Analysis of the Change Feature and Action Mechanism of Coal Pore Structure under the Action of Supercritical CO2

Abstract

In this study, supercritical CO2- (ScCO2) induced permeability enhancement tests were performed on two different-rank coal samples (i.e., coking coal and anthracite) by a self-developed ScCO2-induced permeability enhancement test device for the purpose of exploring the change feature and action mechanism of ScCO2 on coal pore structure during the improvement of coalbed methane extraction. The following results were obtained: after the injection of ScCO2, the original pore structure of coal changes. Specifically, the connectivity between macropores and fractures increases, that between mesopores and small pores decreases and that between small pores and micropores increases. Besides, the total pore volumes of the two types of coal samples both grow primarily due to the increase in macropores and mesopores, and the sample of a higher rank corresponds to a higher growth rate. The growth in the number of effective pores conduces to dredging and enlarging pores in some ways, promoting the surface roughness of macropores and mesopores and reducing that of small pores and micropores. With respect to the action mechanism, ScCO2-induced changes in the coal pore structure are jointly induced by pore adsorption swelling, dissolution-migration, and dissolution-precipitation. Among them, the internal cause of pore adsorption swelling is the surface-free energy of fine pores. The sample of a higher rank contains more fine pores and greater surface-free energy; resultantly, it experiences stronger adsorption swelling and thus greater changes in its pore structure. The carbonic acid generated by ScCO2 and the strong acid minerals existing in the coal matrix and pore space dissolute and migrate, which stands to dredge and expand pore space and meanwhile promote pore connectivity and volume. Moreover, dissolution-precipitation leads to the blockage of pore space and pore channels, hence reducing pore connectivity and pore volume.