Oil recovery from oil and mixed-wet naturally fractured reservoirs is difficult. Standard production mechanisms like water injection and countercurrent imbibition will not work. Gas–oil gravity drainage (GOGD) is then the production mechanism of choice. Gravity drainage with immiscible/ solution gas can result in low production rates and/or high residual oil saturation due to capillary holdup.

When the injected gas is miscible with the oil, density and viscosity reduction will occur. Miscibility adds the advantages of single-phase flow and interfacial tension reduction, which further improves GOGD rates and ultimate recovery.

The paper discusses the modeling and upscaling of first contact miscible gravity drainage as well as the differences from regular gas injection processes. The key parameters for this case were identified to be width over height ratio, layering heterogeneity and gas recycling rate. First contact miscible gas injection benefits the most from the IFT reduction and is most suitable in heterogeneous reservoirs with large capillary hold up and re-imbibition effects.

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