Coalbed Natural Gas
Environments rich in plant material such as swamps, estuaries, and marshes were prolific in Wyoming during the Eocene Epoch (54 to 33 million years ago) and the Paleocene Epoch (65 to 54 million years ago). A large portion of the coal deposits that the U.S. exploits today were formed during these epochs. Coalbed natural gas (CBNG), also called coalbed methane, formed in these coal seams by biogenic or thermal processes. The primarily subbituminous Wasatch (Eocene-aged) and Fort Union (Paleocene-aged) formations are currently the main targets of Wyoming CBNG development. CBNG development in Wyoming first occurred in the late 1970s, but did not boom until the 1990s. Today there are more than 21,000 producing and shut-in CBNG wells in Wyoming, with the bulk of the wells in the Powder River Basin (PRB). Because of competition from unconventional gas reservoirs and lower natural gas prices, Wyoming CBNG production is on the decline.
How is coalbed natural gas formed?
Coalbed natural gas forms in one of two ways. During the earliest stage of coalification (the process that turns plant detritus into coal), biogenic methane is generated as a byproduct of bacterial respiration. Aerobic bacteria (those that use oxygen in respiration) first metabolize any free oxygen left in the plant detritus and the surrounding sediments. In fresh water environments, methane production begins immediately after the oxygen is depleted. Species of anaerobic bacteria (those that do not use oxygen) then reduce carbon dioxide and produce methane through anaerobic respiration. When the temperature of coal underground reaches approximately 122°F (50°C), and after a sufficient amount of time, most of the biogenic methane is fully generated. Also at this time nearly two-thirds of the moisture is expelled from the coal and it reaches a rank of subbituminous.
After the temperature of a coal exceeds 122°F (50°C), due to the geothermal gradient and excessive burial, thermogenic proceses begin to generate additional carbon dioxide, nitrogen, methane, and water. At this point the amount of hydrocarbons or volatile matter has increased and the coal reaches a rank of bituminous. When the temperature of the coal reaches 302°F (150°C), thermogenic production of methane is maximized.
Coalbed natural gas extraction
In the Powder River Basin, most coalbed natural gas wells are completed open-hole. This method involves setting casing to the top of the target coalbed, under-reaming the underlying target zone, and cleaning the coal with a fresh-water flush. A down-hole submersible pump removes water from the coal and depressurizes the aquifer. The methane gas desorbs from the coal, flows up the annulus, and is piped to a metering facility where the gas and water production from each well is recorded. The methane then flows to a compressor station where the gas is compressed and shipped via pipeline. The produced water is either diverted to a central discharge point (called an outfall) and then into a drainage or impoundment, or is re-injected into nearby aquifers.
Rice, DD and Claypool, GE., 1981, Generation, accumulation, and resource potential of biogenic gas: American Association of Petroleum Geologists Bulletin, v. 65, no. 1, p. 5-25.
Rightmire, CT., 1984., Coalbed methane resources, in Rightmire, C.T., Eddy, G.E., and Kirr, J.N., editors, Coalbed methane resources of the United States: American Association of Petroleum Geologists Explorer, v. 21, no.4 (April, 2000), p. 16, 18-20, 22-23.