A new pore classification method based on the methane occurrence and migration characteristics in coal

The multi⁃scale pore structures in coal are the occurrence and migration space of coalbed methane (CBM). Therefore，the research on the pore structures in coal is of great significance for the extraction and utilization of CBM and disaster prevention. In the past few decades，the complex pore structures in coal have been extensively studied and classified. However，with the development of pore testing technology and the further understanding of the CBM oc⁃ currence and migration characteristics in coal，the existing pore classification methods of coal have their own defects. Therefore，previous pore classification methods of coal are reviewed systematically. Based on the authors′research re⁃ sults on the CH4 occurrence and migration characteristics in coal， a new pore classification method is proposed for the coal⁃CH4 system. The pore structures in coal are divided into four types: inaccessible pores ( 100 nm). In addition，the existence mode of different pores in coal (filling pores，diffusion pores and seepage pores) is formed in series with each other，and the existence mode of the same pores is formed in parallel with each other. As the source of CBM migration in coal，the CH4 molecules are mainly adsorbed in the filling pores in the form of mi⁃ cropore filling. The adsorbed CH4 in the filling pores continuously provides constant CH4 molecules for the CBM migra⁃ tion in the diffusion pores. The flowing CH4 molecules in the diffusion pore are also aggregated into the seepage pore and continuously seep into the drainage hole or coal roadway. This new pore classification method more intuitively summarizes the CBM occurrence forms in the pore structures of different sizes (micropore filling adsorption state，mon⁃ olayer adsorption state，and free state) and its migration laws.It provides a theoretical basis for further studying the CBM occurrence and migration in coal from the perspective of combining micro and macro.