Investigation of Wettability Alteration and Oil-Recovery Improvement by Low-Salinity Water in Sandstone Rock

Optimization of any oil-recovery process is based on understanding of the recovery mechanisms, whereas the underlying mechanisms of improving oil recovery by low-salinity-water injection are still debated. Wettability alteration is one of the mechanisms suggested to be the primary mechanism of low-salinity water. Therefore, wettability alteration by low-salinity water in sandstone reservoirs is examined by using the contact-angle technique. The effect of changing ionic strength on the electrokinetic charges is investigated by use of the zeta-potential technique to explain the causes of wettability alteration by low-salinity water. Moreover, coreflood experiments were performed in order to correlate between the wettability alteration and oil-recovery improvement caused by low-salinity water. Mica sheets were used for the contact-angle measurements to represent sandstone rock. The effect of water salinity on wettability was studied by using synthetic water over a wide range of salinities (from 0 to 174 000 mg/L), with two different crude oils at different conditions of pressure and temperature. Zeta-potential tests were conducted to measure the electroldnetic charges for combinations of each of the two crude oils or Berea sandstone and different brines. Berea sandstone cores were used for water-flooding experiments by injecting the same brines tested in the wettability measurements. Low-salinity water showed lower contact angles compared with high-salinity water for the two types of crude oil, which demonstrates the ability of low-salinity water to alter the rock wettability to more water-wet. In addition, low-salinity water made the surface charges at rock/brine and oil/brine interfaces strongly negative. As a result, the repulsive forces between rock and oil surfaces increase, which leads to expansion of the double-layer and, consequently, wettability alteration and oil-recovery improvement as confirmed by coreflood experiments.