Miscible GOGD in Naturally Fractured Reservoirs

Oil recovery from naturally fractured reservoirs is difficult. Especially in the Middle East where carbonate rocks are oil or mixed-wet. In these reservoirs typical production mechanisms like viscous displacement and counter current imbibition will not work. Gas–oil gravity drainage (GOGD) is then the production mechanism of choice. However, gravity drainage with immiscible (equilibrium) gas could result in low oil rates and/or low ultimate recovery due to capillary hold-up. Miscible gas injection could have significant advantages in a GOGD situation. Miscibility might be achieved with for instance CO2, which has typically a low minimum miscibility pressure. When the injected gas is (first contact) miscible with the oil, density and viscosity will be reduced. Miscibility adds the advantages of single-phase flow and interfacial tension reduction, which further improves GOGD rates and ultimate recovery. This paper evaluates the impact of first contact miscible gravity drainage on oil recovery and discusses the main modelling aspects. The key parameters were identified to be matrix block width over height ratio and vertical heterogeneity. First contact miscible gas injection benefits mainly from IFT reduction and has its application in heterogeneous reservoirs with large capillary hold up and re-imbibition effects.