Experimental investigation of CO2 injection into coal seam reservoir at in-situ stress conditions for enhanced coalbed methane recovery

CO2 geological sequestration is an effective method to reduce the concentration of CO2 in the atmosphere. The injection of CO2 into CH4-containing coal seams also named CO2-ECBM (CO2 Enhanced Coalbed Methane Recovery), allows the storage of CO2 and the enhancement of CH4 recovery. Due to complex geological environment of the site, dynamic variation of CH4 and CO2 can hardly be monitored in real time during CO2 injection into coal seams. Therefore, this paper conducted experimental studies were carried out on cuboid coal samples with a size of 300 ∗ 70 ∗ 70 mm via a self-developed experimental device which can simulate in-situ stress and temperature conditions. In the experiment, different adsorbed gases (CH4 and CO2) were applied to determine the variation of permeability values under various load stresses and pore pressures. Results proved that the coal had greater adsorption capacity to CO2 than to CH4. The injected CO2 occupied the original adsorption site of CH4, thus leading to the rapid discharge of CH4. Meanwhile, real-time dynamic monitoring was also performed on gas parameters in coal under different gas injection pressures to obtain variation laws of pore pressure, gas flow rate and concentration at the outlet with the increment of injected CO2. The results reveal that with the increase of injected CO2, pore pressure tended to equilibrium while CH4 flow rate and concentration at the outlet gradually fall to zero. Therefore, CO2 injection into the coal mass can effectively improve the recovery of CH4. In addition, variations of gas flow rate and concentration at the outlet with the displacement ratio of gas volume under different gas injection pressures suggested that the enhancement of gas injection pressure boosted discharge efficiency of CH4 from the coal mass and CO2 consumption. Furthermore, inspired by the competition relationship between engineering efficiency and CO2 consumption, we think a reasonable and effective economic cost model can be established as an effective method to guide the selection of gas injection pressure in the future. Hence, the experimental results have guiding significance for better understanding and application of CO2-ECBM technology.