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.