Ion-induced Wetting Transition during Low Salinity Waterflooding

Low Salinity Waterflooding has been one of the most investigated and used methods of Enhanced Oil Recovery (EOR) in the past decades. Irrespective of the much debated mechanism of such waterflooding, a change in rock-wettability (from oil-wet to water-wet) definitely occurs during the process. We investigate optically the wetting of mineral surfaces by aqueous salt solutions in ambient oil. Monovalent salt solution (NaCl, KCl) at all pH and concentration shows an almost ‘complete’ wetting scenario on various surfaces while divalent salt solutions (CaCl2, MgCl2) above a threshold pH and concentration demonstrates partial wetting on mica. We attribute this wetting transition to the stronger ion-adsorption and subsequent charge reversal at mica/water interface in case of divalent cations. Using Ellipsometry we investigate the molecularly thin aqueous film between mineral and oil phases in such wetting scenarios, and measure the surface charge at the interface using zeta potential measurement technique. Surface Complexation modeling validates the surface charge reversal and we calculate the interface potential Φ in the aqueous film to demonstrate the gradual development of a contact angle (as Φmin). Making the oil more ‘realistic’ by adding polar fatty acid components in the oil phase makes the transition to partial wetting stronger as well as dramatic involving autophobing and self-propelling aqueous drops. We use Atomic Force Microscopy (AFM) to depict the complex adsorption process going on at oil/water as well as mineral/water interface aided by divalent cations. We have also investigated wetting at higher temperatures for the same system to emulate reservoir-like conditions where the combined effort of ion-adsorption and temperature control the conditions necessary for wetting transition.