Wettability alteration by magnesium ion binding in heavy oil/brine/chemical/sand systems—analysis of hydration forces

In laboratory sandpack tests for heavy oil recovery by alkaline flooding, it was found that wettability alteration of the sand had a significant impact on oil recovery. In this work, a heavy oil of 14 API was used to examine the effect of organic acids in the oil and water chemistry on wettability alteration. From interfacial tension measurements and sand surface composition analysis, it was concluded that the water-wet sand became preferentially oil-wet by magnesium ion binding. The presence of Mg in the heavy oil/Na2CO3 solution/sand system increased the oil/water interfacial tension. This confirmed the hypothesis that magnesium ion combined with the ionized organic acids to form magnesium soap at oil/water interface. Under alkaline condition, the ionized organic acids in the oil phase partition into the water phase and subsequently adsorb on the sand surfaces. The analysis of sand surface composition suggested that more ionized organic acids adsorbed on the sand surface through magnesium ion binding. The attachment of more organic acids on the sand surface changed hydration forces, making the sand surface more oil-wet.