A flexible representation of quaternary fluid phase equilibria for use in a chemical flood simulator

Abstract

In order to simulate the enhanced oil recovery process of surfactant flooding it is necessary to be able to represent the phase behaviour produced by the mixing of injected and reservoir fluids. These fluids consist of oil, brine, surfactant and cosurfactant. In practical systems these components may combine to form up to three fluid phases and, when flowing through rock, surfactant and cosurfactant may sometimes be adsorbed on to the rock surface. The model described here has been developed so that this phase behaviour can be represented in calculations. The purpose of the model is to take an overall mixture and compute the number and composition of the fluid phases taking into account the chemical adsorbed by the rock surface. In such a complex situation experimental data are essential and the model has been especially formulated to make direct use of such data. The model is based on subdividing the quaternary space into volumes in which the phase behaviour can be represented in a linear fashion. The approximating functions, which are described, depend on whether the region being represented is in the single, two or three phase part of the quaternary space. The calculational task is to find an appropriate root of the approximation and this has been done using a specially developed continuation method which is also described. Surfactant flood simulation also depends on being able to represent other phase-related properties such as interfacial tension and phase viscosity. The treatment described here integrates these into the phase model again allowing experimental data to be used in a straightforward manner.