Wettability and oil recovery from carbonates: Effects of temperature and potential determining ions

The success of oil recovery from fractured low permeable carbonates by water injection is dictated by the wetting properties of the rock surface. Potential determining ions like Ca2+ and SO42− have influence on the surface charge of the carbonate rock and are thereby linked to its wetting properties. In the present study, zeta potentials of chalk surfaces were measured by changing the sulfate and calcium concentrations in aqueous chalk suspensions. Series of long-term spontaneous imbibition tests were conducted at 70, 100 and 130 °C using oil containing chalk cores which were close to neutral wetting conditions. The concentration of SO42− in the imbibing fluids varied above and below the seawater concentration, while the Ca2+ concentration was kept constant. Sulfate acted as a wettability modifying agent by improving the water wetting nature of the chalk. The major experimental observations were: (1) the zeta potential on chalk was determined by the relative concentration of Ca2+ and SO42− present. (2) The oil recovery was tremendously improved by increasing the sulfate concentration in the imbibing fluid. (3) The wettability modifying property of sulfate increased as the temperature increased. (4) The chemical nature of the wettability modifying imbibition process was discussed.