Gas Generation in the Powder River Basin: By Microbial Fermenters and CO₂ Reducers

Flores, Romeo M.¹; Stricker, Gary D.^1
1 MS 939, US Geological Survey, Denver, CO.

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 (δ¹³C_CH4, δD_CH4, δ¹³C_CO2, δD_H2O), 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 δD_CH4 and δD_H2O, 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