Geochemical and geomechanical alteration of Mt. Simon sandstone due to prolonged contact with CO₂-saturated brine during carbon sequestration

Geologic carbon sequestration is an emerging approach for mitigating climate change due primarily to point-source CO₂ emissions. Sequestration of carbon dioxide in deep saline aquifers prevents the release of CO₂ to the atmosphere. The Mt. Simon formation in the Illinois Basin is the site of a pilot-scale geologic carbon sequestration effort. It is a siliciclasic formation consisting of primarily quartz and potassium feldspar, cemented by illite-smectite and iron-illite clays. During CO₂ injection, carbon dioxide dissolves into pore water, resulting in a low-pH brine that can dissolve clays, carbonates, and other reactive minerals. In this work, the geochemical reactions resulting from the interaction of acidified-brine and reservoir rock are probed during and after aging samples of Mt. Simon sandstone in CO₂-saturated brine under reservoir conditions. During aging, contact angle measurements of supercritical CO₂ suggest the loss of iron-illite and increase of surface roughness. Post-aging, scanning electron microscopy coupled with electron dispersive spectroscopy revealed the loss of clays, greater exposure of quartz and K-feldspar grains, and apparent roughening, confirmed by XRD analysis and laser profilometer surface images respectively. Scratch tests were performed on both unaltered and aged samples, and the latter show a 63% decrease in fracture toughness. I conclude that acidified brine causes dissolution of clays and K-feldspar during carbon sequestration that weakens the Mt. Simon sandstone matrix along quartz and feldspar grain boundaries. This process may contribute to microseismic activity observed at the Mt. Simon sequestration site during and after CO₂ injection.