Foam can increase sweep efficiency in gas-injection IOR processes. Here we develop design criteria for<br>surfactant-alternating-gas foam processes in layered reservoirs. We reach the following conclusions:<br>Trends of foam strength with permeability or surfactant formulation, as measured in conventional<br>coreflood tests at fixed injected water fraction, may not correspond to behavior in a SAG process in the<br>field. In the cases examined, foam strength in a SAG process is much less sensitive to permeability and<br>foam parameters than is foam strength at fixed injected water fraction.<br>Placement of surfactant into low-permeability layers is a key challenge of SAG processes in<br>heterogeneous reservoirs. Gas breakthrough occurs via low-permeability layers that did not receive enough<br>surfactant. For that reason, a stronger foam that more effectively diverts flow away from higherpermeability<br>layers may send more gas to lower-permeability layers that lack surfactant, and thereby<br>accelerate gas breakthrough.<br>Using multiple surfactant and gas slugs allows foam to redistribute surfactant in later slugs.<br>However, this strategy suffers from poor injectivity during liquid injection, which slows the process and<br>promotes gravity segregation.<br>Injection of both gas and surfactant slugs at the maximum allowed injection pressure, rather than<br>at fixed rate, gives best results.<br>Injecting gas from only the bottom of the well offers no significant advantages in the best case,<br>where a high-permeability layers lie at top and bottom of the reservoir, and performs significantly worse if<br>low-permeability layers lie at the top (where lack of surfactant leads to override) and bottom (where low<br>permeability restricts injectivity) of the reservoir.<br>A surfactant slug sized for a homogeneous reservoir is too large for a heterogeneous reservoir,<br>because little surfactant enters lower-permeability layers: much of the injected surfactant goes to waste. A<br>surfactant slug sized to sweep high-permeability layers and a portion of low-permeability layers performs<br>nearly as well as one sized to sweep the entire reservoir.


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