Structure/Performance Relationships for Surfactant and Polymer Stabilized Foams in Porous Media

Abstract

Abstract The addition of polymer has the potential to enhance both the viscosity and the stability of surfactant-stabilized foams. However, the degree to which the bulk properties of polymer-thickened foams are retained or enhanced in porous rock is not well understood and is difficult to predict. We have compared the viscosities, at equivalent shear rates, of two different types of polymer-thickened foams in laminar pipeline (bulk) flow vs. the same foams flowing in consolidated sandstone rock. For one kind of foam, the apparent viscosity in the rock is very similar to that in pipeline flow. However, for another kind of foam, the apparent viscosity in the rock is an order of magnitude greater than that in pipeline flow. Low-energy scanning electron microscopy was used to examine the pore-scale morphology of the two foams in the rock. It was found that the morphologies of the two foams explain at least a large part of the observed differences in foam flow properties between bulk (pipeline) flow and constrained (porous medium) flow. This work is important to the specifications and formulation of the most effective surfactants for varying applications including mobility control, blocking, and diverting. Introduction Aqueous foams are used in a variety of the petroleum industry's enhanced oil recovery flooding techniques(1). For example, surfactant stabilized foams have been used as mobility control agents in gas-flooding(2–4). The foam, which has an apparent viscosity greater than the gas, lowers the gas mobility in the swept and/or higher permeability regions of the formation. Thus, the foam will divert some of the gas into other parts of the reservoir formation that were previously unswept, or poorly swept, to recover additional oil. Significant foam stability is a prerequisite for the successful application of foam flooding. There are also many other applications of foams in the petroleum industry, all requiring controlled stability(5–8). Foams have also been used as blocking agents because of their selective ability to reduce the gas permeability(9). Foam that has been developed for a blocking application must meet different requirements than foam that has been developed for sweep efficiency applications. A blocking foam must possess the ability to completely fill a selected volume in all locations where the gas could travel through, and to act as a barrier to flow. The gas blocking foam must stay in place and possess long-term stability, providing the largest possible gas mobility reduction for the longest periods. For the formulation of either kind of foam, one of the challenges that must be met is the proper selection of foam-forming surfactants. The foaming capability of a surfactant relates to both foam formation and foam persistence, which are influenced by many bulk and interfacial properties(10). Unfortunately, it is generally found that the performance of foams in porous media is not easily predicted based on these physical properties(11), although they can be exploited to increase foamability and foam persistence. Harsh chemical environments are sometimes present in oil reservoirs and several reviews have been published identifying desirable foam-forming characteristics for harsh and less demanding environments(1, 12).