A mechanistic experimental study on the combined effect of Mg²⁺, Ca^2+, and SO₄ ^2- ions and a cationic surfactant in improving the surface properties of oil/water/rock system

More than half of the discovered hydrocarbon reservoirs in the world are the oil-wet carbonates. Enhanced oil recovery (EOR) from these reservoirs is challenging because of their tendency to retain oil at the rock surface. The use of low salinity (LS) water injection to recover more oil from these types of oil reservoirs has been recommended by several researchers. There are possible mechanisms concerning LS water flooding that have been introduced in the literature; a distinct lack of experimental investigation to enhancing the efficiency of this method is still felt. In this work, a detailed and comprehensive study was carried out on using LS + surfactant (LSS) to improve the surface and interface properties of oil + water + calcite rock samples. In the first step, the point of zero charge of the rock sample was determined. Then, the effect of different salts including MgCl₂, CaCl₂, and Na₂SO₄ at different concentrations on improving the oil-water interfacial tension and wettability alteration of the rock surfaces was investigated in the presence and absence of a cationic surfactant, cetyl methylammonium bromide (CTAB), in the operational range of pH. Emulsion formation and adsorption measurement tests were performed for a deep investigation of the performance of the cationic surfactant in the presence of different salts. Finally, waterflooding experiments were carried out and the water/oil relative permeabilities, amount of recovery and wettability alteration were thoroughly investigated for three different injection scenarios. The results showed that the co-presence of magnesium, calcium and sulfate ions all in the presence and absence of CTAB surfactant causes a significant reduction in IFT and contact angle values, especially at the lower ranges of salinity. The addition of divalent cations, i.e., Mg²⁺ and Ca²⁺, could reduce the amount of surfactant adsorbed on the calcite rock surfaces, especially in the low ranges of salinities. Also, in flooding tests, an increase in differential pressure between the inlet and outlet faces of the core samples from about 2.9 MPa for LS to about 4.7 MPa for LSS flooding, at the breakthrough time, was observed which was related to the formation of water in oil emulsions during LSS flooding. Comparison of the ratio of the residual to initial oil saturation (S_or/S_oi) for different flooding scenarios (i.e., injection of saline water (SW), LS and LSS solutions) indicates that as the salinity of water reduces, the efficiency of CTAB surfactant in improving oil recovery increases.