Relationship between multiscale nanopore structure and coal connectivity during coalification process

The complex nanopore structures in coal provide the space for gas adsorption and migration, which is crucial for the development of coalbed methane. However, the mechanism of the evolution of multi-scale nanopore structures during coalification is still unclear. In this work, a combined method of CO2/N2 adsorption and synchrotron radiation Nano-CT experiments were used to reveal the multi-scale pore structure characterization during coalification. The synchrotron radiation Nano-CT experiment reconstructed the 3D pore network model for different rank coal and revealed the effective diameter is less than 0.5 μm, accounting for 97.4%–99.6% of the total number of macropores. The combination of these methods, including CO2/N2 adsorption and Nano-CT, accurately characterizes the multi-scale pore distribution in coal, ranging from <2 nm, 2–300 nm and 64 nm - 3.5 μm. The ultra-micropores occupy the primary advantage, accounting for approximately 60.3%–95.2% of the total pore volume and the micropores, mesopores and macropores are more poorly developed than ultra-micropores. During the coalification process, the proportion of porosity contributed by ultra-micropores to the total porosity gradually increases, with the contribution rising by 57.9%. The proportion of porosity contributed by micropores, mesopores and macropores to the total porosity gradually decreases, with the contribution decreasing by 81.0%, 82.8% and 93.6%, respectively. Besides, with growing coal maturity, the total permeability gradually decreases by 9.26 × 10−3 - 3.05 × 10−1 mD, which is negatively correlated with coal maturity during coalification. And the total permeability is mainly provided by macropores, which account for about 99% of the total permeability. This research provides an in-depth understanding of the storage and transport of coalbed methane in a multi-scale nanopore structure.