Atomistic Insight into the Behavior of Ions at an Oil-Bearing Hydrated Calcite Surface: Implication to Ion-Engineered Waterflooding

This research provides an atomistic picture of the role of ions in modulating the microstructural features of an oil-contaminated calcite surface. This is of crucial importance for the rational design of ion-engineered waterflooding, a promising technique for enhancing oil recovery from carbonate reservoirs. Inspired by a conventional lab-scale procedure, an integrated series of molecular dynamics (MD) simulations were carried out to resolve the relative contribution of the major ionic constituent of natural brines (i.e., Na+, Cl–, Mg2+, Ca2+, and SO42–) when soaking an oil-bearing calcite surface in different electrolyte solutions of same salinity, namely, CaCl2, MgCl2, Na2SO4, MgSO4, and deionized water (DW). In all cases, we observed the gradual detachment of polar oil molecules (mimicked by decanoate) already paired to Na+ cations, covering the calcite substrate in such a way that carboxylate groups were in contact with the treating solution. The appearance of such a negatively charged interface in conjunction with the bare calcite/brine surface is a likely nanometric source for the disparity of surface characteristics of oil-bearing rocks reported in the literature. The MD results showed the affinity of divalent cations (Ca2+ or Mg2+) for pairing with negatively charged carboxylate functional groups, thereby facilitating desorption of decanoates. Having a compact solvation shell, Mg2+ was not as effective as Ca2+ cations; however, its performance was enhanced in the presence of sulfate anions. We further figured out the tendency of SO42– anions to shielding Na+ sites over the calcite surface, thus limiting the chance of readsorption of carboxylate compounds. Consistent with lab experiences, sulfate was found to assist the access of magnesium cations to the calcite surface as well. Altogether, the present results provide us with a molecular-level validation for the well-known multicomponent ion exchange mechanism proposed for the wettability alteration of carbonate rocks through enriching the divalent ionic content of the injection brine solution