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dead oil |
Oil at sufficiently low pressure that it contains no dissolved gas or a relatively thick oil or residue that has lost its volatile components. |
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. |
depositional environment |
The conditions under which sediments were laid down. Depositional environments are divided into five groups: marine (ocean borne), aeolian (windborne, alluvial (river-borne). deltaic (borne by a river at its delta), and interdeltaic (between river deltas). |
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. |
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 cantered. 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. |
derrick |
The structure used to support the crown blocks and the drillstring of a drilling rig. Derricks are usually pyramidal in shape, and offer a good strength-to-weight ratio. |
derrick floor |
DF. The drilling rig floor. sometimes used as depth datum. |
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 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. |
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. However it is clear that diagenetic changes grade into metamorphic ones. sub-aerial weathering is also specifically excluded |
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. |
diapir |
An intrusion which domes the overlying cover after piercing lower layers. Occasionally diapiric bodies pierce the surface. |
dielectric |
A material used in a capacitor to store a charge from an applied electrical field. A pure dielectric does not conduct electricity. |
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 non-conductors. 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 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 |
The spontaneous potential (SP) measured between two electrodes placed close together in the borehole, as opposed to the normal SP, which is measured with one electrode in the borehole and one at surface. |
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. |
dipmeter, dipmeter tool |
A downhole tool used to make a dipmeter log or dip log. |
directional drilling |
Deviated 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 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. |
discovery well |
The first successful 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. |
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 replacing minerals. |
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 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. |
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. |
down time |
The length of time lost during an operation because of non-scheduled stoppages such as failure, delay, etc. |
drainage |
The process of forcing a non-wetting phase into a porous rock. Oil migrates into most reservoirs as the non-wetting phase, so initial charging of the reservoir is a drainage process. |
drainage radius |
The radius, measured from a wellbore, of a circular area of a reservoir which is drained by a single well. |
drawdown |
The difference between static and flowing bottom-hole pressures. |
drawworks |
The machine on the rig consisting of a large-diameter steel spool, brakes, a power source and assorted auxiliary devices. The primary function of the drawworks is to reel out and reel in the drilling line, a large diameter wire rope, in a controlled fashion. The drilling line is reeled over the crown block and travelling block to gain mechanical advantage in a "block and tackle" or "pulley" fashion. This reeling out and in of the drilling line causes the travelling block, and whatever may be hanging underneath it, to be lowered into or raised out of the wellbore. The reeling out of the drilling line is powered by gravity and reeling in by an electric motor or diesel engine. |
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. |
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. |
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 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. |
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. |
dual completion |
A single wellbore having tubulars and equipment that enable production from two segregated zones. In most cases, two tubing strings will be used to provide the necessary level of control and safety for the fluids from both zones. However, in some simple dual completions, the second or upper zone is produced up the tubing-casing annulus. |
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. 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. DLL is a mark of Schlumberger. |
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. |
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. |
