Concentration-dependent Effects of CO2 on Deep Subsurface Microbial Ecology under Carbon Sequestration Conditions

Gulliver, Djuna; Gregory, Kelvin; Lowry, Gregory

Geological carbon sequestration is likely to be part of a comprehensive strategy to minimize the release of greenhouse gasses into the atmosphere. Reservoir storage capacities and long-term security of geologic carbon sequestration will be dependent on the trapping mechanisms and mineral transformation in the deep subsurface. A critical need exists to understand the evolution of microbial populations that influence the biogeochemistry in these reservoirs. As the CO2 front moves through the storage aquifer, microbial communities may preside in residual brine left behind in cracks, dead flow zones, and upstream to the front; this brine will have a gradient of dissolved CO2 concentration. The evolution of microbial ecology along this CO2 concentration gradient was investigated using fluid-slurry samples obtained from a proposed carbon sequestration site. The native species of these samples were investigated with varying pCO2 from 0% to 100% under reservoir temperature and pressure for up to 56 days. Microbial growth occurred with 0% and 1% pCO2, while microbial population decreased by four orders of magnitude with 10% and 100% pCO2. This suggests that any biological processes that may have been involved in the security of the reservoir pre-CO2-injection will be hindered near the CO2 plume, but may be important where CO2 is attenuated. Batch reactors with 0% pCO2, but lowered pH had increased microbial population by one order of magnitude, suggesting that microbial shifts will occur even in highly buffered reservoirs due to exposure to CO2. Halotolerant microorganisms Halomonas and Marinobacter were the most tolerant to the conditions that would follow CO2 injection. Findings provide insight into the populations that may survive in the deep subsurface following the supercritical fronts. These populations will eventually give rise to the community that will impact the trapping mechanisms, storage capacities, and long-term security of the CO2 deposits. Knowledge of the surviving microbial populations will enable improved models for predicting the fate of CO2 following injection and lead to better strategies for ensuring the security of CO2 in the subsurface.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013