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Data rate	The speed at which measurements and data are transmitted and recorded. In MWD, this parameter is important due to its relation to drilling or tripping speeds. If data are recorded slowly by a fast moving sensor, they may be less representative of the environment and difficult to reproduce. (See also Transmission rate)

Data storage capacity	The volume of information which may be saved. In MWD measurements, data are usually transmitted to the surface in real time but on a lower rate. Consequently the real-time log is often replaced by the higher-density, and better resolution data which are stored downhole and down-loaded after the tool is brought to the surface or downlinked from the surface. (See also Downlink)

Deepwater	Generally defined as operations in water depths of 1000 ft. (ca. 300 meters) or greater

Depletion	Depletion describes the declining oil production from oil fields over time. According to the International Energy Agency the oil production of producing fields in 2010 will decline by 47 million barrels per day until 2035.

Derrick	A derrick is a complex set of machines used over oil and gas wells. The structure is used to support the drillstring of a drilling rig. Derricks are usually pyramidal in shape.

Detection radius	The distance at which an MWD sensor in one formation can detect another formation parallel to the borehole, measured from the center of the MWD tool. For resistivity or conductivity tools, this is defined as the distance where the log response changes by 25% of its value in the current formation. For nuclear measurements, it is defined as the distance where the log response changes by at least two standard deviations from the value in the current formation.

Development	After exploration drilling has proven successful and before full-scale oil production the development phase needs to be started. After the well was assessed during an appraisal phase, a plan to fully and efficiently exploit the well is worked out. Mostly additional wells are drilled then.

Deviated hole	A wellbore that is not vertical. The term usually indicates a wellbore intentionally drilled away from vertical (see Directional drilling).

Directional drilling	Directional drilling is a method of increasing a well's productivity while reducing the environmental footprint of an oil and gas operation. New technologies enable to drill laterally or horizontally (see horizontal drilling) beneath the surface, as opposed to vertically, allowing for a wider range of possible well configurations. Because horizontal and directional drilling methods often utilize existing vertical wellbores, additional wells may be drilled without additional disruption to the environment. Utilizing this technique also helps maximize recovery (see recovery factor) from existing reservoirs by penetrating a greater cross-section of the formation, allowing substantially more oil to be produced while reducing the total number of wells required. Directional drilling is common in shale reservoirs because it allows drillers to place the borehole in contact with the most productive reservoir rock.

Directional Survey	A well survey that measures the degree of departure of a borehole from vertical and the direction of departure. Measurements are made of azimuth and inclination of the borehole.

Downhole assembly	The drill collars, subs and drill bit on the bottom of a drillstring.

Downlink	The capability to retrieve data from, and send instructions to the tool when it is located downhole. Four principles are currently used for downlink communications: mechanical (wireline), electrical (inductive coupling), hydraulic (mud pulse) and electromagnetic propagation.

Downstream	Refers to oil and gas operations after the production phase and through to the point of sale. This term is often used to refer to the refining of crude oil, and the selling and distribution of natural gas and products derived from crude oil. The opposite is upstream.

Drill bit	This tool is located on the bottom of a drillstring and is used to cut or crush rock.

Drill collars	Collars are extremely thick-walled drill pipes having a higher weight than regular drill pipes. The drill collar section is the part right above the drill bit and downhole motor.

Drill pipe	A drill pipe is a hollow steel pipe with special threaded ends called tool joints. The drill pipes can be screwed together and connect the rig surface equipment with the bottomhole assembly. Drill pipes need to be differentiated from drill collars.

Drilling fluid type	The type of drilling mud present in the drilling media, such as chemical gel mud, crude oil, caustic (high pH), Gypsum mud, native mud, etc.

Drilling Log	A log of drilling parameters such as penetration rate, rotary speed, weight on the drill bit, pump pressure, pump strokes, etc.

Drilling Motor	The Positive Displacement Motor (PDM) is composed of a stator and a rotor. The stator is made of an extremely rigid elastomere. This material will yield when acted upon by a force, but will return to its original shape when that force is removed. The rotor, supported by bearings on each end, is mounted in the stator. The drill bit is attached, through a mechanical linkage, to the lower end of the rotor. As the drilling mud flows through the system, the cavity within the PDM is pressurized due to the hydraulic energy of the drilling mud. In the motor, the hydraulic energy is converted into rotational energy, which enables rotation of the rotor and the attached drill bit. The design of the PDM is similar to a drill collar.

Drilling motor / downhole motor	The Positive Displacement Motor (PDM) is composed of a stator and a rotor. The stator is made of an extremely rigid elastomere. This material will yield when acted upon by a force, but will return to its original shape when that force is removed. The rotor, supported by bearings on each end, is mounted in the stator. The drill bit is attached, through a mechanical linkage, to the lower end of the rotor. As the drilling mud flows through the system, the cavity within the PDM is pressurized due to the hydraulic energy of the drilling mud. In the motor, the hydraulic energy is converted into rotational energy, which enables rotation of the rotor and the attached drill bit.

Drilling Mud	Drilling mud is a liquid drilling fluid which is needed to drill boreholes into the earth. Drilling mud has several functions, which range from cooling and cleaning the drill bit and carrying out drill cuttings to providing pressure to prevent formation fluids from entering into the well bore. Based on the mud composition, drilling mud can be subdivided into water-based mud, non-water based mud and gaseous drilling mud.

Drilling mud	A liquid circulated through the wellbore during drilling and workover operations. One purpose of the mud is to remove rock cuttings produced by drilling. The mud also helps to cool the drill bit, it prevents the borehole walls from caving in, constrains high-pressure formation fluids, and provides a medium for MWD mud-pulse transmission signals. See also drilling fluid type.

Drillstring	The rotating kelly, drill pipe, drill collars, subs, and drill bit in the well.

Dry gas	Natural gas that occurs without liquid hydrocarbons, or gas that has had condensable hydrocarbons removed (in the U.S. less than 0.1 gallon of condensables per 1,000 cubic feet of produced gas).

Dry hole	Dry hole