Natural-gas processing is a complex industrial process designed to clean raw natural gas by separating impurities and various non-methane hydrocarbons and fluids to produce what is known as pipeline quality dry natural gas.
Natural-gas processing begins at the well head. The composition of the raw natural gas extracted from producing wells depends on the type, depth, and location of the underground deposit and the geology of the area. Oil and natural gas are often found together in the same reservoir. The natural gas produced from oil wells is generally classified as associated-dissolved, meaning that the natural gas is associated with or dissolved in crude oil. Natural gas production absent any association with crude oil is classified as “non-associated.” In 2004, 75 percent of U.S. wellhead production of natural gas was non-associated.
Most natural gas extracted from the Earth contains, to varying degrees, low molecular weight hydrocarbon compounds; examples include methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10). Although these hydrocarbons exist in a liquid state at underground pressures, they will become gaseous at normal atmospheric pressure. Collectively, they are called natural gas liquids (NGLs). The natural gas extracted from coal reservoirs and mines (coalbed methane) is the primary exception, being essentially a mix of mostly methane and about 10 percent carbon dioxide (CO2) .
Natural-gas processing plants purify raw natural gas extracted (a) from underground gas fields or (b) at the surface from fluids produced by oil wells. A fully operational plant delivers pipeline-quality dry natural gas that can be used as fuel by residential, commercial and industrial consumers. Plant processes remove contaminants; the NGL hydrocarbons, part of the contaminants, are recovered for other commercial uses. For economic reasons, however, some plants may be designed to yield an intermediate product typically containing over 90 per cent pure methane and smaller amounts of nitrogen, carbon dioxide, and sometimes ethane. This can be further processed in downstream plants or used as feedstock for chemicals manufacturing.