Robust Isenthalpic Flash for Multiphase Water/Hydrocarbon Mixtures

Abstract

Summary Robust isenthalpic flash is important in compositional simulation of steam injection, which involves at least three phases—oleic, gaseous, and aqueous. However, multiphase isenthalpic flash is challenging because the total enthalpy can be substantially nonlinear, or even discontinuous, with respect to temperature. This type of phase behavior is referred to as narrow-boiling behavior in the literature. This paper presents robust isenthalpic flash for multiphase water-containing hydrocarbon mixtures. The algorithm developed is based on the direct substitution (DS) presented in our previous research for two phases. A detailed analysis is given for narrow-boiling behavior and its effects on the DS algorithm. A new method is also presented for K-value estimates for three phases for water-hydrocarbon mixtures. The thermodynamic model used is the Peng-Robinson equation of state with the van der Waals mixing rules. The narrow-boiling behavior in isenthalpic flash occurs when a small temperature change yields significant changes of equilibrium-phase compositions relative to the overall composition. The system of equations used in the DS algorithm becomes degenerate for narrow-boiling fluids. The multiphase DS algorithm developed in this paper adaptively switches between Newton's iteration step and the bisection step depending on the Jacobian condition number. The bisection algorithm solves for temperature, based solely on the enthalpy constraint only when narrow-boiling behavior is identified. The algorithm is tested with a number of different isenthalpic flash calculations for three and four phases formed by water-containing hydrocarbon mixtures at elevated temperatures. Results show the robustness of the algorithm for narrow-boiling fluids, including the cases with one degree of freedom.