Surface and Interfacial Tension Behavior of Salt Water Containing Dissolved Amphiphilic Compounds of Crude Oil: The Role of Single-Salt Ionic Composition

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 Na₂SO₄ having 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 Na₂SO₄ had 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.