Classification methods of pore structures in coal: A review and new insight

The 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 coal mining. 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 CBM occurrence and migration characteristics in coal, the existing pore classification methods of coal have their defects. Therefore, previous pore classification methods of coal are reviewed systematically in this paper. Based on the CH4 occurrence and migration characteristics in coal, a new pore classification method is proposed in this work. The pore structures in coal are divided into four types: inaccessible pores (<0.38 nm), filling pores (0.38 similar to 1.5 nm), diffusion pores (1.5 similar to 100 nm), and seepage pores (>100 nm). As the source of CBM migration in coal, CH4 molecules are mainly adsorbed in the filling pores in the form of micropore filling. The adsorbed CH4 in the filling pores continuously provides continuous CH4 molecules for the CBM migration in the diffusion pores. The flowing CH4 molecules in the diffusion pore are also aggregated into the seepage pore and continuously flow before being produced. This new pore classification method more intuitively summarizes the CBM occurrence forms in pores of different sizes (micropore filling adsorption state, monolayer adsorption state, free state) and its migration characteristics.

Automated summary

The pore structures in coal are crucial for the occurrence and migration of coalbed methane (CBM), which plays a significant role in CBM extraction, utilization, and coal mining disaster prevention. Existing pore classification methods for coal have their drawbacks, thus this paper systematically reviews previous methods and proposes a new pore classification based on the characteristics of CH4 occurrence and migration in coal. This new method categorizes pore structures into four types and provides a more intuitive understanding of CBM occurrence and migration.