Experimental Investigation of CO ₂ –CH ₄ Displacement Behavior during the Soaking Stage of CO ₂ Huff-and-Puff in Coal

High Resolution Image Download MS PowerPoint Slide CO 2 huff-and-puff is considered a promising technique for enhancing coalbed methane (CBM) recovery from low-productivity wells while enabling simultaneous CO 2 geological storage. However, the gas displacement behavior and phase transition mechanisms between CO 2 and CH 4 during the soaking stage remain unclear. In this study, laboratory-scale CO 2 injection and soaking experiments were conducted on CH 4 -saturated coal samples collected from the Qinshui Basin using a custom-designed experimental setup. The dynamic evolution of free and adsorbed gas components, gas pressure, partial pressure, permeability, and displacement ratio after CO 2 injection was systematically investigated. The results indicate that during the CO 2 soaking stage, CO 2 is preferentially and selectively adsorbed by the coal matrix, continuously displacing adsorbed CH 4 and converting it into the free state. After 66 h of soaking, the CH 4 desorption efficiency reached 19.12%, while the CO 2 storage efficiency attained 15.12%, with the absolute amount of adsorbed CO 2 significantly exceeding that of desorbed CH 4 . Meanwhile, continuous CO 2 adsorption induced coal matrix swelling, resulting in a 61.14% reduction in permeability. These findings elucidate the gas adsorption–desorption and displacement mechanisms during the CO 2 soaking stage and provide experimental evidence to support the optimization of soaking time and the evaluation of CO 2 huff-and-puff technology for enhancing recovery and enabling CO 2 storage in low-production CBM wells.