Pore-scale investigation of CO2-oil miscible flooding in tight reservoir

CO2 miscible flooding is a promising technique for enhancing tight oil development and achieving carbon capture, utilization, and storage. This work has developed a comprehensive pore-scale modeling workflow for CO2 miscible flooding in tight reservoirs. Firstly, Maxwell-Stefan diffusion and equation of state are coupled to calculate Fick diffusion flux, considering composition-dependent effects. Subsequently, the Navier-Stokes and mass transport equations are coupled, incorporating crucial recovery mechanisms in miscible flooding. Furthermore, the effects of essential factors on miscible flow in tight porous media are analyzed. The findings are as follows. The competition between molecular and convective diffusion determines the displacement patterns. Both diffusion mechanisms contributed before the CO2 breakthrough; after that, the oil retained in unswept zones was extracted by molecular diffusion and subsequently displaced by limited convection. The relative contributions of each recovery mechanism are evaluated from a pore-scale perspective. Viscosity reduction primarily affects the oil production rate, while desorption and extraction mainly influence the ultimate oil recovery. A higher Péclet number results in an earlier gas breakthrough, causing a transition from piston-like to ramified displacement, which negatively impacts oil recovery and CO2 storage. Fracture angle and connectivity significantly affect the breakthrough moment and the recovery factor. The adsorption rate has a slight impact on the oil production rate, while the maximum adsorption capacity affects the final oil production and the stability of the miscible flooding front.