We report a simulation study of surfactant-alternating-gas (SAG) foam injection into a waterflooded oil reservoir. We show the effects of oil, and of SAG cycle size and number on sweep efficiency, and the longterm impact of a single surfactant slug on the areal sweep efficiency of a gas-flood. Shan and Rossen (2004) show that a single-cycle SAG flood with fixed injection pressure can effectively overcome gravity override in a homogeneous reservoir with a uniform residual oil saturation. A single cycle works better than multiple cycles. We show that the presence of mobile oil can invalidate this model, but not simply because oil weakens or destroys foam. If foam is weakened by oil, moderately but uniformly, vertical sweep efficiency can still be good. Of course if oil kills foam nearly completely, gravity override occurs. In our simulations, foam collapses where oil saturation is above a certain threshold value greater than waterflood residual. Oil mobilized by the foam bank flows downward. This can lead to an oil bank at the bottom of the reservoir, and in single-cycle SAG this oil bank is not displaced by foam if its oil saturation is sufficient to destroy foam. Meanwhile, gas flows upward and the low-mobility front advances rapidly across the top of the reservoir, leading to an override zone. It is the non-uniformity of the resulting oil saturation and gas mobility that invalidates Shan and Rossen's model in this case. Instead there is an oil-rich zone at the bottom of the reservoir, foam above the oil-rich zone, and a foam-free override zone above the foam. In this case, if foam is injected in several relatively small slugs, oil production can be better than that with fewer relatively large slugs. We also illustrate the impact of injecting a single surfactant slug on the areal sweep efficiency of a long-term gas-flood. By injecting a surfactant slug prior to the gas slug, stronger foam can form in parts of the reservoir with a lower oil saturation. Foam then diverts gas flow to oil-rich areas in the reservoir, in our case the bottom of the reservoir. In a conventional gas flood gas flows primarily across the top of the reservoir with poor sweep efficiency. By injecting a single surfactant slug ahead of gas, higher oil recovery can be achieved at the same injected PV of gas.