Effect of initial water saturation on bypassed oil recovery during CO₂ injection at different miscibility conditions

During gas injection in oil reservoirs, oil might be bypassed in either a micro- or macroscopic scale, because of different kinds of heterogeneities. However, it can be recovered by vaporization, diffusion/dispersion, swelling, and cross-flows (viscous, gravity, and capillary). Oil recovery depends upon the injected gas properties, injection type (immiscible, near-miscible, or miscible), presence of connate water, and frequency of dead-end pores. In this study, a special setup is used to investigate the effect of initial water saturation (S_wi) on the recovery of bypassed oil during CO₂ injection at different miscibility conditions. Further, analytical calculations of dimensionless numbers are used to study the dominant forces and mechanisms. Regardless of the amount of initial water, the maximum amount of oil recovery was achieved at near-miscible injection. Because the main recovery mechanism at immiscible conditions was gravity drainage, the presence of S_wi could hardly change the oil recovery factor. However, at near-miscible and first-contact-miscible regions, the oil recovery factor was decreased in the presence of initial water, because of the reduction of the available oil/gas contact area and the blockage of some pore throats.