Microscopic Control of Coal Quality and Pore Structure on CH ₄ Adsorption of Different-Rank Coals: A Multifractal Perspective

Coal is characterized by low porosity and complex pore structures. A thorough investigation into the pore heterogeneity of coals across different ranks is crucial for the exploration and development of coalbed methane (CBM). From a multifractal characterization perspective, this study explored the quantitative relationships between the fractal parameters of three pore scales (micropores, mesopores, and macropores) and adsorption fractal parameters. The results indicate that (1) Both the pore structures and CH4 adsorption behaviors of coals across different ranks exhibit distinct multifractal characteristics. (2) The multifractal features of the pore structure vary with coal rank and are coinfluenced by the degree of coalification and pore scale. (3) Significant correlations exist between the multifractal parameters of the coal pore structure and CH4 adsorption behavior. Among the influencing factors, micropores exert the most dominant control on adsorption, followed by the coal’s volatile matter content, fixed carbon content, and maximum vitrinite reflectance (Ro,max). This study presents, for the first time, a multifractal characterization of the pore structure and CH4 adsorption behavior across different coal ranks, and reveals the coupled mechanism by which coalification, coal quality, and pore structure jointly influence CH4 adsorption behavior. It elucidates the mechanisms by which coal quality and pores of varying scales influence CH4 adsorption from a multifractal perspective. The findings provide a scientific basis and methodological reference for the safe and efficient exploration and development of CBM resources.