(378h) New Initialization Procedures from Stability Testing in Multiphase Flash Calculations for Water/Hydrocarbon/CO2 Mixtures

Phase equilibrium calculations plays an important role in a wide variety of applications in chemical and petroleum engineering. In recent years, great attention has been paid to applications in carbon capture utilization and storage (CCUS), processes that are intended to reduce the amount of CO2 in the atmosphere. In those applications, complex phase behavior occurs and three or more phases might be present, making the compositional simulation more challenging. In reservoir compositional simulators, a failure in correctly describing the phase behavior will result in either an erroneous physical modeling of the problem, or in significant error propagations that may compromise a simulation run. Moreover, in field-scale simulations, the phase equilibrium routine is called a significant number of times. Therefore, both robustness and efficiency are of utmost importance. The traditional approach for multiphase equilibrium consists in a sequence of phase stability and flash calculations. In this work, highly robust routines are used, based on successive substitution iterations (SSI) in early iteration stages, followed by Newton iterations with modified Cholesky factorization and line search, in both stability and flash calculations.

Stability testing aims to ascertain whether a chemical system maintains equilibrium as a single phase or it splits in two or more phases. It uses the tangent plane distance (TPD) function, and its stationary points are calculated in order to verify the stability of the mixture. Combination of stability tests and split calculations, in a stepwise manner, is the common strategy proposed by Michelsen and different sets of initial guesses were proposed by many authors. A widely used number of trial phases is four plus the number of components in the mixture, for one phase check, and twice this value for two-phase stability test. Therefore, reducing the number of stability tests is a key point in developing an efficient stability-flash algorithm, which can be effectively applied to compositional reservoir simulation.

However, there is a lack of details about how information from stability testing is effectively used. This work aims to develop a detailed procedure for using in a systematic manner all information available from stability tests to initialize two- and three-phase split calculations. Different strategies are presented, pointing out common procedures and their applicability range, as well as new possible lines of action to reduce the computational time in flash calculations and at the same time maintain the robustness of the flash package. We present the limitations and advantages of each methodology, comparing the number of iterations and computational time. Various mixtures containing hydrocarbon components, CO2 and water are studied, and phase diagrams are constructed in P-T, P-Z and T-Z planes, focusing on the number of negative stationary points of the TPD functions found in each step of the multiphase stability-flash algorithm and on how they must be efficiently used in initialization. An extensive Gibbs energy analysis for binary mixtures exhibiting different kinds of phase behavior gives a theoretical foundation of our choice in using the stationary points of the TPD function to initialize two- and three-phase flash calculations. For all the test mixtures considered here, in the proposed stability-flash strategy, the number of calls of the stability and flash routines and the number of iterations in flash calculations are significantly reduced as compared to previous approaches, particularly in three-phase and two-phase liquid-liquid domains, recommending the new approach as an useful tool in compositional simulation.