Gas-aided gravity drainage is a common oil-recovery technique in anticline-shaped oil reservoirs. If the permeability is low and the reservoir is oil-wet, the remaining oil saturation can be quite high. The goal of this work is to mobilize a part of this oil by surfactant injection. An anionic-surfactant formulation was developed to alter wettability and lower interfacial tension (IFT) for a gasflooded, carbonate reservoir. Different coreflood strategies, including gas/water/surfactant/water (GWSW), gas/surfactant/gas (GSG), gas/surfactant/water (GSW), and gas/surfactant/water/gas (GSWG) floods, were investigated. GSG, GWSW, and GSWG corefloods conducted in limestone cores recovered an additional 40–50% of the original oil in place (OOIP) because of the injection of surfactant. GSW corefloods conducted in a vuggy dolomite recovered less: an approximately 20%-of-OOIP incremental recovery. Numerical simulation was used to match GSG and GSW corefloods and estimate multiphase-flow functions. A 2D conceptual simulation model using these functions was built for an anticline reservoir for gas and surfactant-solution injection. GSG flooding using wettability-altering surfactant exhibited high oil recovery at the field scale. IFT reduction, wettability alteration, and foam formation contributed to enhanced oil recovery (EOR).