Laboratory experiments of CO2-enhanced coalbed methane recovery considering CO2 sequestration in a coal seam

CO2-enhanced coalbed methane (CO2-ECBM) recovery has the triple benefits of recovering clean resources, ensuring coal mine safety, and achieving CO2 geological sequestration. To study the synergistic effect of CH4 recovery and CO2 sequestration, we designed a large-scale multifunctional apparatus that can realistically simulate the environment of CO2-ECBM technology. The results showed that the reservoir pressure dropped rapidly during conventional CBM recovery, but remained at a high level for a long time during CO2-ECBM recovery. The reservoir temperature close to the injection well was dominated by CO2 adsorption, whereas that close to the production well was dominated by CH4 desorption. The recovery efficiency of CH4 increased from 66.67% during conventional CBM recovery to 90.86%-93.50% after CO2-ECBM recovery. However, the sequestration efficiency of CO2 decreased from 67.89% to 43.98% with increasing CO2 injection pressure from 1.0 to 1.6 MPa. We then proposed a displacement index to reflect the synergistic effect of CH4 recovery and CO2 sequestration. This index first increased then decreased during CO2-ECBM recovery, dropping significantly at the last stage under increased injection pressure. Therefore, we suggest that a dynamic pressure injection mode may be more effective for ensuring efficient CH4 recovery and CO2 sequestration effect.