Water-based enhanced oil recovery from fractured oil-wet or mixed-wet carbonate rock by water flooding is a great challenge. Surfactant-assisted wettability alteration in combination with the impact of gravity drainage appeared to be one possible technique. The big challenge is to select a surfactant system, which is able to cause wettability modification, and, at the same time, to keep the interfacial tension (IFT) between oil and water high enough to take benefit of capillary forces and low enough to activate gravity forces in the oil displacement process. Furthermore, the surfactant should not adsorb onto the rock, which could decrease the water-wetness. Cationic surfactants of the type alkyltrimethylammonium, R-N(CH3)(3)(+), have the properties needed, IFT in the range of 0.1-1.0 mN/m, not affected by salts, and no adsorption onto the rock. Phase trapping because of complex formation between the cationic surfactant and carboxylic material in the crude oil is a possible way for the loss of chemicals, especially at low temperatures. They act as good wettability modifiers in the presence of sulfate: even at low temperatures. Spontaneous imbibition experiments have been performed at 50 C in oil-wet reservoir limestone cores of quite different permeability (0.46 and 364 mD), and the results were discussed in terms of a previous study in chalk. For the high-permeable core, different impact of capillary and gravity forces resulted in different flow regimes with respect to the imbibing time: (1) capillary forces (approximate to 30% recovery), (2) combined capillary and gravity forces (recovery increased to approximate to 40%), (3) gravity forces [ultimate recovery of 50-60% of original oil in place (OOIP)]. The fraction of oil recovered by gravity forces increased as the IFT decreased from 0.67 to 0.34 mN/m. No oil was recovered from the low-permeable core, indicating that the rock permeability is a very important parameter when judging the efficiency of the process. The time scale for a diffusion process is linked to the square of the length scale of the medium. The time required to achieve 70% of the recoverable oil from the core plug in the laboratory was 1 day. Applying the upscaling equation, the corresponding time required to achieve 70% of the recoverable oil from a 1 m(3) reservoir block would be in the order of 2 years. Consequently, a combined effect of surfactant-enhanced gravity drainage (SEGO) and the wettability alteration in fractured, high-permeable, oil-wet carbonate rock can provide an economically interesting opportunity.