Classical stability theory can be used to estimate the critical velocity of a miscible flood stabilized by gravity forces. However, stability theory for an ultra-low interfacial tension (IFT) surfactant displacement is not well developed or validated. In this paper, a method for predicting the critical velocity for a surfactant flood is proposed taking into account the microemulsion phase. Vertical upward surfactant displacement experiments were performed in sandpacks at velocities of 0.2, 0.4, 0.8 ft/day. The surfactant flood at 0.2 ft/day was a nearly stable displacement whereas the floods at 0.4 and 0.8 ft/day were unstable with visually obvious fingers. The stability theory is in good agreement with experimental results. The proposed theory and experimental results offer new insight into the behavior of surfactant floods stabilized by gravity forces and in particular the importance of the microemulsion phase and its properties, especially its viscosity. It is very important to measure the microemulsion viscosity and account for its effect on the critical velocity. Furthermore, the microemulsion viscosity can be optimized to improve the velocity for a stable displacement. This insight opens up a new pathway for optimizing surfactant floods without mobility control. It is possible to design an efficient surfactant flood without any mobility control if the surfactant solution is injected at a low velocity in horizontal wells at the bottom of the geological zone and the oil captured in horizontal wells at the top of the zone. This approach is practical if the vertical permeability of the geological zone is high. Under favorable reservoir conditions, gravity-stable surfactant floods may be an attractive alternative to surfactant-polymer floods. (C) 2014 Elsevier Ltd. All rights reserved.