Abstract
SummaryExtraction of natural gas from unconventional hydrocarbon reservoirs by hydraulic fracturing raises concerns about methane migration into groundwater. Microbial methane oxidation can be a significant methane sink. Here, we inoculated replicated, sand‐packed, continuous mesocosms with groundwater from a field methane release experiment. The mesocosms experienced thirty‐five weeks of dynamic methane, oxygen and nitrate concentrations. We determined concentrations and stable isotope signatures of methane, carbon dioxide and nitrate and monitored microbial community composition of suspended and attached biomass. Methane oxidation was strictly dependent on oxygen availability and led to enrichment of 13C in residual methane. Nitrate did not enhance methane oxidation under oxygen limitation. Methylotrophs persisted for weeks in the absence of methane, making them a powerful marker for active as well as past methane leaks. Thirty‐nine distinct populations of methylotrophic bacteria were observed. Methylotrophs mainly occurred attached to sediment particles. Abundances of methanotrophs and other methylotrophs were roughly similar across all samples, pointing at transfer of metabolites from the former to the latter. Two populations of Gracilibacteria (Candidate Phyla Radiation) displayed successive blooms, potentially triggered by a period of methane famine. This study will guide interpretation of future field studies and provides increased understanding of methylotroph ecophysiology.
Highlights
Oil and natural gas extraction from organic-rich shale formations has transformed the global energy outlook (Malakoff, 2014)
Aerobic oxidation of methane may lead to increased turbidity resulting from microbial growth, oxygen limitation, anoxic conditions (Cahill et al, 2017), and, in theory, production of sulfide by microbial sulfate reduction
Methanotrophs may leak out metabolites, such as methanol or acetate, which are further oxidized by other methylotrophic bacteria, which may use nitrate as electron acceptor (Nercessian et al, 2005; Chistoserdova et al, 2009; Takeuchi, 2019)
Summary
Oil and natural gas extraction from organic-rich shale formations has transformed the global energy outlook (Malakoff, 2014). More than 100,000 of oil and gas wells completed in the United States and Canada over the past decade were horizontally drilled and hydraulically fractured (Kerr, 2010; McIntosh et al, 2019) In some of these wells, well bore integrity failure leads to the unintentional subsurface release of natural gas— known as fugitive methane or stray gas (Vidic et al, 2013; Darrah et al, 2014). Such release may be followed by gas migration via multiphase fluid flow, through geological profiles, toward groundwater and the water-unsaturated vadose zone, resulting in atmospheric emissions (Cahill et al, 2019). No anaerobic oxidation of methane was observed, and a lack of oxygen led to persistent (i.e. up to 700 days post injection) presence of methane in the aquifer
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