Mineralogical alterations during laboratory-scale carbon sequestration experiments for the Illinois Basin

During geological sequestration of carbon dioxide, the injected CO₂ will react with formation fluids and rocks in the injection zone and overlying cap rocks. The resulting acidification of the fluids may result in the dissolution of solid phases and the formation of new solid phases which can cause changes in rock composition and overall fabric. We are conducting laboratory-scale geochemical and mineralogical studies on reservoir and cap rock samples in the Illinois Basin that complement the on-going Illinois Basin - Decatur Project (IBDP), a large-scale one million tonne demonstration of geologic sequestration in the Mt Simon Sandstone, Illinois USA. Mt. Simon Sandstone, Eau Clare Shale, and KNO_x Supergroup samples from the IBDP injection and deep monitoring wells and locations with rocks analogous to those at the IBDP site have been selected for simulated reactions using synthetic brine and CO₂ in modified Parr pressure reactors at pressure and temperature conditions that correspond to ambient reservoir conditions (9.87 to 20.7 MPa and 38 to 50 °C) and for varying amounts of time (1 to 9 months). Using petrographic techniques and XRD analysis, samples have been analyzed before and after reactor experiments to define the mineralogical and textural baseline and report observed changes. Brine composition has also been analyzed for geochemical changes. The React® and Differential Evolution geochemical modeling programs are being used to simulate changes in mineral mass and brine chemistry. Post-reaction analyses of rock and brine samples from the Mt. Simon Sandstone show evidence of dissolution of diagenetic clays, increased porosity, and possible illitization of clay minerals. Three, six, and nine month post-reaction Eau Claire Shale rock and brine sample analyses indicate some degree of brine-rock-CO₂ reaction by showing weathered illite, mixed clay, feldspar, biotite, and pyrite crystals. Post-reaction Potosi Dolomite rock and brine samples show evidence of dissolution of dolomite. Overall, petrographic and geochemical observations from these experiments suggest that the Mt. Simon Sandstone reservoir and Eau Claire cap rock system serve as good CO₂ sequestration site. The competency of the KNO_x Supergroup as a CO₂ sequestration target is still under investigation.