Three‐dimensional characterization of open and closed coal nanopores based on a multi‐scale analysis including CO2 adsorption, mercury intrusion, low‐temperature nitrogen adsorption, and small‐angle X‐ray scattering

Coal seams in China have poor permeability and a complex pore structure, leading to a low gas extraction efficiency. Studying the pore structure of different metamorphic degrees of coal by using modern technological methods and employing appropriate pumping measures for different pore structures can improve the gas extraction efficiency. In this study, the pore sizes of three coal samples with different metamorphic grades were measured at different pore scales through CO2 adsorption, nitrogen adsorption, small‐angle X‐ray scattering, and mercury pressure methods. The pore structure was three‐dimensionally reconstructed using the lattice Boltzmann method‐based quartet structure generation set algorithm. The optimum pore size measurements performed using the CO2 adsorption, nitrogen adsorption, small‐angle X‐ray scattering, and mercury pressure methods were in the ranges of 0.3‐2, 1.7‐300, 1‐50, and 7.5‐11 000 nm, respectively; this demonstrates that the structure of a full‐scale coal model cannot be accurately tested using a single technique. The pore structure varies depending on the metamorphic degree of the coal; therefore, coal metamorphism should be considered for effective gas extraction.