SmartWater flooding (SWF) has been proven as an effective and successful recovery method for carbonates, in which the injected water alters the carbonate rock wettability to produce incremental oil. Core-scale displacement experiments have demonstrated significant incremental recoveries of SmartWater in both secondary and tertiary modes. Single-well chemical tracer tests have demonstrated this potential in the field at a scale larger than those examined in the laboratory. Still, the underlying mechanisms responsible for SmartWater wettability alteration of carbonates are not well understood. The objective of this work is to understand the effects of individual mono and divalent ions on brine-oil interactions and their role in the observed alteration of carbonate wettability.In previous studies, we have investigated liquid-rock interactions and their role in wettability alteration. At fixed salinities, mono and divalent ions were found to have different effects on the calcite surface potential. Here, we investigate SmartWater effects on liquid-liquid interactions. We perform interfacial tension (IFT) measurements between oil and brines of fixed salinities but varying ionic compositions, and we collect IFT data at different temperature conditions. The different SmartWater recipes, at fixed salinities, have exhibited different IFT values indicating the varying effects of ions on fluid-fluid interactions. SmartWater recipes composed exclusively of magnesium cations exhibited a remarkably low level of IFT values. In contrast, SmartWater recipes with sodium or calcium cations exhibited comparable IFT stabilization levels, while SmartWater recipes that are solely composed of sulfate anions have resulted in higher IFT values. The low IFT values obtained with Mg 2ϩ ions can be attributed to weaker bond dissociation energies and quicker ion accessibility in MgCl 2 when compared to the other salts.In the next stage, these results will be integrated with Zeta potential and contact angle data acquired at similar brine salinities and composition. This systematic integration will allow for a better understanding of individual ion effects, which would eventually help to further optimize the SmartWater recipe for higher oil recovery.

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Results and discussions in this paper relate to a Lower Cretaceous carbonate reservoir located in southeastern Saudi Arabia. It is a heterogeneous carbonate formation with various facies due to diagenetic alteration of the original rock fabric. The reservoir is large and prolific with mixed-wet characteristics. Because of the economic importance and variety of oil-recovery mechanisms operative or possible in the reservoir, the multiphase recovery behavior has been extensively studied. Also, various wettability tests were carried out using Amott and USBM methods. This paper describes the variation in wettability and relative permeability of Lower Cretaceous carbonate reservoir and the multi-phase simulations of the experimental results. It shows that measurements are consistent with recent theories of the relationship between water saturation, relative permeability, and wettability as described by Jadhunadan and Morrow, 1991. However, the results indicate that the wettability of the reservoir changes from water-wet low on structure near the oil/water contact to mixed or neutral wet behavior higher on structure. Oil-wetting character increases towards the top of oil column and is correlated to decreasing water saturation. The results revealed that changes in wettability are accompanied by changes in waterflood efficiency and facies of deposition.