Foam is widely used to divert acid or abandon the high permeable layers. In this type of application foam should considerably reduce gas mobility. The nature of the gas and the surfactant may influence foaming behavior and thus the efficiency of the foam. In this paper an experimental study of the behaviorof CO2 and N2 foams in granular porous media using X-ray Computed Tomography is reported. In the experiments gas is forced through natural porous media initially saturated with a surfactant solution, a process known as SurfactantAlternatingGas (SAG). The CO2 was either under sub- or super-critical conditions whereas N2 remained under subcritical conditions in all experiments. Alpha Olefin Sulfonate (AOS) surfactant was used as foaming agent. We found that injection of gas following a slug of surfactant can considerably reduce gas mobility and promote higher liquid recovery at the experimental conditions investigated. Foaming of CO2 builds-up a lower pressure drop over the core at both low and high pressures than N2. Both gases require a certain penetration depth to develop into foam. This length is longer for N2 (larger entrance effect) and increases with growing gas velocity. Moreover, the ultimate liquid recovery by CO2 foam is always lower than by N2 foam. The possible mechanisms explaining the observed differences in foaming behavior of the two gases are discussed in detail.

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