Enhanced Coal Bed Methane (ECBM) Recovery by Injecting Flue Gas under Varying Injection Pressures

Abstract

Enhancing coalbed methane by injected gas is a significantly effective method to improve the methane extraction efficiency from low-permeability coal seams. A multifield coupling system is independently developed for the gas injection displacement of coal-containing gas under stress loading for conducting flue gas driving and displacing experiments under different injection flue gas pressures. The experimental investigation has systematically analyzed the dynamic variations in gas volume fraction, gas flow rate, and cumulative volume during the replacing process while evaluating pressure-dependent characteristics of displacement efficiency under varying gas injection pressures ranging from 0.7 to 1.1 MPa (specifically at 0.7, 0.8, 0.9, and 1.1 MPa intervals). The results show that the gas composition from the triaxial pressure chamber exhibited distinct temporal sequencing: CH₄ emerged initially, followed sequentially by N₂, O₂, and finally CO₂. CH₄ production exhibited rapid initial peaking, followed by exponential decay, while breakthrough sequences reflected gas-specific adsorption kinetics (CO₂ preceding N₂/O₂), resulting in N₂ and O₂ reaching the triaxial pressure chamber outlet earlier than CO₂, and the N₂ breakthrough time shortening with the increase of injection gas pressure. Simultaneously, the displacement efficiency increased with higher injection gas pressure. The higher the injection pressure, the shorter the time required for the gas composition at the clamp outlet to reach the flue gas component ratio. Consequently, increasing injection gas pressures promote both CH₄ desorption kinetics and seepage dynamics through competitive adsorption and pressure-driven flow mechanisms, providing critical insghts for optimizing enhanced coalbed methane operations in low-permeability coal seams.