Effects of CO2 injection pressure and confining pressure on CO2-enhanced coalbed methane recovery: Experimental observations at various simulated geological conditions

Systematic understanding of the characteristics of fluid transport and occurrence in conventional Coalbed Methane (CBM) recovery and Injecting CO2 enhancing CBM recovery (CO2-ECBM) engineering plays an essential role in improving the recovery rate of CBM and mitigating the greenhouse effect. In this study, the displacement adsorption time, fluid flow rate and cumulative fluid flow rate of different coal rank samples under conventional and CO2 injection conditions were obtained. The efficiency of exchange adsorption was evaluated based on sweep efficiency and displaced volume. It is found that: (a) In conventional CBM recovery, there is no good correlation between the CBM recovery amount and the gas adsorption capacity of different coal rank samples, but CO2 injection can effectively improve the CBM recovery amount of coal reservoir. (b) In CO2-ECBM, the overall flow rate of the mixed gas decreases rapidly in the initial stage, and then when CO2 is detected at the end, the flow rate of the mixed gas gradually increases and becomes stable until the displacement is completed. (c) Due to the fluid velocity sensitivity effect of coal reservoirs, excessive CO2 injection pressure will have a negative effect on sweep efficiency, but confining pressure will weaken this negative effect. (d) Confining pressure changes the pore-fracture structure of coal reservoirs, strongly influencing conventional CBM recovery. However, it does not change the gas occurrence mechanism in the coal reservoir, it only causes a larger number of gas molecules in the coal matrix instead of the pores, which has little influence on the effectiveness parameters of CO2-ECBM. This research is of great significance for the systematic understanding of the occurrence mechanism of multiple fluids in coal reservoirs, the safe and efficient development of CBM resources, and the realization of carbon neutrality.