Deuterium as a quantitative tracer of enhanced microbial coalbed methane production

Kilian Ashley1, Katie Davis2, Anna Martini3, Matthew Fields2, Jennifer McIntosh1

1Department of Hydrology and Atmospheric Sciences, The University of Arizona, Tucson, Arizona

2Center for Biofilm Engineering, Montana State University, Bozeman, Montana

3Department of Geology and Environmental Studies, Amherst College, Amherst, Massachusett

Microbial production of natural gas in subsurface organic-rich reservoirs (e.g. coal, shale, oil) can be enhanced by the introduction of limiting nutrients to stimulate microbial communities to generate “new” methane resources on human timescales. The few successful field experiments of Microbial Enhancement of Coalbed Methane (MECBM) relied on relatively qualitative approaches for estimating the amount of “new” methane produced during the stimulation process (i.e. extrapolation of pre-stimulation gas production curves).  We have developed a tracer, initially in the laboratory, to more directly quantify the amount of “new” methane generated and the effectiveness of MECBM stimulation approaches.

Microorganisms, formation water, and coal were obtained during a previous drilling project in the Powder River Basin, Birney, Montana. We used these materials to set up a series of benchtop stimulation experiments where we added incremental amounts of deuterated water to triplicate sets of stimulated microbes (methanogens). We hypothesized that as MECBM progresses, methanogens will incorporate the heavy water into new methane produced, as methanogens naturally uptake hydrogen during methanogenesis to produce methane.  The amount of hydrogen from water incorporated into methane is dependent on the methanogenic pathway (CO2 reduction vs. acetate fermentation).  During the experiments, we saw a shift in the methanogenic pathway (i.e. more acetate fermentation), which was indicated by a consistent shift in the enrichment of deuterium in the methane produced. The enrichment of the methane as compared to the deuterium content of the water the microbes used followed a narrowly confined, statistically significant range of values.  This predictable enrichment of the methane allows us to quantify the amount of methane produced, as we can compare the change in the overall deuterium content of the methane with the known value before the stimulation.  The success of our proof-of-concept laboratory experiments suggests that deuterium may be used as a tracer of “new” natural gas resources in field- to commercial-scale MECBM projects.  In addition, deuterium may also be useful in bioremediation projects (e.g. oil spills) or microbial enhanced oil recovery.

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