Evaluation of the performance of oil as a membrane during low-salinity water injection; focusing on type and concentration of salts

Highlights

• Hydrocarbons are a membrane for water molecules and pass them from low-salinity to high-salinity water.

• In lower salinities, reducing the salinity of the low-salinity water can reduce the membrane performance of the oil.

• Different hydrocarbon materials exhibit various membrane performances, related to their viscosities.

• Among the important salts in the seawater, KCl is a far better membrane for oils and lead to the most oil displacement.

Abstract

Injection of low-salinity or smart water into the reservoir is one of the most common EOR techniques. This technique leads to more oil recovery in comparison to reservoir water and seawater injection. Several mechanisms explain the reasons for this increased recovery level, the most important of which is the wettability alteration towards water-wet conditions. However, in recent years, further mechanisms have been added to the previous ones as the main or complementary mechanism. One of these newly added mechanisms is the performance of oil as a semi-permeable membrane between connate and low-salinity waters. According to this mechanism, water molecules move from low-salinity water through oil to connate water due to the difference in their osmotic pressure (difference in chemical potential), which works as the driving force, until the system moves toward equilibrium in concentration. Introducing water molecules into connate water causes the swelling of connate water, and eventually moves the oil towards the pore throats. In this study, experiments have been performed on capillary tubes, while ignoring the effect of fluid-rock interactions to investigate the proposed theory. Experimental results confirmed the role of osmotic theory. Furthermore, a decrease in the salinity of the low-salinity water causes the oil-droplets' displacement to increase, whereas further reductions in the salinity of the injected water cause lower displacements. Therefore, there is an optimum value of low-salinity water concentration to obtain the highest possible oil drop displacement. The optimum values for the tested samples in this study were about 5000–10000 ppm. Additionally, it was found that among all of the essential seawater salts, potassium chloride leads to the highest oil drop displacement; and among oil samples, heptane showed better membrane-like performance in comparison to toluene and crude oil.