Impact of Carbonation on Fault Instability During CO₂ Storage in Basalts and Fractured Shales

Mineral carbonation is currently a feasible and promising approach to ensure long-term, thermodynamically stable, and environmentally benign geological carbon dioxide (CO₂) storage. However, precipitated carbonates potentially alter the frictional and stability characteristics of deep fractures/faults and the potential for, and hazard from, seismicity. We conducted fault shear reactivation experiments on simulated faults containing basalt/shale gouge with embedded carbonate patches (areal coverage 0–100%) and homogeneous mixtures (0–100% by volume). Double direct shear was used to define the control of enhanced carbonation on basalt/shale fault friction and stability at room temperature. Frictional strengths of faults in both basalt and shale faults are insensitive to both carbonate contents and gouge morphology (homogeneous or patchy) due to the similar frictional strengths of calcite with basalt/shale gouges. A transition from velocity-weakening (potentially unstable) to velocity-strengthening (inherently stable) response was observed for enhanced carbonation at calcite contents of <60% for homogeneous basalt faults but only <40% for patchy basalt faults in defining the potential limits for seismicity. In addition, increased carbonation degree in homogeneous shale faults has an overall positive influence in reducing the hazard of seismicity in both homogeneously carbonated and patchy faults, except for the sole velocity-weakening response in homogeneous mixture containing 60–80% calcite. These impacts from enhanced carbonation can be explained via localized shears present in the basalt-rich gouges and by granular cataclastic flow in the shale-rich gouges from the gouge microstructures. The above results and disparities have important implications in understanding mechanisms and in defining the impact of enhanced carbonation on fault stability in basalts and shales during geological CO₂ storage.