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darcy, darcys, darcies A unit of measure of permeability. The permeability of a porous medium which will allow a flow of one milliliter per second of fluid of one centipoise viscosity through one square centimeter under a pressure gradient of one atmosphere per centimeter. The commonly used unit is the millidarcy or 1/1000 darcy.
Darcy's equation Sometimes referred to as Darcy's law. A relationship for the fluid flow rate q through a porous medium:

[formula]

where: k = permeability. A = cross-sectional area, µ = viscosity and Dp = pressure difference across the distance Dx.

dead oil (1) An oil that has undergone a lowering of pressure such that there is no longer any gas in solution. Compare live oil.

(2) A residual oil measured at 60°F and 14.7 psia after all gas has been removed by differential liberation at reservoir temperature.

(3) A viscous oil, asphalt. or tar which has undergone a lowering of pressure so that the gases and lighter oil fractions have come out of solution and expanded. Leaving the heavier hydrocarbon fractions behind.

dead time In radioactivity logging. The length of time the system requires to recover from counting an event, in order to count a successive event. Events occurring during dead time are not counted. Usually measured in microseconds.
decay The spontaneous reduction of an effect with time.

(1) The disintegration process of the nucleus of an unstable isotope by the spontaneous emission of charged particles and/or photons.

(2) The equilibration process of heat transfer after the disturbance of thermal equilibrium.

(3) The progressive reduction in amplitude of a transient signal due to damping or energy absorption.

decay time See thermal decay time and half life.
decentralize To eccenter. To purposely force a tool against the borehole or casing wall by means of an arm or bow spring.
decentralizer See eccentering arm.
deep investigation The measurement of formation properties far enough from the well bore that the effects of the invaded zone become minimal.
deep propagation log A well log that provides the resistivity and dielectric constant of the formation. The deep propagation tool (DPT) radiates electromagnetic energy into the formation surrounding the wellbore. Measurements of the attenuation and velocity of this electromagnetic wave provide values to determine the resistivity and dielectric constant of the formation. The tool operates at a frequency in the tens of megahertz range and measures signal level and relative phase at four receivers. Compare electromagnetic propagation tool.
deep well thermometer A logging device for the continuous measurement of temperatures in a well bore. Compare maximum reading thermometer.
deflection (1) The internal movement in a galvanometer, in response to current, which produces the excursion on a logging trace or curve.

(2) The lateral movement or excursion of a curve is often referred to as a deflection.

(3) A change in the drilling angle of the borehole. In directional drilling, it is the angle in degrees from vertical. The angle of deviation.

degrees API (°API) A unit of measurement of the American Petroleum Institute that indicates the weight, or gravity, of oil. See API gravity.
dehydrate (1) To lose water.

(2) To cause to lose water.

Delaware gradient An anomalous effect on guard log and early laterolog curves first observed in the Delaware Basin. It can be recognized as an erroneous high-resistivity gradient in conductive beds when these beds are overlaid by thick high resitivity formations.

Principle of Delaware Effect
delay panel A memorizer panel. An electrical device which stores signals measured by the sonde so that all measured signals can be recorded with the same depth reference. See memorizer.
delayed gamma ray As a result of some nuclear reactions, especially those involving neutrons, a nuclide will emit prompt gamma rays and be left in a ground state, which may be unstable. To decay further (usually by beta emission) to excited states of a different nuclide. Delayed gamma rays are emitted in the decay of these excited states.
delta-t (Dt) The interval transit time from an acoustic log in microseconds per foot or microseconds per meter. It is the inverse of velocity. The official symbol of SPE of AIME and SPWLA for Dt is now the symbol t.
densimeter A device which measures the average density of the fluid mixture in a borehole by means of a vibrating cylinder. The holdups for the heavy and light phases can be determined from the measured average density, when the bottomhole densities are known for each phase and if a contrast exists.
density Mass per unit volume (often expressed as specific gravity). Well-logging units are g/cm3, often written g/cc.
density log A well log that records formation density. The logging tool consists of a gamma-ray source (e.g., Cs137) and a detector shielded from the source so that it records backscattered gamma rays from the formation.The backscattering depends on the electron density of the formation, which is roughly proportional to the bulk density. The source and detector usually are mounted on a skid which is pressed against the borehole wall. The compensated density logging tool includes a secondary detector which responds more to the mud cake and small, borehole irregularities. The response of the second detector is used to correct the measurements of the primary detector. The density log applies primarily to uncased holes. Sometimes called a gamma-gamma log. See also Compton scattering, Z/A effect, and compensated formation density log. Compare nuclear cement log.

Gamma Ray Backscattering
departure In true vertical depth calculations from directional surveys. Departure is the horizontal displacement in an east or west direction from the wellhead of a location or station in the borehole at which directional survey measurements were taken. Compare latitude.
departure curves Graphs which show the influence of various conditions on the basic measurement. Such curves, for example, show the effects of temperature, hole diameter, mud resistivity, bed thickness, adjacent bed resistivity, etc. Sometimes the effects are correctable.
deplete To exhaust a supply. An oil and gas reservoir is depleted when most or all recoverable hydrocarbons have been produced.
depositional environment The conditions under which sediments were laid down. Depositional environments are divided into five groups: marine (oceanborne), aeolian (windborne, alluvial (river-borne). deltaic (borne by a river at its delta), and interdeltaic (between river deltas).
depth column A depth track. A narrow column near the center of the well log in which the depths within the well are recorded. Usually depths are recorded at l00 foot (meter) intervals and sometimes 50-foot (meter) intervals. See API log grid.
depth-control log A well log run in cased holes for the purpose of providing correlation with open-hole logs in order to establish depth control for certain completion operations. Usually the depth-control log is made with a radioactivity logging tool in conjunction with a casing collar locator. The correlation log may be a gamma-ray log and/or neutron log, or in some cases a pulsed neutron capture log.
depth datum The zero-depth reference for well logging. A location on or above the surface (land or water) at which an elevation can be determined for depth reference. The elevation of this datum will be the reference for all depth measurements made in the well bore. Usually, the top of the kelly bushing is used as depth datum in drilling wells, but could be ground level, derrick floor, or any other specific depth reference.
depth of invasion The radial depth from the well bore to which mud filtrate has invaded porous and permeable rock. Usually measured in inches. See also diameter of invasion and invaded zone.
depth of investigation Radius of investigation. The radial distance from the measure point on a downhole tool to a point usually within the formation where the predominant tool-measured response may be considered to be centered. Varies from one type of device to another because of design and techniques of compensation and focusing. May also change from formation to formation because of changes in formation properties.
depth scales See scale.
derrick A tower-like load-bearing structure, usually of bolted construction. In drilling, the standard derrick has four legs standing at the corners of the substructure and reaching to the crown block. The substructure is an assembly of heavy beams used to elevate the derrick and provide space to install blowout preventers, casingheads, etc. Because the standard derrick must be assembled piece by piece, it has largely been replaced by the mast, which can be lowered and raised without disassembly. The derrick is used to raise and lower equipment (pipe, casing, etc.) used in the drilling and testing of a well.
derrick floor DF. The drilling rig floor. sometimes used as depth datum.
detail log A borehole log recorded on larger depth scale than correlation scale of 1 or 2 inches per 100 feet. Specifically, a log recorded at a depth scale of 5 inches of record length per 100 feet of formation logged (sometimes 1 foot per 200 feet). See scale.
detector A sensor used for the detection of some form of energy. Usually this term is used to refer to the device used in nuclear logging tools to detect neutrons and gamma rays. See scintillation counter and Geiger-Mueller counter.
detonate To explode in an extremely violent chemical reaction See high explosives. Compare low explosives.
detonator Electrical blasting cap. Unstable compounds such as high explosives require initial shock or other disturbance to set off the chemical reaction. A blasting cap is usually used to set off Primacord, which in turn sets off shaped charges.
detrital sediment Sediment formed from accumulations of minerals and rocks derived either from mechanical erosion of previously existing rock or from the mechanically weathered products of these sediments.
development well A well drilled in an area in a field to complete a pattern of production. An exploitation well.
deviated hole A borehole which has been intentionally drilled at an angle from vertical by special downhole drilling tools to guide the drill assembly in the desired direction. Deviated holes are drilled to reach a part of a formation or reservoir which cannot be drilled by a straight or vertical hole because of environmental, political, or economic reasons.
deviation (1) Departure of a borehole from vertical. See deviated hole, drift, and directional survey.

(2) Angle measured between tool axis and vertical as in dipmeter or dip log.

deviation angle The inclination of the wellbore from the vertical. The angle of deviation, angle of drift, or drift angle is the angle in degrees that shows the variation of the borehole from the vertical as revealed by a deviation survey or directional survey.
deviation survey An operation made to determine the angle from which a hole drilled by the bit deviated from the vertical during drilling. There are two basic deviation survey, or drift survey, instruments: one reveals the angle of deviation only, the other indicates both the angle and direction of deviation.
dew point The temperature and pressure at which a liquid begins to condense out of a gas. For example, if a constant pressure is held on a certain volume of gas but the temperature is reduced, a point is reached at which droplets of liquid condense out of the gas. That point is the dew point of the gas at that pressure. Similarly, if a constant temperature is maintained on a volume of gas but the pressure is increased, the point at which liquid begins to condense is the dew point at that temperature. Compare bubble point.
d exponent See drilling exponent.
DF Derrick floor.
diagenesis The chemical, physical, and biological changes that a sediment undergoes after initial deposition and burial that convert the sediment to consolidated rock and/or result in the creation of some forms of porosity. Such changes might result from compaction, cementation, recrystallization or replacement, but exclude metamorphism and fracturing resulting from tectonic stresses.
diagenetic porosity Porosity developed in a sediment after its initial deposition as a result of chemical and biological changes and burial. A form of secondary porosity as opposed to primary porosity. Fracture porosity is not thought of as diagenetic porosity.
diameter of invasion The diameter to which drilling-mud filtrate has invaded porous and permeable rock. Usually measured in inches. See also depth of invasion and invaded zone.
diapir A dome, core, or anticlinal fold of plastic or igneous material which has been squeezed upward to pierce overlying rocks. Common diapirs in sedimentary strata are domes of salt or shale.
diatoms A microscopic, single celled plant growing in marine or fresh water. Diatoms have siliceous skeletons of a great variety of forms that may accumulate in sediments in enormous numbers.
diatomite A siliceous sediment consisting of the skeletons of diatoms.
dielectric A material having low electrical conductivity compared to that of metal.
dielectric constant Relative permittivity. A measure of the relative ability of a material to store electric charge for a given applied field strength. Dielectrics are considered nonconductors. A state of electric stress can exist between two conductors separated by a dielectric without a continuous supply of energy from outside the system. For an isotropic medium, the dielectric constant is the ratio of the capacitance of a capacitor filled with a given medium to that of the same capacitor having only a vacuum as dielectric.
differential log A well log which records the depth-rate-of-change of a parameter measureable from the well bore. This kind of log is sensitive to small changes measured in the absolute value of the parameter. An example is the differential temperature survey.
differential pressure The difference between two fluid pressures. For example, the difference between the pressure in a reservoir and in a wellbore drilled into the reservoir.
differential-pressure sticking A condition in which the drill stem becomes stuck against the wall of the wellbore because part of the drill stem (usually the drill collars) has become embedded in the mud cake. Necessary conditions for differential-pressure sticking (or wall sticking) are a permeable permeable formation, a pressure differential across a nearly impermeable mud cake, and drill stem.
differential SP A curve recorded as a result ot simultaneous SP measurements from two electrodes located downhole, each serving as a reference potential for the other. The purpose of a differential SP is to minimize or eliminate unwanted effects induced on the SP measuring circuitry (i.e., electrode and survey conductor). The "twin" circuitry allows spurious efforts induced simultaneously on each segment of the circuit to negate each other at the recording galvanometer. The two downhole electrodes selected for this purpose should be separated by enough distance that their equal and opposite responses do not cancel the valid SP signals produced by different segments of the formation. See SP, downhole ground, and telluric currents.
differential temperature survey A log consisting of a curve which is a continuous record of the temperature gradient in the borehole. It can be measured by two separate sensors having identical thermal characteristics separated by a fixed vertical distance on a sonde; or, by a technique involving the use of a single temperature sensor and placing the measured signal into storage so it can be played back after the sensor has moved a distance in the borehole equal to a predetermined vertical distance. The differential temperature curve enhances small changes in temperature occurring in the borehole. A differential temperature survey should always include a recorded measurement of temperatures in the borehole. See also temperature log.
diffuse layer In liquid. The position of a charge on the outer surface of a solid is rigidly fixed. Adsorbed to this surface may be an essentially immobile layer of oppositely charged ions from the liquid. This is called the Stern layer. Further in the solution is a layer of charge in which the ions having the same charge as the immobile layer outnumber the ions having the opposite charge. This is the diffuse layer. Beyond this layer is a region of neutral charge where anions and cations exist as neutral pairs. Only ions in the diffuse region and neutral region are free to move under the influence of fluid motion.
diffusion (1) Of ions. The spontaneous migration of ions from a more concentrated solution into a more dilute solution. Compare osmosis.

(2) Of thermal neutrons. A net movement or flow of thermal neutrons from regions of higher neutron concentration to regions of lower concentrations. The neutrons are moving with velocities corresponding to their energies in more or less random directions (due to repeated scattering interactions). Thus, there is a tendency for more neutrons to leave the higher-concentration zone and for fewer to enter it.

An example case is when the neutron population in the borehole is much lower than that in the formation because the neutrons are rapidly captured by the abundant chlorine nuclei in the very saline borehole fluid. More neutrons will diffuse from formation to borehole than from borehole to formation, and there will be a net flow of neutrons from formation to borehole.

There is generally a net diffusion flow away from the source (i.e., toward zones of lower neutron concentration) and a tendency for net flow from a zone of smaller capture cross section to one of larger capture cross section.

diffusion effect The contribution of thermal-neutron diffusion to the thermal-neutron die-away or decay measurement. In thick beds, diffusion effects are not readily separable from borehole effects, and appropriate departure curves are used to correct for both at the same time. See diffusion, thermal neutrons.
diffusion potential Liquid junction potential. See electrochemical potential.
digital Representation of quantities in discrete (quantized) units. A digital system is one in which the information is contained and manipulated as a series of discrete numbers, as opposed to an analog system, in which the information is represented as a continuous trace or curve of the quantity constituting the signal.
digitize Convert data from analog trace records to digital, machine-useable numbers.
dilatancy The property of rock to expand.

(1) Consolidated rock can expand as a result of the creation of microfractures.

(2) Unconsolidated rocks, which have a large number of grain contacts, have the property to expand by readjusting grain positions to those positions resulting in fewer contacts.

diode error An unwanted portion of the total electrical conductivity signal sent to the surface from downhole induction logging instruments. The diode error is produced by the measure circuit electronics of the induction cartridge. It is isolated and measured during the calibration operation at a step where the sonde output is zero. Once evaluated, it is cancelled during the survey operation. See also sonde error.
dip The angle that a structural surface (e.g., a bedding or fault plane) makes with the horizontal, measured perpendicular to the strike of the structure. Observe dip patterns on idealized log in illustration. Also see illustration of dip at strike.

Dip Patterns and the Geologic Anomalies Commonly Associated with Them
dip log See dipmeter log.
Diplog See dipmeter log. Diplog is a Dresser Atlas trademark.
dip log tool – 4 arm See high-resolution dipmeter tool.
dipmeter, dipmeter tool A downhole tool used to make a dipmeter log or dip log.

Dipmeter Scheme
dipmeter log A dip log. (1) A well log from which formation dip magnitude and azimuth can be determined. The resistivity dipmeter includes three or four (sometimes eight) micro-resistivity readings made using sensors distributed in azimuth about the logging sonde and a measurement of the azimuth of one of these; a measurement of the hole deviation or drift angle and its bearing; and one or two caliper measurements. The azimuth, deviation, and relative bearing are measured by a system similar to that described for the poteclinometer. The microresistivity curves are correlated to determine the difference in depth of bedding markers on different sides of the hole. See also high-resolution dipmeter and poteclinometer.

(2) Other types of dipmeters use three SP curves, three wall scratchers, etc. to produce logs.

(3) A log showing the formation dips calculated from the above, such as a tadpole plot or stick plot. See illustration of dip patterns at dip.

directional drilling Slant hole drilling. The technique of intentional. controlled drilling at an angle from the vertical by deflecting the drill bit. Although wellbores are normally drilled vertically, it is sometimes necessary or advantageous to drill at an angle from the vertical. Controlled directional drilling makes it possible to reach subsurface areas laterally remote from the point where the bit enters the earth. It involves the use of turbodrill, whipstocks, or other deflecting tools.
directional hole

 Deviated hole. A borehole intentionally drilled at an angle from the vertical. See directional drilling.
directional log See directional survey.
directional survey Measurements of drift, azimuth. and inclination of a borehole with the vertical. A directional survey is often made as part of a dipmeter survey or sometimes as a continuous log with a poteclinometer. Sometimes measurements are made at discrete levels with a photoclinometer.
dirty Containing appreciable amounts of clay and shale of different material from the host rock. Such material may decrease permeability and effective porosity of the rock. Compare clean.
discontinuous phase See internal phase. Compare continuous phase.
discovery well

 The first oil or gas well drilled in a new field. The well that reveals the presence of a petroleum-bearing reservoir. Subsequent wells are development wells.
disequilibrium In radioactivity logging for uranium. After a long period of disintegration, if the radioactive parent and its daughter isotopes remain undisturbed, equilibrium will be attained between the production and decay of the respective radioactive materials. Since U-238 does not emit gamma rays, the gamma-ray activity recorded is due to the quantity of daughter isotopes. When U-238 is in equilibrium with its daughter isotopes, the amount of daughter isotopes detected is an index to the amount of U-238 present. Disequilibrium results when anything has occurred to disturb the relative quantities of radioactive materials (selective leaching, escape of radon gas, etc.).
dispersed A term used in well-logging to refer to solid particles distributed within the interstices of the rock framework. A form of distribution of clays and other small particles and crystals. Detrital and/or authigenic materials often are found dispersed in the form of fines or crystal growths occupying pore space and lining the walls surrounding the pores.
displacement (1) The distance between the logging tool measure points for various parameters measured by a combination logging tool. Also, the correction necessary to record all parameters at the same depth reference.

(2) Pertaining to the dipmeter, it is the vertical distance in the hole between equivalent responses measured at different quadrants in the hole.

(3) Also refers to the replacement of a portion of the interstitial fluids by borehole fluids, near the borehole, during the invasion process.

disposal well A well into which salt water is pumped, usually part of a saltwater disposal system.
dissociation The breaking up of a compound into its simpler components such as molecules, atoms, or ions. Results from the action of some form of energy on gases and from the action of solvents on substances in solutions.
dissolution porosity Secondary porosity which is created when solid materials in sediment dissolve in interstitial solutions. Dissolution porosity results from the dissolution of:  sedimentary constituents, authigenic cementing minerals, and authigenic replacive minerals.
dissolved-gas drive A solution-gas drive. See also reservoir drive mechanism and bubble point.
distillate Liquid hydrocarbons, usually water-white or pale straw color, and high API gravity (above 60 degrees) recovered from wet gas by a separator that condenses the liquid out of the gas stream. (Distillate is an older term for the liquid. More commonly it is called condensate or natural gasoline.) The product obtained from the condensation of vapors in distillation.
ditch gas Is that portion of the hydrocarbons removed from the mud at the flowline by any type of mechanical means.
doghouse A portable, one-room shelter (usually made of light tank-iron) at a well site for the convenience and protection of the drilling crew and others. The doghouse serves as lunch room, change house, dormitory, and for keeping small supplies and records.
dogleg A sharp bend or change in direction of the borehole.
dolomite A type of sedimentary rock similar to limestone but rich in magnesium carbonate. sometimes a reservoir rock for petroleum.  CaMg (CO3)2.
dome A geologic structure resembling an inverted bowl. A short anticline plunging on all sides. See salt dome.
downhole A term used to describe tools, equipment, and instruments which are run into the borehole. Also pertains to techniques, processes, and conditions which apply to the wellbore and its environment.
downhole ground Cable armor or a long electrode often attached to the logging cable some distance (perhaps 100 feet) above an electrical logging sonde, or hung just below the casing, used as the reference electrode instead of an electrode at the surface. Used in the case of bad SP interference from electrical surface facilities or telluric currents. See differential SP.
down time (1) The length of time it takes for drilling mud to travel from a location on the surface to the bottom end of the drill stem (assembly of drill pipe, etc.) during a drilling operation. It is a function of the inside diameter and length of drill pipe and the output of the mud pump.

(2) The length of time lost during an operation because of non-scheduled stoppages such as failure, delay, etc.

(3) The period during which an item of equipment cannot be operated because of ongoing repair or maintenance.

drainage The migration of oil or gas in a reservoir toward a wellbore due to pressure reduction caused by production of reservoir fluids by the well. A drainage point is a wellbore (or in some cases several wellbores) which drains the reservoir.
drainage radius The radius, measured from a wellbore, of a circular area of a reservoir which is drained bv a single well.
drawdown (1) The difference between static and flowing bottom-hole pressures.

(2) The distance between the static level and the pumping level of the liquid in the annulus of a pumping well.

drawworks The hoisting mechanism on a drilling rig. It is essentially a large winch that spools on or off the drilling line and thus raises or lowers the drill stem and bit.
drift The attitude of a borehole. The drift angle or hole deviation is the angle between the borehole axis and the vertical; the drift azimuth is the angle between north and the vertical projection of the borehole on a horizontal surface.
drift angle See deviation angle.
drill bit See bit.
drill collar A heavy, thick-walled pipe, usually steel, used between the drill pipe and the bit in the drill stem to weight the bit in order to improve its performance.
driller One who operates a drilling rig. The person who is in charge of drilling operations and who supervises the drilling crew.
drilling break An increase in the penetration rate of the drill bit caused by a change in the formation, often indicative of penetration into a porous zone.
drilling exponent The drilling exponent (d), the exponent corrected for mud density (dc), and the exponent corrected for mud density and bit wear (dcs) are used to detect undercompacted shaly formations and associated high-pressure zones.

(1) d = uncorrected drilling exponent

when:    R = drilling rate (ft/hr)
   K = rock drillability (~ 1 for shales)
   N = rotary speed (rpm)   
   W = weight on bit (lb)   
   D = bit diameter (in.)
(2) dc = drilling exponent corrected for mud density

where:    d1 = equivalent mud density of normal pore pressure gradient.

   d2 = equivalent mud density of actual pore pressure gradient
(3) dcs = drilling exponent corrected for nmud density and bit wear.
Same as "dc" except that instead of R (drilling rate at time t) the drilling rate Ro (corrected for bit wear) is used.

drilling log A log of drilling parameters such as penetration rate, rotary speed, weight on the bit pump pressure, pump strokes, etc.
drilling mud See mud.
drilling riser See marine drilling riser.
drilling-time log A record of the time to drill a unit thickness of formation.
Drilling Porosity Log DPL. A log derived from a mathematical treatment of drilling parameters (e.g., penetration rate, rotary speed, weight on the bit, pump pressure, pump strokes, etc.) which has been calibrated to indicate porosity in known rock types. Drilling Porosity Log trademark of The Analysts.
drill pipe Heavy, thick-walled, seamless steel pipe used in rotary drilling to turn the drill bit and to provide a conduit for the drilling mud. Joints of drill pipe are about 30 feet long.
Drill Pipe Electric Log A resistivity well log which is obtained from a logging instrument which has a self-contained recording mechanism. The log consists of a SP and short and long normal curves. The tool is lowered through the drill pipe, and the flexible electrode assembly is pumped through a port in the bit. The log is recorded by a tape recorder within the tool during the process of withdrawing pipe from the hole. The tape is played back to obtain the log. Drill Pipe Electric Log is a trademark of Welex.
drill stem The drill stem is comprised of the drill pipe, drill collars, bottomhole assembly, and drill bit. The drilling fluid is pumped down this pipe at a desired pressure and then jetted out the bit. The drill stem imparts certain characteristics on the dynamics of the drilling operation.
drill-stem test DST. A procedure for testing a formation through drill pipe. Often defined as a temporary completion of a well to determine the fluid content of a reservoir and its ability to produce. Formation fluid is recovered in the drill pipe through temporary relief of backpressure imposed on the formation. Hydrostatic, flowing and shut-in pressures are recorded versus time.
drill string The column, or string, of drill pipe, not including the drill collar or kelly. Often, however, the term is loosely applied to include both the drill pipe and drill collars.
dry gas Natural gas from the well free of liquid hydrocarbons. Gas that has been treated to remove all liquids.
dry hole Any well that does not produce oil or gas in commercial quantities. A dry hole may flow water, gas, or even oil, but not enough to justify production.
dry rock Rocks beneath the earth's surface that do not have meteoric or juvenile water supplied to them by an aquifer or any other source.
DST See drill-stem test.
dual completion A single well that produces from two separate zones at the same time. Production from each zone is segregated by running two tubing strings with packers inside the single string of production casing; or one tubing string with a packer may be run through one zone while the other is produced through the annulus.
Dual Dipmeter Four dual electrodes record eight microconductivity curves allowing side-by-side button correlations and pad-to-pad correlations for high-density dip results. The solid state inclinometer system has a triaxial accelerometer and three magnetometers for information on tool deviation and azimuth. Also see dipmeter tool and dipmeter log. Dual Dipmeter is a mark of Schlumberger.
dual guard log A formation resistivity log made from a system consisting of both very deep and shallow investigative guard log schemes. The tool records, in combination, deep and shallow guard log curves and a gamma ray and/or SP curve. The dual guard-FoRxo is a simultaneously recorded dual guard log and FoRxo. See also guard log and FoRxo log.
dual induction log DIL. An induction log consisting of two induction curves representing electrical conductivity measurements taken at different depths of investigation. Usually run in conjunction with a focused resistivity device with a shallow depth of investigation, such as a shallow laterolog or guard log. See also induction log. DIL is a mark of Schlumberger.
dual laterolog DLL. A formation resistivity log made from a system consisting of both very deep and shallow investigative laterolog schemes. The tool records. in combination, deep and shallow laterolog curves and a gamma ray and/or SP curve. The dual laterolog-Rxo, is a simultaneously recorded dual laterolog which also includes flushed zone resistivity information derived from a micro-Spherically Focused Logging device. See also laterolog. DLL is a mark of Schlumberger.

Dual Laterolog Scheme
dual-spaced density log

 See compensated density log.
dual-spaced thermal decay log TDL. A log made with a pulsed neutron tool utilizing two radiation detectors. See Thermal Multigate Decay Log.
dual-spaced neutron log DSN. A well log made with a tool having two thermal 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.
dual porosity CNL DNL. The dual porosity compensated neutron tool has two thermal and two epithermal neutron detectors for separate porosity measurements. The epithermal measurement can also be made in air- or gas-filled holes. Also see compensated neutron log. See Dual Porosity CNL Tool diagram.

Dual Porosity CNL Tool
dual-spacing formation density log

 See compensated formation density log.
Dual-Spacing Thermal Decay Time Log TDT. A well log produced by a thermal decay time tool utilizing two radiation detectors. TDT is a mark of Schlumberger. See Thermal Decay Time Log.
dynamic In a state of motion. In well logging, it usually refers to borehole fluid in motion; specifically, fluid (liquid or gas) moving in the well bore under injecting, producing, or crossflowing conditions.
dynamic measure point A depth reference point on the downhole instrument where measurements are taken. On most instruments, the measure point and the dynamic measure point are found at the same point or place on the sonde. In nuclear tools, lag makes the dynamic measure point appear below the static measure point by the distance of the lag. See lag.
dynamic positioning A method by which a floating offshore drilling rig is maintained in position over an offshore well location. Generally, several motors called thrusters are located on the hull(s) of the structure and are actuated by a sensing system. A computer to which the system feeds signals then directs the thrusters to maintain the rig on location. See illustration of marine drilling rig at marine drilling riser.