Glossary

cable	See survey cable.

cable clamp	A rope socket. Also see weak point.

cable electrode	An electrode mounted on a flexible insulating sheath for the purpose of providing remote current or voltage measurement contact with the borehole fluid. This extension is fastened intermediately between the logging cable and the rigid logging tool with appropriate connectors at each end. See bridle.

cable hanger	A device used at the rotary table to support the weight of the survey cable (usually during fishing operations). The device clamps firmly on the cable and supports cable weight on a cross-bar which rests on the rotary table. This removes all tension from the survey cable between the rotary table and hoist unit.

cable head	A quick-connecting and quick-disconnecting device, mounted at the end of the logging cable, which provides the means to couple a cable electrically and mechanically to a logging tool. See also fishing bell.

cable tool drilling	A drilling method in which the hole is drilled by dropping a sharply pointed bit on the bottom of the hole. The bit is attached to a cable and the cable is picked up and dropped, repeatedly, to deepen or drill the hole.

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

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.

calibration loop	See test loop.

calibration source	See test pill.

calibration tails	Calibration test records documenting calibration procedures run before and/or after a survey and attached to the logs. Sometimes the calibration tail includes a copy of the test film.

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.

calorie	The amount of heat energy necessary to raise the temperature of 1 gram of water 1°C. Compare British thermal unit.

CAL Systems	Computer Analyzed Log Systems. A computed log analysis performed at a computer center. CAL Systems is a Welex trademark.

camera	(1) recorder. An instrument which records traces of light which have been beamed on film by galvanometers responsive to logging tool measurements. Some cameras may use laser technology or fiber optics. (2) borehole camera. A downhole instrument which photographs the interior of the borehole or casing.

capacitor	An electrical device that, when connected in the line of an electrical circuit, stores a charge of electricity and returns the charge to the line when certain electrical conditions occur. It also is called a condenser.

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

capture cross section	(1) 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). (2) "Macroscopic capture cross section" S is the effective cross-sectional area per unit volume of material for capture of neutrons; hence, it depends on the number of atoms pre sent as well as their nuclear capture cross sections. Thus, the macroscopic capture cross section is the sum of the various weighted capture cross sections. The unit of measure for S is cm²/cm³ or reciprocal cm (cm^–1) is often measured in "capture units" or "sigma units." A c.u. = 10^–3 cm^–1. (3) The rate of absorption of thermal neutrons with a velocity v is vS. (4) Of gamma rays. See photoelectric absorption, photoelectric effect, and photoelectric absorption cross section index.

capture gamma ray	A gamma-ray photon produced upon the capture of a thermal neutron by a neutron absorber such as chlorine. Gamma rays of capture are important in some neutron logging methods (n-g) and in pulsed neutron logging methods such as the Thermal Decay Time Log or Neutron Lifetime Log.

capture unit	c.u., 10^–3 cm^–1. A unit of measure of macroscopic capture cross section. Same as sigma unit.

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

carbonate analysis log	A log which presents computed porosity in complex lithology and sometimes includes secondary porosity and movable oil plots.

carbonate reef	See reef.

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. The carbon-oxygen log can be put to nearly the same uses as the pulsed neutron capture logs, but has proved to be useful under some conditions where pulsed neutron capture logs have shown decreased effectiveness; e.g., in rocks where formation water cannot easily be distinguished from oil because of the lack of sufficient contrast in their neutron capture cross sections. Carbon/Oxygen Log is a Dresser Atlas trademark.

cartridge	A package which contains electronic modules or hard ware for the downhole instrument. The package is carried in a protective housing and is attached to, or becomes a part of, the downhole instrument (logging sonde).

cased hole	A wellbore in which casing has been run.

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: (1) to prevent the formation walls from caving or squeezing into the hole during drilling; (2) to protect one formation from damage when heavy mud weights are required in drilling another; (3) to provide a means to isolate porous and permeable formations from one another at the wellbore; (4) to prevent contamination of shallow aquifers by drilling muds. See also casing string.

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. (1) a magnetic casing collar locator. Involves a system of two opposed permanent magnets in two similar magnetic circuits which produce characteristic magnetic fields in which flux lines pass through casing or tubing. A deformation of either of the magnetic fields, caused by the gap between casing joints, packers, sometimes holes, etc. is detected by a winding having a core of high permeability The resulting electromagnetic imbalance, first in one direction and then in another, is telemetered to the surface where it is recorded, depth correlated, as a feature of the downhole hardware arrangement. (2) a mechanical collar locator. May involve feelers or fingers which produce signals sent to the surface when the feelers cross pipe connections or other irregular features inside casing or tubing.

casing collar log	A record of casing collar responses with depth as measured by a casing collar locator. Usually is an intergral 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 potential profile	Detects corrosion by measuring the electrical potential of the casing at various levels to detect current entering or leaving the casing. The amount of cathodic protection needed can be determined and results monitored.

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

casing shoe	A short, heavy, hollow, cylindrical steel section with a rounded bottom that is placed on the end of the casing string to serve as a reinforcing shoe and to aid in cutting off minor projections from the borehole wall as the casing is being lowered. Also called a guide shoe.

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. See surface casing, protection casing, and production casing.

cathead	A spool-shaped attachment on a winch around which rope is wound for hoisting and pulling. The breakout cathead, a rotating spool located on the driller's side of the drawworks, is used as a power source for unscrewing drill pipe. The makeup cathead is a power source for screwing together joints of pipe.

cation	A positively charged ion.

cation-exchange capacity	Also called base-exchange capacity. A measure of the extent to which a substance will supply exchange cations. Compensating cations serve to compensate the excess of (usually negative) charge in clay lattices. The cation exchange capacity is related to the concentration of compensating cations (or counter ions) near clay-layer surfaces, which in the presence of water, can be exchanged for other cations available in solution. The cation-exchange capacity of a clay is expressed in terms of milliequivalents of exchangeable ions per 100 grams of dry clay. See also ion exchange.

cationic membrane	A membrane which permits the passage of cations but not of anions. Clay acts as such a membrane, allowing sodium ions to pass but not chloride ions. Important in generating the electrochemical potential.

cat line	A hoisting or pulling rope operated by the driller at a cathead. On a drilling rig, the cat line is the rope used for lifting well logging tools from the catwalk to the drilling floor and for exerting a pull on pipe tongs in tightening (making up) or loosening (breaking out) joints of pipe.

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. (2) A secondary mineral growth of chemically precipitated material on the surfaces and in the interstices of clastic sediments. Authigenic clay minerals are not considered cement. (3) To fix the casing firmly in the hole with cement, which is pumped through the drill pipe to the bottom of the casing and up into the annular space between the casing and the walls of the borehole. After the cement sets (hardens) it is drilled out of the casing. The casing can be perforated to allow reservoir fluids to enter the well.

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.

cement bond log	Used to determine the presence of cement behind casing and the quality of cement bond to casing or formation wall. Usually an acoustic log. The cement bond log is a continuous measurement of the amplitudes of acoustic pulses after they have traveled a length of the casing. The amplitude of a pulse is strong after traveling along unsupported pipe because there is nothing to restrict the vibration of the casing. On the other hand, the vibration of the casing is dampened by the cement sheath in well-cemented pipe, and amplitude is weak. If the formation bond is poor, acoustic energy traveling through the formation is weak; if both the casing and formation are well bonded, only the formation signal is strong. The log may consist of (1) an amplitude log which represents the amplitude of a portion of the acoustic wave train, or (2) a display of the acoustic wave train in the amplitude-time (wave train, x-y) mode or the intensity modulated-time (variable density, x-z) mode.

cement casing	To fill the annulus between the casing and formation with cement to support the casing, improve zonal isolation, or to prevent migration of fluids between permeable zones.

cementing	The application of a liquid slurry of cement andwater to various points inside or outside the casing. See cement casing.

centigrade (Celsius) scale	The metric system of temperature measurement used universally by scientists. In the centigrade scale, 0° represents the freezing point of water and 100° its boiling point at 760-mm barometric pressure. Degrees centigrade are converted to degrees Fahrenheit by the following equation: °F = 9/5 (°C) + 32.

centipoise	cp. One-hundredth of a poise.

centralized	Positioned in the center of the hole by use of centering devices.

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

chalcedony	A transparent, or more generally, a translucent cryptocrystalline quartz; the material of agate.

channel	(l) A defect in cement quality which prevents.zone isolation, usually in the form of void space in the annulus behind cemented casing. The channel constitutes a conduit for fluid flow between a completed interval and other permeable strata. (2) A course or perceptible depression where surface water has traveled. (3) In a pulse height analyzer, an energy gate in which only pulses occurring within a specific energy range are registered. The difference between the upper and lower limits is the width of the channel. The detail of the spectrum thus produced is related to the width and number of channels in the instrument. (4) A path along which digital or other information may flow in a computer. (5) An allocated frequency or time segment in a data multiplexing system; one of a stream of data bands. (6) The position in a frame or sequence of data on magnetic tape that represents a specific measurement.

character log	A display of the acoustic wave train. See X-Y plane or amplitude-time. Compare intensity modulated-time.

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 Source and receiver positions can be switched; i.e., small energy sources can be used in the borehole with a fixed geophone at the surface. Interval velocities are needed for interpreting seismic reflection profiles.

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.

chert	Cryptocrystalline varieties of quartz regardless of color.

chlorine log	A log based on the counting rate of capture gamma rays produced by capture of thermal neutrons by chlorine in the formation. By limiting the count to a certain energy range, the tool is made more sensitive to chlorine and relatively less sensitive to formation porosity. The interpretation of such curves yields a calculated water saturation. The chlorine log's primary application is in cased holes.

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

Christmas tree	An assembly of valves mounted on the casinghead through which a well is produced. The Christmas tree also contains valves for testing the well and for shutting it in if necessary.

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

chromatograph	See partition gas chromatograph.

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.

circulation	The forced flow of drilling fluid out of the mud pit into a drill hole, down the drill pipe or tubing and up the annulus to return to the surface.

clastic	Sedimentary rock formed from mineral fragments which have been moved individually from their places of origin.

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. Compare dirty.

collar	(1) A coupling device with internal threads used to join two pieces of threaded pipe of the same size. (2) Drill collar. A thick walled steel pipe used to stabilize the bottom-hole drilling assembly. The drill collars are used to provide the weight required to drill the hole. The collars are usually under compression while the drill pipe is always under tension.

collar locator	See casing collar locator.

collar log	See casing collar log.

collimated gamma radiation	Gamma radiation in which the stream of photons is beamed in a single direction.

combination logging tool	A single assembly of logging tool components, often joined in tandem, capable of performing more than one general type of logging service with a single trip into the well bore.

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. In applying the empirical correction to the time-average relationship for obtaining apparent porosities from interval transit times, the relationship becomes: where C_p is the compaction-correction coefficient; other symbols as described in time-average relationship. In consolidated formations C_p = 1 (corresponding to no correction). In unconsolidated formations (which generally occur at shallow depths) C_p has values greater than 1.

compatible scales	The interpretation of well logs often requires a direct comparison of one logging response with another, performed at the same time or at a different time. In order to facilitate this comparison. the same grid type and equal scale sensitivities must be used. Overlay techniques particularly require the use of compatible scales.

compensated density log	See compensated formation density log.

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. See also density log, spine-and-ribs plot, and Z/A effect.

compensated log	A well log made with a tool designed to correct for unwanted effects associated with the borehole. The compensated density log uses the signal from a secondary detector to correct for the effect of mud cake and small irregularities in the borehole wall. The borehole compensated sonic log uses a special arrangement of the transducers to correct tor irregularities in borehole size and sonde tilt.

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.

Compensated Spectral Density	CSD, a Welex trademark. This tool combines the features of the compensated density tool, which measures density by compton scattering cross section of gamma rays, and the lithology effect by measuring the low gamma-ray energies associated with the photoelectric absorption cross-section. This lithology recognition is further enhanced by borehole compensation of the photoelectric gamma response.

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.

completion tool	Any tool or combination of tools designed to complete a well for production; i.e., perforating gun, packersetting tool, etc.

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.

compressibility	The volumetric change in a unit volume ot fluid (usually) when the pressure on that volume is changed. where DV = change in volume, V, due to change in pressure, DP

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 ot 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.

Com-Pro Log	See computed log analysis. Cornputer-processed interpretation produced at a computer center. Com-Pro is a Birdwell trademark.

Compton scattering	The inelastic scattering of photons (gamma rays) by collision with orbital electrons of an atom. When a gamma ray photon having an energy in the intermediate range from about 2 keV to about 2 MeV collides with an atom, it may transfer some energy to one of the orbital electrons. which. as a result, is knocked out of the atom. The photon thereby loses some energy (its frequency is lowered) and changes direction according to its energy loss. The Compton scattering power of a material is proportional to the number of electrons in that material. Important in density logging. One of the three interactions of gamma rays with matter. Compare photoelectric effect and pair production.

computed log analysis	Log analysis derived from computer manipulation of digital log data. Service company organizations have different tradenames for their various. though sometimes similar, machine interpretations.

computer-processed interpretation	See computed log analysis.

condensate	A light hydrocarbon liquid obtained by condensation of hydrocarbon vapors. It consists of varying proportions of butane, propane, pentane, and heavier fractions, with little or no ethane or methane. Condensate generally has an APl gravity of 50 to 120 degrees and is water-white, straw or bluish in color.

condensate reservoir	A reservoir in which both condensate and gas exist in one homogeneous phase. When fluid is drawn from such a reservoir and the pressure decreases below the critical level, a liquid phase (condensate) appears.

conductive solid	Any solid material which exhibits electrical conductivity.

conductivity	The property of a solid or fluid medium which allows the medium to conduct a form of energy; e.g., conductivity or thermal conductivity. In well logging, presently, the conventional use of the term means electrical con. Conductivity, which is the reciprocal of electrical resistivity. Usually expressed as the reciprocal of ohm-meters × , or .

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

conductor pipe	A short string of large-diameter casing used to keep the well bore open and to provide a means to direct the upflowing drilling mud from the well bore to the mud pit.

cone	(1) The rolling mechanism supporting the cutting edge of a drill bit. (2) The liner which is implanted in the shaped cavity of shaped charges. (3) A downward displacement of a gas-liquid isosaturation surface, or upward displacement of an oil-water isosaturation surface. Such displacement occurs around the well bore in some wells with high oil production rates. See coning.

confirmation well	A well drilled to "prove" the formation or producing zone encountered by an exploratory or wildcat well.

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 undesirable process of creating a distortion in the gas-liquid or oil-water isosaturation surfaces at the well bore by the withdrawal of oil at an excessive rate. Production of oil produces pressure gradients in all directions from the depth of withdrawal. Therefore. there is a tendency to draw fluids from above and below (as well as laterally) to the level of withdrawal. That tendency is counterbalanced by the tendency of gas to stay above oil because of its lighter density and water to stay below oil because of its heavier density. Thus, when the pressure gradient required to bring oil to the well bore exceeds the pressure gradient to the gas or water-bearing levels, coning will occur and gas will be drawn down and water will be lifted up toward the withdrawal level.

connate water	Water entrapped in the interstices of the rock (either sedimentary or extrusive igneous) at the time the rock was deposited. It may be derived either from ocean water or land water. Often used incorrectly to denote formation water or interstitial water. The composition of the original water may have been altered during the compaction process by entrapment of ions, extrusion of water, diffusion, osmosis, etc.

consolidated	Pertains to a rock framework provided with a degree of cohesiveness or rigidity by cementation or other binding means.

contact log	A generic term referring to the log produced by any logging tool which uses pad or skid devices to make direct electrical contact with the formation wall.

continuity	Property of being continuous. (1) A term used in trouble-shooting electrical circuitry. Continuity implies continuous electrical circuitry through connections. Broken wires, corroded contacts, cold solder joints, poor contacts all can impair continuity. (2) In geology, continuity implies a continuous property or pattern relating to rock type, beds, formations, or sedimentation. (3) In reference to immiscible fluids in rock, continuity might refer to the condition of the nonwetting phase when droplets or insular globules of the nonwetting phase become connected to form a continuous web throughout the pores of the rock.

continuous flowmeter	A velocimeter which is designed to measure fluid velocities in the casing. Usually this tool is capable of passing through production tubing to make fluid velocity measurements in the casing below. The tool is held in the center of the fluid column usually by spring centralizers and moved at a constant rate of speed against (or with) the direction of flow. The spinner speed, a linear function of fluid velocity relative to the tool, is recorded continually versus depth. Primarily, this tool should be used in monophasic flow regimes; i.e., injection wells and high-flow-rate gas wells or oil wells.

continuous guidance log	The continuous guidance tool provides a continuous gyroscopic directional measurement in cased holes. The measurement is based on a two-axis gyroscope whose spin axis is maintained horizontal and is aligned towards the north The position of the gyro is sensed by an acceleronmeter and a gyro-axis positional resolver. This information is combined with data from another accelerometer to derive the azimuth and inclination of the hole. See also directional survey.

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.

continuous velocity log	A log of the interval transit time of a compressional wave. See sonic log, also acoustic log.

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.

convection	A process of mass movements of portions of any fluid medium (liquid or gas) in a gravitational field as a consequence of different temperatures in the medium and hence different densities. The process thus moves both the medium and the heat, and the term is used to signify either or both.

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. (2) A sidewall core. (3) To obtain a conventional core or to obtain a sidewall core.

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. (2) Drill pipe core barrel. A hollow cylindrical device from 25 to 60 feet in length with a hollow drill bit which can be attached to the bottom of the drill pipe for the purpose of recovering a continuous sample of the formation while the hole is being drilled. The sample recovered is a cylindrical core which, under ideal conditions, might be as long as the core barrel.

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.

coregamma	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.

coregraph	Results of core analysis illustrated or graphed in the form of a log. See core analysis.

core sample	A solid column of rock, usually from two to four inches in diameter, taken from the bottom of a well bore as a sample of an underground formation.

core slicer	A wireline open-hole device having two diamond edged saw blades with converging orientation and vertical travel over several feet when in position. The design allows a wedge-shaped slice of formation to be cut from the side of a smooth borehole for evaluation at the surface.

correlate	To relate subsurface information obtained from one well to that of others so that the formations may be charted and their depths and thicknesses noted. Correlations are made by comparing electrical well logs, radioactivity logs, and cores from different wells.

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. (2) The matching of different well-log curves and other well data either in the same well or in different wells. (3) The mathematical interdependence between variables.

correlation length	The length of the formation interval over which a depth-related correlation is made in order to determine the depth wise displacement between curves on which the same geological events have been logged. The correlation length is an important parameter in the computation of dipmeter measurements. Dipmeter log correlations are made in order to evaluate the displacements between the correlation curves; these displacements are used in turn in the calculation of the formation dip. The correlation length used will depend on the purpose. Long correlation lengths (10 to 20. or even 30 feet are used for evaluating structural dip. Short correlation lengths (1, 2, or 3 feet) are useful in determining finer details of the dip for sedimentation studies over such stratigraphic features as bars, lenses or channels.

corrosion	A complex chemical or electrochemical process by which metal is destroyed through reaction with its environment, For example, rust is corrosion.

coupling	A collar. A short pipe fitting with both ends threaded on the inside circumference used for joining two lengths of line pipe or casing or tubing.

CPI	computer-processed interpretation. See computed log analysis.

critical point	The temperature and pressure at which the properties of a liquid and its vapor become indistinguishable.

critical pressure	The pressure needed to condense a vapor at its critical temperature.

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.

critical temperature	The highest temperature at which a fluid can exist as a liquid or vapor. Above this temperature the fluid is a gas and, regardless of the amount of pressure applied, cannot be liquefied.

critical water saturation	The highest water saturation a rock can maintain while producing hydrocarbons before water will begin to flow.

crooked hole	A wellbore that has deviated from the vertical. It usually occurs where there is a section of alternating hard and soft strata steeply inclined from the horizontal.

crossflow	A condition of fluid flow, in some part of the well bore, from one permeable zone to another at lower pressure.

crossplot	A plot of one parameter versus another.

cross section	(1) A contraction of capture cross section. (2) A diagram showing geological features transected by a vertical plane. (3) A designation for a specific area (e.g., borehole cross section).

crown block	A stationary pully 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 traveling block and hook are attached.

crude oil	Unrefined liquid petroleum. It ranges in gravity from 9 to 55 °API and in color 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 sulfur or sulfur compounds, it is called sour crude; if it has little or no sulfur, 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.

curie	A standard measure of the rate of nuclear transformations, or disintegrations equal to that of one gram of radium. This rate is 3.70 × 10¹⁰ disintegrations per second.

curie temperature	The temperature at which ferromagnetic ef fects are destroyed by thermal agitation in ferromagnetic substances. In common iron alloys, the curie temperature (or curie point) is typically 500 to 700° C.

curve	In well logging, a trace representing a continuous record of some property or occurrence in the wellbore environment versus depth. One or more curves may constitute a well log.

cushion	(1) A column of water or drilling fluid placed inside drill pipe or tubing to prevent it from being crushed by the hydrostatic pressure of the fluid in the annulus of the wellbore. Usually the pipe or tubing is full of liquid, but in certain operations it is necessary to run the pipe or tubing nearly empty to maintain less pressure inside the pipe than in the annulus. In such cases. a cushion may be needed to prevent collapse of the pipe. (2) The column of liquid in the wellbore to insure optimum performance of a perforating gun.

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	(1) Galvanometer cutoff. A point at minimum or maximum deflection on a galvanometer trace where the curve begins or ends. Cutoff is produced by blocking the reflected light beam. (2) Parameter cutoff. 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.

Cyber Log Systems	Computed log analysis systems designed for use at the well site. Cyber Log Systems is a mark of Schlumberger.

cycle skip, cycle skipping	In acoustic transit time or sonic logging. When the amplitude of the first arrival form (cycle of the acoustic wave train is large enough to be detected by the near receiver of a receiver pair) but not large enough to be detected by the far receiver, then one or more cycles will be skipped until a later cycle arrives which has energy above the detection level. This situation is called "cycle skipping." Its onset is characterized by a sharp deflection on the transit time curve corresponding to one or more added cycles of time between receivers. "Short cycle skipping," where the near receiver is triggered a cycle too late can also occur, resulting in an abnormally short travel time.

cyclonite	A powerful, high explosive material (cyclo-trimethylenetrinitramine) used as the main charge in shaped charges. Also called RDX.

cylindrical plot	A graphic presentation of the bed boundaries intersecting the well bore. The beds are usually plotted on a clear plastic which is rolled up to simulate the borehole so that the observer can see the patterns cutting the well bore.