Evolution Laws of Pore Structure in Anthracite with Different Moisture Contents Induced by Liquid CO2 Immersion and Acidification

Water in a coal seam matrix critically dictates the kinetic response and pore enhancement efficacy during liquid CO2 displacement, yet the underlying mechanisms governing full-scale pore structure evolution under varying moisture regimes remain inadequately quantified. To address this issue, this study investigates the impact of the initial moisture content on pore damage in low-permeability anthracite. Low-field nuclear magnetic resonance and low-temperature nitrogen adsorption experiments were conducted. These techniques were combined to systematically evaluate the full-scale pore structure evolution before and after liquid CO2 immersion. The results indicate that the specific surface area, total pore volume, and T2 spectrum integral areas of microscopic pores at all scales exhibit a distinct non-linear, single-peak evolution pattern with increasing initial moisture content. The initial moisture content of 4.8% serves as a critical turning point for full-scale pore-space expansion and closure, at which the T2 spectrum integral areas for micropores, mesopores, and macropores increase by 116.9%, 221.4%, and 243.5%, respectively, compared to the dry group, thereby effectively clearing blind-end pore throats; below this inflection point, moisture primarily mediates the carbonic acid dissolution response. Beyond this critical threshold, the mechanism transitions into a physical antagonism dominated by water-locking truncation and low-temperature frost-heaving damage. Compared to the 4.8% peak group, the T2 spectrum integral areas for macropores, mesopores, and micropores in the 7.2% high-moisture group decrease significantly by 68.8%, 61.6%, and 46.1%, respectively, leading to an overall occlusion of full-scale pores. These findings provide crucial engineering guidance for field practices involving liquid CO2 injection into moisture-bearing coal seams to enhance coalbed methane displacement.