This paper presents theoretical models to predict the magnitude of the minimum pressure gradient (∇p)min to mobilize foam in two cases. In the first case, foam lamallae form by snap-off at pore throats during simultaneous flow of liquid and gas through the pore space. Percolation theory relates (∇p)min to the fraction of pore throats blocked by lamellae, the geometry of the pore throats, and the topology of the pore network. This theory indicates that in field application foam can begin to flow only at the relatively high ∇p found near the well. Effective reservoir sweep with foam therefore depends on the propagation of foam formed near the well.

In the second case a well-established foam flows as "bubble trains" through regions of trapped gas. Theory predicts that fine-textured foams then have substantial (∇p)min (tens of psi/.ft). If the reservoir ∇p cannot sustain (∇p)min from injection well to producer, flow stops and the foam plugs the zone. Thus some coalescence is necessary to avoid plugging and allow foam propagation deep into the formation.

Bubble compressibility increases (∇p)min because bubbles lodge at the pore throats, where capillary resistance is highest. Plateau borders, the rounded films were the lamellae contact the pore wall, reduce (∇p)min, especially for incompressible foams, At low capillary pressures, however, (∇p)min increases due to bubble separation at pore throats.

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