Numerical simulation of multicomponent gas diffusion and flow in coals for CO2CO2 enhanced coalbed methane recovery

This paper presents an alternative model of multicomponent gas diffusion and flow in bulk coals, focusing on CH4–CO2CH4–CO2 counter-diffusion associated with CO2CO2-sequestration enhanced coalbed methane (CO2CO2-ECBM) recovery. The model was developed based on the bidisperse diffusion mechanism and the Maxwell–Stefan (MS) diffusion theory, which provides an improved simulation of multicomponent gas diffusion dynamics. The model was firstly validated under the condition of pure gas diffusion by comparing with the analytical solutions of a bidisperse diffusion model and the experimental data obtained from pure-gas sorption kinetic tests. Then it was numerically solved by considering CH4–CO2CH4–CO2 counter-diffusion and flow in a large coal sample to simulate a laboratory CO2CO2-injection core flush experiment. The simulation shows an excellent agreement with the CO2CO2 flush experiment. A quantitative description of the relationship between micropore diffusivity and concentration has been achieved, which is a deficiency in currently available CBM/ECBM models. The concentration-dependent diffusivities need to be taken into account in modeling the coalbed methane (CBM) recovery, in particular for simulation of ECBM production from and CO2CO2 sequestration in coal seams.