Experimental Investigation of CO₂-CH₄ Displacement Behavior during the Soaking Stage of CO₂ Huff-and-Puff in Coal

CO₂ huff-and-puff is considered a promising technique for enhancing coalbed methane (CBM) recovery from low-productivity wells while enabling simultaneous CO₂ geological storage. However, the gas displacement behavior and phase transition mechanisms between CO₂ and CH₄ during the soaking stage remain unclear. In this study, laboratory-scale CO₂ injection and soaking experiments were conducted on CH₄-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 CO₂ injection was systematically investigated. The results indicate that during the CO₂ soaking stage, CO₂ is preferentially and selectively adsorbed by the coal matrix, continuously displacing adsorbed CH₄ and converting it into the free state. After 66 h of soaking, the CH₄ desorption efficiency reached 19.12%, while the CO₂ storage efficiency attained 15.12%, with the absolute amount of adsorbed CO₂ significantly exceeding that of desorbed CH₄. Meanwhile, continuous CO₂ 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 CO₂ soaking stage and provide experimental evidence to support the optimization of soaking time and the evaluation of CO₂ huff-and-puff technology for enhancing recovery and enabling CO₂ storage in low-production CBM wells.