Biotransformation Processes Relevant to Geologic Carbon Sequestration: Potential Implications for Environmental Fate

The nature and extent of microbial reactions in formations targeted for geologic carbon sequestration (GCS), as well as in nontarget formations that may be impacted by potential CO2 migration, are key to understanding the fate of injected CO2. The dissolution of CO2 into formation waters drives predictable geochemical changes, including pH reduction, shifts in redox conditions, and increased mineral solubility. These changes can alter microbial community composition (e.g., favoring acid-tolerant taxa) and stimulate microbes capable of using CO2 as a carbon source. Resulting biotransformation processes can transfer CO2 into the mineral phase (e.g., microbially facilitated carbonate precipitation), gas phase (e.g., methanogenesis), or organic phase (e.g., biomass formation). However, the extent, rate, and significance of these processes in both target and nontarget environments are not well understood. This paper reviews current knowledge of CO2 biotransformation relevant to GCS, including reactions occurring in deep storage formations and those arising from potential CO2 migration into shallow groundwater aquifers, the vadose zone, and marine environments. Additionally, factors that influence these transformations are summarized, methods for monitoring microbial processes are discussed, and key research gaps that could facilitate improved prediction of the long-term fate of CO2 under varying environmental conditions are identified.