EVALUATING COAL FOR POTENTIAL BIOGENIC METHANE GENERATION USING A BIOASSAY TO ASSESS COAL BIOAVAILABILITY

Secondary biogenic methane generation offers the potential to extend the gas productivity of coal beds as an energy source. A bioassay was developed to determine the amount of coal organic matter microbially available for conversion to methane. A microbial consortium (WBC-2) was shown to produce secondary biogenic methane from several different types of coal in laboratory experiments. WBC-2 is a mixed culture enriched from wetland sediments and includes fermentative, acetogenic and methanogenic microorganisms. The WBC-2 bioassay was used to compare the methane generating potential of 10 subbituminous coal samples collected from beds in the Wilcox Group (Paleocene-Eocene, Zavala Co, TX) and the Fort Union Formation [Paleocene, four counties, Powder River Basin (PRB), WY]. The contribution of desorbed methane in the bioassays was determined in negative controls treated with bromoethane sulfonic acid, a methanogen inhibitor. The bioassay generated the most methane [80 µmole methane per g coal (56 scf/ton)] from the Texas coal samples collected from a non-gas-producing well. Methane production at the well sampled in the Texas coal may be limited by environmental factors such as high sulfate in the coal-associated water. PRB coals contained more desorbed (original) methane than the Texas coal sample and generated much less biogenic methane in the bioassay. The highest production of secondary biogenic methane from a PRB coal sample was from the upper part of the Wyodak-Anderson, a dewatered coal bed (southern Campbell Co, WY), which generated 23 µmole/g (16 scf/ton) methane. In other PRB coal samples tested, methane generation by the microbial consortium varied from 0 to 9 µmole methane per g coal. The bioassay offers a new tool for measuring the potential of coal for secondary methane generation, and provides a platform for studying the mechanisms involved in this economically important activity.