Macro- and Microscopic Studies of "smart Water" Flooding in Carbonate Rocks: An Image-Based Wettability Examination

To date, numerous studies have shown that "smart water" flooding can enhance the oil recovery of carbonate reservoirs by altering the rock wettability. In particular, the Ca²⁺, Mg²⁺, and SO₄ ^2- ions in smart water play important roles in altering the wettability of carbonate rocks, although their symbiotic effects are still under debate. In this study, we employ both macro- and microscopic methods, including ζ-potential measurements, contact angle measurements, and micro X-ray computed tomography (μ-CT) scanning to examine the effects of several "smart waters", e.g., increasing SO₄ ^2- concentrations or decreasing Ca²⁺ and Mg²⁺ concentrations in seawater, in changing the wettability of Austin chalk. ζ-potential results confirm that the surface potentials of chalk samples become more negative in smart waters than in seawater. Contact angle results suggest that smart waters are more effective in making the chalk surface more water-wet than seawater. However, seawater with four times the SO₄ ^2- concentration (SW4SO) and seawater with one-fourth of the Ca²⁺ concentration (SW0.25Ca) show more potential in enhancing the alteration of chalk wettability compared to other smart waters. The μ-CT images offer a microscopic view of the fluid distribution in the porous media of chalk samples after flooding with seawater and followed by SW4SO or SW0.25Ca, which shows that SW4SO contributes to the increase in water-wetness in nanopores (or subscale porous structure), whereas SW0.25 contributes to the increase in water-wetness in micropores. In addition, the "effective contact angles" of chalk samples decreases approximately 10° after SW4SO and SW0.25Ca flooding, resulting in an increase in the "microscopic oil recovery" by 18.6 and 20.2%, respectively. Thus, this result suggests that SW0.25Ca is more effective in enhancing the water-wetness of chalk samples than SW4SO.