Oil & Gas Terms in Category O


An opening of a measured diameter that is used for measuring the flow of fluid through a pipe, the orifice must be of smaller diameter than the pipe diameter.

It is drilled into an orifice plate held by an orifice fitting.

On-off tool

A tool used to open or close a downhole valve; a tool used to set or release a downhole tool, such as a retrievable bridge plug.


A tool that goes downhole and is not retrievable.



Of a wellbore, having no casing.


Of a hole, having no drill pipe or tubing suspended in it.

Opening/closing plug

A rubber plug used in primary cementing operations to displace cement slurry from the casing into the borehole annulus.


Said of a well that has produced more than its allowable.


The surface area overlying an oil reservoir or reservoirs.

The term usually includes not only the surface area, but also the reservoir, the wells, and the production equipment.

Oil-base mud

A drilling or workover fluid in which oil is the continuous phase and which contains from less than 2 percent and up to 5 percent water.

This water is spread out, or dispersed, in the oil as small droplets.

See oil mud.


Abbreviation: outer continental shelf

Oil – petroleum – gas

A fluid of vapor composed of hydrocarbons; dry gas is nearly free of oil and gasoline vapor.

Oil and gas separator

An item of production equipment used to separate liquid components of the well stream from gaseous elements.

Separators are either vertical or horizontal and either cylindrical or spherical in shape.

Separation is accomplished principally by gravity, the heavier liquids falling to the bottom and the gas rising to the top.

A float valve or other liquid-level control regulates the level of oil in the bottom of the separator.

Oil-in-water emulsion mud

Any conventional or special water-base mud to which oil has been added.

The oil becomes the dispersed phase and may be emulsified into the mud either mechanically or chemically.

Also called oil-emulsion mud.

Oil breakout

Oil that has risen to the surface of the mud but which was previously combined with the mud as emulsion.

Oil zone

A formation or horizon of a well from which oil may be produced.

The oil zone is usually immediately under the gas zone and on top of the water zone if all three fluids are present and segregated.


That geographic area which lies seaward of the coastline