Study on the influence mechanism of the supercritical CO2-H2O-coal interaction on the anisotropy of coal permeability via digital rock technology

CO2-enhanced coal bed methane (CO2-ECBM) achieves the dual benefits of increasing methane production and reducing carbon emissions through CO2 geological sequestration. The supercritical CO2 (ScCO2)-H2O-coal interaction critically modifies the seepage behavior of coal reservoirs, directly governing the efficiency of CO2-ECBM. Using micro-CT technology, this study constructs a 3D digital core of deep, highly metamorphosed coal from the Qinshui Basin to investigate how ScCO2-H2O-coal interaction affect flow heterogeneity and anisotropy. Experimental results show mineral dissolution and ScCO2 swelling increased coal porosity fivefold. Mineral content decreased by 41 %, primarily from carbonate mineral dissolution. The heterogeneity of coal influences the development direction of fractures, leading to their extension in the vertical bedding direction. As carbonate minerals dissolve, intergranular pore heterogeneity promotes interconnected pores along parallel bedding planes. The ScCO2-H2O-coal interaction enhanced coal's seepage capacity while reducing anisotropic seepage differences. However, the original development direction of fractures still maintained a seepage advantage. These results offer novel insights into understanding microscale structural transformations and pore-scale fluid transport dynamics within coal reservoirs during CO2-ECBM operations.