The Development of a General Phase Envelope Construction Algorithm for Reservoir Fluid Studies

This paper deals with the development of a general algorithm for phase envelope construction. It extends the pressure-temperature diagram construction method of Michelsen to the generation of phase envelopes on pressure-composition, temperature-composition and composition-composition diagrams. The bubble point and dew point curves are traced in one pass, and an estimate of the critical point is also given. The loci of mixtures having a constant vapor-liquid split (e.g. 25% vapor mole fraction or volume fraction) can also be generated. The algorithm selects internally the primary variables and the points on the diagram to enhance the convergence. Results are presented for simple mixtures, typical reservoir oil-CO/sub 2/ mixtures and a gas condensate. Problems associated with a discontinuity in the phase envelope because of a three-phase region are also presented, and it is shown how an analysis of the Gibbs energy surface can help in discarding unrealistic results.