NEW STRATEGY FOR PHASE-EQUILIBRIUM AND CRITICAL-POINT CALCULATIONS BY THERMODYNAMIC ENERGY ANALYSIS .2. CRITICAL-POINT CALCULATIONS

The thermodynamic energy analysis of Michelsen is reformulated in terms of component molar densities as the primary variables. In this formulation, we can provide analytical expressions for any higher order derivatives for the energy function. This feature enables us to enhance the speed and the reliability of the critical point search method. It is designed to find the critical composition and the critical pressure for fluid mixtures at a fixed temperature (pressure-composition diagram), as well as critical points in the composition space at the specified pressure and temperature (pseudo-ternary diagrams). This has proved to be very useful in finding the miscibility composition for complex and near-critical petroleum reservoir fluids subjected to gas injection. A number of examples illustrate the successful use of this method.