Abstract

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 together with an oleic two phase desaturation curve. 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.

Introduction

After waterflooding an oil reservoir residual oil is trapped by capillary forces as small droplets. These can be remobilised if the interfacial tension (IFT) between the oil and the displacing fluid can be reduced by several orders of magnitude. Optimised surfactant systems can achieve this and are there lore of interest as enhanced oil recovery chemicals by considering the force balance on a trapped droplet, and generalising the result to account for the fact that residual oil droplets will have a spread of sizes, the oil release may be characterised by a capillary number (1), Nc defined by: <Equation Available In Full Paper>

Studies of the mobilisation of oil in various rocks have correlated the results with capillary number (2).

Surfactant systems designed to minimise the effective IFT generate three phases (1), Hence, in principle, there are three possible capillary numbers that should be taken into account corresponding to the three possible phase pairs. However, in optimised systems, the interfacial tensions between the phases are all similar and so to a good approximation only a single capillary number need be considered.

In numerical simulations of surfactant flooding, the oil release mechanism is modelled by making the relative permeabilities functions of capillary number in addition to phase saturations. This allows the residual oil saturation to be reduced as the capillary number is increased, However, the way in which the shape of the relative permeabilities are changed can have an important influence on the predictions of simulations (3).

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