Liquid CO2 injection to enhance coalbed methane recovery: An experiment and in-situ application test

Abstract

In recent times, enhanced deep-buried coalbed methane recovery has increased the interest in coal and unconventional natural gas exploration. This study investigated liquid CO2 (LCO2) injection to enhance coalbed methane recovery, the variation in the gas component and the sweep efficiency of CH4 recovery by LCO2 injection into coal were studied under laboratory conditions. The results demonstrated that the rate of decrease in CH4 concentration initially increased and then stabilized with the increase in the injection pressure of LCO2. This investigation revealed two factors behind the decrease in the CH4 concentration. The first was the carrying effect of CO2 airflow and the competitive adsorption between CO2 and CH4 that enhanced CH4 recovery in coal, while the second was the dilution effect of high-concentration CO2 that diluted the residual CH4 molecules and the experiment converted into an inefficient stage. Additionally, the quantitatively calculated results indicated that the average sweep efficiency of CH4 recovery was greater than 92.0%, which proves the sweep efficiency of CH4 recovery by low-pressure LCO2 injection was obvious. An in-situ test of high-gas-content coalbed in Chinese mining areas concluded that the effective migration radius of CO2 in the test coalbed was beyond 15 m when the cumulative LCO2 injection volume was approximately 5.0 m3. The amount of methane extraction was compared with that obtained by the original gas extraction for 60 consecutive days, the amount of methane extracted from the test coalbed was 2–3 times higher and the time taken for methane extraction to reach the standard was significantly reduced.