Pressure-Controlled Dynamic Conversion of Replacement-Displacement Mechanisms During ECBM Recovery

Gas injection pressure is a key parameter controlling enhanced coalbed methane (ECBM) recovery, as it directly influences gas adsorption-desorption, diffusion-seepage, and the dominance of replacement-displacement mechanisms. In this study, carbon dioxide (CO₂) and nitrogen (N₂) were selected as injection media, and gas-injection simulation experiments were conducted under injection pressures of 0.6, 1.0, and 1.4 MPa. Real-time monitoring of inlet and outlet flow rates, outlet gas composition, and gas-content variations in coal enabled quantitative analysis of gas breakthrough timing, replacement-displacement transition characteristics, and injection efficiency. Increasing injection pressure significantly shortened gas breakthrough time, accelerated outlet gas-concentration evolution, and enhanced gas transport capacity within the coal. When injection pressure increased from 0.6 to 1.4 MPa, the breakthrough time decreased from 406 to 128 min for CO₂ and from 16 to 6 min for N₂, corresponding to reductions of 68.5% and 62.5%, respectively. During CO₂ injection, replacement remained the controlling mechanism, although the cumulative displacement ratio increased from 6% to 24% as pressure increased. In contrast, N₂ injection changed rapidly from replacement-dominated behavior to displacement-dominated behavior, with cumulative displacement ratios of 69%, 83%, and 76% at 0.6, 1.0, and 1.4 MPa, respectively. The maximum final injection efficiency was 23% for CO₂ at 0.6 MPa and 64% for N₂ at 0.6 MPa; CO₂ injection efficiency followed a decrease-then-increase pattern with increasing pressure, whereas N₂ injection efficiency decreased monotonically from 64% to 17%. These findings clarify the pressure-dependent evolution of replacement-displacement dominance for different injection gases and provide experimental guidance for ECBM injection-pressure optimization.