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Oilfield Terminology






Acidize - treat oil-bearing limestone or other formations with acid to increase production.

Acoustic (sonic) well logging - recording the time required for sound to travel a specific distance through rock, using a wireline of LWD instrument. The rate of travel varies with rock composition, porosity, and fluid content.

Along hole depth (AHD) - the distance along the path of the wellbore from a measured depth datum (e.g., the drilling rig floor) to a point in the well.

American Petroleum Institute (API) - an oil industry organization that is the leading standard-setting body for oilfield equipment and products.

Anisotropy - variation of a physical property depending on the direction in which it is measured. In geophysics, a difference between velocity parallel to the bedding plane and velocity perpendicular to the bedding plane for a lithologic unit. In geology, anisotropy usually refers to the differency between vertical and horizontal permeability.

Annulus - in a borehole, the space between the drill pipe and the borehole, between tubing and casing, or between casing and formation.

AutoTrak RCLS - a rotary steerable drilling system that combines an automated downhole guidance system, formation evaluation sensors, advanced drill bits and two-way communication with the surface to deliver high performance drilling and precise geosteering.




Barefoot completion - (also open-hole completion) a method of preparing a well for production in which no casing or liner is set opposite the producing formation.

Bit whirl - the motion a bit makes when it does not rotate about its center, but in a spiral motion. Usually occurring in soft formations, it results in an over gauge (bigger than the bit) hole and excessive bit wear.

Barrel (bbl) - 42 U.S. gallons

Bird - a device with moveable vanes attached to a marine seismic streamer to maintain the streamer at a predetermined depth.

Bull plug - a threaded, cylindrical device with a rounded closed end, used to seal off the casing string or instrument barrel to ensure pressure integrity.




Cake - compacted solid or semisolid material remaining after liquid/solid separation processes.

Casing - a borehole lining (pipe) separating the formation from the borehole, with or without cement between pipe and formation.

Casing hanger - in a well, a circular device with a frictional gripping arrangement of slips and packing rings used to suspend casing from a casinghead.

Casinghead - a flanged steel fitting connected to the first string of casing. It allows suspension of intermediate and production strings of casing and is the means for sealing off the annulus.

Cement -a powder consisting of alumina, silica, lime and other substances. It hardens when mixed with water and is used to bond casing to the walls of the borehole and to prevent fluids from migrating between permeable zones.

Centrifuge - a machine that uses centrifugal force to separate substances of varying densities.

Chemical inhibitor - liquid chemical compounds injected into lines carrying fluids that contain H2S. Most inhibitors coat surfaces to isolate them from corrosive substances. Some react to produce less-destructive compounds.

Choke - a device to restrict the flow of fluids in a pipe.

Christmas tree - control valves, gauges, and chokes at the surface to control oil and gas flow in a completed well.

Closed Chamber Testing - a technique applied to drillstem testing.  Closed chamber testing provides data security and secrecy (minimal or no flare), and can measure minute quantities with exacting accuracy.

CMP - Common midpoint - the assumption that a recorded seismic trace was reflected by a point midway (horizontally) between the source and detector.

Coiled Tubing - long sections of small-diameter tubulars deployed in rolls and used to replace jointed pipe in stimulation, workover and drilling operations.

Collar - a device used to join two lengths of pipe.

Completion - technology used to bring a well to production. Matched to the reservoir and formation for optimum production, completion technology includes perforating, gravel packing, packers, safety valves, flow control, and completion fluids and services.

Conventional production logging - wireline well logs run in vertical production wells for the purpose of determining the wellbore oil, water, and gas inflow. Conventional production logging sensors include center-sampling (center-of-the-borehole) measurements of temperature, pressure, spinner velocity, fluid capacitance, differential pressure, and nuclear fluid density coupled to gamma ray, casing collar locator, and caliper measurements.

Coring Services - Coring is the removal of sample formation material from a wellbore. To the extent possible, core samples are taken in an undamaged, physically unaltered state. The formation material may be solid rock, friable rock, conglomerates, unconsolidated sands, coal shales, gumbos, or clays. Coring can be conducted by various methods with a variety of tools.




Darcy – a unit of permability. That value which will allow flow of 1 milliliter/sec of 1 centipoise viscosity through 1 cm2 under pressure of 1 atmosphere per cm.

Deconvolution – the process of undoing the effect of a filter.

Deepwater – Generally defined as operations in water depths of 1000 ft. or greater.

Deepwater Upper Completions - Offshore wells in water depths greater than 1000 feet are considered "deepwater". These wells are characterized by high drilling and intervention costs. Both "dry tree" and "wet tree" completion methods are common.

Depth migration – special seismic data processing used to focus subsurface events to their proper location in depth. Prestack depth migration, a computing-intensive process, has been highly successful for complex imaging, including subsalt formations.

Desander – a centrifugal device for removing sand from drilling fluid to prevent pump abrasion.

Development well – a well drilled in a proven field to complete a pattern of production.

Deviation – the angle between the wellbore axis (in the direction of the end, or bottom, of the well) and the downward vertical. Deviation values are always positive. Also called Inclination.

Dip – the angle that a refractor or reflector makes with the horizontal. Also, the angle of inclination of a geologic layer or sedimentary bed.

Directional drilling – the method of guiding a well along a predetermined path to a specific target. A directional drilling company provides technology and rig site supervision to efficiently meet directional drilling objectives.

DMO – Dip moveout offset – the difference in arrival times at different seismic detectors due to reflector dip.

Downhole motor - a tool directly above the drill bit in a drill string that converts the hydraulic energy of the circulating drilling fluid into mechanical energy to turn the bit independently of drill string rotation. May include a bent section to perform directional drilling.

Downhole oil/water separation – a system comprising a downhole hydrocyclone and electrical submersible pump that separates oil from water downhole, reinjects water, and produces oil to the surface.

Drill bit - the component at the end of the drill string that cuts the rock and makes hole.

Drill collar - heavy-walled sections of pipe included at the bottom of the drill string to apply weight to the drill bit during drilling.

Drill ship - A vessel designed for drilling in deep water without legs or anchors holding it to the sea floor and using dynamic positioning to hold it over the subsea wellhead.

Drill stem – all components in a rotary drilling assembly from the swivel to the bit.

Drill Stem Testing - A method of determining the potential productivity of a subsurface formation in either open or cased hole.

Drill string – the total string of drill pipe with attached tools and bit.

Drill-in fluid – a specialty drilling fluid designed to minimize formation damage in the reservoir.

Drilling fluid – fluid used in the wellbore to lubricate and cool the bit, control bottom-hole pressures, and remove cuttings.

Drilling mud – see drilling fluid

DZO – Demigration to zero offset – in seismic processing an improvement over DMO in situations where velocity varies significantly as a function of depth.




Elastomer – an elastic synthetic rubber or plastic material – often the main component of packing material in downhole packers.

Electrical submersible pump (ESP) – a system comprised of a downhole pump, a downhole electric motor, cabling, and surface controller to lift larger quantities of fluids from wells that do not flow under their own pressure.

Emulsifier – a material that causes water and oil to form an emulsion, i.e.; fine oil droplets suspended in the water.

Expandables - Expandables refers to any of a series of systems and products based on expandable solid tubular, which relies on forcing an expansion device (i.e., mandrel or pig) through standard oilfield tubulars to permanently expand the diameter of the pipe downhole.

Exploration well – a well drilled in search of an undiscovered reservoir or to greatly extend the limits of a known reservoir.




Fishing – the process of recovering equipment lost or stuck in the wellbore. Tools and services that perform specialty and repair work downhole. Fishing activities include retrieving lost tools and repairing wellbore damage.

Float equipment – Check valves that permit fluid to pass downward, but not upward through the casing. A float collar is a special coupling device inserted one or two joints above the bottom of the casing string. A float shoe is positioned at the end of the casing.

Flocculation – the coagulation of solids in a fluid, produced by special additives or contaminants.

Flow Control Equipment - are mechanical devices that use conventional wireline techniques for the express purpose of directing, managing and controlling the flow of produced or injected fluids in the production tubing.

Fluid – a substance which deforms continuously under the action of a shear force, however small. Wellbore fluids include oil and water (with or without gas in solution) and free gas.

Foaming agent – A chemical used in gas wells to lighten the water column to promote gas production. Also, a chemical used while drilling wells with air or gas as the drilling fluid, to force water with the air and cuttings.

Fold – in seismic processing, the number of traces with different source-to-receiver separations summed into a single trace.

Fracturing (frac) – a method of stimulation production by opening new flow channels in the rock surrounding a production well by pumping proppant and fluid into the well at high pressure and volume.

Full Bore Isolation Valve (FBIV) - is a normally closed disc valve, designed to provide isolation within the tubing string, holding pressure from both above and below.




Gamma-ray log - well log that records natural radioactivity of formations around the wellbore.

Gauge - the diameter of a bit or the hole drilled by the bit.

Gather - a display of the input data to a stacking process arranged to show all the seismic traces corresponding to some criterion are displayed side by side.

Geophone - a device to transform seismic energy (movement) to an electrical voltage - voltage that is proportional to the velocity of the seismic wave motion.

Geosteering - a subset of horizontal drilling in which measurements of formation properties are used to place the wellbore in specific geologic targets.

GeoThermal - High temperature energy from the Earth located where the earth's major Tectonic Plates are joined.

Gravel Pack - a completion technique used to control production of sand from loosely consolidated formations.

Gyroscopic Survey - survey conducted inside a well's casing to measure is position and trajectory. The steel's casing prevents use of magnetic readings to compute the directional and coordinate information.




Highly Deviated Wells - a class of nonvertical wells where the deviation angle is approximately 30-80 degrees.

Holdup - the fraction of the total volume of the borehole sampled by the measurement device that contains a particular phase; more specifically, oil holdup, water holdup, and gas holdup.

Horizontal Direction Drilling - a trenchless construction technique, which uses guided drilling for creating an arc profile.

Horizontal Drilling - a subset of directional drilling in which the angle of deviation of the wellbore reaches at least 80 degrees from the vertical, maximizing the length of wellbore exposed to the formation.

Horizontal Well - a class of nonvertical wells where the wellbore axis is near horizontal (within approximately ten degrees of the horizontal), or undulating (fluctuating above and below 90 degrees deviation).

Horner Plot - Formation parameters are calculated by analyzing the shut-in build-up using the techniques developed by Horner in his classic pressure build-up paper. The Horner Plot is used to extrapolate a build-up that has not yet stabilized to enable us to predict the virgin reservoir pressure. The slope of the extrapolated line (designated ‘m’) is also used in the calculations because it reflects the nature of the reservoir in terms of the rate of the steady state increase of pressure. That is to say, the rate of pressure response is due to the nature of the reservoir rock and the fluids flowing in the system.

Hydrates - Natural gas hydrates, also referred to as clathrates, are crystalline structures of water that surround low molecular weight gases, such as methane, ethane, propane or butane.

Hydrocyclone - a cone-shaped device for separating mixed liquids (e.g., oil and water).

Hydrophone - a pressure-sensitive sensor to transform changes in (water) pressure to an electrical voltage - voltage which is proportional to the velocity of the seismic wave.




Inclination - the measurement of a well's deviation from vertical, in degrees. Also the angle between the direction of the produced fluid flow and the horizontal. Inclination values are positive for fluid flowing upward and negative for fluid flowing downward.

Infill Drilling / Well - wells placed between known producing wells to further exploit a reservoir.

Inflatable Systems - Inflatable tools are designed with a nitrile based bladder that is expanded by means of hydraulic pressure much in the same manner as blowing up a ‘balloon’. 

Intelligent Completions - a completion technology in which formation property measurements are made and the completion dynamically adjusts itself to maximize production from the well.

Intelligent Well Technology - technology that combines sensors and remote control to detect production problems and solve them without costly well intervention.

Isotropic - of equal physical properties in all directions.




Jackup drill rig - a mobile, bottom-supported offshore drilling structure. Legs or columns rest on the seafloor and the platform is raised or adjusted by moving up or down on the legs.

Jet cutter - a fishing tool that uses shaped charges to sever casing, tubing, or drill pipe stuck in a hole.




Kickoff point - the depth in a vertical wellbore at which some type of directional drilling operations (e.g., deviated or slant hole) begin.

Kriging - the geostatistical method of applying known values in one sample to produce an unbiased estimate of values in another.




Lateral bore – normally referred to as the deviated or horizontal extension in the drilling of a horizontal well or multilateral well.

Liner – a string of pipe used to case open hole below existing casing, overlapping inside the upper string and held in place by a liner hanger packer.

Lithology - 1. the study of rocks, usually macroscopic. 2. the individual character of a rock in terms of mineral composition, structure, and so forth.

Logging-While-Drilling (LWD) – a variation of measurement-while-drilling in which the LWD tool gathers information (i.e.; resistivity, density, porosity, gamma ray) about the formation while the well is being drilled.




Main bore – the main casing string from which subsequent directional drilling operations or openhole operations are initiated. Lateral bores extend from the main bore to the desired target depth.

Measurement-While-Drilling (MWD) – measuring directional information (azimuth, inclination, and tool orientation) downhole to adjust the drilling process and guide the wellbore to a specific target.

Migration – a computational process applied to seismic data that returns reflection events to their origin in the subsurface.

Minerals Management Service (MMS) – Branch of the US Department of Interior that governs oil operations in US waters on the Outer Continental Shelf.

Moveout – the difference in arrival times of reflected seismic data at different detectors.

Mud – see drilling fluid

Multicomponent Seismic – a survey conducted using 3-component (3-C) geophones for sensing seismic reflections in the vertical, horizontal, and crossline directions. In the marine environment, a hydrophone is included to acquire 4-component (4-C) data.

Multi-Cycle Tool (MCT) - is primarily a circulating device that is operated hydraulically from the surface using internal tubing pressure.

Multilateral – the construction of two or more wellbores into one or more reservoirs for the purpose of managing and optimizing fluid movement within the reservoir(s). These lateral wellbores are connected back to a common main bore that extends to surface. Various levels of completion systems can be installed in a multilateral well to enable it to produce from several zones simultaneously.




NMO – Normal moveout offset – the difference in arrival times of reflected seismic data at different detectors due to different source and detector separations.

North Sea Brent – crude oil produce from the Brent field in the British sector of the North Sea. Often quoted as a benchmark for world oil prices.




Oilfield chemicals – chemicals used to treat produced fluids and control corrosion and deposition in producing wells.

Open hole – any wellbore in which casing has not been set.

Outer Continental Shelf (OCS) block – a unit of defined area for purposes of management of offshore petroleum exploration and production by the Minerals Management Service (MMS) of the US Department of Interior.




Packer – open and cased-hole devices used to create seals to control fluid flow.

PDC drill bits – use fixed position polycrystalline diamond compact cutters that shear the formation instead of grinding it. In many applications, PDC bits offer higher penetration rates and longer life than Tricone bits.

Perforate – to open holes through casing walls and cement into a formation so that fluids can flow into the borehole, or vice-versa.

Perforating gun – a device fitted with shaped charges or bullets that is lowered to a desired depth in a well and fired to create penetrating holes in casing, cement, and formation.

Permeability – a measure of the ease with which a fluid can pass through the pore spaces of a formation.

Phase – in reference to multiphase flows, the phase refers to oil, water, or gas (for three-phase flow), or liquid and gas, or oil and water (for two-phase flow).

Pick – select an event on a seismic record.

Pipeline Management - Verify the integrity and improve the deliverability of your pipeline network with Direct Assessment Pipeline Inspection and Pipeline Cleaning Services from Baker Hughes PMG.

Polymer – a substance containing large molecules formed from smaller molecules in repeating structural units. In the oilfield, various polymers are used to thicken drilling fluids, fracturing fluids, acid and other liquids. In petrochemical production, polymers are used as a feedstock for plastics.

Production Integrity Management - The systematic management of production pipeline integrity through the prediction, identification and prevention of pipeline failures occuring from corrosion and erosion.

Proppant – a granular substance that is carried into the formation by the fracturing fluid and helps keep the cracks open after a fracture treatment.

PS-wave (or converted wave) – seismic energy which has traveled partly as a P-wave and then as an S-wave; having been converted upon reflection at an interface.

P-wave – a wave in which particle motion is in the direction of source propagation. Also called compressional wave, primary wave, pressure wave, and longitudinal wave.




Rate of penetration (ROP) – the speed (rate) with which the bit drills the formation, measured in feet or meters per hour.

Raypath – a line everywhere perpendicular to wavefronts (in isotropic media).

Resistivity – a measurement of a formation’s resistance to electrical current used to determine whether the formation holds hydrocarbons or water.

Reynolds number – the dimensionless ratio of inertial forces to viscous forces in flowing fluids. It may be viewed as a ratio of the shear stress due to turbulence to the shear stress due to viscosity. Flow with a Reynolds number less than 2000-4000 is laminar flow; that with a Reynolds number greater than 2000-4000 is turbulent flow.

Rotary drilling — the method of drilling oil and gas wells in which the entire drill string is rotated from the surface to turn the drill bit, and cuttings are removed from the hole by a circulating fluid.

Rotary steerable system — a drilling system that can perform directional drilling operations without interrupting drill string rotation. Rotary steerable systems include downhole guidance systems that continually adjust the drill bit’s course. They offer significant advantages over steerable motor systems.

Run in – to go into the hole with tubing, drill pipe, tools, or other devices.




Saddle - for pipe, a fitting made in parts to clamp on to a pipe to stop a leak or provide an outlet.

Safety Valves - are fail-safe shut-off devices located at the surface and/or subsurface which shut-off the produced flow in the event of a catastrophic event, such as fire, collision, or sabotage.

Saturation - the fraction of the effective porosity of the formation that contains a particular phase; more specifically, oil saturation, water saturation, or gas saturation.

Seismic acquisition (2-D, 3-D, 4-D) - seismic data are used to map subsurface formations. A 2-D survey reveals a cross section of the subsurface. In a 3-D survey, seismic data are collected in the inline and crossline directions to create a three-dimensional image of the subsurface. In a 4-D or time-lapse 3-D survey, 3-D surveys are repeated over time to track fluid movement in the reservoir.

Semi-submersible rig - a mobile offshore drilling unit that floats on the water's surface above the subsea wellhead and is anchored in place. The semi-submersible rig gets its name from pontoons at its base which are empty while being towed to the drilling location and are partially filled with water to steady the rig over the well.

Shaped charge - a small container of high explosive that is loaded into a perforating gun. The charge releases a small, high-velocity stream (jet) of particles that penetrate the casing, cement, and formation.

Shot hole - a relatively small hole drilled into the earth to hold a high-explosive charge that creates vibrations in the earth's crust for seismic recording.

Skin factor - The degree of reduction in permeability immediately proximal to the wellbore.

Sliding sleeve - a flow-control device that can be opened or closed to allow or prevent production to flow into the well.

Snell's law - an expression to determine the change of direction of a wave crossing the boundary between two isotropic media.

Sonde - a well logging tool.

Sonic log - a well log of the travel time for acoustic waves per unit of distance.

Sonobuoy - a free-floating sea buoy containing radio equipment to relay data received by one or more detectors to recording equipment typically aboard the source vessel.

Stack - a composite record made by combining traces from different seismic records. Commonly, to combine data from several detectors in a limited area.

Statics - corrections applied to seismic data to determine the reflection arrival times which would have been observed, were no weathering or low-velocity material present.

Steerable motor - a downhole motor used for directional drilling which can turn the drill bit independently of drill string rotation. Placed just above the bit, a steerable motor has a bend in its housing that can be oriented to steer the well's course. During "rotary" mode the entire drill string is rotated from the surface, negating the effect of this bend and causing the bit to drill a straight course. During "sliding" mode, drill string rotation is stopped and the bit drills in the direction that it is oriented, gradually turning the well.

Stoneley wave - a type of seismic wave propagated along an interface. A surface wave in a borehole.

Straddle - the placement of completion equipment across a multilateral junction to achieve hydraulic integrity (level 5), or means to isolate the internals of the junction(s) from fluid flow and/or pressure from the reservoir.

Streamer - a marine cable containing regularly spaced hydrophones. During a marine seismic survey, several of these streamers are towed behind the survey vessel.

Surfacant - a soluble compound that concentrates on the surface boundary between two substances, such as oil and water, and reduces the surface tension between the substances. Surfacants permit mixing of substances that ordinarily remain separate.

Swage tool - a device that passes through a FORMation Junction's preformed leg, reforming the leg into a full, round shape.

S-wave - shear wave or transverse wave. Sometimes called converted waves because the particle motion is perpendicular to the direction of propagation. Shear waves are generated by the incidences of P-waves on surfaces at other than normal incidence.




Tension-leg platform - an offshore drilling platform attached to the seafloor with tensioned steel tubes. The buoyancy of the platform applies tension to the tubes.

Thru tubing - operations performed from inside the production tubing of an existing well.

Trace - a record of one seismic channel.

Transition zone - the region between land and deep water consisting typically of surf, swamps, and bayous.

Transmissibility - The ability of the formation rock to transmit the fluid contained within it.

Tricone Drill Bit - a rotary drill bit employing three cones and either hardened steel teeth or tungsten carbide inserts (TCI). This bit works by grinding away at formation rock as it is turned.

Trip - Hoist (remove) the drill stem from the wellbore to perform one or more operations, such as changing bits, running a logging tool, or taking a core sample, and then return the drill stem to the wellbore.

True vertical depth (TVD) - the vertical distance from a measured depth datum (usually the drilling rig floor) to a point in the well.




Ultra Deepwater - Generally defined as operations in water depths of 5000 ft. or greater.




Water cut - the percent of the total liquid production of the well represented by the water phase.

Water Management - Baker Hughes provides a broad range of solutions to help upstream and downstream customers manage water associated with hydrocarbon production, processing and transportation. We offer drilling, completion, monitoring and workover technologies to manage water production from the reservoir, identify problems as they occur, and take action to limit water production. We also offer chemical solutions to address water-related problems in flow lines, pipelines and during refinery processing.

Weight on bit (WOB) - the amount of downward force placed on a bit by the weight of the drill stem.

Wellbore damage - The ratio of theoretical production rate versus the actual production rate obtained during the drillstem test.

Well log - a record of one or more subsurface formation measurements as a function of depth in a borehole.

Whipstock - a long steel tool that uses an inclined plane to cause the bit to deflect from the original borehole at a slight angle.

Wireline - a slender rod-like or threadlike small-diameter piece of metal used to lower tools, such as logging tools, perforating guns, valves, and fishing tools into a well. May include electrical conductors to power and control instruments and to convey data to the surface.

Workover - maintenance procedures performed on a previously completed well to stimulate or restore production or increase the life of the well.