Effects of Salinity on Oil Recovery (the Dilution Effect): Experimental and Theoretical Studies of Crude Oil/Brine/Carbonate Surface Restructuring and Associated Physicochemical Interactions

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 (time dependence) of these effects, were investigated. Ionic strengths varied from pure water to 350 000 ppm, and temperatures varied from 20 to 75 degrees 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 large-scale, 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 well established 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-Dupre equation 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 water] limit).