Pore-Size-Dependent Competitive CH4/CO2 Adsorption in HRTEM-Informed Short-Chain-Aromatic Organic-Matter Nanopores of Coal-Measure Shale

This study investigates pore-size-dependent competitive CH4/CO2 adsorption in coal-measure shale organic-matter nanopores represented by HRTEM-informed short-chain aromatic fragments. Coal-measure shale from the eastern Yushe Block of the Qinshui Basin was selected as the research object. High-resolution transmission electron microscopy was employed to quantitatively characterize the aromatic lamellar structure of the organic matter. Based on the obtained structural parameters, organic-matter pore models with pore sizes ranging from 1 to 4 nm were constructed. Grand canonical Monte Carlo simulations were subsequently performed to investigate the single-component and competitive adsorption behaviors of CH4 and CO2 at temperatures of 288–308 K and pressures of 0.25–8 MPa. The results indicate that the aromatic structure of the organic matter is characterized by short chains and a low degree of condensation, while the pore system is dominated by micropores and small mesopores. CH4 exhibited its highest nominal-pore-volume-normalized adsorption in the 1 nm pore, consistent with micropore filling, whereas CO2 reached its maximum in the nominal 2–3 nm pore models. These maxima may be influenced by model geometry and normalization because the probe-accessible pore volumes and surface areas after annealing were not quantified. In the smaller 1 and 2 nm pore models, CO2 exhibits a pronounced competitive adsorption advantage. As the pore size increases to 3–4 nm, the relative adsorption capacity of CH4 increases, thereby narrowing the adsorption difference between CH4 and CO2. These findings provide molecular-scale insights into pore-size-dependent equilibrium CH4/CO2 adsorption in experimentally informed organic-matter nanopores, offering a reference for future evaluations of CO2-enhanced gas recovery and storage in coal-measure shale.