The effects of CO2-rock interaction on the failure mode of clay-rich sandstone

Understanding how rock formations respond to CO₂ injection is critical for geological carbon storage. This study examines the impact of CO₂-rock interaction on the failure mode of clay-rich Tuscaloosa sandstone. To simulate CO₂-rock interaction in the subsurface, we conducted CO 2 treatment tests by exposing saturated sandstone to 10 MPa CO₂ at 85 °C for 12 weeks. Triaxial compression tests revealed that the failure mode of the sandstone containing 28 % clay transitioned from semi-brittle to brittle after CO 2 treatment, which can be attributed to CO₂-clay reaction-induced microstructural changes. The microstructural change involves dissolution of pore-filling and pore-lining clays and precipitation of crystals. As the sandstone fails, this microstructural change facilitates sliding between stiff grains like quartz and newly precipitated crystals in the absence of the lubricating effect of soft clay cement, which results in more brittle failure behavior. This transition appears to depend on clay content. A further increase in clay content may suppress brittle failure of the sandstone even after exposure to CO₂ treatment, suggesting that the CO₂–clay reaction is limited and insufficient to promote a transition in failure mode. These findings highlight the importance of considering reaction-induced mechanical changes in clay-rich reservoir rocks to better assess formation integrity for carbon storage.