Pore Structural Complexities and Gas Storage Capacity of Indian Coals with Various Thermal Maturities

Understanding pore structural complexities of coal is essential in coalbed methane (CBM) enhanced recovery and optimization of CO2 sequestration strategies. Coal's micropores play a pivotal role in gas adsorption, while its mesopores and macropores facilitate gas migration and recovery. This study investigates the relationship between thermal maturity, maceral composition, and pore structural attributes in five coal samples with progressing thermal maturity from the Raniganj and Jharia Basins, India, using low-pressure nitrogen (N2) and carbon dioxide (CO2) adsorption techniques. A key focus is to derive fractal dimensions from CO2 adsorption data, which effectively captures micropore complexity and heterogeneity, offering critical insights into the coal's gas storage potential. The results reveal that thermal maturity significantly impacts pore development, with postmature coals exhibiting greater micropore volumes and higher fractal dimensions, indicating higher complexity of the pore surface area and gas storage capacity. The analysis of the CO2 adsorption data proved superior to the N2 ones in characterizing micropores, which contribute significantly in estimating the maximum gas adsorption potential of coal. This study highlights strong correlations between fractal dimensions, maceral composition, and thermal maturity markers obtained from programmed pyrolysis. This work highlights that CO2-derived fractal dimension analysis coupled with organic petrography and the Rock-Eval thermal maturity parameter can be an effective way to understand the surface heterogeneity of micropores in coals and its implications for gas storage.