CO2-H2O-induced mineral-organic response and pore heterogeneity evolution in different-rank coals

The mineral-organic structural response and pore heterogeneity evolution of coal under CO 2 -H 2 O interaction are important for evaluating coal reservoir alteration during CO 2 -ECBM and coalbed CO 2 storage. However, the contrasting responses of coals with different coalification degrees remain insufficiently understood. In this study, two separate single-factor CO 2 -H 2 O treatment series were conducted for bituminous coal and anthracite: a time-effect series at a fixed CO 2 pressure of 2 MPa for 2, 4, 6, and 8 d, and a pressure-effect series at a fixed reaction time of 2 d under gaseous CO 2 pressures of 1, 2, 3, and 4 MPa. X-ray diffraction, Fourier-transform infrared spectroscopy, low-temperature N 2 adsorption–desorption, and multifractal analysis were used to characterize changes in mineral composition, functional groups, N 2 -accessible pore-size distribution, and pore heterogeneity. CO 2 -H 2 O treatment did not generate new crystalline mineral phases but altered their relative proportions, with dolomite showing the most evident response. Hydroxyl groups and oxygen-containing functional groups were more sensitive to treatment than aliphatic structures, and bituminous coal exhibited a stronger functional-group response than anthracite. Bituminous coal possessed a larger N 2 -accessible pore volume and retained a mesopore-dominated structure, although its macropore proportion increased from 8.88 % to 13.01 %-15.44 %. In contrast, anthracite exhibited more pronounced pore-size redistribution, with its macropore proportion increasing from 26.67 % to 60.00 % as reaction time increased, resulting in a transition from a mesopore-dominated to a macropore-dominated structure within the measured pore-size range. Multifractal analysis showed that the pore-size distributions of both coals exhibited multifractal characteristics. Bituminous coal maintained a relatively stable multifractal structure, whereas anthracite showed larger and non-monotonic variations in pore-volume probability distribution and heterogeneity. These findings provide a pore-scale basis for understanding the rank-related structural alteration of bituminous coal and anthracite during CO 2 -H 2 O interaction and offer implications for evaluating coal reservoir alteration associated with CO 2 -ECBM and coalbed CO 2 storage.