Full-scale pore structure characterization of different rank coals and its impact on gas adsorption capacity: A theoretical model and experimental study

Microscopic pores significantly impact the coalbed methane (CBM) storage, hence gas energy recovery and gas-related problems mitigation. However, the quantitative relationship between microscopic pore properties and CBM storage dictated by gas adsorption capacity remains unclear. In this study, high-pressure isothermal gas adsorption experiments were conducted using differently ranked coal samples to investigate gas adsorption characteristics. High-pressure mercury injection (HPMI), low-pressure nitrogen adsorption (LPGA-N2), low-pressure carbon dioxide adsorption (LPGA-CO2) and scanning electron microscopy (SEM) tests were employed for full-scale microscopic pore structure characterization. Considering potential energy induced by CH4 molecules and microscopic pore wall interaction, an improved method was proposed to quantitatively characterize gas adsorption capacity and obtain CH4 occurrence characteristics for different-scale pores. The results show that microscopic properties of differently ranked coal samples vary remarkably with evident heterogeneity. The micropore specific surface area (SSA) is 79.396–232.253 m2/g, accounting for 90.03%–99.45% of the total specific surface area (TSSA). The adsorption capacities of differently ranked coal samples present significant differences and range between 13.38 and 20.08 cc/g, and shows an asymmetric U-shaped trend as coal metamorphism deepens. Based on microscopic pore properties, the Langmuir volume theoretically calculated using the new method ranges between 12.29 and 20.85 cc/g. The calculated results agree well with experimental results with a relative error of less than 10%, proving that this theoretical model can predict gas adsorption capacity with sufficient confidence.