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 degrees at most. At lower concentrations, the effect of CaCl2 solutions is more obvious than that of NaCl solutions, and Na2SO4 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. (C) 2020 Elsevier Ltd. All rights reserved.