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A systematic knowledge of the pore morphology of coal treated with supercritical CO2 (ScCO2) is critical for the process of CO2 geological sequestration. To better understand the desorption mechanism and to evaluate the storage capacity of target coal seams, the changes in pore volume, pore size distribution, fractal dimension, pore shape, and connectivity in high-, middle-, and low-rank coals were analyzed using N2/CO2 adsorption and mercury intrusion porosimetry. The results indicate that micropores of high- and middle-rank coals decreased after ScCO2 treatment, whereas an increasing trend was found in low-rank coals, and ScCO2 promoted the accessibility of the macropore spaces for all coals. With ScCO2 treatment, the roughness of smaller pores in both high- and middle-rank coals decreased, whereas larger pores became more complex for high-rank coals. Although no significant change was observed in the pore shapes, ScCO2 facilitated the development of effective pore spaces and improved the connectivity of the pore system. Additionally, the gas desorption properties of these samples were enhanced by ScCO2, verifying the pore morphology results. A conceptual model was proposed to explain the mechanism of the desorption process in relation to the constricted pore spaces of the coal matrix under ScCO2 and higher-pressure conditions. The results contribute to the understanding of long-term CO2 storage and enhanced coalbed methane recovery.

The diffusion coefficient of methane in coal is a key parameter for the prediction of coalbed methane production. The apparent diffusion coefficient is different from the true diffusion coefficient, which would result in the deviation of methane production. In this study, the particle method using the unipore model and the counterdiffusion method is adopted to measure the methane diffusion coefficients. The results indicated that the true diffusion coefficient obtained by the counterdiffusion experiment decreases first and then increases with increasing methane pressure. The apparent diffusion coefficients obtained by the particle method with two different grain sizes are lower than the true diffusion coefficient. The relationship between the apparent diffusion coefficient and the true diffusion coefficient is analyzed, and the desorption capacity factor (DCF) is proposed to reflect the gap between them. The apparent diffusion coefficient is closer to the true diffusion coefficient when the DCF is small. When using the particle method to estimate the methane diffusion coefficient, experiments with large coal particles and a small methane concentration gradient should be adopted.

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