Foam is a promising method to improve sweep in gas-injection enhanced-oil-recovery (EOR) projects. Where well-to-well distances are large, as in offshore applications, the question arises whether foam can prevent gravity segregation over large distances. For such cases, theoretical studies suggest that the best process is to inject one large slug of surfactant followed by one large slug of gas. Shan and Rossen (2004) present a simple model for such a process that provides an initial assessment whether foam can prevent segregation over large distances. They did not extend their calculations to the large distances between wells in some applications, and they treated only the case where vertical permeability kv equals horizontal permeability kh. Here we extend the model to cases of kv < kh. We derive an analytical solution for the limit as kv approaches zero, which could serve as quick first estimate of feasibility for other cases. Surprisingly, the model predicts that gravity segregation is worse as kv decreases; the reason is that with large kv foam pushes gas downward in response to the pressure difference across the titled foam front.
We illustrate use of the model with an example from a North Sea field, where the issue was whether foam could prevent gravity segregation over a distance of several km. The simple model said this was feasible, and more-detailed simulation then confirmed that segregation was insignificant over an inter-well distance of 6 km.