Characterization and mechanism of multi-scale pore changes in scCO2-water injection into different porosity coal specimen

The process of supercritical CO2 (scCO2) injection into the coal body induces inhomogeneous changes in pore, which has a significant effect mechanical changes of the geological storage of CO2 coal seams. In this study, the effect of scCO2-water intrusion on coal samples with different porosity was investigated by NMR equipment and the change in the mechanical properties were tested by uniaxial compression and acoustic emission techniques after treatment. Results showed that during the treatment of scCO2-water, the pores of different scales showed an exponential change trend of y = a·xb, with the macropore of low-porosity coal samples continuously increasing and the macropore of high-porosity coal samples continuously decreasing. The pore changes after treatment mainly occurred in the micropore range, and the changes of low-porosity coal samples were more than 6%, while the changes of high-porosity coal samples were less than 2.3%. The changes in the fractal dimension of low-porosity coal samples exceeded those observed in high-porosity. The geochemical impact, effective stress of scCO2-water mixed fluid, and the adsorption and expansion mechanism of matrix macromolecules are influenced by varying porosity. This study helps to understand the internal multi-scale pore coupling mechanism and the effect on the coal mechanical properties after CO2 injection.