Insights From Ion Adsorption and Contact-Angle Alteration at Mineral Surfaces for Low-Salinity Waterflooding

Most solid surfaces acquire a finite surface charge after exposure to aqueous environments caused by desorption and/or adsorption of ionic species. The resulting electrostatic forces play a crucial role in many fields of science and technology, including colloidal stability, self-assembly, wetting, and biophysics. Enhanced oil recovery (EOR) is an example of a large-scale industrial process that hinges in many respects on these phenomena. In this paper, we present a series of experiments illustrating fundamental aspects of low-salinity waterflooding in well-defined model systems. We show how pH and ion content of the water phase as well as the presence of model polar components (fatty acids) in the oil phase affect the wettability (i.e., contact-angle distribution) of oil/water/rock systems. Specifically, we discuss high-resolution atomic-force microscopy (AFM) experiments demonstrating the preferential adsorption of multivalent cations to mineral surfaces such as mica and gibbsite. Cation adsorption leads to increased and, in some cases, reversed surface charge at the solid/liquid interface. In particular, the adsorption of divalent cations gives rise to charge reversal and thereby induces a transition from complete water-wetting in the absence to finite contact angles in the presence of such ions. Although already dramatic for pure alkanes as base oil, adding fatty acids to the oil phase enhances the effect of divalent ions on the oil/water/rock wettability even more. In this case, contact-angle variations of more than 70° can be observed as a function of the salt concentration. This enhancement is caused by the deposition of a thin film of fatty acid on the solid surface. AFM as well as surface plasmon-resonance spectroscopy measurement in a microfluidic continuous flow cell directly demonstrate that adsorbed Ca2+ ions promote secondary adsorption of acidic components from the oil phase. The combination of the effects discussed provides a rational scenario explaining many aspects of the success of low-salinity waterflooding.