We report an experimental study of the behavior of CO2 and N-2 foams in granular porous media using X-ray computed tomography. In the experiments either CO2 or N-2 gas is forced through natural porous media initially saturated with a surfactant solution, a process known as surfactant-alternating-gas or SAG. The CO2 was either under sub- or supercritical conditions, whereas N-2 remained under subcritical conditions at all experimental conditions. We found that CO2 injection following a slug of surfactant can considerably reduce its mobility and promote higher liquid recovery at the experimental conditions investigated. Foaming of CO, builds-up a lower pressure drop over the core at both low and high pressures than N-2 Both gases require space to develop into foam. The space is longer for N-2 (larger entrance effect) and increases with increasing gas velocity. Moreover, the ultimate liquid recovery by CO2 foam is always lower than by N-2 foam. The possible mechanisms explaining the observed differences in foaming behavior of the two gases are discussed in detail.