Safer carbon capture and storage

Carbon capture and storage (CCS) is a crucial technology to mitigate the environmental impact of carbon dioxide (CO₂) emissions. It involves the capture of CO₂ from emissions from industrial processes, or from the burning of fossil fuels in power generation, which is then stored underground in geological formations.

The UK government recently selected four sites to develop multi-billion-pound CCS projects as part of its scheme to cut 20-30m tonnes of CO₂ per year by 2030 from heavy industry. Other countries have made similar carbon reduction commitments.

Depleted hydrocarbon reservoirs have a smaller (10%) storage potential than deep saline aquifers but are seen as a critical early opportunity to develop geological CO₂ storage technologies. Fortuitously, CO₂ has historically been injected into numerous depleted hydrocarbon reservoirs as a means of enhanced oil recovery (CO₂-EOR). It provides a unique chance to evaluate injected carbon’s (bio)geochemical behavior over-engineering timescales.

“CCS will be a key tool in our battle to avert climate change. Understanding how CCS works in practice, in addition to computer modelling and lab-based experiments, is essential to provide confidence in safe and secure CO₂ geological sequestration.” Said Dr. Rebecca Tyne, Dept Earth Science, The University of Oxford.

In a paper published in Nature, Dr. Rebecca Tyne and Prof. Chris Ballentine from Oxford University led a team of international collaborators to investigate the behavior of CO₂ within a CO₂-EOR flooded oil field in Louisiana, USA. They compared the (bio)geochemical composition of the CO₂-EOR flooded field with an adjacent field, which was never subjected to CO₂-EOR. Data suggest that up to 74% of CO₂ left behind by CO₂-EOR was dissolved in the groundwater. Unexpectedly, it also revealed that microbial methanogenesis converted as much as 13-19% of the injected CO₂ to methane, a stronger greenhouse gas than CO₂.

“Long-term carbon dioxide (CO₂) trapping mechanisms include structural or stratigraphic trapping in stable geological configurations, dissolution into pore fluids (solubility trapping), carbonate mineralization (precipitation and mineral trapping) or adsorption (for example, onto coal). Typically, during CO₂ enhanced oil recovery (CO₂-EOR), a proportion of the injected CO₂ remains within the reservoir post-injection, providing an analogue for investigating and quantifying processes within CCS sites, over decadal timescales.” Study quotes

This study is the first to integrate state-of-the-art isotopic tracers (noble gas, clumped, and stable isotope data) with microbiological data to investigate the fate of the injected CO₂.

“Methane is less soluble, less compressible, and less reactive than CO₂, so, if produced, it reduces the amount of CO₂ we can safely inject into these sites. However, now this process has been identified, we can take it into account in future CCS site selection.” Said Prof. Chris Ballentine, Dept. Earth Sciences, The University of Oxford.

Additionally, the authors suggest that this process occurs at other CO₂-rich natural gas fields and CO₂-EOR oil fields. Temperature is a critical consideration, and many CCS geological targets will be too deep and hot for microbes to operate. However, this process could occur if CO₂ leaks from deeper hot systems into similar, shallower, colder geological structures where microbes are present. This research is critical for identifying future CCS targets, establishing safe baseline conditions, and long-term monitoring programs essential for low-risk, long-term carbon storage.

Journal Reference

1. Tyne, R.L., Barry, P.H., Lawson, M. et al. Rapid microbial methanogenesis during CO2 storage in hydrocarbon reservoirs. Nature 600, 670–674 (2021). DOI: 10.1038/s41586-021-04153-3