Time-Varying Characteristics of CH4 Displacement–Replacement Effect in Coal Seams During CO2-Enhanced Coalbed Methane Recovery

Carbon dioxide (CO2)-enhanced coalbed methane recovery involves a complex process of mixed-gas adsorption, desorption, and diffusion–transport. The literature suggests that an appropriate range of CO2 injection pressure and an optimal injection time window are critical for coal seams with varying reservoir conditions. That is, higher pressure and longer injection periods do not necessarily lead to better displacement performance. Therefore, in this study, experimental research was conducted on the time-varying characteristics of the displacement–replacement effect of CO2-enhanced methane (CH4) extraction from coal seams, and the following results were obtained. (1) The process of displacement–replacement of CH4 by CO2 in coal seams can be divided into five stages: a stage of spontaneous CH4 desorption caused by partial-pressure effects, a replacement-dominated stage, a stage where replacement and displacement act jointly, a displacement-dominated stage, and a stabilization stage. (2) For all three coal samples (anthracite, coking coal, and long-flame coal), cumulative CH4 desorption increases with increasing CO2 injection pressure below 5 MPa and finally stabilizes. However, when CO2 injection pressure exceeds 5 MPa, the effect weakens, possibly due to the dynamic changes in CO2 partial pressure. (3) The displacement–replacement ratio decreases with increasing CH4 equilibrium pressure. Additionally, the larger the difference between the CO2 injection pressure and the CH4 equilibrium pressure, the better the displacement–replacement effect.