Experimental investigation on synergic effect of salinity and pH during low salinity water injection into carbonate oil reservoirs

Abstract Interaction between rock-fluid and fluid-fluid can have a significant effect on oil recovery. Changing the wettability of reservoir rock toward more water-wet or less oil-wet state is one of the expected mechanisms during low salinity water injection (LSWI). pH and salinity are of the most eminent factors of injection water controlling the wettability state of a crude oil/brine/rock system during any waterflooding operation. A small change in pH can affect the surface charges at the rock/water and oil/water interfaces leading to wettability alteration in a porous medium. In this study, the synergic effect of salinity and pH on the wettability state of carbonate rocks is evaluated through adhesion bottle tests for two different carbonate rock types and two different crude oils. The experimental results reveal that non-adhesion (water wet) state is achieved more easily during LSWI (4,160 ppm) in a broader pH range compared to that in high salinity water injection (166,400 ppm). Also, it is shown that the acquired wettability maps strongly depend on the rock and crude oil surface charges which are directly affected by the rock mineralogy and crude oil effects. The wettability of rock particles in higher pH values is controlled by the rock mineralogy effect, while the wettability is influenced by the crude oil effect in lower pH ranges. It has been shown in this study that the wettability of carbonate rocks strongly is in proportion to the surface charges and electrical double layer (EDL) theory. The findings herein give insight into the underlying mechanism of LSWI contributing undeniably to effectively optimize and control the Low Salinity Effect (LSE) in the porous media.