Surfactant-Induced Wettability Shifts Across Salinity Gradients

This study investigates the mechanisms of wettability alteration in rock-brine-oil systems across a wide salinity range (360–180,000 ppm), both in the presence and absence of surfactant additives. Three surfactants, including anionic and nonionic types, were systematically evaluated through measurements of surface tension, interfacial tension (IFT), zeta potential, and oil contact angle (CA). Classical Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, incorporating the Frumkin-Derjaguin approach, was employed to predict oil CAs based on electrostatic and van der Waals interactions. Oil CA and zeta potential measurements confirm that surfactant addition increases the magnitude of the surface charge over the investigated salinity range. At low salinity, wettability is primarily governed by electrostatic double-layer (EDL) repulsion, while hydrophobic interactions remain less significant, resulting in more water-wet conditions. With increasing salinity, wettability becomes less controlled by electrostatic interactions, and shifts toward more oil-wet conditions, driven primarily by hydrophobic interactions at surfactant-modified interfaces rather than cation bridging, despite the elevated surface charge. In systems without surfactants, DLVO theory effectively captures salinity-dependent wettability trends, which are primarily governed by electrostatic double-layer interactions. In contrast, in surfactant-treated systems, classical DLVO theory consistently overestimates oil CAs, with deviations increasing substantially as salinity rises. This discrepancy is attributed to hydrophobic interactions induced by the presence of surfactants, which are not captured by classical DLVO calculations. To address this limitation, an extended DLVO framework incorporating hydrophobic forces was applied, resulting in significantly improved agreement between theoretical predictions and experimental results across the full salinity range.