Chapter 2: Processing
Gathering systems primarily made up of small-diameter, low-pressure pipelines move raw natural gas from the wellhead to a natural gas processing plant, or fractionator.
Natural gas processing purifies raw natural gas by separating the natural gas liquids; contaminants such as hydrogen sulfide, carbon dioxide, nitrogen, water vapor and oxygen are separated from the natural gas stream before the natural gas is delivered into a mainline transmission system. This pipeline quality “dry” natural gas must meet certain standards as a consumer grade product. Regulated indirectly by the industries that extract, process, distribute and store natural gas, the actual end-user composition of natural gas – almost pure methane – is defined as being within a specific range of heat Btu content range +/- 50 Btu of 1,035 Btu per cubic foot. The gas must be delivered at a specified hydrocarbon dew point temperature level, and can contain only trace amounts of elements, particulate solids or liquid water that could be detrimental to the carrying equipment.
After purification processing, natural gas is pressurized to 6.9 MPa (about 1,000 psi) and sent through transmission networks. About 306,000 miles of wide-diameter, high-pressure interstate pipelines transport natural gas from the producing area to market areas, sometimes more than 1,500 miles from the producing wells.
More than 1,400 compressor stations are located along the length of the pipeline network, keeping the natural gas flowing. More than 200 companies operate these main transmission pipelines. As it nears the consumer, the distribution pressure is reduced to as low as 200 to 600 Pa.
Three principal types of underground storage facilities are used to store natural gas: depleted oil / natural gas fields, aquifer reservoirs and salt caverns. Storage is essential—it creates a backup supply and balances gas supplies on the pipelines operating in the region. In 2007, about 125 natural gas storage operators managed roughly 400 active storage fields. When needed, this reserve is withdrawn to meet additional customer demand during peak usage periods. Above-ground liquefied natural gas storage facilities are also used for this purpose.
Pennsylvania, New York and West Virginia have the largest underground storage capacity in the region. More pipeline capacity exits these states than enters, reflecting their storage capability as a seasonal supply source for the states to the north and east.