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.