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Methane has been observed in coalbeds since underground mining of that resource began. This gas, however, has only recently been recognized as an economically producible resource. Coal underlies approximately 360,000 sq mi of the conterminous United States but because of the paucity of data on coals deeper than 3,000 ft the size of the deep coal and therefore deep gas resource is not well known.
Coal responds to increased temperature over time by increasing in rank or thermal maturity. During this maturation process increased volumes of methane are generated. Coal is identified as a humic, Type III, kerogen and as such yields methane as its primary hydrocarbon product and water, carbon dioxide, and nitrogen as nonhydrocarbon products. It is estimated that more than 7,000 cubic feet of methane is generated for each ton of coal during coalification from lignite to anthracite rank.
Methane is found in coals either adsorbed on the coal surfaces, as free gas in fractures and large pores or dissolved in ground water in coalbeds. The amount of gas stored in the coals is influenced by depth of burial and its related pressure, rank of coal and its related porosity distribution, and a time-maturity relationship. Adsorption isotherm determinations show the maximum amount of methane that can be adsorbed on coals of various ranks. Desorption analyses of coal core samples indicate that for high rank coals adsorbed gas may exceed that predicted by isotherm analyses.
Methane has been produced from coals since the early 1900s. Producibility is influenced by depth, rank, permeability, water saturation, and other hydrogeologic characteristics. Current commercial gas production from coalbeds has been documented in the Warrior Basin, Northern Appalachian Basin, and the San Juan Basin. One well in the San Juan Basin is producing at a rate in excess of 1.5 MMcfd from a 17-ft coalbed.
Possible constraints to coalbed gas production involve completion technology and legal and institutional inconsistencies. Research is currently underway to enhance gas production from deep coals. Legal and regulatory constraints are being addressed through appropriate state and federal channels to resolve questions of gas ownership and price.
Data indicate that the coalbed methane resource is producible at currently economic rates.
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