APPLICATION OF A REDUCTION METHOD TO PHASE-EQUILIBRIA CALCULATIONS

A proper understanding of the phase behaviour of petroleum fluids is essential in the oil industry, both for the design of processing facilities and for the planning of field development. The phase equilibrium of a mixture of N components is described by a set of N nonlinear equations. Standard methods to solve such systems require a computing effort that is at least proportional to N2. Unfortunately, for an accurate description N needs to be rather large (about 20 - 30). Several methods have been proposed to obtain an (approximate) solution with a significantly smaller effort. In this paper we investigate one such method, in which the set is reduced to a much smaller number of equations (about 5). A few years ago it was shown that this approach gives exact results for some specific thermodynamic models. We demonstrate that it can be modified to provide a systematic way of obtaining a very accurate approximation for the models that are commonly used in petroleum phase bebaviour studies. Potential application of this work is in the fields of reservoir and process simulation, where a single study requires many phase equilibrium calculations.