Raw gas from lease facilities is transported through a gathering system of small diameter pipes to a processing facility where it is separated into flammable gases and liquids (methane/natural gas, ethane, propane, butane, and pentane), nonflammable gases (carbon dioxide and nitrogen) and impurities (such as water vapor, sulphur, and sand), which are removed if the quantities of these materials exceed pipeline standards. The natural gas liquids (ethane, propane, butane, and natural gasolines), often called NGLs, are valuable by-products of the processing and in some situations are worth as much or more than the natural gas.
Other by-products such as sulphur and carbon dioxide may also be processed and sold. Processing facilities are usually located on the gathering systems so that the gas is processed and cleaned prior to entering a transmission line. Less frequently, gas is processed directly at the wellhead. It may also be reprocessed on the mainline pipe to further extract NGLs. If the gas contains significant amounts of condensate (gas that becomes liquid when exposed to atmospheric pressure), the condensate is removed early in the flow process and is sold like crude oil.
Natural gas processing can be complex and usually involves several processes, or stages, to remove oil, water, NGLs, and impurities such as sulfur, helium, nitrogen, hydrogen sulfide, and carbon dioxide. The composition of the wellhead natural gas determines the number of stages and the processes required to produce pipeline-quality dry natural gas. These stages and processes may be integrated into one unit or operation, performed in a different order or at alternative locations (lease/plant), or not required at all.
The basic stages of natural gas processing/treatment are:
Gas-oil separation: For associated wells, gas and oil are separated at the lease facility. Water may also be removed.
Condensate separation: Condensates are removed from the natural gas stream at the lease facility with separators. Extracted condensate is sent to storage tanks. Water may also be removed.
Dehydration: A dehydration process removes water that may cause the formation of undesirable hydrates and water condensation in pipelines.
Contaminant removal: Nonhydrocarbon gases — such as hydrogen sulfide, carbon dioxide, water vapor, helium, nitrogen, and oxygen — are removed from the natural gas stream. The most common removal technique is to direct the natural gas though a vessel containing an amine solution. Amines absorb hydrogen sulfide and carbon dioxide from natural gas and can be recycled and regenerated for repeated use.
Nitrogen extraction: Once the hydrogen sulfide and carbon dioxide are reduced to acceptable levels, the natural gas stream is routed to a Nitrogen Rejection Unit (NRU), where it is further dehydrated.
Methane separation: The process of demethanizing the natural gas stream can occur as a separate operation in a natural gas processing plant or as part of the NRU operation. Cryogenic processing and absorption methods are some of the ways used to separate methane from hydrogen gas liquids (HGLs).
Fractionation: Fractionation separates the HGLs into component natural gas liquids (NGLs) using the varying boiling points of the individual gases.
The dry gas then enters a transmission pipeline for delivery to markets, while the NGLs are transported by pipeline or truck to markets.