Three‐Phase Fractional‐Flow Theory of Foam‐Oil Displacement in Porous Media With Multiple Steady States

Understanding the interplay of foam and nonaqueous phases in porous media is key to improving the design of foam for enhanced oil recovery and remediation of aquifers and soils. A widely used implicit‐texture foam model predicts phenomena analogous to cusp catastrophe theory: The surface describing foam apparent viscosity as a function of fractional flows folds backwards on itself. Thus, there are multiple steady states fitting the same injection condition J defined by the injected fractional flows. Numerical simulations suggest the stable injection state among multiple possible states but do not explain the reason. We address the issue of multiple steady states from the perspective of wave propagation, using three‐phase fractional‐flow theory. The wave‐curve method is applied to solve the two conservation equations for composition paths and wave speeds in 1‐D foam‐oil flow. There is a composition path from each possible injection state J to the initial state I satisfying the conservation equations. The stable displacement is the one with wave speeds (characteristic velocities) all positive along the path from J to I. In all cases presented, two of the paths feature negative wave velocity at J; such a solution does not correspond to the physical injection conditions. A stable displacement is achieved by either the upper, strong‐foam state, or lower, collapsed‐foam state but never the intermediate, unstable state. Which state makes the displacement depends on the initial state of a reservoir. The dependence of the choice of the displacing state on initial state is captured by a boundary curve. Plain Language Summary Foam has unique microstructure and reduces gas mobility significantly. Foam injection into geological formations has broad engineering applications: removal of nonaqueous phase liquid contaminants in aquifers and soils, oil displacement in reservoirs, and carbon storage. Key to the success of foam is foam stability in the presence of oil or nonaqueous phase liquid. An experimentally validated foam model describes foam properties as a function of water, oil, and gas saturations. This model predicts that some injected fractional flows of phases correspond to multiple possible injection states with different saturations: strong‐foam state with low mobility, intermediate state, and collapsed‐foam state with high mobility. We show how to determine the unique displacing state, using three‐phase fractional‐flow theory and the wave‐curve method. A physi