Enhanced Oil Recovery with Smart Water Prepared from Treated Produced Water

Over the millions of years, a physico-chemical equilibrium between reservoir crude oil- brine-rock (CBR) has been established in oil reservoirs. At a given saturation level, the equilibrium can only be perturbed by injection of brine different from reservoir brine. Injection of brine similar to reservoir brine may only alter the relative saturation of the fluids and it will hardly influence the existing physico-chemical equilibrium. Therefore, injection of formation water is a secondary recovery technique and injection of low salinity water (LSW) having significantly lower salinity than the reservoir brine is a tertiary recovery technique. Flooding of water having different ionic strength and composition than reservoir brine to increase the recovery factor is termed as smart water (SW) flooding. In that sense, low salinity water flooding also falls under smart water flooding. Additionally, various other adjustments and optimization of the ionic strength and composition of the injection water to perturb the existing CBR equilibrium so that more oil recovery can be realized is also known as SW flooding (Austad et. al. 2013). Smart Water can be made by adjusting the ion composition and concentration of the injected fluid in such a way that the change in the equilibrium of the initial CBR-system will modify the initial wetting conditions. That eventually facilitate displacement of oil from the porous network. The physical principle for enhanced oil recovery by smart water is a change in wetting properties of the CBR-system, which has a positive effect on the capillary pressure and relative permeability of oil and water regarding oil recovery. The technique is cheap, environment friendly, no expensive chemicals are added, and no injection problems are encountered. From an economical point of view, the smartest water should be injected from the start of the water flooding process (Austad et. al. 2013). The principal mode of action of smart water flooding is wettability alteration, although various other mechanisms like pore blockage by fine migration, multi-ion exchange and expansion of electrical double layer etc. also comes into the picture when low salinity water flooding is considered. The mechanism of smart water effect is easier to understand in case of carbonate reservoirs than sandstone reservoirs (Austad et. al. 2013).