Role of sulfate ion on wettability alteration and oil mobilization in chalk reservoirs during modified salinity waterflooding

It has often been reported that an increase in sulfate has a positive effect on altering wettability and mobilizing oil during modified salinity water flooding in carbonate reservoirs. However, this understanding is mostly based on experimental studies using outcrop rock material. Therefore, this study aims to reassess the role of sulfate on altering wettability and recovery performance during modified salinity water injection in oil-saturated porous media of chalk reservoirs using real reservoir material. We compare the results with outcrop samples to evaluate the effects of sulfate and rock material on wettability alteration and oil mobilization. We conduct dynamic contact angle measurements and coreflooding experiments in reservoir conditions. Our findings contradict the previous observations, as we discovered that decreasing the sulfate concentration in the displacing fluid causes a shift in the reservoir chalk's wettability to a more water-wet condition, which significantly improves oil mobilization. We also establish that outcrop chalk samples, typically used in laboratory experiments, cannot be used as reliable analogues for reservoir chalk samples with the same geological age. Finally, we use direct pore scale simulations to demonstrate the impact of observed wettability alteration on oil mobilization. Our simulation results align with our experimental findings, indicating that wettability alteration is the primary mechanism in our experiments, as fluid-fluid interactions were ignored, and flow was mainly controlled by the medium's wettability state.