Pore-network reconstruction, energy evolution, and mechanical damage of shale under supercritical CO2–water/brine interactions

Deep saline aquifers are considered promising sites for large-scale CO2 geological sequestration. However, interactions among supercritical CO2 (ScCO2), pore fluids, and rock minerals may alter shale pore structures and mechanical properties, potentially increasing CO2 leakage risk. In this study, Longmaxi shale was exposed to H2O–ScCO2 and brine–ScCO2 at 90 °C and 12 MPa for 14 days, followed by x-ray diffraction, nuclear magnetic resonance, and uniaxial compressive strength. Both treatments induced mineral dissolution, pore restructuring, and mechanical degradation, with stronger effects under brine–ScCO2 conditions. Dolomite and calcite contents decreased by 42.4%–52.8% and 30.8%–43.1%, respectively, while total porosity increased by 13.2% under H2O–ScCO2 and 18.1% under brine–ScCO2. The increases in the permeation-pore and overall fractal dimensions further indicate enhanced pore-network heterogeneity and geometric complexity, particularly after brine–ScCO2 treatment. Fluid–ScCO2 interactions also altered the pre-failure deformation and crack-evolution processes. Both treatments increased the relative proportions of the crack-closure and stable crack-propagation stages while reducing that of the elastic deformation stage, whereas the unstable crack-growth stage remained nearly unchanged. These changes were more pronounced under brine–ScCO2 conditions. Young's modulus decreased by 7.8% and 11.6%, respectively. Energy analysis revealed a shift from elastic energy storage toward energy dissipation, indicating weakened brittleness and more progressive deformation behavior after treatment. A segmented statistical damage model incorporating porosity related chemical effects and dissipated energy driven mechanical degradation effectively reproduced the initial compaction and pre-peak stress–strain response of chemically altered shale.