Compositional Simulation of Two-Phase Flows for Pipeline Depressurization

Abstract

Summary The simulation of multiphase flow, considered in the case of coexisting vapor and liquid phases, is an important topic in engineering for the design of oil-and-gas production and transportation facilities. This paper presents the development of a compositional approach for the dynamic calculation of multiphase flows in pipelines. This approach can be defined as “full compositional,” because the vapor and liquid phases are described by taking into account the chemical composition, presenting points of interest from both the theoretical and the practical points of view. Physical properties required are calculated at each integration timestep with the actual phase compositions instead of relying on property tables previously generated from a single constant fluid composition. With this approach, in the numerical solution of the conservative two-phase-flow equations, the congruency between the dynamic pressure, calculated by solving the Navier-Stokes equations at constant temperature, and the thermodynamic pressure of the system becomes a critical constraint. In the numerical solution, the overall mass balance defined by means of the vapor- and liquid-phase densities is verified with respect to the mass balance of each chemical component involved, and the system pressure obtained from the solution of the momentum equations is always compared with the thermodynamic value defined by mass balance. Of the numerous test cases created for model validation, three of them (focused on fast depressurizations) are presented and discussed. Similar examples are not available in the literature as solutions of the current “state-of-the-art” commercial pipeline simulators.