Rapid microbial methanogenesis during CO 2 storage in hydrocarbon reservoirs.

Carbon capture and storage (CCS) is a key technology to mitigate the environmental impact of carbon dioxide (CO ₂ ) emissions. An understanding of the potential trapping and storage mechanisms is required to provide confidence in safe and secure CO ₂ geological sequestration ^1,2 . Depleted hydrocarbon reservoirs have substantial CO ₂ storage potential ¹ , ³ , and numerous hydrocarbon reservoirs have undergone CO ₂ injection as a means of enhanced oil recovery (CO 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 ₂ -EOR project in the Olla Field (Louisiana, USA). We show that microbial methanogenesis converted as much as 13-19% of the injected CO ₂ to methane (CH ₄ ) and up to an additional 74% of CO 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 ₄ per cubic metre (standard temperature and pressure) per year for the Olla Field. Similar geochemical trends in both injected and natural CO ₂ fields suggest that microbial methanogenesis may be an important subsurface sink of CO ₂ globally. For CO ₂ sequestration sites within the environmental window for microbial methanogenesis, conversion to CH ₄ should be considered in site selection.
(© 2021. The Author(s).)