Seepage–Deformation Characteristics of Deep Sandstone During CO2 Injection Under Different Axial-to-Confining Pressure Ratios

During CO2 geological storage in deep saline aquifers, pore pressure fluctuations induced by supercritical CO2 (SCCO2) injection alter the local stress state of the reservoir, thereby affecting the seepage and deformation behavior of the surrounding rock. To investigate the mechanisms by which different deviatoric stress paths govern SCCO2 seepage characteristics in sandstone, this study focused on sandstone from the Liujiagou Formation in the Ordos Basin. Using a self-developed multi-field coupled experimental system, SCCO2 seepage–deformation experiments were conducted under two axial-to-confining pressure ratios (η = σ_axial/σ_confining, where η = 1.1 represents a compressive stress path with the axial stress as the maximum principal stress, and η = 0.9 represents a low-deviatoric-stress path with the axial stress as the minimum principal stress) and three injection pressures (10, 12, and 14 MPa). COMSOL Multiphysics 4.1 numerical simulations were further performed to analyze the dynamic responses of effective stress, porosity, and permeability under different stress states. The results indicate that (1) permeability exhibits a three-stage evolution pattern with increasing confining pressure, characterized by rapid decline, gradual decline, and residual stabilization, and follows a negative exponential relationship with effective stress. Under the stress paths investigated, permeability at η = 0.9 consistently exceeds that at η = 1.1 under the same confining pressure. (2) During loading, the specimens generally undergo volumetric compression, with radial compression typically exceeding axial compression. The axial compression rate is relatively lower along the η = 0.9 stress path, indicating that the low-deviatoric-stress path suppresses axial compaction. (3) An empirical exponential relationship between effective stress and permeability was established based on the experimental data, reproducing the main trends of the measured permeability and volumetric strain within the investigated experimental conditions. The model-predicted strain agrees well with the experimental results. These findings demonstrate that the stress state and injection pressure jointly control the SCCO2 seepage–deformation behavior of sandstone. Within the investigated stress-path range, permeability under the low-deviatoric-stress path, where the axial stress is the minimum principal stress, consistently remains higher than that under the compressive stress path, where the axial stress is the maximum principal stress.