Understanding the influence of CO2 on coal mineralogy and pore structure is essential for optimizing CO2-enhanced coalbed methane (CO2-ECBM) recovery, particularly across the full range of CO2 thermodynamic states encountered at varying reservoir depths. In this study, we investigate the pore structure evolution and mineralogical alterations of bituminous coal exposed to gaseous, liquid, and supercritical CO2 under controlled temperature–pressure conditions for 7 and 14 days. Mercury intrusion porosimetry (MIP), low-temperature N2 adsorption (LTNA), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were jointly employed to characterize pore development and mineral reactivity. Results indicate that CO2 induces substantial modifications to the coal pore structure, with supercritical CO2 producing the most pronounced enhancement in pore volume. On average, the treated samples exhibit a 40.1% increase in pore volume relative to the untreated coal. XRD analysis reveals substantial calcite dissolution across all treated samples, without the formation of new mineral phases, which directly contributes to pore enlargement. SEM observations further confirm mineral dissolution and surface restructuring across treated samples. Collectively, these findings demonstrate that the thermodynamic state of CO2 governs the intensity of coal-CO2 interactions and that CO2-induced mineral dissolution is a key mechanism enhancing pore development. The study offers comparative experimental insight into the microstructural evolution of coal under different CO2 thermodynamic states and may assist phase-aware interpretation of CO2-ECBM-related coal behavior.
Ge et al. (Wed,) studied this question.