Reduced method for rapid multiphase isenthalpic flash in thermal simulation

Abstract

Thermal enhanced oil recovery (EOR) involves the complex interplay of mass and energy transport processes with phase behavior. Hydrocarbon and water components partition across multiple fluid phases as a function of composition, pressure and temperature. Thermal compositional simulation thus requires phase-equilibrium calculations for at least three fluid phases. Isenthalpic flash (phase-split) is needed for thermal compositional simulation when the molar variables are used in the solution of the global mass and energy balances. Isenthalpic flash solves for phase compositions and phase amounts, in addition to temperature.Isothermal flash algorithms have been consistently refined over the past four decades, whereas isenthalpic flash remains an outstanding challenge in the study of petroleum reservoir thermodynamics. Isenthalpic flash is more challenging than isothermal flash for a number of reasons. First, temperature is unknown a priori, resulting in uncertainty regarding the phase state. Second, narrow boiling point behavior results in extremely nonlinear behavior of enthalpy with respect to temperature. Finally, thermal EOR involves the injection of steam which results in hydrocarbon–water interactions at elevated temperatures. Water is the thermodynamically dominant component in this context, and cannot be excluded from phase equilibrium calculations in the simulation of thermal recovery processes.In this research we address the isenthalpic flash problem using a reduced variables formulation. For the first time, we extend the use of reduced variables beyond the isothermal-isobaric domain for phase-split calculations. We introduce a fully coupled Newton method, which exhibits rapid convergence even for narrow boiling point problems. Excellent performance of our novel formulation is demonstrated through comprehensive testing of characterized fluids from the literature. We expect the developments in this research to fortify the coupling of phase-equilibrium calculations with the mass and energy conservation laws in thermal compositional simulation.