Influence on the efficiency of liquid CO 2 -ECBM under the different pressure and temperature levels: From experimental to in-situ test

This study focuses on the efficiency of liquid CO 2 (LCO 2 ) displacing CH 4 in the coal under optimal T i and P i. The objective is to study the influence on the efficiency of LCO 2 injection to displace CH 4 under the different temperature (T i ) and pressure (P i ). Thereafter, an in-situ test is conducted on LCO 2 injecting to enhance coalbed methane (LCO 2 -ECBM) in a high-gas coalbed. Results showed that low temperature inhibited the movement activity of CO 2 /CH 4 in coal, a significant delayed effect is exhibited during CO 2 displacing CH 4, and the maximum displacement efficiency (η max ) is only 56.02 %. The competitive-adsorption effect of CO 2 /CH 4 intensified, and η max increased to 84.13 % as T i gradually increased. The CH 4 desorption rate and migration dynamics in the coal intensified as P i increased, resulting in a significant increase in efficiency, and η max reached 92.87 %. At 30 °C, where CH 4 is present in the coal, an appropriate increase in P i caused the maximum replacement ratio to decrease from 1.60:1 to 0.40:1, indicating a higher efficiency. An In-situ test of LCO 2 -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.