Influence on the efficiency of liquid CO2-ECBM under the different pressure and temperature levels: From experimental to in-situ test

This study focuses on the efficiency of liquid CO2 (LCO2) displacing CH4 in the coal under optimal Ti and Pi. The objective is to study the influence on the efficiency of LCO2 injection to displace CH4 under the different temperature (Ti) and pressure (Pi). Thereafter, an in-situ test is conducted on LCO2 injecting to enhance coalbed methane (LCO2-ECBM) in a high-gas coalbed. Results showed that low temperature inhibited the movement activity of CO2/CH4 in coal, a significant delayed effect is exhibited during CO2 displacing CH4, and the maximum displacement efficiency (eta max) is only 56.02 %. The competitive-adsorption effect of CO2/CH4 intensified, and eta max increased to 84.13 % as Ti gradually increased. The CH4 desorption rate and migration dynamics in the coal intensified as Pi increased, resulting in a significant increase in efficiency, and eta max reached 92.87 %. At 30 degrees C, where CH4 is present in the coal, an appropriate increase in Pi caused the maximum replacement ratio to decrease from 1.60:1 to 0.40:1, indicating a higher efficiency. An In-situ test of LCO2-ECBM showed that the concentration and flow rate of methane increased by nearly three times, and the CBM extraction time and borehole layout number have been shortened by one-third and half, respectively.