A multi-scale model for CO_(2) sequestration enhanced coalbed methane recovery

This paper presents a multi-scale model to simulate the multicomponent gas diffusion andflow in bulk coals for CO_(2) sequestration enhanced coalbed methane recovery.The model is developed based on a bi-dispersed structure model by assuming that coal con-sists of microporous micro-particles,meso/macro-pores and open microfractures.The bi-disperse diffusion theory and the Maxwell-Stefan approach were incorporated in the model,providing an improved simulation of the CH4-CO_(2)/CH4-N2 counter diffusion dynamics.In the model,the counter diffusion process is numerically coupled with theflow of the mixture gases occurring within macro-pores or fractures in coal so as to account for the interaction between diffusion andflow in gas transport through coals.The model was validated by both experimental data from literature and our CO_(2)flush tests,and shows an excellent agreement with the experiments.The results reveal that the gas diffusivities,in particular the micro-pore diffusivities are strongly concentration-dependent.