Mechanical, structural, and mineralogical changes of shale under acid-CO₂-rock interactions and their implications for CO₂ storage

Elucidating the underlying chemo-mechanical coupling mechanisms of acid-supercritical carbon dioxide (scCO₂) reactions in shale reservoirs is essential for advancing the efficacy and safety of CO₂ geo-storage and enhanced oil recovery (EOR) techniques. Multi-scale experiments-including X-ray diffraction (XRD), scanning electron microscopy (SEM), nuclear magnetic resonance (NMR), atomic force microscopy (AFM), and nanoindentation-were used to characterize changes in mineralogy, pore structure, and mechanical properties under hydrochloric acid (HCl) pretreatment, followed by scCO₂ exposure. Results demonstrate that acid pretreatment acts as an efficient method to dissolve carbonate minerals, effectively boosting secondary porosity and scCO₂ accessibility. Subsequent exposure to scCO₂ further modifies the mineral composition and induces a marked enhancement in pore-fracture connectivity, as revealed by advanced imaging and topographical analysis. The synergistic effect of acid and scCO₂, validated by nanoindentation and NMR, leads to considerable mechanical softening and superior hydrocarbon displacement efficiency. These findings establish the synergy between acid and scCO₂ in dynamically reshaping the shale pore-fracture system, thereby simultaneously enhancing hydrocarbon recovery and long-term CO₂ storage. This integrated approach offers a novel paradigm for optimizing reservoir stimulation within carbon capture, utilization, and storage CCUS frameworks.