Oil & Gas Terms in Category U

Unconsolidated formation

Formations with insufficient cementing agents between the grains to stop movement of individual grains when fluid flows through the formation.

Usually less than 2 to 10 psi compressive strength.



See valence, unloader.

Same as a circulation valve


Of or relating to a condition in which pressure in the wellbore is less than the pressure in the formation.


A system of operating a certain oil and condensate reservoir in order to conduct some form of pressure maintenance, repressuring, waterflood, or other cooperative form to increase ultimate recovery.

Upper kelly cock

A valve installed above the kelly that can be closed manually to protect the rotary hose from high pressure that may exist in the drill stem

Underground blowout

An uncontrolled flow of gas, salt water, or other fluid out of the wellbore and into another formation that the wellbore has penetrated.


To enlarge the wellbore below the casing.


A soluble, weakly basic, nitrogenous compound used in the manufacture of resins and plastics.

Unloading sub

An unloader; provides a means to equalize tubing and annulus pressure.

Unconventional oil

Unconventional oil is petroleum produced or extracted using techniques other than the traditional oil well method.

Unconventional oil production is less efficient and has more environmental impacts than conventional oil production.

Oil industries and governments are investing in unconventional oil sources due to the increasing scarcity of conventional oil reserves.

Upper crown plug (subsea)

A plug that fits in the bore of a subsea tree to serve as the secondary barrier against reservoir pressure.



Any of various external electrical lines or fluid tubes which connects one portion of a system to another.


A control line attached to a remove piece of equipment, usually a subsea wellhead, to provide hydraulic or electrical control, or inject small amounts of chemicals.

Unloading a well

Removing fluid from the tubing in a well, often by means of a swab, to lower the bottomhole pressure in the wellbore at the perforations and induce the well to flow.

Unit operator

The oil company in charge of development and production in an oilfield in which several companies have joined to produce the field.

Ultraviolet light

Light waves shorter than the visible blue violet waves of the spectrum.

Crude oil, colored distillates, residuum, a few drilling fluid additives, and certain minerals and chemicals fluoresce in the presence of ultraviolet light.

These substances, when present in mud, may cause the mud to fluoresce.