|Fig. 1: Schematic of oil recovery profile showing the impact of modifying water salinity. A typical recovery range of high salinity water injection is 30-50%. Modified water could be injected after production plateau is reached. The new water could increase incremental recovery by 5-20%.|
Water injection is commonly used to produce oil in most of the world's oil fields. The injection process is very effective in displacing oil at relativity low cost. At the end of this stage, which usually takes few decades, there is still significant amount of oil left underground. The remaining oil is trapped either due to poor sweep efficiency or a stronger attraction between rock surface, oil and water in some of the pores. Several methods are used to recover the remaining oil. This includes chemical flooding where some chemicals, such as surfactants and polymers, are added to injection water to improve its ability to displace more oil. Other techniques could involve injecting carbon dioxide or nitrogen instead of water. Oil chemical and physical properties, reservoir conditions, economic feasibility and many other factors dictate the preference of one method over others.
Modifying water salinity to increase oil recovery is a relatively new method that could be applied in many oil fields to unlock millions of barrels of oil. Seawater and saline aquifers are typical sources of water in conventional water injection projects. Several studies, supported by field tests, suggest that additional oil can be recovered by reducing salinity and/or adding some key ions to the water salts. Oil reservoirs are classified into sandstones and carbonates and each has relatively different properties and characteristics. Modifying water salinity could impact oil recovery in both types although the mechanism and approach could be different.
Reducing water salinity in sandstone reservoirs has been demonstrated to be a very effective way to recover more oil. Successful core flooding experiments showed an increase in oil recovery of 5%-20% when water salinity is lower than 5 kppm.  Some of the remaining oil, which has a negative charge, is stuck on the positively charged clay surface. The additional oil production is associated to the release of these oil droplets that are attached to clay particles. 
Pilot tests were conducted on two Alaskan fields and both showed positive impact of low salinity water. The injection water was switched to low salinity water while production was closely monitored from a nearby producer. The production wells from both tests showed an increase in oil production as soon as the low salinity water breakthrough occurs. Production rates doubled on the first year. The reduction in oil saturation in one of these tests was measured to be 10% compared to lab prediction of 13%. [5,6]
The impact of water salinity on oil recovery is different in carbonates as compared to sandstones. Adding divalent ions such as calcium and magnesium along with sulfate at elevated temperature could increase oil production. The release of oil is attributed to ionic interactions between oil, water and rock surface that eventually lead to oil detachment from the rock surface.  Several low salinity core flooding experiments were conducted using carbonate cores from the Middle East and they showed an increase in oil recovery. The incremental gain was 7% only by reducing water salinity from 58 kppm to 29 kppm without adding or removing specific ion concentration. Further dilution of water to 5.8 kppm added another 9% to the recovery.  A pilot test was conducted under similar conditions on a well located on an area flooded with seawater. A slug of twice diluted water was injected followed by another slug of ten times diluted water. Oil saturation was monitored at each staged using a tracer and it showed a reduction of oil saturation units by 3% and 6%, respectively. 
Modifying the salinity of the injection water is a promising technique to increase oil recovery in both sandstone and carbonate reservoirs. The mechanism taking place, when salinity is altered, differs for each type which requires implementing different schemes. Sandstone reservoirs are very sensitive to total salinity and additional oil can be produced by reducing salinity to very low levels. Carbonate reservoirs are sensitive to both total salinity and divalent ions and additional oil can be produced by reducing salinity or adding these ions to the injection water salts.
Â© Mohammed Alshakhs. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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