Role of Brine Composition and Water-Soluble Components of Crude Oil on the Wettability Alteration of a Carbonate Surface

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 Na₂SO₄ saltwater), 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.