottom part of the tank or lower point in a pipeline. |
| Production Testing | bottomhole gas separator | A perforated tubular attached to the subsurface sucker-rod pump that controls the entrance of gas. Since it is the only way for formation fluid to enter the pump, its use increases the efficiency of the subsurface sucker-rod pump. It also helps to prevent the phenomenon called gas lock. A gas anchor is similar to a bottomhole gas separator. |
| Production Testing | BS&W | Abbreviation for basic sediment and water. BS&W is measured from a liquid sample of the production stream. It includes free water, sediment and emulsion and is measured as a volume percentage of the production stream. |
| Production Testing | bundle | Several pipes (production or injection, gas lift) that are jointly insulated to keep together production lines. The bundle minimizes heat transfer and avoids hydrate or wax deposition that could plug the pipelines. Bundles are common in deepwater field developments. |
| Production Testing | BWPD | Abbreviation for barrels of water per day, a common unit of measurement for the daily volume of produced water. The volume of a barrel is equivalent to 42 US gallons. |
| Production Testing | clathrate | Compounds or complex ions that are formed by the union of water with other substances. Hydrates can form in pipelines and in gas gathering, compression and transmission facilities at reduced temperatures and high pressures. Once hydrates are formed, they can plug the pipelines and significantly affect production operations. |
| Production Testing | composite stream | The flow of different fluids such as oil, gas or water, in a single production stream. |
| Production Testing | condensate liquids | Hydrocarbons that are in the gaseous phase at reservoir conditions but condense into liquid as they travel up the wellbore and reach separator conditions. Condensate liquids are sometimes called distillate. |
| Production Testing | condensate ratio | The ratio of the volume of liquid produced to the volume of gas produced. |
| Production Testing | counterbalance weight | Part of rod pumping unit. The counterbalance weight is installed on the end of the walking beam, opposite to the end over the well, and counterbalances the weight of the sucker rods and the fluid being pumped. |
| Production Testing | critical flow rate | In sand control operations, the maximum production rate below which the production of solids along with the produced fluid is uniform. When the flow rate exceeds this threshold, the production of sand and fines increases significantly. Sand-production control is important to avoid formation damage, possible casing collapse and deterioration of surface equipment due to drag forces. |
| Production Testing | critical gas flow rate | The gas flow rate equivalent to the speed of sound in that fluid. Exceeding this limit during gas production accelerates corrosion in the pipelines. |
| Production Testing | cylinder | The barrel of the sucker rod pump. The plunger travels up and down in the cylinder. The plunger and the barrel operate as a piston mechanism to lift reservoir fluids into the subsurface pump. A cylinder is also known as a pump barrel. |
| Production Testing | degree API | A unit of measurement established by the American Petroleum Institute (API) that indicates the density of a liquid. Fresh water has an API density of 10. |
| Production Testing | deliverability test | Tests in an oil or gas well to determine its flow capacity at specific conditions of reservoir and flowing pressures. The absolute open flow potential (AOFP) can be obtained from these tests, and then the inflow performance relationship (IPR) can be generated. A deliverability test also is called a productivity test. |
| Production Testing | densimeter | An instrument that measures the specific gravity of a mixture of gas, liquid and solids. This device is also known as a densitometer. |
| Production Testing | down stroke | The portion of movement of a downhole pump at which the rods are going down and the downhole pump is being filled with fluid. |
| Production Testing | downhole sensors | Mechanical or electronic devices for measuring various properties in the well such as pressure, fluid flow rate from each branch of a multilateral well, temperature, vibration, composition, fluid flow regime, and fluid holdup. These devices can be permanent; in this case, they are left in place for months or even years. |
| Production Testing | downstroke | The portion of movement of a downhole pump at which the rods are going down and the downhole pump is being filled with fluid. |
| Production Testing | draw down | The difference between the average reservoir pressure and the flowing bottomhole pressure. |
| Production Testing | draw down | The difference in height between the static level and the dynamic level in a pumping well, expressed as hydrostatic fluid pressure. |
| Production Testing | drawdown | The difference between the average reservoir pressure and the flowing bottomhole pressure. |
| Production Testing | drawdown | The difference in height between the static level and the dynamic level in a pumping well, expressed as hydrostatic fluid pressure. |
| Production Testing | dynamic fluid level | The level to which the static fluid level drops in the tubing or casing when the well produced under pumping conditions. The dynamic fluid level is also called the pumping fluid level. |
| Production Testing | dynamometer | An instrument used in sucker-rod pumping to record the variation between the polished rod load and the polished rod displacement. |
| Production Testing | dynamometer card | The record made by the dynamometer. An analysis of this survey may reveal a defective pump, leaky tubing, inadequate balance of the pumping unit, a partially plugged mud anchor, gas locking of the pump or an undersized pumping unit. The dynamometer card is also called a dynagraph. |
| Production Testing | electric gas lift valves | A type of gas-lift valve that allows a gas-lift port size to be adjusted remotely from surface to positions from fully open to closed. These valves offer the possibility of changing gas-injection points without well intervention. |
| Production Testing | electric gas-lift valves | A type of gas-lift valve that allows a gas-lift port size to be adjusted remotely from surface to positions from fully open to closed. These valves offer the possibility of changing gas-injection points without well intervention. |
| Production Testing | flow after flow | A type of deliverability test conducted in gas wells to generate a stabilized gas deliverability curve (IPR). In a flow-after-flow test, a well flows under a constant rate until it reaches stabilized conditions (pseudosteady state). After the stabilized rate and pressure are recorded, the rate is changed and the well flows until pressure stabilizes again. The same procedure is repeated three or four times. The stabilization requirement is an important limitation of this type of test, especially in low-permeability formations, which require longer stabilization times. This test is also known as a backpressure or four-point test. |
| Production Testing | flow line | A surface pipeline carrying oil, gas or water that connects the wellhead to a manifold or to production facilities, such as heater-treaters and separators. |
| Production Testing | flow stream | The flow of oil, gas or water through a pipe. |
| Production Testing | flowline | A surface pipeline carrying oil, gas or water that connects the wellhead to a manifold or to production facilities, such as heater-treaters and separators. |
| Production Testing | flowline manifold | A pipe fitting with several lateral outlets for connecting flowlines from one or more wells. This connection directs flow to heater-treaters, separators or other devices. |
| Production Testing | flowstream | The flow of oil, gas or water through a pipe. |
| Production Testing | flowstream sample | The fluid sample from the wellhead that is used to analyze the composition of the flow. The term is analogous to the term flowline sample, except that it refers to the production part of the flowstream. |
| Production Testing | fluid pound | A phenomenon that occurs when the downhole pump rate exceeds the production rate of the formation. It can also be due to the accumulation of low-pressure gas between the valves. On the downstroke of the pump, the gas is compressed, but the pressure inside the barrel does not open the traveling valve until the traveling valve strikes the liquid. Finally when the traveling valve opens, the weight on the rod string can suddenly drop thousands of pounds in a fraction of a second. This condition should be avoided because it causes extreme stresses, which can result in premature equipment failure. Slowing down the pumping unit, shortening the stroke length or installing a smaller bottom hole pump can correct this problem. |
| Production Testing | fluid tester | A tool run on wireline to obtain fluid samples and measure formation pressures. This device is also called a wireline formation tester. |
| Production Testing | flush production | A high flow rate reached with a new well. |
| Production Testing | gas anchor | A perforated tubular attached to the subsurface sucker-rod pump that controls the entrance of gas. Since it is the only way for formation fluid to enter the pump, its use increases the efficiency of the subsurface sucker-rod pump. It also helps to prevent the phenomenon called gas lock. A gas anchor is similar to a bottomhole gas separator. |
| Production Testing | gas interference | A phenomenon that occurs when gas enters the subsurface sucker-rod pump. After the downstroke begins, the compressed gas reaches the pressure needed to open the traveling valve before the traveling valve reaches liquid. The traveling valve opens slowly, without the drastic load change experienced in fluid pound. It does not cause premature equipment failure, but can indicate poor pump efficiency. A bottomhole separator or a gas anchor can correct gas interference. |
| Production Testing | gas lock | A condition sometimes encountered in a pumping well when dissolved gas, released from solution during the upstroke of the plunger, appears as free gas between the valves. On the downstroke, pressure inside a barrel completely filled with gas may never reach the pressure needed to open the traveling valve. In the upstroke, the pressure inside the barrel never decreases enough for the standing valve to open and allow liquid to enter the pump. Thus no fluid enters or leaves the pump, and the pump is locked. It does not cause equipment failure, but with a nonfunctional pump, the pumping system is useless. A decrease in pumping rate is accompanied by an increase of bottomhole pressure (or fluid level in the annulus). In many cases of gas lock, this increase in bottomhole pressure can exceed the pressure in the barrel and liquid can enter through the standing valve. After a few strokes, enough liquid enters the pump that the gas lock in broken, and the pump functions normally. |
| Production Testing | GLR | Abbreviation for gas/liquid ratio, the ratio of produced gas to produced liquids (oil and water). |
| Production Testing | green gas | Untreated gas that leaves the well. This gas could contain impurities such as water, carbon dioxide [CO2], nitrogen [N2] and hydrogen sulfide [H2S], which will be removed in treating facilities. |
| Production Testing | gross production | The total production of oil, gas or water from a well or field over a specific period of time. |
| Production Testing | gunk | Debris, dirt, paraffin, oil, mill scale and rust removed from a pipeline when a scraper or a pig is used for cleaning purposes. |
| Production Testing | hydrate | Compounds or complex ions that are formed by the union of water with other substances. Hydrates can form in pipelines and in gas gathering, compression and transmission facilities at reduced temperatures and high pressures. Once hydrates are formed, they can plug the pipelines and significantly affect production operations. |
| Production Testing | hydraulic pumping | An artificial-lift system that operates using a downhole pump. A surface hydraulic pump pressurizes crude oil called power oil, which drives the bottom pump. When a single production string is used, the power oil is pumped down the tubing and a mixture of the formation crude oil and power oil are produced through the casing-tubing annulus. If two production strings are used, the power oil is pumped through one of the pipes, and the mixture of formation crude oil and power oil are produced in the other, parallel pipe. |
| Production Testing | injection gas | Natural gas injected into a formation to maintain or restore reservoir pressure. Other reasons for gas injection are gas-lift operations, cycling in gas-condensate reservoirs or storing gas. |
| Production Testing | injection pressure | The pressure needed to inject fluid into the formation to pressurize or displace hydrocarbons. |
| Production Testing | injection water | Water injected into the reservoir to pressurize and displace hydrocarbons to producing wells. Injection water is also used in water-storage operations in offshore and remote locations with economic and environmental constraints. |
| Production Testing | injection well testing | The testing of wells in which fluid is being injected into the reservoir. The most common type of test is a falloff test, in which injection is halted and the pressure decline is measured as a function of time. The most common situation is a waterflood. In many reservoirs, the formation pressure is high enough to maintain a full column of fluid in the wellbore and the pressure can be monitored at the surface. The bottomhole pressure is then calculated by adding the weight of the fluid column to the surface pressure. Gas-injection wells, although less common, lend themselves to similar testing. The rise in fluid pressure as a function of time while injection is taking place could theoretically be used also, but this type of approach is rarely used. The equations and theory for these tests are an exact mirror image of those for buildup and drawdown testing. Calculated results for these wells are usually good because the formations are commonly liquid-filled. Frequently water-injection wells are inadvertently fractured at some time in their life and consequently have a negative skin effect. |
| Production Testing | injectivity test | A procedure conducted to establish the rate and pressure at which fluids can be pumped into the treatment target without fracturing the formation. Most stimulation treatments and remedial repairs, such as squeeze cementing, are performed following an injection test to help determine the key treatment parameters and operating limits. Likewise, injection tests are also conducted when pumping secondary recovery fluids such as water, nitrogen [N2], carbon dioxide [CO2], natural gas and steam. |
| Production Testing | instrumented pig | A device made of rubber or polyurethane that has electronic devices. An instrumented pig is run through a pipeline to record irregularities that could represent corrosion. An instrumented pig is also called a smart pig. |
| Production Testing | IPR | A mathematical tool used in production engineering to assess well performance by plotting the well production rate against the flowing bottomhole pressure (BHP). The data required to create the IPR are obtained by measuring the production rates under various drawdown pressures. The reservoir fluid composition and behavior of the fluid phases under flowing conditions determine the shape of the curve. |
| Production Testing | knock out | Liquid condensed by a scrubber following a compression and cooling process. |
| Production Testing | knockout | Liquid condensed by a scrubber following a compression and cooling process. |
| Production Testing | lead acetate test | A test to detect hydrogen sulfide in a fluid by discoloration of a paper moistened with the lead acetate solution. It is important to determine the presence and amount of hydrogen sulfide because this gas is extremely poisonous, highly flammable, explosive and corrosive. |
| Production Testing | leak detection | The determination of the location of a leak in a pipeline. In onshore operations, this can be done by external detection or by using material balance leak-detection systems. In offshore operations, the task is more difficult because of the lack of inlet flow-rate measurements and the considerable solubility of natural gas in seawater at high pressures and low temperatures (seafloor level). In deepwater operations with multiphase flow, indications of a leak may not be present at the ocean surface or it could be considerably displaced from the site of origination. In these circumstances, an energy-balance technique based on the changes in frictional losses through the pipeline is a powerful tool. |
| Production Testing | lean gas | Natural gas that contains a few or no liquefiable liquid hydrocarbons. Lean gas is also called dry gas. |
| Production Testing | lean gas | Residual gas, mainly methane and ethane, that remains after the heavier hydrocarbons have been condensed in the wellhead. When the lean gas is liquefied, it is called liquefied natural gas (LNG). |
| Production Testing | lean gas condensate | A gas condensate with low condensate formation in the reservoir (when the bottomhole pressure is reduced below the dewpoint pressure). |
| Production Testing | lean oil | Liquid hydrocarbon utilized to remove heavier components from the gas stream in a gas processing plant. |
| Production Testing | light crude oil | Crude oil that has a high API gravity, usually more than 40o. |
| Production Testing | light hydrocarbons | Hydrocarbons with low molecular weight such as methane, ethane, propane and butane. |
| Production Testing | liquefied petroleum gas | Gas mainly composed of propane and butane, which has been liquefied at low temperatures and moderate pressures. The gas is obtainable from refinery gases or after the cracking process of crude oil. Liquefied petroleum gas is also called bottle gas. At atmospheric pressure, it is easily converted into gas and can be used industrially or domestically. The term is commonly abbreviated as LPG. |
| Production Testing | liquid hydrocarbons | Liquid compounds such as propanes, butanes, pentanes and heavier products extracted from the gas flowstream. |
| Production Testing | LPG | See liquefied petroleum gas. |
| Production Testing | makeup gas | Gas injected into a gas-condensate reservoir to maintain the pressure level, thus preventing further condensate dropout. |
| Production Testing | measuring tank | A calibrated tank that automatically measures the liquid volume passing through it. Measuring tanks are also called metering tanks or dump tanks. |
| Production Testing | meter | A device used to measure volumes or rates of fluids (liquid or gas). |
| Production Testing | meter calibration | The operation to adjust the meter to a specific standard. |
| Production Testing | meter capacity | The maximum and the minimum rate of flow specified by the manufacturer to maintain accuracy in the readings. |
| Production Testing | meter difference | The difference in gas volume registered using two different meters. |
| Production Testing | meter factor | A correction number for the meter. It is determined by calibrating the meter using an incompressible fluid (liquid). |
| Production Testing | meter slippage | The volume of liquid that is not registered by the meter at a specific flow rate. |
| Production Testing | methane hydrate | A compound or complex ion that is formed by the union of water with methane. Hydrates can form in pipelines and in gas gathering, compression and transmission facilities at reduced temperatures and high pressures. Once hydrates are formed, they can plug the pipelines and significantly affect production operations. |
| Production Testing | million standard cubic feet | A common measure for gas volume. Standard conditions are normally set at 60oF and 14.7 psia, abbreviated MMscf. |
| Production Testing | MMscf | Abbreviation for million standard cubic feet, a common measure for volume of gas. Standard conditions are normally set at 60oF and 14.7 psia. |
| Production Testing | modified isochronal test | A type of deliverability test conducted in gas wells to generate a stabilized gas deliverability curve (IPR). This test overcomes the limitation of the isochronal test, which requires long shut-in times to reach the average reservoir pressure. In the modified isochronal test, the shut-in periods are of equal duration, as are the flowing periods. The final shut-in pressure before the beginning of the new flow is used as an approximation of the average reservoir pressure. The same procedure is typically repeated four times. A stabilized point (pseudosteady state) is usually obtained at the end of the test. Modified isochronal tests are commonly used in gas wells, because they require less time and money to produce results comparable to the isochronal test. |
| Production Testing | mosquito bill | A tubular placed at the bottom of the subsurface sucker-rod pump and inside the gas anchor to drive the formation fluid with little or no gas into the pump. |
| Production Testing | Mscf/d | Abbreviation for a thousand standard cubic feet per day, a common measure for volume of gas. Standard conditions are normally set at 60oF and 14.7 psia. |
| Production Testing | mud anchor | Large diameter pipe placed outside the gas anchor to reduce the amount of solids carried by the formation liquid entering the subsurface sucker-rod pump. |
| Production Testing | multiphase meter | A device that can register individual fluid flow rates of oil and gas when more than one fluid is flowing through a pipeline. A multiphase meter provides accurate readings even when different flow regimes are present in the multiphase flow. When using single-phase meters, the fluid mixture (oil and gas) coming from the wellbore must pass through a fluid-separation stage (separator) prior metering. Otherwise, the readings of the single-phase meters will be inaccurate. Separators are not necessary for multiphase metering, and the meters can support different proportions of gas and oil. Multiphase meters provide the advantage of continuous well monitoring, which is not possible using single-phase meters. Additionally, multiphase meters cost less, weigh less and require less space. Multiphase meters are more common in deepwater operations, where well-intervention operations are often prohibitively expensive. |
| Production Testing | naphthene base crude oil | Crude oil containing asphaltic materials but very little or no paraffin wax. This type of oil is suitable for making gasoline, lubricating oil and asphalt. It is also called asphalt-base crude. |
| Production Testing | naphthene-base crude oil | Crude oil containing asphaltic materials but very little or no paraffin wax. This type of oil is suitable for making gasoline, lubricating oil and asphalt. It is also called asphalt-base crude. |
| Production Testing | natural gas liquids | Components of natural gas that are liquid at surface in field facilities or in gas-processing plants. Natural gas liquids can be classified according to their vapor pressures as low (condensate), intermediate (natural gasoline) and high (liquefied petroleum gas) vapor pressure. Natural gas liquids include propane, butane, pentane, hexane and heptane, but not methane and ethane, since these hydrocarbons need refrigeration to be liquefied. The term is commonly abbreviated as NGL. |
| Production Testing | natural gasoline | A natural gas liquid with a vapor pressure intermediate between condensate and liquefied petroleum gas. This liquid hydrocarbon mixture is recovered at normal pressure and temperature and is much more volatile and unstable than commercial gasoline. |
| Production Testing | net gas production | The volume of gas produced less gas injected. |
| Production Testing | net oil production | The volume of oil produced less oil injected. In hydraulic pumping, the oil injected is known as power oil. |
| Production Testing | NGL | Abbreviation for natural gas liquids. |
| Production Testing | nonhydrocarbon contaminants | Contaminants such as hydrogen sulfide [H2S], carbon dioxide [CO2], nitrogen [N2], and water, which are commonly associated with oil and gas production. |
| Production Testing | orifice meter | An instrument that records the flow rate of gas through a pipeline. The flow rate is calculated from the pressure differential created by the fluid passing through an orifice of a particular size and other parameters such as static pressure, temperature, density of the fluid and size of the pipe. |
| Production Testing | over travel | A condition in downhole pumping operations that occurs when the stroke length at the subsurface sucker-rod pump is longer than the surface stroke length (polished rod). This phenomenon is caused by the elongation of the rod string because of dynamic loads imposed by the pumping cycle. In this situation, when the upstroke begins at the surface, the downhole pump maybe still be moving downward; when the downstroke begins, the downhole pump is still moving upward. |
| Production Testing | overtravel | A condition in downhole pumping operations that occurs when the stroke length at the subsurface sucker-rod pump is longer than the surface stroke length (polished rod). This phenomenon is caused by the elongation of the rod string because of dynamic loads imposed by the pumping cycle. In this situation, when the upstroke begins at the surface, the downhole pump maybe still be moving downward; when the downstroke begins, the downhole pump is still moving upward. |
| Production Testing | paraffin base crude oil | A crude oil containing paraffin wax but very few asphaltic materials. This type of oil is suitable for motor lubricating oil and kerosene |
| Production Testing | paraffin-base crude oil | A crude oil containing paraffin wax but very few asphaltic materials. This type of oil is suitable for motor lubricating oil and kerosene. |
| Production Testing | permanent well monitoring | A situation in which the well and the reservoir are continuously monitored. On the basis of this information, the well completion may be adjusted remotely to adapt to changes in downhole conditions. A permanent well monitoring system is composed of the following: · Inflow control valves that enable choking or shutting off different zones according performance such as drawdown, GOR or water cut · Downhole sensors that register pressure, fluid flow rate and temperature · Control lines for power transmission and transferring of monitored downhole data captured by downhole sensors. · A surface control unit to handle the monitored data and for remote operation of the downhole inflow control valves. Wells with permanent monitoring systems are commonly called intelligent or smart wells. Permanent well monitoring is commonly used in multilateral wells, where hydraulically independent valves control the flow of each lateral and in deepwater wells, where well-intervention operations are often prohibitively expensive. Permanent well monitoring helps improve reservoir management by quickly choking or shutting off zones, avoiding expensive well intervention. It also helps maximize production and optimize recovery. |
| Production Testing | PI | Abbreviation for productivity index. |
| Production Testing | pig | To run a scraper, or pig, through a pipeline for cleaning purposes. |
| Production Testing | pig | A spheroid implement used to displace liquid hydrocarbons from natural gas pipelines. They are also called signaling and batching pigs. |
| Production Testing | pig | A device with blades or brushes inserted in a pipeline for cleaning purposes. The pressure of the oil stream behind pushes the pig along the pipeline to clean out rust, wax, scale and debris. These devices are also called scrapers. |
| Production Testing | pig run | The trip of a pig through a pipeline for cleaning purposes. |
| Production Testing | pigging | The act of forcing a device called a pig through a pipeline for the purposes of displacing or separating fluids, and cleaning or inspecting the line. |
| Production Testing | Pitot tube | A measuring device for determining the gas-flow rate. It is composed of a 1/8-inch tube inserted horizontally along the axis of the gas flowline. The pressure at the end of the tube is compared with the static pressure to determine the final gas flow rate within the flow line. |
| Production Testing | PLT | Abbreviation for Production Logging Tool. |
| Production Testing | plunger overtravel | A condition in downhole pumping operations that occurs when the stroke length at the subsurface sucker-rod pump is longer than the surface stroke length (polished rod). This phenomenon is caused by the elongation of the rod string because of dynamic loads imposed by the pumping cycle. In this situation, when the upstroke begins at the surface, the downhole pump maybe still be moving downward; when the downstroke begins, the downhole pump is still moving upward. |
| Production Testing | pony rod | A rod shorter than usual, usually placed below the polished rod and used to make a rod string of a desired length. |
| Production Testing | power fluid injection rate | Volume of fluid injected in a well during hydraulic pumping. |
| Production Testing | power oil | In hydraulic pumping, the crude oil that is pressurized at surface to energize the bottom pump. |
| Production Testing | power-fluid injection rate | Volume of fluid injected in a well during hydraulic pumping. |
| Production Testing | pressure capability | The maximum pressure an electrical submersible pump can withstand. This pressure is directly related to the differential pressure between the discharge and the suction pressures, and it is always limited by the maximum capacity of the equipment. |
| Production Testing | profile testing | A procedure that involves sampling gas and liquid at different points across the diameter of pipe to evaluate the degree of stratification at a specific location. |
| Production Testing | progressive cavity pumping system | A type of a sucker rod-pumping unit that uses a rotor and a stator. The rotation of the rods by means of an electric motor at surface causes the fluid contained in a cavity to flow upward. It is also called a rotary positive-displacement unit. |
| Production Testing | pump barrel | The cylinder of the downhole pump. |
| Production Testing | pump off | A phenomenon produced when pump submergence into the fluid column is low. A pump-off situation will increase the gas intake, thus reducing the pump efficiency. |
| Production Testing | pump submergence | The difference in hydrostatic head between the pump depth and the dynamic fluid level above the pump. The pump submergence is continuously monitored to adjust the pump flow rate and avoid a pump-off condition. |
| Production Testing | pump volumetric efficiency | The relationship between actual pump displacement and the pump displacement under ideal conditions. The relationship can be expressed as percentage. A reduction in pump volumetric efficiency is an indication of an operational problem in the well. In sucker-rod pumps, the gas lock and gas interference phenomena can significantly reduce the volumetric efficiency of the pump. |
| Production Testing | pump-off | A phenomenon produced when pump submergence into the fluid column is low. A pump-off situation will increase the gas intake, thus reducing the pump efficiency. |
| Production Testing | radioactive tracer | A component of a production-logging tool that carries a radioactive solution (often carnotite) that can be selectively released into a flow stream. When the radioactive solution is released into an injected fluid, the movement of the mixture can be traced by gamma ray detectors located in the tool. Radioactive tracers are generally used in injection wells rather than in production wells to avoid radioactive contamination at the surface. The main applications of radioactive tracers include establishing flow profiles in injection wells, detecting fluid movements behind the pipe, and locating leaking packers and fluid movement between wells. |
| Production Testing | range of load | The difference between the maximum load reached in the upstroke and the minimum load registered in the downstroke. |
| Production Testing | raw crude oil | Crude oil direct from the wellbore, before it is treated in a gas separation plant. It usually contains nonhydrocarbon contaminants. |
| Production Testing | raw natural gas | Gas coming directly from the wellbore containing nonhydrocarbon contaminants and hydrocarbons that can be liquefied. |
| Production Testing | recoverable gas lift gas | Injection gas that has returned to surface and it is not reinjected into the gas-lift system. Instead, it is transferred to a pipeline. This gas is sometimes called spent gas-lift gas. |
| Production Testing | remotely operated vehicle | An unmanned submersible vehicle controlled from surface. In deepwater operations, remotely operated vehicles are used to inspect subsea structures and equipment, and to control or manipulate valves. They can operate at depths from 1500 to 10,000 ft [457 to 3048 m]. This term is commonly abbreviated as ROV. |
| Production Testing | retrograde condensation | The formation of liquid hydrocarbons in a gas reservoir as the pressure in the reservoir decreases below dewpoint pressure during production. It is called retrograde because some of the gas condenses into a liquid under isothermal conditions instead of expanding or vaporizing when pressure is decreased. |
| Production Testing | rich gas | Natural gas containing heavier hydrocarbons than a lean gas. Its liquid content adds important economic value to developments containing this type of fluid. |
| Production Testing | rich gas condensate | Gas condensate comprising significant amounts of heavy hydrocarbon products, which can produce relatively large volumes of condensate. |
| Production Testing | rich oil | Lean oil that has absorbed heavier hydrocarbon components from a gas stream |
| Production Testing | rotational gas lift | A gas-lift system that recycles the injected gas using compressors. This closed system does not require an external source of gas for operating the gas-lift system. |
| Production Testing | ROV | Abbreviation for remotely operated vehicle. |
| Production Testing | sand bailer | A swabbing device used to clean up sand that has accumulated in the wellbore. Because sands abrasiveness is detrimental to the normal operation of production equipment, its production should be minimized. A sand bailer operates by creating a partial vacuum that sucks up the sand. |
| Production Testing | satellite platform | A platform on which surface multiphase pumps can be mounted and connected to subsea multiphase pumps. |
| Production Testing | scrub | To remove impurities, water, liquid hydrocarbons or traces of other gases by passing the gas flowstream through a scrubber, a device in which the gas is mixed with a suitable liquid that absorbs or washes out the constituent to be removed. |
| Production Testing | scrubber | A device to remove dirt, water, foreign matter, or undesired liquids that are part of the gas flowstream. Air can be used to absorb water; also an oil bath might be useful to remove dust, dirt or other liquids. A scrubber is used to protect downstream rotating equipment or to recover valuable liquids from gas. |
| Production Testing | scrubber oil | Oil recovered from a knockout or scrubber device. |
| Production Testing | separator backpressure | The pressure required to force fluids to enter a separator. |
| Production Testing | separator gas | The gas that remains after its separation from condensate. |
| Production Testing | single tank composite sample | The mixture of liquid samples taken from the upper, middle and lower sections of a storage tank. Normally the storage tanks (upright cylindrical or horizontal cylindrical tanks) in the oil field contain crude oil, water and emulsions. |
| Production Testing | single-tank composite sample | The mixture of liquid samples taken from the upper, middle and lower sections of a storage tank. Normally the storage tanks (upright cylindrical or horizontal cylindrical tanks) in the oil field contain crude oil, water and emulsions. |
| Production Testing | slow release inhibitor | A substance added at slow rate to the production fluid stream to prevent corrosion. |
| Production Testing | slow-release inhibitor | A substance added at slow rate to the production fluid stream to prevent corrosion. |
| Production Testing | slugging | Accumulation of a water, oil or condensate in a gas pipeline. These fluids need to be removed using a pig. |
| Production Testing | slugging compound | A chemical used to break emulsions to determine the total amount of sediment and water in the samples. |
| Production Testing | sonolog | An acoustic device that measures the time required for an explosive sound to echo from the annular liquid level in nonflowing wells. The time is proportional to the distance from the surface to the liquid. It is used to determine backpressure in the formation or a static fluid level in the annulus. It is also known as an echo meter. |
| Production Testing | sour crude oil | A crude oil containing hydrogen sulfide, carbon dioxide or mercaptans. |
| Production Testing | sour gas | A gas containing hydrogen sulfide, carbon dioxide or mercaptans, all of which are extremely harmful. |
| Production Testing | stabilized | A term describing a flowing well when its rate of production through a given choke size remains constant, or in the case of a pumping well, when the fluid column within the well remains constant in height. |
| Production Testing | standing valve | In a subsurface sucker-rod pump, a valve that permits flow up the tubing to fill the pump-barrel chamber while preventing downward flow. |
| Production Testing | static fluid level | The level to which fluid rises in a well when the well is shut in. The hydrostatic head of this fluid is equal to the well bottomhole pressure. |
| Production Testing | strainer | A device used to catch and hold the debris flowing in pipelines. Such foreign materials can cause severe damage to meters or other surface equipment. |
| Production Testing | stroke | In a sucker-rod pump, one complete round of the polished rod (surface stroke). On each stroke, a fixed volume of liquid is lifted. This volume is related to the cross-sectional area of the pump and the length of the stroke. The stroke length at the subsurface sucker-rod pump usually differs from the surface stroke because of the stretching in the upstroke and the rebounding in the downstroke. |
| Production Testing | strokes per minute | The number of strokes the polished rod completes in one minute. This determines the rate at which liquid is pumped. If the number of strokes per minute is increased, the pump rate is also increased. This term is also referred to as stroke speed. |
| Production Testing | sulfur light crude | Light crude oil containing sulfur compounds such as hydrogen sulfide |
| Production Testing | sweet crude oil | Oil containing small amounts of hydrogen sulfide and carbon dioxide. |
| Production Testing | sweet gas | Natural gas that contains small amounts of hydrogen sulfide and carbon dioxide. |
| Production Testing | temperature survey | A temperature data set taken at various depths in the wellbore. Temperature surveys are used to determine the top of cement behind the casing, fluid contacts and water influx. It is also useful to check for valve and casing leaks after the well has been temporarily shut down. |
| Production Testing | thousand standard cubic feet per day | A common measure for volume of gas. Standard conditions are normally set at 60oF and 14.7 psia, abbreviated Mscf/d. |
| Production Testing | traveling valve | In a subsurface sucker-rod pump, the valve that closes the barrel chamber allowing the trapped fluid to be lifted in the upstroke of the pump. This valve is similar in configuration to the standing valve. |
| Production Testing | tubing pressure | Pressure on the tubing in a well, as measured at the wellhead. |
| Production Testing | under travel | In sucker-rod pumping, a situation that occurs when the stroke length at the downhole pump is shorter than the surface stroke length. |
| Production Testing | underground gas storage | Gas that is being stored in salt domes, salt layers or depleted oil and gas fields. |
| Production Testing | undertravel | In sucker-rod pumping, a situation that occurs when the stroke length at the downhole pump is shorter than the surface stroke length. |
| Production Testing | upstroke | The stage of downhole pumping at which the polished rod is going up and the downhole pump is pumping fluid. |
| Production Testing | water cut | The ratio of water produced compared to the volume of total liquids produced. The water cut in waterdrive reservoirs can reach very high values. |
| Production Testing | water oil ratio | The ratio of produced water to produced oil, abbreviated WOR. |
| Production Testing | water production | The volume of produced water associated with oil production. In waterdrive reservoirs, water production can be significantly higher than oil production from a field. Consequently, treatment and disposal of produced water, especially in remote locations, have an important impact on the feasibility of a project. |
| Production Testing | water/oil ratio | The ratio of produced water to produced oil, abbreviated WOR. |
| Production Testing | wellhead backpressure | The pressure registered in the wellhead of a producing well. |
| Production Testing | wet gas | Natural gas containing significant heavy hydrocarbons. Propane, butane and other liquid hydrocarbons can be liquefied. |
| Production Testing | wet oil | Oil that contains basic sediment and water (BS&W). |
| Production Testing | WOR | Abbreviation for water/oil ratio, the ratio of produced water to produced oil. |
| Production Testing, Enhanced Oil Recovery | primary recovery | The first stage of hydrocarbon production, in which natural reservoir energy, such as gasdrive, waterdrive or gravity drainage, displaces hydrocarbons from the reservoir, into the wellbore and up to surface. Initially, the reservoir pressure is considerably higher than the bottomhole pressure inside the wellbore. This high natural differential pressure drives hydrocarbons toward the well and up to surface. However, as the reservoir pressure declines because of production, so does the differential pressure. To reduce the bottomhole pressure or increase the differential pressure to increase hydrocarbon production, it is necessary to implement an artificial lift system, such as a rod pump, an electrical submersible pump or a gas-lift installation. Production using artificial lift is considered primary recovery. The primary recovery stage reaches its limit either when the reservoir pressure is so low that the production rates are not economical, or when the proportions of gas or water in the production stream are too high. During primary recovery, only a small percentage of the initial hydrocarbons in place are produced, typically around 10% for oil reservoirs. Primary recovery is also called primary production. |
| Production Testing, Enhanced Oil Recovery | reservoir drive mechanisms | Natural forces in the reservoir that displace hydrocarbons out of the reservoir into the wellbore and up to surface. Reservoir-drive mechanisms include gasdrive (gas cap or solution gasdrive), waterdrive (bottomwater drive or edgewater drive), combination drive, and gravity drainage. Waterdrive is the most efficient drive mechanism, followed by gasdrive and gravity drainage. Reservoir-drive mechanisms are also called natural drives. |
| Production Testing, Production | gas/liquid ratio (GLR) | The ratio of produced gas volume to total produced liquids (oil and water) volume, often abbreviated GLR. |
| Production Testing | spin flowmeter | A production logging method that uses a small propeller turned by fluid movement. The number of turns of the propeller can be related to the amount of fluid passing through the instrument. These devices are used to determine which of several zones contributes the most to the total production or, in the case of an injection well, which zone is receiving the most injected fluids. |
| Production, Well Workover and Intervention | accumulator | A device used in a hydraulic system to store energy or, in some applications, dampen pressure fluctuations. Energy is stored by compressing a precharged gas bladder with hydraulic fluid from the operating or charging system. Depending on the fluid volume and precharge pressure of the accumulator, a limited amount of hydraulic energy is then available independent of any other power source. Well pressure-control systems typically incorporate sufficient accumulator capacity to enable the blowout preventer to be operated with all other power shut down. |
| Reservoir Characterization | additivity | A property of semivariogram models. Any linear combination of admissible models with positive coefficients can be nested or added together. Generally, single models are used for modeling experimental semivariograms that are close in shape to one of the basic admissible models, or for the approximate fitting of complex structural functions. Nested models are used to better fit complex structural functions. Reference: Olea RA: “Fundamentals of Semivariogram Estimation, Modeling, and Usage,” in Yarus JM and Chambers RL (eds): Stochastic Modeling and Geostatistics, AAPG Computer Applications in Geology, no. 3. AAPG, Tulsa, Oklahoma, USA, 1994. |
| Reservoir Characterization | advective transport modeling | A series of techniques that use geostatistical methods to determine fluid and contaminant flow in the subsurface. These techniques are used primarily to study contamination in groundwater in environmental studies. Reference: McKenna SA and Poeter EP: “Simulating Geological Uncertainty with Imprecise Data for Groundwater Flow and Advective Transport Modeling,” in Yarus JM and Chambers RL (eds): Stochastic Modeling and Geostatistics, AAPG Computer Applications in Geology, no. 3. AAPG, Tulsa, Oklahoma, USA, 1994. |
| Reservoir Characterization | arithmetic mean | A mathematical method of finding a central value for a group of data. It is most often referred to as the average but also as the mean. The arithmetic mean is the sum of all the observed values divided by the number of observations. |
| Reservoir Characterization | artificial intelligence | The study of ideas that enable computers to do the things that make people seem intelligent. The term is commonly abbreviated as A.I. Many computer programs written for use in the oil field utilize “rule based” approaches to provide expert systems. The rules are taken from an expert working in the field and are written in a way that attempts to reproduce the knowledge and approaches used by that expert to solve a range of real problems. Most such programs are limited to specific areas such as dipmeter interpretation, electrofacies determination, reservoir characterization, blowout prevention, drilling fluid selection, etc. Sometimes expert systems are written in computer languages that easily handle “rules” such as LISP, but once fully tested are usually translated to BASIC, C or FORTRAN to be compiled into efficient applications or programs.. |
| Reservoir Characterization | azimuth | The angle that characterizes a direction or vector relative to a reference direction (usually True North) on a horizontal plane. The azimuth is usually quoted in degrees from 0 to 359. |
| Reservoir Characterization | Bayesian inference | An inference or deduction made using Bayes’ theorem. Bayesian inference also is used in performing stochastic analyses of geological scenarios or building stochastic models of geological environments. |
| Reservoir Characterization | Bayesian method | A method of updating distributions that requires that prior distributions of the required geological characteristics are defined and that calculation of the posterior distributions be based on an exact stochastic model. |
| Reservoir Characterization | Bayesian probability | A probability based on Bayes’ theorem of interdependent events occurring interdependently. |
| Reservoir Characterization | bivariate analysis | Analysis of two data sets that determines whether or not the data are related and describes the best relationship between them. Crossplots are often used to visualize potential bivariate relationships. Regression methods frequently help determine the best equation to fit to the data and the goodness of the fit. |
| Reservoir Characterization | Boltzmann probability distribution | One of a number of possible distributions that may occur when the results of events are plotted. Boltzmann distributions were originally described from theoretical consideration on the probable interactions of molecules. It has been used in simulation of annealing and can be used for studying perturbations in geostatistical models. |
| Reservoir Characterization | Boolean simulation | The development of a reservoir model by the use of objects. Reservoir models may be developed by adding together a series of objects (such as channel belts) in a fashion that honors the well data (logs, cores, etc.) and satisfies all the geostatistical requirements of the model. Such models may be used to simulate the behavior of the fluids in a reservoir. |
| Reservoir Characterization | box plots | A grid pattern laid over a representation of fractures. The number of boxes that contain a fracture is counted and plotted against the box size on logarithmic scales. The slope of the line is equal to minus the fractal dimension. This is sometimes referred to as the box “dimension.” |
| Reservoir Characterization | Briggs color cube | A system for color-coding three-dimensional information. This system is used in wireline log analysis to provide color shading in which the final color is determined by the values of three curves. One curve dictates the intensity of red, a second the intensity of green, and the third the intensity of blue. The final resulting color is the result of the three input curves. The input curves may be raw curves from the field or computed curves. When used for correlation work on cross sections, the curves must have been normalized to remove the effects of incorrect calibrations and borehole problems. Reference: Doveton JH: Geologic Log Analysis Using Computer Methods, AAPG Computer Applications in Geology, No. 2. AAPG, Tulsa, Oklahoma, USA (1994): 39-41. |
| Reservoir Characterization | Brownian motion | The motion of atoms and molecules in fluids due to the temperature of the fluid. The motion appears to be random, but is described by the relationships derived by Brown. |
| Reservoir Characterization | classical reservoir modeling | A conventional method of mapping reservoir parameters in two dimensions, x and y. The resulting map set usually includes the top and bottom structure map derived from seismic and well data and that are used to generate thickness maps, in addition to maps of other geological and petrophysical parameters produced by standard interpolation techniques. These techniques are appropriate for describing reservoirs that are reasonably continuous and not too heterogeneous. They are usually much faster than full 3D techniques or geostatistical methods, but may be inaccurate when applied to description of complex, heterogeneous strata. |
| Reservoir Characterization | cluster | A group of data points having similar characteristics. These points are usually found by cluster analysis, and are sometimes used to determine electrofacies from wireline data. |
| Reservoir Characterization | cluster | The act of determining clusters from data sets. |
| Reservoir Characterization | cluster analysis | Mathematical techniques for summarizing large amounts of multidimensional data into groups. The two most popular techniques are: hierarchical k-means. The hierarchical system calculates as many clusters as there are data points and displays their relative closeness by means of a dendogram. This system is preferred when there are few data points but the user wishes to see the dendogram to chose an appropriate number of clusters for analysis. Principal Component Analysis (PCA) is a form of hierarchical cluster analysis. The k-means system requires the user to choose the number of cluster to be determined. The computation scatters the centers of the clusters among the data and then moves them until they are gravitationally bound to the larger groups of data and no longer move. The points determined in this way represent the central points of the clusters. This technique is very fast and appropriate for very large data sets. It is most commonly used in electrofacies calculations. Cluster analysis is often used to provide electrofacies from wireline data where each curve is set to be a dimension. |
| Reservoir Characterization | coherence map | A map that displays the degree of correlation between wells as a vector that points from one well to another, the length of the vector being related to the degree of correlation from a correlogram. These maps are used in automatic correlation of well logs across a field and indicate where formations are continuous or are terminated. |
| Reservoir Characterization | coherence vector map | A map that displays the degree of correlation between wells as a vector that points from one well to another, the length of the vector being related to the degree of correlation from a correlogram. These maps are used in automatic correlation of well logs across a field and indicate where formations are continuous or are terminated. Reference: Poelchau HS: Coherence Mapping – An Automated Approach to Display Goodness-of-Correlation Between Wells in a Field, Mathematical Geology 19, no. 8 (1987): 833-850. |
| Reservoir Characterization | cokriging | A form of kriging that involves multiple variables. For example, well data may be used to generate one semivariogram, and three-dimensional seismic data used to generate another. Both semivariograms, along with a cross-variogram model, can then be used to generate a cokriged map. |
| Reservoir Characterization | compartment | The productive segment of an oil or gas field that is not in fluid communication with the remainder of the field. Productive compartments may become isolated at the time of accumulation by depositional processes or become isolated after deposition and burial by diagenesis or by structural changes, such as faulting. |
| Reservoir Characterization | compartmentalization | The geological segmentation of once continuous reservoirs into isolated compartments. Reservoirs that have become compartmentalized require different approaches to interpretation and production than continuous reservoirs. The degree of compartmentalization may vary as a consequence of production. |
| Reservoir Characterization | conceptual model | A hypothetical model characterizing strata, generally strata deposited in one or a related set of environments. Conceptual models usually incorporate rules about possible geometries and successions of facies that can be included in a geological scenario. These often provide limitations to the interpretation of a given reservoir. Conceptual models commonly incorporate sequence stratigraphic concepts such as facies tracts, unconformities, flooding surfaces, erosional surfaces and parasequences. Conceptual models are often used in conjunction with geostatistical and classical technologies for reservoir characterization. |
| Reservoir Characterization | conditional simulation | A geostatistical tool that yields a quantitative measure of the error in a map. It is performed when multiple maps have been created using kriging or cokriging and where each map has similar mean and variance as the control points, has approximately the same semivariogram, and approximately honors the control points. If guide data are used, the average of the conditional simulation images is the kriging with external drift (KED) solution. In general, conditional simulation maps contain more detail than maps produced by kriging or KED, but require much more effort to produce. |
| Reservoir Characterization | conical projection | A projection of data from the apex of a cone in a three-dimensional plot onto a surface at the base of the cone. This projection often is performed in log analysis to remove a dimension and see what a data point would read in the absence of that dimension. For example, removal of shale effects in a plot of neutron, density and gamma ray data helps determine the mineralogy of a sample where the apex of the cone would represent the shale point in the plot. The M-N plot is a plot in which the fluid has been removed by conical projection from the neutron, density and sonic data to provide a porosity-independent plot that can be used to determine lithology. |
| Reservoir Characterization | correlation | A positive relationship between data samples that implies a connection or a relationship between them. |
| Reservoir Characterization | correlogram | A graphical representation of the degree of agreement between segments of curves being correlated between different wells. The degree of lag (required shift), the amplitude of the peaks and the shapes of the peaks are parameters used to calculate the match in a correlogram. |
| Reservoir Characterization | crossplot | A two-dimensional plot with one variable scaled in the vertical (Y) direction and the other in the horizontal (X) axis. The scales are usually linear but may be other functions, such as logarithmic. Additional dimensions may be represented by using color or symbols on the data points. These plots are common tools in the interpretation of petrophysical and engineering data. |
| Reservoir Characterization | cyclothem | A succession of strata deposited during a single cycle of deposition. These sedimentary successions usually occur repeatedly, one above the other. The two main varieties are the cyclic units that are symmetrical cyclothems, and the rhythmic units that are asymmetrical cyclothems. Cyclic groupings of cyclothems are called megacyclothems, and cyclic groupings of megacyclothems are called hypercyclothems. Cyclothems are thought to be due to natural cycles, such as changes in sea levels related to changes in the volume of polar ice caps. |
| Reservoir Characterization | darcy | A standard unit of measure of permeability. One darcy describes the permeability of a porous medium through which the passage of one cubic centimeter of fluid having one centipoise of viscosity flowing in one second under a pressure differential of one atmosphere where the porous medium has a cross-sectional area of one square centimeter and a length of one centimeter. A millidarcy (mD) is one thousandth of a darcy and is a commonly used unit for reservoir rocks. |
| Reservoir Characterization | depth control | The practice of ensuring that all measurements taken in a borehole are matched to the “base depth,” normally the depth determined with the resistivity log. |
| Reservoir Characterization | depth matching | The practice of shifting depths of various data sets to a measurement that is known to be on depth. The general standard that is usually used is the first resistivity logs run, because those logs usually underwent the most rigorous depth control. Depth matching is usually applied to all wireline data, cores, borehole seismic data, and any other data taken in a well. Depth matching is a vital process in any well evaluation or any reservoir characterization exercise, so much so that, in its absence, accuracy and validity of the exercise must be questioned. |
| Reservoir Characterization | derivative logs | Logs that have been calculated from other logs to find the rate at which a log is changing with depth. For example, the derivative caliper (rugosity) calculates the rate at which the caliper is changing from one depth to another. Bad hole conditions that cause the density log to produce incorrect measurements are usually more closely related to the rugosity of the hole than the hole size, so the rugosity curve is the more useful in this regard. |
| Reservoir Characterization | determinism | The use of deterministic methods to solve problems or find solutions to data sets. |
| Reservoir Characterization | deterministic methods | Techniques that use equations or algorithms that have been previously developed for similar situations. These methods do not involve stochastic or statistical approaches. Deterministic methods are generally easier and faster to apply and readily lend themselves to computer applications. However, they may not provide the most detailed or the most accurate reservoir models. |
| Reservoir Characterization | deviation | The angle at which a wellbore diverges from vertical. Wells can deviate from vertical because of the dips in the beds being drilled through. Wells can also be deliberately deviated by the use of a whipstock or other steering mechanism. Wells are often deviated or turned to a horizontal direction to increase exposure to producing zones, intersect a larger number of fractures, or to follow a complex structure. |
| Reservoir Characterization | dip | The angle between a planar feature, such as a sedimentary bed or a fault, and a horizontal plane. True dip is the angle a plane makes with a horizontal plane, the angle being measured in a direction perpendicular to the strike of the plane. Apparent dip is the angle measured in any direction other than perpendicular to the strike of the plane. Given the apparent dip and the strike, or two apparent dips, the true dip can be computed. |
| Reservoir Characterization | dip fault | A fault whose primary movement is in the dip direction. Dip faults are also referred to as dip-slip faults. |
| Reservoir Characterization | directional variograms | Variograms and semivariograms that have a directional component in addition to the normal distance component. Directional variograms and semivariograms are commonly used where geological features are heterogeneous. For example, fluvial environments dominated by valleys, channels and point bars are likely to have directional components that are detectable and that will influence the behavior of fluids in these formations. Geostatistical models that use directional variograms can be expected to be more reliable in these circumstances. |
| Reservoir Characterization | distribution | The variation in the values of a one-dimensional data set. There are a number of readily recognized, possible distributions known to statistics, each with mathematical definitions. Statisticians may endeavor to find whether a data set is a good fit to any of the recognized distributions. Some examples include: bimodal Boltzmann chi-squared general normal Gaussian or standard normal (bell-shaped curve) normal Poisson student’s t |
| Reservoir Characterization | domain | A region of magnetic polarity within a ferromagnetic body. Domains collectively determine the magnetic properties of the body by their arrangement. |
| Reservoir Characterization | domain | A region characterized by a specific feature. |
| Reservoir Characterization | domain | The set of values assigned to the independent variables of a function. |
| Reservoir Characterization | domainal fabric | A structure made up of a number of superposed domains, usually of different size or wavelength. These are used in geostatistical work to describe statistical behaviors on small scales (such as porosity in thin sections) to large scales (such as porosity distributions in reservoirs). |
| Reservoir Characterization | domainal structure | A structure made up of a number of superposed domains, usually of different size or wavelength. These are used in geostatistical work to describe statistical behaviors on small scales (such as porosity in thin sections) to large scales (such as porosity distributions in reservoirs). |
| Reservoir Characterization | effective medium theory | A method for determining the effective properties of random fields, commonly abbreviated EMT. Originally developed to estimate transport coefficients, this theory is based on the idea of replacing the inhomogeneous medium by an equivalent homogenous medium such that the fluctuations induced by restoring the heterogeneity average to zero. EMT is used to upscale parameters such as permeability for use in coarse-grained reservoir simulation studies. This use is controversial in reservoirs that are not homogeneous. Reference Kilpatrick S: Percolation and conduction, Reviews of Modern Physics 45 (1973): 574-614. Mansoori J: A Review of Basic Upscaling Procedures: Advantages and Disadvantages, in Yarus JM and Chambers RL (eds): Stochastic Modeling and Geostatistics, AAPG Computer Applications in Geology , no. 3. AAPG, Tulsa, Oklahoma, USA, 1994. |
| Reservoir Characterization | eigenvector | The vector that best represents the trend of data in multiple dimensions. This is the major component of principal component analysis (PCA). In log analysis, log data plotted in the number of dimensions equal to the number of curves can be subjected to principal component analysis, replotted in PCA space and then subjected to cluster analysis to find usable electrofacies. |
| Reservoir Characterization | empirical | Pertaining to analysis that includes equations or formulae that were derived purely from data analysis and were not derived from a theoretical basis. The majority of equations used in reservoir characterization and reservoir engineering are empirical. |
| Reservoir Characterization | Epanechnikov kernel | A discontinuous parabola kernel that is used in contouring areal density of data points in a crossplot. The kernel function can take many other forms, such as triangular, rectangular or Gaussian. The function determines the shape of the bump or cluster of data under scrutiny. This technology is often used in cluster analysis and statistical graphic techniques. |
| Reservoir Characterization | error | A data value that is not correct. This could be caused by a faulty measurement or by incorrect processing of the data. |
| Reservoir Characterization | error | The difference between a data value and the value predicted by a statistical distribution or other mathematical algorithm. Strictly, normal distributions apply only to random events, but they are often used to describe nonrandom events because their distributions look like approximations to a bell curve. Under these circumstances, the error is only a measure of deviation from a normal distribution and may not have direct physical significance. |
| Reservoir Characterization | Euclidian dimension | A dimension in Euclidian space. Euclidian dimensions are all orthogonal to each other (at right angles to each other) and refer to physical space with X, Y and Z components. |
| Reservoir Characterization | Euclidian distance | The distance between two points in Euclidian space. Euclidian dimensions are all orthogonal to each other (they are all at right angles to each other) and refer to physical space. |
| Reservoir Characterization | eustatic sea level | Global sea level, which changes in response to changes in the volume of ocean water and the volume of ocean basins. |
| Reservoir Characterization | expectation | The results of extrapolating from a known data point to points away from measurements. In geostatistical modeling, generating expectation trends is a fundamental process that requires procedures to estimate geometric changes by using various statistical approaches. Often, trends in surfaces are estimated and used to provide bounds on possible facies extrapolation. |
| Reservoir Characterization | expert system | A computer system that uses a rule-based algorithm to provide expertise on a given subject. Many computer programs have been written for use in the oil field using rule-based approaches to provide expert systems. The rules are taken from an expert working in the field and are written in a way that attempts to reproduce the knowledge and approaches used by that expert to solve a range of actual problems. Most such programs are limited to specific disciplines such as dipmeter interpretation, electrofacies determination, reservoir characterization, blowout prevention or drilling fluid selection. Sometimes expert systems are written in computer languages such as LISP that easily handles rules, but once fully tested, expert systems are usually translated to BASIC, C or FORTRAN to be compiled into efficient applications or programs. |
| Reservoir Characterization | facies | The characteristics of a rock unit that reflect its origin and permit its differentiation from other rock units around it. Facies usually are characterized using all the geological characteristics known for that rock unit. In reservoir characterization and reservoir simulation, the facies properties that are most important are the petrophysical characteristics that control the fluid behavior in the facies. Electrofacies and other multivariate techniques are often used to determine these characteristics. Rock types rather than facies are more likely to be used in reservoir simulation. |
| Reservoir Characterization | facies modeling | The act of modeling a reservoir using knowledge of the facies that make up the reservoir and the depositional environments that the facies represent. The depositional characteristics will suggest rules concerning the geometries of the facies and the possible relationships between facies, especially where the facies have been related to each other within a stratigraphic sequence or a cyclothem. Facies modeling is often an important component of geostatistical reservoir characterization and facilitates construction of superior reservoir models for complex reservoirs. |
| Reservoir Characterization | filters | Devices for selecting or excluding data from a data stream or data set. These devices may be physical (for example to tune an electrical circuit to a particular frequency) or a mathematical algorithm. Mathematical filters take many forms, some of which are used in oilfield data analysis and interpretation. Examples include statistical techniques, geostatistical techniques, clustering, conditional algorithm, etc. A simple example of a conditional algorithm might include using a caliper to determine whether a borehole was rugose, thus requiring special log interpretation through the rugose interval. |
| Reservoir Characterization | fitted variogram | A variogram or semivariogram is said to have been fitted after the best possible model has been applied to it. |
| Reservoir Characterization | flooding surface | A surface exhibiting evidence of an abrupt increase in water depth, separating younger from older strata. The surface may also display evidence of minor submarine erosion. It forms in response to an increase in water depth and typically bounds parasequences. In sequence stratigraphic terminology, it replaces the older, more generic term “trangressive surface,” although it is not a strict equivalent. |
| Reservoir Characterization | flow model | A model of a reservoir in which the steady-state flow and the advective transport are described in two or three dimensions by a computer program. A flow model is an essential component of a reservoir simulator. Flow models are often derived from the petrophysical characteristics of a reservoir (especially porosities and permeabilities) and then the model is adjusted and refined until it correctly predicts the reservoir’s past behavior and can match the historical pressure and production data. |
| Reservoir Characterization | flow simulation | The dynamic simulation of fluids through a reservoir model over time. When the simulation correctly recreates the past reservoir performance, it is said to be “history matched,” and a higher degree of confidence placed in its ability to predict the future fluid behavior in the reservoir. |
| Reservoir Characterization | fluid flow | The movement of fluid through pores and fractures within permeable rocks in a reservoir. Generally, the fluid flow is assumed to follow Darcy’s law, so the fluid flow may be simulated with a model of the reservoir. |
| Reservoir Characterization | Fourier analysis | A mathematical algorithm designed by geometrician and physicist Baron J.B.J. Fourier to determine the frequency distribution within a wave pattern as a series of sine waves. Fourier analysis is also used to study any series of repeated signals or patterns. This analysis is sometimes used to study patterns in images such as thin sections, and in geostatistics and log analysis. |
| Reservoir Characterization | fractal | A special mathematical geometry with properties that reproduce a pattern over a range of scales. They can contain some variations so that the patterns do not perfectly repeat. This geometry claims to represent many natural systems, including plant growth, geological deposition, coastlines and other geographical shapes. Fractal geometry has also led to the recent study of Chaos Theory. This technology is sometimes used in geostatistical studies. Reference: Mandelbrot BB: The fractal geometry of nature. New York, Freeman, 1983. Hewett TA: Fractal Methods for Fracture Characterization, in Yarus JM and Chambers RL (eds): Stochastic Modeling and Geostatistics, AAPG Computer Applications in Geology, no. 3. AAPG, Tulsa, Oklahoma, USA, 1994. |
| Reservoir Characterization | fractal analysis | Analysis of a geometrical system using fractal mathematics. This analysis is sometimes used in geostatistics to describe depositional systems and other geological phenomena. |
| Reservoir Characterization | fractal networks | Networks that are described using the mathematics of fractals. These are useful for describing certain types of fracture systems. |
| Reservoir Characterization | fracture networks | Patterns in multiple fractures that intersect with each other. Fractures are formed when rock is stressed or strained, as by the forces associated with plate-tectonic activity. When multiple fractures are propagated, they often form patterns that are referred to as fracture networks. These networks are studied using a number of mathematical and statistical techniques and may even be represented using fractals. Fracture networks may make an important contribution to both the storage (porosity) and the fluid flow rates (permeability or transmissability) of formations. |
| Reservoir Characterization | Gaussian collocated cosimulation | An algorithm built on a Markov-type hypothesis, whereby collocated secondary information is assumed to screen out secondary data from farther away. The method allows the direct cosimulation of several interdependent variables, integrating several different sources of soft information. Reference: Almeida AS and Frykman P: “Geostatistical Modeling of Chalk Reservoir Properties in the Dan Field, Danish North Sea,” in Yarus JM and Chambers RL (eds): Stochastic Modeling and Geostatistics, AAPG Computer Applications in Geology, no. 3. AAPG, Tulsa, Oklahoma, USA, 1994. |
| Reservoir Characterization | Gaussian techniques | A parametric approach to stochastic imaging or simulation of a reservoir. Simpler than the indicator (nonparametric) approach, Gaussian techniques include a normal score transform of the data to produce a new variable that is, by construction, univariate and normally distributed. |
| Reservoir Characterization | geostatistical methods | Techniques that are applied to reservoir characterization using various statistical approaches to estimate the geological characteristics of formations at a distance from known points, such as within wellbores. These techniques include the use of semivariograms, kriging and multivariate analysis. |
| Reservoir Characterization | geostatistical modeling | The process of generating models of the subsurface using geostatistical methods. The methods derive a detailed model using only sparse data (such as logs and cores from a few wells in a large area) and a knowledge of the depositional systems and structural settings to estimate the subsurface characteristics between the wells. These methods were originally developed in the mining industry, where boreholes are generally much closer, and later were adapted for use in the oil and gas industry. Geostatistical models are used for reservoir simulation and reservoir management. |
| Reservoir Characterization | geostatistics | The study of samples of data from a complete data set (or population) to attempt to estimate the behavior of the population. Typically, geostatistics is applied during the creation of high-resolution subsurface models of mineral deposits or oil reservoirs. The data are available only from well cores and logs (high vertical resolution but laterally infrequent data samples) and from surface seismic (low vertical resolution) data. Geostatistically derived reservoir models, when successful, are used in reservoir simulations and for reservoir management. |
| Reservoir Characterization | glyph | A diagram that displays multivariate data. A good choice of glyph design can aid the recognition of complex similarities or distinctions in a set of data. A well-known example of an oilfield glyph is the Stiff diagram, which shows the patterns of cations and anions in fluids. Reference: Stiff HA Jr: The Interpretation of Chemical Water Analysis by Means of Patterns, Journal of Petroleum Technology 3, no. 10 (1951): 15-16. |
| Reservoir Characterization | greedy algorithm | A mathematical procedure used to improve computer usage in simulated annealing runs. The procedure uses a fast swapping technique to match model semivariograms to actual semivariogram data. |
| Reservoir Characterization | gridding | The act of determining values for grid elements on a map. The grid element values are chosen from nearby data points. Methods are deterministic and use linear and nonlinear interpolation methods, or may be statistical and use geostatistical approaches such as kriging. Gridding is usually applied to one characteristic per map, such as structure, thickness, porosity, permeability or saturation. |
| Reservoir Characterization | gridding algorithm | A computational procedure incorporated in methods for determining the values to be assigned to grid elements on a map. The deterministic gridding algorithms are mostly simpler interpolation; the stochastic approaches include the geostatistical methods, among them kriging. |
| Reservoir Characterization | harmonic | A set of data values related by some function of frequencies and capable of being represented by sine and cosine functions. |
| Reservoir Characterization | harmonic | A particular frequency at which a data set has a resonance, or the frequency has special significance. |
| Reservoir Characterization | hierarchical cluster analysis | A method of cluster analysis in which the distance between every pair of data points is determined and the relative distances displayed on a dendogram. This method is completely accurate but is very CPU intensive when the data set has a large number of data points. For large numbers of data points, the k-means method is usually preferred. This method is sometimes used after the data have first been transformed into their principal components. The method is one possible approach to electrofacies calculations. |
| Reservoir Characterization | highstand systems tract | A systems tract bounded below by a downlap surface and above by a sequence boundary, commonly abbreviated as HST. This systems tract is characterized by an aggradational to progradational parasequence set. |
| Reservoir Characterization | history matching | The act of adjusting a model of a reservoir until it closely reproduces the past behavior of a reservoir. The historical production and pressures are matched as closely as possible. The accuracy of the history matching depends on the quality of the reservoir model and the quality and quantity of pressure and production data. Once a model has been history matched, it can be used to simulate future reservoir behavior with a higher degree of confidence, particularly if the adjustments are constrained by known geological properties in the reservoir. |
| Reservoir Characterization | Hough transform | A method for detecting patterns of points in binary data sets. Data pairs on a plot are assigned slopes and offsets and then replotted in slope and offset space. The method has been used on wireline curve data and on image data, where dips and azimuths are used. The Hough transform can be used to obtain the Buckles number. Reference: Hough PVC: A Method and Means for Recognizing Complex Patterns, U.S. Patent No. 3,069,064, 1962. |
| Reservoir Characterization | indicator methods | An approach to stochastic imaging or simulation of a reservoir that is nonparametric. The Gaussian approach is a simpler approach that includes a normal score transform of the data to produce a new variable that is univariate and normally distributed. |
| Reservoir Characterization | indicator models | Models of reservoirs built using indicator (nonparametric) methods. |
| Reservoir Characterization | information theory | The study, collection and management of information, especially with respect to computer technology. Information theory is an important component in the construction and efficient usage of databases. |
| Reservoir Characterization | interpolation | The assignment of values to points intermediate to two data points. Linear interpolation plots values along a straight line between the values of the two nearest data points. Other interpolation techniques involve other functions. Interpolation is used to fill in missing data in well logs and other data sets. It may also be used to assign values to grid elements in maps. |
| Reservoir Characterization | iterative methods | Mathematical techniques that require an algorithm or equation to be repeated until a condition is met. These methods usually require the convergence of a result to a value. Computers are excellent tools for performing iterative techniques quickly and efficiently. |
| Reservoir Characterization | k-means cluster analysis | A clustering technique that begins with the assignment of the number of clusters to be found. Points that will represent the centroids of these clusters are then evenly dispersed through the data and moved as if by gravity until they settle into positions in the data clouds and cease to move. This technique is much faster than the hierarchical technique but not as accurate, and is often used in electrofacies analysis when large data sets must be analyzed. |
| Reservoir Characterization | Koch curve | A curve used to generate a certain type of fractal geometry. Straight lines are replaced by regular polygons repeatedly. These curves look like a snowflake when displayed graphically and are used to illustrate that a curve has a fractal dimension D>1. |
| Reservoir Characterization | kriging weights | The weights assigned to control points in kriging operations to minimize the variance, thus eliminating systematic estimation errors. |
| Reservoir Characterization | kurtosis | A measure of a curve describing the statistical frequency distribution in the region about its mode; the relative “peakedness” of the distribution. This measure is used in the description of wireline curves and in schemes that attempt to correlate them from well to well. |
| Reservoir Characterization | longitudinal plot | A plot of the longitudinal component of the dip vector computed from a dipmeter. Longitudinal plots are used in the SCAT (Statistical Curvature Analysis Technique) method of interpreting dipmeter data for geological structure. They are especially useful in doubly plunging dip situations. |
| Reservoir Characterization | lowstand systems tract | A systems tract overlying a sequence boundary and overlain by a transgressive surface. Characterized by a progradational to aggradational parasequence set, this systems tract commonly includes a basin-floor fan, a slope fan and a lowstand wedge. It is often abbreviated as LST. |
| Reservoir Characterization | map | A representation, on a plane surface and at an established scale, of the physical features of a part or whole of the Earth’s surface or of any desired surface or subsurface data, by means of signs and symbols, and with the means of orientation indicated. Reservoirs are often represented by a series of maps for each of the layers distinguished within the reservoir. This series of maps may include maps of structure, gross thickness, net thickness, porosity, water saturation and other required petrophysical characteristics. A complete set of petrophysical characteristic maps may constitute a reservoir description, reservoir characterization or reservoir model. |
| Reservoir Characterization | marine flooding surface | Sometimes abbreviated to flooding surface, a surface exhibiting evidence of an abrupt increase in water depth, separating younger from older strata. The surface may also display evidence of minor submarine erosion. It forms in response to an increase in water depth and typically bounds parasequences. In sequence stratigraphic terminology, it replaces the older, more generic term “trangressive surface,” although it is not a strict equivalent. |
| Reservoir Characterization | maximum flooding surface | A widespread marine flooding surface that separates the underlying transgressive systems tract from the overlying highstand systems tract. The surface also marks the deepest water facies within a sequence. The maximum flooding surface represents a change from retrogradational to progradational parasequence stacking patterns. It commonly displays evidence of condensation or slow deposition, such as abundant burrowing, hardgrounds, mineralization and fossil accumulations. On wireline logs, the shales that immediately overlie the maximum flooding surface commonly have different characteristics than other shales and can often be recognized on the basis of resistivity, gamma ray, neutron and density logs. These shales can also be recognized by electrofacies analysis when the analysis is designed to do so. |
| Reservoir Characterization | mean | A mathematical method of finding a central value for a group of data. It is most often referred to as the average but also as the arithmetic mean; it is the sum of all the observed values divided by the number of observations. |
| Reservoir Characterization | median | A mathematical measure of the centrality of a data set. If the data set is arranged in the order of the values, the median is the value of the central data point for an odd number of data, or the mean of the two central data points for an even number of data. The median is often used in place of the mean or average when there are a number of extreme data values or the distribution of data is skewed. |
| Reservoir Characterization | midrange | A mathematical method of finding a central value for a group of data. The midrange is defined as the sum of the lowest value in the data set and the highest value if the data set divided by two. |
| Reservoir Characterization | model | To produce a representation or a simulation of a problem. |
| Reservoir Characterization | model | A conceptual, three-dimensional construction of a reservoir or oil and gas field. The model is constructed from incomplete data and much of the interwell space must be estimated from nearby wells or from low vertical resolution data, such as seismic data. The construction of models can be performed by deterministic methods or geostatistical methods, or a combination of both. |
| Reservoir Characterization | Monte Carlo risk analysis | An approach to performing risk analysis on any project with uncertain input data. Generally, numbers are selected from representative input data and then used in iterative, CPU-intensive calculations to find the most likely outcome and the range of probable outcomes. The uncertainty in the output also provides a measure of the validity of the model. The technique is applied to financial investment portfolio and investment risk analysis as well as scientific applications. Monte Carlo analysis methods are used in the oil field to estimate the risks involved in new exploration projects, evaluation of development schemes and evaluation of validity of reservoir models. |
| Reservoir Characterization | Monte Carlo sampling | The sampling of uncertain data for use in Monte Carlo risk analysis or simulation. |
| Reservoir Characterization | Monte Carlo simulation | The use of Monte Carlo risk analysis techniques to estimate the most probable outcomes from a model with uncertain input data and to estimate the validity of the simulated model. |
| Reservoir Characterization | multiple regression | Regression techniques that find relationships between two or more variables that have a complex (nonlinear) relationship. Porosity and permeability relationships are often of this form in rocks that have multiple porosity types (primary, intergranular, fracture or vugular porosity, for example) or multiple cement types and other variables that affect permeability. |
| Reservoir Characterization | multishot survey | A technique for determining the deviation of a wellbore. The multishot tool provides more accuracy than the single-shot tool and is usually used in highly deviated wells. |
| Reservoir Characterization | nested fractal structures | Any model that incorporates more than one variable that is represented by fractal geometry or a fractal function. These models can become very complex if the variables are interdependent. |
| Reservoir Characterization | neural networks | A concept for advanced computer calculations developed by Alan Turing to mimic some of the operations of the neurons in a brain. Memory elements (neurons) are conceptually interconnected by multiple paths connected with on-off switches to emulate the synapses of the brain. The original intent was to build a data-processing machine. Modern applications reduce the concept to structured digital software processing models. Repeated processing through a neural network allows the network to learn from the data it processes. The learned process obtained from a set of training data with solutions can then be applied to other data sets for which no solution exists. An oilfield example includes training a network with wireline log and core data and then using the network to interpret further log data in terms of the core data. Neural networks are also being used in seismic processing, geological mapping and petrophysical analysis. |
| Reservoir Characterization | parameter | A variable that is given a constant value for the purposes of certain calculations. For example, during log analysis of a particular layer of a reservoir, the water resistivity (Rw) may be set to a particular value and referred to as a parameter. |
| Reservoir Characterization | parameter | A characteristic of a model of a reservoir that may or may not vary with respect to position or with time. Porosity is a petrophysical parameter (or characteristic) that varies with position. |
| Reservoir Characterization | parasequence boundary | A marine flooding surface or its correlative surface. |
| Reservoir Characterization | parasequence set | A succession of genetically related parasequences that form a distinctive stacking pattern, and that are typically bounded by major marine flooding surfaces and their correlative surfaces. Parasequence sets are usually classified as progradational, aggradational or retrogradational. |
| Reservoir Characterization | periodogram | A graphical representation of harmonic information in a data set. Often taken from Fourier analysis of the data, this representation is used to determine periodicities in petrophysical data and in geological depositional sequences. |
| Reservoir Characterization | petrophysical model | A model of a reservoir or a field in which the petrophysical data were the only or the primary data used to construct the model |
| Reservoir Characterization | petrophysical model | A process or procedure used to interpret petrophysical (usually wireline log) data. Usually representing a set of equations, algorithms or other mathematical processes, petrophysical models often have multiple routines. For example, a deterministic model might include routines that: |
| Reservoir Characterization | petrophysical rock type | Rock types that have been classified according to their petrophysical properties, especially properties that pertain to fluid behavior within the rock, such as porosity, capillary pressure, permeabilities, irreducible saturations or saturations. Petrophysical rock types are often calibrated from core and dynamic data, but are usually calculated from wireline logs, where possible, because the wireline logs are generally the only measurements that are available for all wells at all depths. Electrofacies approaches are often used to determine rock types from logs. |
| Reservoir Characterization | Poisson distribution | A probability distribution in which the mean and the variance are identical. This distribution was first described by S.D. Poisson, a French mathematician and physicist (1781-1840). |
| Reservoir Characterization | pressure transient well test | A means of assessing reservoir performance by measuring flow rates and pressures under a range of flowing conditions and then applying the data to a mathematical model. In most well tests, a limited amount of fluid is allowed to flow from the formation being tested. The formation is isolated behind cemented casing and perforated at the formation depth or, in openhole, the formation is straddled by a pair of packers that isolate the formation. During the flow period, the pressure at the formation is monitored over time. Then, the formation is closed (or shut in) and the pressure monitored at the formation while the fluid within the formation equilibrates. The analysis of these pressure changes can provide information on the size and shape of the formation as well as its ability to produce fluids. |
| Reservoir Characterization | principal axis | The axis along which the data in n-dimensional space is primarily distributed. In two dimensions, the first principal axis is the semimajor axis of the ellipse that best fits the data set. Multiple principal axes are always orthogonal. Data are sometimes rearranged to be in principal component space before further analysis (such as cluster analysis) is performed. Analysis on data that have been transformed into principal component space is referred to as principal component analysis, or PCA. |
| Reservoir Characterization | principal component analysis | Analysis of data that has been transformed from the original axes to principal axes, often abbreviated PCA. The first principal axis is the direction in which the data are primarily distributed or the “long” axis of the distribution in n-dimensional space. Data are sometimes rearranged to be in principal component space before further analysis (such as cluster analysis) is performed. |
| Reservoir Characterization | probability | A numerical estimate of the chances of an event occurring given a limited number of opportunities for the event to occur. |
| Reservoir Characterization | quantile map | A map based on quantile values from the probabilities of the values of the data. This map is used to reveal problems with distributions of variables in geostatistical studies. |
| Reservoir Characterization | quartile | Special quantiles at 0%, 25%, 50%, 75% and 100%. |
| Reservoir Characterization | radius of curvature | A method of following a trend between points by connecting the data points by segments of a circle such that the segments “line up” with each other smoothly. This method is used to plot out deviation surveys based on a limited number of survey points to find the path of a wellbore. |
| Reservoir Characterization | random walk method | A method of performing stochastic analysis on dynamic data. This method can be used in stochastic simulation of fluid behavior in reservoirs and fields. |
| Reservoir Characterization | random-walk method | A method of performing stochastic analysis on dynamic data. This method can be used in stochastic simulation of fluid behavior in reservoirs and fields. |
| Reservoir Characterization | recovery | The fraction of hydrocarbons that can or has been produced from a well, reservoir or field; also, the fluid that has been produced. |
| Reservoir Characterization | recovery forecast | A prediction of the amount of production that will occur from a well, reservoir or field. This estimate is sometimes expressed as a fraction of the total hydrocarbons originally in place |
| Reservoir Characterization | regression | The statistical fitting of trend lines to a data set. Many regression methods are available, including linear, iterative, multiple and polynomial. If there is a ‘good’ fit to the data, then the variables are often assumed to be dependent. |
| Reservoir Characterization | regression coefficient | A quantification of the degree of ‘goodness’ of fit of a regression line to a data set. A value of 0.5 represents random data with no dependence, and a coefficient of 1.0 represents a perfect fit with absolute dependence. |
| Reservoir Characterization | regressive | Pertaining to regression, the statistical fitting of trend lines to a data set. Many regression methods are available, including linear, iterative, multiple and polynomial. If there is a ‘good’ fit to the data, then the variables are often assumed to be dependent. |
| Reservoir Characterization | reservoir characterization | The act of building a reservoir model based on its characteristics with respect to fluid flow. |
| Reservoir Characterization | reservoir characterization | A model of a reservoir that incorporates all the characteristics of the reservoir that are pertinent to its ability to store hydrocarbons and also to produce them. Reservoir characterization models are used to simulate the behavior of the fluids within the reservoir under different sets of circumstances and to find the optimal production techniques that will maximize the production. |
| Reservoir Characterization | reservoir communication | The flow of fluids from one part of a reservoir to another or from one reservoir to another. The term is often used to describe crossflow from one reservoir compartment to another. |
| Reservoir Characterization | reservoir description | The act of building a reservoir model based on its characteristics with respect to fluid flow. |
| Reservoir Characterization | reservoir description | Also called reservoir characterization, a model of a reservoir that incorporates all the characteristics of the reservoir that are pertinent to its ability to store hydrocarbons and also to produce them. Reservoir characterization models are used to simulate the behavior of the fluids within the reservoir under different sets of circumstances and to find the optimal production techniques that will maximize the production. |
| Reservoir Characterization | reservoir modeling | The act of producing a model of a reservoir. The model could include any of the geological, fluid or other characteristics of the reservoir. |
| Reservoir Characterization | reservoir simulation | A computer run of a reservoir model over time to examine the flow of fluid within the reservoir and from the reservoir. Reservoir simulators are built on reservoir models that include the petrophysical characteristics required to understand the behavior of the fluids over time. Usually, the simulator is calibrated using historic pressure and production data in a process referred to as “history matching.” Once the simulator has been successfully calibrated, it is used to predict future reservoir production under a series of potential scenarios, such as drilling new wells, injecting various fluids or stimulation. |
| Reservoir Characterization | rock properties | The physical characteristics of reservoir rocks that enable them to store fluids and to allow fluids to flow through them. The main properties of interest are rock porosities and permeabilities. |
| Reservoir Characterization | rock types | A set of characteristics that several rocks have in common. The characteristics of interest are usually those pertaining to fluid movement and fluid storage capacity. |
| Reservoir Characterization | scattergram | A graph in which data points are plotted but not connected. The x and y axes of the scattergram represent the two variables being plotted. Sometimes, the data points are coded by using color or symbols to represent a third dimension. |
| Reservoir Characterization | seals | The geological barriers that isolate fluid compartments within reservoirs or that hydraulically isolate reservoirs from each other. The seals may contain fluids (for example shales) but have very low permeability. The properties of seals can determine the height of hydrocarbon column trapped below them. |
| Reservoir Characterization | sequential Gaussian simulation | A procedure for estimating the reservoir characteristics between data points. Based on the idea of iterating from a first guess and refining through reduction of errors, the procedure generally transforms the model to normality, simulating the normally distributed transform, and then back-transforming to the original variable of interest. |
| Reservoir Characterization | Sierpinski gasket | A form of fractal geometry based on a triangle. It has a fractal dimension D = ln 3/ln 2 = 1.58…. A Sierpinski carpet uses a square instead of a triangle and has a fractal dimension D = ln 8/ln 3 = 1.89…. |
| Reservoir Characterization | simulated annealing | A method for constructing a gridded reservoir model by iterative trial and error. The grid is initially populated randomly with a characteristic (such as facies) so that some property (such as a net/gross ratio) is correct. Then the grids are randomly swapped so that the property is preserved but another property (such as total length) is improved. |
| Reservoir Characterization | single shot survey | A technique for acquiring deviation information from a borehole on a slickline. In high-angle wells, a multishot technique is usually used instead. |
| Reservoir Characterization | single-shot survey | A technique for acquiring deviation information from a borehole on a slickline. In high-angle wells, a multishot technique is usually used instead. |
| Reservoir Characterization | sinusoid | A property or characteristic that has the form of a sine wave. |
| Reservoir Characterization | skewness | The degree to which a distribution has lost the bilateral symmetry of a normal distribution. Skewness is usually expressed qualitatively rather than quantitatively. |
| Reservoir Characterization | Solar terrestrial rhythms | The periodicities in the behavior of the Earth and its climate with respect to the sun. Some rhythms are caused by variations in the elliptically of the Earth’s orbit, rotation of the semimajor axis of the Earth’s orbit, variation on the tilt of the Earth’s axis and rotation in the tilt of the Earth’s axis. Most of these rhythms are predictable over the short term (up to hundreds of millions of years) but can become chaotic due to the influence of other planets (particularly massive Jupiter) or extra solar-system activity. The rhythms affect sea level and other factors that dictate the long-term behavior of depositional systems. |
| Reservoir Characterization | spectral density analysis | A technique for utilizing fractal geometry to produce reservoir descriptions. |
| Reservoir Characterization | spline | A mathematical procedure for connecting data points with a smooth line. The line does not necessarily go through the data points, but its direction and curvature are affected by all the data points. The “stiffness” of the line is controlled by a variable (lambda) in the algorithm. |
| Reservoir Characterization | stationarity | A form of homogeneity in a single characteristic. Local stationarity occurs when two or more adjacent, locally homogeneous samples yield similar values of the property of interest. |
| Reservoir Characterization | statistics | A branch of mathematics that attempts to bring some understanding in the analysis of quantitative measurements. This science deals with the collection, analysis and interpretation of numerical data, often using probability theory. Statistics can also involve the generation of synthetic data between or within a group of measured data. |
| Reservoir Characterization | stochastic analysis | An analysis related to a process involving a randomly determined sequence of observations, each of which is considered as a sample of one element from a probability distribution. |
| Reservoir Characterization | stochastic methods | An analysis related to a process involving a randomly determined sequence of observations, each of which is considered as a sample of one element from a probability distribution. |
| Reservoir Characterization | stochastic modeling | The production of a model of a reservoir or field by using stochastic methods to interpolate between data measurements (usually wells). |
| Reservoir Characterization | strike fault | A fault whose primary movement is in the strike direction (usually horizontal). This type of fault is usually caused by continents or tectonic plates moving laterally with respect to each other, as is happening in California today. The San Andreas fault is a strike-slip fault along which the western side is moving north relative to the eastern side. |
| Reservoir Characterization | structural analysis | An examination of a geological scenario to understand the geometry and spatial arrangement of rocks. The structure or deformation can include many mechanisms, such as folding, faulting and fracturing. Structure can usually be interpreted in terms of the deformation of the crust of the Earth as continents and tectonic plates move and collide. |
| Reservoir Characterization | synthetic seismograms | The seismic traces at a wellbore generated from wireline log data. Synthetic seismograms are generated by calculating reflection coefficients from the sonic and density logs and then applying an ideal or real wavelet to the reflections to obtain the seismic “wiggle” traces. Synthetic seismograms are usually generated to compare with the actual seismic data and identify reflectors with layers and formations already known in the wellbore. |
| Reservoir Characterization | tensor methods | A mathematical entity with components that change in a particular way in a transformation from one coordinate system to another. Tensor methods are used in upscaling reservoir parameters for use in reservoir simulation studies. |
| Reservoir Characterization | theoretical | Pertaining to analysis based on equations or formulae derived from a theoretical basis in science. The majority of equations used in reservoir characterization and reservoir engineering are empirical, but many have been derived from scientific theory. |
| Reservoir Characterization | transgressive surface | A marine flooding surface separating the underlying lowstand systems tract from the overlying transgressive systems tract. Typically, this is the first major flooding surface following the lowstand systems tract. |
| Reservoir Characterization | uncertainty | The degree to which a data set may be in error or stray from predicted values. Sometimes quantified in terms of variance or standard deviation, uncertainty exists in data because of a variety of problems, such as poor calibration or contamination or damage to rocks prior to measurement. Uncertainty is the cause of many problems, which occasionally can be overcome by normalizing the data. |
| Reservoir Characterization | uncertainty principle | A quantum-mechanical principle formulated by German physicist Werner Karl Heisenberg. It proposes that measuring either one of two related quantities, such as position and momentum, or energy and time, produces uncertainty in the measurement of the other. The error in measurement of one (for example, position) times the error in measurement of the other (for example, momentum) must be greater or equal to Planck’s constant. This principle accounts for bizarre behavior of subatomic particles and implies that we can never know everything exactly. |
| Reservoir Characterization | varimax rotation | A method for rotating axes of a plot such that the eigenvectors remain orthogonal as they are rotated. These rotations are used in principal component analysis so that the axes are rotated to a position in which the sum of the variances of the loadings is the maximum possible. |
| Reservoir Characterization | variogram | A two-point statistical function that describes the increasing differences or decreasing correlation, or continuity, between sample values as separation between them increases. The term variogram is sometimes used incorrectly in place of semivariogram. The two differ only in that the semivariogram uses each pair of data elements only once, whereas the variogram uses all possible data pairs. Semivariograms are usually used instead of variograms, but opposite vector directions (for example, north and south) are recognized as representing the same thing and having identical ranges, sills, nugget points and the like. |
| Reservoir Characterization | Walther’s law | A law stating that lithologies that conformably overlie one another must have accumulated in adjacent depositional environments. Exceptions occur where there are erosional breaks. This law allows for transformations from the vertical data to a horizontal set and is often used when a vertical sequence of facies has been identified and characterized (for example, with Markov chain analysis) to estimate the horizontal depositional pattern. |
| Reservoir Characterization, Formation Evaluation | magnetic resonance | A phenomenon by which a nucleus absorbs electromagnetic radiation of a specific frequency in the presence of a strong magnetic field. Isidor Isaac Rabi (1898 to 1988), an American physicist born in Austria, first detected magnetic resonance in 1938. Since then, magnetic resonance has been applied to the detection of light atoms (such as hydrogen in hydrocarbons) and as a nondestructive way to study the human body. |
| Reservoir Characterization, Formation Evaluation | nuclear magnetic resonance | A phenomenon by which a nucleus absorbs electromagnetic radiation of a specific frequency in the presence of a strong magnetic field. Isidor Isaac Rabi (1898 to 1988), an American physicist born in Austria, first detected magnetic resonance in 1938. Since then, magnetic resonance has been applied to the detection of light atoms (such as hydrogen in hydrocarbons) and as a nondestructive way to study the human body. |
| Reservoir Characterization, Formation Evaluation | numerical methods | Mathematical methods that require iterative processing of data rather than applying deterministic equations. Some relationships can be solved only by numerical methods, including most integration problems, some differentials and some statistical processes. |
| Reservoir Characterization, Formation Evaluation | numerical model | A rendering of a model of a reservoir or field in entirely numerical formats. Numerical models, once built, may be used to perform many mathematical operations, including calculations of available reserves and simulations of the behavior of the reservoir. |
| Reservoir Characterization, Formation Evaluation | numerical reservoir simulation | The mathematical simulation of a numerical model of a reservoir’s petrophysical characteristics to analyze and predict fluid behavior in the reservoir over time. |
| Reservoir Characterization, Formation Evaluation, Drilling | log | To continuously measure formation properties with electrically powered instruments to infer properties and make decisions about drilling and production operations. The record of the measurements, typically a long strip of paper, is also called a log. Measurements include electrical properties (resistivity and conductivity at various frequencies), sonic properties, active and passive nuclear measurements, dimensional measurements of the wellbore, formation fluid sampling, formation pressure measurement, wireline-conveyed sidewall coring tools, and others. For wireline measurements, the logging tool (or sonde) is lowered into the open wellbore on a multiple conductor, contra-helically armored wireline cable. Once the tool string (link to ID 2964) has reached the bottom of the interval of interest, measurements are taken on the way out of the wellbore. This is done in an attempt to maintain tension on the cable (which stretches) as constant as possible for depth correlation purposes. (The exception to this practice is in certain hostile environments in which the tool electronics might not survive the downhole temperatures for long enough to allow the tool to be lowered to the bottom of the hole and measurements to be recorded while pulling the tool up the hole. In this case, “down log” measurements might be conducted on the way into the well, and repeated on the way out if possible.) Most wireline measurements are recorded continuously while the sonde is moving. Certain fluid sampling and pressure-measuring tools require that the sonde be stopped, increasing the chance that the sonde or the cable might become stuck. Logging while drilling (LWD) tools take measurements in much the same way as wireline-logging tools, except that the measurements are taken by a self-contained tool near the bottom of the bottomhole assembly and are recorded downward (as the well is deepened) rather than upward from the bottom of the hole. |
| Reservoir Characterization, Formation Evaluation, Drilling | wireline log | A continuous measurement of formation properties with electrically powered instruments to infer properties and make decisions about drilling and production operations. The record of the measurements, typically a long strip of paper, is also called a log. Measurements include electrical properties (resistivity and conductivity at various frequencies), sonic properties, active and passive nuclear measurements, dimensional measurements of the wellbore, formation fluid sampling, formation pressure measurement, wireline-conveyed sidewall coring tools, and others. For wireline measurements, the logging tool (or sonde) is lowered into the open wellbore on a multiple conductor, contra-helically armored wireline cable. Once the tool string (link to ID 2964) has reached the bottom of the interval of interest, measurements are taken on the way out of the wellbore. This is done in an attempt to maintain tension on the cable (which stretches) as constant as possible for depth correlation purposes. (The exception to this practice is in certain hostile environments in which the tool electronics might not survive the downhole temperatures for long enough to allow the tool to be lowered to the bottom of the hole and measurements to be recorded while pulling the tool up the hole. In this case, “down log” measurements might be conducted on the way into the well, and repeated on the way out if possible.) Most wireline measurements are recorded continuously while the sonde is moving. Certain fluid sampling and pressure-measuring tools require that the sonde be stopped, increasing the chance that the sonde or the cable might become stuck. Logging while drilling (LWD) tools take measurements in much the same way as wireline-logging tools, except that the measurements are taken by a self-contained tool near the bottom of the bottomhole assembly and are recorded downward (as the well is deepened) rather than upward from the bottom of the hole. |
| Reservoir Characterization, Geophysics | modeling | The act of constructing a model. |
| Reservoir Characterization, Shale Gas | analog | An example used for comparison. In oil and gas exploration, geoscientists and engineers compare new prospects and fields with fields and surface exposures thought to be similar in depositional environment and reservoir character to guide predictions. Wide variations in shale reservoirs create doubt about the utility of analog comparisons. |
| Reservoir Characterization, Shale Gas | cumulative production | The total amount of oil and gas recovered from a reservoir as of a particular time in the life of the field. Cumulative production can be referenced to a well, a field, or a basin. |
| Reservoir Characterization, Shale Gas | interstitial gas | The gas stored in the pore space of a reservoir rock. Measurement of interstitial gas and adsorbed gas, which is the gas accumulated on the surface of another solid material, such as a grain of reservoir rock, allows calculation of gas in place in a reservoir. |
| Reservoir Characterization, Shale Gas | pore gas | Also known as interstitial gas, the gas stored in the pore space of a reservoir rock. Measurement of interstitial gas and adsorbed gas, which is the gas accumulated on the surface of another solid material, such as a grain of reservoir rock, allows calculation of gas in place in a reservoir. |
| Reservoir Characterization, Well Testing | pressure-transient analysis | The analysis of pressure changes over time, especially those associated with small variations in the volume of fluid. In most well tests, a limited amount of fluid is allowed to flow from the formation being tested and the pressure at the formation monitored over time. Then, the well is closed and the pressure monitored while the fluid within the formation equilibrates. The analysis of these pressure changes can provide information on the size and shape of the formation as well as its ability to produce fluids. |
| roduction | Galling | The tearing of metal when two elements rub against each other. Usually caused by lack of lubrication or extreme contact pressure. |
| Shale Gas | natural gas | Natural gas produced from shale reservoirs is known as shale gas. The composition of the gas stream is a function of the thermal maturity of the rock. Thermally immature rocks will contain heavier hydrocarbon components, possibly even liquid components. Overmature reservoirs typically contain appreciable quantities of carbon dioxide [CO2]. |
| Shale Gas | adsorbed gas | The gas accumulated on the surface of a solid material, such as a grain of a reservoir rock, or more particularly the organic particles in a shale reservoir. Measurement of adsorbed gas and interstitial gas, which is the gas contained in pore spaces, allows calculation of gas in place in a reservoir. |
| Shale Gas | bitumen | The fraction of naturally occurring, inflammable organic matter that is extractable from rock using organic solvents. Many petroleum precursors are composed of bitumen, but most are formed from kerogen in the process of petroleum generation. Bitumen includes hydrocarbons such as asphalt and mineral wax. Typically solid or nearly so, brown or black, bitumen has a distinctive petroliferous odor. Laboratory dissolution with organic solvents allows determination of the amount of bitumen in samples, an assessment of source rock richness. Burial and heating of kerogen yield bitumen, then liquid hydrocarbons, and then hydrocarbon gas. Understanding organic content is especially important in shale reservoirs because the shale is both the source rock and the reservoir rock in the petroleum system. |
| Shale Gas | continuous reservoir | A type of areally extensive reservoir that contains hydrocarbon throughout, rather than containing a water contact or being significantly affected by a water column or a defined structural closure. The areal extent of a continuous reservoir, such as a shale reservoir, can be as large as the extent of the sedimentary basin in which the shale was deposited. |
| Shale Gas | gas shale | Shale that produces natural gas. A shale that is thermally mature enough and has sufficient gas content to produce economic quantities of natural gas. |
| Shale Gas | geomechanics | The geologic specialty that deals with understanding how rocks, stresses, pressures, and temperatures interact. This understanding is used to solve oilfield problems, such as optimizing hydraulic fracturing treatments of shale reservoirs. Geomechanics specialists typically work with experts in geophysics, geology, petrophysics, reservoir engineering, drilling engineering, and rock physics to solve geomechanical problems and address production challenges in shale reservoirs. |
| Shale Gas | horizontal drilling | The intentional deviation of a wellbore from the path it would naturally take to a horizontal trajectory. Horizontal lateral sections can be designed to intersect natural fractures or simply to contact more of the productive formation. Horizontal drilling is accomplished through the use of whipstocks, bottomhole assembly (BHA) configurations, instruments to measure the path of the wellbore in three-dimensional space, data links to communicate measurements taken downhole to the surface, mud motors and special BHA components, including rotary steerable systems and drill bits. While many techniques can accomplish this, the general concept is simple: Direct the bit in the direction that one wants to drill. By placing a bend near the bit in a downhole steerable mud motor, the bend points the bit in a direction different from the axis of the wellbore when the entire drillstring is not rotating. By pumping mud through the mud motor, the bit turns while the drillstring does not rotate, allowing the bit to drill in the direction it points. When a particular wellbore direction is achieved, that direction may be maintained by rotating the entire drillstring (including the bent section) such that the bit does not drill in a single direction off the wellbore axis. Instead, the bit sweeps around and its net direction coincides with the existing wellbore. Rotary steerable tools allow steering while rotating, usually with higher rates of penetration and ultimately smoother boreholes. Horizontal drilling is common in shale reservoirs because it allows drillers to place the borehole in contact with the most productive reservoir rock. |
| Shale Gas | maturation | The process of a source rock becoming capable of generating oil or gas when exposed to appropriate pressures and temperatures. As a source rock begins to mature, it generates hydrocarbons. As an oil-prone source rock matures, the generation of heavy oils is succeeded by medium and light oils and condensates. Above a temperature of approximately 100°C [212°F], only dry gas is generated, and incipient metamorphism is imminent. The maturity of a source rock reflects the ambient pressure and temperature as well as the duration of conditions favorable for hydrocarbon generation. Understanding maturation is especially important in shale reservoirs because of the shales dual role as source rock and reservoir rock. |
| Shale Gas | metagenesis | The last stage of maturation and conversion of organic matter to hydrocarbons. Metagenesis occurs at temperatures of 150° to 200°C [302° to 392°F]. At the end of metagenesis, methane, or dry gas, is evolved along with nonhydrocarbon gases such as CO2, N2, and H2S, as oil molecules are cracked into smaller gas molecules. |
| Shale Gas | mudrock | A fine-grained detrital sedimentary rock formed by consolidation of clay- and silt-sized particles. Mudrocks are highly variable in their clay content and are often rich in carbonate material. As a consequence, they are less fissile, or susceptible to splitting along planes, than shales. Mudrocks may include relatively large amounts of organic material compared with other rock types and thus have potential to become rich hydrocarbon source rocks. The typical fine grain size and low permeability, a consequence of the alignment of their platy or flaky grains, allow mudrocks to form good cap rocks for hydrocarbon traps. However, mudrocks are also capable of being reservoir rocks, as evidenced by the many wells drilled into them to produce gas. |
| Shale Gas | perforate | The creation of holes in the casing or liner to achieve efficient communication between the reservoir and the wellbore. This process is integral to the optimal creation of hydraulic fractures. Geomechanical analysis is commonly conducted before perforating shale reservoirs to account for the relationship between formation stresses and productivity. |
| Shale Gas | petroleum systems modeling | A technique used to represent the history of a sedimentary basin, including the processes and components necessary to form petroleum: a petroleum source rock, a reservoir, a trapping mechanism, a seal, and the appropriate relative timing of formation of these. Using geologic, geophysical, and engineering data, scientists create a 3D model of the subsurface that can be used to understand whether petroleum is present and how much might exist in potential traps. Petroleum systems models can be used to help predict pore pressure and plan well construction and field development. A useful petroleum systems model can be used to identify and explain inconsistencies in the data. The resulting models are valuable during exploration for identifying resource richness, such as sweet spots in unconventional plays such as shale gas, and during field development and production for improving completion efficiency. Petroleum systems modeling is distinct from reservoir simulation in that it covers a larger scale that might include multiple oil and gas fields and considers a geologic time frame of millions of years rather than a production time frame of years or decades. |
| Shale Gas | produced water | A term used to describe water produced from a wellbore that is not a treatment fluid. The characteristics of produced water vary and use of the term often implies an inexact or unknown composition. It is generally accepted that water within the pores of shale reservoirs is not produced due to its low relative permeability and its mobility being lower than that of gas. |
| Shale Gas | refracturing | An operation to restimulate a well after an initial period of production. Refracturing operations attempt to bypass near-wellbore damage, reestablish good connectivity with the reservoir, and tap portions of the reservoir with higher pore pressure. Refracturing operations are also performed after a period of production that can alter the stresses in a reservoir due to depletion; the restimulation can allow the new fracture to reorient along a different azimuth. A successful refracturing operation restores well productivity to near original or even higher rates of production and extends the productive life of a well. |
| Shale Gas | reservoir characterization model | A model of a specific volume of the subsurface that incorporates all the geologic characteristics of the reservoir. Such models are used to quantify characteristics within the subsurface volume that are relatively stable over long periods of time and can, therefore, be considered static. These attributes include the structural shape and thicknesses of the formations within the subsurface volume being modeled, their lithologies, and the porosity and permeability distributions. These last two characteristics often vary significantly from location to location within the volume, resulting in heterogeneity. However, porosity and permeability are stable in the near-geologic timeframe and do not change due to the movement of fluids or gases through any of the formations pore spaces. The result of reservoir characterization is a reservoir characterization model (also known as a static model and sometimes referred to as a geologic model). Shale gas reservoir rocks require the analysis of high-quality seismic data, core, and log measurements and engineering data to produce an accurate reservoir characterization model. This model is then used as input into reservoir simulation, during which reservoir engineers add other reservoir characteristics, such as pressures, temperatures, and fluid and gas compositions. These features can change due to the movement of fluids or gases through any of the formations pore spaces. Since these are dynamic in their nature over short timeframes, once production is initiated these models are referred to as dynamic models. Thorough reservoir simulations (dynamic models) that are based on accurately developed reservoir characterizations (static models) can be of significant value in optimizing well placement and field-development planning. |
| Shale Gas | shale gas | Natural gas produced from gas shale formations. |
| Shale Gas | staged fracturing | An operation in which numerous reservoir intervals are hydraulically stimulated in succession. Staged hydraulic fracturing operations are commonly performed from horizontal wellbores placed in shale gas reservoirs. Using geomechanical data, engineers are able to optimize the placement of perforations and fracturing stages to maximize gas production. |
| Shale Gas | sweet spot | Colloquial expression for a target location or area within a play or a reservoir that represents the best production or potential production. Geoscientists and engineers attempt to map sweet spots enable wellbores to be placed in the most productive areas of the reservoir. Sweet spots in shale reservoirs may be defined by source-rock richness or thickness, by natural fractures, or by other factors, using geological data such as core analysis, well log data, or seismic data. |
| Shale Gas | thermal maturity | The degree of heating of a source rock in the process of transforming kerogen into hydrocarbon. Thermal maturity is commonly evaluated by measuring vitrinite reflectance or by pyrolysis. |
| Shale Gas | total organic carbon | The concentration of organic material in source rocks as represented by the weight percent of organic carbon. A value of approximately 0.5% total organic carbon by weight percent is considered the minimum for an effective source rock, although values of 2% are considered the minimum for shale gas reservoirs; values exceeding 10% exist, although some geoscientists assert that high total organic carbon values indicate the possibility of kerogen filling pore space rather than other forms of hydrocarbons. Total organic carbon is measured from 1-g samples of pulverized rock that are combusted and converted to CO or CO2. If a sample appears to contain sufficient total organic carbon to generate hydrocarbons, it may be subjected to pyrolysis. |
| Shale Gas | vitrinite reflectance | A measure of the thermal maturity of organic matter. This analytical method was developed to rank the maturity of coals and is now used in other rocks to determine whether they have generated hydrocarbons or could be effective source rocks. The reflectivity of at least 30 individual grains of vitrinite from a rock sample is measured under a microscope. The measurement is given in units of reflectance, % Ro, with typical values ranging from 0% Ro to 3% Ro, with values for gas-generating source rocks typically exceeding 1.5%. Strictly speaking, the plant material that forms vitrinite did not occur prior to Ordovician time, although geochemists have established a scale of equivalent vitrinite reflectance for rocks older than Ordovician. |
| Shale Gas | well placement | Activities associated with drilling a wellbore to intercept one of more specified locations. The term usually is used in reference to directional or horizontal wells that are oriented to maximize contact with the most productive parts of reservoirs via hydraulic fracturing or to optimize intersection with natural fractures. Geomechanical analysis of natural fractures and stresses and geological analysis of the reservoir are critical to successful well planning. Advanced formation evaluation and drilling technology support the drilling operation in real time. |
| Shale Gas | well plan | The description of a proposed wellbore, including the shape, orientation, depth, completion, and evaluation. Well plans might be relatively simple for vertical wellbores. Directional or horizontal wellbores require more detailed planning about where to land the well and begin directional drilling, how long the directional or horizontal section should be, and how to evaluate and complete the well. Shale gas wells, many of which are horizontal wells, require highly detailed well plans to optimize production from reservoirs that are vertically and laterally heterogeneous. |
| Shale Gas, Drilling | deviated drilling | The intentional deviation of a wellbore from the path it would naturally take. This is accomplished through the use of whipstocks, bottomhole assembly (BHA) configurations, instruments to measure the path of the wellbore in three-dimensional space, data links to communicate measurements taken downhole to the surface, mud motors and special BHA components and drill bits, including rotary steerable systems, and drill bits. The directional driller also exploits drilling parameters such as weight on bit and rotary speed to deflect the bit away from the axis of the existing wellbore. In some cases, such as drilling steeply dipping formations or unpredictable deviation in conventional drilling operations, directional-drilling techniques may be employed to ensure that the hole is drilled vertically. While many techniques can accomplish this, the general concept is simple: point the bit in the direction that one wants to drill. The most common way is through the use of a bend near the bit in a downhole steerable mud motor. The bend points the bit in a direction different from the axis of the wellbore when the entire drillstring is not rotating. By pumping mud through the mud motor, the bit turns while the drillstring does not rotate, allowing the bit to drill in the direction it points. When a particular wellbore direction is achieved, that direction may be maintained by rotating the entire drillstring (including the bent section) so that the bit does not drill in a single direction off the wellbore axis, but instead sweeps around and its net direction coincides with the existing wellbore. Rotary steerable tools allow steering while rotating, usually with higher rates of penetration and ultimately smoother boreholes. Directional drilling is common in shale reservoirs because it allows drillers to place the borehole in contact with the most productive reservoir rock. |
| Shale Gas, Geology | conventional reservoir | A reservoir in which buoyant forces keep hydrocarbons in place below a sealing caprock. Reservoir and fluid characteristics of conventional reservoirs typically permit oil or natural gas to flow readily into wellbores. The term is used to make a distinction from shale and other unconventional reservoirs, in which gas might be distributed throughout the reservoir at the basin scale, and in which buoyant forces or the influence of a water column on the location of hydrocarbons within the reservoir are not significant. |
| Shale Gas, Geology | fracture | A crack or surface of breakage within rock not related to foliation or cleavage in metamorphic rock along which there has been no movement. A fracture along which there has been displacement is a fault. When walls of a fracture have moved only normal to each other, the fracture is called a joint. Fractures can enhance permeability of rocks greatly by connecting pores together, and for that reason, fractures are induced mechanically in some reservoirs in order to boost hydrocarbon flow. Fractures may also be referred to as natural fractures to distinguish them from fractures induced as part of a reservoir stimulation or drilling operation. In some shale reservoirs, natural fractures improve production by enhancing effective permeability. In other cases, natural fractures can complicate reservoir stimulation. |
| Shale Gas, Geology | natural fracture | A crack or surface of breakage within rock not related to foliation or cleavage in metamorphic rock along which there has been no movement. A fracture along which there has been displacement is a fault. When walls of a fracture have moved only normal to each other, the fracture is called a joint. Fractures can enhance permeability of rocks greatly by connecting pores together, and for that reason, fractures are induced mechanically in some reservoirs in order to boost hydrocarbon flow. Fractures may also be referred to as natural fractures to distinguish them from fractures induced as part of a reservoir stimulation or drilling operation. In some shale reservoirs, natural fractures improve production by enhancing effective permeability. In other cases, natural fractures can complicate reservoir stimulation. |
| Shale Gas, Geology | play | An area in which hydrocarbon accumulations or prospects of a given type occur. For example the shale gas plays in North America include the Barnett, Eagle Ford, Fayetteville, Haynesville, Marcellus, and Woodford, among many others. Outside North America, shale gas potential is being pursued in many parts of Europe, Africa, Asia, and South America. |
| Shale Gas, Geology | tight gas | Gas produced from a relatively impermeable reservoir rock. Hydrocarbon production from tight reservoirs can be difficult without stimulation operations. Stimulation of tight formations can result in increased production from formations that previously might have been abandoned or been produced uneconomically. The term is generally used for reservoirs other than shales. |
| Shale Gas, Well Completions | frac fluid | An abbreviation for fracturing fluid, a fluid injected into a well as part of a stimulation operation. Fracturing fluids for shale reservoirs usually contain water, proppant, and a small amount of nonaqueous fluids designed to reduce friction pressure while pumping the fluid into the wellbore. These fluids typically include gels, friction reducers, crosslinkers, breakers and surfactants similar to household cosmetics and cleaning products; these additives are selected for their capability to improve the results of the stimulation operation and the productivity of the well. |
| Shale Gas, Well Completions | fracture | To perform a stimulation treatment, which is routine for oil and gas wells in low-permeability reservoirs. Specially engineered fluids are pumped at high pressure and rate into the reservoir interval to be treated, causing a vertical fracture to open. The wings of the fracture extend away from the wellbore in opposing directions according to the natural stresses within the formation. Proppant, such as grains of sand of a particular size, is mixed with the treatment fluid to keep the fracture open when the treatment is complete. Hydraulic fracturing creates high-conductivity communication with a large area of formation and bypasses any damage that may exist in the near-wellbore area. |
| Shale Gas, Well Completions | hydraulic fracture monitoring | A technique to track the propagation of a hydraulic fracture as it advances through a formation. Microseisms are detected, located, and displayed in time for scientists and engineers to approximate the location and propagation of the hydraulic fracture. Software provides modeling, survey design, microseismic detection and location, uncertainty analysis, data integration, and visualization for interpretation. Computer imagery is used to monitor the activity in 3D space relative to the location of the fracturing treatment. The monitored activities are animated to show progressive fracture growth and the subsurface response to pumping variations. When displayed in real time, the microseismic activity allows one to make changes to the stimulation design to ensure optimal reservoir contact. Also known as microseismic monitoring, this technique delivers information about the effectiveness of the stimulation of a reservoir that can be used to enhance reservoir development in shale gas completions. |
| Shale Gas, Well Completions, Well Workover and Intervention | hydraulic fracturing | A stimulation treatment routinely performed on oil and gas wells in low-permeability reservoirs. Specially engineered fluids are pumped at high pressure and rate into the reservoir interval to be treated, causing a vertical fracture to open. The wings of the fracture extend away from the wellbore in opposing directions according to the natural stresses within the formation. Proppant, such as grains of sand of a particular size, is mixed with the treatment fluid to keep the fracture open when the treatment is complete. Hydraulic fracturing creates high-conductivity communication with a large area of formation and bypasses any damage that may exist in the near-wellbore area. |
| Well Completions | unload | To initiate flow from a reservoir by removing the column of kill fluid from the wellbore. Several methods of unloading the well are used, including circulation of lower density fluid, nitrogen lifting and swabbing. The method used will depend on the completion design, reservoir characteristics and local availability. |
| Well Completions | upstream | Pertaining to equipment, facilities or systems located in the wellbore or production train before the surface choke or Christmas tree. |
| Well Completions | wellbore diagram | A schematic diagram that identifies the main completion components installed in a wellbore. The information included in the wellbore diagram relates to the principal dimensions of the components and the depth at which the components are located. A current wellbore diagram should be available for any well intervention operation to enable engineers and equipment operators to select the most appropriate equipment and prepare operating procedures that are compatible with any downhole restrictions. |
| Well Completions | vertical lift | The vertical distance between two points in a horizontal or deviated wellbore. Any calculations relating to wellbore pressure or downhole pump performance will be based on the vertical lift rather than the distance traveled through the wellbore. |
| Well Completions | ambient temperature | The temperature at a point or area expressed as an average of the surrounding areas or materials. Ambient surface temperature is generally given to be 70 to 80oF [21 to 27oC]-an average of daily and seasonal variations. |
| Well Completions | annular space | The space surrounding one cylindrical object placed inside another, such as the space surrounding a tubular object placed in a wellbore. |
| Well Completions | annular velocity | The linear velocity of a fluid passing through an annular space. The term critical annular velocity is often used to describe the flow rate or velocity at which entrained solids will be efficiently transported by the annular fluid. If the fluid velocity falls below the critical rate, there will be a risk of particles settling, forming beds or bridges that may obstruct the wellbore. |
| Well Completions | artificial lift | Any system that adds energy to the fluid column in a wellbore with the objective of initiating and improving production from the well. Artificial-lift systems use a range of operating principles, including rod pumping, gas lift and electric submersible pump. |
| Well Completions | atmospheric corrosion | Corrosion (oxidization) resulting from exposure of susceptible materials to oxygen and moisture. Atmospheric corrosion is generally associated with surface storage conditions, or with upper wellbore annuli that may not be fluid-filled. |
| Well Completions | axial loading | The force acting along the axis of an object. In wellbore tubulars, axial loading is typically expressed as tension or compression and may result from applied conditions such as set-down-weight, or be induced by operating conditions or variations such as changes in temperature that cause expansion or contraction of components. |
| Well Completions | back pressure | The pressure within a system caused by fluid friction or an induced resistance to flow through the system. Most process facilities require a minimum system pressure to operate efficiently. The necessary back-pressure is often created and controlled by a valve that is set to operate under the desired range of conditions. |
| Well Completions | back pressure valve | A type of check valve, typically installed in the tubing hanger, to isolate the production tubing. The back-pressure valve is designed to hold pressure from below yet enable fluids to be pumped from above, as may be required for well-control purposes. |
| Well Completions | back up ring | A supporting ring used with an O-ring, or similar seal, to prevent extrusion of the seal material under high differential pressures or excess wear under dynamic sealing conditions |
| Well Completions | back wash | To conduct reverse circulation, that is, to circulate fluid down the wellbore annulus, with returns being made up the tubing string. Reverse circulation is often used to remove debris from the wellbore since the high fluid flow rate inside the tubing string enables the recovery of large or dense debris particles that are difficult or impossible to remove with conventional circulation. |
| Well Completions | back-pressure | The pressure within a system caused by fluid friction or an induced resistance to flow through the system. Most process facilities require a minimum system pressure to operate efficiently. The necessary back-pressure is often created and controlled by a valve that is set to operate under the desired range of conditions. |
| Well Completions | back-pressure valve | A type of check valve, typically installed in the tubing hanger, to isolate the production tubing. The back-pressure valve is designed to hold pressure from below yet enable fluids to be pumped from above, as may be required for well-control purposes. |
| Well Completions | backside | A term used to describe the annulus surrounding a production tubing string above the production packer. |
| Well Completions | back-up ring | A supporting ring used with an O-ring, or similar seal, to prevent extrusion of the seal material under high differential pressures or excess wear under dynamic sealing conditions. |
| Well Completions | backwash | To conduct reverse circulation, that is, to circulate fluid down the wellbore annulus, with returns being made up the tubing string. Reverse circulation is often used to remove debris from the wellbore since the high fluid flow rate inside the tubing string enables the recovery of large or dense debris particles that are difficult or impossible to remove with conventional circulation. |
| Well Completions | balanced plug | A plug of cement or similar material placed as a slurry in a specific location within the wellbore and which has set to provide a means of pressure isolation or mechanical platform. For correct placement, the volume of slurry and the displacement fluid must be carefully calculated and measured. The correct volume ensures that the column of fluid in the tubing string is balanced by the column of fluid in the annulus. |
| Well Completions | ball catcher | A downhole device or assembly used to catch and retain balls used to actuate ball-operated tools or equipment. Following activation, some ball-operated tools incorporate a means of ejecting the activation ball to regain a fullbore flow path. In such cases, the ball can be retained in a ball catcher. |
| Well Completions | ball operated | Describing a mechanism or system that is actuated by a ball that is dropped or pumped through the tubing string. Once located on a landing seat, the tool mechanism is generally actuated by hydraulic pressure. |
| Well Completions | ball-operated | Describing a mechanism or system that is actuated by a ball that is dropped or pumped through the tubing string. Once located on a landing seat, the tool mechanism is generally actuated by hydraulic pressure. |
| Well Completions | barefoot completion | A well completion that has no casing or liner set across the reservoir formation, allowing the produced fluids to flow directly into the wellbore. This type of completion suffers the major disadvantage that the sandface is unsupported and may collapse. Also, without any casing or liner installed, selective treatments or remedial work within the reservoir section are more difficult. |
| Well Completions | barite plug | A plug made from barite weighting materials that is placed at the bottom of a wellbore. Unlike a cement plug, the settled solids do not set solid, yet a barite plug can provide effective and low-cost pressure isolation. A barite plug is relatively easy to remove and is often used as a temporary facility for pressure isolation or as a platform enabling the accurate placement of treatments above the plug. |
| Well Completions | beam pump | An artificial-lift pumping system using a surface power source to drive a downhole pump assembly. A beam and crank assembly creates reciprocating motion in a sucker-rod string that connects to the downhole pump assembly. The pump contains a plunger and valve assembly to convert the reciprocating motion to vertical fluid movement. |
| Well Completions | bean choke | A fixed choke used to control the flow of fluids, usually mounted on or close to the Christmas tree. A bean choke contains a replaceable insert, or bean, made from hardened steel or similar durable material. The insert is manufactured with a precise diameter hole that forms the choke through which all fluids must pass. Choke inserts are available in a complete range of sizes, generally identified by choke diameter stated in 64ths of an inch; for example, a “32 bean” is equivalent to a 1/2-in. choke diameter. |
| Well Completions | blank pipe | A short section of plain tubing used to separate or space-out specialized components in a completion assembly. Blank pipe is commonly used in sand control completions where intervals of screen are separated by short sections of blank pipe. The term is also used to describe unperforated sections of casing or liner. |
| Well Completions | blow down | To vent gas from a well or production system. Wells that have been shut in for a period frequently develop a gas cap caused by gas percolating through the fluid column in the wellbore. It is often desirable to remove or vent the free gas before starting well intervention work. |
| Well Completions | blowdown | To vent gas from a well or production system. Wells that have been shut in for a period frequently develop a gas cap caused by gas percolating through the fluid column in the wellbore. It is often desirable to remove or vent the free gas before starting well intervention work. |
| Well Completions | bottomhole choke | A downhole device used to control fluid flow under downhole conditions. Downhole chokes are generally removable with slickline intervention and are located in a landing nipple in the tubing string. |
| Well Completions | bottomhole pressure | The downhole pressure, measured or calculated at a point of interest, generally the top of the perforated interval. |
| Well Completions | bottomhole sampler | A tool or assembly used to retrieve samples of fluids or fill material from the wellbore. Used as a treatment design aid, the retrieved samples can be checked for compatibility with the selected treatment fluid to verify performance or identify any undesirable reactions |
| Well Completions | bottomhole temperature | The downhole temperature measured or calculated at a point of interest. The BHT, without reference to circulating or static conditions, is typically associated with producing conditions. |
| Well Completions | breakdown pressure | The pressure at which the rock matrix of an exposed formation fractures and allows fluid to be injected. The breakdown pressure is established before determining reservoir treatment parameters. Hydraulic fracturing operations are conducted above the breakdown pressure, while matrix stimulation treatments are performed with the treatment pressure safely below the breakdown pressure. |
| Well Completions | bridge | A wellbore obstruction caused by a buildup of material such as scale, wellbore fill or cuttings that can restrict wellbore access or, in severe cases, eventually close the wellbore. |
| Well Completions | brine | A water-based solution of inorganic salts used as a well-control fluid during the completion and workover phases of well operations. Brines are solids free, containing no particles that might plug or damage a producing formation. In addition, the salts in brine can inhibit undesirable formation reactions such as clay swelling. Brines are typically formulated and prepared for specific conditions, with a range of salts available to achieve densities ranging from 8.4 to over 20 lbm/gal (ppg) [1.0 to 2.4 g/cmo]. Common salts used in the preparation of simple brine systems include sodium chloride, calcium chloride and potassium chloride. More complex brine systems may contain zinc, bromide or iodine salts. These brines are generally corrosive and costly. |
| Well Completions | bring in the well | To prepare a well for production by initiating flow from the reservoir. This is the final phase of a completion or workover process. |
| Well Completions | bubble flow | A multiphase fluid flow regime characterized by the gas phase being distributed as bubbles through the liquid phase. In a producing wellbore where the bubbles are uniformly distributed, there is little relative motion between the phases. Where the bubbles congregate and combine to form a less uniform distribution of the gas phase, some slippage will occur between the phases with the gas tending to cut through the liquid phase. |
| Well Completions | bump the plug | To observe the increase in pump pressure indicating that the top cement plug has been placed on the bottom plug or landing collar. Bumping the plug concludes the cementing operation. |
| Well Completions | buoyancy | The upward force acting on an object placed in a fluid. The buoyancy force is equal to the weight of fluid displaced by the object. Buoyancy can have significant effects over a wide range of completion and workover activities, especially in cases in which the wellbore and tubing string contain liquid and gas. Any change in the relative volumes or fluid levels will change the buoyancy forces |
| Well Completions | buttress thread | A thread profile used on casing or liner tubulars. Buttress threads are square-cut and create a hydraulic seal through the interference fit of the mating threads. |
| Well Completions | calcium carbonate plug | A temporary plug formulated with graded granules or flakes of calcium carbonate that are generally circulated into place as a slurry and allowed to settle out. Calcium carbonate plugs commonly are used to isolate lower production zones, either to enable a column of well control fluid to be placed, or to provide some protection for a lower zone while treating upper zones. Because of their high reaction rate with hydrochloric acid, calcium carbonate plugs are easily removed using common acidizing materials and equipment. |
| Well Completions | cap the well | To regain control of a blowout well by installing and closing a valve on the wellhead. |
| Well Completions | carbon dioxide corrosion | The deterioration of metal components resulting from contact with a gas or solution containing carbon dioxide. |
| Well Completions | carbonate scale | A common type of mineral deposit that is often found on wellbore tubulars and components as the saturation of produced water is affected by changing temperature and pressure conditions in the production conduit. Carbonate scales have a high dissolution rate in common oilfield acids and generally can be effectively removed using acid or chemical treatments. Scale inhibition techniques also may be used to prevent scale formation. In the majority of cases, scale prevention is simpler and more cost-effective than attempting a cure. |
| Well Completions | carrier fluid | A fluid that is used to transport materials into or out of the wellbore. Carrier fluids typically are designed according to three main criteria: the ability to efficiently transport the necessary material (such as pack sand during a gravel pack), the ability to separate or release the materials at the correct time or place, and compatibility with other wellbore fluids while being nondamaging to exposed formations. |
| Well Completions | cased hole | A wellbore lined with a string of casing or liner. Although the term can apply to any hole section, it is often used to describe techniques and practices applied after a casing or liner has been set across the reservoir zone, such as cased-hole logging or cased-hole testing. |
| Well Completions | casing | Steel pipe cemented in place during the construction process to stabilize the wellbore. The casing forms a major structural component of the wellbore and serves several important functions: preventing the formation wall from caving into the wellbore, isolating the different formations to prevent the flow or crossflow of formation fluid, and providing a means of maintaining control of formation fluids and pressure as the well is drilled. The casing string provides a means of securing surface pressure control equipment and downhole production equipment, such as the drilling blowout preventer (BOP) or production packer. Casing is available in a range of sizes and material grades |
| Well Completions | casing bowl | A wellhead component or a profile formed in wellhead equipment in which the casing hanger is located when a casing string has been installed. The casing bowl incorporates features to secure and seal the upper end of the casing string and frequently provides a port to enable communication with the annulus. |
| Well Completions | casing burst pressure | The theoretical internal pressure differential at which a joint of casing will fail. The casing burst pressure value is a key consideration in many well-control and contingency operations and is a major factor in the well design process. |
| Well Completions | casing completion | A completion configuration in which a production casing string is set across the reservoir interval and perforated to allow communication between the formation and wellbore. The casing performs several functions, including supporting the surrounding formation under production conditions, enabling control of fluid production through selective perforation and allowing subsequent or remedial isolation by packers, plugs or special treatments. |
| Well Completions | casing hanger | The subassembly of a wellhead that supports the casing string when it is run into the wellbore. The casing hanger provides a means of ensuring that the string is correctly located and generally incorporates a sealing device or system to isolate the casing annulus from upper wellhead components. |
| Well Completions | casing hardware | A generic term used to describe equipment attached to, and run with, the casing string. Commonly used casing hardware includes guide or float shoes, float or landing collars, centralizers, scratchers and cement baskets. More specialized casing hardware may include stage-cementing collars, differential fill-up equipment and other specialized equipment to help achieve successful placement and cementation of the casing string. |
| Well Completions | casing joint | A length of steel pipe, generally around 40 ft [13 m] long with a threaded connection at each end. Casing joints are assembled to form a casing string of the correct length and specification for the wellbore in which it is installed. |
| Well Completions | casing pressure | A term used in well-control operations, typically during the drilling or workover phases of a well, to describe the pressure in the drillpipe or tubing annulus. |
| Well Completions | casing reciprocation | Movement applied to the casing string during the cementing operation to help in removal of drilling fluid and efficient placement of the cement slurry. |
| Well Completions | casing shoe test | A pressure test applied to the formation directly below a casing shoe. The test is generally conducted soon after drilling resumes after an intermediate casing string has been set. The purpose of the test is to determine the maximum pressures that may be safely applied without the risk of formation breakdown. The results of the test are used to design the mud program for the subsequent hole section and to set safe limits on casing shut-in or choke pressures for well-control purposes. |
| Well Completions | casing spool | A wellhead component used in flanged wellhead assemblies to secure the upper end of a casing string. Casing spools or bowls are available in a wide range of sizes and pressure ratings and are selected to suit the specific conditions. |
| Well Completions | casing swage | A short crossover joint used between two sizes or specifications of casing. A circulating swage is an adapter that enables a temporary circulating line to be rigged to the top of the casing string, allowing circulation of fluids to help properly locate the casing string. |
| Well Completions | casing test | A general term used to describe a drillstem test (DST) performed in cased hole. |
| Well Completions | casing thread | The threadform found on casing joints. In addition to providing mechanical or structural strength, the casing thread must be compatible with the pressures and fluids associated with the application. Some advanced threadforms incorporate a gas seal. |
| Well Completions | casing valve | A valve installed in the wellhead assembly to provide access to the casing annulus of non-producing casings. |
| Well Completions | cement accelerator | A chemical additive mixed with cement slurry to reduce the time required for the set cement to develop sufficient compressive strength to enable drilling operations to continue. Accelerators are generally used in near-surface applications in which the temperature is relatively low. |
| Well Completions | cement additive | Chemicals and materials added to a cement slurry to modify the characteristics of the slurry or set cement. Cement additives may be broadly categorized as accelerators, retarders, fluid-loss additives, dispersants, extenders, weighting agents, lost circulation additives and special additives designed for specific operating conditions. Cement additives are commonly available in powder or liquid form, enabling some flexibility in how the cement slurry is prepared. |
| Well Completions | cement dispersant | A chemical additive that reduces the cement slurry viscosity to improve fluid flow characteristics. Adequately dispersed cement slurries exhibit improved fluid-loss control, can displace drilling fluid more efficiently and be successfully mixed and pumped at higher densities. |
| Well Completions | cement extender | A chemical additive or inert material used to decrease the density or increase the yield of a cement slurry. The slurry yield is typically expressed in cubic feet of slurry per sack of cement. Increasing the yield reduces the cost per volume of cement slurry, while reducing the slurry density reduces the hydrostatic pressure of the cement column, enabling weak zones to be successfully cemented and isolated. |
| Well Completions | cement plug | A balanced plug of cement slurry placed in the wellbore. Cement plugs are used for a variety of applications including hydraulic isolation, provision of a secure platform, and in window-milling operations for sidetracking a new wellbore. |
| Well Completions | cement retainer | An isolation tool set in the casing or liner that enables treatments to be applied to a lower interval while providing isolation from the annulus above. Cement retainers are typically used in cement squeeze or similar remedial treatments. A specially profiled probe, known as a stinger, is attached to the bottom of the tubing string to engage in the retainer during operation. When the stinger is removed, the valve assembly isolates the wellbore below the cement retainer. |
| Well Completions | cement retarder | A chemical agent used to increase the thickening time of cement slurries to enable proper placement. The need for cement retardation increases with depth due to the greater time required to complete the cementing operation and the effect of increased temperature on the cement-setting process. |
| Well Completions | cement squeeze | A remedial cementing operation designed to force cement into leak paths in wellbore tubulars. The required squeeze pressure is achieved by carefully controlling pump pressure. Squeeze cementing operations may be performed to repair poor primary cement jobs, isolate perforations or repair damaged casing or liner. |
| Well Completions | centralizer | A device fitted with a hinged collar and bowsprings to keep the casing or liner in the center of the wellbore to help ensure efficient placement of a cement sheath around the casing string. If casing strings are cemented off-center, there is a high risk that a channel of drilling fluid or contaminated cement will be left where the casing contacts the formation, creating an imperfect seal. |
| Well Completions | charged zone | A formation interval that has become overpressured by the injection of drilling or treatment fluids. |
| Well Completions | check valve | A mechanical device that permits fluid to flow or pressure to act in one direction only. Check valves are used in a variety of oil and gas industry applications as control or safety devices. Check valve designs are tailored to specific fluid types and operating conditions. Some designs are less tolerant of debris, while others may obstruct the bore of the conduit or tubing in which the check valve is fitted. |
| Well Completions | chemical wash | A fluid, generally water-based, to thin and disperse mud in preparation for cementing. The chemical wash is pumped ahead of the cement slurry to help ensure effective mud removal and efficient cement placement. Other specialized chemical washes may be used in the remedial treatment of scales or paraffin deposits in production tubulars. |
| Well Completions | choke | A device incorporating an orifice that is used to control fluid flow rate or downstream system pressure. Chokes are available in several configurations for both fixed and adjustable modes of operation. Adjustable chokes enable the fluid flow and pressure parameters to be changed to suit process or production requirements. Fixed chokes do not provide this flexibility, although they are more resistant to erosion under prolonged operation or production of abrasive fluids. |
| Well Completions | Christmas tree | An assembly of valves, spools, pressure gauges and chokes fitted to the wellhead of a completed well to control production. Christmas trees are available in a wide range of sizes and configurations, such as low- or high-pressure capacity and single- or multiple-completion capacity. |
| Well Completions | circulation device | A completion component, generally included in the lower assembly near the packer, that allows communication between the tubing and annulus. Circulation devices enable the circulation of fluids for well control or kickoff purposes. |
| Well Completions | cleanup | A period of controlled production, generally following a stimulation treatment, during which time treatment fluids return from the reservoir formation. Depending on the treatment, the cleanup period can be relatively short and uncomplicated. However, following more complex treatments such as gravel pack or hydraulic fracturing, the cleanup process should be conducted carefully to avoid jeopardizing the long-term efficiency of the treatment. |
| Well Completions | clear brine | A water-based solution of inorganic salts used as a well-control fluid during the completion and workover phases of well operations. Brines are solids free, containing no particles that might plug or damage a producing formation. In addition, the salts in brine can inhibit undesirable formation reactions such as clay swelling. Brines are typically formulated and prepared for specific conditions, with a range of salts available to achieve densities ranging from 8.4 to over 20 lbm/gal (ppg) [1.0 to 2.4 g/cmo]. Common salts used in the preparation of simple brine systems include sodium chloride, calcium chloride and potassium chloride. More complex brine systems may contain zinc, bromide or iodine salts. These brines are generally corrosive and costly. |
| Well Completions | close in | To close a valve to stop or isolate fluid flow. The term is most commonly applied to “closing-in the well,” meaning isolation of the wellbore by closing the master valve. |
| Well Completions | close-in | To close a valve to stop or isolate fluid flow. The term is most commonly applied to “closing-in the well,” meaning isolation of the wellbore by closing the master valve. |
| Well Completions | coiled tubing completion | A completion that utilizes coiled tubing as the production conduit, or as a means of conveying and installing completion equipment or components. Since the coiled tubing string is continuous, problems associated with connections are avoided. Also, the pressure-control equipment used on coiled tubing operations enables work to be safely conducted on live wells. |
| Well Completions | collar | A threaded coupling used to join two lengths of pipe such as production tubing, casing or liner. The type of thread and style of collar varies with the specifications and manufacturer of the tubing. |
| Well Completions | collar locator | A downhole tool or logging device used to detect and track (log) casing or tubing collars across a zone of interest, typically for correlation purposes. Most collar locators detect the magnetic anomaly created by the mass of the steel collar and transmit a signal to surface-display and depth correlation equipment. |
| Well Completions | collar lock | A type of lock designed to be set in the recess of a tubing collar. Collar locks are compatible only with conventional thread connections where a space exists between the two tubing joints. Premium tubing grades have flush internal surfaces with no space to enable setting of the retaining dogs. |
| Well Completions | collar log | A log showing the depth or relative position of casing or tubing collars that is used to correlate depth for depth-sensitive applications such as perforating or isolation treatments. Indications are provided by a collar locator tool and correlations are made with previous baseline logs, such as the gamma ray log, or the casing or tubing running tally prepared during the installation process. |
| Well Completions | commingled flow | A term used to describe the flow pattern where two or more fluid phases may be present in a relatively even distribution. The flow rate and conduit geometry may cause an apparent mixing of the phases. However, if the flow characteristics are changed through flow rate or conduit geometry, fluid separation may occur. Fine solids also may be entrained in a commingled flow. Commingled flow may also describe the production of fluid from two or more separate zones through a single conduit. |
| Well Completions | completion | A generic term used to describe the assembly of downhole tubulars and equipment required to enable safe and efficient production from an oil or gas well. The point at which the completion process begins may depend on the type and design of well. However, there are many options applied or actions performed during the construction phase of a well that have significant impact on the productivity of the well. |
| Well Completions | completion fluid | A solids-free liquid used to “complete” an oil or gas well. This fluid is placed in the well to facilitate final operations prior to initiation of production, such as setting screens production liners, packers, downhole valves or shooting perforations into the producing zone. The fluid is meant to control a well should downhole hardware fail, without damaging the producing formation or completion components. Completion fluids are typically brines (chlorides, bromides and formates), but in theory could be any fluid of proper density and flow characteristics. The fluid should be chemically compatible with the reservoir formation and fluids, and is typically filtered to a high degree to avoid introducing solids to the near-wellbore area. Seldom is a regular drilling fluid suitable for completion operations due to its solids content, pH and ionic composition. Drill-in fluids can, in some cases, be suitable for both purposes. |
| Well Completions | completion skin | An indicator used to determine the effect that key completion components have on the production efficiency of a well. If one or more of the well-completion components create a localized pressure drop, the effect may be a reduction in the production capability of the well. Such conditions are evident as completion skin. |
| Well Completions | compression-set packer | A type of downhole packer that is activated or set by applying compressive force to the packer assembly. In most cases, this is achieved with set-down weight from the running string, which is controlled by the driller or operator observing the weight indicator on the rig or coiled tubing unit. |
| Well Completions | conductor pipe | A short string of large-diameter casing set to support the surface formations. The conductor pipe is typically set soon after drilling has commenced since the unconsolidated shallow formations can quickly wash out or cave in. Where loose surface soil exists, the conductor pipe may be driven into place before the drilling commences. |
| Well Completions | coning | The change in oil-water contact or gas-oil contact profiles as a result of drawdown pressures during production. Coning occurs in vertical or slightly deviated wells and is affected by the characteristics of the fluids involved and the ratio of horizontal to vertical permeability. |
| Well Completions | contingency plan | A key component of the operational planning process that takes account of reasonably foreseeable events that may prevent completion of normal operations. The formal plans and procedures for any operation should include normal operating procedures, contingency plans and emergency responses |
| Well Completions | continuous gas lift | An artificial-lift method in which the gas-lift system is operated on a continuous basis to sustain liquid production at an efficient rate. |
| Well Completions | control line | A small-diameter hydraulic line used to operate downhole completion equipment such as the surface controlled subsurface safety valve (SCSSV). Most systems operated by control line operate on a fail-safe basis. In this mode, the control line remains pressurized at all times. Any leak or failure results in loss of control line pressure, acting to close the safety valve and render the well safe. |
| Well Completions | corrosion-resistant alloy (CRA) | A specially formulated material used for completion components in wells likely to present corrosion problems. Corrosion-resistant alloys can be formulated for a wide range of aggressive wellbore conditions. However, cost generally determines the viability of any particular completion design. Alloys with a high chrome content are commonly used for tubing strings. |
| Well Completions | cresting | The change in oil-water or gas-oil contact profiles as a result of drawdown pressures during production. Cresting occurs in horizontal or highly deviated wells and is affected by the characteristics of the fluids involved and the ratio of horizontal to vertical permeability. |
| Well Completions | cross over | A short subassembly used to enable two components with different thread types or sizes to be connected. |
| Well Completions | crossflow | A condition that exists when two production zones with dissimilar pressure characteristics are allowed to communicate during production. Reservoir fluid from the high-pressure zone will flow preferentially to the low-pressure zone rather than up the production conduit unless the production parameters are closely controlled. |
| Well Completions | crossover | A short subassembly used to enable two components with different thread types or sizes to be connected. |
| Well Completions | crossover service tool | A specialized tool, frequently used in gravel-pack operations, that enables the circulation of the treatment fluid (slurry) from the internal flow path of the tool string into the annulus area to be packed. The returned carrier fluid enters the internal flow path at the base of the tool before crossing over to the annulus above the packer assembly, isolating the annulus. |
| Well Completions | crown valve | The topmost valve on a Christmas tree that provides vertical access to the wellbore. |
| Well Completions | crystallization temperature | The temperature at which crystals will appear in a brine solution of a given density as it cools. In preparing oilfield brines, the crystallization temperature can be used to indicate the maximum saturation (density) achievable for a brine solution at a given temperature. |
| Well Completions | depleted zone | An isolated section of reservoir in which the pressure has dropped below that of adjacent zones or the main body of the reservoir formation. |
| Well Completions | depth control | The procedures and equipment used to measure and correlate depth to ensure that a treatment is applied at the correct position within the wellbore. |
| Well Completions | depth reference point | A point within the wellbore from which accurate depth measurements can be made, such as the end of the tubing string, or a nipple or similar completion component. |
| Well Completions | down stream | Pertaining to equipment, facilities or systems that are located in the production train after the surface choke or Christmas tree. |
| Well Completions | downhole gauge | A pressure gauge, typically run on slickline, used to measure and record downhole pressure. Downhole gauges are commonly used in assessing the downhole pressure under various flowing conditions, the basis of pressure transient analysis. |
| Well Completions | downhole safety valve (DSV) | A downhole device that isolates wellbore pressure and fluids in the event of an emergency or catastrophic failure of surface equipment. The control systems associated with safety valves are generally set in a fail-safe mode, such that any interruption or malfunction of the system will result in the safety valve closing to render the well safe. Downhole safety valves are fitted in almost all wells and are typically subject to rigorous local or regional legislative requirements. |
| Well Completions | downstream | Pertaining to equipment, facilities or systems that are located in the production train after the surface choke or Christmas tree. |
| Well Completions | drain hole | A hole or short conduit through which fluids can flow. In equipment applications, a drainhole is generally made to avoid the buildup of pressure within a nonpressure area, such as may occur in the event of a leak in a pressure housing within a tool assembly. |
| Well Completions | drainhole | A hole or short conduit through which fluids can flow. In equipment applications, a drainhole is generally made to avoid the buildup of pressure within a nonpressure area, such as may occur in the event of a leak in a pressure housing within a tool assembly. |
| Well Completions | drift | An accurately machined device that is pulled through the casing, tubulars and completion components to ensure minimum-diameter specifications are within tolerance, as described in definition 2. While this tool is usually of a short length, the well planner may specify a special drift that either has a longer length or a nonstandard outside diameter. The large-diameter casing drifts are frequently known as “rabbits. |
| Well Completions | drillable packer | A packer assembly that can be removed from the wellbore only by drilling or milling. Drillable packers, and similar tools such as bridge plugs, are typically made from cast iron, aluminum, plastic or similar brittle materials. |
| Well Completions | driller’s depth | The depth of a well or features within the wellbore as measured while drilling. The measured length of each joint of drillpipe or tubing is added to provide a total depth or measurement to the point of interest. Drillers depth is the first depth measurement of a wellbore and is taken from the rotary table level on the rig floor. In most cases, subsequent depth measurements, such as those made during the well completion phase, are corrected to the wellhead datum that is based on drillers depth. |
| Well Completions | dry gas | Gas produced from a well that produces little or no condensate or reservoir liquids. The production of liquids from gas wells complicates the design and operation of surface process facilities required to handle and export the produced gas. |
| Well Completions | dual completion | A single wellbore having tubulars and equipment that enable production from two segregated zones. In most cases, two tubing strings will be used to provide the necessary level of control and safety for the fluids from both zones. However, in some simple dual completions, the second or upper zone is produced up the tubing-casing annulus. |
| Well Completions | electric submersible pump | An artificial-lift system that utilizes a downhole pumping system that is electrically driven. The pump typically comprises several staged centrifugal pump sections that can be specifically configured to suit the production and wellbore characteristics of a given application. Electrical submersible pump systems are a common artificial-lift method, providing flexibility over a range of sizes and output flow capacities. |
| Well Completions | equalizing valve | A device that is operated to equalize the pressure across a valve, plug or similar pressure or fluid isolation barrier. The operating mechanism on many pressure-sealing devices is rendered inoperable once the mechanism has been activated by pressure. In such cases, the pressure across the pressure barrier must be equalized before the barrier can be removed. |
| Well Completions | expansion joint | A device or completion component designed to enable relative movement between two fixed assemblies in the event of thermal expansion or contraction. The forces generated by thermal expansion or contraction can be significant. Expansion joints within the completion assembly prevent any movement or forces being transmitted to fixed components such as packers or tubing hangers. |
| Well Completions | expendable plug | A temporary plug, inserted in the completion assembly before it is run, to enable pressure testing of the completed string. With the operation complete, the expendable plug can be pumped out of the assembly, thereby avoiding a separate retrieval run. |
| Well Completions | external pulling tool | A downhole tool used to pull or retrieve temporary plugs or similar equipment. The external pulling tool engages on the external surfaces of the item to be retrieved. |
| Well Completions | external upset | A type of tubing connection in which the external diameter of the tubing joint is larger adjacent to the tubing connection to provide the necessary strength. The internal tubing surface is flush to enable good fluid-flow characteristics. |
| Well Completions | filtered brine | A completion or workover fluid that has been treated to remove debris and fine particles that may cause near-wellbore damage if allowed to enter the reservoir formation. |
| Well Completions | fines migration | The movement of fine clay, quartz particles or similar materials within the reservoir formation due to drag forces during production. Fines migration may result from an unconsolidated or inherently unstable formation, or from use of an incompatible treatment fluid that liberates fine particles. Unlike sand migration that is best stabilized, the material mobilized in fines migration should be produced to avoid near-wellbore damage. Fines migration causes particles suspended in the produced fluid to bridge the pore throats near the wellbore, reducing well productivity. Fines can include different materials such as clays (phyllosilicates smaller than 4 microns) and silts (silicates or aluminosilicates with sizes ranging from 4 to 64 microns). Kaolinite and illite are the most common migrating clays. Damage created by fines usually is located within a radius of 3 to 5 ft [1 to 2 m] of the wellbore, but can also occur in gravel-pack completions. In sandstone formations, hydrofluoric acid [HF] mixtures are used to dissolve fines. In carbonate formations, the goal is not to dissolve but rather to disperse fines in the wormholes, so hydrochloric [HCl] acid is used as the treatment fluid. |
| Well Completions | fixed choke | A device used to control the flow of fluids by directing flow through a restriction or hole of a fixed size. The fluid characteristics and the pressure differential across the choke determine the flow rate through a fixed choke. |
| Well Completions | flag joint | A joint of tubing or casing included in the string at a known position to provide a reference point for further operations. A short pup joint that registers clearly in a collar locator log is a common flag joint. |
| Well Completions | flange | A connection profile used in pipe work and associated equipment to provide a means of assembling and disassembling components. Most oilfield flanges feature a bolt-hole pattern to allow the joint to be secured and a gasket profile to ensure a pressure-tight seal. The design and specification of a flange relates to the size and pressure capacity of the equipment to which it is fitted. |
| Well Completions | flange up | The process of assembling flanged components such as pressure-control equipment. |
| Well Completions | float collar | A component installed near the bottom of the casing string on which cement plugs land during the primary cementing operation. It typically consists of a short length of casing fitted with a check valve. This device may be a flapper-valve type, a spring-loaded ball valve or other type. The check-valve assembly fixed within the float collar prevents flowback of the cement slurry when pumping is stopped. Without a float collar, the cement slurry placed in the annulus could U-tube, or reverse flow back into the casing. The greater density of cement slurries than the displacement mud inside the casing causes the U-tube effect. |
| Well Completions | float shoe | A rounded profile component attached to the downhole end of a casing string. An integral check valve in the float shoe prevents reverse flow, or U-tubing, of cement slurry from the annulus into the casing or flow of wellbore fluids into the casing string as it is run. The float shoe also guides the casing toward the center of the hole to minimize hitting rock ledges or washouts as the casing is run into the wellbore. The float shoe reduces hook weight. With controlled or partial fill-up as the string is run, the casing string can be floated into position, avoiding the need for the rig to carry the entire weight of the casing string. The outer portions of the float shoe are made of steel and generally match the casing size and threads, although not necessarily the casing grade. The inside (including the taper) is usually made of cement or thermoplastic, since this material must be drilled out if the well is to be deepened beyond the casing point. |
| Well Completions | flow check | A test performed to ensure stable well conditions or the integrity of a plug, valve or flow-control device. In most cases, the flow check involves observing stable fluid levels or conditions for a prescribed period. |
| Well Completions | flow coupling | A relatively short, heavy-walled completion component installed in areas where turbulence is anticipated. The additional wall thickness prevents early failures due to erosion in the turbulent flow area. Flow couplings are typically installed above and below completion components, such as landing nipples, that may affect the flow. |
| Well Completions | fluid invasion | A general term to describe the presence of a particular fluid in an undesirable area, such as the movement of drilling mud into a section of the reservoir formation. |
| Well Completions | flush joint | A type of tubing connection in which the internal or external surfaces are the same diameter throughout the tubing joint. Internal flush joints are most common, offering no restriction to fluid flow. Externally flush joints are typically used in more specialized applications, such as washover pipe for fishing operations, to allow adequate outer diameter (OD) clearance. |
| Well Completions | formation damage | A general term to describe the reduction in permeability to the near-wellbore area of a reservoir formation. There are several recognized damage mechanisms, such as the invasion of incompatible fluids swelling the formation clays, or fine solids from dirty fluids plugging the formation matrix. Because formation damage can significantly affect the productivity of any well, adequate precautions should be exercised to avoid damage during all phases in the life of a well. |
| Well Completions | formation fluid | Any fluid that occurs in the pores of a rock. Strata containing different fluids, such as various saturations of oil, gas and water, may be encountered in the process of drilling an oil or gas well. Fluids found in the target reservoir formation are referred to as reservoir fluids. |
| Well Completions | free water | The aqueous phase that separates from a slurry or mixture of fluids. In cementing operations, free water is undesirable since channels tend to form through the set cement, providing potential gas migration paths. When processing reservoir fluids, the water that separates easily under gravity separation is known as free water. In some cases, additional water may be locked in an emulsion, contributing to the aqueous phase but not available as free water. |
| Well Completions | fullbore | A description of the internal area and surfaces of a tool or tubular assembly through which there is an unimpeded internal diameter. In some cases, fullbore is used to describe the form of a nominal internal diameter that extends over the length of the tool or interval without any variation. In other applications, the term simply implies an ability to pass a ball or similar item of a stated drift diameter through the assembly. |
| Well Completions | gas bearing | Relating to a formation or interval containing gas, either dissolved in the formation fluid or as free gas. The term is occasionally used to describe wellbore fluids containing dissolved gas. |
| Well Completions | gas cap | The gas that accumulates in the upper portions of a reservoir where the pressure, temperature and fluid characteristics are conducive to free gas. The energy provided by the expansion of the gas cap provides the primary drive mechanism for oil recovery in such circumstances. |
| Well Completions | gas coning | Change in the gas-oil contact profile as a result of drawdown pressures during production. Coning occurs in vertical or slightly deviated wells and is affected by the characteristics of the fluids involved and the ratio of horizontal to vertical permeability. |
| Well Completions | gas drive | A primary recovery mechanism for oil wells containing dissolved and free gas, whereby the energy of the expanding gas is used to drive the oil from the reservoir formation into the wellbore. |
| Well Completions | gas injection | A reservoir maintenance or secondary recovery method that uses injected gas to supplement the pressure in an oil reservoir or field. In most cases, a field will incorporate a planned distribution of gas-injection wells to maintain reservoir pressure and effect an efficient sweep of recoverable liquids. |
| Well Completions | gas lift | An artificial-lift method in which gas is injected into the production tubing to reduce the hydrostatic pressure of the fluid column. The resulting reduction in bottomhole pressure allows the reservoir liquids to enter the wellbore at a higher flow rate. The injection gas is typically conveyed down the tubing-casing annulus and enters the production train through a series of gas-lift valves. The gas-lift valve position, operating pressures and gas injection rate are determined by specific well conditions. |
| Well Completions | gas lift mandrel | A gas-lift system component that is assembled with the production tubing string to provide a means of locating gas-lift valves. The position or depth of the gas lift valves is crucial to the efficient operation of the entire system. Consequently, proper assembly of the gas lift mandrels within the completion tubulars is essential. A port in the gas-lift mandrel provides communication between the lift-gas supply in the tubing annulus and the production-tubing conduit. |
| Well Completions | gas lift valve | A valve used in a gas-lift system to control the flow of lift gas into the production tubing conduit. The gas-lift valve is located in the gas-lift mandrel, which also provides communication with the lift gas supply in the tubing annulus. Operation of the gas lift valve is determined by preset opening and closing pressures in the tubing or annulus, depending on the specific application. |
| Well Completions | gas lock | A condition in pumping and processing equipment caused by the induction of free gas. The compressible gas interferes with the proper operation of valves and other pump components, preventing the intake of fluid. |
| Well Completions | gas separator | A device used to separate entrained gas from production liquids. Surface processing facilities generally use gas separators to render the liquids safe for further processing or disposal. Gas-separation equipment is also used in downhole applications, such as the protection of pumping equipment against gas lock by separating and redirecting free gas at the pump suction or inlet. |
| Well Completions | gas well | A well that primarily produces natural gas. |
| Well Completions | gas-bearing | Relating to a formation or interval containing gas, either dissolved in the formation fluid or as free gas. The term is occasionally used to describe wellbore fluids containing dissolved gas. |
| Well Completions | gasdrive | A primary recovery mechanism for oil wells containing dissolved and free gas, whereby the energy of the expanding gas is used to drive the oil from the reservoir formation into the wellbore. |
| Well Completions | gas-lift mandrel | A gas-lift system component that is assembled with the production tubing string to provide a means of locating gas-lift valves. The position or depth of the gas lift valves is crucial to the efficient operation of the entire system. Consequently, proper assembly of the gas lift mandrels within the completion tubulars is essential. A port in the gas-lift mandrel provides communication between the lift-gas supply in the tubing annulus and the production-tubing conduit. |
| Well Completions | gas-lift valve | A valve used in a gas-lift system to control the flow of lift gas into the production tubing conduit. The gas-lift valve is located in the gas-lift mandrel, which also provides communication with the lift gas supply in the tubing annulus. Operation of the gas lift valve is determined by preset opening and closing pressures in the tubing or annulus, depending on the specific application. |
| Well Completions | gate valve | A type of valve that incorporates a sliding gate to block fluid flow. The design of the valve operating and sealing systems typically requires that gate valves should be operated either fully open or fully closed. |
| Well Completions | gauge pressure | The measured pressure within a system in which the pressure gauge reads 0 psi at nominal atmospheric pressure. |
| Well Completions | gauge ring | A precisely machined test device, typically fabricated from steel or similar durable material, having a specified internal or external diameter. The gauge ring is used to confirm the dimensional compatibility of tools and equipment that must pass through restrictions of a certain diameter. |
| Well Completions | gravel pack | A sand-control method used to prevent production of formation sand. In gravel pack operations, a steel screen is placed in the wellbore and the surrounding annulus packed with prepared gravel of a specific size designed to prevent the passage of formation sand. The primary objective is to stabilize the formation while causing minimal impairment to well productivity. |
| Well Completions | gravel pack screen | A metal filter assembly used to support and retain the sand placed during gravel pack operations. A range of sizes and screen configurations is available to suit the characteristics of the wellbore, production fluid and the formation sand. |
| Well Completions | gravel-pack screen | A metal filter assembly used to support and retain the sand placed during gravel pack operations. A range of sizes and screen configurations is available to suit the characteristics of the wellbore, production fluid and the formation sand. |
| Well Completions | holdup depth | The point or depth at which a tool or drift of a specific size can no longer pass through the wellbore. A higher than expected holdup depth may result from scale, fill, distortion of the wellbore tubulars or formation movement in an openhole completion. |
| Well Completions | hookwall packer | A type of packer than utilizes an assembly of friction blocks and slips to set and anchor the packer on the casing or liner wall. Hookwall packers generally are run on tubing or drillpipe and typically require some rotation of the packer assembly to activate or set the packer slips. Subsequent application of tension or compression, depending on packer design, will set the packer elements. |
| Well Completions | horizontal tree | A Christmas tree design for subsea applications, configured with the master valves and flow-control equipment on a horizontal axis to minimize the assembly height. |
| Well Completions | hot oiling | Circulation of heated fluid, typically oil, to dissolve or dislodge paraffin deposits from the production tubing. Such deposits tend to occur where a large variation in temperature exists across the producing system. |
| Well Completions | hot tapping | The process of drilling a hole through a pressure barrier using special equipment and procedures to ensure that the pressure and fluids are safely contained when access is made. Hot tapping is often used to enable access to the wellbore when wellhead valves jam closed. |
| Well Completions | HUD | The point or depth at which a tool or drift of a specific size can no longer pass through the wellbore. A higher than expected holdup depth may result from scale, fill, distortion of the wellbore tubulars or formation movement in an openhole completion. Alternate Form: holdup depth |
| Well Completions | hydraulic packer | A type of packer used predominantly in production applications. A hydraulic packer typically is set using hydraulic pressure applied through the tubing string rather than mechanical force applied by manipulating the tubing string. |
| Well Completions | hydraulic set | A setting or operating method that uses hydraulic force applied through the tubing or running string to activate a downhole tool. In many cases a drop ball, which lands in a profiled seat, will be used to shift the setting or activation mechanism at predetermined pressures. |
| Well Completions | hydraulic-set | A setting or operating method that uses hydraulic force applied through the tubing or running string to activate a downhole tool. In many cases a drop ball, which lands in a profiled seat, will be used to shift the setting or activation mechanism at predetermined pressures. |
| Well Completions | ICD | Abbreviation for inflow control device, a passive component installed as part of a well completion to help optimize production by equalizing reservoir inflow along the length of the wellbore. Multiple inflow control devices can be installed along the reservoir section of the completion, with each device employing a specific setting to partially choke flow. The resulting arrangement can be used to delay water or gas breakthrough by reducing annular velocity across a selected interval such as the heel of a horizontal well. ICDs are frequently used with sand screens on openhole completions. |
| Well Completions | ICV | Abbreviation for inflow control valve, an active component installed as part of a well completion to partially or completely choke flow into a well. Inflow control valves can be installed along the reservoir section of the completion, with each device typically separated from the next via a packer. Each ICV can be controlled from the surface to maintain flow conformance and, as the reservoir depletes, to stop unwanted fluids from entering the wellbore. A permanent downhole cable provides electric and hydraulic conduits to relay commands from the surface to the ICV. |
| Well Completions | inflatable packer | A type of packer that uses an inflatable bladder to expand the packer element against the casing or wellbore. In preparation for setting the packer, a drop ball or series of tubing movements are generally required, with the hydraulic pressure required to inflate the packer provided by carefully applying surface pump pressure. Inflatable packers are capable of relatively large expansion ratios, an important factor in through-tubing work where the tubing size or completion components can impose a significant size restriction on devices designed to set in the casing or liner below the tubing. |
| Well Completions | inflow control device | A passive component installed as part of a well completion to help optimize production by equalizing reservoir inflow along the length of the wellbore. Multiple inflow control devices can be installed along the reservoir section of the completion, with each device employing a specific setting to partially choke flow. The resulting arrangement can be used to delay water or gas breakthrough by reducing annular velocity across a selected interval such as the heel of a horizontal well. Inflow control devices are frequently used with sand screens on openhole completions. |
| Well Completions | inflow control valve | An active component installed as part of a well completion to partially or completely choke flow into a well. Inflow control valves can be installed along the reservoir section of the completion, with each device typically separated from the next via a packer. Each valve can be controlled from the surface to maintain flow conformance and, as the reservoir depletes, to stop unwanted fluids from entering the wellbore. A permanent downhole cable provides electric and hydraulic conduits to relay commands from the surface to each valve. |
| Well Completions | inflow performance relationship | A mathematical tool used in production engineering to assess well performance by plotting the well production rate against the flowing bottomhole pressure (BHP). The data required to create the IPR are obtained by measuring the production rates under various drawdown pressures. The reservoir fluid composition and behavior of the fluid phases under flowing conditions determine the shape of the curve. |
| Well Completions | inhibitor | A chemical agent added to a fluid system to retard or prevent an undesirable reaction that occurs within the fluid or with the materials present in the surrounding environment. A range of inhibitors is commonly used in the production and servicing of oil and gas wells, such as corrosion inhibitors used in acidizing treatments to prevent damage to wellbore components and inhibitors used during production to control the effect of hydrogen sulfide [H2S]. |
| Well Completions | injection line | A small-diameter conduit that is run alongside production tubulars to enable injection of inhibitors or similar treatments during production. Conditions such as high hydrogen sulfide [H2S] concentrations or severe scale deposition can be counteracted by injection of treatment chemicals and inhibitors during production. |
| Well Completions | injection mandrel | A downhole completion component that enables injection of treatment chemicals or inhibitors into the production conduit. The injection mandrel is equipped with a port- and check-valve system to direct fluid pumped down the annulus or injection line into the production conduit. |
| Well Completions | injection pressure | The pressure at which a treatment or test fluid can be injected into the formation matrix without causing a breakdown, or fracture, of the rock matrix. The injection pressure is commonly described as the surface pump pressure required to achieve injection. However, since the hydrostatic pressure of the fluid column also contributes to the downhole pressure value, the downhole pressure should also be considered. |
| Well Completions | injection pump | Any pump used to inject fluid into the reservoir or production system. Injection pumps vary in volume and pressure capacity, from the large injection pumps used in water-injection wells, to much smaller low-volume injection pumps used in continuous scale-inhibitor treatments. |
| Well Completions | injection well | A well in which fluids are injected rather than produced, the primary objective typically being to maintain reservoir pressure. Two main types of injection are common: gas and water. Separated gas from production wells or possibly imported gas may be reinjected into the upper gas section of the reservoir. Water-injection wells are common offshore, where filtered and treated seawater is injected into a lower water-bearing section of the reservoir. |
| Well Completions | instrument hanger | A downhole tool on which downhole gauges or instruments that are to be temporarily left in the wellbore are attached. The instrument hanger is run into the wellbore on slickline and set in a completion nipple at the required depth. |
| Well Completions | intelligent well | A well equipped with monitoring equipment and completion components that can be adjusted to optimize production, either automatically or with some operator intervention. |
| Well Completions | intermediate casing | A casing string that is generally set in place after the surface casing and before the production casing. The intermediate casing string provides protection against caving of weak or abnormally pressured formations and enables the use of drilling fluids of different density necessary for the control of lower formations. |
| Well Completions | intermittent gas lift | An artificial-lift method, used in relatively low-productivity wells, in which the gas-lift system is operated on an intermittent basis to enable the buildup of liquids in the wellbore. |
| Well Completions | internal pulling tool | A downhole tool used to pull or retrieve temporary plugs or similar equipment. The internal pulling tool engages on the internal surfaces of the item to be retrieved |
| Well Completions | J slot | A type of mechanism commonly used in the setting and unsetting of downhole tools and equipment such as packers. Most conventional downhole tools operate by upward or downward movement, rotation, or a combination of both. The J-slot profile creates the track for an actuating cam or pin that combines rotation and up or down movement to provide a simple yet reliable means of tool activation. |
| Well Completions | jet pump | A type of pump that operates on the principle of a high-pressure fluid jet and the venturi effect it creates. Jet pumps are relatively inefficient but can tolerate a wide range of operating conditions, including easily handling sand-laden or abrasive fluids. |
| Well Completions | J-slot | A type of mechanism commonly used in the setting and unsetting of downhole tools and equipment such as packers. Most conventional downhole tools operate by upward or downward movement, rotation, or a combination of both. The J-slot profile creates the track for an actuating cam or pin that combines rotation and up or down movement to provide a simple yet reliable means of tool activation. |
| Well Completions | landing collar | A component installed near the bottom of the casing string on which the cement plugs land during the primary cementing operation. The internal components of the landing collar are generally fabricated from plastics, cement and other drillable materials. |
| Well Completions | landing nipple | A completion component fabricated as a short section of heavy wall tubular with a machined internal surface that provides a seal area and a locking profile. Landing nipples are included in most completions at predetermined intervals to enable the installation of flow-control devices, such as plugs and chokes. Three basic types of landing nipple are commonly used: no-go nipples, selective-landing nipples and ported or safety-valve nipples. |
| Well Completions | life of the well | The period of time during which economically sustainable production levels may be expected from a well. The anticipated well life and the characteristics of the reservoir fluid are the two main factors in specifying the completion system components. |
| Well Completions | lifting sub | A short drillstring component that is temporarily connected to the top of a tool assembly that is to be lifted vertically, such as when running or retrieving a tool string. The external profile on the upper section of the lifting sub is similar to that of the completion tubing, enabling the rig elevators to lift the assembled tool string safely. |
| Well Completions | liner | Any string of casing in which the top does not extend to the surface but instead is suspended from inside the previous casing string. Many conventional well designs include a production liner set across the reservoir interval. This reduces the cost of completing the well and allows some flexibility in the design of the completion in the upper wellbore, such as when the fluid characteristics make it beneficial to increase the diameter of the conduit and components. |
| Well Completions | liner hanger | A device used to attach or hang liners from the internal wall of a previous casing string. Liner hangers are available in a range of sizes and specifications to suit a variety of completion conditions. |
| Well Completions | live oil | Oil containing dissolved gas in solution that may be released from solution at surface conditions. Live oil must be handled and pumped under closely controlled conditions to avoid the risk of explosion or fire. |
| Well Completions | lock | A downhole device, run and retrieved on slickline, that is placed and anchored within the tubing string to provide a setting point for flow-control equipment such as valves, chokes and plugs. The three main types of lock use different means of locating and securing: a slip lock locates and anchors anywhere within the correct size of tubing; the collar lock locates in the space within tubing collars; and the nipple lock locates within completion nipple profiles. |
| Well Completions | lock mandrel | Another term for lock, a downhole device, run and retrieved on slickline, that is placed and anchored within the tubing string to provide a setting point for flow-control equipment such as valves, chokes and plugs. The three main types of lock use different means of locating and securing: a slip lock locates and anchors anywhere within the correct size of tubing; the collar lock locates in the space within tubing collars; and the nipple lock locates within completion nipple profiles. |
| Well Completions | make up | To connect tools or tubulars by assembling the threaded connections incorporated at either end of every tool and tubular. The threaded tool joints must be correctly identified and then torqued to the correct value to ensure a secure tool string without damaging the tool or tubular body. |
| Well Completions | mandrel | A bar, shaft or spindle around which other components are arranged or assembled. The term has been extended in oil and gas well terminology to include specialized tubular components that are key parts of an assembly or system, such as gas-lift mandrel or packer mandrel. |
| Well Completions | manifold | An arrangement of piping or valves designed to control, distribute and often monitor fluid flow. Manifolds are often configured for specific functions, such as a choke manifold used in well-control operations and a squeeze manifold used in squeeze-cementing work. In each case, the functional requirements of the operation have been addressed in the configuration of the manifold and the degree of control and instrumentation required. |
| Well Completions | marginal well | A well that, for reasons of depletion or natural low productivity, is nearing the limits of viable production and profitability. |
| Well Completions | master valve | A valve located on the Christmas tree that controls all flow from the wellbore. A correctly functioning master valve is so important that two master valves are fitted to most Christmas trees. The upper master valve is used on a routine basis, with the lower master valve providing backup or contingency function in the event that the normal service valve is leaking and needs replacement. |
| Well Completions | microannulus | A small gap that can form between the casing or liner and the surrounding cement sheath, most commonly formed by variations in temperature or pressure during or after the cementing process. Such variations cause small movement of the steel casing, breaking the cement bond and creating a microannulus that is typically partial. However, in severe cases the microannulus may encircle the entire casing circumference. A microannulus can jeopardize the hydraulic efficiency of a primary cementing operation, allowing communication between zones if it is severe and connected. |
| Well Completions | minimum restriction | The smallest diameter present in a wellbore through which a tool string must pass to enable access to the operating depth or zone of interest. The minimum restriction determines the maximum tool string outside diameter and may influence the configuration of the assembled tools or equipment. The minimum restriction should be considered in both running and retrieving modes if any increase in tool string outside diameter is likely, such as when perforating or when using inflatable packers. |
| Well Completions | mist flow | A multiphase fluid-flow regime characterized by the gas phase being distributed as bubbles through the liquid phase. In a producing wellbore where the bubbles are uniformly distributed, there is little relative motion between the phases. Where the bubbles congregate and combine to form a less uniform distribution of the gas phase, some slippage will occur between the phases with the gas tending to cut through the liquid phase. |
| Well Completions | multiphase fluid | A fluid, generally a liquid, comprising more than one phase, such as water- or oil-based liquids, solid material or gas. Multiphase fluids and their behavior are of concern in two main areas, the flow of multiphase fluids and the separation of the various phases at surface. |
| Well Completions | multiphase fluid flow | The commingled flow of different phase fluids, such as water, oil and gas. Multiphase fluid flow is a complex factor, important in understanding and optimizing production hydraulics in both oil and gas wells. Four multiphase fluid flow regimes are recognized when describing flow in oil and gas wells, bubble flow, slug flow, transition flow and mist flow. |
| Well Completions | multiple completion | A single wellbore having tubulars and equipment that enable production from two or more reservoir zones. In most cases, at least two tubing strings will be used to provide the necessary level of control and safety for production fluids. However, in some simple dual completions, the second or upper zone is produced up the tubing-casing annulus. The wellhead and surface flow-control facilities required for multiple completions can be complex and costly; hence, multiple completions are relatively uncommon. |
| Well Completions | natural completion | A completion system designed to utilize the natural flow capability of the reservoir. |
| Well Completions | naturally occurring radioactive materials (NORM) | Materials typically found in certain types of barium or strontium scales that may be deposited in the wellbore or production tubulars. Any attempt to remove and dispose of NORM materials should be performed according to the legislation and policies associated with such potentially hazardous materials. |
| Well Completions | neutral point | The point on a string of tubulars at which there are neither tension nor compression forces present. Below the neutral point, there will be compression forces that build toward the bottom of the wellbore. Above the neutral point, tensile forces build to a maximum applied at the hanger or as hook load. |
| Well Completions | nipple | A completion component fabricated as a short section of heavy wall tubular with a machined internal surface that provides a seal area and a locking profile. Landing nipples are included in most completions at predetermined intervals to enable the installation of flow-control devices, such as plugs and chokes. Three basic types of landing nipple are commonly used: no-go nipples, selective-landing nipples and ported or safety-valve nipples. |
| Well Completions | nipple down | The process of disassembling well-control or pressure-control equipment on the wellhead. Depending on the configuration of the wellhead and casing strings, it may be necessary to nipple-down and nipple-up the blowout preventer (BOP) system as each casing string is run. |
| Well Completions | nipple up | The process of assembling well-control or pressure-control equipment on the wellhead. |
| Well Completions | nipple-down | The process of disassembling well-control or pressure-control equipment on the wellhead. Depending on the configuration of the wellhead and casing strings, it may be necessary to nipple-down and nipple-up the blowout preventer (BOP) system as each casing string is run. |
| Well Completions | nipple-up | The process of assembling well-control or pressure-control equipment on the wellhead. |
| Well Completions | nitrogen kickoff | Another term for nitrogen lift, the use of nitrogen gas circulated into the production conduit to displace liquids and reduce the hydrostatic pressure created by the fluid column. Nitrogen lifting is a common technique used to initiate production on a well following workover or overbalanced completion. A coiled tubing string is generally used to apply the treatment, which involves running to depth while pumping high-pressure nitrogen gas. Once the kill-fluid column is unloaded and the well is capable of natural flow, the coiled tubing string is removed and the well is prepared for production. |
| Well Completions | nitrogen lift | The use of nitrogen gas circulated into the production conduit to displace liquids and reduce the hydrostatic pressure created by the fluid column. Nitrogen lifting is a common technique used to initiate production on a well following workover or overbalanced completion. A coiled tubing string is generally used to apply the treatment, which involves running to depth while pumping high-pressure nitrogen gas. Once the kill-fluid column is unloaded and the well is capable of natural flow, the coiled tubing string is removed and the well is prepared for production. |
| Well Completions | nitrogen unit | A high-pressure pump or compressor unit capable of delivering high-purity nitrogen gas for use in oil or gas wells. Two basic types of unit are commonly available: a nitrogen converter unit that pumps liquid nitrogen at high pressure through a heat exchanger or converter to deliver high-pressure gas at ambient temperature, and a nitrogen generator unit that compresses and separates air to provide a supply of high-pressure nitrogen gas. |
| Well Completions | no go landing nipple | A nipple that incorporates a reduced diameter internal profile that provides a positive indication of seating by preventing the tool or device to be set from passing through the nipple. In many completions, a no-go landing nipple is preferred for the deepest nipple location, providing a no-go barrier to protect against a tool string being run or dropped below the tubing string. |
| Well Completions | NODAL* analysis | An analytical tool used in forecasting the performance of the various elements comprising the completion and production system. NODAL* analysis is used to optimize the completion design to suit the reservoir deliverability, identify restrictions or limits present in the production system and identify any means of improving production efficiency. *NODAL (production system analysis) is a mark of Schlumberger. |
| Well Completions | no-go landing nipple | A nipple that incorporates a reduced diameter internal profile that provides a positive indication of seating by preventing the tool or device to be set from passing through the nipple. In many completions, a no-go landing nipple is preferred for the deepest nipple location, providing a no-go barrier to protect against a tool string being run or dropped below the tubing string. |
| Well Completions | open flow potential | The calculated maximum flow rate that a system may provide in the absence of restrictions. The term may be qualified as relating to a specific zone, such as a perforated interval or be used in referring to the production capability of the well. |
| Well Completions | open-flow potential | The calculated maximum flow rate that a system may provide in the absence of restrictions. The term may be qualified as relating to a specific zone, such as a perforated interval or be used in referring to the production capability of the well. |
| Well Completions | openhole completion | A well completion that has no casing or liner set across the reservoir formation, allowing the produced fluids to flow directly into the wellbore. This type of completion suffers the major disadvantage that the sandface is unsupported and may collapse. Also, without any casing or liner installed, selective treatments or remedial work within the reservoir section are more difficult. |
| Well Completions | openhole gravel pack | A type of sand-control completion in which the gravel pack screen is packed off in an openhole section with no casing or liner to support the producing formation. The openhole interval is often prepared by underreaming a section of reservoir below the last casing shoe. When the treatment is to be applied on an existing well, a section of casing may be milled out. |
| Well Completions | openhole packer | A type of packer designed for use in openhole applications such as drillstem testing. Openhole packers are typically configured with one large element that can be deformed easily to contact the uneven formation surface, yet retain strength and sufficient integrity to withstand the anticipated differential pressures. |
| Well Completions | openhole test | A drillstem test performed in an openhole section of the wellbore. The test once was a popular method of assessing the productivity of exploration wells without the need to run casing or liner across the reservoir interval. Openhole testing now is less common because of the risks and limits associated with fluid flow from an unsupported formation. |
| Well Completions | opening bomb | A device used in stage cementing to open the stage collar or diverting valve through which the second or subsequent cement stage is placed. The opening bomb is dropped down the casing string to land in a seat within the stage collar. Applying pump pressure then activates a sliding collar that opens access ports, enabling circulation across the upper casing interval. |
| Well Completions | operating gas-lift valve (OGLV) | The lowermost gas-lift valve in a gas-lift completion through which the lift gas is injected during normal production. During startup, the upper gas-lift valves open in sequence, from the top down, to enable the tubing fluids to be displaced. At predetermined pressures, each of the upper valves closes to eventually route all lift gas through the operating valve, which is placed at the optimal depth for the reservoir and completion conditions. |
| Well Completions | orifice valve | A type of valve, typically found on small-diameter pipework, that incorporates an orifice or flow-restriction device to control fluid flow. |
| Well Completions | O-ring | A type of circular seal commonly found in downhole tools and a wide range of surface equipment applications. The specification of O-ring material depends on the conditions for which the seal is intended, such as system operating temperature and pressure. Various backup systems are used to support the O-ring seal in either dynamic or static sealing applications. |
| Well Completions | oxygen scavenger | A chemical agent used in some brines and completion fluids to reduce corrosion resulting from, or exacerbated by, dissolved oxygen. Oxygen scavengers capture the dissolved oxygen in a harmless chemical reaction that renders the oxygen unavailable for corrosive reactions. The use of oxygen scavengers is more critical in applications in which fluids are to be circulated in the wellbore. Additional oxygen may be dissolved during agitation at surface. |
| Well Completions | pack off | To effect hydraulic isolation, either with a sealing device, such as a packer, or with a specialized plastic or fluid, such as a sealing compound. |
| Well Completions | packer | A downhole device used in almost every completion to isolate the annulus from the production conduit, enabling controlled production, injection or treatment. A typical packer assembly incorporates a means of securing the packer against the casing or liner wall, such as a slip arrangement, and a means of creating a reliable hydraulic seal to isolate the annulus, typically by means of an expandable elastomeric element. Packers are classified by application, setting method and possible retrievability. |
| Well Completions | packer fluid | The fluid that remains in the tubing-casing annulus above the packer after the completion has been run and all circulation devices have been isolated. Packer fluids are prepared for the requirements of the given completion. Generally, they should be of sufficient density to control the producing formation, be solids-free and resistant to viscosity changes over long periods of time, and be noncorrosive to the wellbore and completion components. |
| Well Completions | packing gland | A device used to seal around a reciprocating or rotating shaft or spindle. A malleable packing compound is forced into place by an adjustable packing nut, or similar arrangement. This enables the seal or packing to be tightened to suit the operating conditions and allows subsequent adjustment to account for wear. |
| Well Completions | paraffin | A hydrocarbon compound that often precipitates on production components as a result of the changing temperatures and pressures within the production system. Heavy paraffins occur as wax-like substances that may build up on the completion components and may, if severe, restrict production. Paraffin is normally found in the tubing close to surface. Nevertheless, it can form at the perforations, or even inside the formation, especially in depleted reservoirs or reservoirs under gas-cycling conditions. |
| Well Completions | pipe stretch | The increase in length resulting from the combination of forces acting on a string within the wellbore. The principal factors resulting in an increase in string length are the weight of the string itself and the effects of thermal expansion. |
| Well Completions | plunger lift | An artificial-lift method principally used in gas wells to unload relatively small volumes of liquid. An automated system mounted on the wellhead controls the well on an intermittent flow regime. When the well is shut-in, a plunger is dropped down the production string. When the control system opens the well for production, the plunger and a column of fluid are carried up the tubing string. The surface receiving mechanism detects the plunger when it arrives at surface and, through the control system, prepares for the next cycle. |
| Well Completions | polished joint | A generic term for a completion component that has been polished or prepared to enable an efficient hydraulic seal. The polished joint may have an internal or external polished surface and is typically configured in a length that enables some movement of the completion string or associated components without compromising the hydraulic seal. |
| Well Completions | polished rod | The uppermost joint in the string of sucker rods used in a rod pump artificial-lift system. The polished rod enables an efficient hydraulic seal to be made around the reciprocating rod string. |
| Well Completions | premium thread | A class of high-performance thread types that are commonly used in modern oilwell and gaswell completions. Premium threads are available in a number of configurations and are typically designed to provide superior hydraulic sealing, improved tensile capacity and ease of make-up. Unlike conventional threads, the sealing areas in premium thread connections are independent of the thread profile and are included as two or three areas within the tool joint, thereby providing some redundancy. |
| Well Completions | pressure buildup analysis | An analysis of data obtained from measurements of the bottomhole pressure in a well that is shut-in after a flow period. The profile created on a plot of pressure against time is used with mathematical reservoir models to assess the extent and characteristics of the reservoir and the near-wellbore area. |
| Well Completions | pressure drawdown | The differential pressure that drives fluids from the reservoir into the wellbore. The drawdown, and therefore the production rate, of a producing interval is typically controlled by surface chokes. Reservoir conditions, such as the tendency to produce sand, may limit the drawdown that may be safely applied during production before damage or unwanted sand production occurs. |
| Well Completions | pressure transient test | A means of assessing reservoir performance by measuring flow rates and pressures under a range of flowing conditions and applying the data to a mathematical model. Fundamental data relating to the interval under test, such as reservoir height and details of the reservoir fluids, are also input. The resulting outputs typically include an assessment of reservoir permeability, the flow capacity of the reservoir and any damage that may be restricting productivity. |
| Well Completions | primary cementing | The process of placing a cement sheath around a casing or liner string. The main objectives of primary cementing operations include zonal isolation to prevent migration of fluids in the annulus, support for the casing or liner string, and protection of the casing string from corrosive formation fluids. |
| Well Completions | primary completion components | The main elements of an oil or gas well, including the production tubing string, that enable a particular type or design of completion to function as designed. The primary completion components depend largely on the completion type, such as the pump and motor assemblies in an electrical submersible pump completion. |
| Well Completions | primary recovery method | The means by which the initial reservoir production is achieved, such as natural production from a gas-drive reservoir. In many cases, a secondary recovery method, such as waterflood, is required to maintain a viable reservoir production rate. |
| Well Completions | produced fluid | A generic term used in a number of contexts but most commonly to describe any fluid produced from a wellbore that is not a treatment fluid. The characteristics and phase composition of a produced fluid vary and use of the term often implies an inexact or unknown composition. |
| Well Completions | production casing | A casing string that is set across the reservoir interval and within which the primary completion components are installed. |
| Well Completions | production packer | A device used to isolate the annulus and anchor or secure the bottom of the production tubing string. A range of production packer designs is available to suit the wellbore geometry and production characteristics of the reservoir fluids. |
| Well Completions | production string | The primary conduit through which reservoir fluids are produced to surface. The production string is typically assembled with tubing and completion components in a configuration that suits the wellbore conditions and the production method. An important function of the production string is to protect the primary wellbore tubulars, including the casing and liner, from corrosion or erosion by the reservoir fluid. |
| Well Completions | production tubing | A wellbore tubular used to produce reservoir fluids. Production tubing is assembled with other completion components to make up the production string. The production tubing selected for any completion should be compatible with the wellbore geometry, reservoir production characteristics and the reservoir fluids. |
| Well Completions | production wing | The portion of a Christmas tree or surface production facility through which production fluids flow. The production wing typically includes a wing valve and a choke to control or isolate flow from the wellbore. |
| Well Completions | productivity index (PI) | A mathematical means of expressing the ability of a reservoir to deliver fluids to the wellbore. The PI is usually stated as the volume delivered per psi of drawdown at the sandface (bbl/d/psi). |
| Well Completions | profile modification | The process of controlling undesirable water production from a well by conducting treatments to prevent coning or cresting. A range of treatment options is available for profile modification applications, most of which are designed to reduce the permeability of the water-bearing zones to encourage preferential flow from the oil-bearing formation. The injection of polymers, or similar chemicals, that form a rigid gel within the formation matrix is a common treatment. |
| Well Completions | rabbit | A slang term for an internal drift diameter gauge typically used to check casing or tubing joints before they are picked up and run into the wellbore. The drift diameter used depends on the size and weight of the tubular being checked. |
| Well Completions | recorder carrier | A downhole tool used to locate or convey a downhole gauge. Recorder carriers may be incorporated into temporary completions, such as for testing purposes, or run on slickline for temporary placement within the wellbore. In some cases, permanent gauges may be installed in recorder carriers run as completion components. |
| Well Completions | relative permeability | A measurement of the ability of two or more fluid phases to pass through a formation matrix. The relative permeability reflects the capability of a specific formation to produce a combination of oil, water or gas more accurately than the absolute permeability of a formation sample, which is measured with a single-phase fluid, usually water. |
| Well Completions | reservoir height | A measurement of the vertical thickness of reservoir formation that is open to flow. The reservoir height is used in calculations and mathematical models to assess reservoir performance or potential productivity. |
| Well Completions | reservoir pressure | The pressure within the reservoir rock. The formation pressure value can be further categorized as relating to flowing well or shut-in conditions. |
| Well Completions | retarder | A chemical additive used to increase the thickening time of cement slurries or similar fluids that may have a limited pumping time. The increased pressure and temperature typically associated with deep wellbores requires the use of such performance-enhancing additives to enable efficient placement without premature setting. |
| Well Completions | retrievable bridge plug | A type of downhole isolation tool that may be unset and retrieved from the wellbore after use, such as may be required following treatment of an isolated zone. A retrievable bridge plug is frequently used in combination with a packer to enable accurate placement and injection of stimulation or treatment fluids. |
| Well Completions | retrievable packer | A type of packer that is run and retrieved on a running string or production string, unlike a permanent production packer that is set in the casing or liner before the production string is run. Retrievable packers are most commonly used in well intervention activities, although some completion designs are more suited to retrievable packers than permanent packers. |
| Well Completions | reverse circulating valve | A downhole tool that is designed to enable communication between the tubing internal diameter and the annulus, typically for reverse circulation purposes, although conventional circulation often can also be undertaken. The valve design may be simple, enabling circulation without the ability to reset the tool until it is retrieved to surface, or be of a more complex design allowing several cycles between open and closed positions. |
| Well Completions | reverse circulation | To circulate fluid down the wellbore annulus, with returns being made up the tubing string. Reverse circulation is often used to remove debris from the wellbore since the high fluid flow rate inside the tubing string enables the recovery of large or dense debris particles that are difficult or impossible to remove with conventional circulation. |
| Well Completions | reverse-circulating valve | A downhole tool that is designed to enable communication between the tubing internal diameter and the annulus, typically for reverse circulation purposes, although conventional circulation often can also be undertaken. The valve design may be simple, enabling circulation without the ability to reset the tool until it is retrieved to surface, or be of a more complex design allowing several cycles between open and closed positions. |
| Well Completions | reversing out | Conducting reverse circulation, that is, circulating fluid down the wellbore annulus, with returns being made up the tubing string. Reverse circulation is often used to remove debris from the wellbore since the high fluid flow rate inside the tubing string enables the recovery of large or dense debris particles that are difficult or impossible to remove with conventional circulation. |
| Well Completions | rigless operation | A well-intervention operation conducted with equipment and support facilities that precludes the requirement for a rig over the wellbore. Coiled tubing, slickline and snubbing activities are commonly conducted as rigless operations. |
| Well Completions | rod pump | An artificial-lift pumping system using a surface power source to drive a downhole pump assembly. A beam and crank assembly creates reciprocating motion in a sucker-rod string that connects to the downhole pump assembly. The pump contains a plunger and valve assembly to convert the reciprocating motion to vertical fluid movement. |
| Well Completions | round thread | A basic threadform commonly found in oilfield applications. The thread profile of a round thread is designed to provide both mechanical strength and a hydraulic seal when made up to the correct torque. |
| Well Completions | running tool | A generic name for a tool or device that is used in the placement or setting of downhole equipment such as permanent packers or plugs. The running tool can be retrieved after the operation or setting process. In some cases, the running tool also is used to retrieve the equipment or tool that has been set in the wellbore. |
| Well Completions | rupture disk | A high-precision component designed to rupture with the application of a predetermined hydraulic pressure. Rupture disks are commonly used in downhole applications in which the controlled application of pump pressure is used to set or operate downhole equipment, such as packers or plugs. In some applications, a rupture disk may be used as a protection device to prevent overpressurizing a vessel or component. |
| Well Completions | salt plug | A temporary plugging agent comprising graded granules of salt that form a physical or hydraulic barrier. The different grain sizes are prepared as a slurry for placement, then allowed to settle into a plug. The resulting plug typically provides good mechanical and hydraulic strength to enable safe treatment of an adjacent zone. On completion of the treatment, the temporary salt plug is easily removed by circulating a water-base fluid to dissolve the plug. |
| Well Completions | sand | A generic term used to describe small formation particles known as fines that may be produced with the reservoir fluid. Sand production generally is undesirable and, if severe, may require some remedial action to control or prevent production, such a gravel packing or sand consolidation. |
| Well Completions | sand bailer | A downhole device, usually run on slickline, used to remove sand or debris from the bottom of the wellbore. In operation, an atmospheric chamber within the tool is opened to create a surge of fluids into the chamber. Debris is then held within the chamber for recovery at surface. |
| Well Completions | sand consolidation | A means of controlling the undesirable production of sand from weak sandstone formations. Sand consolidation chemically binds the grains of sand that make up the formation matrix while maintaining sufficient permeability to achieve viable production rates. |
| Well Completions | sand control | The installation of equipment or application of techniques to prevent migration of reservoir sand into the wellbore or near-wellbore area. In weak formations, sand control may be necessary to maintain the structure of the reservoir around the wellbore. In other formation types, the migration of sand and fines into the near wellbore area may severely restrict production. Each of these conditions requires different treatments. The principal sand-control techniques include gravel packing and sand consolidation. |
| Well Completions | sand production | The migration of formation sand caused by the flow of reservoir fluids. The production of sand is generally undesirable since it can restrict productivity, erode completion components, impede wellbore access, interfere with the operation of downhole equipment, and present significant disposal difficulties. |
| Well Completions | sandface | The physical interface between the formation and the wellbore. The diameter of the wellbore at the sandface is one of the dimensions used in production models to assess potential productivity. |
| Well Completions | scale | A mineral salt deposit that may occur on wellbore tubulars and components as the saturation of produced water is affected by changing temperature and pressure conditions in the production conduit. In severe conditions, scale creates a significant restriction, or even a plug, in the production tubing. Scale removal is a common well-intervention operation, with a wide range of mechanical, chemical and scale inhibitor treatment options available. |
| Well Completions | scale inhibitor | A chemical treatment used to control or prevent scale deposition in the production conduit or completion system. Scale-inhibitor chemicals may be continuously injected through a downhole injection point in the completion, or periodic squeeze treatments may be undertaken to place the inhibitor in the reservoir matrix for subsequent commingling with produced fluids. Some scale-inhibitor systems integrate scale inhibitors and fracture treatments into one step, which guarantees that the entire well is treated with scale inhibitor. In this type of treatment, a high-efficiency scale inhibitor is pumped into the matrix surrounding the fracture face during leakoff. It adsorbs to the matrix during pumping until the fracture begins to produce water. As water passes through the inhibitor-adsorbed zone, it dissolves sufficient inhibitor to prevent scale deposition. The inhibitor is better placed than in a conventional scale-inhibitor squeeze, which reduces the retreatment cost and improves production. |
| Well Completions | screen | A device used in sand control applications to support the gravel pack. To form a screen, a profiled wire is wrapped and welded in place on a perforated liner. Screens are available in a range of sizes and specifications, including outside diameter, material type and the geometry and dimension of the screen slots. The space between each wire wrap must be small enough to retain the gravel placed behind the screen, yet minimize any restriction to production |
| Well Completions | screen out | A condition that occurs when the solids carried in a treatment fluid, such as proppant in a fracture fluid, create a bridge across the perforations or similar restricted flow area. This creates a sudden and significant restriction to fluid flow that causes a rapid rise in pump pressure. |
| Well Completions | screenout | A condition that occurs when the solids carried in a treatment fluid, such as proppant in a fracture fluid, create a bridge across the perforations or similar restricted flow area. This creates a sudden and significant restriction to fluid flow that causes a rapid rise in pump pressure. |
| Well Completions | seal assembly | A system of seals arranged on the component that engages in a sealbore to isolate the production-tubing conduit from the annulus. The seal assembly is typically longer than the sealbore to enable some movement of the components while maintaining an efficient seal. |
| Well Completions | seal receptacle | A profiled completion component designed to accept a mating component equipped with a seal assembly. Completions are designed with seal receptacles to enable the production string to be removed without removing the packer or permanent completion components. |
| Well Completions | sealbore | A polished bore designed to accept a seal assembly, such as may be used in a permanent production packer. |
| Well Completions | sealbore packer | A type of production packer that incorporates a sealbore that accepts a seal assembly fitted to the bottom of the production tubing. The sealbore packer is often set on wireline to enable accurate depth correlation. For applications in which a large tubing movement is anticipated, as may be due to thermal expansion, the sealbore packer and seal assembly function as a slip joint. |
| Well Completions | seating nipple | A completion component fabricated as a short section of heavy wall tubular with a machined internal surface that provides a seal area and a locking profile. Landing nipples are included in most completions at predetermined intervals to enable the installation of flow-control devices, such as plugs and chokes. Three basic types of landing nipple are commonly used: no-go nipples, selective-landing nipples and ported or safety-valve nipples. |
| Well Completions | secondary production | The method used to sustain production levels at viable rates following a fall in flow rate as the efficiency of the primary production methods declines. Secondary production methods frequently involve an artificial-lift system or reservoir injection for pressure maintenance. |
| Well Completions | selective nipple | A type of landing nipple designed to be run in a series throughout the wellbore. Two basic types of selective nipple system may be encountered, a nipple series in which the nipple design or profile determines the selectivity and one in which the running tool is used to find the target nipple. |
| Well Completions | selective running tool | A wireline tool to set and retrieve selectively set downhole equipment such as plugs and similar flow-control devices. The selective running tool enables equipment to be set in a target nipple that may be one of a series placed throughout the wellbore. |
| Well Completions | setting tool | Another term for running tool, a generic name for a tool or device that is used in the placement or setting of downhole equipment such as permanent packers or plugs. The running tool can be retrieved after the operation or setting process. In some cases, the running tool also is used to retrieve the equipment or tool that has been set in the wellbore. |
| Well Completions | shear stock | The material from which shear pins are typically cut. Shear stock is precision-manufactured in a range of sizes and material characteristics to provide predictable performance from the shear pin and the tool in which it is installed. |
| Well Completions | shifting tool | A downhole tool, most commonly associated with slickline operations, that is used to open, close or shift the position of downhole flow control or circulation devices, such as sliding sleeves. The shifting tool generally features some means of engaging the components to be shifted and is typically run with upward or downward operating jars to deliver the necessary force or impact. |
| Well Completions | shoe track | Another term for float joint, a full-sized length of casing placed at the bottom of the casing string that is usually left full of cement on the inside to ensure that good cement remains on the outside of the bottom of the casing. If cement were not left inside the casing in this manner, the risk of overdisplacing the cement (due to improper casing volume calculations, displacement mud volume measurements, or both) would be significantly higher. Hence, the well designer plans on a safety margin of cement left inside the casing to guarantee that the fluid left outside the casing is good-quality cement. A float collar is placed at the top of the float joint and a float shoe placed at the bottom to prevent reverse flow of cement back into the casing after placement. There can be one, two or three joints of casing used for this purpose. |
| Well Completions | shoot a level | To use a special acoustic device to determine the fluid level in a conduit or annular space. The principle of operation relies on accurately recording the time taken for a return echo to be bounced from the fluid in contained area. |
| Well Completions | side pocket mandrel | A completion component that is used to house gas-lift valves and similar devices that require communication with the annulus. The design of a side-pocket mandrel is such that the installed components do not obstruct the production flow path, enabling access to the wellbore and completion components below. |
| Well Completions | side-pocket mandrel | A completion component that is used to house gas-lift valves and similar devices that require communication with the annulus. The design of a side-pocket mandrel is such that the installed components do not obstruct the production flow path, enabling access to the wellbore and completion components below. |
| Well Completions | sieve analysis | The process of analyzing the size distribution of a sand or gravel sample. In sand-control applications, a sample of formation sand is shaken through a series of sieves of known size. The resulting distribution is then used to design an appropriate treatment that will retain the sand, while causing a minimal restriction to production. |
| Well Completions | simultaneous operation (SIMOP) | A term used mainly on offshore platforms, or installations with multiple wellheads, where more than one wellbore is being accessed, such as where a drilling rig, slickline unit or coiled tubing unit may be operating at the same time. Simultaneous operations generally have an impact on the installation safety procedures and contingency planning processes. |
| Well Completions | skin | A dimensionless factor calculated to determine the production efficiency of a well by comparing actual conditions with theoretical or ideal conditions. A positive skin value indicates some damage or influences that are impairing well productivity. A negative skin value indicates enhanced productivity, typically resulting from stimulation. |
| Well Completions | slickline | A thin nonelectric cable used for selective placement and retrieval of wellbore hardware, such as plugs, gauges and valves located in sidepocket mandrels. Valves and sleeves can also be adjusted using slickline tools. Partially collapsed tubing can be repaired using a tubing swage on slickline. |
| Well Completions | sliding sleeve | A completion device that can be operated to provide a flow path between the production conduit and the annulus. Sliding sleeves incorporate a system of ports that can be opened or closed by a sliding component that is generally controlled and operated by slickline tool string. |
| Well Completions | slip joint | A completion component designed to accommodate tubing movement or length changes while maintaining a hydraulic seal between the production conduit and the annulus. The size or length of the slip joint depends on the wellbore conditions and completion characteristics. |
| Well Completions | slip lock | A downhole lock device, run on slickline, that incorporates a slip mechanism that engages on the tubing wall to anchor the lock at the desired setting depth. Slip locks are not depth-dependent and do not require special completion equipment. However, the slip lock has limited function and pressure capacity and is generally less desirable than nipple or collar locks. |
| Well Completions | slug flow | A multiphase-fluid flow regime characterized by a series of liquid plugs (slugs) separated by a relatively large gas pockets. In vertical flow, the bubble is an axially symmetrical bullet shape that occupies almost the entire cross-sectional area of the tubing. The resulting flow alternates between high-liquid and high-gas composition. |
| Well Completions | solution gas | Dissolved gas in wellbore or reservoir fluids. The gas will remain in solution until the pressure or temperature conditions change, at which time it may break out of solution to become free gas |
| Well Completions | solution gas drive | A type of reservoir-drive system in which the energy for the transport and production of reservoir fluids is derived from the gas dissolved in the fluid. As reservoir fluids enter the wellbore, changing pressure conditions cause the gas to break from solution to create a commingled flow of gas and liquid that aids production. |
| Well Completions | sour corrosion | Corrosion associated with the presence of hydrogen sulfide [H2S]. Sour corrosion occurs in several forms of hydrogen embrittlement that cause materials to fail at stress levels below their normal yield strength. This can be problematic in applications where higher strength steels are used. Selection of materials resistant to the effects of H2S is the primary means of controlling sour corrosion. |
| Well Completions | space out | To assemble components to ensure that all critical length dimensions are met, as is required to ensure that the production tubing can be landed in the wellhead and production packer with the desired weight distribution. The term also may apply to surface pressure-control equipment offshore, where well intervention equipment may be required at certain deck levels |
| Well Completions | spacer | A viscous fluid used to aid removal of drilling fluids before a primary cementing operation. The spacer is prepared with specific fluid characteristics, such as viscosity and density, that are engineered to displace the drilling fluid while enabling placement of a complete cement sheath. |
| Well Completions | stab in | To guide and engage components that are designed to couple, such as a seal assembly in a sealbore packer. |
| Well Completions | stab-in | To guide and engage components that are designed to couple, such as a seal assembly in a sealbore packer. |
| Well Completions | standing valve | A downhole valve assembly that is designed to hold pressure from above while allowing fluids to flow from below. Standing valves generally are run and retrieved on slickline with the valve assembly located in an appropriate nipple. Applications for standing valves include testing the tubing string, setting packers, or other applications in which it is desirable to maintain fluid in the tubing string. |
| Well Completions | step rate test | A test performed in preparation for a hydraulic fracturing treatment in which an injection fluid is injected for a defined period in a series of increasing pump rates. The resulting data are used to identify key treatment parameters of the fracturing operation, such as the pressure and flow rates required to successfully complete the treatment. |
| Well Completions | storm choke | A downhole valve that operates by fluid velocity and closes when the fluid flow from the well exceeds preset limits. The forerunner to modern subsurface controlled safety valves, storm chokes were used in offshore applications as a contingency device in the event of a catastrophic failure of surface facilities during a storm or hurricane. |
| Well Completions | storm packer | A heavy-duty retrievable packer assembly that can be run in to isolate the wellbore of a new well in the event of suspended activities, for example, during a severe storm. An on-off disconnect feature enables the storm packer to be set at a safe depth while using the weight of the string below the packer to maintain the set and hang off the drillstring to avoid pulling all the way out of the hole. |
| Well Completions | strap | To measure a running string or assembled components while running in or out of the wellbore. |
| Well Completions | subsea well | A well in which the wellhead, Christmas tree and production-control equipment is located on the seabed. |
| Well Completions | subsurface safety valve (SSSV) | A safety device installed in the upper wellbore to provide emergency closure of the producing conduits in the event of an emergency. Two types of subsurface safety valve are available: surface-controlled and subsurface controlled. In each case, the safety-valve system is designed to be fail-safe, so that the wellbore is isolated in the event of any system failure or damage to the surface production-control facilities. |
| Well Completions | subsurface surface controlled safety valve (SSCSV) | A downhole safety valve designed to close automatically in an emergency situation. There are two basic operating mechanisms: valves operated by an increase in fluid flow and valves operated by a decrease in ambient pressure. |
| Well Completions | subsurface surface-controlled safety valve (SSCSV) | A downhole safety valve designed to close automatically in an emergency situation. There are two basic operating mechanisms: valves operated by an increase in fluid flow and valves operated by a decrease in ambient pressure. |
| Well Completions | sucker rod | A steel rod that is used to make up the mechanical assembly between the surface and downhole components of a rod pumping system. Sucker rods are 25 to 30 ft [7 to 9 m] long and threaded at each end to enable the downhole components to be run and retrieved easily. |
| Well Completions | sucker rod pump | An artificial-lift pumping system using a surface power source to drive a downhole pump assembly. A beam and crank assembly creates reciprocating motion in a sucker-rod string that connects to the downhole pump assembly. The pump contains a plunger and valve assembly to convert the reciprocating motion to vertical fluid movement. |
| Well Completions | surface controlled subsurface safety valve (SCSSV) | A downhole safety valve that is operated from surface facilities through a control line strapped to the external surface of the production tubing. Two basic types of SCSSV are common: wireline retrievable, whereby the principal safety-valve components can be run and retrieved on slickline, and tubing retrievable, in which the entire safety-valve assembly is installed with the tubing string. The control system operates in a fail-safe mode, with hydraulic control pressure used to hold open a ball or flapper assembly that will close if the control pressure is lost. |
| Well Completions | surface-controlled subsurface safety valve (SCSSV) | A downhole safety valve that is operated from surface facilities through a control line strapped to the external surface of the production tubing. Two basic types of SCSSV are common: wireline retrievable, whereby the principal safety-valve components can be run and retrieved on slickline, and tubing retrievable, in which the entire safety-valve assembly is installed with the tubing string. The control system operates in a fail-safe mode, with hydraulic control pressure used to hold open a ball or flapper assembly that will close if the control pressure is lost. |
| Well Completions | swab | To unload liquids from the production tubing to initiate flow from the reservoir. A swabbing tool string incorporates a weighted bar and swab cup assembly that are run in the wellbore on heavy wireline. When the assembly is retrieved, the specially shaped swab cups expand to seal against the tubing wall and carry the liquids from the wellbore. |
| Well Completions | swage | A threaded adapter used to connect a circulating line to a casing or tubing string. A casing or tubing swage generally is required as a contingency option to enable any obstruction or fill to be circulated clear during the running process. |
| Well Completions | sweep pill | A relatively small volume of viscous fluid, typically a carrier gel, that is circulated to sweep, or remove, debris or residual fluids from the circulation system. |
| Well Completions | sweet corrosion | The deterioration of metal due to contact with carbon dioxide or similar corrosive agents, but excluding hydrogen sulfide [H2S]. Sweet corrosion typically results in pitting or material loss and occurs where steel is exposed to carbon dioxide and moisture. |
| Well Completions | sweet crude | Crude oil containing low levels of sulfur compounds, especially hydrogen sulfide [H2S]. The facilities and equipment to handle sweet crude are significantly simpler than those required for other potentially corrosive types of crude oil. |
| Well Completions | sweet gas | Natural gas that does not contain hydrogen sulfide [H2S] or significant quantities of carbon dioxide [CO2]. |
| Well Completions | tag | To contact, or tag, a known reference point or obstruction in the wellbore with the tubing string, wireline or other intervention equipment |
| Well Completions | tail pipe | The tubulars and completion components run below a production packer. The tail pipe may be included in a completion design for several reasons. It can provide a facility for plugs and other temporary flow-control devices, improve downhole hydraulic characteristics, and provide a suspension point for downhole gauges and monitoring equipment. |
| Well Completions | tally | A list containing details of tubulars that have been prepared for running, or that have been retrieved from the wellbore. Each tubing joint is numbered and the corresponding length and other pertinent details noted alongside. |
| Well Completions | tapered string | A string comprising tubing or components of varying size or dimension. A tapered production string may be configured with larger OD tubing joints in the upper wellbore area to optimize the hydraulic performance of the string. Although a tapered coiled tubing string will have the same tubing outside diameter throughout, the upper portion of the string may have a greater wall thickness to support the load of the string below. |
| Well Completions | tell tale | A device used to indicate the position or function of mechanical components that cannot be easily observed, such as to indicate the launch of a cementing plug or dart. |
| Well Completions | tell-tale | A device used to indicate the position or function of mechanical components that cannot be easily observed, such as to indicate the launch of a cementing plug or dart. |
| Well Completions | tension-set packer | A type of packer that is set by applying tension to the running string. These packers are less common than compression-set packers due to the potential difficulties associated with retrieval. However, in applications where there is insufficient string weight to set a compression packer, a tension packer is a useful option. |
| Well Completions | thread protector | A protective sleeve or cap generally made up on the threads of tubular goods during transport and storage. Thread protectors are available in metal, plastic, or a combination of both. |
| Well Completions | thread rule | A pocket-size thread gauge used in field operations to correctly identify or confirm the thread type and size of tubular goods. |
| Well Completions | through flowline (TFL) | Pertaining to treatments performed on subsea wells where the fluids and associated pump-down equipment, such as plugs or darts, are pumped through the flowline normally used for production fluids. |
| Well Completions | through-flowline (TFL) | Pertaining to treatments performed on subsea wells where the fluids and associated pump-down equipment, such as plugs or darts, are pumped through the flowline normally used for production fluids. |
| Well Completions | through-tubing | Pertaining to any reservoir or wellbore treatment performed through the tubing string. Through-tubing treatments are generally associated with live-well operations, thereby causing minimal interruption to production and eliminating the need to kill the well. |
| Well Completions | tie back liner | A section of liner that is run from a liner hanger back to the wellhead after the initial liner and hanger system have been installed and cemented. A tie-back liner may be required to provide the necessary pressure capacity during a flow-test period or for special treatments, and is typically not cemented in place. In some cases, a tie-back liner will be installed as a remedial treatment when the integrity of the intermediate casing string is in doubt. |
| Well Completions | tie back packer | A specially designed packer assembly used in conjunction with a tie-back liner. The tie-back packer can be integral to the original liner hanger, or if the tie-back is a remedial treatment, it can be a separate component set above the liner hanger. |
| Well Completions | tie-back liner | A section of liner that is run from a liner hanger back to the wellhead after the initial liner and hanger system have been installed and cemented. A tie-back liner may be required to provide the necessary pressure capacity during a flow-test period or for special treatments, and is typically not cemented in place. In some cases, a tie-back liner will be installed as a remedial treatment when the integrity of the intermediate casing string is in doubt. |
| Well Completions | tie-back packer | A specially designed packer assembly used in conjunction with a tie-back liner. The tie-back packer can be integral to the original liner hanger, or if the tie-back is a remedial treatment, it can be a separate component set above the liner hanger. |
| Well Completions | tie-back string | Another term for tie-back liner, a section of liner that is run from a liner hanger back to the wellhead after the initial liner and hanger system have been installed and cemented. A tie-back liner may be required to provide the necessary pressure capacity during a flow-test period or for special treatments, and is typically not cemented in place. In some cases, a tie-back liner will be installed as a remedial treatment when the integrity of the intermediate casing string is in doubt. |
| Well Completions | transition flow | A multiphase flow regime in near-vertical pipes in which large, irregular slugs of gas move up the center of the pipe, usually carrying droplets of oil or water with them. Most of the remaining oil or water flows up along the pipe walls. The flow is relatively chaotic, producing a frothy mixture. Unlike slug flow, neither phase is continuous. The gas slugs are relatively unstable, and take on large, elongated shapes. Also known as churn flow, this flow is an intermediate flow condition between slug flow and mist flow, and occurs at relatively high gas velocity. As the gas velocity increases, it changes into annular flow. |
| Well Completions | travel joint | Also known as slip joint, a completion component designed to accommodate tubing movement or length changes while maintaining a hydraulic seal between the production conduit and the annulus. The size or length of the slip joint depends on the wellbore conditions and completion characteristics. |
| Well Completions | tubing broach | A downhole tool used to repair damaged or collapsed tubing. The tubing broach incorporates a cutter profile that is forced inside the tubing by jarring or hydraulic force to re-form the tubing wall by removing tubing wall material and forcing the tubing wall into place. |
| Well Completions | tubing end locator (TEL) | A downhole tool frequently used in slickline or coiled tubing tool assemblies to confirm or correlate the tool position on depth-sensitive applications. With the end of the production tubing as a known reference point, any error in measurement that may occur in reaching the treatment depth will be significantly less than what may have resulted if measuring from surface. |
| Well Completions | tubing grade | A system of classifying the material specifications for steel alloys used in the manufacture of tubing. |
| Well Completions | tubing hanger | A device attached to the topmost tubing joint in the wellhead to support the tubing string. The tubing hanger typically is located in the tubing head, with both components incorporating a sealing system to ensure that the tubing conduit and annulus are hydraulically isolated. |
| Well Completions | tubing head | A wellhead component that supports the tubing hanger and provides a means of attaching the Christmas tree to the wellhead. |
| Well Completions | tubing joint | A single length of the pipe that is assembled to provide a conduit through which the oil or gas will be produced from a wellbore. Tubing joints are generally around 30 ft [9 m] long with a thread connection on each end. The specification of the tubing material, geometry of the tubing, and design of the connection thread are selected to suit the reservoir fluid and wellbore conditions. |
| Well Completions | tubing performance curve (TPC) | A mathematical tool used in production engineering to assess the performance of the completion string by plotting the surface production rate against the flowing bottomhole pressure. The fluid composition and behavior of the fluid phases in the specific completion design will determine the shape of the curve. The TPC is used with the inflow performance relationship to predict the performance of a specific well |
| Well Completions | tubing seal assembly | A system of seals arranged on the component that engages in a sealbore to isolate the production-tubing conduit from the annulus. The seal assembly is typically longer than the sealbore to enable some movement of the components while maintaining an efficient seal. |
| Well Completions | tubing thread | The threaded connection used to assemble the tubing string from individual tubing joints. Various tubing thread types have evolved to suit the wellbore conditions and functions required of the tubing string, both during installation and while the well is in production. |
| Well Completions | tubingless completion | A completion design in which the reservoir fluids are produced through small-diameter casing. The absence of a separate tubing string significantly limits the operating and contingency options available for the well. |
| Well Completions | tubing-retrievable safety valve (TRSV) | A type of subsurface safety valve that is run and retrieved as part of the production tubing string. The TRSV body is integral part of the completion that enables the internal components to be configured to provide near fullbore access through the valve. An external control line is secured to the running string for connection to a surface-control system. |
| Well Completions | turbulent flow | A fluid-flow regime characterized by swirling or chaotic motion as the fluid moves along the pipe or conduit. The linear velocity of the fluid particles is similar regardless of position in the conduit, although particles close to the conduit walls have a lower velocity. This characteristic makes turbulent flow an efficient flow regime for the pickup and transport of solids. However, the potential for erosion may be significant, especially with abrasive fluids and a tortuous flow path. |
| Well Completions | velocity string | A small-diameter tubing string run inside the production tubing of a well as a remedial treatment to resolve liquid-loading problems. As the reservoir pressure in a gas well depletes, there may be insufficient velocity to transport all liquids from the wellbore. In time these liquids accumulate and impair production. Installing a velocity string reduces the flow area and increases the flow velocity to enable liquids to be carried from the wellbore. Velocity strings are commonly run using coiled tubing as a velocity string conduit. Safe live-well working and rapid mobilization enable coiled tubing velocity strings to provide a cost effective solution to liquid loading in gas wells. |
| Well Completions | wait on cement | To suspend operations while a cement slurry to develops sufficient compressive strength to allow drilling or other wellbore activity to continue. The WOC time is generally used to test the surface pressure-control equipment, such as the BOP stack. Attempting to drill out the float or guide shoe before the cement has developed sufficient bond strength may result in backing off a casing joint. |
| Well Completions | water block | A condition caused by an increase in water saturation in the near-wellbore area. Water block typically forms during the drilling phase of a well, when the near-wellbore area is exposed to a relatively high volume of filtrate from the drilling fluid. The increased presence of water causes fine clay crystals that may be present in the formation, such as illite, to swell and cause a reduction in permeability. Water-block treatments typically use surfactants to reduce the surface tension between the oil and water, helping to displace the water from the near-wellbore area. |
| Well Completions | water coning | The change in the oil-water contact profile as a result of drawdown pressures during production. Coning occurs in vertical or slightly deviated wells and is affected by the characteristics of the fluids involved and the ratio of horizontal to vertical permeability. |
| Well Completions | water cushion | A volume of water placed in a tubing string prior to conducting a drillstem test or opening a well to flow. The water cushion is designed to reduce and control the pressure drawdown applied to the reservoir when the downhole valve or tester valve is opened to initiate flow. |
| Well Completions | water drive | A reservoir-drive mechanism whereby the oil is driven through the reservoir by an active aquifer. As the reservoir depletes, the water moving in from the aquifer below displaces the oil until the aquifer energy is expended or the well eventually produces too much water to be viable. |
| Well Completions | waterdrive | A reservoir-drive mechanism whereby the oil is driven through the reservoir by an active aquifer. As the reservoir depletes, the water moving in from the aquifer below displaces the oil until the aquifer energy is expended or the well eventually produces too much water to be viable. |
| Well Completions | wellhead | The surface termination of a wellbore that incorporates facilities for installing casing hangers during the well construction phase. The wellhead also incorporates a means of hanging the production tubing and installing the Christmas tree and surface flow-control facilities in preparation for the production phase of the well. |
| Well Completions | wing valve | A valve located on the side of a Christmas tree or temporary surface flow equipment, such as may be used for a drillstem test. Two wing valves are generally fitted to a Christmas tree. A flowing wing valve is used to control and isolate production, and the kill wing valve fitted on the opposite side of the Christmas tree is available for treatment or well-control purposes. The term wing valve typically is used when referring to the flowing wing. |
| Well Completions | wire wrapped screen | A type of screen used in sand control applications to support the gravel pack. The profiled wire is wrapped and welded in place on a perforated liner. Screens are available in a range of sizes and specifications, including outside diameter, material type and the geometry and dimension of the screen slots. The space between each wire wrap must be small enough to retain the gravel placed behind the screen, yet minimize any restriction to production. |
| Well Completions | wireline retrievable safety valve (WRSV) | A type of safety valve in which the principal components can be run and retrieved by wireline or slickline. The valve assembly is landed in a ported nipple that is equipped with a control line connected to the surface control system. This configuration enables the safety valve to be easily retrieved for repair or maintenance, but the resulting internal bore of the WRSV must be relatively small. |
| Well Completions | wireline-retrievable safety valve (WRSV) | A type of safety valve in which the principal components can be run and retrieved by wireline or slickline. The valve assembly is landed in a ported nipple that is equipped with a control line connected to the surface control system. This configuration enables the safety valve to be easily retrieved for repair or maintenance, but the resulting internal bore of the WRSV must be relatively small. |
| Well Completions | wire-wrapped screen | A type of screen used in sand control applications to support the gravel pack. To form the screen, a profiled wire is wrapped and welded in place on a perforated liner. Screens are available in a range of sizes and specifications, including outside diameter, material type and the geometry and dimension of the screen slots. The space between each wire wrap must be small enough to retain the gravel placed behind the screen, yet minimize any restriction to production. |
| Well Completions, Drilling Fluids, Well Workover and Intervention | acetic acid | An organic acid used in oil- and gas-well stimulation treatments. Less corrosive than the commonly used hydrochloric acid, acetic acid treatments can be more easily inhibited or retarded for treatments of long duration. This is necessary particularly in applications requiring the protection of exotic alloys or in high-temperature wells. In most cases, acetic acid is used in conjunction with hydrochloric acid and other acid additives. It can also be used as a chelating agent. |
| Well Completions, Drilling Fluids, Well Workover and Intervention | sequestering agent | Another term for chelating agent, a chemical used to bind metal ions to form a ring structure. Chelating agents stabilize or prevent the precipitation of damaging compounds. In the oil field, chelating agents are used in stimulation treatments and for cleaning surface facilities. They are also used to treat or remove scale or weighting agents in reservoir drilling fluids. During acid or scale-removal treatments, various compounds may be dissolved in the treatment fluid. As the acid reacts and the pH increases, reaction products may precipitate as a gelatinous, insoluble mass. Should this occur within the formation matrix, it is almost impossible to remove and permanent permeability damage may occur. Chelating agents prevent precipitation by keeping ions in a soluble form until the treatment fluid can be flowed back from the formation during cleanup. Typical oilfield chelating agents include EDTA (ethylenediamine tetraacetic acid), HEDTA (hydroxyethylenediamine triacetic acid), NTA (nitriolotriacetic acid) and citric acid. |
| Well Completions, Enhanced Oil Recovery | breakthrough | A description of reservoir conditions under which a fluid, previously isolated or separated from production, gains access to a producing wellbore. The term is most commonly applied to water or gas breakthrough, where the water or gas injected to maintain reservoir pressure via injection wells breaks through to one or more of the producing wells |
| Well Completions, Enhanced Oil Recovery | water flood | A method of secondary recovery in which water is injected into the reservoir formation to displace residual oil. The water from injection wells physically sweeps the displaced oil to adjacent production wells. Potential problems associated with waterflood techniques include inefficient recovery due to variable permeability, or similar conditions affecting fluid transport within the reservoir, and early water breakthrough that may cause production and surface processing problems |
| Well Completions, Enhanced Oil Recovery | waterflood | A method of secondary recovery in which water is injected into the reservoir formation to displace residual oil. The water from injection wells physically sweeps the displaced oil to adjacent production wells. Potential problems associated with waterflood techniques include inefficient recovery due to variable permeability, or similar conditions affecting fluid transport within the reservoir, and early water breakthrough that may cause production and surface processing problems. |
| Well Completions, Enhanced Oil Recovery | water-wet | Pertaining to the adhesion of a liquid to the surface of a solid. In water-wet conditions, a thin film of water coats the surface of the formation matrix, a condition that is desirable for efficient oil transport. Treatments that change the wettability of the formation from water-wet to oil-wet can significantly impair productivity. |
| Well Completions, Geophysics | blasting cap | A device containing primary high-explosive material that is used to initiate an explosive sequence. The two common types of detonators are electrical detonators (also known as blasting caps) and percussion detonators. Electrical detonators have a fuse material that burns when high voltage is applied to initiate the primary high explosive. Percussion detonators contain abrasive grit and primary high explosive in a sealed container that is activated by a firing pin. The impact force of the firing pin is sufficient to initiate the ballistic sequence that is then transmitted to the detonating cord. Several safety systems are used in conjunction with detonators to avoid accidental firing during rig-up or rig-down. Safety systems also are used to disarm the gun or ballistic assembly if downhole conditions are unsafe for firing. |
| Well Completions, Geophysics | blasting cap | A small, electrically activated explosive charge that explodes a larger charge. Detonators, also called caps, seismic caps or blasting caps, are used for seismic acquisition with an explosive source to achieve consistent timing of detonation. |
| Well Completions, Production Logging | churn flow | A multiphase flow regime in near-vertical pipes in which large, irregular slugs of gas move up the center of the pipe, usually carrying droplets of oil or water with them. Most of the remaining oil or water flows up along the pipe walls. The flow is relatively chaotic, producing a frothy mixture. Unlike slug flow, neither phase is continuous. The gas slugs are relatively unstable, and take on large, elongated shapes. Also known as transition flow, this flow is an intermediate flow condition between slug flow and mist flow, and occurs at relatively high gas velocity. As the gas velocity increases, it changes into annular flow. |
| Well Completions, Production Logging | froth flow | A multiphase flow regime in near-vertical pipes in which large, irregular slugs of gas move up the center of the pipe, usually carrying droplets of oil or water with them. Most of the remaining oil or water flows up along the pipe walls. The flow is relatively chaotic, producing a frothy mixture. Unlike slug flow, neither phase is continuous. The gas slugs are relatively unstable, and take on large, elongated shapes. Also known as transition flow, this flow is an intermediate flow condition between slug flow and mist flow, and occurs at relatively high gas velocity. As the gas velocity increases, it changes into annular flow. |
| Well Completions, Shale Gas | completion | A generic term used to describe the events and equipment necessary to bring a wellbore into production once drilling operations have been concluded, including but not limited to the assembly of downhole tubulars and equipment required to enable safe and efficient production from an oil or gas well. Completion quality can significantly affect production from shale reservoirs. |
| Well Completions, Shale Gas | completion quality (CQ) | A prediction of how effectively rock may be stimulated using hydraulic fracturing. Completion quality (CQ) is an engineering assessment of factors that determine the effectiveness of hydraulic fracture treatments and includes the ability to initiate and create an induced fracture network, the degree of reservoir contact of the newly created fractures, the level of connection to the natural fracture system of those created fractures and ability of the stimulated reservoir to deliver gas or oil into the well. Important factors that affect stimulation design include the rock’s mineralogy, porosity, mechanical properties, compressive strength and tensile strength. In addition, the presence and state of natural fractures, the in situ stresses and the formation pore pressure are input into the derivation of CQ. Mineral content, porosity and natural fractures determine the mechanical properties and strength of rock. The direction of the minimum stress strongly affects the hydraulic fracture propagation direction. The difference between the maximum and minimum stresses influences the amount of branching in the induced fracture network; a low stress difference favors maximal branching. Variations in the stress and mechanical properties between layers control the height of fracture growth. Pore pressure affects the stresses. Pressure from nearby hydraulic fracture treatments or pressure depletion from production in nearby wells can alter the stresses and influence the effectiveness of the current hydraulic fracture stimulation. |
| Well Completions, Shale Gas | fracturing fluid | A fluid injected into a well as part of a stimulation operation. Fracturing fluids for shale reservoirs usually contain water, proppant, and a small amount of nonaqueous fluids designed to reduce friction pressure while pumping the fluid into the wellbore. These fluids typically include gels, friction reducers, crosslinkers, breakers and surfactants similar to household cosmetics and cleaning products; these additives are selected for their capability to improve the results of the stimulation operation and the productivity of the well. |
| Well Completions, Shale Gas | microseismic monitoring | A technique to track the propagation of a hydraulic fracture as it advances through a formation. Microseisms are detected, located, and displayed in time for scientists and engineers to approximate the location and propagation of the hydraulic fracture. Software provides modeling, survey design, microseismic detection and location, uncertainty analysis, data integration, and visualization for interpretation. Computer imagery is used to monitor the activity in 3D space relative to the location of the fracturing treatment. The monitored activities are animated to show progressive fracture growth and the subsurface response to pumping variations. When displayed in real time, the microseismic activity allows one to make changes to the stimulation design to ensure optimal reservoir contact. Also known as hydraulic fracture monitoring, this technique delivers information about the effectiveness of the stimulation of a reservoir that can be used to enhance reservoir development in shale gas completions. |
| Well Completions, Shale Gas, Well Testing, Well Workover and Intervention | formation fracture pressure | Pressure above which injection of fluids will cause the rock formation to fracture hydraulically. |
| Well Completions, Shale Gas, Well Workover and Intervention | frac job | Another term for hydraulic fracturing, a stimulation treatment routinely performed on oil and gas wells in low-permeability reservoirs. Specially engineered fluids are pumped at high pressure and rate into the reservoir interval to be treated, causing a vertical fracture to open. The wings of the fracture extend away from the wellbore in opposing directions according to the natural stresses within the formation. Proppant, such as grains of sand of a particular size, is mixed with the treatment fluid to keep the fracture open when the treatment is complete. Hydraulic fracturing creates high-conductivity communication with a large area of formation and bypasses any damage that may exist in the near-wellbore area. |
| Well Completions, Well Workover and Intervention | acid stimulation | The treatment of a reservoir formation with a stimulation fluid containing a reactive acid. In sandstone formations, the acid reacts with the soluble substances in the formation matrix to enlarge the pore spaces. In carbonate formations, the acid dissolves the entire formation matrix. In each case, the matrix acidizing treatment improves the formation permeability to enable enhanced production of reservoir fluids. Matrix acidizing operations are ideally performed at high rate, but at treatment pressures below the fracture pressure of the formation. This enables the acid to penetrate the formation and extend the depth of treatment while avoiding damage to the reservoir formation. |
| Well Completions, Well Workover and Intervention | acidize | To pump acid into the wellbore to remove near-well formation damage and other damaging substances. This procedure commonly enhances production by increasing the effective well radius. When performed at pressures above the pressure required to fracture the formation, the procedure is often referred to as acid fracturing. |
| Well Completions, Well Workover and Intervention | frac balls | Another term for ball sealers, small spheres designed to seal perforations that are accepting the most fluid, thereby diverting reservoir treatments to other portions of the target zone. Ball sealers are incorporated into the treatment fluid and pumped with it. The effectiveness of this type of mechanical diversion to keep the balls in place is strongly dependent on the differential pressure across the perforation and the geometry of the perforation itself. |
| Well Completions, Well Workover and Intervention | hydrofluoric hydrochloric acid | A mixture of hydrofluoric acid [HF] and hydrochloric acid [HCl] or organic acid used as the main fluid in a sandstone matrix treatment. Hydrochloric acid or organic acid is mixed with HF to keep the pH low when it spends, thereby preventing detrimental precipitates. The name mud acid was given to these mixtures because they were originally developed to treat damage from siliceous drilling muds. |
| Well Completions, Well Workover and Intervention | hydrofluoric-hydrochloric acid | A mixture of hydrofluoric acid [HF] and hydrochloric acid [HCl] or organic acid used as the main fluid in a sandstone matrix treatment. Hydrochloric acid or organic acid is mixed with HF to keep the pH low when it spends, thereby preventing detrimental precipitates. The name mud acid was given to these mixtures because they were originally developed to treat damage from siliceous drilling muds. |
| Well Completions, Well Workover and Intervention | mud acid | A mixture of hydrofluoric acid [HF] and hydrochloric acid [HCl] or organic acid used as the main fluid in a sandstone matrix treatment. Hydrochloric acid or organic acid is mixed with HF to keep the pH low when it spends, thereby preventing detrimental precipitates. The name mud acid was given to these mixtures because they were originally developed to treat damage from siliceous drilling muds. Mud acid is also called hydrofluoric-hydrochloric acid. |
| Well Completions, Well Workover and Intervention | release joint | A downhole tool that is designed to part under controlled conditions. A release joint enables part of the tool string to be left in the wellbore while the running string is retrieved. |
| Well Completions, Well Workover and Intervention | safety joint | Another term for release joint, a downhole tool that is designed to part under controlled conditions. A release joint enables part of the tool string to be left in the wellbore while the running string is retrieved. |
| Well Completions, Well Workover and Intervention | screenout | A condition encountered during some gravel-pack operations whereby the treatment area cannot accept further pack sand and a sudden increase in treatment pressure occurs. Under ideal conditions, this should signify that the entire void area has been successfully packed with sand. However, if screenout occurs early in the treatment, it may indicate an incomplete treatment and the presence of undesirable voids within the pack zone. |
| Well Completions, Well Workover and Intervention, Drilling Fluids | chelating agent | A chemical used to bind metal ions to form a ring structure. Chelating agents stabilize or prevent the precipitation of damaging compounds. In the oil field, chelating agents are used in stimulation treatments and for cleaning surface facilities. They are also used to treat or remove scale or weighting agents in reservoir drilling fluids. During acid or scale-removal treatments, various compounds may be dissolved in the treatment fluid. As the acid reacts and the pH increases, reaction products may precipitate as a gelatinous, insoluble mass. Should this occur within the formation matrix, it is almost impossible to remove and permanent permeability damage may occur. Chelating agents prevent precipitation by keeping ions in a soluble form until the treatment fluid can be flowed back from the formation during cleanup. Typical oilfield chelating agents include EDTA (ethylenediamine tetraacetic acid), HEDTA (hydroxyethylenediamine triacetic acid), NTA (nitriolotriacetic acid) and citric acid. |
| Well Completions, Well Workover and Intervention, Drilling Fluids | chemical diverter | A chemical agent used in stimulation treatments to ensure uniform injection over the area to be treated. Chemical diverters function by creating a temporary blocking effect that is safely cleaned up following the treatment, enabling enhanced productivity throughout the treated interval. In matrix acidizing of injection wells, benzoic acid is used as a chemical diverter, while oil-soluble resins are employed in production wells. Both compounds are slightly soluble or inert in the acidic medium [HCl], but after functioning as diverters, they dissolve with water injection or oil production, respectively. Stable, viscous foams generated in the rock matrix are also considered to be chemical diverters. |
| Well Completions | bottomhole injection pressure (bhip) | The downhole pressure at which a treatment fluid can be injected into a zone of interest. The bottomhole injection pressure is typically calculated by adding the hydrostatic pressure of the fluid column to the surface pump pressure measured during an injection test. |
| Well Completions | complete a well | To perform activities in the final stages of well construction to prepare a well for production. The well is completed once zones of interest have been identified. |
| Well Completions | disposal well | A well, often a depleted oil or gas well, into which waste fluids can be injected for safe disposal. Disposal wells typically are subject to regulatory requirements to avoid the contamination of freshwater aquifers. |
| Well Testing | early time transient data | The data observed before the start of radial flow (middle-time transient data). When not dominated by wellbore storage, these data may reveal near-wellbore reservoir features including limited entry, hydraulic fracture and dual-porosity or layering effects. Downhole shut-in greatly improves the quality of the early-time transient data. |
| Well Testing | early-time transient data | The data observed before the start of radial flow (middle-time transient data). When not dominated by wellbore storage, these data may reveal near-wellbore reservoir features including limited entry, hydraulic fracture and dual-porosity or layering effects. Downhole shut-in greatly improves the quality of the early-time transient data. |
| Well Testing | non-Darcy flow | Fluid flow that deviates from Darcy’s law, which assumes laminar flow in the formation. Non-Darcy flow is typically observed in high-rate gas wells when the flow converging to the wellbore reaches flow velocities exceeding the Reynolds number for laminar or Darcy flow, and results in turbulent flow. Since most of the turbulent flow takes place near the wellbore in producing formations, the effect of non-Darcy flow is a rate-dependent skin effect. |
| Well Testing | adjusted flow time | The approximated flow time used for a well-test analysis when the flow rate varies before or during the test period. It is calculated as t = cumulative well production since the last extended shut-in period divided by the flow rate just before a well is shut in for a buildup test. |
| Well Testing | afterflow | The flow associated with wellbore storage following a surface shut-in. When a well is first shut in at the surface, flow from the formation into the bottom of the wellbore continues unabated until compression of the fluids in the wellbore causes the downhole pressure to rise. If the wellbore fluid is highly compressible and the well rate is low, the afterflow period can be long. Conversely, high-rate wells producing little gas have negligible afterflow periods. |
| Well Testing | anisotropic formation | A formation with directionally dependent properties. The most common directionally dependent properties are permeability and stress. Most formations have vertical to horizontal permeability anisotropy with vertical permeability being much less (often an order of magnitude less) than horizontal permeability. Bedding plane permeability anisotropy is common in the presence of natural fractures. Stress anisotropy is frequently greatest between overburden stress and horizontal stress in the bedding plane. Bedding plane stress contrasts are common in tectonically active regions. Permeability anisotropy can sometimes be related to stress anisotropy. |
| Well Testing | average reservoir pressure | The pressure that would be obtained if all fluid motion ceases in a given volume of reservoir. It also is the pressure to which a well will ultimately rise if shut in for an infinite period. |
| Well Testing | back flow | Fluid flow in the borehole from one zone into another in response to pressure differences between the zones. Any time the wellbore pressure rises above the average pressure in any zone, backflow will occur. Analysis of buildup tests involving backflow is either impossible or extremely difficult and usually requires expert input to determine useful information from such tests. |
| Well Testing | backflow | Fluid flow in the borehole from one zone into another in response to pressure differences between the zones. Any time the wellbore pressure rises above the average pressure in any zone, backflow will occur. Analysis of buildup tests involving backflow is either impossible or extremely difficult and usually requires expert input to determine useful information from such tests. |
| Well Testing | big-hole charge | A flow regime resulting from combined simultaneous linear flow in perpendicular directions. This flow regime is seen most commonly in tests of hydraulically fractured wells and occurs for finite-conductivity fracture where linear flow exists both in the fracture and to the fracture plane. This flow regime is recognized as a 1/4 slope in the pressure derivative on the log-log diagnostic plot. Its presence enables determination of the fracture conductivity. |
| Well Testing | bottomhole pressure | The pressure measured in a well at or near the depth of the producing formation. For well-test purposes, it is often desirable to refer the pressure to a datum level chosen at a reference depth by calculating the pressure that would occur if the pressure measurement were made at the datum level rather than at the actual depth of the gauge. |
| Well Testing | bottomhole shut in | A well shut in slightly above the producing formation by use of special downhole tools containing a valve that can be preprogrammed or controlled from the surface. This practice is commonly associated with drillstem tests. Technology exists to employ bottomhole shut-in in suitably equipped completed wells. |
| Well Testing | bottomhole shut-in | A well shut in slightly above the producing formation by use of special downhole tools containing a valve that can be preprogrammed or controlled from the surface. This practice is commonly associated with drillstem tests. Technology exists to employ bottomhole shut-in in suitably equipped completed wells. |
| Well Testing | boundary conditions | The flux (flow rate) or pressure states assigned to the theoretical boundaries used in developing and solving the differential equations that apply to well testing and in specifying a model to match to pressure-transient data. |
| Well Testing | bounded reservoir | Reservoirs with sealed or apparent outer boundaries that result in pressure depletion. Mathematical treatments differ between bounded and infinite reservoirs. |
| Well Testing | buildup test | The measurement and analysis of (usually) bottomhole pressure data acquired after a producing well is shut in. Buildup tests are the preferred means to determine well flow capacity, permeability thickness, skin effect and other information. Soon after a well is shut in, the fluid in the wellbore usually reaches a somewhat quiescent state in which bottomhole pressure rises smoothly and is easily measured. This allows interpretable test results. |
| Well Testing | closed chamber testing | A type of drillstem testing conducted with the drillstring in the hole and the surface valve closed to create a closed chamber of known volume into which the reservoir fluid can flow. The drillstring is sometimes filled with nitrogen at a relatively low pressure prior to testing. When the well begins to flow, the nitrogen or air is compressed and the volume of fluid inflow can be calculated as a function of time by monitoring the surface pressure in the drillstring. The bottomhole valve is closed to halt flow when the surface pressure reaches a value calculated prior to testing. This ensures that a precisely known amount of production has taken place. |
| Well Testing | closed in well | A well with a valve closed to halt production. Wells are often closed in for a period of time to allow stabilization prior to beginning a drawdown-buildup test sequence. |
| Well Testing | closed-chamber testing | A type of drillstem testing conducted with the drillstring in the hole and the surface valve closed to create a closed chamber of known volume into which the reservoir fluid can flow. The drillstring is sometimes filled with nitrogen at a relatively low pressure prior to testing. When the well begins to flow, the nitrogen or air is compressed and the volume of fluid inflow can be calculated as a function of time by monitoring the surface pressure in the drillstring. The bottomhole valve is closed to halt flow when the surface pressure reaches a value calculated prior to testing. This ensures that a precisely known amount of production has taken place. |
| Well Testing | closed-in well | A well with a valve closed to halt production. Wells are often closed in for a period of time to allow stabilization prior to beginning a drawdown-buildup test sequence. |
| Well Testing | commercial rate | A rate, or production volume, sufficient to satisfy project economics. |
| Well Testing | commingled completion | A wellbore completed in two or more reservoir zones that are not in hydraulic communication in the reservoir. Backflow (often incorrectly referred to as crossflow) is common during rate cutbacks and buildup tests on these types of completions. Analysis of buildup data acquired from a commingled completion can be difficult or impossible. |
| Well Testing | constant rate | A flow rate that does not change appreciably during a test period. Flow rates are never truly constant, but changes of only a few percent do not affect the results of well-test analysis appreciably if the rate is averaged over the flow period. |
| Well Testing | continuity equation | Material balance expressed in a differential equation. |
| Well Testing | convolution | A mathematical operation that uses downhole flow-rate measurements to transform bottomhole pressure measurements distorted by variable rates to an interpretable transient. Convolution also can use surface rates to transform wellhead pressures to an interpretable form. Convolution assumes a particular model for the pressure-transient response, usually infinite-acting radial flow. This operation is similar to what is done to account for the flow history in rigorous pressure-transient analysis. |
| Well Testing | convolve | To perform a convolution, which is a mathematical operation that uses downhole flow-rate measurements to transform bottomhole pressure measurements distorted by variable rates to an interpretable transient. Convolution also can use surface rates to transform wellhead pressures to an interpretable form. Convolution assumes a particular model for the pressure-transient response, usually infinite-acting radial flow. This operation is similar to what is done to account for the flow history in rigorous pressure-transient analysis. |
| Well Testing | correcting pressure to a datum | A procedure for correcting pressure measurements in a reservoir to a common datum level. This is not required for calculating kh, permeability thickness, and s, skin effect, but is required for determining average reservoir pressures or for any comparison of pressures in one area of the reservoir to those in another area. The correction is done by determining the average pressure for a given well test in which the pressure gauge is at a known level, and then adding or subtracting the calculated weight of the column of reservoir fluid in pounds per square inch from the difference in elevation between the pressure gauge and the datum level. |
| Well Testing | cushion | A fluid column (usually water or nitrogen) put in the drillstem to provide the desired backpressure at the start of a drillstem test. The cushion usually serves to limit the differential pressure across the test string and packer to avoid flow below the bubblepoint pressure (in which case water is the usual cushion) or to enable a depleted reservoir to flow (nitrogen is the likely cushion). |
| Well Testing | Darcy units | Units of atm, cm3/s, cp and D, as originally used by Darcy in flow experiments. |
| Well Testing | datum level | The depth to which pressures are corrected to adjust for differences in elevations at which pressure measurements are made in different wells or at different times. |
| Well Testing | deconvolution | A mathematical operation that uses downhole flow-rate measurements to transform bottomhole pressure measurements distorted by variable rates to an interpretable transient. Deconvolution also can use surface rates to transform wellhead pressures to an interpretable form. Deconvolution has the advantage over convolution that it does not assume a particular model for the pressure-transient response. However, the simplest form of deconvolution often gives a noisy result, and more complex approaches may be computing intensive. |
| Well Testing | depletion | The drop in reservoir pressure or hydrocarbon reserves resulting from production of reservoir fluids. At times, a strong waterdrive will maintain reservoir pressure to a substantial degree so that reserves diminish without a corresponding pressure decline. |
| Well Testing | diffusion equation | A fundamental differential equation obtained by combining the continuity equation, flow law and equation of state. Most of the mathematics of well testing were derived from solutions of this equation, which was originally developed for the study of heat transfer. Fluid flow through porous media is directly analogous to flow of heat through solids. Solutions used in well testing usually assume radial flow and homogenous, isotropic formations. |
| Well Testing | directional permeability | Permeability that varies with direction of flow through the porous medium. Lateral permeability contrast can be particularly important in fractured formations, where effective permeability in the direction of the fractures may be many times greater than the matrix permeability. If the permeability in one direction is significantly more than in the other, the flow pattern may more closely approximate linear flow than radial flow. This can be detected from well-test data. Likewise, laminations in most clastic formations cause the permeability normal to the bedding plane to be less than the lateral permeability parallel to bedding. This is called vertical to horizontal permeability anisotropy. |
| Well Testing | drainage area | The reservoir area or volume drained by the well. The terms drainage area, reservoir area and drainage volume are often incorrectly used interchangeably. When several wells drain the same reservoir, each drains its own drainage area, a subset of the reservoir area. |
| Well Testing | drainage volume | The portion of the volume of a reservoir drained by a well. In a reservoir drained by multiple wells, the volume ultimately drained by any given well is proportional to that well’s production rate: Vi = Vt x qi/qt, where Vi is the drainage volume of Well i, Vt is the entire drainage volume of the reservoir, qi is the production rate from Well i, and qt is the total production rate from the reservoir. |
| Well Testing | drawdown test | The measurement and analysis of pressure data taken after a well is put on production, either initially or following an extended shut-in period. Drawdown data are usually noisy, meaning that the pressure moves up and down as fluid flows past the gauges and minute variations in flow rate take place. This is especially true for new wells, in which well cleanup commonly occurs for days after production has begun. Such data are difficult to interpret, and the noise often obscures regions of interest to the analyst. Transient downhole flow rates measured while flowing can be used to correct pressure variations through convolution or deconvolution calculations that enable diagnosis and interpretation, analogous to that done for the pressure change and derivative. |
| Well Testing | drillstem test | Well tests conducted with the drillstring still in the hole. Often referred to as DST, these tests are usually conducted with a downhole shut-in tool that allows the well to be opened and closed at the bottom of the hole with a surface-actuated valve. One or more pressure gauges are customarily mounted into the DST tool and are read and interpreted after the test is completed. The tool includes a surface-actuated packer that can isolate the formation from the annulus between the drillstring and the casing, thereby forcing any produced fluids to enter only the drillstring. By closing in the well at the bottom, afterflow is minimized and analysis is simplified, especially for formations with low flow rates. The drillstring is sometimes filled with an inert gas, usually nitrogen, for these tests. With low-permeability formations, or where the production is mostly water and the formation pressure is too low to lift water to the surface, surface production may never be observed. In these cases, the volume of fluids produced into the drillstring is calculated and an analysis can be made without obtaining surface production. Occasionally, operators may wish to avoid surface production entirely for safety or environmental reasons, and produce only that amount that can be contained in the drillstring. This is accomplished by closing the surface valve when the bottomhole valve is opened. These tests are called closed-chamber tests. Drillstem tests are typically performed on exploration wells, and are often the key to determining whether a well has found a commercial hydrocarbon reservoir. The formation often is not cased prior to these tests, and the contents of the reservoir are frequently unknown at this point, so obtaining fluid samples is usually a major consideration. Also, pressure is at its highest point, and the reservoir fluids may contain hydrogen sulfide, so these tests can carry considerable risk for rig personnel. The most common test sequence consists of a short flow period, perhaps five or ten minutes, followed by a buildup period of about an hour that is used to determine initial reservoir pressure. This is followed by a flow period of 4 to 24 hours to establish stable flow to the surface, if possible, and followed by the final shut-in or buildup test that is used to determine permeability thickness and flow potential. |
| Well Testing | DST | Well tests conducted with the drillstring still in the hole. Often referred to as DST, these tests are usually conducted with a downhole shut-in tool that allows the well to be opened and closed at the bottom of the hole with a surface-actuated valve. One or more pressure gauges are customarily mounted into the DST tool and are read and interpreted after the test is completed. The tool includes a surface-actuated packer that can isolate the formation from the annulus between the drillstring and the casing, thereby forcing any produced fluids to enter only the drillstring. By closing in the well at the bottom, afterflow is minimized and analysis is simplified, especially for formations with low flow rates. The drillstring is sometimes filled with an inert gas, usually nitrogen, for these tests. With low-permeability formations, or where the production is mostly water and the formation pressure is too low to lift water to the surface, surface production may never be observed. In these cases, the volume of fluids produced into the drillstring is calculated and an analysis can be made without obtaining surface production. Occasionally, operators may wish to avoid surface production entirely for safety or environmental reasons, and produce only that amount that can be contained in the drillstring. This is accomplished by closing the surface valve when the bottomhole valve is opened. These tests are called closed-chamber tests. Drillstem tests are typically performed on exploration wells, and are often the key to determining whether a well has found a commercial hydrocarbon reservoir. The formation often is not cased prior to these tests, and the contents of the reservoir are frequently unknown at this point, so obtaining fluid samples is usually a major consideration. Also, pressure is at its highest point, and the reservoir fluids may contain hydrogen sulfide, so these tests can carry considerable risk for rig personnel. The most common test sequence consists of a short flow period, perhaps five or ten minutes, followed by a buildup period of about an hour that is used to determine initial reservoir pressure. This is followed by a flow period of 4 to 24 hours to establish stable flow to the surface, if possible, and followed by the final shut-in or buildup test that is used to determine permeability thickness and flow potential |
| Well Testing | DST pressure chart | The characteristic plot of pressure versus time obtained from the mechanical recording of pressure gauges in a DST tool. Pressure rises as the tool is lowered into the hole and the hydrostatic head above the tool increases. The pressure stabilizes when the tool reaches bottom and then moves when the packer is set. Pressure drops immediately upon opening of the downhole valve to match the pressure in the drillstring, and then rises as fluid flows into the string. When the downhole valve is closed, the pressure buildup period begins immediately and continues until the valve is closed again. |
| Well Testing | dual completion | A wellbore with simultaneous production of hydrocarbons, water or both from more than one producing zone. Although the term refers to cases in which only two separate zones are present, in actuality there may be multiple zones involved. This completion technique avoids backflow from one reservoir zone to another in the wellbore. |
| Well Testing | dual permeability reservoir | A dual-porosity reservoir in which flow to the well occurs in both primary and secondary porosity systems. |
| Well Testing | dual porosity reservoir | A rock characterized by primary porosity from original deposition and secondary porosity from some other mechanism, and in which all flow to the well effectively occurs in one porosity system, and most of the fluid is stored in the other. Naturally fractured reservoirs and vugular carbonates are classified as dual-porosity reservoirs, as are layered reservoirs with extreme contrasts between high-permeability and low-permeability layers. |
| Well Testing | dual-permeability reservoir | A dual-porosity reservoir in which flow to the well occurs in both primary and secondary porosity systems. |
| Well Testing | dual-porosity reservoir | A rock characterized by primary porosity from original deposition and secondary porosity from some other mechanism, and in which all flow to the well effectively occurs in one porosity system, and most of the fluid is stored in the other. Naturally fractured reservoirs and vugular carbonates are classified as dual-porosity reservoirs, as are layered reservoirs with extreme contrasts between high-permeability and low-permeability layers. |
| Well Testing | effective wellbore radius | The value of wellbore radius that produces equivalent results to those obtained using a skin factor of zero. It is possible to represent departures from ideal (zero skin) behavior by using the skin effect, or alternatively by changing the value of the effective wellbore radius. For a positive skin effect, effective wellbore radius is smaller than actual wellbore radius. For a negative skin effect, such as often obtained after well stimulation, effective wellbore radius is larger than actual well radius. |
| Well Testing | fall off test | The measurement and analysis of pressure data taken after an injection well is shut in. These data are often the easiest transient well-test data to obtain. Wellhead pressure rises during injection, and if the well remains full of liquid after shut-in of an injector, the pressure can be measured at the surface, and bottomhole pressures can be calculated by adding the pressure from the hydrostatic column to the wellhead pressure. Since most water-injection wells are fractured during injection, and injection wells often go on vacuum, the fluid level can fall below the surface. Dealing with this complication requires reverting to bottomhole pressure gauges or sonic devices. |
| Well Testing | falloff test | The measurement and analysis of pressure data taken after an injection well is shut in. These data are often the easiest transient well-test data to obtain. Wellhead pressure rises during injection, and if the well remains full of liquid after shut-in of an injector, the pressure can be measured at the surface, and bottomhole pressures can be calculated by adding the pressure from the hydrostatic column to the wellhead pressure. Since most water-injection wells are fractured during injection, and injection wells often go on vacuum, the fluid level can fall below the surface. Dealing with this complication requires reverting to bottomhole pressure gauges or sonic devices. |
| Well Testing | final flow period | The final flow sequence in a drillstem test. The initial shut-in period is usually followed immediately by an extended period of flow. The well is then shut in for the final buildup period. Data obtained during the final buildup period are analyzed to determine permeability thickness, kh, and skin effect, s. Final flow periods commonly range from 4 to 24 hours, and final buildup periods from 6 to 48 hours. |
| Well Testing | final flow rate | The production rate just prior to shut-in for a buildup test. |
| Well Testing | final flowing pressure | The pressure determined at the formation face just prior to shut-in for a buildup test. This value is required to determine the skin effect. |
| Well Testing | final shut in period | The final buildup sequence in a drillstem test. The initial shut-in period is usually followed immediately by an extended period of flow. The well is then shut in for the final buildup period. Data obtained during this period are analyzed to determine permeability thickness, kh, and skin effect, s. Final flow periods commonly range from 4 to 24 hours, and final buildup periods from 6 to 48 hours. |
| Well Testing | final shut-in period | The final buildup sequence in a drillstem test. The initial shut-in period is usually followed immediately by an extended period of flow. The well is then shut in for the final buildup period. Data obtained during this period are analyzed to determine permeability thickness, kh, and skin effect, s. Final flow periods commonly range from 4 to 24 hours, and final buildup periods from 6 to 48 hours. |
| Well Testing | finite conductivity fracture | A planar crack penetrated by a well or propagated from a well by hydraulic fracturing with nonzero pressure drop in the fracture during production. |
| Well Testing | finite wellbore solution | The solution to the diffusion equation that results when the well (inner) boundary condition is treated as a cylinder of finite radius instead of treating the well as a line source |
| Well Testing | finite-conductivity fracture | A planar crack penetrated by a well or propagated from a well by hydraulic fracturing with nonzero pressure drop in the fracture during production. |
| Well Testing | finite-wellbore solution | The solution to the diffusion equation that results when the well (inner) boundary condition is treated as a cylinder of finite radius instead of treating the well as a line source. |
| Well Testing | flow after flow tests | Gas-well tests, often required by law, in which one flow rate immediately follows another, with each flow period reaching stabilized flow. The bottomhole pressure at the end of each flow period is used to calculate gas-well deliverability. |
| Well Testing | flow efficiency | The value that results when the actual productivity index is divided by the productivity index predicted from Darcy’s law. Flow efficiency is greater than 1 in a stimulated well (skin < 0) and less than 1 for a damaged completion (skin> 0). |
| Well Testing | flow period | Part of a well test when the well is flowing. It is usually specified prior to tests to ensure that a stable flow situation has been reached, or that the pressure disturbance has reached far enough into the formation to allow determination of a representative value for kh, or that any nearby boundaries could be recognized in a subsequent buildup. |
| Well Testing | flow regime | The predominant flow geometry reflected in a pressure-transient response that is most easily recognized in the log-log presentation of the pressure-change derivative. The most easily recognized flow regime is radial flow, which produces a constant or flat derivative. Spherical flow, which may result from a limited-entry completion, has a characteristic -1/2 slope in the derivative. Wellbore storage starts as a unit slope in pressure change and then the derivative bends over in a characteristic hump shape. Linear flow produced by flow to a fracture or a long horizontal well has a derivative slope of +1/2. Bilinear flow results when the fracture has finite conductivity and has a derivative slope of +1/4. |
| Well Testing | flow unit | A rock volume with identifiable fluid flow characteristics that can be modeled, including heterogeneity or anisotropy. |
| Well Testing | flow-after-flow tests | Gas-well tests, often required by law, in which one flow rate immediately follows another, with each flow period reaching stabilized flow. The bottomhole pressure at the end of each flow period is used to calculate gas-well deliverability. |
| Well Testing | flowing pressure | The pressure determined at the formation face during the flowing periods of a well test. |
| Well Testing | flowing well | A well in which the formation pressure is sufficient to produce oil at a commercial rate without requiring a pump. Most reservoirs are initially at pressures high enough to allow a well to flow naturally. |
| Well Testing | fluid compressibility | The relative change in fluid volume related to a unit change in pressure. This is usually expressed as volume change per unit volume of fluid per psi of pressure change. Gas has higher compressibility than liquid (oil or water). |
| Well Testing | fracture conductivity | Product of fracture permeability times fracture width for a finite-conductivity fracture. |
| Well Testing | fracture half-length | Radial distance from the wellbore to the outer tip of a fracture penetrated by the well or propagated from the well by hydraulic fracturing. |
| Well Testing | fractured well analysis | Analysis of a well that passes through a natural fracture or that has been hydraulically fractured. The fracture is treated as a slab of high permeability that is an effective extension of the actual wellbore. Flow is from the reservoir to the fracture and through the fracture to the well. The pressure-transient analysis for a fractured well can determine the fracture half-length and the fracture conductivity, as well as a fracture-face skin. The skin factor for the fracture is negative and usually ranges from -1.5 to -5, with an absolute minimum of -6 in rare cases. For effectively infinite-conductivity fractures, the apparent wellbore radius is half the fracture half-length, or xf/2. |
| Well Testing | fractured-well analysis | Analysis of a well that passes through a natural fracture or that has been hydraulically fractured. The fracture is treated as a slab of high permeability that is an effective extension of the actual wellbore. Flow is from the reservoir to the fracture and through the fracture to the well. The pressure-transient analysis for a fractured well can determine the fracture half-length and the fracture conductivity, as well as a fracture-face skin. The skin factor for the fracture is negative and usually ranges from -1.5 to -5, with an absolute minimum of -6 in rare cases. For effectively infinite-conductivity fractures, the apparent wellbore radius is half the fracture half-length, or xf/2. |
| Well Testing | free gas | Gas that exists in the reservoir in the gaseous phase rather than in solution. As soon as formation pressure drops below the bubblepoint, gas is evolved. This is referred to as free gas while it is in the reservoir. |
| Well Testing | gas condensate | Hydrocarbon liquid dissolved in saturated natural gas that comes out of solution when the pressure drops below the dewpoint. |
| Well Testing | gas deviation factor | The factor z, defined by the real (nonideal) gas equation of state, pV = znRT. Values for z have been correlated for mixtures of reservoir gases over pressure and temperature ranges encountered in well tests. |
| Well Testing | gas formation volume factor | Gas volume at reservoir conditions divided by gas volume at standard conditions. This factor is used to convert surface measured volumes to reservoir conditions, just as oil formation volume factors are used to convert surface measured oil volumes to reservoir volumes. |
| Well Testing | gas gravity | Hydrocarbon gas density expressed as the ratio of the molecular weight of the gas to the molecular weight of air. |
| Well Testing | gas solubility factor | The volume of gas (at standard conditions) liberated from a volume of oil (at standard conditions). |
| Well Testing | gas well | A producing well with natural gas as the primary commercial product. Most gas wells frequently produce some condensate (natural gas liquids such as propane and butane) and occasionally produce some water. |
| Well Testing | gas well deliverability | The calculated or measured rate a gas well will produce for a given bottomhole or wellhead pressure. |
| Well Testing | gas-solubility factor | The volume of gas (at standard conditions) liberated from a volume of oil (at standard conditions). |
| Well Testing | gas-well deliverability | The calculated or measured rate a gas well will produce for a given bottomhole or wellhead pressure. |
| Well Testing | heterogeneous formation | Formation with rock properties changing with location in the reservoir. Some naturally fractured reservoirs are heterogeneous formations. |
| Well Testing | homogeneous formation | Formation with rock properties that do not change with location in the reservoir. This ideal never actually occurs, but many formations are close enough to this situation that they can be considered homogeneous. Most of the models used for pressure-transient analysis assume the reservoir is homogeneous. |
| Well Testing | Horner slope | The slope of the chosen straight-line section of a Horner plot. It is used to determine permeability thickness, kh, of the producing zone in the vicinity of the wellbore. |
| Well Testing | humping | The abnormal behavior in a buildup curve caused by phase redistribution in a wellbore. This behavior is most noticeable in oil wells producing a substantial amount of gas and having a substantial skin effect. Analysis of buildup curves for wells exhibiting this behavior can be difficult or impossible because the “hump” obscures the reservoir response. |
| Well Testing | ideal gas | A gas defined by the fundamental equation of state, pV = nRT, where pressure, p, times volume, V, equals moles of gas, n, times gas constant, R, times temperature, T. The units are arbitrary and are accommodated by the value of the gas constant R, which is different for every set of units. |
| Well Testing | image well | A virtual well used to mathematically create the effect of a flow barrier. The pressure transient behavior both at the well and in the reservoir is identical for the following two cases: 1) a well near a barrier represented by a plane normal to the bedding, or 2) a well producing or injecting at the same rate as the tested well. In the second case, the effect is of a barrier bisecting the space between the two wells. |
| Well Testing | impermeable barrier | A single, impenetrable barrier to fluid flow in a reservoir that causes a change of a factor of two in the slope of buildup or drawdown curves. These are often observed in a normal test if the barrier is close (a few hundred feet or less) to the tested well. Most tests are not long enough to detect the presence of distant barriers. Two perpendicular barriers cause a change in the slope of a factor of four, and so forth, and models exist for a variety of geometries, typically for up to four barriers. |
| Well Testing | infinite acting radial flow | Flow into the wellbore during a well test, from a reservoir with no apparent outer boundary limit affecting fluid flow during the test period, the direction of flow being perpendicular to the axis of the well. Unless an outer boundary, such as a nearby fault, is close to the wellbore, it usually takes a day or more for outer boundaries to affect well-test results. Since most tests are of relatively short duration, outer boundaries usually do not affect test results. |
| Well Testing | infinite acting reservoir | A reservoir with no apparent outer boundary limit affecting fluid flow during a test period. Unless an outer boundary, such as a nearby fault, is close to the wellbore, it usually takes a day or more for outer boundaries to affect well-test results. Since most tests are of relatively short duration, outer boundaries usually do not affect test results. |
| Well Testing | infinite conductivity fracture | A planar crack penetrated by a well or propagated from a well by hydraulic fracturing with zero pressure drop in the fracture during production. |
| Well Testing | infinite-acting reservoir | A reservoir with no apparent outer boundary limit affecting fluid flow during a test period. Unless an outer boundary, such as a nearby fault, is close to the wellbore, it usually takes a day or more for outer boundaries to affect well-test results. Since most tests are of relatively short duration, outer boundaries usually do not affect test results. |
| Well Testing | infinite-conductivity fracture | A planar crack penetrated by a well or propagated from a well by hydraulic fracturing with zero pressure drop in the fracture during production. |
| Well Testing | initial flow period | A short flow period at the beginning of a drillstem test. This period is followed immediately by a longer shut-in period to allow the pressure to closely approach initial reservoir pressure. The initial flow period is commonly 5 to 10 minutes, and the initial shut-in period is commonly 30 minutes to one hour. When plotted on a pressure buildup plot, extrapolation of the best straight line gives what is usually accepted as the best obtainable value of initial formation pressure |
| Well Testing | initial reservoir pressure | The reservoir pressure measured in a discovery well, usually referred to as pi. This value is necessary for many reservoir engineering calculations, such as reserve determination. |
| Well Testing | initial shut in period | The comparatively short shut-in period following the initial flow period of a drillstem test. This period is followed immediately by much longer flow and shut-in periods to allow the pressure to closely approach initial reservoir pressure. The initial flow period is commonly 5 to 10 minutes, and the initial shut-in period is commonly 30 minutes to one hour. When plotted on a pressure buildup plot, extrapolation of the best straight line gives what is usually accepted as the best obtainable value of initial formation pressure. |
| Well Testing | initial shut-in period | The comparatively short shut-in period following the initial flow period of a drillstem test. This period is followed immediately by much longer flow and shut-in periods to allow the pressure to closely approach initial reservoir pressure. The initial flow period is commonly 5 to 10 minutes, and the initial shut-in period is commonly 30 minutes to one hour. When plotted on a pressure buildup plot, extrapolation of the best straight line gives what is usually accepted as the best obtainable value of initial formation pressure. |
| Well Testing | injection-well testing | The testing of wells in which fluid is being injected into the reservoir. The most common type of test is a falloff test, in which injection is halted and the pressure decline is measured as a function of time. The most common situation is a waterflood. In many reservoirs, the formation pressure is high enough to maintain a full column of fluid in the wellbore and the pressure can be monitored at the surface. The bottomhole pressure is then calculated by adding the weight of the fluid column to the surface pressure. Gas-injection wells, although less common, lend themselves to similar testing. The rise in fluid pressure as a function of time while injection is taking place could theoretically be used also, but this type of approach is rarely used. The equations and theory for these tests are an exact mirror image of those for buildup and drawdown testing. Calculated results for these wells are usually good because the formations are commonly liquid-filled. Frequently water-injection wells are inadvertently fractured at some time in their life and consequently have a negative skin effect. |
| Well Testing | interference testing | The pressure variation with time recorded in observation wells resulting from changes in rates in production or injection wells. In commercially viable reservoirs, it usually takes considerable time for production at one well to measurably affect the pressure at an adjacent well. Consequently, interference testing has been uncommon because of the cost and the difficulty in maintaining fixed flow rates over an extended time period. With the increasing number of permanent gauge installations, interference testing may become more common than in the past. |
| Well Testing | isochronal test | A multirate test designed as a series of drawdown and buildup sequences at different drawdown flow rates, with each drawdown of the same duration and each buildup reaching stabilization at the same pressure as at the start of the test. The purpose of the test is to determine well deliverability. This type of test is most commonly done in gas wells. |
| Well Testing | isotropic formation | A type of formation whose rock properties are the same in all directions. Although this never actually occurs, fluid flow in rocks approximates this situation closely enough to consider certain formations isotropic. |
| Well Testing | isotropic permeability | Permeability that is the same in all directions. This never really occurs, but permeability along various directions of a formation is often close enough for calculation purposes. |
| Well Testing | kh | The product of formation permeability, k, and producing formation thickness, h, in a producing well, referred to as kh. This product is the primary finding of buildup and drawdown tests and is a key factor in the flow potential of a well. It is used for a large number of reservoir engineering calculations such as prediction of future performance, secondary and tertiary recovery potential, and potential success of well-stimulation procedures. Obtaining the best possible value of this product is the primary objective of transient well tests. To separate the elements of the product, it is necessary to have some independent measurement of one of them, usually the estimation of producing formation thickness from well logs. Permeability is then calculated, provided that the fluid formation volume factor and viscosity are known. The accuracy of the calculated permeability is entirely dependent on the accuracy of the estimated formation thickness and the fluid properties. |
| Well Testing | late time transient data | The portion of the pressure transient occurring after radial flow. Analysis of the late-time transient data provides characterization of outer boundaries such as faults or fluid contacts. This portion of the data appears only in transient tests of sufficient duration. |
| Well Testing | late-time transient data | The portion of the pressure transient occurring after radial flow. Analysis of the late-time transient data provides characterization of outer boundaries such as faults or fluid contacts. This portion of the data appears only in transient tests of sufficient duration. |
| Well Testing | layer stripping | Anything in the wellbore that is not supposed to be there. The term is usually reserved for small pieces of steel such as hand tools, small parts, bit nozzles, pieces of bits or other downhole tools, and remnants of milling operations. |
| Well Testing | A tool run into the wellbore to retrieve junk from the bottom of the hole. | |
| Well Testing | layer-cake geometry | A large, rectangular steel box, usually with sides made of expanded metal to facilitate seeing what is inside. The junk basket is used by the rig crew to store an assortment of relatively small parts of the drilling rig, ranging from drill bits to crossover subs to lifting subs to spare kellys. Dimensions vary, but a typical junk basket on a land rig is 8 ft wide [2.5 m] by 3 ft [1 m] deep by 30 ft [9 m] long. |
| Well Testing | limited entry | A completion with only a portion of the productive interval open to flow, either by design or as a result of damage. Limited-entry completions in vertical wells are designed to avoid unwanted fluid production, such as gas from an overlying gas cap or water from an underlying aquifer. The effects of limited entry may be seen in perforated and gravel-packed wells when some of the perforations fail to clean up. This is also called a partial completion. The limited entry also results from partial penetration, which occurs when the productive formation is only partly drilled. This partial penetration represents a near-well flow restriction that results in a positive skin effect in a well-test analysis. |
| Well Testing | line source solution | The solution to differential equations treating the well as a vertical line through a porous (link to ID 406) medium. The solution is nearly identical to the finite-wellbore solution. At very early times during a well test, there is a notable difference in the solutions, but the differences disappear soon after a typical well is opened to flow or shut in for a buildup test, and in practice the differences are masked by wellbore storage. |
| Well Testing | linear flow | A flow regime characterized by parallel flow lines in the reservoir. This results from flow to a fracture or a long horizontal well, or from flow in an elongated reservoir, such as a fluvial channel, or as a formation bounded by parallel faults. Linear flow is recognized as a +1/2 slope in the pressure derivative on the log-log diagnostic plot. Its presence enables determination of the fracture half-length or the channel or reservoir width, if permeability can be determined independently. |
| Well Testing | line-source solution | The solution to differential equations treating the well as a vertical line through a porous medium. The solution is nearly identical to the finite-wellbore solution. At very early times, there is a notable difference in the solutions, but the differences disappear soon after a typical well is opened to flow or shut in for a buildup test, and in practice the differences are masked by wellbore storage. |
| Well Testing | liquid level | The depth at which the first liquid is found in a well. |
| Well Testing | material balance | An expression for conservation of mass governed by the observation that the amount of mass leaving a control volume is equal to the amount of mass entering the volume minus the amount of mass accumulated in the volume. Through material balance, reservoir pressures measured over time can be used to estimate the volume of hydrocarbons in place. |
| Well Testing | middle time transient data | A common term for the infinite-acting radial flow regime. This portion of the pressure-transient response is between wellbore-dominated flow regimes in the early-time transient data and boundary-dominated flow regimes in the late-time transient data. This most common flow regime is the most central to pressure-transient analysis because its presence enables determination of permeability and skin. |
| Well Testing | middle-time transient data | A common term for the infinite-acting radial flow regime. This portion of the pressure-transient response is between wellbore-dominated flow regimes in the early-time transient data and boundary-dominated flow regimes in the late-time transient data. This most common flow regime is the most central to pressure-transient analysis because its presence enables determination of permeability and skin. |
| Well Testing | modified isochronal test | A multirate test designed as a series of drawdown and buildup sequences at different drawdown flow rates, with each drawdown and buildup of the same duration. The purpose of the test is to determine well deliverability, and this type of test is most commonly done in gas wells. |
| Well Testing | multiphase flow | The simultaneous flow of more than one fluid phase through a porous medium. Most oil wells ultimately produce both oil and gas from the formation, and often produce water. Consequently, multiphase flow is common in oil wells. Most pressure-transient analysis techniques assume single-phase flow. |
| Well Testing | multiple rate tests | Tests conducted at a series of different flow rates for the purpose of determining well deliverability, typically in gas wells where non-Darcy flow near the well results in a rate-dependent skin effect. Multiple-rate tests are sometimes required by regulatory bodies. |
| Well Testing | multiple-rate tests | Tests conducted at a series of different flow rates for the purpose of determining well deliverability, typically in gas wells where non-Darcy flow near the well results in a rate-dependent skin effect. Multiple-rate tests are sometimes required by regulatory bodies. |
| Well Testing | naturally flowing well | A well in which the formation pressure is sufficient to produce oil at a commercial rate without requiring a pump. Most reservoirs are initially at pressures high enough to allow a well to flow naturally. |
| Well Testing | nonideal gas | A gas described by an equation of state of the form pV = znRT, where z is the gas deviation factor dependent on pressure, temperature and gas composition. |
| Well Testing | oil formation volume factor | Oil and dissolved gas volume at reservoir conditions divided by oil volume at standard conditions. Since most measurements of oil and gas production are made at the surface, and since the fluid flow takes place in the formation, volume factors are needed to convert measured surface volumes to reservoir conditions. Oil formation volume factors are almost always greater than 1.0 because the oil in the formation usually contains dissolved gas that comes out of solution in the wellbore with dropping pressure |
| Well Testing | oil well | A producing well with oil as its primary commercial product. Oil wells almost always produce some gas and frequently produce water. Most oil wells eventually produce mostly gas or water. |
| Well Testing | open flow potential | The theoretical flow capacity of gas wells if the bottomhole pressure could be reduced to atmospheric pressure. Test procedures to determine the AOF (Atmospheric Open-Flow) potential were often prescribed by law and enforced by state agencies. The open-flow potential capacity of the well was then used to determine the maximum rate that the gas well would be allowed to produce into a pipeline. During that period when there was an excessive amount of gas available for sale, this approach allowed an orderly method of allocating production rights to operators. The methodology is still used in some areas. The test required use of multiple rates, usually three or four, with measurement or calculation of the bottomhole pressure at the end of each flow period. The values of the difference between the square of the average reservoir pressure and the square of the bottomhole pressure were computed for each flow rate. These values were then plotted on the y-axis of a log-log plot versus the rate on the x-axis. The value of the open-flow potential is obtained by extrapolating the best straight line to the value of the average reservoir pressure squared minus atmospheric pressure squared, and then reading the corresponding rate off the x-axis. |
| Well Testing | open-flow potential | The theoretical flow capacity of gas wells if the bottomhole pressure could be reduced to atmospheric pressure. Test procedures to determine the AOF (Atmospheric Open-Flow) potential were often prescribed by law and enforced by state agencies. The open-flow potential capacity of the well was then used to determine the maximum rate that the gas well would be allowed to produce into a pipeline. During that period when there was an excessive amount of gas available for sale, this approach allowed an orderly method of allocating production rights to operators. The methodology is still used in some areas. The test required use of multiple rates, usually three or four, with measurement or calculation of the bottomhole pressure at the end of each flow period. The values of the difference between the square of the average reservoir pressure and the square of the bottomhole pressure were computed for each flow rate. These values were then plotted on the y-axis of a log-log plot versus the rate on the x-axis. The value of the open-flow potential is obtained by extrapolating the best straight line to the value of the average reservoir pressure squared minus atmospheric pressure squared, and then reading the corresponding rate off the x-axis. |
| Well Testing | partial completion | Completion of or flow from less than the entire producing interval. This situation causes a near-well flow constriction that results in a positive skin effect in a well-test analysis. |
| Well Testing | partial penetration | An incompletely drilled portion of the productive interval. |
| Well Testing | permeability | The capability of a rock to allow passage of fluids through it. The term was basically defined by Darcy, who showed that the common mathematics of heat transfer could be modified to adequately describe fluid flow in porous media. |
| Well Testing | permeability thickness | The product of formation permeability, k, and producing formation thickness, h, in a producing well, referred to as kh. This product is the primary finding of buildup and drawdown tests and is a key factor in the flow potential of a well. It is used for a large number of reservoir engineering calculations such as prediction of future performance, secondary and tertiary recovery potential, and potential success of well-stimulation procedures. Obtaining the best possible value of this product is the primary objective of transient well tests. To separate the elements of the product, it is necessary to have some independent measurement of one of them, usually the estimation of producing formation thickness from well logs. Permeability is then calculated, provided that the fluid formation volume factor and viscosity are known. The accuracy of the calculated permeability is entirely dependent on the accuracy of the estimated formation thickness and the fluid properties. |
| Well Testing | phase redistribution | A pressure phenomenon caused in a wellbore by rise of gas and fall of liquids trapped in a wellbore after a surface shut-in. This phenomenon can cause a “hump” in the buildup curve, and frequently leads to incorrect analysis of buildup test results because the entire early portion of the transient is adversely affected by this pressure response. |
| Well Testing | porous medium | A rock or soil with interconnected pores that permit flow of fluids through the medium. |
| Well Testing | pressure | The force distributed over a surface, usually measured in pounds force per square inch, or lbf/ in.2, or psi, in US oilfield units. |
| Well Testing | pressure buildup | A rise in well pressure as a function of time observed after a well is shut in or after the production rate is reduced. Buildup pressures are normally measured at or near the bottom of the hole. |
| Well Testing | pressure depletion | The drop in average reservoir pressure from fluid production. All bounded reservoirs have pressure depletion (a drop in average reservoir pressure) associated with fluid production. Water influx counters this effect in reservoirs that are surrounded or underlain by aquifers. Likewise, the presence of a gas cap can slow pressure depletion. |
| Well Testing | pressure falloff | The pressure decline after halting or reducing fluid injection in a well. Pressure falloff tests in injection wells are analogous to pressure buildup tests in production wells. |
| Well Testing | pressure gauge | A device used to measure pressure. Many different types of pressure gauges have been developed for use in well testing over the years. For bottomhole pressure measurements, these include helical bourdon tube gauges, strain gauges, quartz crystal gauges and surface readout gauges. All have their roles, and are still in use. Digital memory gauges are popular at the moment, since the data can be printed out or input directly into a computer for immediate use. Proper use of the data often requires specific knowledge of the possible idiosyncrasies of the particular gauge used in a test. |
| Well Testing | pressure squared plot | A plot of p2 versus time function used to analyze low-pressure gas-well drawdown and buildup tests. The square term arises from substituting a gas-law equation into the differential equations where required to account for fluid compressibility. This allows an approximation for the differential equations that approaches the linear form required to use the classical solutions of the diffusion equation. |
| Well Testing | pressure transient well tests | Well tests in which pressure is recorded as a function of time and interpreted using various analysis methods. These include buildup tests and drawdown tests in production wells and falloff tests in injection wells. Pressure-transient well-test analysis procedures are based on classical mathematical relationships between flow rate, pressure and time, which are directly analogous to the theory of heat transfer. |
| Well Testing | pressure-drawdown analysis | The analysis of pressure-transient behavior observed while the well is flowing. Results are generally much less accurate than those from pressure buildup tests because the bottomhole pressure fluctuates rapidly with even slight changes in the surface flow rate. Therefore, pressure buildup tests are much preferred, and analysis of drawdown test data is usually relegated to backup status unless the buildup data are flawed. |
| Well Testing | pressure-squared plot | A plot of p2 versus time function used to analyze low-pressure gas-well drawdown and buildup tests. The square term arises from substituting a gas-law equation into the differential equations where required to account for fluid compressibility. This allows an approximation for the differential equations that approaches the linear form required to use the classical solutions of the diffusion equation. |
| Well Testing | pressure-transient well tests | Well tests in which pressure is recorded as a function of time and interpreted using various analysis methods. These include buildup tests and drawdown tests in production wells and falloff tests in injection wells. Pressure-transient well-test analysis procedures are based on classical mathematical relationships between flow rate, pressure and time, which are directly analogous to the theory of heat transfer. |
| Well Testing | producing formation | An underground rock formation from which oil, gas or water is produced. Any porous rock will contain fluids of some sort, and all rocks at considerable distance below the Earth’s surface will initially be under pressure, often related to the hydrostatic column of ground waters above the reservoir. To produce, rocks must also have permeability, or the capacity to permit fluids to flow through them. |
| Well Testing | producing well | A well producing fluids (gas, oil or water). |
| Well Testing | production period | The flow period before a buildup. The duration of the production period should be specified in the test design to assure that a stable flow situation is reached, and that the pressure disturbance has reached far enough into the formation to allow determination of a representative value for kh. For reservoir-limits testing, the production period must be long enough for the pressure disturbance to have reached the boundaries of interest. |
| Well Testing | prorated well | A well in which the maximum production rate is fixed by law. These laws were developed by producing states primarily to control the market and avoid periodic price collapses. |
| Well Testing | pseudopressure plot | A plot of real gas pseudopressure (pseudopotential) m(p) versus time function used to analyze gas-well drawdown and buildup tests. The use of the real gas pseudopressure linearizes the diffusion equation for gas flow. This form enables rigorous analysis over all pressure ranges. The pressure-squared plot can be used for low pressure (p < ~2000 psi), and pressure can be used to analyze gas-well tests at high pressure (p> ~3000 psi). |
| Well Testing | pseudosteady state | Behavior observed when a well reaches stabilized production from a limited drainage volume. For constant-rate production, under pseudosteady state, the difference between the flowing wellbore pressure and the average reservoir pressure in the drainage volume is constant, and the pressure drawdown is a linear function of time, resulting in a unit slope in the log-log pressure derivative. The late-time buildup pressure will level off to the average reservoir pressure if the buildup duration is sufficient long, resulting in a sudden drop in the log-log pressure derivative. Pressure depletion occurs with continued pseudosteady-state production. |
| Well Testing | pumping well | A well produced by use of some kind of downhole pump. Pumps are required when the formation pressure is not sufficient to allow flowing production of fluids at the desired or necessary rate. The performance of well tests on pumping wells is always complicated by the presence of the pump, which often must be removed to take downhole pressure measurements. Downhole pressure measurements in pumping wells are usually made by measuring the rise in liquid level in the well. This is often accomplished by sonic devices, like well sounders, that measure the response time of sound waves bounced off the downhole liquid surface. Most oil wells are eventually put on pumps as pressure declines during production. The exceptions are in strong waterdrive reservoirs or in settings where pressure maintenance by gas or water injection is sufficient to maintain a high reservoir pressure. |
| Well Testing | pumping well tests | Testing that is accomplished by measuring pressure in the annulus, or by pulling the pump and running a pressure gauge in the hole. The preferred method is usually to measure the pressure in the annulus if no packer is present. This is best done by monitoring the rise in fluid level with an echo-sounding device and calculating the bottomhole pressure by assuming a fluid density. Several excellent devices and associated services are available. Any time a well is shut in gradually, as is the case for pumping wells, some kind of multirate analysis is usually required to obtain acceptable results. |
| Well Testing | radius of investigation | The calculated maximum radius in a formation in which pressure has been affected during the flow period of a transient well test. While not absolutely accurate, the value has meaning in relation to the total volume of reservoir that is represented by calculated reservoir parameters, such as kh, the permeability thickness. This may also be termed transient drainage radius. |
| Well Testing | rate dependent skin effect | Skin effect with a magnitude that depends on the flow rate of the wellbore fluid. It is caused by non-Darcy flow, fluid flow that deviates from Darcy’s law, which assumes laminar flow in the formation. Non-Darcy flow is typically observed in high-rate gas wells when the flow converging to the wellbore reaches flow velocities exceeding the Reynolds number for laminar or Darcy flow, and results in turbulent flow. Since most of the turbulent flow takes place near the wellbore in producing formations, the effect of non-Darcy flow is a rate-dependent skin effect. |
| Well Testing | rate-dependent skin effect | Skin effect with a magnitude that depends on the flow rate of the wellbore fluid. It is caused by non-Darcy flow, fluid flow that deviates from Darcy’s law, which assumes laminar flow in the formation. Non-Darcy flow is typically observed in high-rate gas wells when the flow converging to the wellbore reaches flow velocities exceeding the Reynolds number for laminar or Darcy flow, and results in turbulent flow. Since most of the turbulent flow takes place near the wellbore in producing formations, the effect of non-Darcy flow is a rate-dependent skin effect. |
| Well Testing | single phase flow | The flow of a single-phase fluid, such as oil, water or gas. |
| Well Testing | single-phase flow | The flow of a single-phase fluid, such as oil, water or gas. |
| Well Testing | skin | The zone of reduced or enhanced permeability around a wellbore, often explained by formation damage and mud-filtrate invasion during drilling or perforating, or by well stimulation. |
| Well Testing | skin effect | An increase or decrease in the pressure drop predicted with Darcy’s law using the value of permeability thickness, kh, determined from a buildup or drawdown test. The difference is assumed to be caused by the “skin.” Skin effect can be either positive or negative. The skin effect is termed positive if there is an increase in pressure drop, and negative when there is a decrease, as compared with the predicted Darcy pressure drop. A positive skin effect indicates extra flow resistance near the wellbore, and a negative skin effect indicates flow enhancement near the wellbore. The terms skin effect and skin factor are often used interchangeably. In this glossary, the term skin effect refers to the numerical value of the skin factor. |
| Well Testing | skin factor | A numerical value used to analytically model the difference from the pressure drop predicted by Darcy’s law due to skin. Typical values for the skin factor range from -6 for an infinite-conductivity massive hydraulic fracture to more than 100 for a poorly executed gravel pack. This value is highly dependent on the value of kh. For example, a 20-psi [138-kPa] total pressure drop related to skin effect could produce almost any skin factor, depending on the value of kh. For any given pressure drop from skin effect, the skin factor increases proportionally as kh increases. |
| Well Testing | spherical flow | A flow regime that occurs when the predominant flow pattern in the reservoir is toward a point. Spherical flow occurs for partial penetration and limited-entry completions. This flow regime is recognized as a -1/2 slope in the pressure derivative on the log-log diagnostic plot. Its presence enables determination of the spherical permeability. When spherical flow is followed by radial flow, both horizontal and vertical permeability can be quantified. |
| Well Testing | stabilization | A state that a producing well reaches when the flow rate and well pressure are apparently constant for a reasonable period of time, such as a few hours or a day or more. The actual time period is rather arbitrary and depends on the location and the people involved. For shut-in wells, stabilization refers to a reasonably constant pressure. Stabilized pressures are commonly used as starting points for well tests. In reality, the time to stabilization at truly constant pressure and rate is infinite. The target stabilization for rigorous testing in gas wells is pseudosteady-state flow, and this may be recognized as the pressure change versus time predicted from formation properties and drainage area size. The onset of radial flow may produce an apparent stabilization that is acceptable for analysis when multirate flow periods are of equal duration. |
| Well Testing | stabilized gas well | A gas well producing at a constant rate in which wellhead pressure changes no more than a small amount as a function of time. The actual amount of change permitted in a given time period to allow a well to be designated as stabilized may be fixed by law. Alternatively, the target stabilization for rigorous flow-after-flow testing in gas wells is pseudosteady-state flow, and this may be recognized as the pressure change versus time predicted from formation properties and drainage area size. |
| Well Testing | steady state behavior | Simultaneously constant pressure (wellhead or bottomhole) and flow rate. This behavior can result when there is pressure support, either naturally through an aquifer or gas-cap drive, or artificially through water or gas injection. |
| Well Testing | steady-state behavior | Simultaneously constant pressure (wellhead or bottomhole) and flow rate. This behavior can result when there is pressure support, either naturally through an aquifer or gas-cap drive, or artificially through water or gas injection. |
| Well Testing | subsurface pressure | Any pressure measured in a well below the surface. |
| Well Testing | superposition | A mathematical technique based on the property that solutions to linear partial equations can be added to provide yet another solution. This permits constructions of mathematical solutions to situations with complex boundary conditions, especially drawdown and buildup tests, and in settings where flow rates change with time. |
| Well Testing | superposition in space | A mathematical computation that accounts for production from multiple wells. Image wells are used to model the effect of impermeable barriers. |
| Well Testing | superposition in time | A mathematical computation that accounts for the flow-rate history in analytical models generated to match with pressure-transient test data. The pressure-derivative response can be distorted in late-time data by the effects of superposition in time, except in data acquired from an initial drawdown. Analysts must be aware of this limitation in diagnosing reservoir features from the pressure-derivative response. |
| Well Testing | surface pressure | The pressure measured at or near the surface in a well. This measurement of pressure is usually performed by inserting a gauge into the production string just below the shut-in valve, and is also referred to as tubing-head pressure. |
| Well Testing | surface shut-in | A well shut in at the surface, rather than downhole. Most transient well tests are conducted in this manner for convenience. |
| Well Testing | three phase flow | The simultaneous flow of oil, free gas and water into a wellbore. Stratified flow is the rule rather than the exception |
| Well Testing | three-phase flow | The simultaneous flow of oil, free gas and water into a wellbore. Stratified flow is the rule rather than the exception. |
| Well Testing | transient drainage radius | The calculated maximum radius in a formation in which pressure has been affected during the flow period of a transient well test. While not absolutely accurate, the value has meaning in relation to the total volume of reservoir that is represented by calculated reservoir parameters, such as kh, the permeability thickness. This may also be termed radius of investigation. |
| Well Testing | transient pressure | The change in pressure with time. In well testing, this refers to the pressure measured as a function of time after the test is initiated. |
| Well Testing | transient rate and pressure test analysis | The analysis of transient rate and pressure data taken while a well is flowing at variable rates. The analysis uses either deconvolution or convolution to correct for the flow-rate variations and can make drawdown data interpretable. It has also been applied to correct for afterflow during the buildup. |
| Well Testing | transient-pressure response | The pressure response resulting from changes in a well’s production rate. This includes drawdown, in which the pressure falls in response to the production of fluids; buildups, in which the pressure rises after a well is shut in; and falloffs, in which the pressure falls after an injection well is shut in. |
| Well Testing | transient-pressure testing | The pressure measurements recorded as a function of time, usually in the wellbore near the productive interval, after the flow rate of the well is changed. These form the basis for transient well-test analysis, and are primarily used for determining reservoir-rock properties and producing-formation limits. |
| Well Testing | transient-rate and pressure-test analysis | The analysis of transient rate and pressure data taken while a well is flowing at variable rates. The analysis uses either deconvolution or convolution to correct for the flow-rate variations and can make drawdown data interpretable. It has also been applied to correct for afterflow during the buildup. |
| Well Testing | two phase flow | The simultaneous flow of both oil and free gas into a wellbore. This is thought to occur through common pore spaces, where the fluids flow simultaneously. In actuality, the situation is much more complicated, and much two-phase flow occurs in a stratified manner, with lighter fluids flowing primarily through the top of a producing zone and heavier ones flowing through the bottom layers. |
| Well Testing | two-phase flow | The simultaneous flow of both oil and free gas into a wellbore. This is thought to occur through common pore spaces, where the fluids flow simultaneously. In actuality, the situation is much more complicated, and much two-phase flow occurs in a stratified manner, with lighter fluids flowing primarily through the top of a producing zone and heavier ones flowing through the bottom layers. |
| Well Testing | type curve analysis | A method for quantifying well and reservoir parameters such as permeability, skin, fracture half-length, dual-porosity parameters, and others, by comparing the pressure change and its derivative of the acquired data to reservoir model curve families, called type curves. When a match is found between data and a type curve, the parameters that characterize the behavior of the model providing a match are thereby determined. Originally, type-curve analysis was done manually using only the pressure change. With the introduction of the pressure derivative, the analysis requires matching both pressure change and its derivative. Computer-assisted matching permits rigorous accounting for superposition in time due to flow-rate variations before and even during (in the case of drawdown analysis) the transient data acquisition, as well as providing a continuum of solutions instead of a type-curve family derived from discrete values for the governing parameters. |
| Well Testing | type curves | Families of the paired pressure change and its derivative computed from a model. The model is usually generated from an analytical solution of the diffusion equation with boundary conditions strategically defined to enable observation of theoretical trends in the pressure-transient response. The boundary conditions that can be defined near the well include constant or variable wellbore storage, limited entry (partial penetration), radial composite (damage skin due to permeability alteration), and a fracture extending the cylindrical wellbore to a extended plane. The borehole trajectory can be vertical, angled, or horizontal. The distant boundary conditions include a sealing or partially sealing planar boundary (fault), intersecting faults and rectangular boundaries (sealing or constant pressure). Further, the diffusion equation can be adjusted to accommodate reservoir heterogeneity in the form of dual porosity or layering. Finally, when generated with computer assistance, the type-curve family can account for superposition in time due to flow-rate variations before and even during the transient data acquisition. Originally, type-curve families were printed on specialized (usually log-log) coordinates with dimensionless parameters defining the x and y axes. Today, commercial software can generate the type-curve families on the computer screen, enabling a much more flexible and user-friendly analysis. Further, automated regression (usually least squares) permits an optimized match between the acquired data and a selected model. Type curves have greatly enriched the ability of interpreters to extract potential explanations for transient data trends that differ from the radial-flow behavior required for conventional semilog (Horner buildup) analysis. |
| Well Testing | type-curve analysis | A method for quantifying well and reservoir parameters such as permeability, skin, fracture half-length, dual-porosity parameters, and others, by comparing the pressure change and its derivative of the acquired data to reservoir model curve families, called type curves. When a match is found between data and a type curve, the parameters that characterize the behavior of the model providing a match are thereby determined. Originally, type-curve analysis was done manually using only the pressure change. With the introduction of the pressure derivative, the analysis requires matching both pressure change and its derivative. Computer-assisted matching permits rigorous accounting for superposition in time due to flow-rate variations before and even during (in the case of drawdown analysis) the transient data acquisition, as well as providing a continuum of solutions instead of a type-curve family derived from discrete values for the governing parameters. |
| Well Testing | unitized production | Pooled production from wells or a reservoir. The proceeds of this pooled production are distributed to the participants according to an agreed-upon formula. |
| Well Testing | units conversion factor | A constant provided in an equation that applies only for a particular system of units. This is useful for analysts who must work in different units systems to satisfy local preferences. |
| Well Testing | variable rate | A condition that results when the flow rate varies appreciably during a test period. This can be contrasted to multirate conditions, which describe the step changes in rate demanded by certain test designs. The variable rates observed during drawdown can be measured by a flowmeter at the sandface level in the well. Elegant mathematical methods have been developed to analyze well tests conducted at variable rates. Although, from a practical standpoint, it is much more desirable to conduct constant-rate tests, in reality the only truly constant rate is zero. Sandface flow-rate measurements provide considerable information from drawdown data that cannot be learned from a buildup. |
| Well Testing | water formation volume factor | Water and dissolved gas volume at reservoir conditions divided by water volume at standard conditions. This value can often be neglected, since it is always close to 1.0. |
| Well Testing | water influx | The replacement of produced fluids by formation water. Most petroleum reservoirs are underlain by water, and water influx into a reservoir almost always takes place at some rate when gas or oil is produced. Whether appreciable water is produced along with gas or oil depends on the proximity of the productive interval to the oil-water contact or gas-water contact and whether the well is coning (vertical well) or cresting (horizontal well). |
| Well Testing | well cleanup | That period when drilling debris and fluids are still coming out of the formation and perforations. During this time, the skin effect is changing and any well-test results may reflect temporary obstruction to flow that will not be present in later tests. |
| Well Testing | well flow rate | The well production or injection rate. |
| Well Testing | well interference | The change in pressure at one well caused by production from one or more other wells. |
| Well Testing | well interference testing | The pressure variation with time recorded in observation wells resulting from changes in rates in production or injection wells. In commercially viable reservoirs, it usually takes considerable time for production at one well to measurably affect the pressure at an adjacent well. Consequently, interference testing has been uncommon because of the cost and the difficulty in maintaining fixed flow rates over an extended time period. With the increasing number of permanent gauge installations, interference testing may become more common than in the past. |
| Well Testing | well potential | The flow rate at which a well is theoretically capable of producing. This is usually defined by a mathematical formula related to Darcy’s law, often at maximum theoretical pressure drawdown. These theoretical rates were, and still are to some extent, used to set the production quota for an individual well in prorated or unitized production situations. |
| Well Testing | well production rate | The volume of produced fluid per unit of time. |
| Well Testing | wellbore damage | Any restriction to flow from near-well reductions in flow capacity. This damage is thought to result from reductions in near-well permeability caused by perforating debris or from the solids or mud filtrate invasion caused by the drilling process. |
| Well Testing | wellbore fill up | Following a surface shut-in, the flow into a well caused by the compressibility of the fluids in the wellbore. Most of the flow occurs from compression of gas in the wellbore. The practical result is that the sandface flow rate is not zero and, therefore, not constant. This gives rise to one form of the wellbore-storage effect. |
| Well Testing | wellbore fill-up | Following a surface shut-in, the flow into a well caused by the compressibility of the fluids in the wellbore. Most of the flow occurs from compression of gas in the wellbore. The practical result is that the sandface flow rate is not zero and, therefore, not constant. This gives rise to one form of the wellbore-storage effect. |
| Well Testing | wellbore storage effects | Distortions in the reservoir response due to wellbore storage. The characteristic trends are an early unit slope trend with pressure change and the derivative overlain on the log-log plot, followed by a “hump” in the pressure derivative that gradually disappears as reservoir trends become recognizable. Complex behavior in the wellbore, such as wellbore phase distribution, can result in a more complex transient trend. A crucial part of the transient analysis is to distinguish the effects of wellbore storage from the interpretable reservoir response. |
| Well Testing | wellbore-storage effects | Distortions in the reservoir response due to wellbore storage. The characteristic trends are an early unit slope trend with pressure change and the derivative overlain on the log-log plot, followed by a “hump” in the pressure derivative that gradually disappears as reservoir trends become recognizable. Complex behavior in the wellbore, such as wellbore phase distribution, can result in a more complex transient trend. A crucial part of the transient analysis is to distinguish the effects of wellbore storage from the interpretable reservoir response. |
| Well Testing | well-interference testing | The pressure variation with time recorded in observation wells resulting from changes in rates in production or injection wells. In commercially viable reservoirs, it usually takes considerable time for production at one well to measurably affect the pressure at an adjacent well. Consequently, interference testing has been uncommon because of the cost and the difficulty in maintaining fixed flow rates over an extended time period. With the increasing number of permanent gauge installations, interference testing may become more common than in the past. |
| Well Testing | wireline formation test | Test taken with a wireline formation tester. The wireline formation pressure measurement is acquired by inserting a probe into the borehole wall and performing a minidrawdown and buildup by withdrawing a small amount of formation fluid and then waiting for the pressure to build up to the formation pore pressure. This measurement can provide formation pressures along the borehole, thereby giving a measure of pressure with depth or along a horizontal borehole. The trend in formation pressure with depth provides a measure of the formation-fluid density, and a change in this trend may indicate a fluid contact. Abrupt changes in formation pressure measurements with depth indicate differential pressure depletion and demonstrate barriers to vertical flow. Lateral variation in formation pressure measurements along a horizontal well or in multiple vertical wells indicate reservoir heterogeneity. |
| Well Testing | wireline formation tester | A tool run on an electric logging cable that pushes a probe into the formation, which then allows production into a small closed chamber. The tool is primarily used to obtain formation pressures at chosen locations in an interval, and, with an accurate quartz gauge, permeability estimates may be obtained. Modern variations on this tool have been developed to acquire formation-fluid samples. |
| Well Testing | zone | A slab of reservoir rock bounded above and below by impermeable rock. |
| Well Testing, Enhanced Oil Recovery | bubble point | The pressure and temperature conditions at which the first bubble of gas comes out of solution in oil. At discovery, all petroleum reservoir oils contain some natural gas in solution. Often the oil is saturated with gas when discovered, meaning that the oil is holding all the gas it can at the reservoir temperature and pressure, and that it is at its bubblepoint. Occasionally, the oil will be undersaturated. In this case, as the pressure is lowered, the pressure at which the first gas begins to evolve from the oil is defined as the bubblepoint. |
| Well Testing, Enhanced Oil Recovery | dew point | The pressure at which the first condensate liquid comes out of solution in a gas condensate. Many gas condensate reservoirs are saturated at initial conditions, meaning that the dewpoint is equal to the initial reservoir pressure. Condensate dissolution is called retrograde condensation because this is counter to the behavior of pure substances, which vaporize when the pressure drops below thesaturation pressure under isothermal (constant temperature) conditions. |
| Well Testing, Enhanced Oil Recovery | mobility | The ratio of effective permeability to phase viscosity. The overall mobility is a sum of the individual phase viscosities. Well productivity is directly proportional to the product of the mobility and the layer thickness product. |
| Well Testing, Enhanced Oil Recovery | reservoir heterogeneities | The variations in rock properties in a reservoir. The variations can result in directional variations in permeability. Geological processes, such as sedimentation, diagenesis and erosion, act to produce nonuniformities in rock formations. Because there are so many types of reservoir heterogeneities, a unique interpretation of test results from pressure data alone is often impossible. Expert test interpreters rely heavily on experience, core analysis, well logs and knowledge of the geology specific to the region. |
| Well Testing, Production Testing | gas/oil ratio (GOR) | The ratio of produced gas to produced oil, commonly abbreviated GOR. |
| Well Testing, Production Testing | GOR | Abbreviation for gas/oil ratio, the ratio of produced gas to produced oil. |
| Well Testing, Reservoir Characterization | pressure transient analysis | The analysis of pressure changes over time, especially those associated with small variations in the volume of fluid. In most well tests, a limited amount of fluid is allowed to flow from the formation being tested and the pressure at the formation monitored over time. Then, the well is closed and the pressure monitored while the fluid within the formation equilibrates. The analysis of these pressure changes can provide information on the size and shape of the formation as well as its ability to produce fluids. |
| Well Testing, Well Workover and Intervention, Shale Gas | fracturing pressure | Pressure above which injection of fluids will cause the rock formation to fracture hydraulically. |
| Well Testing | static pressure | The pressure measured in a well after the well has been closed in for a period of time, often after 24 or 72 hours. When a reservoir is first discovered, the static pressure equals the initial pressure. After production begins, the static pressure approaches the average reservoir pressure. |
| Well Workover and Intervention | chrome tubing | Tubing manufactured from an alloy containing a high proportion of chrome, typically greater than 13%. Chrome tubing is classified as a corrosion-resistant alloy (CRA) and is used where the wellbore conditions or reservoir fluid create a corrosive environment that conventional tubing cannot safely withstand. Wells that produce hydrogen sulfide, and similar corrosive fluids, typically require chrome tubing. |
| Well Workover and Intervention | abrasive jetting | A wellbore treatment in which a fluid laden with solid particles is used to remove deposits from the surface of wellbore tubulars and completion components. The treatment fluid is pumped at high pressure through a downhole tool equipped with nozzles that direct a jet, or jets, of fluid onto the target area. Most tool designs use a controlled rotary motion to ensure complete circumferential treatment of internal surfaces. Abrasive jetting techniques can also be used to cut completion or wellbore components. For this application, highly abrasive particles, such as sand, are carried in a fluid and jetted at the target area over an extended period to erode the tubular. |
| Well Workover and Intervention | absolute filter | A type of high-specification fluid filter frequently used to remove small solid particles from workover or treatment fluids that may be injected into, or placed adjacent to, the reservoir formation. In using absolute filters, all particles larger than the micron rating of the filter element in use will be removed from the treated fluid. |
| Well Workover and Intervention | accelerator | A downhole tool used in conjunction with a jar to store energy that is suddenly released when the jar is activated. The energy provides an impact force that operates associated downhole tools or, in a contingency role, helps release a tool string that has become stuck. Depending on the operating mode, the energy in tension or compression can be stored by means of a mechanical spring or a compressible fluid such as nitrogen gas. Accelerators should be selected on the basis of their compatibility with the jar to be used. |
| Well Workover and Intervention | acid | A generic term used to describe a treatment fluid typically comprising hydrochloric acid and a blend of acid additives. Acid treatments are commonly designed to include a range of acid types or blends, such as acetic, formic, hydrochloric, hydrofluoric and fluroboric acids. Applications for the various acid types or blends are based on the reaction characteristics of the prepared treatment fluid. |
| Well Workover and Intervention | acid frac | A hydraulic fracturing treatment performed in carbonate formations to etch the open faces of induced fractures using a hydrochloric acid treatment. When the treatment is complete and the fracture closes, the etched surface provides a high-conductivity path from the reservoir to the wellbore. |
| Well Workover and Intervention | acid inhibitor | A chemical additive used to protect wellbore components and treatment equipment from the corrosive action of an acid. The type and concentration of acid inhibitors are determined by the type of metal to be protected and the specific wellbore conditions, such as temperature and the length of exposure time anticipated during the treatment. To ensure efficient protection, the inhibitor should be consistently blended throughout the treatment fluid. |
| Well Workover and Intervention | acid tank | The rubber-lined vessel used to transport raw or concentrated acid to the wellsite. Some acid additives attack or degrade rubber. Consequently, acid treatment fluids are not generally mixed or transported in acid tanks, but are instead mixed in special batch tanks or continuously mixed as the treatment is pumped. |
| Well Workover and Intervention | acid wash | A wellbore acid treatment designed to remove scale or similar deposits from perforations and well-completion components. Acid-wash treatments generally do not include injection of treatment fluid into the reservoir formation. |
| Well Workover and Intervention | antifoam | A chemical additive used to prevent the formation of foam during the preparation of a treatment fluid or slurries at surface. Excess foam created during the mixing process may cause handling and pumping difficulties and may interfere with the performance or quality control of the mixed fluid. Antifoam agents may also be used to break foams returned from the wellbore, following a treatment, in preparation for disposal of the fluids. |
| Well Workover and Intervention | antifoam agent | A chemical additive used to prevent the formation of foam during the preparation of a treatment fluid or slurries at surface. Excess foam created during the mixing process may cause handling and pumping difficulties and may interfere with the performance or quality control of the mixed fluid. Antifoam agents may also be used to break foams returned from the wellbore, following a treatment, in preparation for disposal of the fluids. |
| Well Workover and Intervention | back up | To hold one end of a threaded connection while the other is turned to make up the joint. To ensure a secure connection, many types of threaded joints are made up to specific torque requirements in oil- and gas-well applications. This process requires the controlled application of force to the rotating component and a means of stabilizing and securing the corresponding stationary component to which it is being connected. |
| Well Workover and Intervention | balance point | The point at which the forces acting on a tubing string suspended in a live wellbore are equal. Under these conditions, the weight of the tubing string is balanced by the wellbore pressure acting to expel the string from the wellbore. The friction caused by the tubing string passing through the stripper or wellhead sealing device acts to extend the interval over which the balance point is apparent. |
| Well Workover and Intervention | ball diverter | Small spheres designed to seal perforations that are accepting the most fluid, thereby diverting reservoir treatments to other portions of the target zone. Ball sealers are incorporated into the treatment fluid and pumped with it. The effectiveness of this type of mechanical diversion to keep the balls in place is strongly dependent on the differential pressure across the perforation and the geometry of the perforation itself. |
| Well Workover and Intervention | ball dropper | The device used to inject ball sealers into the treatment fluid as it is pumped through the surface treating lines. |
| Well Workover and Intervention | ball out | A condition that may occur during ball diversion treatments in which all open perforations capable of receiving fluid are sealed. Ballout is signified by a rapid increase in treating pressure. Maintaining the treatment pressure may result in the breakdown and subsequent treatment of plugged perforations. If no further injection is possible, the pressure must be released to unseat the ball sealers, at which time the treatment will be terminated. |
| Well Workover and Intervention | ballout | A condition that may occur during ball diversion treatments in which all open perforations capable of receiving fluid are sealed. Ballout is signified by a rapid increase in treating pressure. Maintaining the treatment pressure may result in the breakdown and subsequent treatment of plugged perforations. If no further injection is possible, the pressure must be released to unseat the ball sealers, at which time the treatment will be terminated. |
| Well Workover and Intervention | barrel pump | A small pump with an extended suction duct that is designed to pump fluid from barrels. Barrel pumps are commonly used to decant liquid additives during the preparation of treatment fluids at the wellsite. |
| Well Workover and Intervention | basket | A downhole device or tool component designed to catch debris or objects, such as balls, darts or plugs dropped to actuate downhole equipment or tools. |
| Well Workover and Intervention | batch mixer | A vessel and mixing system used to prepare treatment fluids. A batch mixer is generally equipped with a means of adding dry and liquid chemicals, an agitation or circulation system and a manifold system to deliver the prepared fluid to storage tanks or treating pumps. |
| Well Workover and Intervention | bed wrap | The first layer of coiled tubing, slickline or wireline to be wound on the core of a reel drum or spool. The bed wrap helps secure the tubing string or slickline to the reel core and provides the foundation upon which subsequent wraps are laid as the drum is filled. A neat and secure bed wrap is necessary for proper spooling that will allow the drum to hold the maximum capacity without damaging the string. |
| Well Workover and Intervention | belt effect | A condition in deviated wellbores in which an additional friction component is applied as the slickline, wireline or coiled tubing is drawn to the inside radius of the curve. The effect is largely dependent on the load on the string, with the resultant friction forces being of most influence when high loads are encountered under static or slow-moving conditions. |
| Well Workover and Intervention | bias weld | A technique used in the assembly of coiled tubing strings at the manufacturing plant. Prior to being formed, the string is assembled from flat steel strips joined by a bias weld that is angled across the strip joint at 45 degrees. When the tubing string is milled, the helical weld form provides enhanced characteristics of the tube at the weld site. These are significantly better than those achievable with the alternative butt weld technique. |
| Well Workover and Intervention | bleed off | To equalize or relieve pressure from a vessel or system. At the conclusion of high-pressure tests or treatments, the pressure within the treatment lines and associated systems must be bled off safely to enable subsequent phases of the operation to continue. The bleedoff process must be conducted with a high degree of control to avoid the effect of sudden depressurization, which may create shock forces and fluid-disposal hazards. |
| Well Workover and Intervention | bleedoff line | A section of manifold containing the valves and piping necessary to bleed off pressure from a vessel or system. Bleedoff lines may be exposed to widely fluctuating pressures. They must be adequately secured, and consideration must be given to safe handling or disposal of the resulting fluids. |
| Well Workover and Intervention | blender | The equipment used to prepare the slurries and gels commonly used in stimulation treatments. The blender should be capable of providing a supply of adequately mixed ingredients at the desired treatment rate. Modern blenders are computer controlled, enabling the flow of chemicals and ingredients to be efficiently metered and requiring a relatively small residence volume to achieve good control over the blend quality and delivery rate. |
| Well Workover and Intervention | blind box | A simple slickline tool used to dislodge or push tools or equipment down the wellbore. The blind box is generally of heavy construction and is hardened to reduce damage when jarring is required. |
| Well Workover and Intervention | blind-shear ram | A blowout preventer (BOP) closing element fitted with hardened tool steel blades designed to cut the drillpipe or tubing when the BOP is closed, and then fully close to provide isolation or sealing of the wellbore. A shear ram is normally used as a last resort to regain pressure control of a well that is flowing. Once the pipe is cut (or sheared) by the shear rams, it is usually left hanging in the BOP stack, and kill operations become more difficult. The joint of drillpipe or tubing is destroyed in the process, but the rest of the string is unharmed by the operation of shear rams. |
| Well Workover and Intervention | braided line | A type of multistrand wireline used for slickline applications in which higher tension or weight-carrying ability is required. The most common size of braided line is 3/16-in. diameter, although special heavy applications use 1/4-in. and 5/16-in. sizes. When larger sizes are used, it may be necessary to kill the well due to the effect of wellhead pressure on the relatively large cross-sectional area of the line entering the wellbore. |
| Well Workover and Intervention | breaker | A chemical used to reduce the viscosity of specialized treatment fluids such as gels and foams. Breaking down the fluid viscosity may be desirable either as part of a treatment, such as allowing flow back of the spent treatment fluid, or following a treatment as part of the fluid-disposal process. Depending on the application, a breaker of predictable performance may be incorporated into the treatment fluid for downhole activation, or be added directly to the returned fluid for immediate effect at surface. |
| Well Workover and Intervention | bridge off | The accumulation or buildup of material, such as sand, fill or scale, within a wellbore, to the extent that the flow of fluids or passage of tools or downhole equipment is severely obstructed. In extreme cases, the wellbore can become completely plugged or bridged-off, requiring some remedial action before normal circulation or production can be resumed. |
| Well Workover and Intervention | bridge-off | The accumulation or buildup of material, such as sand, fill or scale, within a wellbore, to the extent that the flow of fluids or passage of tools or downhole equipment is severely obstructed. In extreme cases, the wellbore can become completely plugged or bridged-off, requiring some remedial action before normal circulation or production can be resumed. |
| Well Workover and Intervention | bridging agent | Material of a coarse, fibrous or flaky composition used to form an impermeable barrier across a formation interface or perforation. Bridging materials are most commonly used when lost circulation occurs during drilling. They are also used in workover operations in preparation for killing a well when the kill fluid is likely to be lost to the perforations. The selection of an appropriate bridging material is critical during workover operations since the barrier should be completely removed in preparation for placing the well back on production. |
| Well Workover and Intervention | bridging material | Material of a coarse, fibrous or flaky composition used to form an impermeable barrier across a formation interface or perforation. Bridging materials are most commonly used when lost circulation occurs during drilling. They are also used in workover operations in preparation for killing a well when the kill fluid is likely to be lost to the perforations. The selection of an appropriate bridging material is critical during workover operations since the barrier should be completely removed in preparation for placing the well back on production. |
| Well Workover and Intervention | broach | A downhole tool used to repair the internal diameter of the production tubing where a slight collapse or a dent has occurred. Cutting profiles on a broach removes the tubing-wall material to allow subsequent passage of tools and equipment of a prescribed diameter. |
| Well Workover and Intervention | buffer | A chemical used to adjust and control the pH of stimulation fluids. Gels and complex polymer fluids are sensitive to pH changes, especially during the mixing phase when the dispersion and hydration of some polymers require specific pH conditions. In addition, the performance of crosslinked fluids is optimized over a relatively narrow pH range. Buffers, added to the aqueous phase before mixing, adjust the base-fluid pH to achieve a stable treatment fluid with the desired characteristics and predictable performance. |
| Well Workover and Intervention | bull plug | A solid plug used as an isolation device in piping systems, conduits or wellbore tubulars. |
| Well Workover and Intervention | burn over | The use of a mill or burn shoe to remove the outside area of a permanent downhole tool or fish. Burning over the obstruction provides a profile on which fishing or retrieval tools can be engaged to pull the obstruction from the wellbore. |
| Well Workover and Intervention | burn shoe | A downhole tool routinely used in fishing operations to prepare the top and outside surface of a fish, generally to allow an overshot or similar fishing tool to engage cleanly on the fish. In some cases, the outer portion of a fish may be milled out to allow the body and remaining debris to be pushed to the bottom of the wellbore. |
| Well Workover and Intervention | burn-over | The use of a mill or burn shoe to remove the outside area of a permanent downhole tool or fish. Burning over the obstruction provides a profile on which fishing or retrieval tools can be engaged to pull the obstruction from the wellbore. |
| Well Workover and Intervention | butt weld | A welding technique used to join two tubes in which the squared and prepared ends are butted together in preparation for welding. The resulting circumferential weld has relatively good strength characteristics but has limitations where the tube is to be plastically deformed or bent, such as occurs on a coiled tubing string. Consequently, butt welds performed on a coiled tubing string should be checked carefully using hardness and radiographic testing methods and their locations detailed in the string record. The anticipated fatigue life in the butt-weld area must also be reduced to compensate for the weakness of the weld. |
| Well Workover and Intervention | C pump | A type of pump commonly used in the handling and mixing of oilfield fluids. The rotary motion of a profiled impeller in combination with a shaped pump housing or volute applies centrifugal force to discharge fluids from the pump. Centrifugal pumps generally operate most efficiently in high-volume, low-output-pressure conditions. Unlike a positive-displacement pump, the flow from centrifugal pumps can be controlled easily, even allowing flow to be completely closed off using valves on the pump discharge manifold while the pump is running. This pump is known as a “centrifugal pump. |
| Well Workover and Intervention | casing patch | A downhole assembly or tool system used in the remedial repair of casing damage, corrosion or leaks. Casing patches are most frequently used as short- to medium-term repairs that enable production to be resumed until a major workover operation is scheduled. In some cases, such as in depleted wells nearing the end of viable production, a casing patch may be the only economic means of safely returning the well to production. |
| Well Workover and Intervention | casing roller | A heavy-duty downhole tool used to restore the internal diameter of collapsed or buckled casing. Casing rollers generally are configured with an incremental series of rollers that act to gradually form the damaged casing to the desired size. Depending on the degree of damage and the requirement for wellbore access below the site of damage, the nominal diameter of the casing roller and repaired wellbore may be significantly less than the nominal drift diameter of the original casing string. |
| Well Workover and Intervention | casing scraper | A downhole tool incorporating a blade assembly that is used to remove scale and debris from the internal surface of a casing string. Generally run on tubing or drillpipe, casing scrapers are routinely used during workover operations to ensure that the wellbore is clean before reinstalling the completion string. |
| Well Workover and Intervention | centrifugal pump | A type of pump commonly used in the handling and mixing of oilfield fluids. The rotary motion of a profiled impeller in combination with a shaped pump housing or volute applies centrifugal force to discharge fluids from the pump. Centrifugal pumps generally operate most efficiently in high-volume, low-output-pressure conditions. Unlike a positive-displacement pump, the flow from centrifugal pumps can be controlled easily, even allowing flow to be completely closed off using valves on the pump discharge manifold while the pump is running. This pump is sometimes known as a “C pump. |
| Well Workover and Intervention | chelate | To combine a metal ion and a complexing agent to form a ring structure. |
| Well Workover and Intervention | chelation | An equilibrium reaction between a metal ion and a complexing agent. Chelation reactions are characterized by the formation of more than one bond between the metal and a molecule of the complexing agent. Chelation results in the formation of a ring structure incorporating the metal ion. In the oil field, chelation is often used to enhance stimulation treatments and to clean surface facilities. |
| Well Workover and Intervention | chemical diversion | Use of a chemical agent to achieve diversion during matrix stimulation or similar injected treatments. |
| Well Workover and Intervention | circulation sub | A downhole tool typically used with motors or assemblies that restrict the allowable fluid-circulation rates. When operated, the circulation sub allows a higher circulation rate to be established by opening a path to the annulus in the top section of the tool string. This is especially useful in applications such as drilling in slim-diameter wells, where a higher circulation rate may be necessary to effect good cuttings transport and hole cleaning before the string is retrieved. |
| Well Workover and Intervention | circulation valve | A downhole device that enables circulation through the tubing string and associated annulus. As a completion accessory, a circulation valve is included to circulate fluid for well kill or kickoff. Circulation valves typically are operated by slickline tools and are generally capable of several opening and closing cycles before requiring service. |
| Well Workover and Intervention | clay stabilizer | A chemical additive used in stimulation treatments to prevent the migration or swelling of clay particles in reaction to water-base fluid. Without adequate protection, some water-base fluids can affect the electrical charge of naturally occurring clay platelets in the formation. Modifying the charge causes the platelets to swell or migrate in the flowing fluid and, once these are dispersed, it is likely that some clay plugging of the formation matrix will occur. Clay stabilizers act to retain the clay platelets in position by controlling the charge and electrolytic characteristics of the treatment fluid. |
| Well Workover and Intervention | clean out | To remove wellbore-fill material such as sand, scale or organic materials, and other debris from the wellbore. Many reservoirs produce some sand or fines that may not be carried to surface in the produced fluid. Accumulations of fill material may eventually increase in concentration within the lower wellbore, possibly restricting production. Cleanouts using coiled tubing, snubbing or hydraulic workover techniques are performed routinely. |
| Well Workover and Intervention | cleanout | The removal of wellbore-fill material, such as sand, scale or organic materials, and other debris from the wellbore. Many reservoirs produce some sand or fines that may not be carried to surface in the produced fluid. Accumulations of fill material may eventually increase in concentration within the lower wellbore, possibly restricting production. Cleanouts using coiled tubing, snubbing or hydraulic workover techniques are performed routinely. |
| Well Workover and Intervention | closing unit | A generic term given to the hydraulic power pack and accumulators used to control the blowout preventers on a drilling or workover rig. |
| Well Workover and Intervention | closure pressure | An analysis parameter used in hydraulic fracture design to indicate the pressure at which the fracture effectively closes without proppant in place. |
| Well Workover and Intervention | coiled tubing connector | The downhole device used to connect the tool string to the coiled tubing string. Several types of devices with varying principles of operation are commonly used. The primary requirement is provision of an adequate mechanical connection capable of withstanding the necessary tensile and compressive forces, while ensuring efficient hydraulic isolation of the connection between the tool string and the coiled tubing string. |
| Well Workover and Intervention | coiled tubing string | A continuous length of low-alloy carbon-steel tubing that can be spooled on a reel for transport, then deployed into a wellbore for the placement of fluids or manipulation of tools during workover and well-intervention operations. The process of spooling and straightening a coiled tubing string imparts a high degree of fatigue to the tube material. Therefore, a coiled tubing string should be regarded as a consumable product with a finite service life. Predicting and managing the factors that affect the safe working life of a coiled tubing string are key components of the string-management system necessary for ensuring safe and efficient coiled tubing operations. |
| Well Workover and Intervention | coiled tubing unit | The package of equipment required to run a coiled tubing operation. Four basic components are required: the coiled tubing reel to store and transport the coiled tubing string, the injector head to provide the tractive effort to run and retrieve the coiled tubing string, the control cabin from which the equipment operator controls and monitors the operation, and the power pack that generates the necessary hydraulic and pneumatic power required by the other components. The dimensions and capacities of the coiled tubing unit components determine the size and length of coiled tubing string that can be used on the unit. Pressure-control equipment is incorporated into the equipment to provide the necessary control of well pressure fluid during normal operating conditions and contingency situations requiring emergency control. |
| Well Workover and Intervention | collapse pressure | The pressure at which a tube, or vessel, will catastrophically deform as a result of differential pressure acting from outside to inside of the vessel or tube. The collapse-pressure rating of perfectly round tubing is relatively high. However, when the tubing is even slightly oval, the differential pressure at which the tube will collapse may be significantly reduced. This is an important factor in determining the operating limits of coiled tubing strings since the action of spooling the string tends to induce some ovality. |
| Well Workover and Intervention | collector | The electrical device used on the axle of a spool or reel to provide electrical continuity between the rotating reel core and the stationary reel chassis. When using a coiled tubing string equipped with an electrical conductor, such as required during coiled tubing logging operations, a collector is fitted to the reel axle to allow connection of the surface data-acquisition equipment. |
| Well Workover and Intervention | combi BOP | A type of blowout preventer (BOP) in which each ram set combines two conventional ram functions, such as blind/shear and pipe/slip. The principal advantage of the combi-BOP is the reduced height required for rig up of the required ram functions. |
| Well Workover and Intervention | consistometer | A laboratory device used to determine the thickening time of cement slurries under simulated downhole pressure and temperature conditions. |
| Well Workover and Intervention | contaminant | A chemical or fluid that alters the performance of an engineered slurry or treatment fluid. Some remedial cementing treatments require unpredictable volumes of cement slurry to achieve the desired results. When excess slurry is left in the wellbore, it may not be possible to remove the excess slurry by conventional means, such as reverse circulation, before the slurry thickens and becomes immovable. Mixing the contaminant with the slurry in the correct proportions increases the thickening time of the slurry, allowing it to be safely removed from the wellbore. |
| Well Workover and Intervention | core testing | Laboratory analyses performed on formation core samples as part of a stimulation-treatment design process. Tests such as the formation flow potential, fracture orientation and fluid compatibility tests are commonly run in preparation for stimulation treatments. |
| Well Workover and Intervention | corrosion inhibitor | A chemical additive used in acid treatments to protect iron and steel components in the wellbore and treating equipment from the corrosive treating fluid. Corrosion inhibitors generally are mixed with the treatment fluid and are formulated to be effective in protecting the metal components the fluid is likely to contact. This protection must remain effective under the anticipated pressure and temperature environment for the duration of the treatment. |
| Well Workover and Intervention | counterbalance winch | The lifting device on a snubbing unit used to pick up and lay down the tool string and running-string tubulars. |
| Well Workover and Intervention | coupon | An abbreviation for electrical coupon, an instrument used in a corrosion test to determine metal loss. It directly measures the increase in resistance of a metal as its cross-sectional area is reduced by corrosion. At suitable times, once the readings are obtained, these numbers are converted into corrosion rates (mpy). An electrical coupon is also called an electrical resistance probe. |
| Well Workover and Intervention | crosslinker | A compound, typically a metallic salt, mixed with a base-gel fluid, such as a guar-gel system, to create a viscous gel used in some stimulation or pipeline cleaning treatments. The crosslinker reacts with the multiple-strand polymer to couple the molecules, creating a fluid of high, but closely controlled, viscosity. Treatments using crosslinkers should take account of the conditions needed to break the gel structure to ensure satisfactory cleanup and disposal. |
| Well Workover and Intervention | CT | A generic term relating to the use of a coiled tubing string and associated equipment. As a well-intervention method, coiled tubing techniques offer several key benefits over alternative well-intervention technologies. The ability to work safely under live well conditions, with a continuous string, enables fluids to be pumped at any time regardless of the position or direction of travel. This is a significant advantage in many applications. Installing an electrical conductor or hydraulic conduit further enhances the capability of a coiled tubing string and enables relatively complex intervention techniques to be applied safely. |
| Well Workover and Intervention | dart | A device dropped or pumped through a tubing or coiled tubing string to activate downhole equipment and tools. |
| Well Workover and Intervention | defoamer | A chemical additive used to prevent the formation of foam during the preparation of a treatment fluid or slurries at surface. Excess foam created during the mixing process may cause handling and pumping difficulties and may interfere with the performance or quality control of the mixed fluid. Antifoam agents may also be used to break foams returned from the wellbore, following a treatment, in preparation for disposal of the fluids. |
| Well Workover and Intervention | densitometer | A device installed on a mixing or pumping system manifold to measure the density of fluids. The density of fluids pumped into a well is frequently a key operating parameter, requiring constant monitoring and control. This is especially true when mixing slurries and transport fluids for solids, such as fracturing or gravel-pack fluids. |
| Well Workover and Intervention | deployment system | An assembly of pressure-control equipment that enables the running and retrieval of long tool strings on a coiled tubing string in a live wellbore. The deployment system is configured to provide two barriers against well pressure as the tool string is assembled and run into the wellbore. Once fully assembled, the coiled tubing equipment is connected and the tool string is run into the wellbore. The process is reversed for tool retrieval. |
| Well Workover and Intervention | diversion | A technique used in injection treatments, such as matrix stimulation, to ensure a uniform distribution of treatment fluid across the treatment interval. Injected fluids tend to follow the path of least resistance, possibly resulting in the least permeable areas receiving inadequate treatment. By using some means of diversion, the treatment can be focused on the areas requiring the most treatment. To be effective, the diversion effect should be temporary to enable the full productivity of the well to be restored when the treatment is complete. There are two main categories of diversion: chemical diversion and mechanical diversion. |
| Well Workover and Intervention | dosing pump | A low-volume fluid pump with controllable discharge rate used to inject chemical additives to the mixing or pumping system. Dosing pumps frequently are used to inject fluids that may be difficult to mix efficiently in batch-tank systems because of their low volume. |
| Well Workover and Intervention | dummy valve | A blank gas-lift valve placed in a gas-lift mandrel to isolate the tubing string from the annulus. Gas-lift valves frequently are replaced with dummy valves during intervention work on wells with gas-lift completions. |
| Well Workover and Intervention | dump bailer | A wireline or slickline tool used to place small volumes of cement slurry, or similar material, in a wellbore. Typically, the slurry is placed on a plug or similar device that provides a stable platform for the low-volume cement plug. |
| Well Workover and Intervention | duplex pump | A type of fluid pump, commonly used on workover rigs, that has two plungers or pistons. As a positive-reciprocating pump, the fluid flow rate is typically calculated from the number of strokes per minute that the pump makes and the displacement volume per stroke. Such a level of accuracy usually is sufficient for general workover purposes. |
| Well Workover and Intervention | electrical coupon | An instrument used in a corrosion test to determine metal loss. It directly measures the increase in resistance of a metal as its cross-sectional area is reduced by corrosion. At suitable times, once the readings are obtained, these numbers are converted into corrosion rates (mpy). An electrical coupon is also called an electrical resistance probe. |
| Well Workover and Intervention | entrained gas | The gas present in the fluids of a wellbore circulatory system. Many well-intervention operations are conducted with the well live or held on balance. Fluids circulated within the wellbore under these conditions are likely to pick up reservoir fluid and gas. The entrained gas and fluid require special handling and processing before the base fluid can be safely recirculated in the wellbore or prepared for disposal. |
| Well Workover and Intervention | equalizing loop | The high-pressure piping and valves configured around the stripping rams of a snubbing unit. The equalizing loop enables the wellhead pressure to be applied on both sides of the rams when closed, a process required before the rams can be opened. |
| Well Workover and Intervention | equalizing valve | A high-pressure valve, generally of small diameter, located on a conduit that runs between the two sides of an isolation valve or blowout preventer ram set. The forces acting on isolation devices such as blowout preventer rams can be extremely high, preventing the rams from being opened, or causing damage to the ram set seals during the opening process. The equalizing valve allows the pressure to be equalized across the ram set or isolation valve, enabling the device to be opened safely. |
| Well Workover and Intervention | excess cement | The cement slurry remaining in the wellbore following a cement squeeze in which the objective is to squeeze slurry into the perforations and behind the casing or liner. The volume of slurry required to effect a successful squeeze is often difficult to estimate. In most cases, an excess allowance is made since a shortage of slurry would result in failure of the operation. Removal of the excess cement slurry before it sets has been a key objective in the development of modern cement-squeeze techniques. |
| Well Workover and Intervention | external cutter | A downhole tool used to cut tubing or similar tubulars that have become stuck in the wellbore. The external cutter slips over the fish or tubing to be cut. Special hardened metal-cutters on the inside of the tool engage on the external surfaces of the fish. External cutters are generally used to remove the topmost, possibly damaged, portion of a fish to enable an overshot, or similar fishing tools, to engage on an undamaged surface. |
| Well Workover and Intervention | field weld | A welding technique used to join two tubes in which the squared and prepared ends are butted together in preparation for welding. The resulting circumferential weld has relatively good strength characteristics but has limitations where the tube is to be plastically deformed or bent, such as occurs on a coiled tubing string. Consequently, butt welds performed on a coiled tubing string should be checked carefully using hardness and radiographic testing methods and their locations detailed in the string record. The anticipated fatigue life in the butt-weld area must also be reduced to compensate for the weakness of the weld. |
| Well Workover and Intervention | fishing | The application of tools, equipment and techniques for the removal of junk, debris or fish from a wellbore. The key elements of a fishing operation include an understanding of the dimensions and nature of the fish to be removed, the wellbore conditions, the tools and techniques employed and the process by which the recovered fish will be handled at surface. |
| Well Workover and Intervention | fishing diagram | A diagram noting the major profiles and dimensions of tools and equipment run into a wellbore. A fishing diagram should be prepared for every tool operation, enabling contingency plans to be implemented efficiently if the tool string becomes stuck |
| Well Workover and Intervention | fishing neck | The surface on which a fishing tool engages when retrieving tubing, tools or equipment stuck or lost in a wellbore. Tools and equipment that are temporarily installed in a wellbore are generally equipped with a specific fishing-neck profile to enable the running and retrieval tools to reliably engage and release. |
| Well Workover and Intervention | flag | A mark or marker applied to a sand line or similar wire rope to indicate a specific depth or as a means of indicating the end of the line is nearing surface during retrieval. The term may also be used for magnetic or physical marks applied to wireline or coiled tubing strings. |
| Well Workover and Intervention | flow assurance | The design, strategies and principles for ensuring that there is uninterrupted hydrocarbon production flowing from the reservoir to the point of sale. Impediments to hydrocarbon flow in wellbores and flowlines may arise from an interrelated combination of effects involving flow dynamics—single and multiphase fluid flow—and production chemistry. At reservoir pressure and temperature conditions, fluids are single phase. As they travel toward the production facility, the fluids experience changes in pressure and temperature that result in multiple fluid phases and the formation, accumulation and dispersal of inorganic and organic solids that may become impediments to production. Multiphase flow may cause phenomena such as slugging in subsea flowlines and risers. Solids deposition may cause flowline plugging anywhere in the system. Historically, production impediments in wellbores and flowlines are well-known in onshore and shallow water environments, where they have been managed using thermal, mechanical and chemical means. The long flowlines connecting a wellhead or manifold to a production facility are exposed to low temperatures and high pressures. In these deepwater environments, intervention technologies and operations are expensive because they typically require deepwater vessels or a rig. Personnel responsible for flow assurance should have advanced knowledge of flow dynamics and production chemistry. |
| Well Workover and Intervention | flow back | The process of allowing fluids to flow from the well following a treatment, either in preparation for a subsequent phase of treatment or in preparation for cleanup and returning the well to production. |
| Well Workover and Intervention | flow meter | A device installed in a pump manifold or treating line to measure the fluid flow rate. Flowmeters can be used to measure the flow rates of liquid or gas and are available in various configurations and with differing operating principles. |
| Well Workover and Intervention | flowback | The process of allowing fluids to flow from the well following a treatment, either in preparation for a subsequent phase of treatment or in preparation for cleanup and returning the well to production. |
| Well Workover and Intervention | flowmeter | A device installed in a pump manifold or treating line to measure the fluid flow rate. Flowmeters can be used to measure the flow rates of liquid or gas and are available in various configurations and with differing operating principles. |
| Well Workover and Intervention | fluid compatibility test | A test, or series of tests, performed to check that no undesirable reactions occur with a specific fluid. The testing process may include checks for compatibility with other treating fluids, wellbore fluids, reservoir fluids and the reservoir formation. In extreme cases, the mixing of seemingly benign fluids can create significant reactions that may damage the reservoir permeability permanently. |
| Well Workover and Intervention | fluid level | The depth, or distance from surface, that the fluid in a well incapable of natural flow will reach under static conditions. |
| Well Workover and Intervention | fluid-friction reducer | A chemical additive that alters fluid rheological properties to reduce friction created within the fluid as it flows through small-diameter tubulars or similar restrictions. Generally polymers, or similar friction reducing agents, add viscosity to the fluid, which reduces the turbulence induced as the fluid flows. Reductions in fluid friction of 50 to 60% are possible. |
| Well Workover and Intervention | foam breaker | A chemical additive used to prevent the formation of foam during the preparation of a treatment fluid or slurries at surface. Excess foam created during the mixing process may cause handling and pumping difficulties and may interfere with the performance or quality control of the mixed fluid. Antifoam agents may also be used to break foams returned from the wellbore, following a treatment, in preparation for disposal of the fluids. |
| Well Workover and Intervention | foam diversion | The use of foam as a diverting agent during staged stimulation treatments. Stable foam is relatively viscous and the effect within a reservoir matrix can be used to divert subsequent acid stages from the zones already treated. Following the treatment, the foam breaks, with little risk of formation damage, to form a mixture of liquid and gaseous nitrogen that facilitates the cleanup process. |
| Well Workover and Intervention | foam generator | A device fitted in surface treatment lines that helps distribute a liquid foamer phase in a stream of nitrogen gas. The foam generator creates a consistent mixture that becomes a stable foam under downhole pressure and temperature conditions. |
| Well Workover and Intervention | formic acid | An organic acid used in the stimulation of high-temperature oil and gas wells in which conventional hydrochloric acid systems cannot be adequately inhibited, or where contact time with tubulars is likely to be extended. |
| Well Workover and Intervention | frac crew | Collective term for the personnel required to run a successful hydraulic fracturing operation. Members of the frac crew prepare the equipment on the wellsite prior to the operation, mix and pump the necessary chemicals and fluids during the frac job and render the wellsite location safe following the completion of the operation. |
| Well Workover and Intervention | frac gel | The primary fluid used in hydraulic fracturing operations. Several chemical additives generally will be added to the frac gel to form a treatment fluid specifically designed for the anticipated wellbore, reservoir and operating conditions. |
| Well Workover and Intervention | frac gradient | The pressure gradient, generally stated in psi/ft [kPa/m], at which a specific formation interval breaks down and accepts fluid. Determining the frac gradient is a key requirement in designing and analyzing a hydraulic fracturing treatment. |
| Well Workover and Intervention | frac pump | A high-pressure, high-volume pump used in hydraulic fracturing treatments. |
| Well Workover and Intervention | frac valve | A high-pressure isolation valve fitted to the top of the wellhead on a well that is about to be hydraulically fractured. The frac valve can be closed to isolate the treating equipment from the wellbore. |
| Well Workover and Intervention | free point | The depth at which a tubing or coiled tubing string that is stuck in the wellbore is free to move. When the tubing string must be cut to enable recovery, the free point should be known to ensure retrieval of the cut tubing. This enables remedial action to be taken to resolve the sticking mechanism on the portion of the string below the cut. |
| Well Workover and Intervention | free point indicator | A wireline tool used to determine the free point on a stuck string. The free-point indicator operates by detecting stretch in the tubular when tension is applied at surface. If stretch is not detected, the string must be stuck above the tool; if stretch is detected, the string is free above the free-point indicator tool. |
| Well Workover and Intervention | free-point indicator | A wireline tool used to determine the free point on a stuck string. The free-point indicator operates by detecting stretch in the tubular when tension is applied at surface. If stretch is not detected, the string must be stuck above the tool; if stretch is detected, the string is free above the free-point indicator tool. |
| Well Workover and Intervention | friction reducer | An additive, generally in slurry or liquid form, used to reduce the friction forces experienced by tools and tubulars in the wellbore. Friction reducers are routinely used in horizontal and highly deviated wellbores where the friction forces limit the passage of tools along the wellbore. |
| Well Workover and Intervention | gas buster | A simple separator vessel used to remove free or entrained gas from fluids circulated in the wellbore, such as mud used during drilling operations. The gas buster typically comprises a vessel containing a series of baffles with a liquid exit on the bottom and a gas-vent line at the top of the vessel. |
| Well Workover and Intervention | gauge tank | A small tank with accurate volume markings used to measure flow into or out of a well. Treatments that require accurate volume tracking of fluids, such as squeeze cementing, generally use a gauge tank to measure fluid volumes. |
| Well Workover and Intervention | gin pole | A lifting device, similar in function to a crane jib, that is used in a number of oilfield applications, such as for handling tubulars on a snubbing unit, tool strings on a slickline unit or on a winch truck, and for general lifting at the wellsite. |
| Well Workover and Intervention | goose neck | The assembly mounted on a coiled tubing injector head that guides the tubing string as it passes through an arc from the reel into a vertical alignment with the injector-head chains and wellbore. The radius of the guide arch is generally designed to be as large as practicable since the plastic deformation created in the coiled tubing string induces material fatigue in the tube. |
| Well Workover and Intervention | gooseneck | The assembly mounted on a coiled tubing injector head that guides the tubing string as it passes through an arc from the reel into a vertical alignment with the injector-head chains and wellbore. The radius of the guide arch is generally designed to be as large as practicable since the plastic deformation created in the coiled tubing string induces material fatigue in the tube. |
| Well Workover and Intervention | grapple | A generic name given to tools that engage on the outer surface of a tubing string or tool assembly, generally for fishing purposes. |
| Well Workover and Intervention | grease injection system | An assembly of components used to contain wellhead fluids and pressure during braided-line or wireline operations. The wireline passes through a close-tolerance tube assembly as it leaves the wellbore. High-pressure grease is pumped into the surrounding annulus to effect a pressure-tight dynamic seal that is maintained during the operation by injecting more grease as required. A slight leakage of grease is normal, and the addition of fresh grease enables the consistency of the seal to be maintained at an effective level. |
| Well Workover and Intervention | grease-injection system | An assembly of components used to contain wellhead fluids and pressure during braided-line or wireline operations. The wireline passes through a close-tolerance tube assembly as it leaves the wellbore. High-pressure grease is pumped into the surrounding annulus to effect a pressure-tight dynamic seal that is maintained during the operation by injecting more grease as required. A slight leakage of grease is normal, and the addition of fresh grease enables the consistency of the seal to be maintained at an effective level. |
| Well Workover and Intervention | gripper blocks | The profiled blocks attached to the drive chains of a coiled tubing injector head. The gripper blocks are arranged in opposing pairs to secure the coiled tubing string in the injector-head chains. As the hydraulic drive system rotates the chains, the gripper blocks feed the tubing string into, or out of the well. |
| Well Workover and Intervention | heavy pipe | An operating condition during a snubbing operation in which the force resulting from the weight of the pipe or tubing string is greater than the wellhead pressure and the buoyancy forces acting to eject the string from the wellbore. In the heavy-pipe condition, the string will drop into the wellbore if the gripping force is lost. |
| Well Workover and Intervention | hesitation squeeze | A technique used in squeeze cementing whereby a portion of the slurry is pumped, then pumping stops to expose the slurry to differential pressure against the zone of interest in stages over a period from several minutes to several hours. This pressure, higher than necessary for fluid movement, is applied to force the cement slurry into the area requiring repair. This staged procedure is repeated until all the slurry has been pumped or until no further slurry can be placed into the treatment zone. The cement remaining in the zone forms an effective hydraulic seal with a high compressive strength. |
| Well Workover and Intervention | high pressure squeeze | A squeeze-cementing technique involving the application of treatment pressure that is higher than the fracture pressure of the formation. This procedure may be necessary to force the slurry into microcracks or annuli that surround the wellbore. The characteristics of a fracture are dependent on the fluid flow rate when the fracture is initiated; consequently, high-pressure squeeze operations must be conducted with a high degree of control to place the slurry in the desired location. |
| Well Workover and Intervention | high-pressure squeeze | A squeeze-cementing technique involving the application of treatment pressure that is higher than the fracture pressure of the formation. This procedure may be necessary to force the slurry into microcracks or annuli that surround the wellbore. The characteristics of a fracture are dependent on the fluid flow rate when the fracture is initiated; consequently, high-pressure squeeze operations must be conducted with a high degree of control to place the slurry in the desired location. |
| Well Workover and Intervention | hot oiler | A truck- or skid-mounted unit used to heat oil or treatment fluid. Hot oilers are routinely used in the removal of wax deposits from the upper wellbore section of wells in cold climates where low wellhead temperatures increases the susceptibility of heavy crude oil to wax precipitation. |
| Well Workover and Intervention | hydraulic bypass | A design feature on packers and similar downhole tools that occupy a large proportion of the drift diameter of the wellbore. When running and retrieving such tools, the hydraulic bypass allows the wellbore fluid to flow through part of the tool assembly to reduce the forces applied to the tool and reduce any damaging swab or surge effect on the reservoir formation. |
| Well Workover and Intervention | hydraulic centralizer | A type of tool-string centralizer, generally used in through-tubing applications, that employs hydraulic force to energize the centralizer arms or bows. Through-tubing operations sometimes require the tool string to be centralized within the casing or liner below the tubing. The relatively large expansion required for this is not generally within the operating range of conventional centralizer models. |
| Well Workover and Intervention | hydraulic disconnect | A downhole tool designed to allow the lower and upper tool string sections to be parted to enable retrieval of the running string. Hydraulic disconnects rely on the application of a predefined pressure through the running string to activate a release mechanism. In some cases, a ball or dart is plugged to block circulation through the tool string and enable the application of the release pressure. |
| Well Workover and Intervention | hydraulic power pack | An assembly of components and controls necessary to provide a hydraulic power supply. In modern oilfield activities, many systems are hydraulically powered, including the majority of mobile systems such as slickline units, coiled tubing units and snubbing units. In most cases, a diesel engine is the prime mover, providing an independent power supply that is harnessed to the necessary hydraulic pump and control systems. |
| Well Workover and Intervention | hydraulic release tool | A downhole tool designed to allow the lower and upper tool string sections to be parted to enable retrieval of the running string. Hydraulic disconnects rely on the application of a predefined pressure through the running string to activate a release mechanism. In some cases, a ball or dart is plugged to block circulation through the tool string and enable the application of the release pressure. |
| Well Workover and Intervention | hydrochloric acid | An acid type commonly used in oil- and gas- well stimulation, especially in carbonate formations. The reaction characteristics of hydrochloric acid enable it to be used in a wide range of treatments, often with chemical additives that enhance its performance or allow greater control of the treatment. Treatments are most commonly conducted with 15% or 28% solutions of hydrochloric acid. |
| Well Workover and Intervention | hydrostatic bailer | A slickline tool generally used for the removal of sand or similar small particles around the fishing necks of downhole tools or equipment. The hydrostatic bailer incorporates a sealed atmospheric chamber and a shear pin, or similar activation mechanism, to allow communication with the wellbore. When the tool is activated, there is a fluid surge into the atmosphere as the pressure is equalized. A shroud arrangement at the base of the tool contains and directs the fluid surge to dislodge and capture any debris in the area. |
| Well Workover and Intervention | inhibited acid | An acid treatment fluid that has been mixed with chemical additives to control the corrosive effect on the mixing and pumping equipment, as well as on any wellbore tubulars and completion equipment that the fluid may contact. Almost all acid treatments require the addition of an inhibitor to protect against undesirable reactions. |
| Well Workover and Intervention | injection test | A procedure conducted to establish the rate and pressure at which fluids can be pumped into the treatment target without fracturing the formation. Most stimulation treatments and remedial repairs, such as squeeze cementing, are performed following an injection test to help determine the key treatment parameters and operating limits. |
| Well Workover and Intervention | injector head | One of the principal equipment components of a coiled tubing unit. The injector head incorporates special profiled chain assemblies to grip the coiled tubing string and a hydraulic drive system that provides the tractive effort for running and retrieving the string from the wellbore. The base of the injector head is secured to the wellhead pressure-control equipment by the stripper assembly mounting system. The gooseneck mounted on top of the injector head feeds the tubing string from the reel around a controlled radius into the injector head. |
| Well Workover and Intervention | intensifier | A downhole tool used with a jar to increase the impact force imparted as the jar is fired. Similar in function to an accelerator, intensifiers typically use compressed gas rather than a mechanical spring to store the energy released during operation. |
| Well Workover and Intervention | internal flash | The excess material formed on the internal surface of a coiled tubing string by the longitudinal weld during manufacture. The internal flash can be removed in some larger sizes of string to make a fullbore string less susceptible to localized corrosion that can occur in the area of the longitudinal weld. |
| Well Workover and Intervention | jacking frame | A support structure used to stabilize the injector head and pressure-control equipment on some offshore, or special onshore, coiled tubing units. The jacking frame is hydraulically controlled to enable the injector head to be located at a safe and secure working height. Additional features, such as the ability to skid the injector head to the side for access to the wellbore are included in some of the more complex designs of jacking frame. |
| Well Workover and Intervention | jet cutter | A downhole tool, generally run on wireline or coiled tubing, that uses the detonation of a shaped explosive charge to cut the surrounding tubing or casing wall. The cutting action leaves a relatively clean cut surface, although the explosive action tends to flare the cut ends, making retrieval of cut tubular difficult if the fishing tool engages on the external surfaces. |
| Well Workover and Intervention | junk pusher | A downhole tool similar in function to a casing scraper. A junk pusher is run to ensure an unobstructed wellbore before setting a packer or similar fullbore device. |
| Well Workover and Intervention | junk sub | A downhole tool with a profiled external surface designed to catch and retrieve junk or debris from the wellbore. The debris is carried up the tool-string annulus in the circulation fluid. An indented profile creating a larger annular area causes the fluid flow rate to drop and allows debris to drop into a basket or receptacle located at the base of the tool. |
| Well Workover and Intervention | kill pump | A high-pressure pump designated for well-kill purposes. Depending on the application, the kill pump may need to be connected to a ready supply of kill fluid should well control be required at short notice. |
| Well Workover and Intervention | layered reservoir testing | A downhole tool similar in function to a casing scraper. A junk pusher is run to ensure an unobstructed wellbore before setting a packer or similar fullbore device. |
| Well Workover and Intervention | lb/bbl | A downhole tool with a profiled external surface designed to catch and retrieve junk or debris from the wellbore. The debris is carried up the tool-string annulus in the circulation fluid. An indented profile creating a larger annular area causes the fluid flow rate to drop and allows debris to drop into a basket or receptacle located at the base of the tool. |
| Well Workover and Intervention | levelwind | The assembly on a coiled tubing reel that guides the tubing string onto the drum. Accurate spooling is necessary to avoid damaging the tubing and to ensure that the entire string can be run and retrieved without jamming. The levelwind functions automatically, although it incorporates a manual override to facilitate minor corrections. |
| Well Workover and Intervention | lifting frame | A lifting device used when performing coiled tubing operations from a semisubmersible rig or drillship. The coiled tubing injector and pressure-control equipment are positioned within the lifting frame, which is attached to the flow head and running string and supported by the traveling blocks. This configuration enables the heave-compensation system of the rig to counteract the vessel motion. |
| Well Workover and Intervention | light pipe | An operating condition during a snubbing operation in which the wellhead pressure and buoyancy forces are greater than the force resulting from the weight of the pipe or tubing string. In the light-pipe condition the string will be ejected from the wellbore if the gripping force of the slips is lost. |
| Well Workover and Intervention | liner patch | A downhole assembly or tool system used in the repair of liner damage, corrosion or leaks. Liner patches are most frequently used as short- to medium-term repairs that enable production to be resumed until a major workover operation is scheduled. In some cases, such as in depleted wells nearing the end of viable production, a liner patch may be the only economic means of safely returning the well to production. |
| Well Workover and Intervention | live cement | A term used to describe a cement slurry that remains liquid but is still capable of thickening or setting to become an unmovable solid mass. Some remedial operations treat the excess live cement slurry with a contaminant to extend the thickening time and allow its safe removal from the wellbore. |
| Well Workover and Intervention | load cell | The sensor component in a weight-indicator system that detects the tensional or compressional forces being imparted to the running string at surface. Load cells are hydraulically or electronically operated and are connected to the weight-indicator display system on the equipment operator’s console. |
| Well Workover and Intervention | load oil | Oil pumped into a wellbore in preparation for, or as part of, a treatment. Some treatments, such as hydraulic fracturing, involve pumping large volumes of fluid. Using load oil, often produced and processed from adjacent wells in the field, reduces the cost of fluids and can enhance the cleanup process when the treatment is complete. |
| Well Workover and Intervention | lock-up | A condition that may occur when a coiled tubing string is run into a horizontal or highly deviated wellbore. Lock-up occurs when the frictional force encountered by the string running on the wellbore tubular reaches a critical point. Although more tubing may be injected into the wellbore, the end of the tool string cannot be moved farther into the wellbore. |
| Well Workover and Intervention | lubricator | A term initially applied to the assembly of pressure-control equipment used on slickline operations to house the tool string in preparation for running into the well or for retrieval of the tool string on completion of the operation. The lubricator is assembled from sections of heavy-wall tube generally constructed with integral seals and connections. Lubricator sections are routinely used on the assembly of pressure-control equipment for other well-intervention operations such as coiled tubing. |
| Well Workover and Intervention | marker joint | joint of tubing used in a workover or completion tubing string that serves as a position or depth indicator. In most cases, a marker joint is significantly shorter than other joints in the string so that it is easily noticeable on correlation logs or when retrieving a work string, such as on a snubbing or hydraulic workover unit. |
| Well Workover and Intervention | mast unit | A well-servicing unit for slickline, wireline or coiled tubing operations that is equipped with a mast rather than a crane or gin pole. The mast provides a means of lifting and stabilizing tools, and running pressure-control and other equipment. |
| Well Workover and Intervention | maximum treating pressure | The surface-pump pressure limit below which a treatment should be performed. The maximum treating pressure is determined to avoid fracturing the formation or damaging completion components. The maximum treating pressure is generally calculated to ensure that the pump-pressure limit equates to downhole and reservoir conditions that are within the design limits of the treatment. |
| Well Workover and Intervention | mechanical diversion | The use of mechanical devices, such as ball sealers, packers and straddle-packer assemblies, to divert reservoir treatments to the target zone. Ball sealers and solid-particle diverting agents incorporated into the treatment fluid form a temporary plug in the perforations accepting the most fluid flow, thereby diverting the remaining treatment fluid to the less permeable zones. Packers and straddle-packer assemblies function by performing several short treatments over a longer interval to help ensure an even treatment over the entire zone. |
| Well Workover and Intervention | mechanical jar | A type of jar that incorporates a mechanical trip or firing mechanism that activates only when the necessary tension or compression has been applied to the running string. In slickline operations, the term is often used to describe any jar that does not contain a hydraulic trip mechanism, such as link and tubular jars that do not incorporate a firing mechanism. |
| Well Workover and Intervention | memory gauge | A type of electronic pressure gauge that samples and records downhole pressures, with the data being stored, ready for downloading to acquisition equipment when the tool assembly has been retrieved to surface. Memory gauges are generally used to measure bottomhole pressures and temperatures in response to various production rates in tests to assess well productivity and reservoir performance. |
| Well Workover and Intervention | minifrac | A small fracturing treatment performed before the main hydraulic fracturing treatment to acquire critical job design and execution data and confirm the predicted response of the treatment interval. The minifrac procedure provides key design data from the parameters associated with the injection of fluids and the subsequent pressure decline. The final job procedures and treatment parameters are refined according to the results of the minifrac treatment. |
| Well Workover and Intervention | mutual solvent | A chemical additive for stimulation treatments that is soluble in oil, water and acid-based treatment fluids. Mutual solvents are routinely used in a range of applications, such as removing heavy hydrocarbon deposits, controlling the wettability of contact surfaces before, during or after a treatment, and preventing or breaking emulsions. A commonly used mutual solvent is ethyleneglycolmonobutyl ether, generally known as EGMBE. |
| Well Workover and Intervention | neutralizing solution | A fluid prepared to counteract the corrosive effect of acids or acidic treatment fluids. Neutralizing solutions generally are used when the components to be protected cannot be adequately flushed or when there is a risk that residual fluids may cause problems through prolonged exposure. Neutralizing solutions are commonly formulated with soda ash to provide an inexpensive, nondamaging alkaline fluid that does not create excessive disposal difficulties. |
| Well Workover and Intervention | nitrified fluid | A multiphase fluid incorporating a liquid base and gaseous nitrogen. Nitrified fluids are frequently used in stimulation treatments to enhance the performance of the treatment fluid and improve the cleanup process following the treatment. |
| Well Workover and Intervention | nitrogen lifting | The injection of nitrogen into the fluid column within the production conduit to initiate fluid flow from the wellbore and production from the reservoir. Nitrogen lifting through a coiled tubing string is a common technique used in well kickoff. |
| Well Workover and Intervention | nominal filter | A classification of filter used in the cleaning and treatment of brines and solids-free fluids. Nominal filters trap or remove most particles of equal or larger size than the given filter specification. |
| Well Workover and Intervention | ovality limit | The maximum distortion permitted on the cross-sectional profile of a coiled tubing string. The mechanical performance of oval tubing deteriorates as the degree of ovality increases. The most critical effect is the ability of the tube to resist collapse under differential pressure. String ovality limits are generally determined by the maximum diameter that can pass through the primary pressure-control equipment. In high-pressure operations, the ovality limits will generally be reduced to maintain an adequate safety margin against string collapse. |
| Well Workover and Intervention | overflush | A specially prepared fluid used to displace matrix acid treatments away from the wellbore at the conclusion of a stimulation treatment. The overflush is typically formulated from a weak acid solution or brine to maintain a low pH environment in the near-wellbore formation that prevents the precipitation of reaction products as the treatment fluids are flowed back. The overflush is normally a weak acid or brine pumped behind the main treating fluid (mixture of hydrofluoric [HF] and hydrochloric [HCl] or organic acids). The overflush has several purposes: · displacement of the nonreacted mud acid into the formation. · displacement of mud-acid reaction by-products such as amorphous silica. The minimum overflush volume should have at least 3 ft [1 m] of radial penetration inside the formation to displace potential problems away from the critical matrix. · removal of potential oil-wet relative-permeability problems caused by some corrosion inhibitors. |
| Well Workover and Intervention | overshot | A downhole tool used in fishing operations to engage on the outside surface of a tube or tool. A grapple, or similar slip mechanism, on the overshot grips the fish, allowing application of tensile force and jarring action. If the fish cannot be removed, a release system within the overshot allows the overshot to be disengaged and retrieved. |
| Well Workover and Intervention | packoff | To effect hydraulic isolation, either with a sealing device, such as a packer, or with a specialized plastic or fluid, such as a sealing compound. |
| Well Workover and Intervention | paraffin scraper | A downhole tool, generally run on slickline, used to remove paraffin and soft wax deposits from the internal wall of production tubulars and completion equipment. |
| Well Workover and Intervention | paraffin scratcher | A downhole tool, generally run on slickline, used to remove paraffin and soft wax deposits from the internal wall of production tubulars and completion equipment. |
| Well Workover and Intervention | pickle | To use a relatively weak, inhibited acid to remove scale, rust and similar deposits from the internal surfaces of equipment such as treating lines, pumping equipment or the tubing string through which an acid or chemical treatment is to be pumped. The pickling process removes materials that may react with the main treatment fluid to create undesirable secondary reactions or precipitates damaging to the near-wellbore reservoir formation. |
| Well Workover and Intervention | pickling fluid | A relatively weak, inhibited acid used to remove scale, rust and similar deposits from the internal surfaces of equipment such as treating lines, pumping equipment or the tubing string through which an acid or chemical treatment is to be pumped. The pickling process removes materials that may react with the main treatment fluid to create undesirable secondary reactions or precipitates damaging to the near-wellbore reservoir formation. |
| Well Workover and Intervention | pill | A relatively small volume of specially prepared fluid placed or circulated in the wellbore. Fluid pills are commonly prepared for a variety of special functions, such as a sweep pill prepared at high viscosity to circulate around the wellbore and pick up debris or wellbore fill. In counteracting lost-circulation problems, a lost-circulation pill prepared with flaked or fibrous material is designed to plug the perforations or formation interval losing the fluid. |
| Well Workover and Intervention | pilot mill | A downhole milling tool designed with an extended pilot or central stinger section that is inserted in the bore of the packer, tubular or equipment being milled. This design helps ensure that the mill follows the desired path and does not damage the casing or liner wall as the milling operation progresses. |
| Well Workover and Intervention | pipe heavy | An operating condition during a snubbing operation in which the force resulting from the weight of the pipe or tubing string is greater than the wellhead pressure and the buoyancy forces acting to eject the string from the wellbore. In the heavy-pipe condition, the string will drop into the wellbore if the gripping force is lost. |
| Well Workover and Intervention | pipe light | An operating condition during a snubbing operation in which the wellhead pressure and buoyancy forces are greater than the force resulting from the weight of the pipe or tubing string. In the light-pipe condition the string will be ejected from the wellbore if the gripping force of the slips is lost. |
| Well Workover and Intervention | polymer plug | A volume of polymer slurry placed in a wellbore, which, in time and under the correct temperature conditions, will develop to provide a high-viscosity platform on which a cement plug can be placed. Polymer plugs are typically used when a cement plug must be set accurately within the wellbore, The viscous material prevents the dense cement slurry from fingering through the lighter wellbore fluid during placement, helping to ensure that cement is placed over the desired interval. |
| Well Workover and Intervention | poppet valve | A type of check valve often used in the lines or manifolds associated with kill and choke lines or pressure-control equipment. |
| Well Workover and Intervention | pressure sender | The sensor component in a system used to measure and display the pressure within a vessel or system. The pressure sender may be hydraulically or electrically connected to a remote gauge or display. |
| Well Workover and Intervention | pulling tool | A slickline or coiled tubing tool used to retrieve temporary devices, such as plugs and flow-control equipment, from the wellbore. Pulling tools are available in a range of sizes and profiles and must be compatible with the equipment to be retrieved. A contingency release system in the pulling tool allows the tool to be released and retrieved if the equipment to be retrieved cannot be released. |
| Well Workover and Intervention | pump cavitation | A condition affecting an operating pump whereby the pump space is not fully charged with fluid being pumped. Pump cavitation may result from inadequate or restricted supply or from the introduction of air or gas into the fluid stream. The effect of cavitation depends on the type of pump. However, in most cases, it is an undesirable condition that causes a reduction in pump efficiency and excessive wear or damage to pump components. |
| Well Workover and Intervention | pump manifold | The arrangement of lines and valves used to direct and control fluid on a pumping unit. The manifold on the pump suction is generally known as the inlet or low-pressure manifold. The corresponding manifold located on the pump discharge is commonly known as the high-pressure or discharge manifold. In most cases, reference to the pump manifold relates to the high-pressure manifold. |
| Well Workover and Intervention | pumper | A mobile high-pressure pumping unit commonly used for cementing or stimulation operations. Most pump units are configured with a high-pressure triplex pump and one or more centrifugal pumps to precharge the triplex pump and handle displacement fluids. |
| Well Workover and Intervention | pumping schedule | A document prepared to list the sequence, type and volume of fluids to be pumped during a specific treatment. |
| Well Workover and Intervention | ram preventer | A classification of blowout preventer in which the pressure-control functions are achieved through the operation of hydraulically operated ram sets. Each ram set is configured as an opposing pair and, depending on function, are designed to close within the bore of the preventer. Ram preventers are commonly available in single-, double-, triple- and quad-ram configurations. |
| Well Workover and Intervention | reciprocating pump | A type of fluid pump in which reciprocating pistons or plungers displace the fluid. |
| Well Workover and Intervention | recirculating mixer | A type of mixer used in various oil- and gas-well service activities to prepare treatment fluids or slurries. Recirculating mixers are configured with a tank or vessel, a pump to circulate the fluid, and a manifold system to control recirculation and delivery of the fluid to storage or to the high-pressure treatment pumps. Recirculating the fluid induces turbulence to help ensure a homogeneous fluid. |
| Well Workover and Intervention | reel | The device used to store and transport a coiled tubing string ready for use at the wellsite. The coiled tubing reel incorporates a manifold and swivel arrangement to enable fluids to be pumped through the coiled tubing string at any time, a levelwind assembly to ensure the string is correctly spooled and a treatment system to apply inhibitor or similar protective coatings to the coiled tubing string. The reel functions are hydraulically powered and controlled from the unit control cabin. |
| Well Workover and Intervention | reel back tension | The tension applied to a coiled tubing string as it passes between the reel and the injector head. An adequate back-tension must be maintained to ensure that the string spools correctly on or off the reel. |
| Well Workover and Intervention | reel back-tension | The tension applied to a coiled tubing string as it passes between the reel and the injector head. An adequate back-tension must be maintained to ensure that the string spools correctly on or off the reel. |
| Well Workover and Intervention | reeled tubing | A generic term relating to the use of a coiled tubing string and associated equipment. As a well-intervention method, coiled tubing techniques offer several key benefits over alternative well-intervention technologies. The ability to work safely under live well conditions, with a continuous string, enables fluids to be pumped at any time regardless of the position or direction of travel. This is a significant advantage in many applications. Installing an electrical conductor or hydraulic conduit further enhances the capability of a coiled tubing string and enables relatively complex intervention techniques to be applied safely. |
| Well Workover and Intervention | remedial cementing | Cementing operations performed to repair primary-cementing problems or to treat conditions arising after the wellbore has been constructed. The two main categories of remedial cementing include squeeze cementing and the placement of cement plugs. |
| Well Workover and Intervention | residual bend | The natural form that a section of coiled tubing string will take if spooled from the reel and allowed to rest without any tension applied. The residual bend results from the plastic deformation imparted as the string is spooled around the radius of the reel and guide arch. |
| Well Workover and Intervention | rod elevators | Lightweight elevators designed for running and retrieving the sucker-rod string in wells equipped with a rod pump. Rod elevators can be used on a workover rig or a rod unit specifically designed for running and retrieving rod strings. |
| Well Workover and Intervention | rod string | An assembled length of sucker rods used to connect and power a rod pump with the reciprocating power source at surface. |
| Well Workover and Intervention | rod unit | A lightweight workover unit specifically designed for running and retrieving rod strings and rod pumps. Rod units are generally truck-mounted and configured to suit the relatively light work associated with rod-pump servicing. |
| Well Workover and Intervention | roller stem | A downhole tool used on slickline operations conducted on a deviated wellbore to provide additional mass to the tool string. Incorporated in the assembly, rollers reduce the friction encountered as the roller stem is run along the wellbore. This, together with the additional mass, helps achieve deeper slickline penetration of deviated wellbores. |
| Well Workover and Intervention | running squeeze | A cement-squeeze technique in which the cement slurry is continuously injected until the desired squeeze pressure is achieved. When pumping stops, the final squeeze pressure is monitored. If the pressure falls, additional slurry is squeezed to increase the pressure back to the final squeeze value. This process is repeated until the final squeeze pressure can be sustained. |
| Well Workover and Intervention | running tool | A downhole tool used to run and set downhole plugs or similar equipment. The term applies to a range of tools used in workover activities, such as coiled tubing, snubbing or rig-based applications. However, the term is most commonly associated with slickline operations, referring to the tools used to run and set slickline locks, plugs and similar downhole equipment. |
| Well Workover and Intervention | safety clamp | A mechanical device attached to tool strings or flush surface tubulars as they are assembled or disassembled. The safety clamp prevents the tool string from being dropped downhole accidentally if the slips or elevators securing the string lose their grip |
| Well Workover and Intervention | safety head | Another term for shear-seal BOP, an item of pressure-control equipment often fitted to the wellhead during well-intervention operations on live wells. Most commonly associated with coiled tubing operations, the shear-seal BOP is a ram-type preventer that performs the dual functions of shearing or cutting the tubing string and then fully closing to provide isolation or sealing of the wellbore. Shear-seal BOPs are most commonly used in offshore or high-pressure applications where an additional contingency pressure barrier is required. |
| Well Workover and Intervention | sand bailer | Another term for a hydrostatic bailer, a slickline tool generally used for the removal of sand or similar small particles around the fishing necks of downhole tools or equipment. The hydrostatic bailer incorporates a sealed atmospheric chamber and a shear pin, or similar activation mechanism, to allow communication with the wellbore. When the tool is activated, there is a fluid surge into the atmosphere as the pressure is equalized. A shroud arrangement at the base of the tool contains and directs the fluid surge to dislodge and capture any debris in the area. |
| Well Workover and Intervention | sand cleanout | The process of removing sand or similar fill from a wellbore. Many wells produce sand that may accumulate and restrict production if not removed from the wellbore by the production fluid. Coiled tubing and snubbing units are routinely used for sand-cleanout operations, enabling the well condition to be treated without removing the completion equipment or even killing the well. |
| Well Workover and Intervention | sand lifting | Another term for sand cleanout, the process of removing sand or similar fill from a wellbore. Many wells produce sand that may accumulate and restrict production if not removed from the wellbore by the production fluid. Coiled tubing and snubbing units are routinely used for sand-cleanout operations, enabling the well condition to be treated without removing the completion equipment or even killing the well. |
| Well Workover and Intervention | sand line | A long cable, installed on most drilling and workover rigs, used when swabbing or bailing in the production tubing or wellbore tubulars. The sand line is typically stored and operated on a winch drum that is part of the rig drawworks. The sand line is capable of significantly higher tensile forces than slickline or electric wireline. |
| Well Workover and Intervention | sand out | A condition encountered during some hydraulic fracturing operations whereby the fracture cannot accept further sand or proppant and only the carrier fluid is injected into the formation. A sandout occurs when the concentration of proppant within the tubing string rapidly increases, creating a corresponding sudden increase in pump pressure. |
| Well Workover and Intervention | sandout | A condition encountered during some hydraulic fracturing operations whereby the fracture cannot accept further sand or proppant and only the carrier fluid is injected into the formation. A sandout occurs when the concentration of proppant within the tubing string rapidly increases, creating a corresponding sudden increase in pump pressure. |
| Well Workover and Intervention | scraper | Also called a pig, a device with blades or brushes inserted in a pipeline for cleaning purposes. The pressure of the oil stream behind pushes the pig along the pipeline to clean out rust, wax, scale and debris. To clean downhole tubulars a similar device, called a casing scraper or paraffin scraper, may be used. |
| Well Workover and Intervention | secondary cementing | Another term for remedial cementing, cementing operations performed to repair primary-cementing problems or to treat conditions arising after the wellbore has been constructed. The two main categories of remedial cementing include squeeze cementing and the placement of cement plugs. |
| Well Workover and Intervention | setting tool | A downhole tool used to run and set downhole plugs or similar equipment. The term applies to a range of tools used in workover activities, such as coiled tubing, snubbing or rig-based applications. However, the term is most commonly associated with slickline operations, referring to the tools used to run and set slickline locks, plugs and similar downhole equipment. |
| Well Workover and Intervention | shear seal BOP | An item of pressure-control equipment often fitted to the wellhead during well-intervention operations on live wells. Most commonly associated with coiled tubing operations, the shear-seal BOP is a ram-type preventer that performs the dual functions of shearing or cutting the tubing string and then fully closing to provide isolation or sealing of the wellbore. Shear-seal BOPs are most commonly used in offshore or high-pressure applications where an additional contingency pressure barrier is required. |
| Well Workover and Intervention | shear stock | The bar or rod from which shear pins are cut for use in downhole slickline tools. Shear stock is prepared from carefully monitored materials to precise dimensions to ensure predictable and repeatable performance of shear pins. |
| Well Workover and Intervention | shear-seal BOP | An item of pressure-control equipment often fitted to the wellhead during well-intervention operations on live wells. Most commonly associated with coiled tubing operations, the shear-seal BOP is a ram-type preventer that performs the dual functions of shearing or cutting the tubing string and then fully closing to provide isolation or sealing of the wellbore. Shear-seal BOPs are most commonly used in offshore or high-pressure applications where an additional contingency pressure barrier is required. |
| Well Workover and Intervention | shifting tool | A downhole tool used to adjust the position of sliding sleeves or similar production and completion equipment. Shifting tools are typically run on slickline, although they may be used with coiled tubing in deviated or horizontal wellbores. Shifting tools are generally prepared or dressed for use with a specific model and size of sliding sleeve, requiring careful selection of the appropriate shifting tool. |
| Well Workover and Intervention | sinker bar | Also known as stem, the weight bar used in slickline operations to overcome the effects of wellhead pressure and friction at the surface seal where the wire enters the wellbore. In addition to a solid steel stem, a special high-density stem is available with internal cavities filled with lead, tungsten or mercury alloys. |
| Well Workover and Intervention | slickline | A single-strand wireline used to run and retrieve tools and flow-control equipment in oil and gas wells. The single round strand of wire passes through a stuffing box and pressure-control equipment mounted on the wellhead to enable slickline operations to be conducted safely on live wellbores. |
| Well Workover and Intervention | slug | A small volume of fluid, often of a higher density than the main body of fluid, within the circulating or production-fluid system that influences the flow or production characteristics of the well. A slug may be placed to ensure that fluids are naturally drained from a tubing string as it is pulled from the wellbore. The term may also be applied to a small volume of liquid produced from a gas well. Similarly, it is used to describe the flow characteristics that occur when a mixture of liquid and gas result in a sporadic production regime as the liquids are unloaded erratically. |
| Well Workover and Intervention | slugging pill | Another term for a slug, a small volume of fluid, often of a higher density than the main body of fluid, within the circulating or production-fluid system that influences the flow or production characteristics of the well. A slug may be placed to ensure that fluids are naturally drained from a tubing string as it is pulled from the wellbore. |
| Well Workover and Intervention | snub | To force a pipe or tubular into a well against wellbore pressure. Well-intervention techniques in live wells, such as coiled tubing and snubbing, use equipment designed to apply the necessary forces while supporting the tubing and safely containing wellbore pressure and fluids. |
| Well Workover and Intervention | snubbers | The slips used to grip the pipe during a snubbing operation. |
| Well Workover and Intervention | snubbing | The act of forcing a pipe or tubular into a well against wellbore pressure. Well-intervention techniques in live wells, such as coiled tubing and snubbing, use equipment designed to apply the necessary forces while supporting the tubing and safely containing wellbore pressure and fluids. |
| Well Workover and Intervention | snubbing basket | The work area at the top of a snubbing unit that houses the unit controls and a means of handling the tubulars and tool string to be run or retrieved. |
| Well Workover and Intervention | snubbing force | The force required to insert a tool or tubing string into a live wellbore. Two main components act to determine the snubbing force: the force resulting from the wellhead pressure acting on the cross-sectional area of the tubing, or the outside diameter of the tool and the force required to overcome the friction resulting from the stripper or similar sealing device containing the wellbore pressure and fluids. |
| Well Workover and Intervention | snubbing jack | The components of a snubbing unit that provide the vertical stroke or movement required to run or retrieve the work string. Snubbing jacks are hydraulically operated and can apply extremely high forces to the tubing string and the wellhead to which they are attached. |
| Well Workover and Intervention | spooler | A device used to handle and temporarily store a coiled tubing string. Spoolers generally are configured with a removable drum that allows transport spools to be inserted, allowing a new string to be spooled onto a reel. The term is also occasionally used to describe the levelwind assembly on a tubing reel. |
| Well Workover and Intervention | spot | To accurately place a fluid, or fluid interface, at a given position within the wellbore. Treatment fluids such as cement slurries and stimulation fluids for localized treatment often require accurate placement. Correctly calculating and pumping the appropriate volume of displacement fluid while taking account of well production, wellbore returns and fluid-density variations are key factors in achieving accurate placement of fluids. |
| Well Workover and Intervention | spotting | Accurately placing a fluid, or fluid interface, at a given position within the wellbore. Treatment fluids such as cement slurries and stimulation fluids for localized treatment often require accurate placement. Correctly calculating and pumping the appropriate volume of displacement fluid while taking account of well production, wellbore returns and fluid-density variations are key factors in achieving accurate placement of fluids. |
| Well Workover and Intervention | squeeze | The careful application of pump pressure to force a treatment fluid or slurry into a planned treatment zone. In most cases, a squeeze treatment will be performed at downhole injection pressure below that of the formation fracture pressure. In high-pressure squeeze operations, performed above the formation fracture pressure, the response of the formation and the injection of treatment fluid may be difficult to predict. |
| Well Workover and Intervention | squeeze cementing | The process of forcing a cement slurry through holes or splits in the casing or liner. Once the slurry encounters a permeable formation, the cement solids are filtered out of the slurry as the liquid phase is forced into the formation matrix in the form of cement filtrate. A properly designed squeeze-cement operation will fill the relevant holes and voids with cement filter cake that will cure to form an impenetrable barrier. |
| Well Workover and Intervention | squeeze job | An operation that requires the careful application of pump pressure to force a treatment fluid or slurry into a planned treatment zone. In most cases, a squeeze treatment will be performed at downhole injection pressure below that of the formation fracture pressure. In high-pressure squeeze operations, performed above the formation fracture pressure, the response of the formation and the injection of treatment fluid may be difficult to predict. |
| Well Workover and Intervention | squeeze manifold | A manifold connected within the surface treating lines that is configured to enable control and routing of fluids during a squeeze operation. Most squeeze manifolds have treating line connections with the tubing string, annulus, pit line and pump unit. Isolation valves enable the appropriate flowpath to be selected, and pressure sensors included in tubing and annulus lines monitor the key treatment pressures. In some squeeze treatments, such as squeeze cementing, it may be desirable to reverse-circulate excess cement from the tubing string. The squeeze manifold enables a change in fluid routing to be quickly and easily achieved from one station. |
| Well Workover and Intervention | squeeze packer | A type of retrievable packer used in squeeze-cementing operations. Key features include a bypass system and hold-down slips. The bypass system prevents surge and swab effects when running and retrieving the packer and enables circulation of the cement slurry to the proximity of the packer before closing the bypass for injection into the treatment zone. The hold-down slip assembly enables application of high squeeze pressure without the risk of the packer unsetting or moving up the wellbore |
| Well Workover and Intervention | squeeze pressure | The final or maximum pressure that can be applied during a squeeze operation. When conducting a squeeze-cement job, it is generally desirable to achieve a high final squeeze pressure that indicates the target holes and voids are filled with cement filter cake |
| Well Workover and Intervention | squeeze tool | A type of retrievable packer used in squeeze-cementing operations. Key features include a bypass system and hold-down slips. The bypass system prevents surge and swab effects when running and retrieving the packer and enables circulation of the cement slurry to the proximity of the packer before closing the bypass for injection into the treatment zone. The hold-down slip assembly enables application of high squeeze pressure without the risk of the packer unsetting or moving up the wellbore. |
| Well Workover and Intervention | stabbing valve | A valve that is connected to the work string in the event that the well starts to flow when running or retrieving the string. A stabbing valve generally is kept on the rig floor as a contingency against unexpected well flow. On snubbing operations, a stabbing valve, or safety valve, is kept in the workbasket to protect against tubing plug or backpressure valve failure. |
| Well Workover and Intervention | stationary slips | The slip set on a snubbing unit located at the base of the jack. Two sets of stationary slips are available, one set for pipe-heavy conditions and another for pipe-light conditions. |
| Well Workover and Intervention | stationary snubbers | The stationary slip set on a snubbing unit used when operating under light-pipe conditions. Under these conditions, the wellhead pressure is sufficient to eject the tubing string from the wellbore. Therefore, the slips are oriented in a hold-down position to grip with the force acting upward on the string. |
| Well Workover and Intervention | stem | The weight bar used in slickline operations to overcome the effects of wellhead pressure and friction at the surface seal where the wire enters the wellbore. In addition to a solid steel stem, a special high-density stem is available with internal cavities filled with lead, tungsten or mercury alloys. |
| Well Workover and Intervention | stimulation byproduct | A compound formed by a secondary reaction of stimulation fluids with fluids or solids present in the reservoir matrix. The most damaging stimulation byproducts are the insoluble precipitates that can form when the pH of the treatment fluid increases during the reaction process. Precipitates of iron compounds can be particularly problematic if conditions allow the formation of gelatinous, insoluble ferric compounds in the near-wellbore area. |
| Well Workover and Intervention | stimulation fluid | A treatment fluid prepared for stimulation purposes, although the term most commonly is applied to matrix stimulation fluids. Most matrix stimulation fluids are acid or solvent-based, with hydrochloric acid being the most common base due to its reaction characteristics and its relative ease of control. |
| Well Workover and Intervention | stripper rubber | The sealing element used in coiled tubing or snubbing stripper systems. The stripper element is a consumable product and generally should be replaced for each operation. Coiled tubing elements can be replaced with the tubing in place, enabling a worn or leaking element to be replaced during an operation. Snubbing stripper rubbers are of single-piece construction and cannot be changed with the work string in place. |
| Well Workover and Intervention | stripping ram | A ram-type blowout preventer used to provide primary pressure control in high-pressure snubbing operations. Stripping rams are used when the wellhead pressure is higher than the limitations of a stripper bowl. |
| Well Workover and Intervention | swaging tool | A downhole tool, generally run on slickline, that is used to open collapsed or damaged tubing. Configured with a tapered profile, the swaging tool acts as a circular wedge to force the tubing wall out as it is driven through a collapsed or restricted area. A jar is included in the tool string to provide the impact force necessary to push the swaging tool through the tubing restriction. |
| Well Workover and Intervention | taper tap | A fishing tool used to engage on the internal diameter of a hollow fish, such as drillpipe or drill collar. By rotating the taper tap when it is in contact with the fish, a threaded profile is cut, enabling the taper tap to securely engage the fish before retrieval. |
| Well Workover and Intervention | tapered string | A tubing string or work string that is made up from tubulars of different outside diameters (OD). In production applications, this may be used to improve the flow and production characteristics of a well. In drilling applications, a tapered string may be used to enable a small hole section to be drilled without changing the entire string. In coiled tubing operations, tapered strings are configured with a constant OD but with varying wall thickness. |
| Well Workover and Intervention | thickening time | The duration that a cement slurry remains in a fluid state and is capable of being pumped. Thickening time is assessed under downhole conditions using a pressurized consistometer that plots the viscosity of a slurry over time under the anticipated temperature and pressure conditions. |
| Well Workover and Intervention | through-tubing | Pertaining to a range of products, services and techniques designed to be run through, or conducted within, the production tubing of an oil or gas well. The term implies an ability to operate within restricted-diameter tubulars and is often associated with live-well intervention since the tubing is in place. |
| Well Workover and Intervention | travelling slips | The slip set on a snubbing unit that is located at the top of the jack assembly. Two sets of travelling slips are available, one set for heavy-pipe conditions and another for light-pipe conditions. |
| Well Workover and Intervention | travelling snubbers | The travelling slip set on a snubbing unit that is used when operating under light-pipe conditions. Under these conditions, the wellhead pressure is sufficient to eject the tubing string from the wellbore. Therefore, the slips are oriented in a hold-down position to act against the upward force applied to the tubing string. |
| Well Workover and Intervention | treating iron | The temporary surface piping, valves and manifolds necessary to deliver a fluid treatment to the wellbore from the mixing and pumping equipment. |
| Well Workover and Intervention | treatment fluid | A fluid designed and prepared to resolve a specific wellbore or reservoir condition. Treatment fluids are typically prepared at the wellsite for a wide range of purposes, such as stimulation, isolation or control of reservoir gas or water. Every treatment fluid is intended for specific conditions and should be prepared and used as directed to ensure reliable and predictable performance. |
| Well Workover and Intervention | triplex pump | A positive-displacement reciprocating pump that is configured with three plungers. Triplex pumps are the most common configuration of pump used in both drilling and well service operations. Pumps used in well service activities generally are capable of handling a wide range of fluid types, including corrosive fluids, abrasive fluids and slurries containing relatively large particulates. |
| Well Workover and Intervention | tubing job | The process of removing and replacing the production tubing in an oil or gas well. The term is commonly used when conducting a major workover of a well. |
| Well Workover and Intervention | tubing testing tool | A downhole tool used to plug the bottom of a production tubing string when pressure testing the assembled string. Slickline-deployed tools and plugs are most commonly used in vertical or slightly deviated wellbores. |
| Well Workover and Intervention | tubing-end locator | A downhole tool used on slickline or coiled tubing operations to identify the end of the production tubing, or similar well features. This information is used to correlate the position of the tool string for accurate placement of depth-critical treatments, plugs or downhole equipment. |
| Well Workover and Intervention | tubular jar | A downhole tool used on slickline operations. The tubular jar is a relatively simple mechanical jar that is extended or collapsed by manipulation of the slickline at surface. The impact force delivered by the jar depends on the weight of the tool string above the jar, the density of the wellbore fluid and the stroke length of the jar. |
| Well Workover and Intervention | unloading valve | Another term for circulation valve, a downhole device that enables circulation through the tubing string and associated annulus. As a completion accessory, a circulation valve is included to circulate fluid for well kill or kickoff. Circulation valves typically are operated by slickline tools and are generally capable of several opening and closing cycles before requiring service. |
| Well Workover and Intervention | wash over | A type of milling operation in which the outer surfaces of a plug or similar fish are milled with a circular hollow mill. By including wash pipe in the tool string, the mill face can reach over the body of the fish until it can be pushed to bottom, or until the slips or retaining device can be milled out and the fish retrieved. |
| Well Workover and Intervention | wash pipe | A tool-string component used with a burn shoe for washover operations. The wash pipe is a relatively large internal-diameter tubular that can be washed over a fish in preparation for engaging and retrieving the fish. |
| Well Workover and Intervention | washover | A type of milling operation in which the outer surfaces of a plug or similar fish are milled with a circular hollow mill. By including wash pipe in the tool string, the mill face can reach over the body of the fish until it can be pushed to bottom, or until the slips or retaining device can be milled out and the fish retrieved. |
| Well Workover and Intervention | water block | A production impairment that can occur when the formation matrix in the near-wellbore area becomes water-saturated, thereby decreasing the relative permeability to hydrocarbons. Water block may result from the invasion of water-base drilling or completion fluids or from fingering or coning of formation waters. The most extreme cases of water block occur in low-pressure, low-permeability gas formations, where alcoholic acid systems are recommended because they promote water vaporization in the produced gas. Alcoholic acid formulations are a mixture of acid and alcohol. The acids normally employed are usually either hydrochloric acid [HCl], mud acid [HF-HCl or HF-organic acid (formic or acetic)]. The alcohol is either methyl or isopropyl. Alcohol lowers the surface tension of acid and allows deeper penetration of the acid into the matrix of the rock. Alcohol is somewhat soluble in both acid and water, and penetration of low-surface-tension volatile alcohol into a water block will aid in its removal. |
| Well Workover and Intervention | water control | A treatment conducted within a reservoir or perforated interval to reduce water production. Water-control treatments may be necessary when the production efficiency of a well, or the process capability of surface facilities, is compromised by the volume of water produced with the oil or gas. Treatment options include selective isolation of the water-producing perforations or localized treatment of the formation matrix. |
| Well Workover and Intervention | weight indicator | A device or system used to measure, display and record the weight of a tubing string, slickline string or coiled tubing string in the wellbore. The weight indicator is the principal means by which the equipment operator monitors the function of downhole tools and equipment. Factors such as fluid density, which affects buoyancy and wellhead pressure, also impact the forces measured at surface. These factors can influence the apparent string weight significantly. |
| Well Workover and Intervention | well servicing | The maintenance procedures performed on an oil or gas well after the well has been completed and production from the reservoir has begun. Well service activities are generally conducted to maintain or enhance the well productivity, although some slickline and coiled tubing applications are performed to assess or monitor the performance of the well or reservoir. Slickline, coiled tubing, snubbing and workover rigs or rod units are routinely used in well service activities. |
| Well Workover and Intervention | wire clamp | A safety device attached to the slickline at surface between the hay pulley and stuffing-box pulley. The wire clamp generally is applied when the slickline is to be stationary for a period of time. This prevents the tool string from dropping down the wellbore if the winch unit fails or the slickline becomes damaged at surface. |
| Well Workover and Intervention | wireline | A general term used to describe well-intervention operations conducted using single-strand or multistrand wire or cable for intervention in oil or gas wells. Although applied inconsistently, the term commonly is used in association with electric logging and cables incorporating electrical conductors. Similarly, the term slickline is commonly used to differentiate operations performed with single-strand wire or braided lines. |
| Well Workover and Intervention | wireline cutter | A downhole tool used to cut slickline from a tool string that is stuck or jammed in a wellbore. The wireline cutter is attached to the slickline at surface and dropped down the wellbore. When the cutting tool impacts the tool string, a cutting mechanism cuts the slickline and enables recovery of the line in preparation for further fishing operations. |
| Well Workover and Intervention | wireline grab | A fishing tool used for the retrieval of broken or cut slickline from the wellbore. Wireline grabs are intended to catch and engage wireline that has been bunched or nested in the wellbore. For that reason, they are often run after a blind box or similar fullbore tool has been used to nest the wireline. |
| Well Workover and Intervention | work basket | The work area at the top of a snubbing unit that houses the unit controls and a means of handling the tubulars and tool string to be run or retrieved. |
| Well Workover and Intervention | work string | A generic term used to describe a tubing string used to convey a treatment or for well service activities. Both coiled and jointed tubing strings are referred to as work strings. |
| Well Workover and Intervention | workover | The process of performing major maintenance or remedial treatments on an oil or gas well. In many cases, workover implies the removal and replacement of the production tubing string after the well has been killed and a workover rig has been placed on location. Through-tubing workover operations, using coiled tubing, snubbing or slickline equipment, are routinely conducted to complete treatments or well service activities that avoid a full workover where the tubing is removed. This operation saves considerable time and expense. |
| Well Workover and Intervention | workover fluid | A well-control fluid, typically a brine, that is used during workover operations. Since the wellbore is in contact with the reservoir during most workover operations, workover fluids should be clean and chemically compatible with the reservoir fluids and formation matrix. |
| Well Workover and Intervention | workover string | Another term for work string, a generic term used to describe a tubing string used to convey a treatment or for well service activities. Both coiled and jointed tubing strings are referred to as work strings. |
| Well Workover and Intervention, Drilling | blind shear ram | A blowout preventer (BOP) closing element fitted with hardened tool steel blades designed to cut the drillpipe or tubing when the BOP is closed, and then fully close to provide isolation or sealing of the wellbore. A shear ram is normally used as a last resort to regain pressure control of a well that is flowing. Once the pipe is cut (or sheared) by the shear rams, it is usually left hanging in the BOP stack, and kill operations become more difficult. The joint of drillpipe or tubing is destroyed in the process, but the rest of the string is unharmed by the operation of shear rams. |
| Well Workover and Intervention, Drilling | mill | A tool that grinds metal downhole. A mill is usually used to remove junk in the hole or to grind away all or part of a casing string. In the case of junk, the metal must be broken into smaller pieces to facilitate removal from the wellbore so that drilling can continue. When milling casing, the intent is to cut a window through the side of the casing or to remove a continuous section of the casing so that the wellbore may be deviated from the original well through the window or section removed. Depending on the type of grinding or metal removal required, the shape of the cutting structures of mills varies. Virtually all mills, however, utilize tungsten carbine cutting surfaces. |
| Well Workover and Intervention, Drilling | mill shoe | A downhole tool routinely used in fishing operations to prepare the top and outside surface of a fish, generally to allow an overshot or similar fishing tool to engage cleanly on the fish. In some cases, the outer portion of a fish may be milled out to allow the body and remaining debris to be pushed to the bottom of the wellbore. |
| Well Workover and Intervention, Drilling | milling | The use of a mill or similar downhole tool to cut and remove material from equipment or tools located in the wellbore. Successful milling operations require appropriate selection of milling tools, fluids and techniques. The mills, or similar cutting tools, must be compatible with the fish materials and wellbore conditions. The circulated fluids should be capable of removing the milled material from the wellbore. Finally, the techniques employed should be appropriate to the anticipated conditions and the likely time required to reach the operation objectives. |
| Well Workover and Intervention, Drilling | shear ram | A blowout preventer (BOP) closing element fitted with hardened tool steel blades designed to cut the drillpipe or tubing when the BOP is closed, and then fully close to provide isolation or sealing of the wellbore. A shear ram is normally used as a last resort to regain pressure control of a well that is flowing. Once the pipe is cut (or sheared) by the shear rams, it is usually left hanging in the BOP stack, and kill operations become more difficult. The joint of drillpipe or tubing is destroyed in the process, but the rest of the string is unharmed by the operation of shear rams. |
| Well Workover and Intervention, Shale Gas | proppant | Sized particles mixed with fracturing fluid to hold fractures open after a hydraulic fracturing treatment. In addition to naturally occurring sand grains, man-made or specially engineered proppants, such as resin-coated sand or high-strength ceramic materials like sintered bauxite, may also be used. Proppant materials are carefully sorted for size and sphericity to provide an efficient conduit for production of fluid from the reservoir to the wellbore. |
| Well Workover and Intervention, Well Completions | swab valve | The topmost valve on a Christmas tree that provides vertical access to the wellbore. |
| Well Workover and Intervention, Well Completions | acid job | The treatment of a reservoir formation with a stimulation fluid containing a reactive acid. In sandstone formations, the acid reacts with the soluble substances in the formation matrix to enlarge the pore spaces. In carbonate formations, the acid dissolves the entire formation matrix. In each case, the matrix acidizing treatment improves the formation permeability to enable enhanced production of reservoir fluids. Matrix acidizing operations are ideally performed at high rate, but at treatment pressures below the fracture pressure of the formation. This enables the acid to penetrate the formation and extend the depth of treatment while avoiding damage to the reservoir formation. |
| Well Workover and Intervention, Well Completions | acidizing | The pumping of acid into the wellbore to remove near-well formation damage and other damaging substances. This procedure commonly enhances production by increasing the effective well radius. When performed at pressures above the pressure required to fracture the formation, the procedure is often referred to as acid fracturing. |
| Well Workover and Intervention, Well Completions | ball sealers | Small spheres designed to seal perforations that are accepting the most fluid, thereby diverting reservoir treatments to other portions of the target zone. Ball sealers are incorporated into the treatment fluid and pumped with it. The effectiveness of this type of mechanical diversion to keep the balls in place is strongly dependent on the differential pressure across the perforation and the geometry of the perforation itself. |
| Well Workover and Intervention, Well Completions | chelating agent | A chemical added to an acid to stabilize iron. The injected acid dissolves iron from rust, millscale, iron scales or iron-containing minerals in the formation. Iron can exist as ferric iron [Fe+3] or ferrous iron [Fe+2]. If the iron is not controlled, it will precipitate insoluble products such as ferric hydroxide and, in sour environments, ferrous sulfide [FeS], which will damage the formation. Chelating agents associate with iron [Fe+3 or Fe+2] to form soluble complexes. Citric acid, acetic acid and EDTA are effective chelating agents and can be used at temperatures up to 400oF [204oC]. |
| Well Workover and Intervention, Well Completions | diverter | A chemical agent or mechanical device used in injection treatments, such as matrix stimulation, to ensure a uniform distribution of treatment fluid across the treatment interval. Injected fluids tend to follow the path of least resistance, possibly resulting in the least permeable areas receiving inadequate treatment. By using some means of diversion, the treatment can be focused on the areas requiring the most treatment. To be effective, the diversion effect should be temporary to enable the full productivity of the well to be restored when the treatment is complete. There are two main categories of diversion: chemical diversion and mechanical diversion. Chemical diverters function by creating a temporary blocking effect that is safely cleaned up following the treatment, enabling enhanced productivity throughout the treated interval. Mechanical diverters act as physical barriers to ensure even treatment. |
| Well Workover and Intervention, Well Completions | diverting agent | A chemical agent used in stimulation treatments to ensure uniform injection over the area to be treated. Diverting agents, also known as chemical diverters, function by creating a temporary blocking effect that is safely cleaned up following the treatment, enabling enhanced productivity throughout the treated interval. In matrix acidizing of injection wells, benzoic acid is used as a chemical diverter, while oil-soluble resins are employed in production wells. Both compounds are slightly soluble or inert in the acidic medium [HCl], but after functioning as diverters, they dissolve with water injection or oil production, respectively. Stable, viscous foams generated in the rock matrix are also considered to be chemical diverters. |
| Well Workover and Intervention, Well Completions | HF | A poisonous liquid acid composed of hydrogen and fluorine. Hydrofluoric acid [HF] is used primarily because it is the only common, inexpensive mineral acid that can dissolve siliceous minerals. HF is typically mixed with hydrochloric acid [HCl] or organic acid to keep the pH low when it spends, thereby preventing detrimental precipitates. These mixtures, also called mud acids, are considered the main fluid in a sandstone acid treatment because they remove formation damage. Hydrofluoric acid should not be used in sandstone formations with high carbonate content because of the high risk of calcium fluoride precipitation [CaF2]. |
| Well Workover and Intervention, Well Completions | iron stabilizer | Also known as a chelating agent, a chemical added to an acid to stabilize iron. In the oil field, acid is used in stimulation treatments and to treat or remove scale or weighting material in reservoir drilling fluids.The injected acid dissolves iron from rust, millscale, iron scales or iron-containing minerals in the formation. Iron can exist as ferric iron [Fe+3] or ferrous iron [Fe+2]. If the iron is not controlled, it will precipitate insoluble products such as ferric hydroxide and, in sour environments, ferrous sulfide [FeS], which will damage the formation. Chelating agents associate with iron [Fe+3 or Fe+2] to form soluble complexes. Citric acid, acetic acid and EDTA are effective chelating agents and can be used at temperatures up to 400oF [204oC]. |
| Well Workover and Intervention, Well Completions | lubricator valve | The topmost valve on a Christmas tree that provides vertical access to the wellbore. |
| Well Workover and Intervention, Well Completions | matrix acidizing | The treatment of a reservoir formation with a stimulation fluid containing a reactive acid. In sandstone formations, the acid reacts with the soluble substances in the formation matrix to enlarge the pore spaces. In carbonate formations, the acid dissolves the entire formation matrix. In each case, the matrix acidizing treatment improves the formation permeability to enable enhanced production of reservoir fluids. Matrix acidizing operations are ideally performed at high rate, but at treatment pressures below the fracture pressure of the formation. This enables the acid to penetrate the formation and extend the depth of treatment while avoiding damage to the reservoir formation. |
| Well Workover and Intervention, Well Completions | matrix stimulation | A treatment designed to treat the near-wellbore reservoir formation rather than other areas of the production conduit, such as the casing across the production interval, production tubulars or the perforations. Matrix stimulation treatments include acid, solvent and chemical treatments to improve the permeability of the near-wellbore formation, enhancing the productivity of a well. Matrix stimulation is a process of injecting a fluid into the formation, either an acid or solvent at pressures below the fracturing pressure, to improve the production or injection flow capacity of a well. The goal of a matrix treatment is different in sandstones than in carbonates. In sandstones, matrix treatments restore or improve the natural formation permeability around the wellbore by removing formation damage, by dissolving material plugging the pores or by enlarging the pore spaces. In carbonates, matrix stimulation creates new, highly conductive channels (wormholes) that bypass damage. Because of these differences, the selection criteria for the treating fluid are also distinct. For sandstone treatments, knowledge of the extent, type of damage, location, origin, reservoir mineralogy (petrographic study) and compatibility of the treating fluid with the formation are especially important. In carbonate treatments, reservoir temperature, pumping rate and fluid type become more significant because these parameters directly affect the reactivity of the treating fluid with the reservoir rock. A sandstone matrix stimulation treatment is generally composed of a hydrochloric acid [HCl] preflush, a main treating fluid (HCl-HF mixtures) and an overflush (weak acid solution or brine). The treating fluid is maintained under pressure inside the reservoir for a period of time, after which the well is swabbed and returned to production. In carbonate reservoirs, HCl is the most common fluid used. Organic acids such as formic and acetic acid are used in either sandstone or carbonate acidizing, mainly in retarded-acid systems or in high-temperature applications. Matrix stimulation is also called matrix treatment or matrix acidizing. |
| Well Workover and Intervention, Well Completions | screen out | A condition encountered during some gravel-pack operations whereby the treatment area cannot accept further pack sand and a sudden increase in treatment pressure occurs. Under ideal conditions, this should signify that the entire void area has been successfully packed with sand. However, if screenout occurs early in the treatment, it may indicate an incomplete treatment and the presence of undesirable voids within the pack zone. |