Summary
The ionic strength of injection water can have a major impact on oil recovery resulting from the use of low-salinity brines. Understanding how the water and oil chemistry affects the final recovery from a physicochemical point of view is necessary in order to optimize low-salinity waterflooding. It is clear from the literature that wettability is a key factor in achieving the low-salinity effect. Optimum ionic strength and conditions for low-salinity flooding with respect to wettability are still uncertain.
In this paper, we studied fluid/rock interactions at different salinity levels and elevated temperature conditions in terms of wettability and surface charge. Wettability is determined by a high-temperature/high-pressure (HT/HP) contact-angle method and zeta-potential technique. Outcrop rocks and stock-tank crude-oil samples were used in all experiments. Synthetic formation brine water, aquifer water, and seawater were evaluated under high-pressure conditions. Zeta potential of sandstone rocks and selected clay minerals was measured as a function of ionic strength.
Wettability of oil/brine/sandstone systems depends on salinity, temperature, and rock mineralogy. Using aquifer water in Berea sandstone improved the wettability toward a water-wet condition. The same aquifer water behaved in a different way when a different sandstone surface was tested. In Scioto sandstone, aquifer water changed the wettability to a neutral state. Low-salinity water expanded the double-layer thickness and eventually increased the zeta-potential magnitude. As a result of this expansion, it provides a greater opportunity to alter the wettability and enhance oil recovery. This study indicates that clay content in sandstone rocks can significantly alter the wettability either toward water-wet or intermediate. On the basis of the results obtained from this study, it is clear that low-salinity waterflooding can improve oil recovery in the field.
