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type     June, 2005

Vol 2 Chapter 30: Impact of CO2 Injections on Deep Subsurface Microbial Ecosystems and Potential Ramifications for the Surface Biosphere

T.C. Onstott

Abstract: Based upon the calculated potential microbial power for microbial redox reactions, the most readily identified impact of CO2 injections on the subsurface microbial communities was the reduction of one pH unit for the ground water hosted in the siliclastic reservoir. The slightly lower pH is based upon the assumption, yet to be verified, 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 were 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 were 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 (as is usually the case) 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. A dolomitic or carbonate aquifer may be more severely impacted by the simulated CO2 injection because the dissolution of the carbonate failed to restore the pH to a range that is more commensurate with the pH ranges of some of the microorganisms. If mafic igneous rocks host the groundwater and contain Fe bearing clinopyroxene, then the lower pH will automatically stimulate the release of H2 by the oxidation of this ferrous iron to Fe(OH)3. This, in turn, would lead to stimulation of methanogenic and acetogenic communities and a reduction of the injected CO2. Fe(III) reducing microbial reactions may also be stimulated by the appearance of Fe(OH)3 leading to Fe(III) reduction and an eventual increase in pH. For rhizosphere and surface biosphere the most obvious impact would be due to a potential increase in crustal CH4 flux for carbonate and mafic rock hosted aquifers and a decrease in H2 flux in all cases. Since the fluxes of both gaseous species from fermentative communities in shallower, organic-rich aquitards are 10–100 times greater than the deep subsurface flux, this probably is not a showstopper.

Carbon Dioxide Capture for Storage in Deep Geologic Formations – Results from the CO2 Capture Project Geologic Storage of Carbon Dioxide with Monitoring and Verification - Volume 2
Edited by:
Sally M. Benson, Lawrence Berkeley Laboratory, Berkeley, CA, USA

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