Ion-induced oil–water wettability alteration of rock surfaces. Part I: Polar interactions between oil and solid

We experimentally study the influence of inorganic salt ions on the wettability of originally oil-wet rock surfaces, formed by adsorbing polar molecules via polar interactions. It is found that the ions can alter the surface to a more water-wet state with a reduction in the water contact angle of about 20° at most. At lower concentrations, the effect of CaCl 2 solutions is more obvious than that of NaCl solutions, and Na 2 SO 4 solutions are more effective than NaCl solutions. Through a series of characterization, underlying mechanisms are revealed as: (1) the octadecylamine protonates at oil–water interface and attracts with the anions, reducing oil–water interfacial tension; (2) the cations are adsorbed on the negatively charged quartz surface, weakening the electronegativity of quartz surface and replacing the adsorbed octadecylamine molecules. Finally, a quantitative theoretical model is established based on the Young equation to predict the dependence of contact angle on the ion concentration.