Correlation analysis of injection parameters for low-medium pressure injection of liquid CO2 for CH4 displacement in coal seams

Abstract

The injection parameters for liquid CO2 to displace CH4 in a coal seam depend on engineering analogy and construction experience. To explore the relationship between key pressure injection parameters, such as liquid injection pressure and liquid injection volume, a combination of theoretical analysis and a downhole pilot test was used. The effective displacement radii of the CO2 and CH4 extraction under different liquid injection pressures were compared. The correlations between the volume, flow, time, and pressure of the injection liquid were analyzed. The results showed that the influence radius of CO2 effective displacement and influence period of CO2 pressure injection were proportional to the injection pressure. Low-pressure tank injection effectively promoted the extraction of coal seam CH4, whereas medium-pressure pump injection greatly enhanced the extraction of CH4. Therefore, in a low-to-medium pressure injection range, the higher the injection pressure of the liquid CO2, the better the displacement effect. A correlation analysis of the liquid injection parameters reveals that the liquid injection volume shows a concave nonlinear increasing trend with an increase in the liquid injection pressure, the liquid injection flow shows a parabolic trend of the upper opening with the increase of the liquid injection pressureand the liquid injection time shows a parabolic trend of the lower opening with an increase in liquid injection pressure. A comprehensive analysis shows that the optimal injection pressure and volume for the low-medium pressure injection of liquid CO2 to displace coal seam CH4 are 7.5 MPa and 7.4 m3, respectively.