Effects of Supercritical CO 2 Fluids on Pore Morphology of Coal: Implications for CO 2 Geological Sequestration

A systematic knowledge of the pore morphology of coal treated with supercritical CO 2 (ScCO 2 ) is critical for the process of CO 2 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 N 2 /CO 2 adsorption and mercury intrusion porosimetry. The results indicate that micropores of high- and middle-rank coals decreased after ScCO 2 treatment, whereas an increasing trend was found in low-rank coals, and ScCO 2 promoted the accessibility of the macropore spaces for all coals. With ScCO 2 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, ScCO 2 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 ScCO 2, 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 ScCO 2 and higher-pressure conditions. The results contribute to the understanding of long-term CO 2 storage and enhanced coalbed methane recovery.