Critical Point and Saturation Pressure Calculations for Multipoint Systems

Abstract

Abstract Calculation of fluid properties and phase equilibria is important as a general reservoir engineering tool and for simulation of the carbon dioxide or rich gas multiple-contact-miscibility (MCM) mechanisms. Of particular interest in such simulations is the near-critical region, through which the compositional path must go in an MCM process. This paper describes two mathematical techniques that enhance the utility of an equation of state for phase equilibrium calculations. The first is an improved method of estimating starting parameters (pressure and phase compositions) for the iterative saturation pressure (bubble-point or dew-point) solution of the equation of state. Techniques previously have been presented for carrying out this iterative solution; however, the previously described procedure for obtaining initial parameter values was not satisfactory in all cases. The improved method utilizes the equation of state to estimate the parameter values. Since the same equation then is used to calculate the saturation pressure, the method is self-consistent and results in improved reliability. The second development is the use of the equation of state to calculate directly the critical point of a fluid mixture, based on the rigorous thermodynamic criteria set forth by Gibbs. 1 The paper presents an iterative method for solving the highly nonlinear equations. Methods of obtaining initial estimates of the critical temperature and pressure also are presented. The techniques described are illustrated with reference to a modified version of the Redlich-Kwong equation of state (R-K EOS); however, they are applicable to other equations of state. They have been used successfully for a wide variety of reservoir fluid systems, from a simple binary to complex reservoir oils.