Isenthalpic Flash for Thermal Recovery

Simulation of thermal recovery processes requires an energy balance in the governing equations. It is often advantageous to specify enthalpy along with pressure and composition for a grid block and solve the isenthalpic flash problem. It typically involves multiple components and three fluid phases: gas, oil and water. Full thermodynamics can be used for the fluid description. It is also common to describe the phase equilibrium with simplified K-factor correlations in the simulation. A typical challenge in isenthalpic flash is so-called narrow boiling behaviour which is frequently encountered for water containing systems. A general isenthalpic flash algorithm has recently been developed for multicomponent mixtures with any number of phases (SPE-179652-MS). The algorithm uses Newton's method for rapid convergence and a Q function maximisation using a nested isothermal flash to handle non-convergent situations. The algorithm developed is tailored to thermal recovery processes: the Wilson K-factor equation has been modified to incorporate a separate water phase; low solubility components in certain phases can be excluded from the equations to simplify the problem; stability analysis is customised for detecting the water phase; the failure check step in the original algorithm has been simplified. The resulting algorithm is much faster and is convergent in the majority of narrow boiling cases without the need for Q function maximisation. For cases where temperature dependent K-factors are used, the algorithm can be further simplified. A formulation as an extension of the multiphase Rachford-Rice equation is used for convergence in the majority of cases. A simplified Q function maximisation step is used as a fallback approach for convergence. The developed algorithms are tested with typical conditions in thermal recovery and prove to be efficient and robust, even for narrow boiling cases. The two algorithms are best suited for integration into a simulator for thermal recovery processes.