Nanoscale observations of specific ion effects on the interactions between calcite and oil during ion tuned water flooding

We studied the impacts of Ca²⁺, Mg²⁺, and SO₄^2- on the interactions of calcite with oil by direct force measurements using atomic force microscopy (AFM) and “soft tips.” The force results indicated that the attraction between calcite and oil increased with the increase of salt concentration from 0.5 % to 2.0 %, while a concentration threshold (∼1.0 %) existed where a significant change in adhesion was observed. When comparing with the force results in NaCl solutions, it was found that SO₄^2- ions enhanced the repulsions between calcite and oil, whereas Ca²⁺ and Mg²⁺ ions introduced attractions and large adhesions between calcite and oil, and the impact of Ca²⁺ ions was more significant than that of Mg²⁺ ions. The underlying reasons for the specific ion effects on calcite-oil interactions were comprehended by surface forces calculations based on the extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and surface complexation modeling in terms of the zeta potential results. It was found that the adsorption of SO₄^2- ions on calcite surface not only enhanced the electrical repulsion between calcite and oil but also introduced a repulsive hydration force, making the calcite surface repel the oil droplet and producing small adhesions. However, the bridging of Ca²⁺ or Mg²⁺ ions in between calcite and oil nearly eliminated the electrical repulsion and even triggered an attractive hydrophobic force, making the oil droplet attracted to the calcite surface and producing large adhesions. The force results were further correlated to the wettability of the calcite surface with regards to the Young-Dupré equation with the measured contact angle and interfacial tension results. It suggested that the hydrophilicity of the calcite surface decreased with the increase in salt concentration, especially at a salt concentration higher than 1.0 %. The presence of SO₄^2- ions could enhance the hydrophilicity of the calcite surface, whereas Mg²⁺ ions, especially Ca²⁺ ions made the calcite surface less water-wet. To this end, the AFM force measurements have provided nanoscale evidence on the specific ion effects of Ca²⁺, Mg²⁺, and SO₄^2- ions in changing the wettability of calcite surface during ion tuned water flooding in carbonate reservoirs.