Salinity and specific ion induced wettability changes on glass surfaces by direct force measurements and surface forces modeling

The wettability of reservoir rock can be modified to water-wet by manipulating the salinity and ionic composition of the injection water, which has been confirmed by macroscopic laboratory experiments and field tests. However, few studies have tried to probe into the molecular mechanisms of the wettability alteration due to salinity and changes. To fill this gap, this paper examined the wettability changes on glass surfaces induced by salinity (0–6.0 % NaCl solutions) and specific ions (Ca²⁺, Mg²⁺, and SO₄^2-) using a novel force spectroscopy method and custom-made “soft tips.” The force results suggested that the approach forces between the glass surface and the oil droplet were repulsive in 0(water)-6.0 % NaCl solutions, which can be attributed to the synthetic effects of hydration and electrical double layer repulsions according to the extended Derjaguin-Landau-Verwey-Overbeek theory. In addition, the adhesion and adhesion energy of glass surface with oil droplet increased with the increase of brine salinity; there existed a salinity threshold (∼2 % NaCl) from which significant change in adhesion and adhesion energy was observed. The adhesion energies obtained by AFM force measurements were correlated to the wettability of the glass surface using the Young-Dupré equation, which suggested that the hydrophilicity of the glass surface can be enhanced by lowering the NaCl concentration to less than 2 %. The force results also indicated that the approach forces, adhesions, and adhesion energies between the glass surface and the oil droplet were more attractive in CaCl₂ and MgCl₂ solutions than those in NaCl solution owing to that Ca²⁺ and Mg²⁺ modifying the glass surface towards less negative and additionally, introducing an attractive hydrophobic force. In contrast, the approach forces in Na₂SO₄ solution were extremely repulsive, and the adhesion and adhesion energy in Na₂SO₄ solution were far smaller than those in NaCl solution as the SO₄^2- modified the glass surface to more negative, thus enhancing the electrical double layer repulsion between the glass surface and the oil droplet. Therefore, this study suggested that the wettability of the glass surface was sensitive to brine salinity and ionic composition. The hydrophilicity of the glass surface can be enhanced by lowering the salinity to a certain level and smartly decreasing the concentrations of Ca²⁺ and Mg²⁺ but increasing the concentration of SO₄^2-.