Methane Adsorption During Pore Evolution and Its Microscale Impact on Coalbed Methane Recovery: A Case Study of Middle- and High-Rank Coals in the Western Guizhou

The relationship between the pore structure characteristics and methane adsorption behavior evolution during coalification is vital for elucidating coalbed methane (CBM) storage and the impact on gas production. The middle-high rank coals collected from the Western Guizhou were analyzed by the full-scale pore structure characterization and methane adsorption isotherms. The evolution of pore, including pore type, structure, and fractal dimension, and gas adsorption behavior were established. Based on the quantitative characterization of coal samples' desorption, diffusion, and permeability capabilities, the impact of the gas storage mechanism on the gas production at the microscale and the geology-adapted technologies for gas recovery was elucidated. The results show that as the coal rank increases, the methane adsorption spaces and sites within coal undergo a substantial expansion primarily due to the enhanced development of micropores. During this process, the quantity of mesopores remains low, but their proportion increases while macropores gradually diminish. Coal petrographic and quality parameters related to the pore structure parameter exhibit a strong correlation with saturated adsorption capacity (SAC), with micropores playing a dominant role in controlling methane molecule adsorption. Coalification, on the one hand, increases the methane adsorption site, coupled with an increase in gas-solid interaction due to the condensation of macromolecular structures, leading to an increase in SAC. On the other hand, it results in a reduction in the micropore diameter and an intensification of monolayer molecular adsorption, causing a significant decrease in average adsorbed molecular layers (AAML). Therefore, the increase in SAC accompanies a decrease in AAML. Although high-rank coals exhibit higher methane desorption volume, desorption efficiency, and diffusion capacity, their low permeability characteristics hinder fluid seepage. To facilitate efficient development of high-rank CBM, it is imperative to implement geological compatibility techniques aimed at reducing solid-gas interactions within coal reservoirs and enhancing the connectivity of the pore network.