GEOCHEMISTRY OF GEOLOGIC SEQUESTRATION OF CO2: SOURCES OF MAJOR UNCERTAINTIES

Combustion of fossil fuels currently releases approximately 30 Gt CO₂ to the atmosphere annually, and this is projected to increase to ~43 Gt by 2030. Increased anthropogenic emissions of CO₂ have raised its atmospheric concentrations in the last 100 years from ~280 ppmv to 390 ppmv, and based on several defined scenarios, CO₂ concentrations could increase to 1,100 ppmv by 2100. There is now a broad scientific consensus that global warming and the resulting climate changes are caused mainly by these atmospheric CO₂ increases. Carbon dioxide sequestration, especially its geologic storage in sedimentary basins is now considered necessary to stabilize atmospheric levels of greenhouse gases and global temperatures at acceptable values.

Successful sequestration of large volumes of anthropogenic CO₂, however, requires an in depth understanding and accurate predictions of CO₂-brine-oil-mineral interactions from pore space to basin scales. These geochemical and biogeochemical interactions would determine the long-term storage security, reservoir performance and environmental impacts. In this introduction, we discuss recent results and insights obtained from many laboratory and pilot field experiments, natural analogues, EOR and commercial CO₂ sequestration operations. Considerable uncertainties and scientific gaps, however, still exist in understanding and predicting CO₂ phases, transport and interactions in heterogeneous reservoirs and cap rocks, because supercritical CO₂ is buoyant, displaces huge volumes of formation water and becomes reactive to minerals, well pipes and cements when dissolved in brine. A major gap in geochemical knowledge relates to obtaining accurate kinetic rate laws for mineral dissolution, precipitation, and transformation reactions under the full range of subsurface and CO₂-saturation conditions. Detailed site characterization and a comprehensive measurements, monitoring and validation (MMV) program are also needed to minimize environmental impacts, including contaminating underground sources of drinking water.