Mechanism of Methane Adsorption and Diffusion and its Influencing Factors Based on the Fractal Structure of Coal-based Porous Media

The adsorption–diffusion process of methane (CH4) in coal seams directly affects the amount of extractable gas (i.e., the free gas content). To clarify the relationship between the CH4 multiscale migration process and complex pore structure characteristics, the adsorption and diffusion characteristics of coal samples with different metamorphic degrees were taken as the research object, subdivided the gas adsorption structure scale, and quantitatively characterized the adsorption capacity of coal under different adsorption modes. Then, combined with the fractal characteristics of the coal adsorption pore complex structure, the evolution law of methane nonsteady-state diffusion was introduced to calculate the effective diffusion coefficient of methane under the unsteady state, elucidated the mathematical quantitative relationship between diffusion characteristics and structure characteristics, and explored the influence mechanism of coal structural parameters on the methane diffusion law. The results verified that the specific surface area was the dominant factor in the case of monolayer adsorption, while the complexity of the microporous structure was the most important factor in the case of microporous filling. Based on the calculation results of the fractal capillary bundle diffusion model, it was found that the specific surface area indirectly affects the diffusion movement by affecting the gas adsorption process, thus playing a leading role in the influence of fractal diffusion capacity of methane in coal. Overall, our research results provide a theoretical reference for the efficient mining of coalbed methane and mine gas disaster prevention.