Impacts of Smart Waters on Calcite–Crude Oil Interactions Quantified by “Soft Tip” Atomic Force Microscopy (AFM) and Surface Complexation Modeling (SCM)

The interactions of calcite surface with crude oil are influenced by the ionic chemistry of the injection water in “smart water” flooding; however, the molecular evidence of their influences is missing in the current studies. To address this issue, we measured the calcite–oil interactions in four kinds of smart waters by driving a droplet of crude oil (“soft tip”) toward and away from the calcite surface using atomic force microscopy (AFM). The force behaviors showed that the calcite–oil interactions were repulsive in smart waters containing low concentrations of Ca²⁺ ions, such as seawater (SW) and seawater with one-fourth of Ca²⁺ concentration (SWCa), which have produced extremely weak adhesions and works of adhesion in these two brines. In contrast, the force curves showed “jump-in” behavior in seawater with four times increased SO₄^2– concentration (SWSO) or Mg²⁺ concentration decreased to one-fourth (SWMg) compared to SW, which has produced extremely large adhesions and works of adhesion. The underlying reasons for calcite–oil interactions in response to different smart waters were investigated by the method of surface complexation modeling (SCM), which combined a charge-distribution multisite ion complexation (CD-MUSIC) model and a basic Stern model (BSM) to describe the complex electrochemical reactions of calcite, smart water, and crude oil. The adhesion energies were calculated based on the surface complexations, which showed good agreements with the AFM force results. The SCM results suggested that the weak calcite–oil interactions in SW and SWCa solutions can be attributed to the mitigation of cation (Ca²⁺) bridging interactions owing to a deficiency in surface Ca²⁺ ions. The strong calcite–oil interactions in SWSO and SWMg solutions are caused by a dramatic increase of cation (Ca²⁺) bridging interactions due to an excess of surface Ca²⁺ ions. Overall, the attractions of calcite surface with crude oil follow the order SWCa < SW ≪ SWMg < SWSO, which infers that the oil molecules may be easily displaced from the calcite surface with smart waters containing low concentrations of Ca²⁺ and SO₄^2– ions but high concentrations of Mg²⁺ ions.