MICROBIAL METHANOGENESIS IN LABORATORY INCUBATIONS OF SUBSURFACE COALS

Methane produced from subsurface coal seams currently supplies about 8 % of the natural gas consumed in the United States and this number is expected to increase as a growing proportion of energy consumption is supported by this resource. It is estimated that 13 % of the U.S. land mass is underlain by coal, some of which contains adsorbed methane gas held in place by elevated pressures existing in the subsurface. However, the typical coalbed methane well produces economically feasible quantities of gas for less than 8 years and technological limitations restrict current exploration to coal seams that are relatively permeable and shallow. Thus, development of new methodologies that increase gas productivity of methane-bearing coalbeds is an area of active research. We examined methane production in coal incubations amended with either an inorganic salts medium, gaseous H₂/CO₂, or more complex nutrients that could potentially provide carbon and energy for methanogenesis. The fastest rates of methanogenesis –0.26 standard cubic feet/ton/day– were observed (after a 60-day lag) in incubations amended with H₂/CO₂. In addition, periodic reamendment with H₂/CO₂, resulted in sustained production of stoichiometric amounts of methane, indicating the development of a stable methanogenic community. Chloroform extracts of coal samples showed a notable variation in the amount of extractable organic matter ranging from <0.4 to 51.5 mg/g coal. Laboratory incubations demonstrated a positive trend in the rate and extent of methane production among coals containing increasing amounts of extractable organic matter. These findings indicate ongoing methanogenesis in subsurface coals may be a viable resource for domestic natural gas production.