Water-alternating gas injection (WAG) is a common EOR method used at different reservoirs around the world. In this paper we present a novel method called a nanofluid alternating gas (NWAG) injection to improve the ability of the conventional WAG process for the oil-wet carbonate reservoirs. Different experimental tests of water-alternating gas (WAG), as well as nanofluid alternating gas (NWAG) injection were performed on carbonate core samples saturated with crude oil. SiO2 nanoparticles, with spherical morphology and two different sizes of 11-14 and 30-40 nm, were used to improve the performance of the WAG injection method. The WAG process in the core-flooding experiments was carried out by the injection of gas and SiO2 water-based nanofluids. Also, different characterization investigations, such as interfacial tension and contact angle measurements, were completed to study the nanoparticles effect on the recovery mechanisms.Our experiments showed that nano-silica adsorption on the rock surface changed the wettability of reservoir rock from oil-wet to strongly water-wet. Moreover, nanoparticles were located at the oil/water interface, which leads to a reduction in interfacial tension (IFT) between oil and water. Also, by adding SiO2 nanoparticles to the aqueous phase, the viscosity of the injected fluid was increased. Increasing viscosity and changing wettability affect the viscous and capillary forces and increase the capillary numbers; hence, the recovery is improved. Our tests showed that the dominant mechanism is the wettability alteration. These mechanisms enhanced the oil production from the porous media compared to the conventional WAG process.The results indicated a better recovery factor and efficiency for smaller nanoparticle size.Moreover, the experiments showed higher recovery factor for the core sample with lower permeability in comparison with the other.
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