We have measured crude oil-brine-mineral surface (calcite) interactions using a variety of dynamic techniques including contact angles, Surface Forces Apparatus (SFA), AFM, interfacial tension, XPS and other physical and chemical surface characterization techniques. Different brine (ionic electrolyte) solutions and temperatures were investigated, as well as dynamic (time-dependent) effects. The primary aim of the experiments was to investigate the “dilution effect”, where dilution of the ionic concentration of the fluid injected into oil wells is found to enhance oil recovery. Ionic strengths were varied from 350,000 ppm to pure water; and temperature varied from 20 to 75°C. We conclude that the “dilution effect” is due to a combination of colloidal (electric double-layer, hydration and van der Waals) forces, based on well-established theories for these interactions. In addition, our experiments reveal electro-chemical reactions involving dissolution, 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 film) which in turn reduces the water contact angle (increasing the water-wettability and, presumably, oil recovery) with increasing dilution. These two processes appear to be essential for the dilution effect to be effective at all solution concentrations from formation water to pure water. We also find that three different processes occur that have very different time scales: (1) the initial, rapid (seconds – 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 electro-chemical dissolution and restructuring of the surfaces (minutes – hours), which is also often accompanied by the adsorption of an organic-ionic layer on the mineral surface that comes off as visible flakes with the oil, and (3) large-scale, long-times, diffusion-rate controlled restructuring leading to macroscopic changes in rock morphology.

Effects of salinity on oil recovery: Experimental and theoretical studies of crude oil-water-calcite surface restructuring and associated physical and (electro)chemical interactions

253rd National Meeting (2017)

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