Changes in Pore Structure of Coal Associated with Sc-CO2 Extraction during CO2-ECBM

Supercritical CO2 (Sc-CO2), a supercritical solvent, can extract small organic molecules (fluid) from coal, changing pore structures to affect gases storage and migration in the coal matrix. Five undeformed coals before and after the second coalification jump were collected to simulate Sc-CO2 extraction performed with supercritical extraction equipment. Pore structures of the samples before and after Sc-CO2 extraction were characterized using mercury porosimetry. The results show that there are significant changes in pore size distribution of samples. Delta V-Ma and Delta V-Me of coal samples are positive, Delta V-Tr and Delta V-Mi are positive for most coals, and Delta V-Mi of higher coals are negative; the Delta S-Ma and Delta S-Me are positive with small values, the Delta S-Tr and Delta S-Mi are positive and negative before and after the second coalification jump; thus, the pore connectivity is improved. These results indicate that Sc-CO2 extraction not only increases the numbers of micropores, but also enlarges the pore diameter size; these changes in the pore structure are influenced by the second coalification. The changes in the pore structure by Sc-CO2 extraction provide more spaces for gas storage and may improve the pore throats for gas migration.