Impact research of CH4 replacement with CO2 in hydrous coal under high pressure injection

Purpose. Based on high-pressure gas injection technology that enhances coal seam gas drainage, the effect of CH4 replacement with CO2 in aquiferous coal has been studied. Methods. Using the laboratory experimental method and the self-built high-pressure gas injection experimental device, highpressure CO2 is injected into coal with different moisture contents to replace CH4 under different adsorption equilibrium pressures. Findings. With an increase in coal moisture content, the adsorption capacity of coal for CH4 and CO2 gradually weakens, the adsorption capacity for CO2 is always greater than that of CH4, and the CH4 replacement rate and the CO2 injection ratio gradually decrease. It is concluded that the CH4 replacement rate and the CO2 injection ratio are negatively correlated with the water content of coal. With an increase of the pre-adsorption equilibrium CH4 pressure (0.5, 0.75, 1.0, 1.3 and 2.0 MPa), the CH4 replacement rate and the CO2 injection ratio first sharply and then slowly increase. The transition point is 1.3 MPa (pre adsorption equilibrium pressure of CH4). Originality. Based on the adsorption characteristics of coal seam gas injection, the influence of coal water content and gas injection pressure on CH4 replacement rate and CO2 injection ratio is analyzed, and the mechanism is studied. Practical implications. The experimental results have important guiding significance for selecting reasonable gas injection pressure and the source of gas to drive its injection into underground coal seam.