Oil and Gas Glossary

cake thickness

The thickness of the mud cake.

calcite

See calcium carbonate.

calcium carbonate

A chemical combination of calcium, carbon, and oxygen. the main constituent of limestone. It forms a tenacious scale in water-handling facilities and is a cause of water hardness. Chemical symbol is CaCO3.

calcium sulphate

A chemical combination of calcium, sulphur, and oxygen. It sometimes contaminates drilling fluids but may be added to provide certain properties. Like calcium carbonate, calcium sulphate (CaSO4) forms a hard-to-remove scale in some water-handling facilities

calibration

The process wherein the zero and sensitivity of the measuring circuit is adjusted to meaningful units so that the recorded measurements will be accurate with respect to an industry standard.

caliper log

A well log which is a record of hole diameter. Hole caliper logging tools sometimes have 1, 2, 3, 4 or 6 arms. Some caliper logging tools use acoustic methods of determining hole dimensions.

cap rock

Cap rock or seal. Impermeable rock overlying an oil or gas reservoir that tends to prevent the upward migration of fluids form the reservoir.

capture cross section

The nuclear capture cross section for neutrons is the effective area within which a neutron passes in order to be captured by an atomic nucleus. It is a probabilistic value dependent on the nature and energy of the particle as well as the nature of the capturing nucleus. Nuclear capture cross section is often measured in barns (1 barn = 10–24 cm2).

carbonate

A compound containing the radical CO3+2 or rock composed of carbonate minerals. See carbonate rock.

carbonate rock

A sedimentary rock primarily composed of calcium carbonate (limestone) or calcium magnesium carbonate (dolomite).

carbon-oxygen log

A log which presents a measure of the relative abundance of carbon to oxygen derived from the detection of the gamma rays produced from both elements by the inelastic scattering of 14-Mev neutrons. The gamma rays are measured within energy spectrum windows representing the gamma-ray escape peaks of carbon and oxygen. The ratio of counting rates provides a means of predicting the relative amounts of hydrocarbons and water. The log is an alternate means for detecting hydrocarbons (particularly oil) behind casing in formations not subject to flushing or reinvasion by borehole fluids. The C/O ratio is relatively independent of formation water salinity and shaliness. In order to differentiate carbon in hydrocarbon molecules from that in the rock framework (i.e., carbonate solid matter), a Si/Ca ratio is also determined.

casing

Steel pipe cemented in place in an oil or gas well as the drilling progresses or when the well is completed. The casing serves several purposes:

casing collar locator

Used to locate casing collars and other features of downhole hardware (e.g., packers, etc.) which often serve as reference depths in subsequent completion operations.

casing collar log

A record of casing collar responses with depth as measured by a casing collar locator. Usually is an integral part of all well logs run in the cased borehole. The casing collar log provides a means for depth control for other measurements and responses which cannot in themselves be accurately correlated with the formation behind casing.

casinghead

The top of the first string of casing set in a well. Usually a heavy, flanged steel fitting which provides a means to attach various fittings or assemblies.

casing inspection log

Uses a method of relating the effects of eddy currents on a magnetic field to casing wall thickness. The instrument consists of two radial coils – an exciter and a pickup coil. The exciter coil is fed from an AC voltage source at the surface, in turn producing a magnetic field downhole. This field sets up eddy currents in the casing wall. These currents cause the magnetic field to be attenuated and shifted in phase. The resulting magnetic field is detected by the pickup coil and transmitted to the surface. The magnetic field as detected by the pickup coil is then compared with the original field generated by the exciter coil, and the resulting phase shift in the magnetic field that has occurred is recorded. The phase shift can indicate casing wall thickness, splits, and holes.

casing point

The depth in a well at which casing is set, generally the depth at which the casing shoe rests.

casing pressure

Annular pressure between the casing and the well's tubing.

casing shoe

The bottom of the casing string, including the cement around it, or the equipment run at the bottom of the casing string.

casing string

Casing is manufactured in lengths of about 30 ft, each length or joint being joined to another as casing is run in a well. The entire length of the joints of casing is called the casing string.

cation-exchange capacity

Also called base-exchange capacity. A measure of the extent to which a substance will supply exchange cations.

catwalk

A raised, narrow walkway extending from the front of a drilling derrick or mast which serves as a staging area or work area from which equipment is raised to the derrick.

cement

(1) A powder consisting of alumina, silica, lime, and other substances which hardens when mixed with water. Extensively used in the oil industry to bond casing to the walls of the wellbore.

cement evaluation log

CET. A cased hole cement evaluation log that displays data processed from ultrasonic transducers in such a way that channels in the cement sheath can be detected. The quality of the cement is given in eight radial segments, and the orientation of a channel can be determined from a recording of the well deviation and the relative bearing of the first transducer. An acoustic caliper measurement is provided from eight radii measurements. Compare cement bond log. CET is a Schlumberger mark.

cementation

The process of precipitation or growth of a binding material around grains or fragments of rocks. In clastic rocks, cementation is necessary before the rock framework can become consolidated. Chemically the cementing material may be the same as, or different from, the host rock.

cementation factor

The porosity exponent (m) in Archie's formation resistivity factor-porosity relationship.

cement bond

The adherence of casing to cement and cement to formation. When casing is run in a well, it is set, or bonded, to the formation by means of cement.

centralizer

A device which positions the logging tool in the centre or near centre of the well bore. aligned with the wellbore axis. Compare decentralize, eccentering arm and standoff.

check shot survey

A check shot survey determines formation seismic wave velocities over specified depth intervals. Travel times from a surface energy source to a downhole geophone located at different depths are measured. Distance between successive geophone locations, d. divided by the difference in travel time to the geophones at the successive locations, Dt, is the average formation velocity between the geophone locations; v = d/Dt

chemical treatment

Any of many processes in the oil industry that involve the use of a chemical to affect an operation. Some chemical treatments are: Acidizing, crude-oil demulsification, corrosion inhibition. paraffin removal, scale removal, drilling-fluid control, refinery and plant processes, cleaning and pumping operations, waterflood injection, and water purification.

choke

An orifice with a small-diameter bore, installed at the wellhead in a line to restrict and control the rate of production.

chromatogram

The recorder chart response to an analysis of a gas air mixture. A chromatogram records separated hydrocarbons in order of molecular size.

circulate

To pass from one point throughout a system and back to the starting point. Drilling fluid circulates from the suction pit through the drill pipe to the bottom of the well and returns through the annulus between pipe and formation.

circulate out

The movement of drilling fluid from the bottom of the hole to the surface (without drilling) for one or more of the following reasons: to reduce levels of produced and/or liberated gas from the drilling fluid, to obtain a rock cuttings sample from a formation just penetrated, to check for high or excessive levels of hydrocarbons from formations just penetrated by drilling.

clastic

Rocks built up of fragments of pre-existing rocks which have been produced by the process of weathering and erosion and in general transported to a point of deposition.

clay minerals

Fine, crystalline, hydrous silicates which form as a result of the weathering and decomposition of such silicate minerals as feldspar, pyroxene, and amphibole.

clean

Containing no appreciable amount of clay or shale. Applied to sandstones and carbonates.

collar

(1) A coupling device with internal threads used to join two pieces of threaded pipe of the same size.

compaction correction

An empirical correction applied to porosity derived from the sonic log in uncompacted formations such as sands at shallow depths or formations under abnormally high pressures.

compensated formation density log

A dual-spacing formation density log. The compensated formation density logging device employs two detectors spaced at different distances from the source. The detector at the shorter spacing is particularly sensitive to the density of material immediately adjacent to the face of the pad. The contribution of this material. which includes mud cake and drilling mud filling minor borehole wall irregularities, affects the response of each detector to a different degree. The measurements from both detectors provide a means for making a correction for the influence of drilling mud and mud cake on the measurements. This correction is automatically added to the uncompensated density measurement from the detector at the longer spacing. Through the use of appropriate instrumentation, the parameters recorded are: a corrected or compensated value of bulk density, a measure of the correction, Dr, used in making the compensation, and a caliper curve. With the unwanted borehole effects removed, the measurement is recorded directly in terms of bulk density on a linear scale.

compensated neutron log

A well log made with a mandrel type neutron logging tool having two neutron detectors. The neutron porosity is derived from the ratio of the counting rates of the two detectors. Use of the count-rate ratio greatly minimizes borehole effects. This tool can be run in liquid-filled holes, both cased and uncased, but is not usually recommended for use in gas-filled holes. See dual spaced neutron.

complete a well

To finish work on a well by making it ready to produce oil or gas. After reaching total depth (TD) casing is run and cemented, casing is perforated opposite the producing zone, tubing is run and control and flow valves are installed at the wellhead. Well completions vary according to the kind of well, depth and the formation from which it is to produce.

completion fluid

A special drilling mud used when a well is being completed. It is selected not only for its ability to control formation pressure, but also for its properties that minimize formation damage.

composite log

Several well logs of the same or similar types, usually from different logging runs, which have been spliced together to form a single continuous record from the shallowest to the deepest log reading. Composite logs are valuable for correlation and documentation purposes.

compression wave, compressional wave

P-wave, longitudinal wave. An acoustic wave propagated parallel to the direction of particle displacement. Substances which tend to resist compression support the propagation to compression waves (e.g., liquids and solids). Compare shear wave.

compressive strength

The degree of resistance of a material to a force acting along one of its axes in a manner tending to collapse it. Usually expressed in pounds of pressure per square inch (psi) of surface affected.

condensate

A low-density, high-API gravity liquid hydrocarbon phase that generally occurs in association with natural gas. Its presence as a liquid phase depends on temperature and pressure conditions in the reservoir allowing condensation of liquid from vapour. The production of condensate reservoirs can be complicated because of the pressure sensitivity of some condensates: During production, there is a risk of the condensate changing from gas to liquid if the reservoir pressure drops below the dew point during production. Reservoir pressure can be maintained by fluid injection if gas production is preferable to liquid production. Gas produced in association with condensate is called wet gas. The API gravity of condensate is typically 50 degrees to 120 degrees

conductivity

The ability of a material to conduct electricity. It is the inverse of resistivity and is measured in Siemens per meter (S/m) or mho/m. The conductivity is a property of the material, whereas the conductance also depends on the volume measured. The two are related by a system constant, which in simple cases is the length between the measurement electrodes divided by the area. In the most general case, the conductivity is the current density divided by the electric field and depends on the frequency of the applied signal.

conductivity derived porosity

A calculation of the water filled porosity from electrical conductivity or resistivity.

conductor pipe

The casing string that is usually put into the well first, particularly on land wells, to prevent the sides of the hole from caving into the wellbore. This casing, sometimes called drive pipe, is generally a short length and is sometimes driven into the ground. Conductor pipe is run because the shallow section of most wells onshore is drilled in unconsolidated sediment or soil rather than consolidated strata typically encountered deeper. Offshore, the drive pipe or structural casing may be installed prior to the conductor for similar reasons.

conglomerate

A sedimentary rock composed of coarse-grained rock fragments, pebbles, or cobbles cemented together in a fine-grained matrix. A cemented gravel.

coning

The change in oil-water or gas-oil contact profiles as a result of drawdown pressures during production. Coning occurs in vertical or slightly deviated wells and is affected by the characteristics of the fluids involved and the ratio of horizontal to vertical permeability.

connate water

Water trapped in the pores of a rock during formation of the rock. The chemistry of connate water can change in composition throughout the history of the rock. Connate water can be dense and saline compared with seawater. Formation water, or interstitial water, in contrast, is simply water found in the pore spaces of a rock, and might not have been present when the rock was formed. Connate water is also described as fossil water.

continuous phase

The liquid in which solids are suspended or droplets of another liquid are dispersed. Sometimes called the external phase. In a water-in-oil emulsion, oil is the continuous phase. Compare internal phase.

contour map

A map that has lines marked to indicate points or areas that are the same elevation above or below sea level. often used to depict subsurface features.

core

(1) A cylindrical sample of rock taken from a formation for analysis. Usually, a conventional core barrel is substituted for the bit and obtains the sample as it penetrates the formation.

core analysis

A laboratory analysis of recovered formation samples for the purpose of measuring porosity, directional permeability, residual fluid saturations, grain size, density, and other properties of the rock and contained fluids. Core analysis aids formation evaluation, reservoir development, and reservoir engineering.

core barrel

(1) A hollow projectile used for obtaining sidewall cores. See sidewall core and sidewall coring tool.

core bit

A special drill bit for cutting and removing a plug-shaped rock sample from the bottom of the well bore. The core bit is attached to the bottom of the drill pipe core barrel.

core gamma

A measurement of the intensity of natural gamma radiation of a core. Its primary use is as a correlation aid with borehole gamma-ray log over the cored interval.

correlation

(1) The equivalence in stratigraphic positions of formations in different wells. Similarities in the character of well-logging responses and the occurrence of distinctive features which serve as markers from one well to the next are used.

critical saturation

The value of saturation of the specific liquid (or gas) phase at which the liquid (or gas) will first begin to flow as the saturation is increased. The ability to flow is related to the continuity of the phase. A discontinuous phase will not flow under normal producing conditions.

cross flow

A condition that exists when two production zones with dissimilar pressure characteristics are allowed to communicate during production. Reservoir fluid from the high-pressure zone will flow preferentially to the low-pressure zone rather than up the production conduit unless the production parameters are closely controlled.

cross plot

A plot of one parameter versus another.

cross section

A diagram of a vertical section through a volume, as opposed to the surface, "bird's eye," or plan view of a map. Cross sections are useful for displaying the types and orientations of subsurface structures and formations.

crown block

A stationary pulley system located at the top of the derrick used for raising and lowering the string of drilling tools. The sheaves and supporting members to which the lines of the travelling block and hook are attached.

crude oil

Unrefined liquid petroleum. It ranges in gravity from 9 to 55 °API and in colour from yellow to black, and it may have a paraffin, asphalt or mixed base. If a crude oil, or crude, contains a sizable amount of sulphur or sulphur compounds, it is called sour crude; if it has little or no sulphur, it is a sweet crude. In addition, crude oils may be referred to as heavy or light according to API gravity, the lighter oils having the higher gravities.

crystalline

Having regular molecular structure; contrasted with amorphous.

cushion

A fluid column (usually water or nitrogen) put in the drillstem to provide the desired backpressure at the start of a drillstem test. The cushion usually serves to limit the differential pressure across the test string and packer to avoid flow below the bubblepoint pressure (in which case water is the usual cushion) or to enable a depleted reservoir to flow (nitrogen is the likely cushion).

cut-and-thread fishing technique

Strip-over technique. A method of fishing for stuck logging tools which consists of supporting the survey cable at the rig table, cutting the survey cable, and threading the cable through the drill pipe until the overshot attached to the end of the drill pipe engages the fish. The survey cable is then broken free at the weak point and retrieved from the hole. The fish is withdrawn from the well with the drill pipe.

cutoff

Parameter cut-off. Upper or lower limiting value of a reservoir-size parameter. Values outside these limits are not used in computation or other data-handling manipulation.

cuttings

Fragments of rock which are a result of the cutting action of the drill bit on the formation. These cuttings are transported to the surface by the drilling fluid.