Pore Structure, Fractal Dimension, and Methane Adsorption Capacity of Coals: A Case Study from the Huaibei Coalfield, China

Abstract The Huaibei Coalfield hosts abundant coalbed methane (CBM) resources, yet its heterogeneous pore structure poses challenges for efficient development. In this study, the multiscale pore structure was characterized using field-emission scanning electron microscopy (FE-SEM), mercury intrusion porosimetry (MIP), and N2/CO2 adsorption experiments, and fractal dimensions were quantified to reflect pore heterogeneity, and methane adsorption capacity was tested with the improved Langmuir equation. The results indicate that the coal samples are medium-rank bituminous coal, with a maximum vitrinite reflectance (Ro,max) of 0.73%–0.96%. Vitrinite constitutes the major maceral component, while mineral contents remain low. FE-SEM and EDS analyses reveal highly heterogeneous pore fractures and mineral compositions. Of the total pore volume, micropores and macropores together account for approximately 90.81%, with volumes of 0.72–1.86 cm3/100 g (average of 1.24 cm3/100 g) and 0.32–4.99 cm3/100 g (average of 1.54 cm3/100 g), respectively. The specific surface area of micropores is 18.13–48.94 m2/g (average of 32.21 m2/g), which accounts for 99.55% of the total on average. In addition, the coal rank promotes micropore development, whereas the inertinite content negatively correlates with macropore volume. Methane adsorption capacity is 0.81–1 mmol/g (average of 0.92 mmol/g), which is primarily controlled by the volume and specific surface area of micropores but shows no relationship with the maceral or mineral component. Moreover, methane adsorption capacity shows positive correlations with fractal dimensions DN21 and DCO2 but a negative correlation with DN22. From a kinetic perspective, mesopores and macropores can accelerate gas transport and shorten equilibrium time. This study offered scientific support for CBM assessment and exploitation in the Huaibei Coalfield.