CO₂-Rock-Brine Interactions in Feldspar-Rich Sandstones That Underwent Intense Heating

The success of any carbon capture and storage method largely depends on, among other factors, its safety, reliability, and thorough understanding of the interactions among CO₂, underground geological formation, and resident brine. Upon injection into the subsurface rock formation, CO₂ interacts with the host geological formation and brine, initiating complex geochemical reactions that are often poorly understood and could potentially affect the overall stability and storage capacity of the geological formation, particularly those in close proximity to an intense heat source. For instance, the impact of intense and prolonged heat due to, say, magmatic intrusion on sandstones' framework, authigenic mineralogies, and CO₂-storage potentials is still poorly understood. Consequently, in this study, we have investigated the impact of firing on CO₂-rock-brine reactions in the Bandera Gray (BG) sandstone. Prior to the CO₂ injection using 60 000 ppm brine at 75 °C and 28.7 MPa for 30 days, two samples of the BG sandstone were fired for 6 h in a muffle furnace at 700 and 1100 °C each. The BG samples were then studied for XRD, SEM, and ICP-OES analyses before and after the CO₂ injection, mainly to investigate any changes in mineralogical compositions and fluid chemistry. To determine the impact of the CO₂-rock-brine interactions on the authigenic and framework mineralogies of the BG sandstones under low pH (∼3) conditions, powdered samples of the pre- and postfired BG sandstones were treated with nitric acid. The findings of the study indicate that there were no observable reactions involving rock-forming minerals and carbonate cement in the unfired and fired (at 700 °C) sandstones after the CO₂ injection. However, pervasive feldspar-dissolution porosity was formed in the postfired BG sandstone (1100 °C) after CO₂ injection. This was mainly because albite was partly to pervasively transformed into anorthite during firing at 1100 °C, making the feldspar highly susceptible to dissolution under CO₂ conditions. This implies that the conversion of albite into chemically unstable anorthite in natural sandstones that underwent intense and prolonged heating could develop significant amounts of secondary dissolution porosity due to CO₂ injection, thereby impacting their storage capacities and overall petrophysical properties. This dissolution was separately corroborated using a nitric acid treatment. The findings of the study will provide a better understanding of the CO₂-rock-brine reactions involving sandstones that experienced intense heat due to, for instance, magmatic activity over a long geologic time scale, which has largely transformed the chemistry of their feldspars, particularly plagioclase.