Gas Generation in the Powder River Basin: By Microbial Fermenters and CO2 Reducers
Total cumulative gas production from 1990-2008 in the Powder River Basin (PRB) in Wyoming and Montana was >3.1 TCF. Methanogenesis resulting in gas generation in the PRB, as determined by isotopic analyses (δ13CCH4, δDCH4, δ13CCO2, δDH2O), was caused entirely by early-stage microbial CO2 reducers and by late-stage microbial methyl-type fermenters (Flores et al., 2008). Gas generated by fermenters was aided by infiltration of nutrient-rich groundwater recharged from basin-edge clinker areas. About 25% of the gas was generated by fermenters in shallow (190-650 ft) depths along the northwest and east margins of PRB, and about 75% was generated by CO2 reducers in deeper parts (depths 650-2,000 ft) of the basin.
One hundred and ten wells were studied with respect to volumes of produced gas and water and their chemical compositions, coal reservoir depths, coal rank, and geology. Produced water from shallow wells completed in Wyodak-Anderson coal reservoirs is of a type that contains mixed cations (Ca, Mg, Na) and anions (SO4, HCO3) with high HCO3 and low SO4. This composition, along with fractionation between δDCH4 and δDH2O, indicates that microbial gas was generated by methyl-type fermentation. During the life span (3-8 years) of production, wells with gas generated by fermenters produced less gas (3.1 BCF) than wells with gas generated by CO2 reducers (8.3 BCF). During the same period, water production from wells with gas generated by fermenters was 11.2 million barrels and from wells with gas generated by CO2 reducers was 22.8 million barrels. Thus, the ratio of the arithmetic mean of total cumulative gas and water production by well was 1:3.4 for wells with fermenters and 1:2.8 for wells with CO2 reducers; these differences may be controlled by local variations. For example, wells with CO2 reducers have higher coal rank (subbituminous A-B), gas content, and groundwater head and longer residential time for formation water, microbes, and coal reservoir interactions than do wells with fermenters.
Late-stage methanogenesis involving microbial fermenters appears to generate gas that replaces early-stage gas. This is consistent with observations of wells in which initial gas isotopic analyses indicated generation by CO2 reducers but 1-2 years later, after resampling, analyses show that gas was regenerated by fermenters. Thus, the active role of microbial fermenters in bioconversion of coal to methane is the key to sustainability of this resource.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009