In the upstream oil industry, a gas–oil separation plant (GOSP) is temporary or permanent facilities that separate wellhead fluids into constituent vapor (gas) and liquid (oil and produced water) components.

Temporary plant

Temporary gas–oil separation facilities are associated with newly drilled or newly sidetracked wells where the production potential of the well is being assessed. The plant, comprising a test separator vessel, is connected to the

wellhead A wellhead is the component at the surface of an oil or gas well that provides the structural and pressure-containing interface for the drilling and production equipment. The primary purpose of a wellhead is to provide the suspension point and ...

after the

choke valve In internal combustion engines with carburetors, a choke valve or choke modifies the air pressure in the intake manifold, thereby altering the air–fuel ratio entering the engine. Choke valves are generally used in naturally aspirated engines to ...

. The separator allows the fluids to separate by gravity into its component phases: solids such as sand (the densest phase) settle to the bottom of the separator, then produced water and oil which are drawn separately from the base of the separator, and vapor or gas (the lightest phase) separates to the top of the separator vessel from where it is withdrawn. Each of the three fluid phases is metered to determine the relative flow-rates of the components and production potential of the well. In temporary facilities the vapor is generally flared; produced water is disposed of overboard after treatment to reduce its oil content to statutory levels; and the crude oil phase may be diverted to tote tanks for removal and treatment onshore. Alternatively, if the temporary GOSP plant is associated with a permanent production facility, the oil phase may be treated in the installation's permanent gas–oil separation plant.

Permanent plant

Permanent gas–oil separation plant is associated with permanent offshore production facilities. For a full description of such a plant, see

Oil production plant An oil production plant is a facility which processes production fluids from oil wells in order to separate out key components and prepare them for export. Typical oil well production fluids are a mixture of crude oil, oil, natural gas, gas and pr ...

. A gas–oil–and–water separator is called a 3-phase separator. The gas and oil or condensate are pumped through designated pipelines, while the sand and other solids are washed from the separator and disposed of overboard.

Reasons for processing

Multi-phase production

Water need not be separated, and a single liquid (oil and water) phase produced together with a separate gas phase. Chemicals are added so that the crude and water emulsify. This process is then reversed at the storage and processing facility by adding demulsifiers that make the water separate out, and is drawn from the bottom of the tank. After storage, the crude oil can be sold to refineries, which produce fuels, chemicals, and energy products.

Pressure

The well fluids at the wellhead are at high pressure. Production pressures of greater than are not uncommon, but typically are lower than this. The high pressure is reduced at the choke valve to typically 7 to 30 bar at the separator, although the first stage separator could operate at higher pressure c. 250 bar. Modern oil recovery practice may place a hydro-cyclone to replace the temporary GOSP, allowing the water to be removed immediately and re-injected into the reservoir. The hydro-cyclone will vary the flows according to the water content and can also separate condensate from the gas where separate storage and export can be provided for the products close to the production well (e.g. on offshore platforms).

Contaminants

Crude oil leaving the well may contain quantities of sulfur (e.g.

hydrogen sulfide Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...

and

thiol In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl grou ...

s) and/or carbon dioxide, and is known as "sour" crude. The gas–oil separator will typically partition the hydrogen sulfide and carbon dioxide preferentially into the vapor or gas phase, where it may be further treated. The most usual "crude sweetening packages" use amines to remove the sulfur and CO₂ content. Crude that contains water is called "wet", and the water can then be bound in an emulsion in the crude to allow pumping through a pipeline. The crude is processed and treated to make it acceptable for the entry and transportation specification of the pipeline, before it can be transported to a refinery for processing.

Phase separation

It is often appropriate to separate gases and liquids for separate processing. This also involves the separation of oily and water liquid phases.

Gas recovery

In the past, and in some places today, the gas is considered a waste product and was flared off (burned). Collecting the gas reduces carbon emissions, and produces a marketable commodity.

References

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External links

The Gas-Oil Separation Process
Petroleum technology