Mineral Precipitation Behavior of Depleted Reservoirs for CO ₂ Sequestration

As geological CO 2 sequestration is applied toward more diverse storage scenarios, water-deficient depleted reservoirs with water saturation below 20% are attracting increasing attention; however, their long-term sealing behavior remains much less understood than that of conventional water-rich formations. This study combines CO 2 -soaking experiments, petrophysics, and geomechanical testing with long-term multiphysics coupling simulations to investigate the mineral evolution of sandstone–shale and sandstone–dolostone reservoir-caprock systems in the Nanpu Sag, northern China. Under water-deficient conditions, the CO 2 –rock interaction increases clay and secondarily formed carbonate contents and preferentially modifies macropores. The porosity reduction is lithology-dependent, being most pronounced in sandstone, where porosity decreases from 16.09% to 14.53%, compared with shale, where it decreases from 5.47% to 4.33%, and dolostone, where it decreases from 1.31% to 1.14%. However, the change in permeability is insignificant. The compressive and tensile strengths also increase to different extents among lithologies, which depend on the lithology and sequestration conditions. In the water-deficient simulations, the pore pressure rises to 37.5 MPa in the sandstone–shale system and 42.1 MPa in the sandstone–dolostone system at 30 years, both remaining below the corresponding caprock failure thresholds. The peak CO 2 saturation remains approximately 60% throughout the 45 year period. In contrast, the water-rich simulations show broader pressure propagation and lower peak CO 2 saturation, indicating that water-deficient depleted reservoirs may favor more localized CO 2 retention and precipitation-associated sealing enhancement.