Impact of CO₂ Injections on Deep Subsurface Microbial Ecosystems and Potential Ramifications for the Surface Biosphere

This chapter discusses the impact of CO₂ injections on the subsurface microbial communities, which is the reduction of one pH unit for the ground water hosted in the siliclastic reservoir. The slightly lower pH is based upon the assumption that alteration of detrital feldspars to clay in equilibrium with calcite occurs on the time scale of the injection. The power levels for many of the microbial redox reactions are generally larger than in the original ground water systems but because of this reduction of one pH unit in the ground water, microbial Fe(III) reduction reactions are significantly enhanced over the expected ambient conditions. If sufficient electron donors are available for both biotic and abiotic Fe(III) reducing reactions and sufficient Fe(III) bearing oxides are present in the aquifer then these reactions will restore the aquifer's pH to its initial, pre-injection value. CO₂ injection should cause a short-term stimulation of Fe(III) reducing communities. For long-term storage of CO₂ in siliclastic reservoirs the short-term enhancement of Fe(III) reducing microorganisms will increase the pH and most likely lead to the precipitation of various carbonates. As readily available Fe(III) is depleted it can be introduced. If this is not feasible and sulfate is not a major constituent in the ground water, then methanogenic activity will begin to dominate and the proportion of CO₂ converted to CH4 will depend upon the H2 and acetate fluxes.