The immiscible CO2 displacement process appears to be a very promising enhanced oil recovery technique for thin or marginal heavy-oil reservoirs where thermal recovery methods are generally unsuitable. This paper discusses results of laboratory and simulation studies for determining oil recovery mechanisms and displacement characteristics for this recovery method. Phase behaviour, linear coreflood, and scaled model studies were conducted with a medium heavy oil (14°API gravity) from a thin, sandy reservoir. The effects on oil recovery of reservoir operating pressure, CO2 injection rate, waterflood rate, CO2 slug size, and secondary and tertiary WAG (water-alternating-gas) schemes were examined. Oil recovery behaviour for a CO2 presoak process (which involves secondary or tertiary CO2 preflush followed by extended waterflood) was also investigated.

Study results indicated that oil swelling and viscosity reduction were the predominant recovery mechanisms for the subcritical CO2 displacement process (operating at 2 to 5 MPa, 28°C) with the oil. The CO2 presoak process improved the oil recovery because of higher mass transfer of CO2 into oil. A reduction in either waterflood rate (or CO2 injection rate) resulted in an increase in oil recovery and showed the interplay of viscous, capillary, and diffusive forces. The incremental oil recovery data, in conjunction with the CO2 requirements, showed that a slug size of approximately 0.4 pore volume (PV) was suitable for laboratory CO2 flooding studies. An increase in the operating pressure improved oil recovery for the CO2 water-alternating-gas (WAG) processes in sandpacks saturated with stock tank oil. However, the recovery in reservoir-fluid-saturated sandpacks was relatively insensitive to the operating pressures investigated, indicating mobility effects to be dominant in laboratory coreflood studies. Poorer oil recoveries for the secondary than for the tertiary WAG process were attributed to the high mobility of the injected CO2 gas. The CO2 presoak and WAG processes appear to offer strong potential for enhancing oil recovery from thin heavy oil reservoirs.

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