A laboratory study to optimize ion composition for the hybrid low salinity water/polymer flooding

Nowadays, different studies are conducted to design new methods to improve oil recovery efficiency. The study focuses on experimental work to evaluate synergy between low salinity water and polymer flooding in carbonate formations. The main objective of this paper is to develop a hybrid method to enhance oil recovery by a possible combination of low salinity water and polymer flooding. The Caspian Sea water was used as a base brine, and the water composition of the Caspian Sea was altered by diluting and adjusting the concentration of active ions such as Ca²⁺, Mg²⁺, SO₄^2-, to observe the impact of modified seawater on wettability through contact angle measurements. The compatibility of the modified seawater and partially hydrolyzed polyacrylamide (HPAM) based polymer solutions is investigated by performing different rheological experiments. Different governing parameters such as water salinity, the concentration of active ions, and HPAM solutions concentration are experimentally studied to achieve the optimized rheological behavior. The different diluted water compositions of Caspian Sea water were prepared to identify the best solution in terms of wettability alteration. Later, the diluted solutions were modified by adding active ions such as Ca²⁺, Mg²⁺, and SO₄^2-. The wettability alteration to the more water-wet condition was observed when the concentrations of Ca²⁺ and SO₄^2- have been increased 4 times, respectively. However, no significant change in wettability was observed by only increasing the concentration of Mg²⁺ due to low-temperature condition. The effect of modified seawater on polymer performance compatibility was studied by conducting a series of rheological experiments for various salinity levels in smart water composition, temperature range from 25-80 °C, and polymer concentration at 1000 and 3000 ppm. All the polymer solutions showed typical non-Newtonian rheological characteristics with shear thinning behavior. Further, the results based on contact angle measurement and rheological experiments were combined to identify proper optimal conditions for the potential synergy of modified seawater and polymer. Our results showed a clear dependence of the polymer viscosity on the divalent ions concentration and temperature. The target viscosity of 4 cp at a shear rate of 10 s^-1, which approximately corresponds to the shear rate in porous media, was obtained with 500 ppm polymer concentration in 20 times diluted Caspian Sea Water having 4 times of Ca²⁺ and SO₄^2-. Through the series of experiments, we determined the optimum concentration of active ions in water that results in wettability alteration without compromising the effectiveness of polymer in the hybrid low salinity water-polymer flooding technique. The combination of low salinity water and polymer flooding is found to improve the performance of both methods.