Oil & Gas Terms in Category D

Drill stem

All members in the assembly used for rotary drilling from the swivel to the bit, including the kelly, drill pipe and tool joints, drill collars, stabilizers, and various specialty items.

Compare drill string.


Pertaining to the wellbore.


A centrifugal device for removing sand from drilling fluid to prevent abrasion of the pumps.

It may be operated mechanically or by a fast-moving stream of fluid inside a special cone-shaped vessel.

Compare desilter.

Disposal well

A well through which water (usually salt water) is returned to subsurface formations.

Drill pipe pressure gauge

An indicator, mounted in the mud circulating system, that measures and indicates the amount of pressure in the drill stem.

See drill stem.

Drill ship

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

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

See floating offshore drilling rig.

Directional drilling


Intentional deviation of a wellbore from the vertical.

Although wellbores are normally drilled vertically, it is sometimes necessary or advantageous to drill at an angle from the vertical.

Controlled directional drilling makes is possible to reach subsurface areas laterally remote from the point where the bit enters the earth.

It often involves the use of turbodrills, dyna-drills, whipstocks, or other deflecting rods.

2. the intentional deviation of a wellbore from the path it would naturally take.

This is accomplished through the use of whipstocks, bottomhole assembly (bha) configurations, instruments to measure the path of the wellbore in three-dimensional space, data links to communicate measurements taken downhole to the surface, mud motors and special bha components and drill bits.

The directional driller also exploits drilling parameters such as weight on bit and rotary speed to deflect the bit away from the axis of the existing wellbore.

In some cases, such as drilling steeply dipping formations or unpredictable deviation in conventional drilling operations, directional-drilling techniques may be employed to ensure that the hole is drilled vertically.

While many techniques can accomplish this, the general concept is simple: point the bit in the direction that one wants to drill.

The most common way is through the use of a bend near the bit in a downhole steerable mud motor.

The bend points the bit in a direction different from the axis of the wellbore when the entire drillstring is not rotating.

By pumping mud through the mud motor, the bit turns while the drillstring does not rotate, allowing the bit to drill in the direction it points.

When a particular wellbore direction is achieved, that direction may be maintained by rotating the entire drillstring (including the bent section) so that the bit does not drill in a single direction off the wellbore axis, but instead sweeps around and its net direction coincides with the existing wellbore. rotary steerable tools allow steering while rotating, usually with higher rates of penetration and ultimately smoother boreholes.

Dispersed phase

That part of a drilling mud–clay, shale, barite, and other solids–that is dispersed throughout a liquid or gaseous medium, forming the mud.

Dynamic positioning

A method by which a floating offshore drilling rig is maintained in position over an offshore well location without the use of mooring anchors.

Generally, several propulsion units, called thrusters, are located on the hulls of the structure and are actuated by a sensing system.

A computer to which the system feeds signals directs the thrusters to maintain the rig on location.



A small house used for keeping lease records, changing clothes, or any other use around a lease.

2. the steel-sided room adjacent to the rig floor, usually having an access door close to the driller’s controls.

This general-purpose shelter is a combination tool shed, office, communications center, coffee room, lunchroom and general meeting place for the driller and his crew.

It is at the same elevation as the rig floor, usually cantilevered out from the main substructure supporting the rig.

Drillable packer

A packer assembly that can be removed from the wellbore only by drilling or milling.

Drillable packers, and similar tools such as bridge plugs, are typically made from cast iron, aluminum, plastic or similar brittle materials.

Diamond bit

A tool for drilling rock that works by scraping industrial grade diamonds against the bottom of the hole.

The diamonds are embedded into the metal structure (usually a sintered or powdered carbide base matrix) during the manufacture of the bit.

The bit designer has virtually unlimited combinations of bit shape, the placement of hydraulic jetting ports, the amount of diamonds and the size of the diamonds used (usually expressed as diamonds per carat).

In general, a diamond bit that drills faster has a shorter lifetime.

Similarly, a bit designed for extremely long life will typically drill at a slower rate.

If a bit has a relatively high number of diamonds compared with other bits, it is said to be “heavy-set” and has higher durability.

A “light-set” bit, on the other hand, drills more aggressively, but wears out faster because fewer diamonds do the work.



The spontaneous movement and scattering of particles of liquids, gases, or solids.


The migration of dissolved substances from an area of high concentration to an area of low concentration.


N:a unit of measure of permeability.

A porous medium has a permeability of 1 darcy when differential pressure of 1 atmosphere across a sample 1 centimeter long and 1 square centimeter in cross section will force a liquid of 1 centipoise of viscosity through the sample at the rate of 1 cubic centimeter per second.

The permeability of reservoir rocks is usually so low that it is measured in millidarcys.


A spring-loaded finger in a tubing end locator.