document

Abstract

This paper details the application of transient multiphase compositional tracking in a pipeline flow simulation study. Comparisons of results between the compositional modeling approach and results obtained using a precalculated table-based fluid property approach is presented. Both simulation models are based on the dynamic two-fluid theory commonly used to simulate transient multiphase flow in wells and pipelines. When comparing the simulation results with pipeline depressurization field data for one field case, the gas outlet rate and pressure is simulated with greater accuracy using the compositional tracking approach. For a second field case the simulation results are comparable for gas outlet rates and pressures although the compositional tracking overall is better. For liquid volumes the results are of comparable quality for both field cases.

Introduction

Transient multiphase flow in wellbores and pipeline systems is typically analyzed using a dynamic two- or three-fluid modeling technique (). To determine pressure drop, temperature changes and flow regime, the model essentially solves conservation equations for mass, momentum and energy for the gas and liquid phase or phases as a function of time. For hydrocarbon systems with water, the model can handle water either as an integral part of the hydrocarbon phase or as a separate liquid phase. The most important fluid physical properties and transport properties that are required by the model are density, viscosity, thermal conductivity and heat capacity, surface tension and phase mass fractions.

The traditional representation of the required fluid properties is through precalculated tabular values with an upper and lower boundary for pressure (P) and temperature (T). Based on an overall total fluid composition the properties are tabulated at a number of (P, T) points. During the course of the simulation the model interpolates in the properties table as needed. This method is referred to as a table-based approach. The primary purpose of using a table-based approach is to save simulation time.

Recent advances of () in the numerical handling of flash calculations for multiphase flow simulations have enabled a more rigorous simulation approach to be developed.

The approach, compositional tracking (), accurately takes into account the fluid compositional changes in space and time, and continuously calculates physical properties based on the in-situ hydrocarbon and aqueous compositions. The compositional tracking approach consequently does not have the same limitations in application as the table-based approach.

Potential application areas for the compositional tracking approach are for simulations involving some degree of phase separation of gas and liquid for example:

• Shut-in (liquid settles in low spots)

• Restart (potential slugging)

• Depressurization (gas removed from pipeline potentially together with liquid slugs)

• Slugging (alternate flow of gas and liquid)

• Mixing of fluids from pipelines that individually are in a transient state resulting in varying transient state in the merge pipeline

• Hot oil circulation and well start-up

Whereas it is intuitively obvious that a compositional modeling approach will allow a more accurate fluid description in a system with varying mixing of different fluids from different pipelines with time, it is less obvious that a compositional model is required for a simpler system consisting of, for instance, a single pipeline. Even if there are recognized compositional effects during slugging, is it enough to justify the more rigorous compositional approach considering the uncertainty in the calculation of the properties of the slug flow regime () in the first place? Even if there are compositional effects during the depressurization of a pipeline, is it enough to justify the compositional tracking approach to be applied for all such simulations or should it only be mandated when certain extreme conditions apply?