Effect of Ion Type on the Interaction between Polar Model Oil and Mica Substrate: A Chemical Force Microscopy Study

Abstract

Low salinity waterflooding is an effective approach to improving oil recovery, behind which the microscopic mechanisms have been debated over the past decade, and especially, the role of polar organic components of crude oil is almost ignored but proposed to dominate the adhesion force of oil-rock in specific cases. In this study, chemical force microscopy (CFM) was used to directly measure the adhesion forces between polar model oil and mineral surface in the presence of electrolyte solution with different ionic type and concentration. By the use of classic Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, the van der Waals and electrostatic forces were calculated, and it was found that there were some additional interactions, denoted as structural forces, playing an important part in the resultant forces. By the aid of extended DLVO (EDLVO) theory, the adhesion force of polar tip–substrate was found to increase in the order: Na⁺ < Ca²⁺ < Mg²⁺, and the low salinity effect (LSE) potential shows the opposite order, attributed to the ligand bridging and strong solvation of multivalent cations enhancing the electrostatic attraction of the tip–surface pair. In the case of sulfate ion as the anion, the adhesion force was smaller than that in the case of chloride ion because the SO₄²– ion can favor the formation of electrostatic repulsion between the tip and surface and accordingly enhance the LSE potential. On the basis of the experimental results, we propose a molecular interaction model between oil and solid substrate, which will provide an in-depth understanding of the effect of brine composition and concentration on oil detaching from a solid surface at the molecular level.