Objective This study aims to reveal the evolutionary patterns of reservoir parameters during the CO₂-enhanced coalbed methane (CO₂-ECBM) recovery and the impact of initial reservoir pressure on CBM recovery by gas injection.

Methods Based on the multi-field coupling-based physical simulation experiment system of gas injection into coal seams for production growth, this study conducted experiments of CH₄ displacement by CO₂ under a constant gas injection pressure of 2.0 MPa and initial reservoir pressures of 1.5 MPa, 1.0 MPa, and 0.5 MPa. Accordingly, this study explored the spatiotemporal evolutionary patterns of multiphysical field parameters such as reservoir pressure, temperature, and volumetric strain during CH₄ displacement by CO₂, as well as the displacement effects. Moreover, this study divided the displacement process into three stages (i.e., stages 1, 2, and 3) by analyzing the interaction mechanisms.

Results and Conclusions  The results indicate that in the displacement process, the reservoir pressure in the injection well was higher than that in the production well. Their pressure difference increased with the initial reservoir pressure, with a maximum of 0.34 MPa. In contrast, the reservoir equilibrium pressure decreased with an increase in the initial reservoir pressure. The reservoir temperature rose earlier at a location closer to the injection well, and it rose at a higher rate under a lower initial reservoir pressure. Furthermore, the reservoir equilibrium temperature decreased with an increase in the initial reservoir pressure. The evolutionary process of reservoir volumetric strain was divided into three stages: slow increase, rapid increase, and stabilization, and the reservoir volumetric strain decreased with an increase in the initial reservoir pressure. During the displacement, as the initial reservoir pressure increased from 0.5 MPa to 1.0 MPa and then to 1.5 MPa, the CH₄ recovery decreased from 91.00 % to 88.48 % and then to 86.81 %, showing a decreasing trend with increasing initial reservoir pressure. In contrast, the CO₂ breakthrough time and CO₂ storage efficiency increased with the initial reservoir pressure. The displacement process exhibited varying mechanisms in various stages. In stage 1 (CO₂ pre-breakthrough stage) and stage 2 (CO₂ breakthrough stage), the cumulative CH₄ volume and CO₂ storage capacity increased with the initial reservoir pressure, both representing over 80 % of the corresponding total volumes of the whole displacement process. The results of this study provide a theoretical basis for developing an integrated technology for efficient CBM recovery and CO₂ geological storage.