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.