Tacite: Contribution Of Fluid Composition Tracking On Transient Multiphase Flow Simulation
Authors: Henriot V. , Pauchon C. 
Published: October 2008

Abstract
The composition tracking approach allows an improved estimation of the local fluid properties. It enables computation of the local thermodynamic equilibrium. An optimized two component representation of the fluid is shown to be sufficient in a number of applications. This paper illustrates possible applications of the composition tracking approach as applied to production allocation problems. In this case more than two components are used to describe the two-phase flow mixture.

Introduction
The TACITE code developed by I.F.P., ELF and TOTAL is a compositional transient multiphase flow simulation tool, for the design and control of oil and gas production pipelines and wells. One of the main objectives of this code is to predict accurately the propagation of liquid slugs. These slugs may occur during transient flow due to boundary conditions modification such as inlet flowrates variation, outlet depressurisation, shutdown or restart. They may also appear due to transients created by heavy topography changes: terrain slugging or severe slugging phenomena.
In the first version of TACITE and in other transient simulation codes for multiphase flow in pipelines such as OLGA. PLAC the fluid properties and thermodynamic equilibrium are based on a fixed inlet global composition.
This papers presents the TACITE composition tracking feature. This approach allows a local determination of the fluid composition, hence it provides a better estimate of the fluid properties and local equilibrium conditions, possibly leading to the formation of hydrates, and/or wax deposition.
The main advantages of this approach are as follows:Variations of sources composition versus time can be taken into account.For network simulations, it accounts for the varying compositions in the different links.The code provides the liquid and gas compositions at the outlet of the pipe, which can be useful for allocation purposes or downstream plant operation.The use of an internal flash algorithm ensures robustness and accuracy of the simulation, in particular for conditions lying in the vicinity of the phase envelope (bubble or dew lines).Phenomena such as local accumulation of the heavy components in the low points of the line and subsequent changes in the fluid properties can be predicted through this compositional approach.For coupled simulations between the reservoir and flowline, or between the flowline and process facilities, the compositional tracking approach makes better use of the information already available when the other code operates in compositional mode.
The first commercial version of TACITE computes a lumped composition with two pseudo-components. The algorithm uses compositional lumping as a pre-processor to optimize the characterisation of the pseudo-components. The version under development of TACITE can take more than 2 components into account.
In the first part of this paper we describe the lumping method. Then we present the mathematical model: transport equations for mass, momentum and energy, closure laws (hydrodynamic and thermodynamic models), and the treatment of initial and boundary conditions. Next, the main resolution algorithms are presented. Finally, the numerical scheme that has been chosen is presented briefly.