Non-monotonicity of the Contact Angle from NaCl and MgCl2 Concentrations in Two Petroleum Fluids on Atomistically Smooth Surfaces

Wetting and alteration of wetting are among the most important material properties of fluid fluid substrate systems in biological and industrial systems. An important industrial application of wetting and wetting alteration is related to displacement of crude oil by water injection in porous media. Water injection in oil reservoirs has been used since the early periods of oil production. Recently, it has been discovered that the salt concentration in the injected water may have a significant effect on the oil recovery. The process is under active research for the need of an improved understanding. In this work, we investigate the governing elements of surface wettability with two different crude oils on two atomistically smooth surfaces (mica and quartz) and one smooth surface (calcite) as a function of the salt concentration (0-3 M) and type (mono- versus divalent). We investigate the change of wettability from NaCl and MgCl2 salts over a wide concentration for the first time. The measurements are based on long enough aging times and droplet sizes that give equilibrium and size-independent contact angles. Our measurements show a non-monotonic behavior, in that, as NaCl concentration increases, there is a decrease (increase of water-wetting) and then an increase (decrease of water-wetting) of the contact angle in all of the systems that we have studied. MgCl2 salt shows two trends with an increasing concentration. For mica and quartz, there is first a decrease of the contact angle and then an increase followed by a second sharp decrease at high MgCl2 concentrations. For calcite substrate, we observe an increase of the contact angle reaching a maximum and then a decrease with an increasing salt concentration. These observations have profound implications on the effect of salts on wettability alteration. The measurements have set the stage for atomistic simulations for a molecular understanding of the salt effect in complex fluids.