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

Abstract

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.