Pore-Scale Investigation of Carbon Dioxide-Enhanced Oil Recovery

Carbon dioxide (CO₂) enhanced oil recovery is a green and promising way to produce oil and reduce the rapid growth of carbon dioxide released to atmosphere. A pore-scale understanding of CO₂ displacement phenomena is important to enhance oil recovery in porous media. In this work, a direct numerical simulation method is employed to investigate the drainage process of CO₂ in an oil-wet porous medium. The interface between the oil and CO₂ is tracked by the phase field method. The capacity and accuracy of the model are validated using a classic benchmark: the process of a bubble rising. A series of numerical experiments were performed over a large range of values of the gravity number, capillary number, and viscosity ratio to investigate the flooding process of CO₂ in a porous medium. The results show that the pressure in the main CO₂ flow path decreases dramatically after CO₂ breaks through the outlet. Oil begins to reflow into large pores that were previously occupied by CO₂. This phenomenon has an important impact on the final saturation distribution of CO₂. Increasing the viscous force is the dominant mechanism for improving oil recovery. Selecting an appropriate depth is the primary consideration for reaching the maximum recovery before CO₂ is injected into the subsurface. Abnormal high-pressure formations represent a good choice for CO₂ sequestration. Gravity fingers improve the sweep area of CO₂ when the viscous force is small. The oil recovery increases with increasing contact angle. It is difficult to reach the final steady state of saturation because of the “snap-off and supplement” dynamic balance in porous media when both the injection velocity and the contact angle are small.