Influence of moisture on coal apparent permeability and CO2 sequestration capacity: Coupled effects of hygroscopic swelling and water film

Abstract

Moisture is spread throughout the coal seams, significantly influencing coalbed methane extraction efficiency and CO₂ sequestration capacity. However, existing research often neglects the impact of moisture on gas migration behavior. This study develops an apparent permeability evolution model for binary gases based on the three competing mechanisms of effective stress, gas-induced matrix strain and thermal swelling, and considering the dual role of water on fracture aperture. On this basis, the thermo-hydro-mechanical coupled gas mass transfer theory is constructed. The theory was utilized to obtain the relationship between gas-induced matrix strain and fracture water film thickness under varying water content conditions. And the mechanism of moisture’s role in characterizing the evolution of apparent permeability, CH₄ recovery and CO₂ storage is further discussed. Results indicate that moisture significantly suppresses the competitive adsorption behavior of binary gases within coal seams, leading to a notable reduction in matrix strain. Simultaneously, increased water content intensifies the development of water film along fracture walls. Additionally, through fixed-point monitoring method, it was elucidated that the apparent permeability of coalbeds showed a more obvious decreasing trend with the increase of water content, but the evolution of dramatic showed a significant decrease. Meanwhile, both CH₄ recovery and CO₂ cumulative storage capacities also exhibit a downward trajectory as water content levels increase. Inspired by these observations, the principles and advantages of intermittent pressurized CO₂ injection are discussed, offering novel theoretical insights to support CO₂ sequestration methods in deep, water-bearing coalbeds.