Glossary

Accommodation platform/rig

a platform or semi-submersible rig that has been specially built or adapted to act as a 'hotel' for offshore personnel.

Acidisation

a stimulation technique whereby acid (generally some form of hydrochloric) is injected into the reservoir rock to remove cement and improve permeability, thereby increasing the flow of oil or gas.

Acoustic log

a record of the time taken by an acoustic (sound) wave to travel over a certain distance through the geological formations. Also called a sonic log. See also seismic survey.

Acreage

area covered by a lease granted for oil and gas exploration and for possible future production.

Air gun

chamber from which compressed air is released to produce shock waves in the earth. The air gun array is the most common technique used for seismic surveying at sea.

Alkanes

the simplest homologous series of saturated aliphatic hydrocarbons, consisting of methane (CH4), ethane (C2H6), propane (C3H8), etc., also known as the paraffins.

Alkenes

the homologous series of aliphatic hydrocarbons containing one double (unsaturated) carbon-to-carbon bond, consisting of ethene (C2H4), propene (C3H6), butene (C4H8),etc., also known as olefins.

Annulus

the name given to the annular space between the drill string and the well bore.

Anticlinal tap

a hydrocarbon trap formed by the upward bowing of strata into a dome or arch.

Anticline

a fold in layered rocks originating below the surface in the form of an elongated dome. Anticlines make excellent drilling prospects since any oil in the deposit will naturally rise to the highest point of the structure because oil has a lower specific gravity than water. See tectonic process.

API gravity

the universally accepted scale adopted by the American Petroleum Institute for expressing the specific gravity of oils: API gravity = _________141.5________ _ 131.5 specific gravity at 60 deg F Water at 4 deg C has an API gravity of 10. Crude oil gravities generally range from 10 - 50 deg.

Appraisal drilling

drilling carried out to determine the physical extent, reserves and likely production rate of a field.

Appraisal well

a well drilled as part of an appraisal drilling programme.

Aqueous

watery. Usually applied to solutions indicating water is present.

Aquifer

an underground zone of permeable rock saturated with water under pressure. For gas storage applications, an aquifer will need to consist of a permeable lower layer of rock and an impermeable upper layer (or 'cap') with a cavity for storing gas.

Asphalt

the mixture of bitumen and aggregate used for road surfacing.

Assets

property, inventory or stock, plant, money, etc. owned by a company.

Associated gas

natural gas found in association with oil, either dissolved in the oil at reservoir conditions (solution gas) or as a cap of free gas above the oil (gas cap).

Associated liquids

liquid hydrocarbons found in association with natural gas