Theory and experimental study of CBM recovery driven by energy boosting

It is an important issue to enhance coalbed methane (CBM) recovery in the development of CBM industry. This paper systematically elaborates the method of energy boosting for enhancing CBM recovery. The model of the en-ergy transformation was built and the feasibility of this method was analyzed by thermodynamics. Furthermore,a large number of experiments were conducted to investigate the CO2 enhanced CBM recovery. The results show that ① the adsorption potential of CO2 is greater than that of CH4 on the surface of coal matrix. The variations of surface free ener-gy and adsorption heat of coal due to CO2 adsorption are greater than that due to CH4 adsorption. ② Under constant confining stress,the increase of the CO2 injection pressure promotes the change of the surface free energy and the ad-sorption heat. Meanwhile,the effective stress decreases due to the progressive injection pressure,which increases the permeability,resulting in the promotion of the CO2 / CH4 displacement rate. On the contrary,under constant injection pressure,the effective stress increases due to the progressive confining stress,leading to the decrease of coal permeabil-ity,which has a negative effect on the displacement rate. ③ Supercritical CO2 is more adsorptive to coal than gaseous CO2 ,leading to a high CO2 / CH4 replacement rate under great effective stress. ④ Increasing the CO2 injection tempera-ture will contribute to the desorption of some adsorbed CH4 ,but it has a negative effect on CO2 absorption,leading to the decrease of the CO2 / CH4 replacement rate. The study provides a theoretical and technical method to improve the existing CBM extraction technology and enhance the CBM recovery.