Oil and Gas Terms Beginning with “R”

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 formationconductivity 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.

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.

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-emulsionmud, the basis for the Chenevert Method for testing oil muds.

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.

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

Abbreviation for remotely operated vehicle.

Abbreviation for Recommended Practice.

The closing and sealing component on a blowout preventer. Types include blind, pipe, or shear rams.

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.

A tool that enlarges holes to specified size, stabilizes the bit, straightens wellbores, and maintains even diameter in boreholes.

Abbeviation for horizontal resistivity.

A marineseismic 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.

A zero-phasewavelet commonly convolved with a reflectivitytrace to generate a synthetic seismogram.

Abbreviation for vertical resistivity.

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.

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.

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.

An array of sources or receivers radiating outward from a central point, usually a borehole.

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.

The inversion of resistivity logs with differing depths of investigation into a model of the formationinvasion 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.

A boreholeseismic 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.

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.

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 signalprocessing. 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.

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.

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.

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.

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 productionlogging 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.

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.

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.

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.

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.

A nonreproducible error that is generally imputable to the physics of the measurement. For example, the statistical errors in nuclear measurements are random errors.

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.

A method of performing stochastic analysis on dynamic data. This method can be used in stochastic simulation of fluid behavior in reservoirs and fields.

The difference between the maximum load reached in the upstroke and the minimum load registered in the downstroke.

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.

(noun) A component of the total skin factor in a well that increases with flow rate, caused by non-Darcy (turbulent or inertial) flow in the near-wellbore region, particularly in high-rate gas wells. The rate-dependent skin is typically expressed as Dq, where D is the non-Darcy flow coefficient and q is the flow rate.

Extra hole drilled at the bottom of the hole to leave expendable completion equipment, such as the carriers for perforating gun charges.

Crude oil direct from the wellbore, before it is treated in a gas separation plant. It usually contains nonhydrocarbon contaminants.

Gas coming directly from the wellbore containing nonhydrocarbon contaminants and hydrocarbons that can be liquefied.

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.

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.

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.

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.

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.

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.

A type of fluid pump in which reciprocating pistons or plungers displace the fluid.

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.

(verb) To detect, measure, and store geophysical or well data using electronic instruments for subsequent processing and interpretation. In seismic acquisition, recording involves capturing the electrical signals generated by geophones or hydrophones in response to reflected seismic energy.

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.

(noun) A downhole or surface instrument that records physical measurements such as pressure, temperature, flow rate, or depth over time during well testing, production logging, or reservoir monitoring operations. Modern recorders use electronic memory gauges with high-resolution sensors.

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.

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.

The fraction of hydrocarbons that can or has been produced from a well, reservoir or field; also, the fluid that has been produced.

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.

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

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.

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.

A cementslurry made with less mix water than is customarily used without modifying additives.

A cementslurry made with less mix water than is customarily used without modifying additives.

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.

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.

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.

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.

The position on a loggingtool 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.

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.

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:Typical values of R are approximately -1 from water to air, meaning that nearly 100% of the energy is reflected and none is transmitted; ~ 0.5 from water to rock; and ~ 0.2 for shale to sand. At non-normal incidence, the reflection coefficient defined as a ratio of amplitudes depends on other parameters, such as the shear velocities, and is described as a function of incident angle by the Zoeppritz equations.

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.

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.

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 a salt dome within surrounding rocks of lower velocities.

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.

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.

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 porepressure. 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.

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.

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.

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.

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.

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.

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.

A measurement of the ability of two or more fluid phases to pass through a formationmatrix. 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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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)

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.

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.

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.

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.

The act of building a reservoirmodel based on its characteristics with respect to fluid flow.

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.

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.

(noun) A comprehensive characterisation of a petroleum reservoir that integrates geological, geophysical, petrophysical, and engineering data to define the spatial distribution of rock properties, fluid properties, and flow behaviour. Reservoir description forms the foundation for building simulation models and optimising field development plans.

A measurement of the vertical thickness of reservoirformation that is open to flow. The reservoir height is used in calculations and mathematical models to assess reservoir performance or potential productivity.

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.

The act of producing a model of a reservoir. The model could include any of the geological, fluid or other characteristics of the reservoir.

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. 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.

A computer run of a reservoirmodel 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.

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.

(noun) The average duration that a fluid element or particle remains within a defined volume, such as a separator vessel, reactor, or section of a wellbore. In surface facilities, adequate residence time is essential for effective phase separation, and in chemical treatments, it determines the extent of reaction.

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.

Oil that does not move when fluids are flowed through the rock in normal conditions, for example primary and secondary recovery, and invasion.

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.

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.

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.

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.

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.

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.

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.

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.

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.

A chemical additive used to increase the thickening time of cementslurries 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.

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 separatorpressure. 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

To circulate fluid down the wellbore annulus, with returns being made up the tubing string. Reverse circulation often is 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.

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.

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.

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.

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 fluidmodel 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.

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.

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.

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.

Natural gas containing heavier hydrocarbons than a lean gas. Its liquid content adds important economic value to developments containing this type of fluid.

Gas condensate comprising significant amounts of heavy hydrocarbon products, which can produce relatively large volumes of condensate.

In a glycol dehydrator, glycol that contains water released by wet gas while percolating upward in the absorber.

Lean oil that has absorbed heavier hydrocarbon components from a gas stream

To pull apart the Earth's crust.

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.

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.

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.

To make ready for use. Equipment must typically be moved onto the rig floor, assembled and connected to power sources or pressurized piping systems.

The characteristic of a cementslurry 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.

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.

The characteristic of a cementslurry 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.

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.

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.

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.

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.

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.

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.

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.

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.

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 workoverrig or a rod unit specifically designed for running and retrieving rod strings.

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.

An assembled length of sucker rods used to connect and power a rod pump with the reciprocating power source at surface.

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.

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.

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.

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.

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.

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.

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.

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.

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-receiveroffset approaches zero and layers are horizontal and isotropic.

A method of making hole that relies on continuous circular motion of the bit to break rock at the bottom of the hole. This 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.

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.

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.

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.

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.

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.

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 collarconnection. 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.

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.

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.

A percentage share of production, or the value derived from production, paid from a producing well.

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.

Pertaining to a borehole wall that is rough.

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.

With reference to logging, an operation in which a logging tool is lowered into a borehole and then retrieved from it while recording measurements.

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.

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.

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.

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.