ONSET OF MOBILIZATION AND THE FRACTION OF TRAPPED FOAM IN POROUS-MEDIA

Usually, foam in a porous medium flows through a small and spatially v arying fraction of available pores, while the bulk of it remains trapp ed. The trapped foam is under a pressure gradient corresponding to the pressure gradient imposed by the flowing foam and continuous wetting liquid. The imposed pressure gradient and coalescence of the stationar y foam lamellae periodically open flow channels in the trapped foam re gion. Foam lamellae in each of these channels flow briefly, but channe ls are eventually plugged by smaller bubbles entering into the trapped region. The result is a cycling of flow channels that open and close throughout the trapped foam, leading to intermittent pulsing of foam f low in that region. The dynamic behavior of foam trapped in porous med ia is modeled here with a pore network simulator. We predict the magni tude of the pressure drop leading to the onset of flow of foam lamella e in the region containing trapped foam. This mobilization pressure dr op depends only on the number of lamellae in the flow path and on the geometry of the ports that make up this path. The principles learned i n this study allow us to predict the fraction of foam that is trapped in a porous medium under given flow conditions. We present here the fi rst analytic expression for the trapped foam fraction as a function of the pressure gradient, and of the mean and standard deviation of the pore size distribution. This expression provides a missing piece for t he continuum foam flow models based on the moments of the volume-avera ged population balance of foam bubbles.