Dissolution-induced pore-matrix-fracture characteristics evolution due to supercritical CO2

Geological carbon dioxide (CO2) storage in deep, unmineable coal seams represents a promising strategy for carbon emissions reduction. This approach involves pore and fracture alteration due to injecting supercritical CO2 (SCCO2), which is crucial for long-term safe storage of CO2 and extracting coalbed methane. This study quantitatively characterized pores and fractures before and after SCCO2 saturation using nuclear magnetic resonance (NMR). The results show an average 89% increase in total porosity after SCCO2 treatment. The proportion of macropores significantly increased, resulting in a wider range of pore sizes, with the average of macropore porosity increased by more than seven times. Furthermore, SCCO2 exposure reduced the fractal dimension, resulting in smoother pores conducive to gas transport. The alterations in pore type induced by SCCO2 were discussed, in which original fractures exhibited increased apertures after SCCO2 exposure, accompanied by new Y-shaped secondary fractures, while XRD analysis explained mineral dissolution and precipitation. A conceptual model considering the swelling coefficient in matrix-fracture development under SCCO2 dissolution is proposed based on the correlation between seepage pores and adsorption pores. Furthermore, the influence of pore morphology on the development of pores and fractures under SCCO2 exposure was analyzed, offering valuable insights into the CO2-ECBM project.