Copyright 1998, Society of Petroleum Engineers, Inc. This paper was prepared for presentation at the 1998 SPE/DOE Improved Oil Recovery Symposium held in Tulsa, Oklahoma, 19—22 April 1998. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435. Abstract Previous studies of foam generation and transport were conducted, mainly, in one-dimensional and homogeneous porous media. However, the field situation is primarily heterogeneous and multidimensional. To begin to bridge this gap, we have studied foam formation and propagation in an annularly heterogeneous porous medium. The experimental system was constructed by centering a 5.0 cm diameter cylindrical Fontainebleau sandstone core inside an 8.9 cm acrylic tube and packing clean Ottawa sand in the annular region. The sandstone permeability is roughly 0.1 d while the unconsolidated sand permeability is about 7 d. Experiments with and without crossflow between the two porous media were conducted. To prevent crossflow, the cylindrical face of the sandstone was encased in a heat-shrink Teflon sleeve and the annular region packed with sand as before. Nitrogen is the gas phase and an alpha olefin sulfonate (AOS 1416) in brine is the foamer. The aqueous phase saturation distribution is garnered using X-ray computed tomography. Results from this study are striking. When the heterogeneous layers are in capillary communication and cross flow is allowed, foam fronts move at identical rates in each porous medium as quantified by the CT-scan images. Desaturation by foam is efficient and typically complete in about 1 PV of gas injection. When cross flow is prohibited, foam partially plugs the high permeability sand and diverts flow into the low permeability sandstone. The foam front moves through the low permeability region faster than in the high permeability region. Introduction Foam is applied broadly as a mobility-control and profile modification agent for flow in porous media. Foams are usually formed by nonwetting gases such as steam, carbon dioxide (CO 2 ), or nitrogen (N 2 ) dispersed within a continuous surfactant-laden liquid phase. Typical applications include aqueous foams for improving steam-drive 1-3 and CO 2 -flood performance 4 , gelled-foams for plugging high permeability channels 5 , foams for prevention or delay of gas or water coning 6 , and surfactant-alternating-gas processes for clean up of ground-water aquifers 7,8 . All of these methods have been tested in both the laboratory and the field. An unfoamed gas displays low viscosity relative to water or crude oil and is thereby very mobile in porous media. However, dispersing the gas phase within a surfactant solution where the surfactant stabilizes the gas/liquid interface can substantially reduce gas mobility in porous media. Mobility is reduced because pore-spanning liquid films (foam lamellae) and lenses block some of the flow channels. Additionally, flowing lamellae encounter significant drag because of the presence of pore walls and constrictions. One aspect of foams that makes them attractive is that a relatively small amount of surfactant chemical can affect the flow properties of a very large volume of gas. The volume fraction of gas in a foam frequently exceeds 80 percent and stable foams up to 99 percent volume fraction are not uncommon. Recent reviews of foam flow phenomena and mechanisms are given in refs. 9-11 . Laboratory studies on foam generation and transport have aided greatly in formulating and improving both our microscopic and macroscopic understanding of foam flow in porous media. They have focused, for the most part, on one- dimensional and homogeneous porous media. These studies, however, leave gaps in our knowledge of foam behavior because the field situation is primarily heterogeneous and multidimensional. While much work has been conducted in homogeneous systems, the literature on flow in stratified systems is sparse. Notable experiments in stratified systems include Casteel and Djabbarah who performed steam and CO 2 displacements with foaming agents in two parallel porous media 12 . Robin studied foam generation and transport in layered beadpacks that simulated reservoir strata 13 . In these experiments he surmised that foam blocked the high permeability layer. Llave et al observed that foam can divert gas flow from high permeability layers to low permeability layers when the layers are isolated 14 . More recently, Hirasaki et al performed foam displacements in layered porous media to study the removal of organic liquids from groundwater aquifers 15 . Gas was injected at a fixed pressure gradient rather than a specified rate. By dyeing the various fluids, they observed displacement patterns directly. They found that injection of gas slugs into a porous SPE 39678 Foam Flow in Heterogeneous Porous Media: Effect of Crossflow H. J. Bertin, LEPT -ENSAM, O. G. Apaydin, L. M. Castanier, and A. R. Kovscek, Stanford U.