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In this study, the effects of amphiphilic water-soluble components of crude oil and ionic composition of water were investigated on the wettability alteration of a carbonate surface. Four different types of saltwater, which are made up of NaCl, MgCl2, CaCl2, and Na2SO4, with 0.5 M ionic strength, besides distilled water, were contacted with crude oil to be saturated with water-soluble components. The post-contact (after contact with crude oil) water properties were characterized by Fourier transform infrared spectroscopy, total organic carbon, and pH analyses. The results showed that some polar compounds of crude oil with basic and mostly acidic functional groups were transferred to saltwater and distilled water. Afterward, the oil-wet carbonate surface was soaked in both freshwater (not contacted with crude oil) and post-contact water, and their effects on the wettability of the surface were investigated through contact angle and scanning electron microscopy experiments. The results of the contact angle indicated that generally, soaking of the oil-wet carbonate surface in both freshwater and post-contact water altered the wettability of the carbonate surface toward more water-wetness. However, the presence of water-soluble components in post-contact water (except for Na2SO4saltwater), significantly reduced its wettability alteration efficiency in comparison to freshwater. Similarly, when water contains the Ca2+ion, the calcite surface could not be dissolved in the aqueous phase, and accordingly, wettability alteration would be seriously retarded in both freshwater and post-contact water. Moreover, several mechanisms may jointly contribute in wettability alteration of the carbonate surface, which include organic–ionic layer dissolution into the aqueous phase, dissolution of the calcite surface alongside its adsorbed amphiphilic compounds, expansion of the electrical double layer, pairing of the Mg2+ion with surface-absorbed compounds, Mg2+substitution for Ca2+on the calcite surface, and interaction of cationic components with absorbed acidic compounds.

A detailed investigation into the effect of dissolved amphiphilic compounds of crude oil in salt water on its surface tension/interfacial tension (ST/IFT) behavior has been conducted in this paper. To simulate water–oil contact during water injection into oil reservoirs, 16 single-salt aqueous solutions of NaCl, CaCl2, and Na2SO4having ionic strengths of up to 2.0 M were mixed with a natural acidic–basic crude oil. The mixing was performed by a rocking mechanism in a visual pressure–volume–temperature cell at an elevated temperature. After the mixing process, two phases were completely separated from each other using a high-temperature centrifuge. Finally, in addition to pH measurements, the air–water ST and decane–water IFT of salt water were measured using the pendant-drop method. The results reveal that the dissolution of crude oil amphiphilic compounds, especially the acidic or basic compounds, in salt water can considerably decrease its ST/IFT. It seems that there is the same optimal ionic strength for the aqueous solutions of the three studied salts, in which both the air–water ST and the decane–water IFT of salt water are minimized after its contact with crude oil. It was observed that, among the three studied salts, the aqueous solutions of Na2SO4had the lowest ST/IFT values for all ionic strengths after their contact with crude oil. This result can be because the crude oil used was more basic than acidic.