Multiphase Flow Through Chokes - An Evaluation of Frozen, Equilibrium, and Nonequilibrium Flow Models

Abstract

This work has evaluated the most common published mechanistic multiphase choke models and compared them with a newly developed model. A typical assumption for most of the models is that the flow through the choke is adiabatic because it is argued that there is limited amount of time for heat and mass transfer. Such models are often referred to as “frozen flow” models and are widely used in the industry. The new model deviates from the frozen flow concept because it includes phase transfer. The depressurization through the choke is fast; hence, thermodynamic equilibrium will not be established and a nonequilibrium coefficient will account for this intermediate state. The goal was to investigate whether frozen flow and adiabatic expansion through the choke is a reasonable simplification or whether the concept of nonequilibrium flow could provide higher predictive ability.The predicted mass rates from all models have been calculated for two experimental multiphase datasets, and the mean relative error, mean absolute percentage error, root-mean-square deviation, and standard deviation have been calculated to compare the performances. The new model seems to be more robust and performs better than the more common frozen flow models. It has better overall predictive potential with a mean absolute percentage error of 2.6% and 5.2% for the multiphase datasets.The new model is generic and can be used with any given thermodynamic process, e.g., equilibrium flow, nonequilibrium flow, or frozen flow. For frozen flow, both adiabatic and polytropic gas expansion can be used, and the model performance will then be similar to previously published frozen flow models such as Al-Safran and Kelkar or Mwalyepelo.Further, a methodology for highly viscous flow has been proposed. All the present models originally assume frictionless flow, and high viscous flow is therefore not accounted for. A Reynolds number correction has been incorporated by adjusting the choke discharge coefficient. The correction is based on a study of highly viscous flow through safety valves.