Two-Phase Gas-Condensate Flow in Pipeline Open-Network Systems
- F.F. Martinez A. (Corpoven S.A.) | M.A. Adewumi (Pennsylvania State U.)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- November 1997
- Document Type
- Journal Paper
- 218 - 224
- 1997. Society of Petroleum Engineers
- 5.2.1 Phase Behavior and PVT Measurements, 5.6.5 Tracers, 5.3.2 Multiphase Flow, 4.2 Pipelines, Flowlines and Risers, 4.6 Natural Gas, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 3.3.6 Integrated Modeling, 4.6.3 Gas to liquids
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Even if gas transmission occurs after the necessary processing of the gas has been completed, condensation still occurs in the natural gas transmission and/or distribution systems. The quantity of the condensate formed will not only depend on composition, pressure, and temperature, but also on the unequal splitting phenomenon that takes place at T-junctions in a network system. This paper investigates the splitting phenomenon in horizontal-branching T-junctions. The compositional hydrodynamic model developed at Pennsylvania State U. is used to evaluate gas-condensate flow in a pipeline under steady-state conditions. Using a double-stream model for splitting analysis at T-junctions, the mass-liquid-intake fractions are determined. The junction is considered as a separator and the new compositions are calculated at the run and at the branch of the junction. Although quantitative validation of the model is limited by the incompleteness of the available data, a reasonable qualitative match of experimental data is achieved. The results demonstrate the predictive capability of liquid route preference in two-phase natural-gas/condensate flow at T-junctions. In addition to liquid split, compositional split is tested using polychlorinated biphenyl (PCB) as the focal point. It is found that the concentration of PCB is distributed in direct proportion to the liquid preference route, and the PCB concentration in the delivery points can be higher or lower than the inlet concentration at the supply point.
The importance of and tremendous potential associated with natural gas as an energy source make a compelling case for the accurate design of gas-pipeline-network systems, the main vehicle for natural-gas transportation. Engineers should provide optimal design of gathering, transmission, and distribution systems to keep consumer prices at reasonable levels. A gas-pipeline network is formed by pipes of various sizes and lengths, each crossing undulating terrain. The complexity of gas-network analysis is apparent even if the fluid is single-phase gas. The problem is more complicated when liquid, mainly from condensation, is introduced into the analysis.
The problem of partial condensation in gas pipelines leads to multiphase flow in pipes. Multiphase fluid flow in pipes has been studied by many researchers for many years. More sophisticated methods of analysis were initiated by the nuclear industry, where two-phase flow occurs in reactor-cooling systems and affects heat transfer. Several correlations exist for predicting pressure drop and liquid holdup in horizontal, vertical, and inclined pipeline. Also, the fundamental-fluid-mechanics approach has been studied to model two-phase flow in gas pipelines. One of the recent improvements in this field has been the development of compositional multiphase hydrodynamic models for analyzing gas-condensate pipelines.1,10 This model couples a two-fluid hydrodynamic two-phase flow model with a phase-behavior model for single pipelines.
Although this constitutes a major improvement in the mathematical description of this process, a number of hurdles remain. Some of these are the ability to handle single-phase/two-phase transitions along the pipeline, the limited flow-regime handling capability, and the inability to handle pipeline networks. Before the last problem could be addressed with any reasonable chance of success, the first two must be handled. These two aspects were resolved recently.4 In spite of this, using fundamental hydrodynamic models for network analysis remains a challenge. The only published analyses of two-phase pipeline networks use semiempirical two-phase flow correlations.10 This paper is an attempt to implement a two-fluid multiphase hydrodynamic model in a pipeline-network-analysis algorithm.
Over the past 20 years, several studies have been reported that have attempted to understand condensate movement at junctions in two-phase systems.2,7,11 Simultaneous flows of gas and liquid present peculiarly different behaviors at junctions, but the most striking is the fact that the liquid phase has a preferred route in the network that depends on the interrelationship between flow rates to the branch and main pipes downstream of the junction and the pressure loss between the single main inlet flow and the two outlet flows.
The main focus of this work is to study two-phase flow in a gas-pipeline open network using a compositional hydrodynamic model, including flow-split evaluation in network junctions. To the best of the authors' knowledge, no integrated model of the kind described in this work is available in the literature to date, although there is a number of published works on splitting phenomenon for two-phase flow.8,9 The development of a unified model to analyze and design gas-condensate-distribution pipeline-network systems with the prediction of condensate movement through the network is the main objective of this work.
Formulation of the Model
When gas and liquid move simultaneously through the network system, one has to consider that both the gas and the liquid streams will lose a part of their initial energy because of frictional resistance. Also, the mass transfer between gas and liquid has to be considered because of the phase behavior. In addition, the fluid distribution in the junctions must be evaluated because neither phase, gas nor liquid, splits proportionally. The formulation of a pipeline-network model consists of four parts: the open-network model, the hydrodynamic model, the phase-behavior model, and the splitting model. These four parts are coupled together to build a unified model for predicting the flow behavior of natural-gas condensate in pipeline networks.
The open-network model involves an algorithm that enables the calculations to be performed in the simplest way. In this approach, pipelines and T-junctions are evaluated consecutively until the whole system is completed. The model is restricted to a network that consists of one source node, a main pipe, and branch pipes. The interconnection of the network cannot close a path of branches. This network configuration is commonly found in gas-transmission and/or distribution systems.
The version of the hydrodynamic model used in this work was developed at Pennsylvania State U.,4 based on fundamental conservation equations of mass and momentum of each phase. The equations governing steady-state flow are written below.
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