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 T_i and P_i. The objective is to study the influence on the efficiency of LCO₂ injection to displace CH₄ under the different temperature (T_i) and pressure (P_i). 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 (η_max) is only 56.02 %. The competitive-adsorption effect of CO₂/CH₄ intensified, and η_max increased to 84.13 % as T_i gradually increased. The CH₄ 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 CH₄ 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 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.