Fractal Analysis of Coal Pore Structure Based on Low-Pressure Gas Adsorption and Its Influence on Methane Adsorption Capacity: A Perspective from Micropore Filling Model

Understanding pore structure heterogeneity is crucial for gaining insights into the occurrence, transport, and recovery of methane from the coal stratum. The pore structure of coal directly affects the spatial distribution of methane in coal. The influence of the pore structure on methane adsorption has attracted considerable research attention, primarily because of the multiscale characteristics of the pore system and its diverse adsorption behaviors. This study analyzes fractal characteristics to reveal the heterogeneity of the pore structure and pore surface via low-pressure gas adsorption experiments. An adsorption capacity calculation model based on micropore filling is applied and validated by using CH4 isothermal adsorption experiments. Furthermore, the variation in gas adsorption with fractal dimensions is studied via correlation analysis. Results show that high-rank coal exhibits well-developed micropores with larger micropore fractal dimensions than low-rank coal, and low-rank coal exhibits more meso/macropores and higher meso/macropore fractal dimensions than high-rank coal. The fractal dimensions of meso/macropores and micropores could describe the surface and the volumetric properties of pore structure, respectively. The micropore filling model based on the pore structure can be used to quantitatively calculate the methane adsorption capacity of coal, with errors of −32.6, 2.9, 3.2, and 14.3%. This offers significant advantages over the monolayer adsorption hypothesis in understanding the adsorption behavior of coal. The fractal dimensions of the pore structure are positively correlated with the adsorption capacity in each adsorption form, with correlation coefficients of 0.792 and 0.842 in meso/macropores and 0.821 and 0.743 in micropores, respectively. Because most methane molecules are stored in micropores, the fractal dimensions of micropores play a critical role in governing the methane adsorption capacity of coal. These findings hold substantial implications for understanding the mechanisms that influence methane occurrence in coal under diverse geological conditions, contributing to subsequently assessing the risk of coal and gas outbursts and estimating coalbed methane resources.