Methods for analyzing the flow of mixtures containing gas and liquid phases that are of interest to the petroleum industry have traditionaly been handled by empirical approaches, and more recently, through the use of mechanistic models. Mechanistic models for multiphase flow calculations can improve our ability to predict pressure drop and holdup in pipes especially in situations that cannot easily be modeled in a laboratory, and for which reliable empirical correlations are not available.

The brief introductory discussion that follows is based on excerpts from the report, "A Mechanistic Model for Stabilized Multiphase Flow in Pipes" by N. Petalas and K. Aziz, Petroleum Engineering Department, Stanford University (August, 1997). In addition, all the analytical results that are presented here are based on the mechanistic model described in this report.

Most mechanistic models begin by assuming that a particular flow regime is present. By solving the momentum balance equations for certain quantities that determine its characteristics, the stability of the flow pattern is examined. If the chosen flow pattern is shown to be stable, the procedure is terminated, the pressure drop and phase volume fractions being obtained directly from the momentum balance equations. If the flow pattern cannot exist under the specified conditions, a new flow pattern is assumed and the procedure is repeated until a stable flow pattern is determined. By performing these calculations over a range of gas and liquid flow rates, we can construct a Flow Pattern Map. Such a Flow Pattern Map is shown for an air/water system at atmospheric conditions.

At each calculated point we can also derive certain other quantities which help us with our understanding of Multiphase Flow. One such quantity is the frictional pressure loss in the pipe. This is plotted for the same set of conditions used in the Flow Pattern Map above. The colours relating to the flow pattern are also shown so that we may better observe any discontinuities that arise from the flow pattern transitions.

Another important quantity in Multiphase Flow analysis is the Phase Volume Fraction. Here is shown a surface plot of the Volume Fraction Liquid , i.e., the in situ fraction of fluid in the pipe that is liquid. This is sometimes referred to as the "Liquid Holdup".