Numerical study of CO2 enhanced natural gas recovery and sequestration in shale gas reservoirs

Due to the ultra-fine pore sizes, Darcy's law is not applicable in shale gas reservoir. On the basis of the multiple binary gas transport mechanisms including viscous flow, Knudsen diffusion and ordinary diffusion, a new dual-porosity mathematical model is established to investigate the CO2 sequestration with enhanced natural gas recovery (CSEGR) in shale gas reservoirs. Meanwhile, the transfer function and well flow model considering the multiple mechanisms are developed. The mathematical model is implemented by the finite element simulation software COMSOL, and the accuracy of the numerical solution is verified by comparison with the classic analytical solution. Three sets of simulations were conducted to investigate the influence of CO2 injection on the development of shale gas reservoirs. The results show that CSEGR is feasible to achieve CO2 sequestration and enhance CH4 recovery in shale gas reservoirs, the CO2 storage and natural gas production rate can be improved highly with the increase of injection pressure. The CSEGR process can be divided into three stages including the early depressurization production period, the intermediate CO2 adsorption and CH4 replacement period and the late period of CH4 and CO2 produced simultaneously. In addition, the concentration variation and storage pattern of CH4 and CO2 gas are analyzed during the CSEGR process in shale gas reservoirs.