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The salt content and composition of the water that is either produced with the oil or injected in an oil reservoir can change the composition of the oil at the oil/water interface. Having identified these compositional changes is key to designing the most optimized water ionic recipe to be used in waterflooding of an oil reservoir. Hence, there is a need to better understand the physicochemical interactions at the oil/water interface because of salinity effects. This study elucidates the effect of salinity on the interfacial interactions through pendant drop measurements of crude oil/water interfacial tension, surface charge evaluation by zeta potential, water content measurements by Karl Fischer titration, Fourier transform infrared (FT-IR), and ultraviolet–visible spectroscopy analyses. The interfacial tension results indicate that the interfacial tension increases with reduction in water salinity, which is shown for the first time by FT-IR and water content measurements to be proportional to the spontaneous formation of water microdispersion as the main mechanism of low salinity water injection. Formation of microdispersions and partitioning of surface-active materials by conjugated acidic compounds and/or acidic asphaltenes and low-molecular weight acidic compounds, respectively, are the main parameters controlling the crude oil/water interactions. Asphaltenes and acidic materials are shown to be the underlying compounds in the crude oil phase promoting the microdispersion formation.

The long-held industry view that the Low Salinity Effect (LSE) depends mainly on rock−fluid interactions has led to failures and successes that can be explained by fluid−fluid interactions. Therefore, elevating our knowledge about the microscopic interactions occurring in the crude oil/brine/rock system appears to be of paramount importance. This paper chooses to outline various analytical tools in combination with a microfluidic instrument (Micromodel) to identify these interactions at simulated reservoir conditions for the first time (temperature and pressure of 50 °C and 2000 psi). In this study, six crude oil samples have undergone testing for microdispersion quantification and surface charge evaluation. Microdispersion is a term referring to the spontaneous formation of water clusters (in micrometer sizes) within the crude oil during low salinity water injection (LSWI), which will be elaborated in this study. Despite all samples showing the same trend regarding the negative surface charges, they showed an entirely different propensity toward formation of water microdispersion. The analysis of the oil/water interface by Fourier-transform infrared spectroscopy (FT-IR) led to the understanding that conjugated acidic compounds within the crude oil are the main compounds for the creation of water microdispersions. The Micromodel results revealed the predominant role of microdispersions in oil swelling and wettability alteration in a porous medium leading to an increase in the microscopic sweeping efficiency, thus leading to improved oil recovery. Also highlighted is the pivotal importance of water microdispersion as a screening method for oil reservoirs before waterflooding operation.

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