Salt precipitation-driven rock failure mode transition during CO2 geological sequestration

Salt precipitation has emerged as a critical factor influencing injectivity, reservoir stability, and the risk of induced seismicity in the near-wellbore region during geological CO2 sequestration (GCS). While previous studies have primarily focused on the brine acidification induced by CO2 injection, triggering geochemical reactions in carbonate rocks and leading to mechanical degradation, the mechanical behaviour associated with salt precipitation in drying zones, particularly the fracture mechanisms, remains poorly understood. In this work, we designed a reservoir-condition displacement system to mimic near-wellbore drying progress and further investigated the rock failure modes due to salt precipitation in red sandstone samples. Our study demonstrates that, despite the densification of the pore structure due to salt precipitation, the overall mechanical performance of the rock undergoes significant deterioration. More importantly, for the first time, we observe a distinct transition of failure mode from shear-dominated to extension-dominated under uniaxial compression. Microstructural analysis further shows that the growth of polycrystalline and bulk crystals induces microcrack initiation and propagation, with the failure mechanism of rocks subjected to salt precipitation primarily characterized by intercrystalline damage at weak bonding interfaces under external loading.