Rapid methanogenesis as a significant control on carbon capture and storage

Abstract:

Carbon capture and storage (CCS) is a key strategy in mitigating the environmental impact of CO₂ emissions. However, the potential trapping/storage mechanisms required for CCS to effectively sequester CO₂ on geologic timescales remains unclear. Independently, several hydrocarbon reservoirs have undergone CO₂ injection as a means of enhanced oil recovery (EOR), providing an opportunity to evaluate the (bio)geochemical behaviour of injected carbon.

Here we present noble gas, stable isotope, and clumped isotope data from two comparable oil fields with contrasting production histories. The Olla field (Louisiana, USA, n=7) was CO₂-flooded during the 1980s. Data from this site are compared to the nearby Nebo-Hemphill field (n = 7), which has never been subjected to EOR injection. This integrated approach allows us to place constraints on both physical and biogeochemical processes resulting from CO₂ injection. From noble gas data we estimate that there is a minimum CO₂ loss of ~90% within the Olla field, and that solubility trapping is the dominant CO₂ sink. However, bulk and clumped isotope constraints on the presence of microbial methane show that, even with the most conservative estimates, CO₂ consumption via methanogenesis is significant within the Olla field. Although this is quantitatively less than the amount of CO₂ that has been dissolved into water, it nonetheless represents a substantial CO₂ loss mechanism, at rates of which are significant on engineering timescales. This presents a significant consideration for CCS, as CH₄ is less soluble and more mobile than CO₂, thus there is an enhanced potential of gas loss associated with CH₄ production from sites where CO₂ injection could drive significant methanogenesis. Similar geochemical trends in other injected and natural CO₂ fields suggest that microbial methanogenesis may be a globally significant process and therefore a paramount factor in site selection for future CCS projects.