Difference between of coal and shale pore structural characters based on gas adsorption experiment and multifractal analysis

Coal and shale reservoirs are characterized by low porosity, low permeability, and complex pore structure, which are the limiting factors for dual gas production. Studying the pore structure of shale and coal informs gas migration, adsorption mechanisms, and storage modes, guiding the assessment of reservoir quality and development plans, crucial for dual gas recovery and underground H2 and CO2 storage. Hence, in this study, we aimed to comparatively analyze the pore structures of different rock types. Gas adsorption and field-emission scanning electron microscope coupled with multiple fractal theory analyses were performed to characterize the porosity and pore structure of the over-mature Longmaxi Formation (Lower Silurian system) and coal of the Benxi Formation (Upper Carboniferous system). The organic pores in coal were more developed than in shale. Micropores provided pore volume and specific surface area in coal, whereas meso-macropores were dominant in shale. Micropores and meso-macropores exhibited multifractal pore-size distributions in coal and shale. Micropores had higher connectivity but lower heterogeneity than meso-macropores. The permeability, connectivity, and heterogeneity of coal rock were related to vitrinite reflectance, ash content, and moisture content, respectively, whereas the permeability, connectivity, and heterogeneity of shale were influenced by total organic carbon content and mineral composition. In the shale of the Longmaxi Formation, gas molecules were predominantly adsorbed in microscopic pores, whereas in the coal of the Benxi Formation, they were primarily free, with a few in microscopic pores. Desorption of adsorbed gas into free gas in the microcavities of coal beds was key to coal bed methane development, whereas migration of shale gas from the matrix reservoir space to natural or engineered fracturing was controlled by pore connectivity. Thus, the study reveals the differences between coal and shale rocks that impact gas distribution and emphasize the importance of pore evaluation for effective development and storage strategies in both reservoirs. Collectively, this research has important implications for future dual-gas and tri-gas co-extraction and selection layers as well as underground hydrogen storage.