Unraveling the ScCO2–Induced Mineral-Pore-Mechanics Interplay in Coal and Caprock with Integrated Nanoindentation, SEM-EDS, and BET Characterization

Abstract The long-term security of geological carbon storage (GCS) in coal seams requires a predictive understanding of ScCO2 (supercritical carbon dioxide)-induced alterations across the reservoir–caprock system. This study quantifies the differential response of coal, sandstone, and shale to ScCO2 exposure through integrated nanoindentation, X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), and Brunauer–Emmett–Teller (BET) analysis. Coal undergoes the most severe weakening, quantified by a 42.6% reduction in reduced elastic modulus (from 3.317 ± 0.155 GPa to 1.903 ± 0.511 GPa) and a 61.8% reduction in hardness (from 0.403 ± 0.043 GPa to 0.154 ± 0.058 GPa) after 5-day exposure. Sandstone exhibits moderate, localized weakening, while siliceous shale remains mechanically stable. This divergence stems from lithology-controlled geochemical pathways. In coal, the preferential dissolution of carbonate minerals (calcite dropping by 27.7%) by in situ formed carbonic acid drives a profound pore-structure transformation, characterized by a 69.9% increase in pore volume and a 30.1% increase in average pore diameter, accompanied by a 32.1% decrease in specific surface area (SSA) after 5-day exposure. Concurrently, deformation mechanism analysis reveals that coal transitions toward more heterogeneous and plasticity-dominated behavior, with both elastic recovery and creep displacement increasing significantly after treatment. In contrast, the inert silicate-dominated frameworks of sandstone and shale impart pronounced chemical resistance, resulting in minimal pore alteration and mechanical change─though sandstone exhibits localized, unpredictable creep responses at grain boundaries. Our findings suggest that, under the tested conditions, the primary integrity risk may shift from the caprock to the coal reservoir, and raise the hypothesis that differential deformation at coal-rock interfaces could induce shear failure─an implication requiring validation through future shear strength and coupled geomechanical modeling. These findings provide a mechanistic, data-driven framework for stability assessment and injection strategy design in CO2 storage while enhancing coalbed methane recovery (CO2-ECBM).