Effects of supercritical CO2 fluids on pore structure and fractal characteristics of bituminous coal.

Enhanced coalbed methane recovery with CO₂ coal seam storage (CO₂-ECBM) technology is an important way to achieve China's strategic goals of carbon peak and carbon neutrality. Presently, to date there has been rarely research conducted on the effect of coal sample scale on pore structure under supercritical CO₂ (ScCO₂) fluids. In this study, a high-pressure geological environment simulation system was adopted to analyze coal samples of different scales for ScCO₂ saturation. Subsequently, low-pressure nitrogen gas adsorption (LP-N₂GA), mercury intrusion porosimetry (MIP), and low-field nuclear magnetic resonance (LF-NMR) were used to analyze the pore structure and fractal dimension changes in saturated coal samples at different scales. The experimental results show that the mesopore ratios of cylindrical and granular coal decrease by an average of 1.68% and 2.30%, respectively, after the saturation of ScCO₂. The proportion of macropores in cylindrical coal increased by an average of 5.50% after ScCO₂ saturation, while the proportion of macropores in granular coal changed by 176.86% compared to cylindrical coal. The fractal dimension of the ScCO₂ saturated coal samples obtained with LP-N₂GA, MIP, and LF-NMR all show a decreasing trend, again confirming the modification of the coal pore surface by ScCO₂. Finally, a conceptual model is presented to analyze the mechanism of the effect of coal sample scale on the pore structure under ScCO₂. The difference in the transport paths of ScCO₂ molecules at different coal scales is the main reason for the difference in the evolution of the pore structure. In addition, the impact of the amount of adsorption obtained in the laboratory using coal samples of different scales on the assessment of the CO₂ storage capacity was discussed. Therefore, the results of this study are expected to provide a reference for the CO₂ storage capacity assessment of the CO₂-ECBM project. [ABSTRACT FROM AUTHOR]