Foam is a promising means of increasing sweep in miscible- and immiscible-gas enhanced oil recovery. SAG (surfactant-alternating-gas) is a preferred method of injection. Numerous studies verify that the water relative-permeability function krw(Sw) is unaffected by foam. This paper shows a connection between the krw(Sw) function and SAG foam effectiveness that is independent of the details of how foam reduces gas mobility. The success of SAG depends on total mobility at a point of tangency to the fractional-flow curve, which defines the shock front at the leading edge of the foam bank. Geometric constraints limit the region in the fractional-flow diagram in which this point of tangency can occur. For a given krw(Sw) function, this limits the mobility reduction achievable for any possible SAG process.
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