Effect of monovalent and divalent salts on the interfacial tension of pure hydrocarbon-brine systems relevant for low salinity water flooding

Interfacial tension between hydrocarbon liquids and salt water is a very important property for many industrial applications, especially in petroleum industry. Interfacial tension, in addition to other factors, has direct impact on displacement process in porous media so thus on oil recovery. Very recently, the salinity of injection brine has been regarded as a key factor in oil recovery using low salinity water flooding process. Decrease in the interfacial tension between crude oil and injection brine at low salinity condition might be a reason behind recovery improvement as suspected by some researchers; but there are also other effects associated with low salinity water flooding like wettability alteration, fine migration, mineral dissolution etc. It is suspected that the interaction of polar components at oil-water interface lead to the reduction of interfacial tension. However, the actual mechanism is not known and still under research. The main objective of this work is to determine the effect of salt concentration and type of ions present in an aqueous phase on the interfacial tension between pure hydrocarbon liquids and water. Different hydrocarbon liquids, such as aliphatic and aromatics, have been tested to understand the interaction of monovalent and divalent salts on the interfacial tension. The study reports the interfacial tension of five pure hydrocarbon liquids against solutions of three different salts (NaCl, MgCl2 and CaCl2) over a wide range of salinities. The interfacial tension measurements were done using Wilhelmy plate method by a dynamic contact angle tensiometer. All the experiments were conducted at room temperature and atmospheric pressure. The results lead us to the view that there is low a salinity concentration where the hydrocarbon/brine interfacial tension shows a minimum value. The type of salt also has a significant effect on interfacial tension of aliphatic and aromatic hydrocarbons. Monovalent salt found to be effective in reducing interfacial tension of aliphatic hydrocarbons while divalent salts were found to be effective for aromatic hydrocarbons. The possible mechanism for the reduction in IFT at low salt concentration has also been explained using Gibb's adsorption isotherm. In addition, the trend in IFT has been explained in the light of well-known Jones-Ray effect.