Compositional Two-Phase Flow in Pipelines
Authors: Gould Thomas L. 
Published: December 2007

This paper accurately calculates the interrelated behavior of pressure, temperature, and liquid dropout for multicomponent two-phase flow. The computational method developed has been used in field-scale applications where pressure and temperature profiles were calculated for a gas-gathering system in which liquid dropout was occurring.

Introduction

Engineering analysis of two-phase fluid flow in pipes has focused primarily on the problem of predicting pressure drop. Phase-behavior aspects of the problem traditionally have been simplified to obtain manageable calculation methods. During the last 10 to 15 years, it has be come possible to treat two-phase pipeline problems with empirical numerical techniques that yield reasonably accurate pressure drops. When used with high-speed computers, it seems reasonable to expect that these methods might profit from a more rigorous treatment of the hydrocarbon phase-behavior aspects of the problem.

Nearly all two-phase pipeline simulation currently is performed using "black-oil" simulators. A black-oil model's validity rests on the assumption that the hydrocarbon mixture is composed of only two components (denoted oil and gas), each with fixed composition. The gas is said to be dissolved in the oil, with the amount of dissolved gas decreasing with decreasing pressure. The oil component in the pipeline generally is defined as stock-tank oil. A black-oil model usually treats PVT properties (solution gas, densities, and viscosities) as properties (solution gas, densities, and viscosities) as single-value functions of pressure. More sophisticated models include the temperature effect on fluid properties as well. A linear temperature profile sometimes is considered, but laboratory data are seldom available to calibrate the thermal effects.

The multicomponent or compositional approach is designed for gas condensate and volatile oil systems that are not handled easily using the black-oil method. These fluids are represented as N-component hydrocarbon mixtures, where N might equal 9 with components methane, ethane, propane, i-C4, n-C4, i-C5, n-C5, C6, and C7 +. A single equation of state then is used to determine physical properties.

The term "compositional" is borrowed from reservoir engineering and implies that the overall or in-situ fluid composition varies point-by-point with distance. In other words, the mole fraction of each component is a function of pressure, temperature, and slip between the phases, which in turn vary with distance. The compositional problem then is distinguished from a multicomponent-type problem then is distinguished from a multicomponent-type problem that assumes uniform composition with distance. problem that assumes uniform composition with distance. When a multicomponent gas-liquid mixture flows through a pipe, the composition, pressure, temperature, and liquid holdup distributions are related. The purpose of this study is to calculate each distribution accurately, while properly accounting for the steady-state interaction between them. The proposed model computes changing compositions of liquid and gaseous phases in the pipeline in accordance with the resulting pressure and temperature profiles. These profiles are calculated using the principles of heat balance, momentum balance, and principles of heat balance, momentum balance, and phase equilibrium. phase equilibrium. Phase Behavior of Hydrocarbon Fluids Phase Behavior of Hydrocarbon Fluids Hydrocarbon fluids usually are classified as to the phase behavior exhibited by the mixture.

JPT

P. 373