Naturally fractured reservoirs with low permeability and mixed-wet properties usually have poor waterflood performance because the imbibition of water into matrix is not very significant. Surfactants could be used to change matrix wettability to more water-wet and thereby improving the imbibition of water into matrix. Surfactants can also decrease water/oil interfacial tension (IFT) resulting in mobilization of residual oil. The goal of this study is to analyze the wettability alteration and IFT decrease in the imbibition process in fractured reservoirs by simulating surfactant imbibition at core scale. The simulator Eclipse is used to simulate wettability alteration and IFT decrease separately or simultaneously. A 3D model is set up to model a core surrounded by water/surfactant solution in a container with an injector and a producer. The injection and production rates are 0.01 cm3/hr, therefore, the viscous force can be ignored. Three different kinds of surfactant with different properties are used. Two of them have only one mechanism each, either wettability alteration or IFT decrease, and the other has both mechanisms. In the simulation study of the injection of the surfactant that only changes the wettability to more waterwet, the ultimate oil recovery is larger and oil recovery rate is bigger than for the case with no surfactant injection. The simulation study with the surfactant that only reduces the IFT shows that ultimate oil recovery is increased when the IFT at critical micelle concentration (CMC), which is expressed as IFTCMC, is lower than 0.05 mN/m. However, at the beginning of the imbibition, the lower IFT leads to a lower recovery rate. The study of the surfactant that has both mechanisms shows that there exist optimal IFTCMC's at different wettability conditions. If the matrix is rendered strongly water-wet by surfactant, the optimal IFTCMC is 19 mN/m. If the surfactant has a weak capability of wettability alteration (the matrix is still mixed-wet), the IFTCMC needs to be reduced to ultralow value. The present studies show that wettability alteration is a more important parameter for the surfactant imbibition in fractured reservoirs than changes in IFT. This work improves the understanding of the interplay between the main mechanisms of surfactant enhanced oil recovery (EOR) from fractured reservoirs, and provides a reference for the surfactant screening for improving oil recovery in naturally fractured mixed-wet or oil-wet reservoirs.
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