Oilfield glossary

Average seismic velocity

Effective vertical seismic speed from a specified datum to a target, calculated as vertical distance divided by one-way travel time under the stated ray-path assumptions. For a vertical two-way reflection time t, the equivalent average is 2z/t; it is not generally the arithmetic mean of the interval velocities.

Worked example: two layers, travel time and depth

Also called: Average velocity (seismic)

Depth conversion

Transformation of seismic interpretations from travel-time coordinates into estimated depth using a velocity model and consistent reference datums. The result depends on the velocity information and imaging assumptions; changing the axis label alone does not establish a depth model.

Worked example: two layers, travel time and depth

Also called: Seismic time-to-depth conversion

Global Boundary Stratotype Section and Point GSSP

An internationally ratified reference point in a specified stratigraphic section that fixes a boundary on the geological time scale. Correlation markers help identify its time-equivalent level in other successions.

Field context A local fossil occurrence or log pick is evidence for correlation, not a new GSSP. Check the ratified reference and supporting markers.

Worked example: a fixed boundary and a revised age

Also called: Golden spike

Global Standard Stratigraphic Age GSSA

An internationally agreed numerical age used to define a boundary on the geological time scale, particularly in parts of the Precambrian. Its defining reference is a number rather than a designated point in a stratigraphic section.

Field context Distinguish a defining GSSA from the estimated numerical age of a GSSP. Consult the current ICS chart for the standard assigned to a particular boundary.

Worked example: a fixed boundary and a revised age

Graded bedding

A systematic change in grain size through the thickness of a sedimentary bed. Describe the direction and scale of the change before interpreting the depositional process.

Worked example: grading within one bed

Also called: Sedimentary grading

Normal grading

An upward decrease in grain size within a bed, with coarser particles below finer particles. Upward refers to stratigraphic younging, which need not coincide with present vertical.

Field context Determine stratigraphic way-up separately from present vertical: a normal graded bed fines toward its original top even after overturning. Confirm the depositional interpretation with other indicators before assigning younging.

Worked example: which way is stratigraphic up?

Also called: Normally graded bedding

Inverse grading

An upward increase in grain size within a bed. The observation describes a texture; it does not by itself identify a unique transport mechanism or depositional environment.

Worked example: grading within one bed

Also called: Reverse grading, Inversely graded bedding

Absolute open flow potential AOFP

Theoretical maximum deliverability extrapolated to zero sandface backpressure under a specified model; not a normal operating rate.

Also called: AOF

Absolute permeability

Rock permeability measured with a single fluid phase occupying its connected pores.

Field context In the single-phase Darcy-flow context, this corresponds to intrinsic permeability of the pore network. Hydraulic conductivity additionally incorporates fluid properties: K = kρg/μ. This comparison assumes unchanged pore geometry and a fully saturated medium.

Worked example: permeability versus hydraulic conductivity

Also called: Intrinsic permeability

Absorption

Transfer of a substance into the bulk of another material, such as water vapor entering glycol during gas drying.

Accommodation space

Space available for sediment accumulation, influenced by subsidence and changes in sea level.

Also called: Accommodation

Accumulator

System storing pressurized hydraulic fluid used to actuate well-control equipment.

Acetic acid

Organic acid used in stimulation formulations where slower reaction or corrosion control is useful.

Acid corrosion inhibitor

Treatment additive intended to reduce acid attack on exposed metal components during well servicing.

Acid fracturing

A stimulation treatment that creates fractures and uses acid to alter fracture surfaces, improving their ability to conduct fluids.

Also called: fracture acidizing

Acid gas

Gas constituent or separated stream, commonly hydrogen sulfide or carbon dioxide, capable of forming acidic water solutions.

Acid wash

Acid treatment directed at deposits in the wellbore, perforations, or completion equipment rather than deep formation stimulation.

Acid wormhole

Preferential dissolution channel formed when acid reacts nonuniformly with carbonate rock.

Also called: wormhole

Acidizing

Treatment using acid to remove reactive deposits or alter near-wellbore rock and improve fluid flow.

Acoustic impedance

Product of rock bulk density and compressional-wave velocity; contrasts between layers influence seismic reflections.

Field context For a P wave, Z = bulk density × P-wave velocity. Use compatible units: kg/m³ × m/s gives kg/(m²·s). Equal impedance can occur in different lithologies; impedance is not a unique rock identifier. For example, 2000 kg/m³ × 3000 m/s and 2500 kg/m³ × 2400 m/s both give 6 million kg/(m²·s), despite different densities and speeds.

Worked example: different properties, equal impedance

Also called: Seismic impedance

Acute exposure

Brief contact with a harmful agent, distinguished from repeated long-term exposure.

Administrative control

Work rule, scheduling practice, or procedure used to reduce exposure to hazards.

Adsorption

Attachment of molecules to a material's surface, used to remove selected constituents from process streams.

Aggradation

Upward accumulation of sediment with relatively little overall advance or retreat of the depositional system.

Air drilling

Drilling method using compressed gas as the principal circulating medium for bit cooling and cuttings transport.

Air-purifying respirator APR

Respirator that removes specified contaminants from surrounding air using appropriate filters or sorbents.

Alkaline flooding

Injection of alkaline solution that can react with suitable oil components to form surfactants in the reservoir.

Alkalinity

Acid-neutralizing capacity of a fluid; mud alkalinity measurements help assess its chemical condition.

Alkylate

High-octane hydrocarbon blendstock produced by an alkylation unit.

Alkylation

Refinery process combining isobutane and light olefins to produce high-octane gasoline components.

Along-hole axis

Local coordinate direction tangent to the wellpath, positive toward increasing measured depth. It is distinct from the vertical direction.

Amine regenerator

Equipment releasing absorbed acid gases so the solvent can be reused.

Amine treating

Gas treatment using an amine solution to absorb acid gases.

Amplitude variation with offset AVO

Change in seismic reflection amplitude with source-receiver separation, analyzed for information about lithology and fluids but subject to nonunique interpretation.

Also called: Amplitude versus offset

Angular unconformity

Erosional or nondepositional surface separating younger beds from older beds tilted at a different angle.

Anhydrite

Anhydrous calcium-sulfate mineral and associated evaporite rock that may form sealing intervals.

Anisotropy

Variation of a material property with measurement direction, such as different seismic velocities parallel and perpendicular to bedding.

Field context Name the property and scale: hydraulic anisotropy concerns conductivity, not necessarily seismic velocity or electrical resistivity. Isotropic beds can form an anisotropic layered unit.

Worked example: conductivity along and across layers

Annular flow

Multiphase regime with a lighter central stream and a heavier liquid film along the pipe wall.

Annular preventer

Pressure-control device with a deformable sealing element that closes around suitable tubulars or the bore.

Also called: Annular BOP

Annular velocity

Average linear speed of fluid moving through the annular space, distinct from volumetric flow rate.

Annulus

Space between concentric tubulars, or between a tubular and the surrounding borehole.

Anticline

Fold with older beds in its core, commonly forming an upward arch that can create a structural hydrocarbon trap.

Field context Separate fold shape from stratigraphic age: antiform and synform describe closure, whereas anticline and syncline require age evidence. Record the profile orientation before measuring limb angles.

Worked example: fold profile and bed age

Antiwhirl bit

Bit designed to direct imbalance forces in a way that stabilizes its motion against the borehole. The design aims to reduce destructive lateral motion rather than remove every vibration mode.

API gravity °API

Petroleum-liquid density scale on which higher values indicate lower relative density at the specified reference temperature.

Apparent dip

Inclination of a geological plane seen in a vertical section not aligned with its maximum-dip direction.

Field context Specify the section direction or borehole-coordinate convention. A dip seen along the well direction can differ from maximum geological dip; using the wrong projection changes the interpreted stratigraphic position. For a nonvertical plane, tan(apparent dip) = tan(true dip) × cos(theta), where theta is the horizontal angle from the down-dip direction to the section heading. A negative result means the surface rises along that heading. Two perpendicular signed section slopes of the same planar surface can recover its dip: combine their tangents as orthogonal components, rather than averaging the angles.

Worked example: recover dip from perpendicular sections

Apparent viscosity AV

Effective viscosity at a stated shear rate and temperature; its value may vary with measurement conditions.

Aquifer

Water-bearing permeable rock or sediment capable of transmitting water; an adjoining aquifer may support reservoir pressure.

Worked example: aquifers separated by a leaky layer

Archie equation

Empirical relationship linking formation electrical behavior to porosity and, in saturation applications, water saturation, primarily for electrically clean rocks.

Also called: Archie's equation

Areal sweep efficiency

Fraction of a flood pattern's plan area contacted by the injected fluid at a specified stage.

Aromatic hydrocarbon

Hydrocarbon containing an aromatic ring system, such as benzene, toluene, or xylene.

Array induction

Induction-resistivity measurement combining multiple coil-array responses to obtain selected investigation depths and vertical resolutions.

Array laterolog

Electrode-resistivity measurement using multiple focusing configurations to investigate different distances into the formation.

Artificial lift AL

Equipment or injected gas that supplies additional energy to move well fluids toward surface.

Asphalt

Bitumen-rich petroleum product used as a binder in paving and other applications.

Atmospheric crude distillation

Initial separation of crude oil into boiling-range fractions at approximately atmospheric pressure.

Attapulgite

Needle-shaped clay used to develop suspension and carrying capacity, particularly in saline drilling fluids.

Attenuation resistivity

Resistivity derived from the reduction in amplitude of an electromagnetic signal between receivers.

Authigenic

Describes minerals formed in place within a rock, rather than transported into it as sediment grains.

Autoignition temperature

Lowest temperature at which a material ignites without an external spark or flame under specified conditions.

Average linear groundwater velocity

Mean advective groundwater speed estimated as v = q/ne, where q is Darcy flux and ne is effective flow porosity. This averages pore paths and does not include dispersion or solute retardation.

Field context For uniform steady flow, t = L ne/q. State the flow-porosity assumption and test its sensitivity; this time is not a first-arrival or contaminant-breakthrough prediction.

Worked example: flow porosity and travel-time sensitivity

Also called: Seepage velocity, Average pore-water velocity

Average reservoir pressure

Representative volumetric average of reservoir pressure at a specified depletion stage.

Axial vibration

Oscillating motion or load along the drillstring axis. At the bit, severe axial response can interrupt rock contact and contribute to damage.

Azimuth AZI

Horizontal direction measured from a stated north reference; true, grid and magnetic north are not interchangeable.

Field context Record whether the heading is magnetic, true or grid referenced. Converting between references requires the applicable declination and convergence conventions.

Azimuthal deep resistivity ADR

Directional resistivity measurement used to detect contrasts around the wellbore and support boundary interpretation. Different tools and processing schemes provide different sensitivities and useful ranges.

Azimuthal density

Density measurement resolved into sectors around the borehole. Sector differences may reflect formation variation, tool position or borehole conditions, so they require environmental review.

Azimuthal gamma ray

Gamma-ray response resolved by direction around the tool. Changes between sectors can help describe formation geometry when orientation and data quality are reliable.

Azimuthal laterolog

Focused electrical resistivity measurement using segmented electrodes to distinguish response by direction around the borehole, rather than averaging the entire circumference.

Azimuthal measurement

Measurement with directional sensitivity around the tool rather than a fully circumferential average. Its orientation reference and angular coverage determine how the result should be interpreted.

Background gas BGG

Baseline mud-gas level under reasonably steady drilling conditions, used to compare departures from the prevailing trend.

Backward whirl

Lateral orbital motion of the BHA opposite to its own rotational direction. It can generate large cyclic bending loads and rapid component fatigue.

Balanced cement plug

Cement plug placed using balanced fluid columns inside and outside the placement tubing.

Barite

Dense barium-sulfate mineral commonly added to increase drilling-fluid density.

Barite sag

Settling and redistribution of weighting solids that create mud-density differences within the circulating system or well.

Also called: Weighting-material sag

Basement

Older rock foundation beneath a sedimentary succession, commonly crystalline igneous or metamorphic rock.

Battery site

Field location containing equipment that separates, measures, treats, or stores produced fluids.

Also called: Battery, Tank battery

Bearden units of consistency Bc

Dimensionless units expressing cement-slurry consistency in pumpability testing; they are not directly interchangeable with viscosity units.

Bed

A distinct sedimentary layer; when formally named, the smallest ranked sedimentary lithostratigraphic unit.

Bedding

Arrangement of sediment or rock into layers separated by identifiable boundaries.

Bell nipple

Enlarged well-top conduit that guides tools and directs returning drilling fluid into the flowline.

Bend angle

Specified angular offset in a bent housing or directional sub. Actual directional response also depends on BHA geometry, bit, formation and drilling conditions.

Also called: Motor bend

Bent housing

Angled section within a steerable motor assembly that offsets the bit direction relative to the upper assembly. The bend setting influences steering response but does not directly equal achieved hole curvature.

Bent sub

Angled drillstring connector used to create an offset for directional drilling. Its position in the assembly distinguishes it from a bend incorporated inside a motor housing.

Bentonite

Swelling clay rich in montmorillonite, added to water-based drilling fluids for viscosity and filtration control.

Benzene

Six-carbon aromatic hydrocarbon, C6H6, occurring in petroleum and used as a petrochemical feedstock.

Bicenter bit

Bit with an integral eccentric reaming section that drills a larger hole than its pass-through diameter.

Bilinear flow

Concurrent linear flow through a finite-conductivity fracture and from the reservoir toward that fracture.

Binary-cycle geothermal plant

Power plant transferring geothermal heat to a separate working fluid that drives a turbine.

Bingham plastic model

Idealized rheology model with a yield-stress intercept and a linear shear-stress increase after yielding.

Biocide

Chemical treatment used to control microorganisms in oilfield fluids and equipment.

Also called: Bactericide

Biostratigraphy

Use of fossils to date and correlate sedimentary intervals and help interpret depositional environments.

Field context Check assemblage consistency and possible reworking before assigning a host-bed age from a fossil occurrence. State whether a reported range is local or compiled across sections; correlation requires an explicit zonal criterion. Reworking is a depositional history; drilling cavings are sample contamination. Do not interpret a mixed fossil assemblage until these possibilities have been considered.

Worked example: reworking versus drilling cavings

Bioturbation

Disturbance and mixing of sediment by organisms, often expressed as burrows or disrupted bedding.

Bit bounce

Repeated loss and re-establishment of bit contact with the bottom of the hole during axial vibration. Surface measurements may not reveal its full severity.

Bit breaker

Profile-matched tool that holds a drill bit stationary while its threaded connection is made or released.

Bit sub

Short connector installed immediately above the drilling bit.

Bitumen

Very viscous petroleum that generally needs heating, dilution, or another recovery aid to flow readily.

Also called: crude bitumen

Bland coring fluid

Coring fluid selected to minimize changes to the core's original wettability and fluid composition.

Blast joint

Heavy-wall tubing positioned opposite perforations to resist erosion from incoming formation fluids.

Blind ram

Ram preventer element intended to close an empty bore; it is distinct from equipment designed to shear a tubular.

Blowout

Uncontrolled movement of formation fluids from a well, potentially to surface or another subsurface formation.

Blowout preventer BOP

Wellhead pressure-control equipment designed to close and seal the well under specified conditions.

Borehole compensation

Tool-transducer arrangement and processing designed to reduce effects of borehole size variations or tool tilt on wave-based measurements.

Borehole correction

Adjustment to a log measurement that accounts for the contribution of the borehole and its contents.

Bottomhole assembly BHA

Lower drillstring components used to cut, guide, stabilize and measure the borehole.

Field context Record component order, dimensions and sensor positions as well as tool names. Two assemblies carrying similar tools can behave differently because their geometry and stabilization differ.

Bottomhole pressure BHP

Pressure at a stated downhole location; its value depends on fluid and operating conditions.

Bottoms-up

Fluid, cuttings, or circulation volume associated with bringing material from the bottom of the well to surface.

Bound fluid

Pore fluid that remains immobile under the reservoir conditions considered; nuclear magnetic resonance estimates depend on the chosen interpretation model.

Boundary mapping

Interpretation of the position and geometry of nearby geological or fluid boundaries from suitable measurements. The result depends on sensor sensitivity, model assumptions and available contrast.

Bow-spring centralizer

Casing-centering device using flexible outward-bowed strips that contact the borehole wall.

Box connection

The internally threaded female end of a tubular connection.

Also called: box end

Braided line

Multistrand mechanical wireline used where greater tensile capacity is needed than conventional slickline provides.

Break out

Unscrew a threaded tubular connection.

Breakdown pressure

The pressure at which a new hydraulic fracture starts under the particular downhole stress and well conditions.

Also called: fracture initiation pressure

Breccia

Rock composed of angular fragments bound together; the fragment origin and surrounding material determine its geological interpretation.

Bridge plug

Downhole barrier set across the wellbore to isolate the interval below it.

Bridging agent

Sized particles selected to span pore openings or fractures and initiate a sealing layer.

Bright spot

Locally strong seismic amplitude that may indicate hydrocarbons but can also result from lithology or thin-bed interference.

Bubble flow

Flow regime with dispersed gas bubbles moving within a continuous liquid phase.

Bubblepoint pressure

Pressure at which gas first begins to leave solution from oil as pressure falls at a specified temperature.

Build rate BUR

Increase in inclination per unit of drilled length, commonly stated in degrees per 30 m or 100 ft. It describes the inclination component of curvature, not total three-dimensional dogleg severity.

Also called: Build-up rate

Build section

Part of a trajectory where inclination increases.

Bull plug

Solid end closure used to seal a tubular, pipe, or equipment passage.

Burn shoe

Hollow milling tool used to prepare the outside or top of a fish for retrieval.

Also called: mill shoe, washover shoe

Butane

Four-carbon saturated hydrocarbon, C4H10; commercial usage includes its straight-chain and branched forms.

Buttress thread

A casing-connection thread form designed to carry axial load, with sealing dependent on the connection design and fit.

Butylene

Four-carbon olefin, C4H8, produced during refining or petrochemical processing.

Calcimetry

Estimation of carbonate content from a sample's reaction under a defined analytical method. Report the method and whether the result represents calcite, dolomite or total carbonate.

Calcite

Calcium-carbonate mineral that is a major constituent of limestone and a common cement in sedimentary rocks.

Field context Calcite and aragonite share the ideal formula CaCO3 but differ in crystal structure. Calcite identified in a fossil is not by itself proof that the organism originally secreted calcite.

Worked identification: composition versus crystal structure

Calcium bromide

Soluble salt used to formulate dense, solids-free completion and workover brines.

Calcium chloride

Soluble salt used in completion and drilling brines and in the aqueous phase of some emulsion muds.

Calcium contamination

Unwanted calcium-ion influx that alters clay or polymer behaviour in a drilling-fluid system.

Calcium hydroxide

Alkaline calcium compound used in lime muds and in certain nonaqueous-fluid formulations.

Also called: Slaked lime

Caliper log

Depth-based record of borehole diameter used to assess hole condition and support log interpretation and volume calculations.

Capillary entry pressure

Threshold pressure difference at which a non-wetting fluid first invades an accessible, initially wetting-fluid-filled pore connection under a specified displacement path. Geometry, interfacial tension and wetting conditions control the threshold.

Field context Check whether a measured throat size is radius or diameter. In the ideal tube model, a 10 μm diameter means r = 5 μm, giving 12 kPa for γ = 0.030 N/m and water-side θ = 0°. Treating that diameter as radius incorrectly gives 6 kPa. Preserve phase order and angle convention when transferring software inputs.

Worked example: radius versus diameter

Also called: Entry capillary pressure

Capillary number

Dimensionless measure of viscous forces relative to interfacial forces in flowing fluids.

Capillary pressure

Pressure difference across a fluid interface. Here Pc is non-wetting-phase pressure minus wetting-phase pressure; always state the phase order because sign conventions differ.

Capillary entry: worked pore-throat example

Also called: Pc

Capillary suction time test CST test

Filter-paper test that times water movement from a mud sample to assess filtration and clay-fluid behaviour.

Caprock

A low-permeability geological barrier above or around a reservoir that helps contain fluids and pressure.

Also called: reservoir containment

Carbonate ion

Dissolved CO3²⁻ species whose concentration and interaction with other ions affect mud chemistry and treatment.

Carboxymethylcellulose CMC

Cellulose-derived polymer used principally to reduce drilling-fluid filtrate loss; viscosity depends on its grade.

CAS registry number

Unique identifier assigned to a chemical substance by the Chemical Abstracts Service.

Cascading heat use

Sequential use of a heat stream for applications requiring progressively lower temperatures.

Casing CSG

Pipe installed in a well to support the hole and contribute to isolation between formations.

Casing centralizer

Device that helps hold casing away from the borehole wall to improve annular cement placement.

Casing coupling

Internally threaded sleeve that joins the externally threaded ends of two casing joints.

Also called: casing collar

Casing grade

Designation identifying specified material and strength requirements for a casing product.

Casing gun

Perforating gun sized to run directly inside casing before restrictive production tubing is installed.

Casing hanger

Wellhead device that supports a casing string and commonly incorporates annular sealing features.

Casing head

The lower wellhead adapter attached to the first casing string and supporting subsequent wellhead or pressure-control equipment.

Also called: casinghead

Casing interference

Magnetic influence of nearby steel casing on a magnetic survey. It is especially relevant near existing wells and cannot be dismissed solely because the measurement appears repeatable.

Casing joint

An individual length of casing pipe that is connected with other joints to make a casing string.

Casing point

The depth at which an openhole interval ends and the next casing string is planned to be set.

Casing scraper

Tool with scraping elements that remove deposits and debris from the inside of casing.

Casing scratcher

Brushlike casing attachment intended to disturb mud cake on the borehole wall during cement preparation.

Casing shoe

Component at the lower end of a casing string that guides it into the hole.

Casing spool

Wellhead section that houses support and sealing arrangements for a casing string.

Also called: casing bowl

Casing thread

Machined helical connection geometry joining casing components and contributing to structural and pressure integrity.

Catagenesis

Burial-related thermal alteration of organic matter and sediments beyond early diagenesis, including important stages of petroleum generation.

Catalytic cracking

Conversion of heavy hydrocarbons into lighter products using a catalyst.

Catalytic hydrocracking

Conversion of heavier petroleum fractions into lighter products using hydrogen and catalysts.

Also called: Hydrocracking

Catalytic hydrotreating

Hydrogen-and-catalyst treatment of petroleum fractions to improve quality and remove unwanted constituents.

Also called: Hydrotreating

Catalytic reforming

Catalytic rearrangement of naphtha hydrocarbons to produce higher-octane components and petrochemical feedstocks.

Catwalk

Staging and transfer area used to move tubulars and long tools between storage and the rig floor.

Caustic soda

Alkaline chemical used to raise and maintain the pH of water-based drilling fluids.

Also called: Sodium hydroxide

Cavings

Fragments detached from the borehole wall rather than newly drilled at the bit; they can contaminate cuttings samples.

Field context Older material falling from an unstable interval can appear with newly drilled cuttings. Do not assign every recovered fragment to the current bit formation; retain the evidence for a mixed sample. Material from an interval penetrated earlier need not be geologically older. In an upright, unfaulted succession, younger fossils can cave from above into a sample collected while drilling older beds. Folded or faulted sections require their own age-order model.

Worked example: reworking versus drilling cavings

Ceiling exposure limit

Exposure concentration that is not to be exceeded at any time.

Cement accelerator

Additive that speeds the development of cement strength, commonly useful at lower temperatures.

Cement additive

Material blended into a cement slurry to change its placement behavior or the properties of the hardened cement.

Cement bond log CBL

An acoustic log used to assess cement bonding around casing as part of evaluating annular isolation.

Cement channeling

Uneven annular cement placement that leaves a continuous fluid pathway and can compromise zonal isolation.

Cement dispersant

Additive that reduces cement-slurry viscosity by improving dispersion of its solid particles.

Cement extender

Material used to lower cement-slurry density or increase the volume obtained from a given quantity of cement.

Cement head

Surface equipment connected to casing that holds and releases cementing plugs and provides fluid connections.

Cement plug

A localized cement barrier placed in a wellbore for isolation, support, or subsequent well construction.

Cement retainer

Casing-mounted isolation tool with a valve arrangement that allows treatment below it while isolating the upper annulus.

Cement retarder

Additive that extends the time a cement slurry remains pumpable for placement.

Cement sack

A customary cement quantity unit; the conventional US oilwell-cement sack is commonly 94 lb.

Cement slurry density

Mass per unit volume of the mixed cement slurry, governing its contribution to hydrostatic pressure.

Cement slurry yield

Mixed-slurry volume obtained per unit quantity of dry cement at the specified water and additive proportions.

Cement squeeze

Remedial cement placement intended to seal unwanted fluid pathways around or through a well completion.

Also called: squeeze cementing

Cementation exponent m

Exponent describing how formation factor depends on porosity in an Archie-type electrical model.

Also called: Porosity exponent

Cementing CMT

Placing cement slurry in a well, commonly around casing, to provide support and zonal isolation.

Cementing plug

Elastomeric separator that travels inside casing to limit mixing between cement slurry and other fluids.

Also called: wiper plug

Cementing spacer

Engineered viscous fluid placed between drilling fluid and cement to improve displacement and limit contamination.

Also called: spacer fluid, cement spacer

Central control room CCR

Facility area where operators monitor and direct process operations.

Centrifugal pump

Pump whose rotating impeller transfers energy to fluid, increasing its velocity and discharge pressure.

Cesium formate

Highly soluble formate salt used to make dense brines for selected drilling, completion, and workover applications.

Chalk

Fine-grained marine limestone largely composed of microscopic calcareous skeletal material.

Check valve

Valve admitting flow in one direction and closing against reverse flow.

Checkshot survey

Borehole survey measuring travel time from a surface seismic source to receivers at known depths to calibrate time-depth relationships.

Field context Checkshot first arrivals provide source-to-receiver times, not automatically surface-reflection two-way times. Survey geometry, well deviation and seismic reference datum corrections must be reconciled before using the resulting time–depth relationship for a well tie.

Worked example: two layers, travel time and depth

Also called: Check-shot survey

Chemical cutter

Specialist downhole tool that uses a localized chemical reaction to sever a tubular.

Chemical flooding

Recovery processes using injected chemical solutions to improve displacement or mobility control.

Chemical stability

Ability of a material to remain chemically unchanged under specified storage and handling conditions.

Chert CHT

Hard, very fine-crystalline silica-rich rock, often found as nodules or layers within sedimentary successions.

Chloride test

Analytical test measuring chloride-ion concentration in water or mud filtrate, commonly by titration.

Chlorite

Platy green silicate mineral that can occur as pore-lining coatings in sandstone and influence reservoir quality.

Field context Chlorite can belong to an earlier low-grade assemblage or form during later replacement. Report its texture and associated minerals before interpreting metamorphic history.

Worked guide: mineral evidence, isograds and overprints

Choke line

Pressure-rated flow path connecting the BOP arrangement to the choke system for controlled well-fluid returns.

Choke manifold

Arrangement of valves, flow paths and chokes used to route and control pressurized well-fluid returns.

Chronic exposure

Repeated or sustained contact with a harmful agent over an extended period.

Chronostratigraphic correlation

Correlation intended to connect intervals of equivalent geological age. It differs from matching similar rock types, which may occur at different times in different locations.

Chronostratigraphy

Classification and correlation of rock bodies according to their relative or absolute ages.

Circulating swage

Threaded adapter that connects a circulating line to the end of a casing or tubing string.

Also called: casing swage, tubing swage

Circulation

Movement of drilling fluid through the drillstring and back to surface through the return path.

Clast imbrication

An overlapping, preferentially inclined arrangement of sedimentary clasts. In the common upstream-dipping fabric, the broad clast surfaces dip against the inferred flow direction; long-axis orientation must be recorded separately.

Field context Average dip azimuths as circular directions, not ordinary numbers. Keep measured dip directions, inferred flow directions and undirected long-axis orientations distinct; a circular mean is not a bedding-restoration calculation.

Worked example: averaging bearings across north

Also called: Pebble imbrication, Imbricated clasts

Clay aggregation

Formation of particle clusters, including face-to-face clay associations, that changes mud behaviour and separation response.

Also called: Agglomeration

Clay swelling

Expansion of reactive clay minerals on contact with incompatible water, potentially constricting pore passages.

Clay-bound water CBW

Water associated with clay surfaces or interlayers that is generally immobile and commonly excluded from log-derived effective porosity.

Clear-water drilling

Drilling with a water-based circulating fluid maintained at very low solids content.

Closed isotopic system

A mineral or rock domain that has neither gained nor lost the relevant parent or daughter isotopes over the interval being dated, apart from internal radioactive decay. Closure is specific to an isotope system and geological history.

Field context A lower retained daughter/parent ratio can reflect daughter loss rather than a younger formation event. The worked loss example holds parent constant; it is not a universal correction for disturbed samples.

Worked example: daughter loss and apparent age

Closed mud system

Fluid-processing arrangement designed to recover and recirculate mud while separating drilled solids without a conventional reserve pit.

Also called: Closed-loop mud system

Coal

Organic-rich sedimentary rock formed as buried plant material changes during coalification. It may contain or generate natural gas.

Coalescence

Merging of small dispersed liquid droplets into larger droplets, which can promote phase separation.

Cogeneration CHP

Producing useful heat and electricity from a shared energy source, such as at a steam-intensive oil-sands facility.

Also called: combined heat and power

Coiled tubing CT

Continuous small-diameter tubing deployed from a reel to convey fluids, tools, or both into a well.

Also called: coil tubing

Coiled tubing unit CTU

The reel, injector, pressure-control equipment, and controls used to deploy and recover a coiled-tubing string.

Coking

Thermal conversion of heavy refinery fractions into lighter products and carbon-rich petroleum coke.

Collar lock

Lock designed to anchor in the internal recess between tubing ends at a compatible threaded coupling.

Colloidal solids

Very fine suspended particles that are difficult to remove by ordinary mechanical solids-control equipment.

Combustible dust

Fine solid particles capable of fire or explosion when dispersed at sufficient concentration in air.

Commingled completion

Completion that allows fluids from separate reservoir zones to enter a common wellbore flow path.

Common midpoint CMP

Surface midpoint shared by multiple source-receiver pairs whose seismic traces are grouped for processing.

Compaction

Reduction of sediment volume as grains pack more closely or deform, commonly decreasing pore space during burial.

Field context A pore-pressure decline can increase effective stress even without additional burial. Predicting deformation also requires compressibility, stress history and drainage times; the stress change alone does not give a settlement.

Effective stress and compaction: worked example

Company representative

Operator's onsite representative responsible for coordinating and overseeing wellsite operations.

Also called: Company man, Wellsite supervisor

Completion fluid

Fluid selected for completion work to support well operations while limiting unwanted reactions or damage to the producing formation.

Completion packer

A downhole sealing device that isolates the annular space around the production tubing at a chosen depth.

Also called: production packer, packer

Composite well log

Combined presentation of selected geological observations and measurements on a common well and depth reference. Data sources and any alignment adjustments should remain identifiable.

Compressed natural gas CNG

Natural gas stored under high pressure while remaining gaseous, unlike refrigerated LNG.

Compressibility

Fractional volume change of a fluid or rock per unit change in pressure.

Compression-set packer

Packer activated by axial compression applied through its running string.

Compressional wave P-wave

Elastic body wave in which particle motion is parallel to the direction of propagation.

Also called: P wave

Compressor

Machine that raises gas pressure for transport, processing, or injection.

Compressor station

Pipeline facility restoring gas pressure lost during transport.

Conchoidal fracture

A mineral or glass breakage texture with curved, shell-like surfaces rather than repeated planar cleavage. Quartz commonly shows this texture, but the texture alone does not establish mineral identity.

Mineral identification: cleavage, fracture and parting

Also called: Shell-like fracture

Condensed section

Thin sedimentary interval representing a long time span because deposition was unusually slow.

Confined space

Enclosed or partly enclosed space with restricted access and hazards, not intended for continuous occupancy.

Confined-space attendant

Person stationed outside a confined space to monitor entrants and coordinate assistance.

Also called: safety watch, standby person

Confined-space entry permit

Document recording authorized entry, assessed hazards, controls, personnel, and validity for a particular space.

Confined-space rescue plan

Prearranged personnel, equipment, communication, and methods for responding to an emergency during confined-space work.

Conglomerate CGL

Sedimentary rock containing abundant rounded gravel-sized fragments within a finer matrix or cement.

Field context Describe clast orientation separately from roundness and sorting. An imbricated fabric can constrain flow interpretation, but a conglomerate name alone does not identify the transport process.

From grain shape to clast fabric

Coning

Drawdown-driven distortion of a fluid contact toward a vertical or slightly deviated producing well.

Connate water

Water retained within sediment pores from the time the sediment formed; distinct from all present-day formation water.

Connection gas CG

Gas entering or being drawn into the well around a pipe connection, commonly recognized later at surface after circulation lag.

Consequence severity

Seriousness of the potential harm arising from a hazardous event.

Contact angle

Angle where two fluid phases meet a solid surface, measured through a specified phase to characterize wetting.

Field context For a cylindrical, water-wet throat, entry pressure scales with interfacial tension and the cosine of the contact angle measured through water, and inversely with throat radius. A larger pore body does not remove a narrow connecting throat. At one ideal flat contact, water-side and oil-side angles sum to 180°. Record which phase advances or recedes; a single smooth-surface measurement is not a complete rock wettability description.

Worked example: supplementary contact angles

Continuous gas lift

Artificial-lift method using sustained gas injection to help produced liquids reach the surface.

Continuous phase

Connected liquid phase surrounding dispersed droplets in an emulsion.

Also called: External phase

Continuous survey

Series of directional measurements acquired while the tool moves through the hole or during drilling. Sampling, motion corrections and quality control determine how it can be combined with stationary surveys.

Control actuator

Device converting a control command into mechanical movement of equipment.

Control line

Small hydraulic conduit used to operate downhole completion equipment from the surface.

Conventional petroleum resources

Petroleum in discrete buoyancy-controlled accumulations that can generally be produced using conventional recovery methods.

Convolution

Mathematical operation combining an input signal with a response function; used to model a seismic trace from reflectivity and a wavelet.

Core

Rock sample recovered largely intact to examine geological fabric and measure properties that cuttings or logs may not resolve.

Reading burrowed sediment: core-description example

Core analysis

Laboratory evaluation of recovered rock to measure properties such as porosity, permeability, saturation and fabric.

Correlation marker

Recognizable feature used to connect intervals between logs or wells. A similar curve feature is evidence for a correlation, not proof that two intervals are identical in age or properties.

Corrosion

Deterioration of metal through chemical or electrochemical reactions with its environment.

Corrosion inhibitor

Chemical treatment that reduces metal corrosion under specified fluid, temperature, and exposure conditions.

Cresting

Drawdown-driven movement of a fluid contact toward a horizontal or highly deviated producing well.

Cross-bedding

Sets of inclined internal layers within a bed, commonly produced as ripples or dunes migrate.

Field context Terminology varies: dune-scale cross-stratification is commonly called cross-bedding and ripple-scale structures cross-lamination. Identify the set boundary that truncates older laminae before inferring stratigraphic way-up; internal lamina dip alone is insufficient.

Worked example: cross-bed truncation and way-up

Crown block

Fixed sheave assembly near the top of a mast or derrick through which the drilling line is reeved.

Crude distillation unit

Refinery equipment that heats crude and separates its components into different boiling-range fractions.

Crude oil

Naturally occurring petroleum mixture recovered as liquid, commonly containing dissolved gas and nonhydrocarbon constituents.

Crustal magnetic anomaly

Local magnetic-field contribution from magnetized rocks in the Earth's crust. It can make a regional or global reference insufficient for a particular drilling location.

Cuttings

Fragments generated as the bit breaks rock and carried to surface in the drilling fluid.

Cuttings description

Written observations of recovered rock fragments, including lithology, texture and shows, tied to a stated lag-corrected interval.

Cuttings morphology

Observed size and shape of recovered drilled fragments. Bit action, transport and wall failure affect morphology, so shape alone is not a unique indicator of formation type.

Cuttings sample interval

Measured-depth range assigned to a collected cuttings sample. It must be distinguished from collection time and from the bit depth when the material reached surface.

Cuttings texture

Arrangement and appearance of grains, matrix and other fabric visible in recovered fragments. Drilling damage and small fragment size can limit how faithfully it represents the intact rock.

Cyclic steam stimulation CSS

A repeating recovery cycle in which one well injects steam, rests while heat spreads, then produces heated oil and water.

Also called: huff and puff, huff-and-puff

Daily drilling report DDR

Time-bounded record of well operations, progress and notable conditions for communication between the rig and office.

Darcy flux

Volumetric discharge divided by the bulk cross-sectional area normal to flow: q = Q/A. For one-dimensional saturated Darcy flow, q = −K dh/dx. Its units are length per time, but it is not the velocity within connected pores.

Field context Use the bulk area for q = Q/A; dividing by flow porosity gives an average advective speed, not a local pore-channel speed.

Worked example: flow porosity and travel-time sensitivity

Also called: Specific discharge, Darcy velocity

Daughter isotope

An isotope produced by radioactive decay of a parent isotope.

Field context Measured daughter atoms can include both initial and radiogenic components. Treating all daughter atoms as decay products can bias an age.

Radiometric dating: half-life and age interpretation

De-energization

Disconnection and isolation of energy sources, including control of remaining stored energy.

Debutanizer

Fractionation column separating butanes-rich overhead from pentanes-and-heavier material.

Deconvolution

Signal processing that estimates or removes a filtering effect to improve seismic wavelet shape or suppress predictable reverberations.

Deethanizer

Fractionation column separating ethane-rich overhead from heavier hydrocarbons.

Definitive survey

Selected final trajectory record, potentially assembled from several survey datasets. Its tie-ons, corrections, station status and uncertainty basis must remain traceable.

Deflocculant

Additive that reduces attraction between suspended particles, lowering flocculation and often mud viscosity.

Also called: Thinner

Defoamer

Chemical additive used to break foam and help trapped gas escape from a liquid.

Also called: Antifoam, Foam breaker

Degasser

Equipment that removes entrained gas from drilling fluid.

Dehydrator

Equipment removing water vapor from gas, commonly using glycol or a solid desiccant.

Delayed coking

Batch-drum thermal process converting heavy petroleum residues into lighter fluids and petroleum coke.

Delta

Sedimentary accumulation where a sediment-bearing river enters standing water and loses transport capacity.

Demulsifier

Chemical that promotes separation of the immiscible liquid phases in an emulsion.

Density

Mass per unit volume of a material or fluid.

Field context State the mass and volume basis, units and material condition. A heavier specimen is not necessarily denser if its volume is larger.

Worked example: density, equal volume and specific gravity

Density log

Radiation-based measurement used to estimate formation bulk density and, with assumptions, porosity.

Depletion

Reduction in reservoir pressure or remaining hydrocarbon quantities as reservoir fluids are produced.

Depositional channel

Elongated depression that carries water and sediment and can become filled with a distinctive sediment body.

Depositional environment

Physical, chemical and biological setting in which sediment accumulated; interpreted from multiple rock and fossil observations.

Also called: DE, Sedimentary environment

Depropanizer

Fractionation column separating propane-rich overhead from heavier hydrocarbons.

Depth matching

Alignment of logs, cores or other well data to a consistent depth reference before comparison or integration.

Field context A visually improved alignment should retain its reference curve, method and applied shifts. Keep acquisition depths available so a later reviewer can reproduce or challenge the match.

Depth migration

Seismic imaging that places reflections in depth using a velocity model, particularly where velocity varies laterally.

Depth of investigation DOI

Characteristic distance into the formation contributing to a tool measurement; its meaning depends on the response and stated signal fraction.

Field context The quoted distance depends on how sensitivity is defined and on formation conditions. It should not be interpreted as a sharp boundary beyond which a tool has zero response.

Depth reference

Specified reference point or plane from which well depths are measured; comparisons require consistent datums.

Also called: Depth datum

Depth shift

Adjustment applied to align one dataset with a chosen depth reference. Preserve the original depths and the shift history so the aligned record remains traceable.

Derrick

Load-bearing rig structure that supports the hoisting system and suspended tubulars.

Derrickhand

Crew member who handles pipe aloft and may also monitor the drilling-fluid system.

Also called: Derrickman

Desander

Solids-control equipment that separates sand-size particles from circulating drilling fluid.

Desiccant

Material that removes moisture from a gas stream; gas plants use regenerable liquid or solid desiccants.

Desilter

Solids-control equipment intended to remove finer particles than a desander.

Detonating cord

Specialized explosive cord that transmits initiation between components of a perforating assembly.

Also called: primer cord

Detrital

Describes rock or mineral particles derived from breakdown of pre-existing rocks and subsequently transported or deposited.

Also called: Clastic

Deviated hole

Borehole whose trajectory departs from vertical, commonly by intentional directional drilling.

Dewpoint pressure

Pressure at which the first liquid condensate appears from a gas mixture at a specified temperature.

Diagenesis

Physical, chemical and biological changes to sediment after deposition, generally below metamorphic conditions. These changes can alter reservoir quality.

Diapir

Mobile material, commonly salt or shale, that intrudes surrounding rock and can deform overlying beds.

Differential pressure Delta P

Difference between fluid pressures at two specified locations or across a component.

Differential sticking

Pipe held against a permeable interval by a pressure difference acting over the contact area, often across filter cake.

Dilbit

A transport blend of bitumen and a lighter hydrocarbon diluent.

Also called: diluted bitumen

Diluent

A lighter hydrocarbon liquid blended with viscous oil to improve its ability to flow during transport.

Dim spot

Locally weakened seismic reflection amplitude, potentially caused by reduced impedance contrast associated with fluids or lithology.

Dip direction

Compass bearing of the steepest downward line in an inclined geological plane. It is perpendicular to strike and identifies which way the plane descends; it is an azimuth, not the dip angle.

Worked example: strike and apparent dip

Also called: Dip azimuth

Directional driller DD

Specialist who guides well trajectory through drilling parameters, steering equipment, and directional measurements.

Also called: DD

Directional drilling DD

Deliberately controlling a well trajectory to reach a subsurface objective.

Directional survey

Measurement of wellbore inclination and azimuth at a specified depth, used to reconstruct its path.

Field context A station measures direction at a location. The continuous path shown between stations is calculated, so survey spacing and the reconstruction method affect what small-scale bends can be resolved.

Also called: Deviation survey

Disconformity

Break in the geological record between broadly parallel sedimentary beds, reflecting erosion or nondeposition.

Displacement efficiency

Fraction of the original oil removed from the portion of reservoir actually contacted by a displacement process.

Displacement front

Moving transition between injected fluid and the reservoir fluid it displaces.

Also called: flood front

Distance to bit DTB

Along-assembly separation between a sensor's measurement reference and the drill bit. It creates a depth offset between the bit's current position and the rock sampled by that sensor.

Also called: Sensor-to-bit offset

Distance to boundary DTB boundary

Estimated separation between a measurement or well position and an interpreted formation boundary. Specify the direction and reference point; the abbreviation DTB can also mean distance to bit.

Distillate fuel oil

Petroleum fuel fraction produced by distillation, including grades used for diesel engines and heating.

Distributed control system DCS

Plant control architecture distributing control functions among linked controllers and operator stations.

Doghouse

Crew shelter or control-support room located beside the rig floor.

Dogleg

Localized change in wellbore direction that can increase friction, contact loads, and difficulty passing rigid equipment.

Dogleg severity DLS

Change in wellbore direction per unit length, commonly expressed as degrees per 30 m or 100 ft.

Field context Build rate describes inclination change; dogleg severity includes both inclination and azimuth change. Values normalized per 30 m and per 100 ft are close but not numerically identical.

Also called: DLS

Dolomite mineral

Calcium-magnesium carbonate mineral, CaMg(CO₃)₂, that is the principal constituent of dolostone.

Also called: Dolomite

Dolomitization

Replacement of calcite by dolomite during diagenesis, potentially changing pore structure and reservoir properties.

Dolostone

Carbonate rock composed mainly of dolomite mineral. The rock name distinguishes it from the mineral itself.

Also called: Dolomite rock

Downhole junk

Unwanted loose material in a well, commonly small metal fragments or dropped objects.

Also called: Junk

Downhole mill

Rotary cutting tool used to grind away downhole equipment, debris, or casing.

Also called: mill

Downhole telemetry

Transfer of measurements or commands between downhole tools and surface systems. Transmission method, bandwidth and latency affect which data are available during drilling.

Downlap

Termination of inclined strata against an underlying surface with a shallower apparent dip.

Field context Read the termination relative to the lower bounding surface and original depositional geometry. In the standard usage, onlap ends updip and downlap ends downdip; present apparent dip alone may be misleading after deformation.

Worked interpretation: reflection terminations and uncertainty

Downlink

Command sent from the surface to a downhole tool. The signaling method and confirmation procedure depend on the tool and telemetry system.

Drainage area

Plan area of reservoir contributing fluids to a particular well under stated conditions.

Drainage displacement

Increase of nonwetting-phase saturation as that phase enters a porous rock.

Field context Identify the wetting fluid before classifying displacement. Track saturation reversals when comparing capillary-pressure data; the worked example is conceptual, not a field calibration.

Worked example: saturation and displacement history

Drawworks

Rig winch that raises or lowers suspended equipment through the drilling line.

Drill bit

Cutting tool at the bottom of the assembly that breaks or shears rock.

Drill collar DC

Thick-walled tubular used to add stiffness and weight near the drill bit.

Drill pipe DP

Tubulars that convey drilling fluid and transmit rotation between surface equipment and the bottomhole assembly.

Drill-in fluid

Fluid designed for drilling the reservoir interval while limiting formation damage and supporting the intended completion.

Driller

Crew leader who operates and supervises the rig's principal drilling and hoisting systems during a shift.

Drilling break

Abrupt change in penetration rate that may reflect a formation, pressure, or drilling-condition change.

Drilling contractor

Company providing and operating a drilling rig and crew under contract to the well operator.

Drilling foaming agent

Additive that promotes formation and persistence of a gas-in-liquid foam used as the circulating drilling medium.

Drilling gooseneck

Curved rigid pressure conduit connecting sections of the rig's drilling-fluid supply path.

Drilling hook

High-capacity suspension device beneath the travelling block for supporting elevators, swivel, or other hoisting equipment.

Drilling mud

Circulating fluid used to transport cuttings, cool equipment and help manage pressure and hole stability.

Also called: Mud, Drilling fluid

Drilling starch

Plant-derived polymer used to limit fluid loss from water-based drilling muds.

Drilling-fluid contamination of samples

Presence of introduced fluid, additives or recirculated material in a geological sample. It can distort mineral, fluorescence, gas or chemical observations and should be documented.

Drillship

Ship equipped with drilling machinery and station-keeping systems for offshore well operations.

Drillstem test DST

A temporary formation test using a downhole test assembly to observe pressure and controlled fluid flow.

Also called: drill stem test

Drillstring

Connected drillpipe, bottomhole assembly, and associated tools extending from the surface drive to the bit.

Also called: Drill string

Drillstring rocking

Controlled back-and-forth surface rotation intended to reduce friction while retaining downhole steering orientation. The effective rocking depth and toolface response depend on the string and operating conditions.

Drillstring sub

Short drillstring component used for connection, spacing, or a specific mechanical function.

Also called: Sub

Drop ball

Ball conveyed down a tubular to engage a seat and enable operation of a downhole tool.

Drop rate DOR

Decrease in inclination per unit of measured depth. State whether a report uses a positive drop magnitude or a negative signed build rate; the two conventions convey the same tendency differently.

Also called: Drop-off rate

Dry natural gas

Processed natural gas after removal of recoverable hydrocarbon liquids and enough nonhydrocarbon material to meet its applicable sales specification.

Dry-steam geothermal plant

Plant generating electricity directly from naturally produced geothermal steam.

Dry-tree riser

Riser protecting production tubing between the seabed and an above-water tree.

Dual completion

A well arrangement that enables separately controlled production from two reservoir zones.

Dump bailer

Wireline or slickline tool that carries and releases a small quantity of cement or other material downhole.

Dynamic filtration

Filtration while fluid moves across the filter cake, allowing deposition and erosion to occur together.

Dynamic positioning DP

Computer-controlled use of vessel thrusters to maintain a desired offshore position and heading.

Dynamometer

Instrument recording rod-pump load versus displacement to evaluate pumping behavior.

Dynamometer card

Plot of pump-system load against displacement used to diagnose rod-pumping performance.

Also called: Dynagraph

East-west displacement E/W

Horizontal offset along the adopted east axis from the specified origin, commonly positive eastward. Keep the origin, units and north-reference convention with the value.

Echo spacing

Time interval between successive echoes in an NMR pulse sequence, affecting sensitivity to fast-relaxing components.

Effective permeability

Permeability available to one fluid phase when other immiscible phases also occupy the rock.

Effective porosity

Interconnected pore fraction available for fluid flow; petrophysical conventions commonly exclude clay-bound water, so the interpretation basis should be stated.

Field context State the measurement convention: dry-core connected porosity excludes isolated pores; a common log convention subtracts clay-bound water from total porosity. These are not interchangeable corrections. Retain sample preparation, method and bulk-volume basis with the result.

Worked example: porosity measurement conventions

Effective stress

Stress acting on the solid framework of a porous material. For a saturated soil under the Terzaghi convention, effective normal stress equals total normal stress minus pore-water pressure; compression is positive.

Field context When a falling water table reduces the weight of overlying sediment, total stress also changes. At a point that remains saturated, use Δσ′v = Δσv − Δu rather than treating the pressure decline alone as the effective-stress increase.

Worked example: falling water table and changing load

Also called: Intergranular stress

Electric submersible progressing cavity pump ESPCP

A progressing-cavity pump powered by a downhole electric motor rather than a surface-driven rod string.

Also called: electric submersible progressive cavity pump

Electric submersible pump ESP

A downhole electric motor drives a multistage centrifugal pump to lift produced fluids.

Also called: electrical submersible pump

Electrical anisotropy

Directional variation of electrical resistivity, commonly expressed as a difference between horizontal and vertical formation resistivity.

Field context For continuous horizontal isotropic layers in ideal DC experiments, resistivity parallel to bedding is the thickness-weighted harmonic mean; normal resistivity is the thickness-weighted arithmetic mean. Name the axes and scale. These equivalent material values are not automatically apparent resistivities measured by a surface array or logging tool, and do not determine hydraulic conductivity.

Worked example: electrical versus hydraulic anisotropy

Electrical stability test ES test

Oil-mud test measuring electrical breakdown voltage as an indicator of emulsion condition and oil wetting.

Electromagnetic skin effect

Attenuation and phase change of an electromagnetic field within conductive material, affecting induction-tool responses.

Electromagnetic telemetry EM telemetry

Transmission of encoded downhole information using electromagnetic signals through the surrounding earth. Its usable range and signal quality depend on the formation and installation conditions.

Elevators

Lifting equipment that grips a tubular so it can be raised or lowered.

Ellipse of uncertainty

Two-dimensional projection of a position-uncertainty ellipsoid onto a specified plane. Always identify the plane and confidence convention when comparing ellipses.

Also called: Error ellipse

Ellipsoid of uncertainty EOU

Three-dimensional uncertainty region around a reported well position, defined for a stated statistical model and confidence convention. It is not a physical hole envelope.

Emulsifier

Chemical that helps form and stabilize droplets of one immiscible liquid within another.

Emulsion

Dispersion of droplets of one liquid in another liquid with which it does not freely mix.

Energy-isolating device

Mechanical device that physically blocks energy transmission, such as a disconnect or isolating valve.

Engineering control

Physical design or equipment change that removes a hazard or separates people from it.

Enhanced geothermal system EGS

Engineered geothermal reservoir with improved fluid pathways for extracting subsurface heat.

Enhanced oil recovery EOR

Processes that change fluid or rock-fluid behavior to recover additional oil, including thermal, chemical, and miscible methods.

Also called: tertiary recovery

Eolian

Describes sediment transport and deposition by wind.

Also called: Aeolian

Equalizing valve

Valve that provides controlled communication across a pressure barrier to reduce the pressure difference.

Equivalent circulating density ECD

Effective fluid density representing static fluid head plus annular friction pressure at a specified depth during circulation.

Estimated ultimate recovery EUR

Total quantity expected to be economically produced over the life of a well, field, or reservoir.

Estuary

Partly enclosed coastal setting where river water mixes with seawater.

Ethane

Two-carbon saturated hydrocarbon, C2H6, recovered chiefly from natural gas.

Ethanol

Two-carbon alcohol, C2H5OH, used among other applications as a gasoline blending component.

Ethylene

Two-carbon olefin, C2H4, widely used as a petrochemical feedstock.

Evaporite

Mineral deposit or rock produced by precipitation as saline water concentrates, commonly containing halite, gypsum or anhydrite.

Expansion joint

Completion component that accommodates relative movement caused by thermal expansion or contraction.

Expendable gun

Perforating assembly designed to disintegrate when fired, reducing the size of components that must be retrieved.

Export riser

Riser carrying production from an offshore facility down to a seabed pipeline.

Extended-reach drilling ERD

Drilling that pushes substantial reach or construction-complexity limits, often with long horizontal displacement relative to vertical depth.

External upset

Thickened outer end section of a tubular that provides extra material for a stronger connection.

Extraction loss

Gas-volume reduction associated with removing liquid hydrocarbon constituents during natural gas processing.

Facies

A rock body or sediment distinguished by a characteristic combination of composition, texture, structures or fossils.

Fault

Fracture or fracture zone across which rock bodies have moved relative to one another.

Field guide: fault core, damage zone and slickenlines

Feldspar

Group of common silicate minerals present in many sandstones and crystalline rocks; alteration can produce clay minerals.

Field context In ideal plagioclase, replacing Na+ with Ca2+ is balanced by replacing Si4+ with Al3+: both paired contributions total +5. An30 denotes 30 mole percent anorthite component, not 30 percent calcium by mass.

Worked example: plagioclase charge balance

Fence diagram

Perspective display of intersecting geological cross sections used to show three-dimensional relationships among rock units.

Field instrumentation

Installed sensors and switches measuring process conditions such as pressure, temperature, and flow.

Field-level risk assessment FLRA

On-site review of task hazards and controls as work conditions change.

Fieldbus

Digital communication network connecting field instruments and control devices.

Filter cake

Deposit of retained solids that develops where fluid filters through a permeable surface, such as the borehole wall.

Also called: Mud cake, Wall cake

Filter press

Pressurized test cell used to measure filtrate loss and examine the resulting mud filter cake.

Filtrate

Liquid phase that passes through a filter medium or deposited filter cake during filtration.

Filtration

Separation in which liquid passes through a porous medium while suspended solids are retained.

Fines migration

Movement of small mineral particles through reservoir pores, potentially plugging narrower passages and reducing flow.

Fingerboard

Slotted structure that organizes and restrains the upper ends of pipe stands racked in the derrick.

Firing head

Downhole initiation device used with a perforating system, commonly in tubing-conveyed applications.

First-contact miscibility FCM

Condition in which injected fluid and oil form one phase at any mixing ratio on initial contact.

Fish

Equipment or material left in a well that is the target of a recovery operation.

Fishing

Operations to recover equipment or material left unintentionally in the well, using tools suited to the object and hole conditions.

Fishing neck

Designed engagement profile on downhole equipment for a compatible running or retrieval tool.

Fishing tool

Downhole equipment designed to engage, retrieve or help free an object from a well.

Five-spot pattern

Pattern with a central producer and four surrounding injection wells at square corners.

Flame ionization detector FID

Instrument that measures an electrical response from hydrocarbons burning in a hydrogen flame; used for total-gas detection.

Flare

Burner system that disposes of combustible gas by controlled burning.

Flash point

Lowest test temperature at which a liquid produces enough vapor to ignite briefly with an ignition source.

Flash steam

Vapor formed when hot water experiences a pressure reduction.

Flat gels

Mud gel strengths that change little between shorter and longer static test intervals.

Flexicoking

Proprietary fluidized-bed residue conversion combining thermal cracking with gasification of most produced coke into fuel gas.

Float collar

Casing component containing a check valve and a landing surface for cementing plugs.

Float shoe

Rounded casing-end component that guides the string and contains a check valve limiting reverse flow.

Flocculant

Chemical that encourages suspended particles to form clusters, often helping their separation from water.

Flocculation

Association of fine suspended particles into loosely connected clusters that can change fluid rheology.

Floorhand

Rig-crew member who handles tubular connections and supports rig-floor equipment operation and maintenance.

Also called: Roughneck, Floor hand

Flow assurance

Engineering discipline addressing conditions that hinder reliable transport of produced fluids from reservoir to delivery.

Flow check

Observation of well flow or fluid-level behaviour to assess whether conditions are stable.

Flow coupling

Heavy-wall tubing section installed near flow disturbances to provide additional allowance for internal erosion.

Flowline sample

Sample of returning drilling fluid collected before solids removal, potentially containing drilled cuttings. Record its collection point because screening changes the solids represented.

Fluid breakthrough

Arrival of previously separated or injected water or gas at a producing well.

Fluid coking

Continuous thermal conversion of heavy oil using fluidized coke particles.

Fluid holdup

The fraction of the local wellbore volume occupied by a particular fluid phase; it need not equal that phase’s flow-rate fraction.

Also called: phase holdup

Fluid pound

Abrupt load change when a rod-pump plunger encounters liquid in a partly filled pump barrel.

Fluid saturation

Fraction of a defined pore volume occupied by a particular fluid.

Fluid-loss control

Management of filtrate movement through a filter cake using fluid chemistry, additives, and cake properties.

Fluorescence

Visible light emitted under ultraviolet illumination; hydrocarbon interpretation must consider mineral and drilling-fluid fluorescence.

Flushed zone

Near-wellbore region where mud filtrate has displaced the movable original fluids, while residual hydrocarbons may remain.

Fluvial

Describes sediment transport, deposition or landforms associated with rivers and streams.

Foam flooding

Use of foam to restrict preferential gas flow and improve the sweep of an injected fluid.

Foamed cement

Lightweight cement containing stabilized gas bubbles, commonly nitrogen, to reduce slurry density.

Fold

Bend in originally straighter rock layers produced by deformation.

Field context A hinge line, an intersection lineation and a stretching lineation are not interchangeable. Compare orientations and fabric relationships before using any lineation as a fold-axis proxy.

Worked example: separate lineation mechanisms

Formate brine

Aqueous fluid weighted with formate salts, such as sodium, potassium, or cesium formate, for drilling or completion service.

Formation

Primary lithostratigraphic unit: a distinguishable, mappable body of rock defined by lithological characteristics.

Field context A lithological boundary is not automatically a time surface. Track the rock pick and its age interpretation separately.

Worked example: rock boundary versus time surface

Formation damage

Reduction in near-wellbore flow capacity caused by physical or chemical changes during operations.

Formation dip

Inclination of a geological surface from horizontal, specified together with the direction of maximum downward slope.

Field context For a planar bed, maximum true dip is measured perpendicular to strike. A single apparent dip from one section does not determine the full three-dimensional attitude. True dip is the maximum inclination below horizontal. It is measured perpendicular to strike; an oblique section generally shows a smaller angle.

Worked example: strike and apparent dip

Also called: Dip, True dip

Formation factor F

Ratio of the resistivity of a fully water-saturated rock to the resistivity of its pore water.

Formic acid

Organic acid used in selected high-temperature stimulation treatments or where extended contact with tubulars is expected.

Forward modelling

Calculation of the measurements a sensor should produce for a specified subsurface model, tool configuration and environment.

Field context The predicted result belongs to a specified sensor, geological model and environment. A generic curve-shape match is not automatically a physical electromagnetic forward model.

Also called: Forward modeling

Forward whirl

Lateral orbital motion in the same sense as drillstring rotation. Imbalance and borehole contact can contribute to the motion and uneven wear.

Fossil

Preserved remains or evidence of past organisms, such as shells, casts, tracks or burrows.

Field context Record preservation and host sediment separately; an older fossil can be redeposited into a younger bed.

Fossil preservation and reworking: worked interpretation

Fossil disarticulation

Separation of once-connected skeletal elements, such as bones or shell valves. It describes preservation and does not by itself establish transport distance.

Fossil preservation and reworking: worked interpretation

Also called: Disarticulated remains

FPSO turret

Rotating vessel interface permitting weathervaning around a fixed mooring connection.

Fractionation

Separation of a hydrocarbon mixture into product fractions, such as ethane, propane, and butanes.

Field context This entry concerns product separation in processing. Isotope fractionation describes redistribution of isotopes and is a separate concept.

Fracture closure pressure

The pressure associated with induced fracture faces closing as fluid pressure declines; interpretation reflects the local stress conditions.

Fracture conductivity

The product of fracture permeability and width, expressing its ability to transmit fluid along the fracture.

Fracture half-length

Distance from the wellbore to one outer tip of a hydraulic fracture.

Fracture porosity

Pore volume contained in natural fractures; even small fracture volumes can strongly affect permeability.

Fracturing fluid

The treatment fluid that transmits pressure into the formation and, when applicable, transports proppant.

Also called: frac fluid

Free fluid in rock

Pore fluid capable of moving under the reservoir conditions considered; NMR estimates use relaxation-time interpretation and calibration.

Free point

Depth marking the transition between freely responding tubulars and the section restrained by a sticking mechanism.

Free water in cement

Water that separates from a cement slurry rather than remaining uniformly incorporated, potentially leaving unwanted channels.

Also called: cement free water

Free-point indicator

Tool that measures tubular response to applied loading to help locate the transition between free and stuck pipe.

Fresh feed

Material entering a particular refinery process unit for its first pass, excluding recycled feed.

Funnel viscosity

Measured drainage time through a Marsh funnel, used for quick comparative assessment rather than a true viscosity value.

Gamma ray log GR

Record of natural formation radioactivity used in correlation and lithology interpretation; low values alone do not prove reservoir quality.

Also called: Gamma-ray log

Gas anchor

Downhole intake assembly that helps separate free gas before liquid enters a rod pump.

Also called: Bottomhole gas separator

Gas cap

Accumulation of free gas above an oil column within the same reservoir.

Gas chromatography GC

Separation and measurement of individual gas components, allowing composition to be evaluated alongside total-gas readings.

Gas component ratio

Ratio calculated from measured gas components, commonly used to compare changes in gas character. Sampling, detector response and drilling-fluid contamination can affect its interpretation.

Gas coning

Downward distortion of a gas-oil contact toward a producing interval due to drawdown.

Gas deviation factor Z-factor

Dimensionless correction z in the real-gas equation that measures departure from ideal-gas behavior.

Also called: gas compressibility factor

Gas drive

Reservoir drive in which expanding free or dissolved gas provides energy for oil production.

Also called: gasdrive

Gas formation volume factor Bg

Volume occupied by a quantity of gas at reservoir conditions divided by its volume at standard conditions.

Also called: gas FVF

Gas hydrate

An ice-like crystalline solid in which water structures trap gas molecules under suitable pressure and temperature conditions.

Also called: natural gas hydrate

Gas injection

Introduction of gas into a reservoir to support pressure or improve hydrocarbon displacement.

Gas lift

Injected gas lowers the effective weight of the fluid column, helping reservoir fluids rise through the well.

Gas lock

Loss of effective pumping when free gas interferes with liquid intake or valve operation.

Gas show

Increase in detected mud gas relative to the surrounding trend, evaluated at lagged depth and against drilling conditions.

Gas sweetening

Removing unwanted acid-gas constituents such as hydrogen sulfide and carbon dioxide from a natural-gas stream.

Also called: natural gas sweetening

Gas-cap drive

Oil production supported by expansion of an existing free-gas cap.

Gas-cut mud

Drilling fluid containing gas bubbles that lower its apparent unpressurized bulk density.

Also called: Air-cut mud

Gas-lift mandrel

Tubing-string component supporting a gas-lift valve and providing its annulus-to-tubing flow passage.

Gas-lift valve

Downhole valve controlling admission of lift gas from the annulus into production tubing.

Gasification

Conversion of carbon-containing feedstock into a combustible gas for power, fuels, or chemical manufacture.