The Mechanism of Gas and Liquid Flow Through Porous Media in the Presence of Foam
Authors: Holm L.W. 
Published: December 2007

This study shows that in the presence of foam, gas and liquid flow separately through porous media representative of reservoir rock. These results were obtained by using tracer techniques to measure the flow of the gas and liquid comprising the foam.

Foam does not flow through the porous medium as a body even when the liquid and gas are combined outside the system and injected as foam Instead the liquid and gas forming the foam separate as the foam films break and then re-form in the porous system. Liquid moves through the porous medium via the film network of the bubbles and gas moves progressively through the system by breaking and re-forming bubbles throughout the length of the flow path. The flow rates of the gas and liquid are a function of the number and strength of the films in the porous medium. There is no free flow of gas; i.e., no continuous gas phase.

On the basis of these results, foam can be expected to improve a waterflood or gas drive by decreasing the permeability of the reservoir rock to a displacing liquid or gas. This improves the mobility ratio and thus the conformance of the flood.

Introduction

Foam is formed when gas and a solution of a surface active agent are injected into a porous medium either simultaneously or intermittently. During the past few years, several papers have been published on the subject of foam flow in porous media. Foam has been used successfully in the removal of capillary water blocks from producing formations. The use of foam in gas storage reservoirs to reduce gas leaks and to increase storage capacity has been considered in recent years. Foam has also been investigated as an oil displacing agent, and as an agent to improve the mobility ratio in a waterflood. However, the mechanism by which the gas and liquid phases comprising the foam flow through a porous medium has not been described adequately.

Normally, when two immiscible phases (gas and liquid) flow concurrently through a porous medium, each phase follows separate paths or channels. At given saturations of the two phases, a certain number of channels are available to each phase, and as saturations change, the number and configuration of the channels available for each phase also change. The effective permeability of each phase is a function of the saturation of that phase only, and the flow of each phase can be described by Darcy's law. When foam is present, the effective permeability of the porous medium to each phase is greatly reduced compared with permeabilities measured in the absence of foam.

Based upon the observed flow of surfactant solutions and gas in capillaries, it has been concluded that the gas and liquid may flow separately or they may flow combined as foam. At least four mechanisms have been postulated to explain how fluids flow with foam present:A large portion of the gas is trapped in the porous medium and a small fraction flows as free gas, following Darcy's law.The foam structure moves as a body; the rate of gas flow is the same as the rate of liquid flow.Gas flows as a discontinuous phase by breaking and re-forming films. Liquid flows as a free phase.A portion of the liquid and gas move as a foam body while excess surfactant solution moves as a free phase.

It also has been suggested that different flow mechanisms exist for high quality (dry) foams made from dilute surfactant solutions and for foams made from more concentrated solutions.

Studies conducted on the flow of foam through capillaries have shown that a plug-type flow occurs and that foam flows as a body.