This paper presents laboratory studies to apply a new approach based on combined foam EOR processes to a naturally fractured carbonate reservoir (NFR) located in Oman. Applications of EOR techniques in fractured reservoirs, despite their attractive potential, have always been challenged by the inability to efficiently control EOR fluid mobility in fractures, which results in inefficient flooding of the reservoir matrix and poor economics of the process. This work discusses the maturation of efficient foam EOR processes for NFRs. The foam is aimed at blocking aqueous solution flow in fractures on the one hand while allowing low interfacial tension (IFT) solution to enter the matrix on the other hand.

We use an extensive experimental workflow to develop such solution in the challenging salinity conditions of the considered reservoir. Using robotics, we first combine low-IFT and foam boosting surfactants to come up with the most adapted chemical cocktail in terms of solubility, IFT with crude oil and foaming properties in hard brine. The selected formulation is then quantitatively characterized for IFT with crude oil, phase behavior with live oil, foam stability in reservoir pressure and temperature conditions, and foaming properties in model porous media. Dedicated coreflood experiments mimicking flow in fractured reservoirs are finally used to quantitatively evaluate the process using the designed formulation. This includes evaluation of foam-induced pressure drop, effluent fluid composition and oil recovery in artificially fractured cores.

The designed combined foam EOR formulation is perfectly soluble in hard brine and yields an IFT with crude oil well below 10-2 mN/m at 65°C. It is able to generate and stabilize foam both in absence and in presence of crude oil. Process evaluation in artificially fractured core shows good control by foam of aqueous solution mobility in fracture, and efficient imbibition of aqueous solution from fracture to matrix. Interestingly, a filtration effect is observed whereby only aqueous solution enters the matrix from the fracture, while foam only exists in fracture. This, combined with the sensitivity of foam to presence of oil, enables an efficient production of oil from the matrix through the fracture, as measured during recovery experiments.

This paper presents the first steps toward a potential pilot application of a new process aimed at making chemical EOR in fractured carbonate technically and economically feasible. The approach presented here allows the design of a performing process in challenging conditions of water salinity and hardness.

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