Rapid microbial methanogenesis during CO.sub.2 storage in hydrocarbon reservoirs

Carbon capture and storage (CCS) is a key technology to mitigate the environmental impact of carbon dioxide (CO.sub.2) emissions. An understanding of the potential trapping and storage mechanisms is required to provide confidence in safe and secure CO.sub.2 geological sequestration.sup.1,2. Depleted hydrocarbon reservoirs have substantial CO.sub.2 storage potential.sup.1,.sup.3, and numerous hydrocarbon reservoirs have undergone CO.sub.2 injection as a means of enhanced oil recovery (CO.sub.2-EOR), providing an opportunity to evaluate the (bio)geochemical behaviour of injected carbon. Here we present noble gas, stable isotope, clumped isotope and gene-sequencing analyses from a CO.sub.2-EOR project in the Olla Field (Louisiana, USA). We show that microbial methanogenesis converted as much as 13-19% of the injected CO.sub.2 to methane (CH.sub.4) and up to an additional 74% of CO.sub.2 was dissolved in the groundwater. We calculate an in situ microbial methanogenesis rate from within a natural system of 73-109 millimoles of CH.sub.4 per cubic metre (standard temperature and pressure) per year for the Olla Field. Similar geochemical trends in both injected and natural CO.sub.2 fields suggest that microbial methanogenesis may be an important subsurface sink of CO.sub.2 globally. For CO.sub.2 sequestration sites within the environmental window for microbial methanogenesis, conversion to CH.sub.4 should be considered in site selection.