Microbial Methanogenesis In Laboratory Incubations Of Coal: Implications For A Sustainable Energy Resource In Subsurface Coalbeds

Methane desorbed from subsurface coalseams contributes about 8% of the total natural gas produced in the US. This value is expected to increase over the next several years as a growing proportion of energy demands are supplied from unconventional reservoirs. Isotopic analyses of gas samples from several geographically separate coalbeds indicates a substantial proportion of the sorbed methane is biogenic in origin. Furthermore, previous studies have shown the ability of microbial consortia to degrade coal in aerobic laboratory incubations. These findings suggests the stimulation of microbial methane production in subsurface coals may provide a sustainable source of domestic energy. To address this prospect, we assessed the ability of indigenous microbial populations to produce methane in coal maintained under anaerobic conditions in the laboratory and investigated factors that influenced the rate and extent of the process. Several freshly collected coals of different rank were examined for their ability to support methanogenesis in mineral medium alone or amended with different nutrients such as hydrogen (4 kPa), formate (20 mM), or acetate (25mM). Microbial methane production was distinguished from abiotic desorption by subtracting methane generated in replicate incubations that contained bromoethanesulfonic acid (5 mM), an inhibitor of methanogenesis. The extent and rate of methane production varied among the different coals. A relatively shallow (400 m), immature coal exhibited a rate of 700 nmole CH4*day-1*g coal-1, a value comparable to previous observations of contaminated sediments. Methane production was negligible in a deeper, relatively mature (650 m) coal obtained from the same borehole although the same material exhibited a rate of about 80 nmole CH4*day-1*g coal-1 after a formate amendment. In contrast, hydrogen proved to be ineffective as a methanogenic substrate, although this electron donor was rapidly consumed in coal incubations. A filter-sterilized warm water extract of spent coal renewed methanogenesis in incubations no longer generating methane, suggesting the cessation of methane production was not due to moribund cells or the accumulation of an inhibitory compound, but rather the lack of suitable electron donor. Viable methanogenic consortia were present in most of the coal samples examined in this study and their activity could be enhanced by electron donor amendment, which presumably supports microbial growth. Furthermore, the observation of rapid hydrogen consumption uncoupled from methanogenesis suggests competition exists for this compound. The success of efforts to stimulate methanogenesis in subsurface coalbeds will likely be influenced by the nature of the electron donor.