Relationship Between Recharge, Redox Conditions, and Microbial Methane Generation in Coal Beds

Abstract

Natural gas is an important transitional energy source to replace more carbon intensive coal combustion in the face of climate change and increasing global energy demands. A significant proportion of natural gas reserves (~20%) were recently generated by microorganisms that degrade organic-rich formations (i.e. coal, shale, oil) in-situ to produce methane. Recent studies have shown that these microbial communities may be potentially stimulated to generate more methane to extend the lifetime (~10 years) of existing biogenic gas wells. This dissertation investigates how microbial coalbed methane (CBM) systems are impacted by geochemical conditions, microbial community composition, and groundwater recharge. The first study is a review and synthesis of existing basic research and commercial activities on enhancement of microbial CBM generation, and identification of key knowledge gaps that need to be addressed to advance stimulation efforts. The second study couples water and gas geochemistry with characterization of microbial communities in coalbeds in the Powder River Basin (PRB), Wyoming to investigate the influence of microbiology on water and gas geochemistry. Geochemistry results indicated that nutrients are likely source in situ from coal, and that all sulfate must be removed from the system before methanogenesis will commence. Increased archaeal (i.e. methanogens) diversity was observed with decreasing sulfate concentration, while sulfate reducing bacterial communities were different in wells with high sulfate concentrations (sulfate reducing conditions) when compared to wells with low sulfate concentrations (methanogenic conditions). The third study uses noble gases to constrain the residence time of groundwater associated with CBM in the PRB. Measured diffusional release rates of 4He from PRB coals were ~800 times greater than typical rates observed in sandstone or carbonate aquifers, and measured 4He values far exceeded expected values from in-situ decay of U and Th. Groundwater 4He residence times ranged from <1 to ~800 years using the measured diffusion rates versus ~130 to 190,000 years using a standard model. Coal waters with the longest residence time had the highest alkalinity concentrations, suggesting greater extents of microbial methanogenesis, although there was no relationship between groundwater "age" and methane concentrations or isotopic indicators of methanogenesis. Constraining the relationship between microbial activity (e.g. mechanisms of coal biodegradation and methane generation), environmental geochemical conditions, and groundwater flow is important to better understand subsurface hydrobiogeochemical processes and to ensure the success of future projects related to stimulation of microbial CBM.