The ionic strength of injection water can have a major impact on oil recovery resulting from the use of low-salinity brines. Understanding how the water and oil chemistry affects the final recovery from a physicochemical point of view is necessary in order to optimize low-salinity waterflooding. It is clear from the literature that wettability is a key factor in achieving the low-salinity effect. Optimum ionic strength and conditions for low-salinity flooding with respect to wettability are still uncertain.In this paper, we studied fluid/rock interactions at different salinity levels and elevated temperature conditions in terms of wettability and surface charge. Wettability is determined by a high-temperature/high-pressure (HT/HP) contact-angle method and zeta-potential technique. Outcrop rocks and stock-tank crudeoil samples were used in all experiments. Synthetic formation brine water, aquifer water, and seawater were evaluated under highpressure conditions. Zeta potential of sandstone rocks and selected clay minerals was measured as a function of ionic strength.Wettability of oil/brine/sandstone systems depends on salinity, temperature, and rock mineralogy. Using aquifer water in Berea sandstone improved the wettability toward a water-wet condition. The same aquifer water behaved in a different way when a different sandstone surface was tested. In Scioto sandstone, aquifer water changed the wettability to a neutral state. Low-salinity water expanded the double-layer thickness and eventually increased the zeta-potential magnitude. As a result of this expansion, it provides a greater opportunity to alter the wettability and enhance oil recovery. This study indicates that clay content in sandstone rocks can significantly alter the wettability either toward water-wet or intermediate. On the basis of the results obtained from this study, it is clear that low-salinity waterflooding can improve oil recovery in the field.

High-salinity water such as seawater, or formation brines, is frequently injected in carbonate reservoirs. Ion interactions between injection water, reservoir fluids, and rock surface are quite complex. It has recently come to be believed that the chemistry of injection water can significantly enhance oil recovery. Several reaction mechanisms were suggested, including rock dissolution, change of surface charge, and/or sulfate precipitation.This study attempts to characterize the electrokinetics of limestone and dolomite suspensions at 25 and 50°C. In addition, reaction mechanisms at the water/rock interface were established. Synthetic formation brine, seawater, and aquifer water were chosen from Middle East reservoirs. Carbonate particles were soaked in high-and low-salinity water. A phase-analysis-light-scattering (PALS) technique was used to determine the zeta potential (surface charge) of carbonate particles over a wide range of pH, ionic strength, and temperature.Zeta potential of limestone particles was significantly affected by calcium ion. Low-salinity water created more negative charges on limestone and dolomite particles by expanding the thickness of the diffuse double layer. Individual divalent cations decreased the zeta potential of limestone particles in sodium chloride solutions, while sulfate ions showed a negligible effect. Limestone particles in high-salinity water had decreased zeta potential. The solubility of calcium ions increased as temperature was increased and thus created additional negative charges. The absence of sulfate in aquifer water strongly influenced the dolomite surface charge. In summary, surface-charge adjustment from positive to negative can alter the wettability of carbonate rock from preferentially oil-wet to waterwet. As a result, residual-oil saturation should be decreased. IntroductionInterfacial phenomena at carbonate/water interfaces are controlled by the electrical-double-layer 1 (EDL) forces. Therefore, it is necessary to understand the behavior of the ions' interactions with the rock surface. Charged species 2 are transferred across any solid/liquid interface only until reaching equilibrium. The interface can be visualized as a semimembrane 3 that allows the common charged species between solid and solution to pass through. These species are called potential-determining ions. As a result of the relative motion between the charged dispersed phase and the bulk liquid, the EDL is sheared. The potential, at this shear plane, is commonly called electrokinetic or zeta potential (). Various methods are applied to measure the potential at the shear plane. However, the most commonly used technique is the electrophoresis method (Pierre et al. 1990).

The primary aim of this study was to investigate the "dilution effect", where dilution of the ionic concentration of the fluid injected into oil wells has been found to enhance oil recovery. We have measured crude oil/brine/carbonate surface (calcite) interactions using a variety of dynamic techniques including contact angles, surface forces apparatus, atomic force microscopy, interfacial tension, X-ray photoelectron spectroscopy, and other physical and chemical surface characterization techniques. The effects due to different brine (ionic electrolyte) solutions and temperatures, as well as the dynamics (timedependence) of these effects, were investigated. Ionic strengths varied from pure water to 350 000 ppm, and temperatures varied from 20 to 75 °C. We found that upon exchanging solutions (as occurs for waterflooding using dilute solutions), three different dynamic processes occur that have very different time scales: (1) the initial, rapid (seconds to minutes) physical ion exchange with the surfaces that locally changes the surface charge/potential and, hence, the double-layer and hydration forces, (2) the local electrochemical dissolution and restructuring of the surfaces (minutes to hours), which is also often accompanied by the desorption of preexisting organic−ionic layers on the mineral surface that come off as visible flakes with the oil, and (3) the largescale, diffusion-rate-controlled restructuring leading to macroscopic changes in rock morphology (months to years). We conclude that the "dilution effect" is in part due to the well-known colloidal interaction forces (electric double-layer, hydrophilic-hydration, and van der Waals). In addition, our experiments reveal (electro)chemical reactions involving dissolution, pitting, adsorption, and restructuring of the calcite surfaces, which increases their roughness (cf. the geological process of "pressure solution"). Both the colloidal forces and surface roughening and restructuring act to reduce the adhesion of the crude oil/brine interface to the calcite/brine interface (across the thin aqueous or "water" film), which in turn reduces the water-side contact angle (increasing the water-wettability and, presumably, oil recovery), with increasing dilution. These two contributionsreduced colloidal forces and surface rougheningappear to be essential for the "dilution effect" to be effective at all solution concentrations from formation water to pure water. We propose a semiquantitative model to explain the "dilution effect" based on a form of the wellestablished extended-Derjaguin−Landau−Verwey−Overbeek theory for the colloidal interactions between the crude oil and carbonate surface across brine of different concentrations and a modified Young−Dupréequation that accounts for the effects of surface roughness. We present the "dilution effect" in terms of "wettability maps" for the calculated (effective) adhesion energy of the crude oil/brine/carbonate system as a function of brine concentration (from formation water down to the infinite-dilution [i.e., pure ...