Effect of Thermo-Physical Properties on Multiphase Flow Modeling

Abstract

An important, but relatively unexplored topic in multiphase flow modelling is the effect of thermos-physical properties on multiphase models. This is relevant in the context of 1-D modelling of large multiphase systems such as pipelines from subsea wellheads to the onshore processing facility. Millions are spent on multiphase flow simulation models, but not enough attention is given to the thermophysical models which can affect the results just as much as multiphase flow correlations. A wide variety of field data have been compared to various multiphase models. More often, the multiphase flow model needs custom "Tuning" of the thermophysical properties to closely match the production data from the field. In this presentation, we discuss the impact of water content, composition of hydrocarbons, gas to oil ratio (GOR), surface tension, liquid density, and fluid enthalpy. Typically, improving accuracy of these properties will increase the prediction of multiphase models significantly. For example, error in the temperature prediction can be reduced from 10 C to 1 C, pressure uncertainties from 25% to 5%, and liquid holdup from 30% to 10%.This presentation will present examples of each using field data, before and after this improvement. This paper will also discuss using four different equations of state (EOS) for the calculations of different properties: phase equilibrium, gas density, liquid density, and enthalpy departures. This approach is used instead of the traditional approach of using 1 cubic EOD for all properties. This talk will also present error uncertainty bands for some commercial thermodynamic simulators and their corresponding impact on the multiphase flow predictions. Among all the properties mentioned above, surface tension needs particular attention since most of the multiphase models use it as a parameters. However, it is seldom measured for most of the hydrocarbon systems. Off-the-shelf thermodynamic simulators are not able to predict the surface tension accurately because of the variable content of naturally-existing surfactants in the hydrocarbon fluids. Hence, through this presentation, we raise the question if this is the limitation of the multiphase models for design, especially since most multiphase flow correlations have not been compared over a wide range of surface tensions.