Effects of inorganic salt ions on the oil-water wettability of quartz surfaces

Abstract

The oil-water wettability in reservoir rock plays an important role in the recovery of crude oil. However, the mechanism of wettability transition on reservoir rock surfaces after low salinity water injection is still controversial. In this paper, the influence of inorganic salt ions in water on the wettability of the rock surfaces with the adsorption of polar molecules and its mechanisms were studied. Amines commonly found in crude oil were selected to represent base polar components in oil, and a highly idealized experimental model was used to exclude the influence of various components and other effects in crude oil. By measuring the oil-water contact angle on the quartz surface, the influence of various salt ion concentration in water on the wettability of quartz in the presence of base polar molecule in oil was obtained, and the mechanism of wettability alteration was analyzed by Zeta potential and AFM measurement. The experiment result showed that, the cleaned surface of quartz was water-wet, when 0.1 mmol/L octadecylamine was added to decane, the contact angle rose to about 42°, that is, the surface of the quartz became more oil-wet. When salts with different kinds and concentrations were added to water, the water contact angles decreased sharply with the increase of the concentration of ions. The contact angles of NaCl solutions and KCl solutions were basically the same at the same concentrations. However, at low concentrations, the CaCl2 solutions reduced the contact angles more than the NaCl solutions. The contact angles of Na2SO4 solutions were smaller than those of NaCl solutions at low concentrations, while the contact angles of Na2SO4 solutions were similar to those of NaCl solutions at higher concentrations. When the ion concentrations reached a certain level, the contact angles of different solutions all tended to be about 15°, which indicated that the ions desorbed basically all of the adsorbed polar molecules, leading to the change of wettability of quartz surfaces to be water-wet. The clean quartz powder was highly electronegative when dispersed in pure water. After soaking in octadecylaminedecane, the surface of quartz powder was positively charged. As a contrast, the Zeta potential of the quartz powder after soaking in pure decane changed little, indicating that the potential change was caused by the adsorption of positive octadecylamine on the quartz surface. When the quartz powder soaked with octadecylaminedecane was reversely extracted into NaCl solution, the Zeta potential of the quartz powder would decrease, and the electropositivity would weaken. In high concentration NaCl solution, the Zeta potential of the quartz powder would quickly return to negative charge, indicating the desorption effect of ions in the solution on polar molecules. From the AFM roughness analysis, it can be seen that the surface roughness of clean quartz was small, after soaking in octadecylaminedecane, the roughness increased, indicating that octadecylamine was fully adsorbed on the surface of quartz. After the soaking of droplet, the surface roughness of quartz plate decreased, indicating that the partial desorption of octadecylamine under the effect of ions. Based on the Zeta potential measurement of quartz powder and AFM roughness analysis of quartz surfaces, two mechanisms were proposed: (1) The octadecylamine in decane adsorbed and protonated at oil-water interface, resulting in an electrostatic attraction with the anion, thus reducing the oil-water interfacial tension; (2) the cations in water could be adsorbed on the negatively-charged quartz surfaces, weakening the polarity of quartz surfaces and thus replacing part of adsorbed octadecylamine molecules on the quartz surfaces.