In surfactant flooding the form of the relative permeabilities at high capillary numbers has a crucial role in determining the transport of equilibrium phases through the reservoir. A conventional assumption is that a system which produces ultra-low interfacial tensions is close to miscibility, making the use of straight line relative permeabilities appropriate. In gas/oil systems this assumption may be justified, however, in surfactant systems it is unclear that the ultra-low interfacial tensions are associated in the same way with miscibility. There is little conclusive data in the literature to clarify this. This paper describes experimental work undertaken to complement the development of an alternative three phase relative permeability model for surfactant flooding based on the concept of droplet flow.
Two and three phase relative permeabilities have been determined for equilibrium phases from an optimal surfactant system. All relative permeabilities have been measured under steady state conditions and the pressure drop held constant to ensure that the relative permeabilities are determined at a constant capillary number, as defined within chemical flooding simulators. Initial measurements suggested that the relative permeabilities were linear functions of saturation. However, a careful investigation showed that these results arose from gravity segregation in the core endcaps. This was overcome using an improved endcap design and the relative permeabilities were then found to be a nonlinear function of saturation. In particular the micellar phase relative permeabilities were concave functions of saturation, in agreement with our model.