It is well-established that smartwater flooding through tuning of injection water chemistry and ionic composition, has a significant impact on the recovered oil, but the exact underlying mechanism by which this occur, is not well understood, and is supposed to be caused by a complex interactions occurring at the fluid/fluid and fluid/rock interfaces. Most of the laboratory studies reported so far have been focused on characterization of crude oil/brine/carbonate system and wettability alteration at micro- and macroscale using classic measurements, including contact angle, IFT, NMR, zeta potential and coreflooding. However, those techniques depend strongly on carbonate heterogeneities, roughness and fluids distribution inside the pores. Thus, a direct visualization at pore scale is needed to identify fluids distribution in-situ, wettability state at pore scale and wettability alteration by injection water composition tuning. We used Broad Ion Beam slope-cutting in combination with Scanning Electron Microscopy under Cryogenic conditions (Cryo-BIB-SEM) to study crude oil/brine/carbonate interfaces. Direct imaging at nanoscale allows investigation of the porosity, in-situ preserved fluids and, combined with energy dispersive spectroscopy (EDS) identify crude oil and brine distribution and quantifies wettability state by measuring contact angle at pore level. In this study, we compare carbonate rocks that have been aged in crude oil and saturated with brines at high and low ionic strength brines. In both samples, we investigate oil and brine distribution in the carbonate porous matrix. Results show that ion milling at cryogenic conditions allows the preparation of a large smooth cross-section. The presence of pinning points contribute to the crude oil adherence to carbonate surface. Scanning electron microscope images indicate that in presence of high ionic strength brine, large trapped oil patches have an elongated shape, following the carbonate surface morphology. While oil droplets have a pseudo-spherical shape with low ionic strength brine in addition to a distinct boundaries between oil and brine phase. Statistical analysis of in-situ contact angle and carbonate/oil/brine interface demonstrate the sensitivity of cryo-BIB-SEM approach to sub-micron scale wettability alteration caused by ionic strength variations.

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