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Oil recovery from carbonate reservoirs can be enhanced by altering the wettability from oil-wet toward water-wet state. Recently, silica nanoparticle (SNP) suspensions are considered as an attractive wettability alteration agent in enhanced oil recovery applications. However, their performance along with the underlying mechanism for wettability alteration in carbonate rocks is not well discussed. In this work, the ability of SNP suspensions, in the presence/absence of salt, to alter the wettability of oil-wet calcite substrates to a water-wet condition was investigated. In the first step, to ensure that the properties of nanofluids have not been changed during the tests, stability analysis was performed. Then, low concentration nanofluids were utilized, and transient as well as equilibrium behavior of wettability alteration process were analyzed through contact angle measurement. Moreover, a mechanism for a wettability alteration process was proposed and verified with different tools. Results showed that the SNP suspensions could effectively change the wetness of strongly oil-wet calcite to water wet (e.g., from 156° to 41.7° at 2000 mg/L nanofluid). This ability was enhanced by increasing concentration, time, and salinity. Two equations were proposed to predict the equilibrium and transient contact angles with a good agreement. Analyzing the transient behavior of the wettability alteration indicated that the rate constant increased from 0.0019 to 0.0021 h–1 with the increase in nanofluid concentration from 500 to 1000 mg/L. It was further increased to 0.0026 h–1 for 1000 mg/L in 0.05 M electrolyte solution. The partial release of carboxylate groups from the oil-wet calcite surface and their replacement with SNP was suggested to be the responsible mechanism for wettability alteration. Surface equilibria and interaction studies, Fourier transform infrared spectroscopy, and scanning electron microscopy provided verification in support of the proposed mechanism. The enhanced wettability alteration in the electrolyte media was attributed to the role of Na+ ions facilitating the adsorption and release of SNP and stearates, respectively. In addition, the presence of electrolyte favorably affected the position of the system’s equilibria.

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