Experimental Studies of Capillary Pressure Effects of Foams in Porous Media

Abstract

Abstract Although foams have been recognized for improving in oil recovery process, detailed descriptions of the relative importance of viscous and capillary effects are still lacking. Previous work has largely been concerned with the measurement of effective viscosities or dispersed phase permeabilities which masks both viscous and capillary effects. A first step in separating such effects requires the measurement of capillary pressure for foam systems in porous media. To date, only isolated data of this type have been reported. In this work, a capillary pressure cell has been designed to investigate capillary behavior of foams in bead packs. This experimental setup has several unique and versatile features such as: (1) Hydrophobic and hydrophilic membranes placed at the entrance and exit ends of the bead pack to selectively restrict the flow of gas and liquid to and from the cell and hence insure correct capillary pressure (PC = PG-PL) measurement; (2) a compression spring imposed on the top of the bead pack to insure a stable bead pack and good capillary contact between the bead pack and the membranes; and (3) back pressure control to allow high pressure testing and to minimize in situ bubble size changes (compressibility effects) during testing. With the above system, sequential drainage and imbibition cycles can be repeatedly traversed without interruption. Multicycle drainage and imbibition experiments were performed under conditions where the foam was generated in situ, and the static capillary effects were obtained for both non-dispersed phase (gas-water system with no film stabilizing surface active agents) and dispersed phase (surface active agent present) systems. While only two drainage-imbibition cycles are necessary for non-dispersed phase systems to achieve invariant capillary pressure curves, 4 to 6 cycles are required for dispersed phase systems. In all cases, the irreducible and residual wetting phase saturations at the end of drainage and imbibition are lower than for those of non-dispersed phase systems. Studies of pore structure effects provide direct evidence that dispersed phase systems effectively reduce the degree of heterogeneous behavior in a bed. Studies of wetting effects show that both the irreducible and residual saturations at the end of drainage and imbibition are significantly decreased as the degree of oil wettability is increased. Finally, from tests at different surfactant concentrations, the largest changes in gas trapping occur as the concentration is increased below the CMC (critical micelle concentration); at higher concentrations additional trapping takes place but the effect is less dramatic.