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Oil and gas processing
Oil and gas wells produce a mixture of hydrocarbon gas, condensate or oil; water with dissolved minerals, usually including a large amount of salt; other gases, including nitrogen, carbon dioxide (CO2), and possibly hydrogen sulfide (H2S); and solids, including sand from the reservoir, dirt, scale, and corrosion products from the tubing. The purpose of oil and gas processing is to separate, remove, or transform these various components to make the hydrocarbons ready for sale.
For the hydrocarbons(gas or liquid) to be sold, they must be:
- separated from the water and solids
- transported by pipeline, truck, rail, or ocean tanker to the user
The goal is to produce oil that meets the purchaser’s specifications that define the maximum allowable amounts of the following:
- other impurities
Similarly, the gas must be processed to meet purchaser’s water vapor and hydrocarbon dewpoint specifications to limit condensation during transportation.
The produced water must meet the following requirements:
- regulatory requirements for disposal in the ocean if the wells are offshore
- reservoir requirements for injection into an underground reservoir to avoid plugging the reservoir
- technical requirements for other uses, such as feed to steam boilers in thermal-flood operations, or in special cases, for irrigation
The equipment between the wells and the pipeline, or other transportation system, is called an oilfield facility. An oilfield facility is different from a refinery or chemical plant in a number of ways. The process is simpler in a facility, consisting not of chemical reactions to make new molecules, but of:
- phase separation
- temperature changes
- pressure changes
In a refinery, the feed-stream flow rate and composition are defined before the equipment is designed. For a facility, the composition is usually estimated based on drillstem tests of exploration wells or from existing wells in similar fields. The design flow rates are estimated from well logs and reservoir simulations. Even if the estimates are good, the following change over the life of the field as wells mature and new wells are drilled:
- flow rates (gas, oil, and water)
Facilities have a design rate that is a best-guess maximum flow based on:
- the number of wells
- production profiles
- total oil or gas that can be produced from the reservoir
Function of a facility
The main function of an oil facility is to:
- separate the oil, gas, water, and solids
- treat the oil to meet sales specifications (e.g., BS&W, salt content, vapor pressure)
- measure and sample the oil to determine its value
- deliver it to the transportation system (i.e., the pipeline, truck, ship, or railroad car)
The gas must be treated for sales or disposal. In the past, disposal sometimes meant flaring or venting, but now gas that can’t be transported is usually compressed for reinjection into the reservoir. Gas treating may involve only separation from the liquids, or it may include additional processes such as:
- removing H2S and CO2
- gas processing to condense heavier components that can be transported as a liquid
The following pages included more details:
In addition to processing the oil and gas for sale, the produced water and solids must be treated for disposal. For produced water, treating usually includes removal of dispersed and dissolved hydrocarbons and, in addition to separation or oil skimming, may include:
If treating of solids is required, it may include water washing and agitating the solids to remove the oil and then separating the water from them.
In addition to the process systems, auxiliary process heating and cooling may be required. Process heat is usually needed for oil treating and superheating fuel gas for use in gas turbine generators or compressors. Process cooling is usually required for gas compression.
While, if necessary, facilities can be run without electric power, power generation and electrical systems will usually be included for a facility that is large or complex or for living quarters that are provided for personnel.
All facilities require safety systems, including:
- safety instrumentation and shutdown system
- fire and gas detection
- fire-fighting equipment
- a means of evacuation, such as life rafts and escape capsules for offshore
- other equipment, depending on the location and complexity of the facility and whether it is manned
The actual production rates for a facility increase as the wells are completed up to the design rate. This rate will be maintained as long as possible by drilling additional wells; then,
- oil and gas rates will decline
- water rates will increase
- flowing pressure will decrease as the reservoir is depleted
The equipment must be designed to operate over a range of flow rates with uncertain compositions and temperatures.
Gas is usually restricted to pipeline transportation but can also be shipped in pressure vessels on ships, trucks, or railroad cars as compressed natural gas or converted to a liquid and sent as a liquefied natural gas (LNG).
Noteworthy papers in OnePetro
Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read
Smith, Graeme. 2015. Upgrading Crude Oil Separators and Systems for Mature Oil Fields. SPE PFC Webinar. https://webevents.spe.org/products/upgrading-crude-oil-separators-and-systems-for-mature-oil-fields
Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro