Multiphase flow and wettability effects of surfactants in porous media

The concept of enhancing oil recovery from petroleum reservoirs, by reducing oil-water interfacial tension (IFT) using surfactants, is an old one. The main mechanism involved there is the alteration of oil-water relative permeability characteristics, due to reduction in interfacial tension between the two immiscible fluids competing to flow through the porous medium of reservoir rocks. In order for this mechanism to yield effective results, in terms of oil recovery enhancement, the surfactants used must be capable of lowering the oil-water interfacial tension by four to six orders of magnitude. This imposes such requirements of surfactant type, concentration and quantity that the process becomes uneconomical in practice. Another aspect of the use of surface-active agents that has received little attention is their ability to alter wetting preference of the rock surface in relatively dilute concentrations. Such wettability alterations could yield considerable shifts in oil-water distribution and flow characteristics in porous media resulting in far more beneficial effects than can be realized by reduction of interfacial tension alone. In this paper, we present the results of experiments involving flow through porous media using different rock-fluid systems, so chosen as to allow us to differentiate between the relative effects of interfacial tension reduction and wettability alteration on oil-water flow behavior and oil recovery. In addition to confirming and quantitating the beneficial effects of wettability alteration by surfactants on oil recovery, this study has identified an ability of some surfactants to render a unique type of fluid distribution in porous media that enables the preferential draining of the oil phase through formation of oil films on the rock surface. (C) 2004 Elsevier B.V. All rights reserved.