Absorber Unit

Process unit that uses the property of some liquids or solids to soak up (absorb) water or other fluids. The unit brings gas and liquid phases, or in some cases solids in contact so that contaminants can be removed because of this interaction. Commonly used in amine treating processes where sour gases such as Carbon Dioxide (CO2),carbonyl sulfide, and Hydrogen Sulfide (H2S) are removed from production gas. The amine solution absorbs H2S and CO2 . This rich amine solution is then routed to a regenerator. For glycol dehydration absorber units use liquid glycol to absorb water vapor resulting in a dehydrated gas product. Light oil, (absorption oil) is also used certain absorber units to remove heavier liquid hydrocarbons from a wet gas stream resulting in a dry gas product to obtain dry gas.

Acid Job

A well stimulation process where the wellbore is treated with acid to improve productivity or injectivity of the formation. An acid job may include acid washing, matrix acidization or fracture acidization.

Active Fault

Fault along which there is current movement or where there has been periodic displacement, slip or seismic activity. 


Organic compounds that consist of single-bonded carbon and hydrogen atoms. Alkanes are divided into three groups: cycloalkane, linear straight chain alkanes, and branched alkanes with the general formula CnH2n+2. 

Amine Gas Treatment

Processes including amine scrubbing, gas sweetening and acid gas removal that use aqueous solutions of amines to remove contaminants such as H2S, and CO2. 


Organic chemicals commonly used in gas treatment and processing. Amines most used in oil and gas treatment and processing are Diethanolamine (DEA), Monoethanolamine (MEA), and Methyl diethanolamine (MDEA). 

Amine Process

Processes that utilize an amine solution for the removal of hydrogen sulfide (H2S) and carbon dioxide (CO2) from produced gases. 

Anisotropic Stress

Forces that have directionally dependent properties. In an anisotropic formation properties such as permeability and stress are directionally dependent.


The space between any layer of piping, drill pipe, drill collars, tubing, or casing and the next layer of piping, drill pipe, drill collars, tubing, casing or the wellbore where fluid can flow. 


An environment depleted of or lacking free oxygen. In an aquatic environment it would be demonstrated by a dissolved oxygen (DO) concentration less than 0.5 mg/l. 


Structural feature of folded rocks formed because of compressional tectonic forces. Strata dip away from the center (axis) of the fold with the oldest rock beds (strata) in the center of the structure.


A geologic unit that contains water in pores spaces that can yield usable and economic amounts of water to a well or spring. 

Area of Review

Extent of the area to which the CO2 plume will migrate in a carbon sequestration project.

Artesian Spring

A source of water formed where groundwater flows freely to the surface because the aquifer is under pressure greater than at the land surface.

Artesian Well

A hole or boring where groundwater is under sufficient hydrostatic head to rise above the top of the aquifer to the ground surface through a well casing to equalize the pressure.

Balanced Drilling

Procedure used to drill a wellbore where the pressure in the wellbore is equal to the reservoir's formation pressure.

Band Brake

A flexible steel band that grips a drum when tightened which is part of the draw works braking system. When applied it controls the motion the of draw works.


The mineral barium sulfate mineral (BaSO4). It is commonly used as a weighting agent for drilling fluids. When used as a drilling additive barite is required to have a minimum specific gravity of 4.2 g/cm3.

Bell Nipple

Large section of pipe found at the top of the casing string and just above the blowout preventer stack. It acts as a guide to move the drill string and tools into the top of a well. 


A rock material composed largely of the clay mineral montmorillonite and other minor amounts of clays in the smectite group. Bentonite is derived from the alteration in place of volcanic materials and is widely used in drilling mud. Bentonite absorbs water and swells when exposed to water making it a good material to protect formations from the invasion of drilling mud. 


A level space, shelf or raised barrier to separate two items and often provide containment. Oil and gas containment can be provided by permanent or temporary berms (a.k.a. dikes), often made of earth or concrete.

Biogenic Production

One of the main byproducts of the bacterial degradation of organic matter in sediments formed by microbial fermentation and carbon dioxide reduction.


An extremely viscous liquid or semi-solid form of petroleum. Bitumen occurs naturally but can also be distilled from crude oil. 


A piece of equipment with a lower inlet pressure than the outlet pressure designed to move air through a wide variety of industrial applications such as exhausting air and other gases, cooling, and ventilating. 


Uncontrolled flow of formation fluids including water, oil and gas from a wellbore during drilling, workover or other well operations. 

Blowout Preventer (BOP)

One or more specialized valves or devices used to seal, control and monitor oil and gas wells to prevent blowouts.

Blowout Preventer (BOP) - Annular Type

Valve designed to form a seal in the annular space between the drill pipe and the wellbore. Annular type BOPs are mounted at the top of the BOP stack above any ram-type preventers. See BOP Stack and BOP Ram-Type preventer. 

Blowout Preventer (BOP) - Ram Type

Type of BOP that uses rubber faced rams (opposing steel plungers) that come together to seal off pressure. The four major types of rams are blind, pipe, shear and blind shear (sealing shear).

Blowout Preventer (BOP) - Stack

BOP installation consisting of, at a minimum, both annular and ram-type BOPs assembled into a stack. 

Bottomhole Assembly

The bottom portion of the drill string. The bottomhole assembly includes the drill bit, drill collars, jarring devices and may also include mud motors, logging while drilling (LWD) or measurement while drilling (MWD) tools.

Bulk Porosity

The ratio of void space, commonly called pore volume, to bulk volume.


Fitting or adaptor with an external thread larger than the internal thread that allows for the connection of pipes of different sizes and thread types. 


A layer of rock that caps or seals fluids, including hydrocarbons, in deeper rock. Commonly a rock with low permeability such as shale. 


Large diameter, steel pipe, placed in a wellbore to prevent, the wall of the borehole from caving, the movement of fluids from one formation to another and, to aid in well control. Casing should be designed to handle several different forces include burst, collapse, tensile failure and exposure to chemically active brines and treatment fluids. 

Casing Hanger

A device that is part of the wellhead assembly consisting of slips and packing rings used to support the casing string in the wellbore. 

Casing Head

Flanged, steel, fitting connected to the first string of casing. It allows for the suspension of production and intermediate casing strings that provides a housing for slips and packing assemblies, and a means to seal off the annulus. 


A positively charged ion with fewer electrons than protons. 

Chemical Sedimentary Rock

An inorganic sedimentary rock that forms when minerals precipitate out of solution such as oolitic limestone and rocks composed of evaporite minerals like halite (rock salt), and gypsum.

Christmas Tree

Pressure gauges, chokes, fittings and control valves assembled at the top of a well after the well has been completed that controls the flow of produced or injected fluids. 

Christmas Tree Choke

Choke assembled with Christmas Tree that controls the flow rate of injected or produced fluids. 


Fragments or grains that make up clastic sediment. 


Sedimentary rock composed of clasts which are fragments or grains of weathered rocks.

Clastic Sediment

Sediments consisting of rock fragments derived from pre-existing rocks transported from place of origin and redeposited to form another rock. 


Sedimentary rock having a composition like shale but lacking the fine thin layering (fissile) of a shale. Claystone normally contain at least 50% clay minerals. 

CO2 Plume

Area of a formation occupied by flowing supercritical CO2 during and after injection.

Compton Scattering

A gamma ray interaction in which a gamma ray collides with an electron, transferring part of its energy to the electron, while itself being scattered at a reduced energy. It is a key process in the measurement of formation bulk density. During the logging operation gamma rays collide with electrons in the formation, giving off energy and scattering.

Conductor Casing

A casing string, also referred to as conductor pipe or drive pipe, that, if used is the first casing string installed in a wellbore. Conductor pipe controls caving and washing out of unconsolidated surface formations and can be installed by pile driving or with a separate smaller auger rig. It is the largest diameter casing and generally short in length. 

Cone Of Depression

A depression, roughly conical in shape, that occurs in an aquifer when water is pumped from a well. As water flows from the higher to lower pressure the gradient produces flow from surrounding aquifer into the well. 

Confining Zone

A geological formation, group of formations, or part of a formation stratigraphically overlying the injection zone(s) that acts as barrier to fluid movement.


Coarse, clastic sedimentary rock composed of gravel sized grains (>2mm) within a matrix of finer grains. The larger clasts of a conglomerate are rounded.

Core Barrel

A tube or piece of pipe, inserted inside the drill pipe, that holds rock samples being extracted from the drill hole.

Core Plug

A small sample, or plug, cut from a conventional core. A typical core plug is cut perpendicular to the axis and is 1.0 to1.5 inches in diameter and 1.0 to 2.0 inches long. 

Core Sample

Cylindrical rock or sediment sample collected from a borehole. 

Coring While Drilling

Collection of a core sample while drilling the wellbore using specialized tools including a coring bit, core barrel and core catcher. 

Correlative Rights

Legal principle that states each mineral owner and producer in a common pool or source of supply of oil and gas shall have an equal opportunity to obtain and produce their just and equitable share of the oil and gas underlying such pool or source of supply. 

Crown Block

Assembly of pulleys (sheaves) mounted on the top of the derrick. The drill line is run over the pulleys down to the hoisting drum. 

Darcy’s Law

A formula that describes the flow of fluids through a porous media used to calculate permeability. This formula states that the flow rate or velocity of oil, gas or water movement between two points in a reservoir is proportional to the pressure gradient in the direction of flow.


A centrifugal or hydrocyclone device used to remove sand from drilling mud to prevent abrasion of the pumps and other surface equipment. 


A centrifugal device that removes fine grained, silt sized solids from drilling fluid. 


Drilling line from the crown block sheave to the anchor; named deadline because it does not move. 

Delay Rental

Fee or other consideration paid at the end of each lease year during the primary term if no production has been established on the lease to keep the lease in effect during the primary term.

Depositional Environment

An area such as river channel, tidal inlet, or reef where a limited range of physical, chemical, and biological depositional processes take place, resulting in the accumulation of sedimentary deposits that have distinctive characteristics. Geologists derive information regarding geologic history of the area through analysis of these characteristics.

Depth Of Investigation

The distance from the face of the formation or wellbore into a rock formation that a logging tool can make measurements of fluid and rock properties. 


Tall structure that is part of a drilling rig that supports the weight of the drill string and associated equipment and allows pipes to be hoisted completely out of the well in sections of up to around 100 feet. It generally has a pyramidal shape as opposed to a mast which serves the same purpose on different types of rigs but has a rectangular shape and generally handles less weight (see mast). 

Diamond Coring

A process that uses specialized tools such as a core bit, core cutter, cylindrical core barrel and catcher, to collect and retrieve samples of the formation. The cutting tools are fixed with poly crystalline diamond compact (PDC) or natural occurring diamonds. 

Diesel-Electric Drilling Rig

Power supplied to a drilling rig by internal combustion engines driving d-c generators, that convert mechanical energy into electricity for individual electric motors. 

Draw works

Hoisting mechanism on the drilling rig consisting of a large winch, spool, brakes, and a power source. The draw works spools off or takes in the drilling line and lowers or raises the drill string and bit. 

Drill Collar

Part of the drill string that is a thick-walled piece of pipe usually made of steel. It is placed between the drill pipe and the drill bit to add weight on bit for drilling. 

Drill String

The column of drill pipe, including drill collars, tools and the bottomhole assembly, that transmits drilling fluid and torque to the drill bit. 

Drilling Line

A twisted wire rope that works with the traveling block and crown block to lift or lower drill string out of a wellbore. 

Drilling Mud

Material used while drilling a wellbore also referred to as drilling fluids. Drilling mud may include liquids, gaseous fluids, emulsions, and suspended solid mixtures. Drilling fluid is generally classified by the primary component and function such as: water based, oil based, and gaseous (such as air). 

Effective Porosity

The pore volume or void spaces of a reservoir that are interconnected and contribute to fluid flow or permeability. Effective porosity equals total porosity minus isolated porosity and the fraction of the pore space occupied by water absorbed on shale or clay particles. 

Electrical Resistivity

Property (ρ) of a material to resist the flow of an electrical current that is measured in ohmmeters. Electrical resistivity is the inverse of conductivity.


A substance (natural or chemical) used as an additive to combine one immiscible liquid such as water into another immiscible liquid like oil. Stability of the two liquids is reached by reducing the interfacial tension between the two liquids.


A mixture of two or more liquids that are generally unmixable or cannot be blended (immiscible). 

Enhanced Oil Recovery (EOR)

Processes that operators employ to increase recovery of oil-in-place. Also called tertiary recovery EOR processes can be employed at any stage of reservoir development. EOR includes three broad categories including gas, chemical and thermal injection, that alter the properties of oil in the reservoir to increase production. 

Enhanced Recovery Technique

Oil recovery method that uses methods to alter the chemical properties of the oil to make it easier to extract.


The point on Earth’s surface directly above the focus of an earthquake. 


A colorless volatile flammable liquid C2H5OH which is produced by the natural fermentation of sugars.


Demonstration of moral competence, expertise and knowledge. 


In the petroleum industry it is the initial phase of finding oil and gas deposits below the Earth’s surface. Exploration may include several processes including geologic mapping, geophysical analysis, and drilling stratigraphic tests and exploration wells.


Rock characteristics and attributes (composition, color, fossil content, sedimentary structures and texture) that that imply information related to the rock’s depositional environment. Fluid behavior within facies can be evaluated based on a facies petrophysical characteristics.


Reasoning that fails to meet the requirements of a logical proof or to provide a logical argument.


A fracture or zone of fractures between two blocks of rock where there has been movement or displacement. 

Fire Flooding

An EOR process that uses heat to move viscous oil underground. In fire flooding combustion is used directly in the producing formation to reduce oil viscosity and turn reservoir water into steam. 

Fish or Junk

Tools, including hand tools, drill string or junk that is unintentionally left in a wellbore. 


Operation to remove fish (tools, drill string, or unwanted material) from the wellbore.

Flowback (Fluids)

Fluids from hydraulic fracturing or other well stimulation activities that return to the surface that include water and chemical additives. 

Flowback (Process)

Process that follows hydraulic fracturing or other well stimulation activities where stimulation fluids return to the surface. 

Fluid Transport Behavior Model

Computer models that integrate geology and chemical reactions to estimate the transport of fluids through the Earth’s crust.


Bend or curve in sedimentary strata resulting from geologic forces.

Formation Damage

A condition most often caused by exposure of the producing formation to drill fluids during drilling operations. The fluids may decrease pore volume and permeability thereby reducing flow of oil and gas from the reservoir rock into the wellbore. 

Formation Evaluation

The analysis and determination of fluid and formation characteristics through review of drill cuttings and well log data.

Formation Invasion

The movement of drilling fluids into permeable formations during the drilling process when wellbore pressures are higher than formation pressure due to excessive mud weights. 


A colloquial term for hydraulic fracturing. See hydraulic fracturing. 

Fracture Acidizing

Well stimulation process where highly pressurized acid is pumped into a wellbore above the formation-fracture pressure to physically fracture the producing formation. This process creates pathways and channels in rock through which hydrocarbons can flow.

Fracture (frac) Gradient

Pressure gradient at which a formation breaks. Also referred to as frac gradient. The frac gradient should be calculated to identify bottom-hole treating pressure before starting a hydraulic fracturing job. It is calculated by dividing the true vertical depth by formation fracture pressure.

Friction Reducer

Slurry or liquid additive used to reduce friction in tubulars and tools; most often applied during hydraulic fracturing of horizontal or directional wells.

Gas Hydrate

Solid, ice-like crystalline minerals formed under low temperature and moderate pressure when low molecular weight gases like methane, carbon dioxide or ethane combine with water and freeze. Gas hydrates are typically formed from methane (CH4). 

Gas Window

The temperature range (150–200 °C) within the earth's crust at which kerogen releases natural gas. 

Geologic Structure

A geological feature or spatial arrangement of rocks resulting from tectonic forces that cause deformation such as bending, folding and faulting.


Heat within the earth or the utilization the earth's interior heat. 

Geothermal Gradient

Temperature increase or rate of increase per unit depth within the earth. The geothermal gradient is variable from place to place but averages 25 °C per km of depth or 1 °F per 70 feet of depth. 


A curved, inverted U-shaped pipe that connects the standpipe to a Kelly hose that provides a conduit for high pressure drilling fluid. 

Gross Rock Volume (GRV)

Gross Rock Volume (GRV) in an oil field is calculated by multiplying the thickness of a rock unit times its area of distribution. The area is limited by the oil/water contact. The calculation is an integral part of estimating hydrocarbons in place. 

Horst and Graben

Structural feature, formed by extension in the earth’s crust, that results in the formation of a distinctive type of topography, A horst is a block of earth bounded by normal faulting on both sides that has moved up relative the blocks on either side and a graben is a block of earth, also bound on both sides by normal faults, that have moved down relative to other blocks. The Basin and Range physiographic province in the western United States is an example of horst and graben topography. 

Hydraulic Fracturing

A well stimulation process where liquids and proppants are injected into an unconventional reservoir to create fractures for increased conductivity and communication within the producing formation. Colloquially known as fracking. 

Hydraulic Head

A pressure measurement describing the force exerted by the height, or distance of a column of liquid, above the point at which the pressure is measured, where the force of the liquid column is directly proportional to its height.

Hydrogen Concentration Index (HI)

The fluid filled porosity of a formation can be measured with neutron logging tools that detect hydrogen atoms. The Hydrogen Index (HI) is the ratio of the concentration of hydrogen atoms per cm3 in the material, to that of pure water at 75 °F. As hydrogen atoms are present in both water and oil-filled reservoirs, measurement of the HI allows estimation of the amount of liquid-filled porosity.

Hydrogen Sulfide (H2S)

Colorless, flammable, extremely hazardous gas with a “rotten egg” smell. It occurs naturally in crude petroleum, natural gas. It is also produced by bacterial breakdown of organic materials and human and animal wastes (e.g., sewage). Industrial activities that can produce the gas include petroleum/natural gas drilling and refining, and wastewater treatment. Hydrogen sulfide is both an irritant and a chemical asphyxiant with effects on both oxygen utilization and the central nervous system.

Hydrogen Sulfide (H2S) Sensor

Instrument or device that detects hydrogen sulfide (H2S) gas.

Hydrostatic Gradient

The rate of change (pressure gradient) of fluid formation pressure per unit of depth; the hydrostatic pressure gradient can vary based on region and relative to formation fluids. The hydrostatic pressure gradient of freshwater is 0.433 psi/ft and 0.465 psi/ft for fluids with a total dissolved solids content of 100,000 parts per million.

Hypocenter (focus)

Point within Earth’s crust where an earthquake originates.

Igneous Rock

Igneous rocks are one of the three main classes of rock including sedimentary and metamorphic. Crystalline rock formed by the cooling and solidification of lava or magma. Igneous rocks include extrusive or volcanic rocks formed at the Earth’s surface and intrusive or plutonic rocks formed at depth.

Inactive Fault

Fault which has not moved or created seismic activity with-in the last 10,000 years. 

Induced Fracture

Fracture caused by excessive mud weight.

Induced Seismicity

Earthquakes that are typically minor and related to human activity such as wastewater injection, reservoir impoundment, or geothermal energy development. 

Intermediate Casing

Casing string set after surface casing but before production casing. It is used to isolate zones and provide protection against sluffing, caving or abnormal formation pressures. 

Isolated Porosity

The volume or percentage of total porosity not connected to the permeable pore system. 


Compounds with the same molecular formula but different molecular structure and different properties. 


The square or hexagonal steel bar suspended from the swivel through the rotary table and connected to the topmost joint of drill pipe. The Kelly transmits motion (rotary) from the rotary table or Kelly bushing to the drill string, while allowing the drill string to be lowered or raised during rotation.

Kelly Bushing

A device fitted to the rotary table by four large steel pins through which the Kelly passes. The Kelly bushing transmits the motion of the rotary table to the Kelly which turns the drill string. 


Naturally occurring, solid, insoluble organic fraction of certain sedimentary rocks that can yield oil and gas upon heating. Algae and woody plant material are typical organic constituents of kerogen. 


The entry of water, gas, oil, or other formation fluids into the wellbore during drilling. Kicks may be caused by an in-balance in the weight of the column of drilling fluids to overcome the pressure exerted by formation fluids, or by the dynamic and transient fluid pressure effects created from the motion of drilling or casing. If a kick is not controlled a blowout may occur. 


A person that negotiates mineral and surface leases, and business deals with the persons holding mineral or other property rights, on behalf of oil and gas companies. 

Leakoff Rate

The pressure exerted on a formation that would cause fluids to flow into pore spaces or cracks within the formation. 

Leakoff Test

Test that determines the fracture pressure of the open formation. The results are used to identify the maximum mud weight that can be applied during drilling operations. 

Lifecycle Analysis (LCA)

A process that assesses the environmental impact that occurs throughout a product’s lifecycle, from raw materials acquisition through production, use, final treatment, recycling, and disposal.


A sedimentary rock composed primarily of calcium carbonate such as calcite and aragonite that precipitates from water or a biological sedimentary rock composed of fragments of marine organisms. Limestones are usually deposited in warm shallow marine environments.

Line Heaters

A piece of equipment used to heat liquids or gas in a pipeline or a well-stream. Typically used in gas flowlines found near a wellhead or gas transmission lines to reduce the formation of ice and gas hydrates. 


Complex process in which newly deposited sediments are compacted under pressure and converted into solid rock. 

Logging While Drilling (LWD)

A technique used to evaluate formation properties. LWD tools are integrated with bottomhole assembly and transmit formation data to the surface in real time. 


Logic behind an argument.

Lost Return

The loss or reduction of fluids flowing up the annulus when fluid is pumped through the drill string. The loss or reduction may be total lost returns, partial lost return or seepage. 


Re-enforced steel, tower used to support the crown block on a drilling rig. It also supports drilling loads and allows hoisting of drill string through a system of pulleys. Masts are generally rectangular or trapezoidal in shape and provides a similar function as a derrick. 

Master Bushing

A piece of equipment that fits inside the rotary table that holds slips or the Kelly Bushing in its center and transmits rotational power to the Kelly.

Matrix Acidizing

Process where acid is injected into the formation at pressures below fracture gradient used to improve flow or to remove damage.

Maximum Sand Free Rate

The maximum hydrocarbon flow rate, during production operations that a weakly consolidated reservoir will produce without producing sand.

Metamorphic Rock

One of the three major classes of rocks. Metamorphic rocks are formed by the transformation or alteration of existing rock by heat and pressure.


Hydrocarbon consisting of 1 carbon and 4 hydrogen atoms CH4. It is a colorless, odorless and flammable gas that is the major component of natural gas. 

Mineral Rights

The ownership rights to underground resources such as water, oil and gas, or coal. The ownership rights include the right to produce, or mine the subsurface minerals and to use the surface as reasonably necessary to exploit the minerals. 


Two liquids are miscible if they combine in any ratio to form a homogeneous solution.

Miscible Gas Injection

A common EOR technique used to maintain reservoir pressure and to improve oil displacement through the injection of a gas at a temperatures and pressures where a single phase of oil and the injected gas is formed. The most common gas used in this process is CO2 but nitrogen and hydrocarbon gases such as field separator gas, propane and butanes may also be employed. 

Monkey Board

Platform about halfway up the derrick or mast. It is essentially a rack that is used to secure the upper end of a stand of pipe when it is removed from the well. The worker who guides the pipes into the rack is called the derrickman.


Cavity or void space created through the dissolution of shells, rock fragment or other pre-existing constituents of the rock where pore space preserves the shape of the dissolved fragment. 


A monocline is a step-like fold in rock strata consisting of a zone of steeper dip within an otherwise horizontal or gently-dipping sequence.


An opening in the rig floor near the rotary table that enables rapid connections while drilling.

Mud Log

A detailed record of the wellbore created through the analysis and examination drill cuttings brought to the surface with returned drilling fluids. 

Mud Logger

A person that uses a variety of techniques to examine drill cuttings, and other data such as records maintained by the driller of drilling operations and the status of the well.

Mud Logging

The creation of a detailed record (mud log) of a borehole by examining the cuttings of rock brought to the surface by the circulating drilling medium. 

Mud Mixer

Device on a drilling rig that mixes, blends, and agitates solids and liquids to form drilling fluids. The mud mixer consists of paddles, stirrers, jet hoppers, mud guns, and chemical barrels.

Mud Motor

A motor that uses the hydraulic horsepower of the drilling mud at the bottom of the hole to drive the drill bit. 

Mud Pit

An open pit dug in the ground to hold drilling fluid. The mud pit may be used for settling of mud sediments, storage of reserve mud and for suction to the mud pumps (see also mud tank).

Mud Pumps/Suction Line

Pump used to circulate drilling fluids downhole. Drilling fluids are carried from the mud pit/tank to the mud pump through the suction line.

Mud Return Line

A large diameter pipe that acts as a conduit for the gravity flow of mud, that surfaces under the rotary table, to the mud tanks or mud gas separators.

Mud Tank

A tank used during drilling operations to hold drilling fluid. The mud tank may be used for settling of mud sediments, storage or reserve mud and for suction to the mud pumps. In some cases, depending on site set up an earthen pit may be used in place of the mud tank (see mud pit). 

Mud Weight Window

The range of mud weight from the upper limit where tensile failure would occur and the lower limit where compressive failure occurs. Drilling with mud within this window increases wellbore stability. 


Very fine-grained (grain size < 0.06 mm), clastic, sedimentary rock composed of a mixture of clay and muds. Mudstones differ from shales due to lack of the parallel layering (fissile). 

Normal Fault

Rupture in the Earth's crust where one side of the fault moves downward (hanging wall) with respect to the other side (footwall).

Normal Faulting Stress Regime

A tectonic area or regime that causes normal faults to form. In this regime the primary stresses are S1=Sv (vertical stress at depth), S2 =Shmax (maximum horizontal stress), and S3 = Shmin (minimum horizontal stress), where S1>S2>S3.

Normal Pore Pressure Gradient

Pore pressure gradient is considered normal when the change in pressure per unit of depth is equivalent to hydrostatic pressure. The normal hydrostatic pressure gradient for freshwater is 0.433 pounds per square inch per foot (psi/ft), or 9.792 kilopascals per meter (kPa/m), and 0.465 psi/ft for water with 100,000 ppm total dissolved solids.


Hydrocarbon compound (CH3(CH2)6CH3) that is part of the alkane series consisting of eight carbon atoms. Octane is a colorless and flammable hydrocarbon derived through petroleum refining. 

Oil and Gas Window

A range of both depth (pressure) and temperature where both oil and gas may be generated through the alteration of type I and II kerogens. 

Oil Window

Temperature range (50-150 °C) in the earth's crust at which kerogen releases crude oil.


Thickness or weight of rock, soil and other materials of an overlying a rock layer of interest, such as a reservoir or source rock.


Tool used during fishing operation that consists of a friction device such as a grapple or slip mechanism attached to tubing or drill pipe. It is lowered over the outside wall of lost or stuck equipment and the gripping device attaches to the fish allowing it to be pulled from the wellbore. 

Pad Stage

The stage in hydraulic fracturing that generally follows the initial acid stage. During the pad stage carrier fluids, without proppants, are injected to open the formation to initiate the hydraulic fracturing which facilitates flow and placement of the proppant during the prop stages. Sometimes referred to as The Pad as in pumping The Pad. 

Plug and Abandoned (P&A)

The process of permanently closing a formerly producing oil or gas well. P&A operations include removing and potentially salvaging casing and tubulars, setting and testing cement plugs to prevent migration of formation fluids between different formations, and cutting and capping the borehole.  


First of two elastic body waves to arrive at a seismic station after an earthquake; P-waves are compressional waves; thus, their oscillation moves in the direction the wave travels.


A device used downhole that expands externally to seal a wellbore including test packers and inflatable packers.


Appeal to the emotions of the audience.

Percussive Sidewall Coring

A core taken from the side wall of the wellbore; this coring process includes firing hollow, retrievable bullets into the formation and collecting the samples. 


To create holes in the liner or casing to allow for communication between the wellbore and the producing formation.

Perforating Gun

A device used to perforate (create holes) in the liner or casing of oil and gas wells.

Petroleum Exploration Geologist

A scientist (geologist) that uses geological, geochemical and geophysical techniques to determine where extractable oil and gas deposits may be located.

Petroleum System

A system that includes essential geologic elements and processes for the accumulation of oil and gas. For conventional resources, the elements include source rock, migration pathway, reservoir, and a trap or seal. 


A professional that studies the physical, chemical and geological aspects of rock formations to identify reservoir rocks and calculates the potential for hydrocarbons to flow through the reservoir. 

Plate Tectonics

Geologic theory that the Earth’s lithosphere is divided into several hard and rigid plates that move over the upper layer of the mantle. These interactions result in observed large-scale changes in the Earth.


A plan, map or chart, drawn to scale showing divisions and actual or proposed features of tracts of land.


A chemical substance that has a molecular structure consisting of several similar units bonded together.

Plug and Perf

Plug and perf is one of the most common well completion practices for horizontal wells. The process includes a five basic steps; 1) a plug and perforating gun are pumped to the appropriate depth on a wireline tool near the toe of the well, 2) a plug is set, the zone perforated and the tools removed from well, 3) zones below the plug are isolated by pumping a ball down the wellbore, 4) fracture stimulation treatment is pumped and a ball-activated plug diverts fracture fluids through the perforations and into the formation, and 5); the next plug is set, perforations delivered and the process is repeated moving up the well from the toe to the heel.

Pore Pressure

The pressure of fluids within pores of a reservoir, usually hydrostatic pressure, or the pressure exerted by a column of water from the formation's depth to sea level.

Pore Throat

Small pore space where two grains meet and connect two larger pores within a rock matrix.


Total void space in the rock whether or not it contributes to fluid flow.

Pressure Gradient

Change in pressure divided by the distance (depth) over which that change occurs. The pressure gradient is generally stated in psi/ft, or kPA/m; deviations from normal pressure are described as high or low pressure. 

Pressure Gradient Limit

Also referred to as overburden gradient or overburden stress it is the pressure on a subsurface point resulting from the total weight of soils, materials and rock overlying it. Often specified by a pressure gradient dependent on rock density. The pressure gradient limit tends to increase with depth. 


The first ranking regulatory agency with authority to implement rules, policies and administrative penalties. Primacy can be granted to a different agency through formal agreement with the original first ranking agency. 

Primary Recovery

The first stage of hydrocarbon production where natural reservoir pressures cause hydrocarbons to move into the well and up to the surface. Pumps and other forms of equipment may also be used to create artificial lift during primary recovery. 

Primary Term

The length of time that a lease remains effective after which the lease expires. Lease expiration may be affected by absence of production, drilling or failing to complete other terms of the lease. 

Produced Water

Water that is produced along with hydrocarbons during production operations. Often referred to as brine it may contain heavy metals, varying amounts of naturally occurring radioactive materials (NORM) and dissolved hydrocarbons. Characteristics of produced water may vary significantly by reservoir or basin.

Production Casing

Casing string set across the intended producing formation that allows for primary completion components to be installed. 

Production Tubing

Tubing used to produce wellbore fluids. Production tubing along with other components complete the production string.


A colorless, flammable hydrocarbon gas within the alkane series with a chemical formula of CH3CH2CH3.


A solid material that is mixed with hydraulic fracturing fluids. Proppant is typically sand, ceramics or treated sand that holds fractures open after hydraulic fracturing and flowback.

Rayleigh Wave

Surface waves that “rolls” the Earth’s surface through a combination of compressional and transverse motion.


A process vessel used to provide heat to the bottom of industrial distillation column. Liquid is boiled from the bottom of a distillation column and the vapors generated are returned to the column to drive the separation. Frequently used in oil and gas production operations to remove H2S, CO2 or water from the gas production stream.


Process vessel where rich amine solution is transferred to be heated in order to break the bond between amine and contaminants such as H2S and CO2 allowing the amine to be regenerated for re-use. 

Reservoir Drive

Natural forces (drive mechanisms) that supply energy in a reservoir to displace hydrocarbons toward the wellbore as fluids are produced to the surface. Some of the major drive mechanism include water drive, gravity drainage, gas drive, and combination drive.

Reservoir Rock

Rock with a subsurface volume that has sufficient porosity and permeability to allow hydrocarbons to migrate, accumulate and drain in economic quantities. 

Reverse Fault

A type of fault where the hanging wall moved up relative to the footwall. Reverse faults occur where the Earth's crust is compressed. 

Rift Valley

Linear depression or lowland that forms from the downward displacement of blocks of the Earth’s crust at a divergent plate boundary which is subsequently further deepened by the forces of erosion.

Rigging Down

Process of dismantling a drilling rig, along with the supporting equipment for power, pressure and storage of drilling fluids, and fuel, for transport to another well site or a laydown yard.

Rigging Up

Process where the drilling rig is moved on to location along with the supporting equipment for power, pressure and storage or drilling fluids, and fuel, is set up to drill one or more boreholes. 

Rock Volume

Can be short for gross rock volume. Gross Rock Volume (GRV) in an oil field is calculated by multiplying the thickness of a rock unit times its area of distribution. The area is limited by the oil/water contact. The calculation is an integral part of estimating hydrocarbons in place. 

Rotary Sidewall Coring

Process of obtaining core samples from the side wall of a borehole using a rotary drilling tool. Sidewall cores provide enough sample material for geological and engineering studies. 

Rotary Table

Also called master bushing it is part of the rotary machine that is used to turn the drill stem and support the drilling assembly. 

Royalty Payment

The amount paid from an oil and gas lease to a mineral owner derived from the value of the oil and gas from produced from a well. 

Rule of Capture

A general rule related to the development of natural resources where others may capture or lawfully take resources such as oil or natural gas by drilling a well on their own land and the well draws the oil and gas away. 


The second of two body waves to arrive at a seismic station after an earthquake. Because S-waves are shear waves; their oscillation moves in a direction perpendicular to the direction the wave travels.

Salt Bed

A type of sedimentary rock formed in arid environments also referred to as an evaporite.

Sand Out

A condition that may occur during hydraulic fracturing operations where fractures will only accept carrier fluids and no sand or other proppants. It occurs when proppant concentration in the tubing string increases creating an increase in pump pressure. 

Sanding Off

A situation that can happen during hydraulic fracturing in which proppant completely plugs the fractures being opened, preventing fluid from entering the formation.


Sedimentary rock composed primarily of sand size particles ranging from 0.0625 to 2 mm that consist mostly of silicate minerals such as quartz and feldspar. 

Screen Out

A condition that may occur during well completion operation when solids create a bridge across a perforated zone and restricts fluid flow. It can also occur during well treatment or well intervention operations when a treatment zone can no longer accept further pack sand. This condition would be identified by a sudden increase in treatment pressure.


The process of removing water, liquid hydrocarbons, and other impurities by directing the production gas stream through a process unit (scrubber) in which gas is mixed with a suitable liquid to absorb or wash out the constituents to be removed.

Seal Rock

A rock layer with very-low permeability that may trap gas or oil. Also referred to as cap rock. The seal is an essential element of a conventional petroleum system.

Secondary Recovery Stage

The second of the three major stages of oil and gas production where processes such water and gas injection are implemented to displace and drive hydrocarbons towards nearby production wells. 

Sedimentary Basin

Geologically depressed area with thick sediments in the interior and thinner sediments at the edge. Formed through long term subsidence that allowed for the infill of sediments. 

Sedimentary Rock

Rocks formed by the compaction and lithification of sediments and other materials. The sedimentary rock classification includes clastic rocks and rocks formed by biogenic activity and through chemical processes and chemical precipitation. Sedimentary rocks are one of the three main classes of rocks.


A device used to detect and record seismic energy related to earthquakes, volcanic eruptions and explosions. The output of the device, either printed or in digital form, is called a seismogram.


An instrument that responds to ground noises and shaking using digital instruments or weights on a frame with a stylus to record the land movement. A seismometer is combined with a timing device and a recording device to form a seismograph.

Self Contained Breathing Apparatus

An atmosphere-supplying respirator for which the breathing air source is designed to be carried by the user.


A pressure vessel used in oil and gas production to separate fluid components into gas and liquid phases. Separators can be classified in several ways based on degree of separation (2 phase, 3 phase) and the design (vertical, horizontal). 

Sequence Stratigraphy

Field of study based on one of the major unifying theories of the geosciences that includes facies mapping, applied geology, and geophysics to understand the chronostratigraphic framework where the relationship of rocks and the repetitive related strata bounded by uniformities. 


[Geologic] sequestration is the injection of carbon dioxide into deep geologic formations that are suitable for long-term or permanent subsurface storage of CO2.

Shale Gas

Natural gas found in shale reservoirs. The primary constituent is methane but may also contain heavier gases such as propane, ethane, butane.

Shale Shaker

The shale shaker, or shaker, is a vibrating screen or sieve that removes cuttings and drilling solids from the circulating fluid.


A very-fine grained, fissile, clastic sedimentary rock formed through compaction of clay minerals and silt-sized particles of other minerals, especially quartz and calcite. 


Also referred to as tangential stress, it is where two adjacent solids slip past each other along a plane parallel to the imposed stress. 


The change or difference in the volume of oil produced to the surface relative to the volume of oil produced in the reservoir oil. Shrinkage occurs when solution gas comes out of the oil due to the drop in pressure near surface. 


A type of sedimentary rock, also referred to as mudstone, that contains at least 50% silt (grain size <0.06 mm) by composition.


In well completion skin is the production efficiency of a well where positive skin may indicate damage and decreased well productivity and negative skin may indicate improved productivity, generally related to well stimulation. For well testing skin refers to the zone where there has been either there is enhanced permeability around a wellbore or a reduction and may be related to formation damage by mud-filtrate invasion during drilling or perforating, or by well stimulation operations.

Skin Effect

A phenomenon in which permeability is enhanced or reduced in the vicinity of a drill hole by factors that occur during drilling or the completion process. 

Skin Factor

Pressure drop due to damage or alteration of the oil and gas formation around the wellbore.

Skin Region

Pressure drop due to damage or alteration of the oil and gas formation around the wellbore.


Nonelectric, single-strand cable used to deliver tools, and to place hardware, plugs, gauges downhole. Also used to adjust valves and sleeves and to retrieve tools and equipment. 

Slickwater Solution

A low-viscosity fluid pumped at high rates to generate narrow fractures with a low concentration of proppant.

Sliding Sleeve

Device that allows for communication between the tubular and annulus for circulation or selective-zone production in a completed oil or gas well. A sliding sleeve can be open or closed using standard wireline tools. 

Smallest Stress

The least stress acting on a surface.

Social Network

The networking of personal relationships and social interactions. 

Source Rock

A rock containing approximately 1% organic matter and 0.5% total organic carbon (TOC) from which hydrocarbons have been generated or are capable of being generated; source rocks are typically fine-grained, relatively impermeable materials such as shale.


To begin drilling a well, such as, spud in or to force a wireline tool or tubing down the hole by using a reciprocating motion. 


An additive to solutions that inhibits chemical degradation. Stabilizer may also refer to equipment and processes used in to reduce the vapor pressure of liquid hydrocarbons to stabilize and fractionate natural gas condensate to remove C1-C4 hydrocarbons in a single column, or in stages sequentially removing one or more of ethane (C2), propane (C3) and butane (C4) in multiple columns (fractionation). 


A vertical pipe rising along the side of the derrick or mast, which joins the discharge line leading from the mud pump to the rotary hose through which mud is pumped going into the hole.


The connected lengths of pipes (joints) racked in the derrick or mast when making a trip; the usual stand is two to three lengths of pipe. 

Steam Flooding

An EOR process in which steam is injected into a reservoir to drive oil towards producing wells. The steam reduces the viscosity of the crude oil causing it to flow more freely and the heat vaporizes lighter hydrocarbons. As the lighter hydrocarbons move ahead of the steam, they cool and condense into liquids that dissolve and displace crude oil. The steam also provides additional gas drive. 


A scientist that studies sedimentology and stratigraphy.

Stratigraphic Sequence

A chronological succession of sedimentary beds occurring between, or existing between two or more regions arranged with older rocks below and younger above.


A branch of geology that studies sedimentary rocks and their layers to understand earth history. 

Strike-slip Fault

Fault where two blocks of rock slide past each other horizontally without any significant vertical movement.

Structural Trap

A sealed geologic structure, such as a fold or fault, that creates an impedance to hydrocarbon migration. 


A layer beneath the earth's surface.


The underlying structure of a drilling rig that supports the platform, derrick, and drill string.

Surface Casing

A large diameter pipe set in shallow formations, designed to protect freshwater aquifers, and to allow a diverter or BOP to be installed at the top of the casing string. Surface casing also provides a mechanism for attaching casing strings. 

Subsurface Containment

Practice of containing fluids within their respective zones during oil and gas operations.


A mechanical device on a drilling rig that hangs below the traveling block and directly above the Kelly. It suspends the weight of the drill string and is designed to allow the rotation of the drill string as the drill string conveys high volumes of drilling mud.

Swivel Pipe

Mechanical device that suspends the weight of the drillstring designed to allow rotation of the drillstring beneath it conveying high volumes of high pressure drilling mud between the rigs circulating system and the drillstring.


A structural fold in rock formed through compressional forces where the strata dip inward toward the axis of the fold. Younger strata compose the core area and become progressively older way from the core. 

Tectonic Force

Forces that subject rock to stress and strain because of the movement of the earth's tectonic plates.

Tertiary Recovery

Also referred to as enhanced recovery (EOR) it includes various operations where fluids consisting or brine, freshwater, steam, polymers, carbon dioxide and other gases are injected into oil-bearing formations to recover residual oil in place. Tertiary recovery was traditionally the third phase of oil production, however, EOR operations now occur during any phase of production. 

The Pad

A preparatory stage before high-pressure fracturing begins in which slickwater (without proppant) is put in the well to lubricate and slightly open fractures.

Thermogenic Production

Thermogenic gases, such as natural gas and alkanes (C1-C4), generated from the cracking of organic molecules. Natural gas may be formed through primary cracking of kerogen or secondary cracking of oil.

Three-Phase Separator

A vessel that separates produced fluids into three phases including gas, oil, and water. Frequently referred to as a free-water knockout because one of the primary uses of three phase separators is to remove water from produced fluids.

Top Drive

Device used in place of a rotary table to turn the drill stem. It is suspended from the hook to allow the rotary mechanism to travel up and down the derrick. The advantages of top drive compared to conventional rotary table/Kelly method are, the ability to drill with three joints instead of single joint, and it allows the driller to quickly engage the pumps or the rotary while tripping pipe.

Traveling Block

Part of the block and tackle system used to raise and lower the drill string. It consists of several pulleys (sheeves) in sequence, and it is raised and lowered along with the drill string.

Trip (Tripping)

An operation to remove or replace the entire drill string including bottomhole assembly from the wellbore. 

Trip Out

The process of removing the entire drill string from the hole.

Trip Tank

A mud storage tank used during tripping operations. Fluid from the trip tank is continually circulated through the well during tripping and typically has a capacity of 10-15 bbl. It is used to monitor and maintain the required amount of mud for well control during tripping operations. 

Tripping Out

The process of removing the entire drill string from a well thousands of feet deep.

Two-Stage Separator

A vessel that separates produced fluids into a liquid phase, that may include water, oil, emulsion, and a gas phase.

Unconventional Resource

A term used for reservoirs with characteristics, such as low permeability, that require significant well stimulation and special recovery techniques for it to produce economic volumes of hydrocarbons. Examples of unconventional resources include shale gas, shale oil, tight gas, tight oil, coalbed methane, and gas hydrates.

Underground Injection Control (UIC)

An USEPA program designed to protect underground sources of drinking water (USDW) through the regulation of the construction, operation, permitting, decommissioning and closure of wells used to inject fluids and dispose of wastewater. 

Underground Source of Drinking Water (USDW)

An aquifer that is currently being used as a source of drinking water or those potentially capable of supplying a public water system.

Vertical Pressure

One of the principal stresses in addition to minimum and maximum horizontal stress experienced by confined underground formations. 

Vertical Stress Gradient

Stress acting vertically on a horizontal plane as function of depth and porosity. It is typically 1.0 psi/ft to 1.1psi/ft. Vertical stress in a formation is higher with lower porosity at greater depth, and is lower in formations with higher porosity at shallower depths. 


A fluid that is thick or sticky and does not flow easily.


To increase the viscosity of a fluid.


The measure of a fluid's resistance to flow.

Vitrinite Reflectance

The ranking of the thermal maturity of organic matter in coal and rocks. The ranking is based on an analytical method that uses a microscope to provide data on the reflectivity of vitrinite grains (a type of woody material) and whether the materials have generated hydrocarbon or have the potential to be effective source rocks. The reflectivity changes predictably and consistently under heating.


Cavities, voids or large pore spaces formed when minerals are removed through dissolution or erosion. Vugs are typically lined with mineral precipitates whose composition is different than the surrounding rock. 

Vuggy Porosity

Secondary porosity in calcareous rocks formed by the solution of vugs. Similar to moldic (mold) porosity.  

Wastewater Injection

A process that entails the injection of wastewater for EOR or disposal of waste fluid by subsurface injection. Wastewater injection is strictly regulated through the USEPA Underground Injection Control (UIC) program that classifies wells used for wastewater injection into six specific classes.

Water Table

The underground boundary marking the upper surface where the pores and fractures of the ground are saturated with water (zone of saturation) and the water pressure is equal to the atmospheric pressure.


A secondary recovery process implemented to increase production of oil in place where water is injected to re-energize and maintain pressure in the reservoir. The injected water also sweeps oil towards nearby production wells. 

Waterflood Technique

When water is pumped into an injection well located near the producing well.

Well Control

Procedures, technology and equipment used to control kicks and to minimize the potential for a blowout during drilling, workover, well completion, and abandonment operations Techniques and procedures include monitoring and management of drilling fluids and use of BOPs.

Well Log

A record, of the physical properties of rock formations in or around a wellbore displayed versus depth and/or time based on data generated with downhole tools or visual observations of drill cuttings. 

Well Logging

The process of collecting and recording downhole information using a variety of sensors that are lowered into the wellbore or used during drilling operations. 


Equipment installed at the surface of an oil and gas well that provides the structural and pressure-containing interface for the drilling and production equipment. Important wellhead components are the casing head, casing stringer, tubing head, and tubing adaptors. 

Wellhead/Casing Head

The wellhead is the primary pressure containment system for a well. It is where surface equipment interfaces with the borehole. Typically, the well head is welded to the first string of casing cemented into the hole; drilling continues through the top of the wellhead and BOP, which is mounted on the wellhead.


A tool used in horizontal and directional drilling to start a new path for the borehole by deflecting the drill bit. The tool, consisting of a long steel piece of casing, uses an inclined plane to deflect the bit slightly from the original path. Also used to sidetrack and straighten boreholes. 


A textile that is conical or tube shaped, mounted on a pole, mast or other equipment to indicate basic information on wind direction and strength.

Wireline Logging

Logging method that uses tools lowered into the borehole on an electrical cable to make a detailed record of the formations penetrated by a borehole. Wireline logging does not include measurement while drilling or logging while drilling tools or processes. 

Wireline Tool

Instrument that is lowered into a borehole by the wireline cable. Wireline tools provide and transmit data and information for identification and evaluation of rock properties and formation pressures.