Oil & Gas Terms in Category D


Any chemical that prevents of lessens frothing or foaming in another agent.


A substance added to cement that chemically wets the cement particles in the slurry, allowing the slurry to flow easily without much water.


To detect, locate and remove mistakes from a routine or malfunctions from a computer.

Drilling spool

A fitting placed in the blowout preventer stack to provide space between preventers for facilitating stripping operations, to permit attachment of choke and kill lines, and for localizing possible erosion by fluid flow to the spool instead of to the more expensive pieces of equipment.

Deviated well

A wellbore that is not vertical.

The term usually indicates a wellbore intentionally drilled away from vertical.

Driller’s depth

The depth of a well or features within the wellbore as measured while drilling.

The measured length of each joint of drillpipe or tubing is added to provide a total depth or measurement to the point of interest.

Drillers depth is the first depth measurement of a wellbore and is taken from the rotary table level on the rig floor.

In most cases, subsequent depth measurements, such as those made during the well completion phase, are corrected to the wellhead datum that is based on drillers depth.

Drilling contractor

An individual or group of individuals that own a drilling rig and contract their services for drilling wells.


The phase of petroleum operations that occurs after exploration has proven successful, and before full-scale production.

The newly discovered oil or gas field is assessed during an appraisal phase, a plan to fully and efficiently exploit it is created, and additional wells are usually drilled.


Equipment designed to remove small quantities of liquids from a gas stream.


A type of sedimentary rock similar to limestone but containing more than 50 percent magnesium carbonate; sometimes a reservoir rock for petroleum.

Drive-in unit

A type of portable service or workover rig that is self-propelled, using power from the hoisting engines.

The driver’s cab and steering wheel are mounted on the same end as the mast support; thus the unit can be driven straight ahead to reach the wellhead.

See carrier rig.

Dry gas

Natural gas that is produced without liquids; also a gas that has been treated to remove all liquids.


A worker who never stay long in one place.

Dart-type inside blowout preventer

A dart-shaped drill pipe inside blowout preventer installed on top of the drill stem when the well is kicking through the drill stem.

It is stabbed in open then closed against pressure.

Downhole safety valve

A downhole device that isolates wellbore pressure and fluids in the event of an emergency or catastrophic failure of surface equipment.

The control systems associated with safety valves are generally set in a fail-safe mode, such that any interruption or malfunction of the system will result in the safety valve closing to render the well safe.

Downhole safety valves are fitted in almost all wells and are typically subject to rigorous local or regional legislative requirements.