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.
Although wettability alteration has been shown to be the main control mechanism of Low Salinity and Smart Water (LS-SmW) injection, our understanding of the phenomena resulting in wettability changes still remains incomplete. In this study, more attention is given to direct measurement of wettability through contact angle measurement at ambient and elevated temperatures (28 °C and 90 °C) during LS-SmW injection to identify trends in wettability alteration. Zeta potential measurement is utilized as an indirect technique for wettability assessment in rock/brine and oil/brine interfaces in order to validate the contact angle measurements. The results presented here bring a new understanding to the effect of temperature and different ions on the wettability state of dolomite particles during an enhanced oil recovery process.
Our observations show that increasing temperature from 28 °C to 90 °C reduces the contact angle of oil droplets from 140 to 41 degrees when Seawater (SW) is injected. Besides, changing crude oil from crude-A (low asphaltene content) to crude-B (high asphaltene content) contributes to more negative surface charges at the oil/brine interface. The results suggest that the sulphate ion (SO42-) is the most effective ion for altering dolomite surface properties, leading to less oil wetness. Our study also shows that wettability alteration at ambient and elevated temperatures during LS-SmW injection can be explained by Electrical Double Layer (EDL) theory.
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