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Carbon capture and storage (CCS) is an important component in many national net-zero strategies. Ensuring that CO 2 can be safely and economically stored in geological systems is critical. To date, CCS research has focused on the physiochemical behavior of CO 2 , yet there has been little consideration of the subsurface microbial impact on CO 2 storage. However, recent discoveries have shown that microbial processes (e.g., methanogenesis) can be significant. Importantly, methanogenesis may modify the fluid composition and the fluid dynamics within the storage reservoir. Such changes may subsequently reduce the volume of CO 2 that can be stored and change the mobility and future trapping systematics of the evolved supercritical fluid. Here, we review the current knowledge of how microbial methanogenesis could impact CO 2 storage, including the potential scale of methanogenesis and the range of geologic settings under which this process operates. We find that methanogenesis is possible in all storage target types; however, the kinetics and energetics of methanogenesis will likely be limited by H 2 generation. We expect that the bioavailability of H 2 (and thus potential of microbial methanogenesis) will be greatest in depleted hydrocarbon fields and least within saline aquifers. We propose that additional integrated monitoring requirements are needed for CO 2 storage to trace any biogeochemical processes including baseline, temporal, and spatial studies. Finally, we suggest areas where further research should be targeted in order to fully understand microbial methanogenesis in CO 2 storage sites and its potential impact.

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).)