In many large oil fields low oil saturation transition zones encompass significant hydrocarbon volumes. In some fields this retained oil below the water-oil contact in the water column is referred to as paleo oil. Gas injection represents one of the potential recovery methods to mobilize, displace and produce this remaining oil from transition zones.

The simulation model was built using geological description characteristic for the oil field water zone. The reservoir model for the water column above paleo water-oil contact was established based on core and log data from two wells. Down dip gas injection with vertical and horizontal wells was evaluated for drainage and recovery of paleo oil.

CO2 injection has been compared to flue gas and hydrocarbon gas. The immiscible or developed miscibility by CO2 injection process was more efficient due to compositional effects such as vaporization and swelling. Compositional simulation with fine grid vertical resolution in the reservoir model was performed. A large number of hydrocarbon components in the fluid description (17) were used in order to account properly for vaporization, swelling and gravity effects. The remaining oil saturation in the paleo zone may be significantly reduced by secondary and tertiary CO2 injection.

Simulations confirmed the formation of the tertiary oil bank and updip oil displacement. The CO2 liquid fraction in the oil bank reaches a level of about 50%. The oil viscosity in the moving oil bank is reduced significantly by dissolved CO2 compared to the viscosity of the original oil.

Simulations showed that high oil recovery factors could be achieved by gas injection in the transition paleo zone. The injection/production strategies can be further optimized to reduce high water-cut production period.

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