Oil & Gas Terms in Category D

Dead well

A well that will not flow.

Drill string

The column, or string, of drill pipe with attached tool joints that transmits fluid and rotational power from the kelly to the drill collars and bit.

Often, especially in the oil patch, the term is loosely applied to both drill pipe and drill collars.

Compare drill stem.

Duplex pump

A reciprocating pump with two pistons or plungers and used extensively as a mud pump on drilling rigs.

Drill ship

A self-propelled floating offshore drilling unit that is a ship constructed to permit a well to be drilled from it.

While not as stable as semisubmersible, drill ships are capable of drilling exploratory wells in deep, remote waters.

They may have a ship hull, a catamaran hull, or a trimaran hull.

See floating offshore drilling rig.

Depth reference

The point in a well from which depth is measured.

Alternatively, the depth reference is the point at which the depth is defined as being zero. it is typically the top of the kelly bushing or the level of the rig floor on the rig used to drill the well.

The depth measured from that point is the measured depth (md) for the well.

Even when the drilling rig has been removed, all subsequent measurements and operations in the well are still tied in to the same depth reference.

However, for multiwell studies, the depths are normally shifted to the permanent datum (generally mean sea level).

The depth reference and its elevation above the permanent datum are recorded on the log heading.

In some contexts, the term may refer to any point from which depth is measured.

Dst tool

Drill stem test tool; used for formation evaluation.


Liquid added to dilute or thin a solution

Drag blocks

Spring-loaded buttons on a packer that provide friction with casing to retard movement of one section of a packer while another section rotates for setting.


A powered machine for earthwork excavations.

Directional survey

A logging method that records rift angle, or deflection from the vertical, and direction of the drift.

A single-shot directional-survey instrument makes a single photograph of a compass reading of the draft direction and the number of degrees the hole is off vertical.

A multishot survey instrument obtains numerous readings in the hole as the device is pulled out of the well.

See directional drilling.

Drill pipe

Heavy seamless tubing used to rotate the bit and circulate the drilling fluid.

Joints of pipe approximately 30 feet (9 meters) long are coupled together by means of tool joints.


The separation of a molecule into two or more fragments (atoms, ions) by interaction with another body or by the absorption of electromagnetic radiation.



Dogleg severity

Drilled show

Oil or gas in the mud circulated to the surface


The removal of water or water vapor from gas or oil