Glossary Of Upstream Terms

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Prelude Floating LNG Terminal


Probably the most exciting project in the world of Oil & Gas. The platform will be the largest ship ever built



  • To cease work on a well which is non-productive, to plug off the well with cement plugs and salvage all recoverable equipment Also used in the context of field abandonment. See also decommissioning


  • The space between the drillstring and the well wall, or between casing strings, or between the casing and the production tubing.

Appraisal Well:

  • A well drilled as part of an appraisal drilling programme which is carried out to determine the physical extent, reserves and likely production rate of a field.

Associated Gas:

  • Natural gas associated with oil accumulations, which may be dissolved in the oil at reservoir conditions or may form a cap of free gas above the oil.



  • A unit of volume measurement used for petroleum and its products (7.3 barrels = 1 ton: 6.29 barrels = 1 cubic metre).


  • One barrel of oil; 1 barrel = 35 Imperial gallons (approx.), or 159 litres (approx.); 7.5 barrels = 1 tonne (approx.); 6.29 barrels = 1 cubic metre.


  • Billion cubic feet; 1 bcf = 0.83 million tonnes of oil equivalent.


  • Billion cubic metres (1 cubic metre = 35.31 cubic feet).


Condensate and gas is produced simultaneously from the outset of production.

Blow-out preventers

(BOPs) Are high pressure wellhead valves, designed to shut off the uncontrolled flow of hydrocarbons.


When well pressure exceeds the ability of the wellhead valves to control it. Oil and gas "blow wild" at the surface.


The hole as drilled by the drill bit.


Capital expenditure.

Casing string

The steel tubing that lines a well after it has been drilled. It is formed from sections of steel tube screwed together.

Central estimate

A range of exploration drilling scenarios from which the following activity levels, based on recent historical experience, are adopted as the central estimates.

Christmas tree

The assembly of fittings and valves on the top of the casing which control the production rate of oil.

Commercial field

An oil and/or gas field judged to be capable of producing enough net income to make it worth developing.


The installation of permanent wellhead equipment for the production of oil and gas.


Hydrocarbons which are in the gaseous state under reservoir conditions and which become liquid when temperature or pressure is reduced. A mixture of pentanes and higher hydrocarbons.

Connate water

Salt water occurring with oil and gas in the reservoir.


Taking rock samples from a well by means of a special tool — a "core barrel".

Crane barge

A large barge, capable of lifting heavy equipment onto offshore platforms. Also known as a "derrick barge".

Creaming Theory

A statistical technique which recognises that in any exploration province after an initial period in which the largest fields are found, success rates and average field sizes decline as more exploration wells are drilled and knowledge of the area matures.

Cubic foot

A standard unit used to measure quantity of gas (at atmospheric pressure); 1 cubic foot = 0.0283 cubic metres.


Rock chippings cut from the formation by the drill bit, and brought to the surface with the mud. Used by geologists to obtain formation data.


Preferred term (rather than Abandonment) for the re-use, recycling and disposal of redundant oil and gas facilities


The tower-like structure that houses most of the drilling controls.

Development phase

The phase in which a proven oil or gas field is brought into production by drilling production (development) wells.


An Exploration well which has encountered hydrocarbons.

Drilling rig

A drilling unit that is not permanently fixed to the seabed, e.g. a drillship, a semi-submersible or a jack-up unit. Also means the derrick and its associated machinery.

Dry Gas

Natural gas composed mainly of methane with only minor amounts of ethane, propane and butane and little or no heavier hydrocarbons in the gasoline range.

Dry hole

A well which has proved to contain no hydrocarbons.


Abbreviation for exploration and appraisal.


Abbreviation for exploration and production.

Enhanced oil recovery

A process whereby oil is recovered other than by the natural pressure in a reservoir.

Exploration drilling

Drilling carried out to determine whether hydrocarbons are present in a particular area or structure.

Exploration phase

The phase of operations which covers the search for oil or gas by carrying out detailed geological and geophysical surveys followed up where appropriate by exploratory drilling.

Exploration well

A well in an unproven area or prospect, may also be known as a "wildcat well".

Farm in

When a company acquires an interest in a block by taking over all or part of the financial commitment for drilling an exploration well.


A geographical area under which an oil or gas reservoir lies.


Retrieving objects from the borehole, such as a broken drillstring, or tools.


  • A pipe, laid on the seabed, which allows the transportation of oil/gas production or injection of fluids. Its length can vary from a few hundred meters to several kilometers.

Formation pressure

The pressure at the bottom of a well when it is shut in at the wellhead.

Formation water

Salt water underlying gas and oil in the formation.


A method of breaking down a formation by pumping fluid at very high pressures. The objective is to increase production rates from a reservoir.




Gas Condensate.

Gas field

A field containing natural gas but no oil.

Gas injection

The process whereby separated associated gas is pumped back into a reservoir for conservation purposes or to maintain the reservoir pressure.

Gas/oil ratio

The volume of gas at atmospheric pressure produced per unit of oil produced.


  • The method of making a connection to an existing pipeline containing hydrocarbons under pressure.


A compound containing only the elements hydrogen and carbon. May exist as a solid, a liquid or a gas. The term is mainly used in a catch-all sense for oil, gas and condensate.

Injection well

A well used for pumping water or gas into the reservoir.


The lower section, or "legs", of an offshore platform.


Complex mixture of compounds with large molecules containing mainly hydrogen and carbon but also oxygen, nitrogen, and sulfur.


A well is said to "kick" if the formation pressure exceeds the pressure exerted by the mud column.

Lay barge

A barge that is specially equipped to lay submarine pipelines.

Liquefied natural gas (LNG)

Oilfield or naturally occurring gas, chiefly methane, liquefied for transportation.

Liquefied petroleum gas (LPG)

Light hydrocarbon material, gaseous at atmospheric temperature and pressure, held in the liquid state by pressure to facilitate storage, transport and handling. Commercial liquefied gas consists essentially of either propane or butane, or mixtures thereof.


Million Barrels Oil Equivalent. Metric tonne Equivalent to 1000 kilos, 2204.61 lbs; 7.5 barrels.


Millions of cubic feet per day (of gas).


An aperture in the centre of a drillship or semi-submersible drilling rig, through which drilling and diving operations can be conducted.


Million tonnes.


A mixture of base substance and additives used to lubricate the drill bit and to counteract the natural pressure of the formation.

Natural gas

Gas, occurring naturally, and often found in association with crude petroleum.


Natural gas liquids. Liquid hydrocarbons found in association with natural gas.




Oil and Gas.


A mixture of liquid hydrocarbons of different molecular weights.

Oil field

A geographic area under which an oil reservoir lies.

Oil in place

An estimated measure of the total amount of oil contained in a reservoir, and, as such, a higher figure than the estimated recoverable reserves of oil.


The company that has legal authority to drill wells and undertake production of hydrocarbons are found. The Operator is often part of a consortium and acts on behalf of this consortium.


Operating expenditure.


Rock in which oil and gas are found in exploitable quantities.


The property of a formation which quantifies the flow of a fluid through the pore spaces and into the wellbore.


A generic name for hydrocarbons, including crude oil, natural gas liquids, natural gas and their products.


An offshore structure that is permanently fixed to the seabed.


The percentage of void in a porous rock compared to the solid formation.

Possible reserves

Those reserves which at present cannot be regarded as 'probable' but are estimated to have a significant but less than 50% chance of being technically and economically producible.

Primary recovery

Recovery of oil or gas from a reservoir purely by using the natural pressure in the reservoir to force the oil or gas out.

Probable reserves

Those reserves which are not yet proven but which are estimated to have a better than 50% chance of being technically and economically producible.

Proven field

An oil and/or gas field whose physical extent and estimated reserves have been determined.

Proven reserves

Those reserves which on the available evidence are virtually certain to be technically and economically producible (i.e. having a better than 90% chance of being produced).

Recoverable reserves

That proportion of the oil and/gas in a reservoir that can be removed using currently available techniques.

Recovery factor

The ratio of recoverable oil and/or gas reserves to the estimated oil and/or gas in place in the reservoir.


The underground formation where oil and gas has accumulated It consists of a porous rock to hold the oil or gas, and a cap rock that prevents its escape.

Riser (drilling)

A pipe between a seabed BOP and a floating drilling rig.

Riser (production)

The section of pipework that joins a seabed wellhead to the Christmas tree.


Drill crew members who work on the derrick floor, screwing together the sections of drillpipe when running or pulling a drillstring.


Drill crew members who handle the loading and unloading of equipment and assist in general operations around the rig.

Royalty payment

The cash or kind paid to the owner of mineral rights.

Secondary recovery

Recovery of oil or gas from a reservoir by artificially maintaining or enhancing the reservoir pressure by injecting gas, water or other substances into the reservoir rock.


A production hiatus during which the platform ceases to produce while essential maintenance work is undertaken.

Significant Discovery

A BERR definition of a well which flow tested, or would have flowed, at a rate of 1000 barrels of oil a day or 15 million cubic feet of gas a day.


The operation of drilling the first part of a new well.

Steam Assisted Gravity Drainage (SAGD)

  • Steam Assisted Gravity Drainage is a form of "in situ" bitumen recovery, used where the bitumen deposits are too far below ground to extract them with open cast mining

Suspended well

A well that has been capped off temporarily. tcf Trillion Cubic Feet (of gas).


Second-in-command of a drilling crew under the drilling superintendent. Responsible for the day-to-day running of the rig and for ensuring that all the necessary equipment is available.


The superstructure of a platform. Well log A record of geological formation penetrated during drilling, including technical details of the operation.


  • An assembly of steel tubes and/or thermoplastic hoses which can also include electrical cables or optic fibres used to control subsea structures from a platform or a vessel.

Wildcat well

A well drilled in an unproven area. Also known as a "exploration well". [Ety. The term comes from exploration wells in West Texas in the 1920s. Wildcats were abundant in the locality, and those unlucky enough to be shot were hung from oil derricks.]


Remedial work to the equipment within a well, the well pipework, or relating to attempts to increase the rate of flow.

Adapted from UK Government Oil & Gas Website

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