CO₂ displacing CH₄ under different initial reservoir pressure in triaxial stress

Objective This study aims to reveal the evolution laws of reservoir parameters in the process of Carbon dioxide Enhanced Coalbed Methane (CO₂-ECBM) and the effect of different initial reservoir pressure on coalbed methane extraction. Methods Using physical simulation test system for multi-field coupled stimulation of coal seam gas injection to conduct constant CO₂ displacing CH₄ experiments with gas injection pressure of 2.0 MPa and initial reservoir pressures of 1.5, 1.0 and 0.5 MPa. The experiments were conducted to study the spatiotemporal evolution laws of multi-physical field parameters such as reservoir pressure, temperature and volumetric strain and the displacement effect during the CO₂ displacing CH₄ process. Based on the analysis of the mechanism, the CO₂ displacing CH₄ process was divided into stages. Results and Conclusions Key findings are as follows: (1) During the displacement process, the reservoir pressure of gas injection well is higher than that of production well at the same moment, and the pressure difference increases with the increase of the initial reservoir pressure, with a maximum value of 0.34 MPa, while the reservoir equilibrium pressure decreases with the increase of the initial reservoir pressure; (2) The reservoir temperature where located closer to the gas injection well rises first, and the reservoir temperature rises faster when the initial reservoir pressure is smaller. The reservoir equilibrium temperature decreases with the increase of initial reservoir pressure; (3) The reservoir volumetric strain can be divided into three stages: slow rise, rapid rise and leveling off. The reservoir volumetric strain decreases with the increase of initial reservoir pressure; (4) During the displacement process, when the initial reservoir pressure increases from 0.5 MPa to 1.0 MPa and then to 1.5 MPa, the CH₄ recovery efficiency decreases from 91.00% to 88.48% and then to 86.81%, showing a decreasing trend with the increase of the initial reservoir pressure. On the contrary, the CO₂ breakthrough time and CO₂ storage efficiency increases with the increase of initial reservoir pressure. In each stage of the displacement process, the mechanism of action is different. The CH₄ cumulative volume and CO₂ storage volume in stage 1 and stage 2 increase with the increase of initial reservoir pressure and both accounted for more than 80% of the whole displacement process. The research results provide theoretical basis for the development of integrated technology for efficient coalbed methane recovery and CO₂ geological storage.