Miscible CO2 flooding is a proven field development technology, which has been widely applied to many fields to enhance the oil recovery. It can efficiently expand oil volume, reduce oil viscosity and eliminate oil-gas interfacial tension. In this paper, a case study is detailed on CO2 flooding in a giant low-permeability (2-5md) and low-viscosity (0.3mpa.s) carbonate oil reservoir in Middle East, which has been extensively developed since 1990's with natural depletion, water flooding, and hydrocarbon gas (HC) flooding as well as HCWAG. The demand for further enhancing oil recovery and reducing greenhouse gas emission requires to assess the potential of CO2-EOR flooding.

In this paper, production history is firstly reviewed as well as the CO2 pilot test, including production performance, gas/water injection profile and remaining oil distribution in a five-point high-angle horizontal well pattern. MMP of hydrocarbon gas and CO2 is compared in the carbonate reservoir with the miscibility with oil, miscible area, residue oil and also, the gas intake profile as well as gas breakthrough. The density difference between gas/CO2 and oil results in a gravity override, gas/CO2 channeling, and early breakthrough and poor vertical conformance problems. Therefore, CO2-alter-Water injection with optimized WAG cycle as well as tapered WAG are detailed to improve production performance and increase the oil recovery, which is considered as a potential way with high swept efficiency of water flooding (70-80%) in a low-viscosity and high displacement efficiency of CO2 miscible flooding (85-90%).

The discussions of simulation schemes in this study can be applied to determine the optimal strategy for the future development of this large carbonate reservoir in the Middle East.

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