Summary. An experimental study of two-phase flow was conducted to investigate liquid holdup in wet-gas pipelines. The liquid-holdup data were obtained by passing spheres through a 1,333-ft [406.3-m] -long, 3.068-in. [77.93-mm] -ID horizontal pipe and measuring the liquid volumes removed. Three different two-phase mixtures were used. The holdup data were compared with predicted holdup values and were used to evaluate a mechanistic model for stratified flow. None of the methods could accurately predict liquid holdup in this low-holdup region. Two new empirical liquid-holdup correlations for horizontal flow were proposed. The first is strictly for wet-gas pipelines (0 less than yL less than 0.35); the second is for any horizontal pipeline (0 less than yL less than 1.0). Introduction The simultaneous flow of gas and liquid in pipes is commonly encountered in the oil and gas industry. The complexity of this two-phase flow, however, has hindered the development of an accurate and completely theoretical model for predicting pressure drop and liquid holdup. As a result, numerous empirical correlations have been published and widely used. These correlations are usually published and widely used. These correlations are usually based on experimental data, taken in small-scale facilities operating at relatively low pressures and using different fluids from the ones encountered in the field. The accuracy of the correlations during simulation of behavior in large-diameter, high-pressure, and long pipelines is questionable. The liquid present from retrograde condensation in wet-gas transmission lines is usually small, leading to low liquid-holdup values. Accurate prediction of liquid holdup for these lines is not possible because all existing correlations were based on relatively high liquid-holdup data. Accuracy of the data diminishes rapidly as measured holdups approach zero. The inaccuracy is a result of the liquid-holdup measurement techniques used by investigators. In this study, a horizontal test loop 3.068 in. [77.93 mm] in diameter and 1,333 ft [406.3 m] long was used to investigate liquid holdup in the low-liquid-holdup region. Three different two-phase mixtures were used: kerosene/air, water/air, and water/surfactant/air. The liquid holdup was averaged over the entire length of the pipeline, because liquid was removed by pigging with rubber pipeline, because liquid was removed by pigging with rubber spheres. The collected data were used to evaluate the correlations of Dukler et al., Eaton et al., Beggs and Brill, and Mukherjee and Brill, as well as the Taitel and Dukler theoretical model for predicting liquid level in stratified flow. Experimental Program Fig. 1 is a schematic of the experimental facility used in this study. A brief description of the various components of the system and the testing procedure follows. A more detailed description was given in Ref. 6. Experimental Equipment. Air was supplied by a compressor rated at 800 Mscf/D [22.65 std m3/d] at 125 psig [861.8 kPa]. A pressure-regulator valve was located downstream of three connected storage tanks to maintain the pressure at 80 psig [551.6 kPa]. The air was then piped through one of two meter runs before going to the mixer. The temperatures were read from a thermometer installed upstream from the orifice. Kerosene and water were stored in two separate steel tanks. Each liquid had its own single-stage centrifugal pump with a 6,800-B/D [1081-m3/d] capacity. After the pump with a 6,800-B/D [1081-m3/d] capacity. After the liquid went through the pump, it was metered by either an orifice meter or a rotameter, depending on the flow rates. A quick-closing ball valve was provided just before the mixing tee. The two-phase test section consisted of a horizontal flow loop 3.068 in. [77.93 mm] in diameter and 1,333 ft [406.3 m] long. The length was measured from the pig launcher to the pig catcher. Special joints were used to ensure constant-diameter connections. Two transparent PVC sections located at 200 and 656 ft [61 and 200 m] from the pig launcher enabled visual observation of flow pattern pig launcher enabled visual observation of flow pattern and liquid level. At the outlet end, a pig catcher that consisted of a cylinder with baffle plates was used to catch the pig. Then quick-closing ball valves allowed deviation of the flow into either the separator or the weighing tank. Pressure transducers and pressure gauges were located at 14, 200, 656, and 1,299 ft [4, 61, 200, and 396 m] from the pig launcher and at the separator. The transducer signals were demodulated and recorded on an oscillograph. Direct pressure reading was also possible by use of a multimeter pressure reading was also possible by use of a multimeter with the demodulators. A tension load cell was used to weigh the amount of liquid in the weighing tank. This cell was connected to a digital electronic weight indicator. Testing Procedure. The data points were selected so that they would be evenly distributed on the Mandhane et al. flow-pattern map. For each test, air and liquid were allowed to flow at the desired flow rates through the separator until a steady-state condition was reached. SPEPE P. 36
Summary A mechanistic model is developed for the prediction of annular two-phaseflow behavior in gas wells. The model can predict annular flow characteristics, such as liquid film thickness, gas void fraction, and pressure gradient. It wasevaluated against a data bank of 75 wells for pressure gradient. It wasevaluated against a data bank of 75 wells for which the pressure drop betweenthe bottom of the well and the wellhead was available. The model was alsocompared with data from two wells with measured pressure profiles. Modelpredictions are in agreement with these data and superior to most commonly usedcorrelations. Introduction Gas/liquid two-phase flow occurs in a wide range of engineering applicationsin the petroleum, nuclear, chemical, and geothermal industries. Two-phase flowin the petroleum industry is very common because of the simultaneous productionof gas, oil, and water from hydrocarbon reservoirs. When gas and liquid flow simultaneously in a pipe, various flowconfigurations or patterns may form, each with different spatial dis-tributionsof the gas/liquid interface. The existing flow patterns depend on theoperational variables-namely, the liquid and gas flow rates, the physicalproperties of the two phases, and the geometry of the conduit (diameter andinclination angle). Knowledge of the flow regime that will occur in a pipe isessential to the design engineer because the flow hydrodynamics and thetransfer mechanisms of momentum, mass, and heat differ significantly from onepattern to another. For upward vertical flow, the flow patterns encountered art bubble, dispersed bubble, slug, churn, and annular flow. Flow-pattern prediction hasbeen studied extensively by two-phase-flow prediction has been studiedextensively by two-phase-flow researchers. Recent models enable prediction offlow patterns for all inclination angles. These models, which are givenelsewhere beyond the scope of this paper. Annular flow occurs under conditions of high gas flow raw and low to mediumliquid flow rate. The liquid flows as a film around the pipe wall, surroundinga high-velocity gas core, which may contain entrained liquid droplets. Theinterface between the gas and the liquid film is very wavy. Atomization anddeposition of liquid droplets occur through this interface. Predicting flow behavior under annular conditions is important for properdesign of systems, such as gas wells. Earlier predictive means for two-phaseflow were empirical correlations. These correlations have been usedextensively, but their validity is limited because they depend strongly on theconditions under which the experimental data were taken. An attempt has beenmade recently to develop models to predict flow behavior on the basis ofphysical principles. These models can be applied for different flow principles. These models can be applied for different flow conditions with more confidencebecause they include such important parameters as pipe diameter, inclinationangle, physical properties, parameters as pipe diameter, inclination angle, physical properties, and flow rates. Wallis and Hewitt and Hall-Taylor gave general discussions of annular flow. Dikler presented an early attempt to model annular flow that considered fallingfilm in vertical pipes. Hewitt later extended this work to upward flow. Morerecently, other models have been published for vertical upward annular flow. Hasan and Kabir developed a comprehensive model for predicting two-phase flowin wellbores that includes treatment of annular predicting two-phase flow inwellbores that includes treatment of annular flow with previously publishedmethods. Yao and Sylvester presented a simple model to predict film thicknessand pressure drop presented a simple model to predict film thickness andpressure drop for vertical flow. The Oliemans et al model is based on atwo-fluid model supported by empirical correlations far both entrainment andinterfacial shear stress. Finally, Caetano presented a model for annular flowin an annulus configuration. Physical mechanisms associated with annular flow have also been studiedextensively. Turner et al. and Ilobi and Ikoku studied the minimum gas velocityrequired for liquid removal from verbical pipes. Taitel et. al.. used this sameconcept to predict the tran-sition boundary to annular flow. Wallis, Henstockand Hanratty, Whalley and Hewitt, and Asali et al developed interfacial shearrelationships. Various studies" have focused on the entrainment process. These phenomena are essential to model development. Oliemans et al gave acomprehensive review of both interfacial shear and entrainment. They concludedthat most of the correlations for entrainment do not give satisfactory resultsand apply only in the test range in which they were derived. The objective of this study is to present a general model for the predictionof annular flow behavior in vertical pipes. This includes prediction of annularflow behavior in vertical pipes. This includes the prediction of liquid filmthickness, gas void fraction, and pressure bent. The model is presented in adimensionless form, thus pressure bent. The model is presented in adimensionless form, thus making its application easy and straightforward to anyflow condition. Analysis Fig. 1a is a schematic of annular flow, and Fig. lb shows the generalapproach of separating the flow into two fluids. This includes the liquid filmflowing around the pipe wall and the core, which consists of the gas phase andentrained liquid droplets. The proposed model is for fully developed annular flow in vertical andoff-vertical upward pipes under isothermal conditions. The liquid film isassumed to have a uniform thickness, hF, and to be free of gas. The fluid inthe core is considered homogeneous i.e., no slip occurs between the liquiddroplets and the gas phase. The average-velocity concept is used in both theliquid film and gas core regions. Although the model assumes incompressibleflow, gas compressibility can be taken into account in the numericalintegration to calculate total pressure drop. From the above simplified assumptions, the conversation laws of linearmomentum and mass are used to derive the model The approach taken is similar to Taitel and Dukler's for horizontal and near-horizontal stratified flow and Barnea et al. for downward vertical annular flow. Linear Momentum. Eqs. 1 and 2 represent the conservations of the linearmomentum applied to the liquid film and gas core, respectively (refer to Fig.1b): Assuming equilibrium condition between the liquid phase and the gas core, wecan combine Eqs. 1 and 2 by eliminating the pressure gradient. SPEPE P. 435
Significant part of the Brazilian oil reserves is located in ultradeepwater fields (WD > 1500 m). In this scenario, flow assurance plays a crucial role due to the existing high pressures and low temperatures.This paper focuses on the strategies concerning flow assurance issues for the near future. These strategies are strongly based on our specific field experiences as well as on the foreseeable technological scenario. The future, as treated in this work, refers to the short and mid-term future and not to the prospective long term one. Thus, it will mainly discuss current R&D in flow assurance activities to tackle existing problems and the ones foreseen for the discoveries already in development.In order to facilitate the understanding of our vision, a brief summary about todays flow assurance issues is presented. Currently, the main flow assurance concerns are related to hydrate formation and wax deposition. Accordingly, design criteria and operational procedures to avoid these problems are briefly described.There is also an increasing concern about heavy and extraheavy oil production, since a relevant part of recent offshore discoveries involves this type of oil.Among the initiatives that are being taken aiming at optimizing flow assurance design in the near future, one can mention:-rheology of heavy oil and water in oil emulsion; -heavy oil multiphase flow (gas-liquid) simulation; -hydrate slurry transportability; -improvements on wax deposition modeling; and -improvements on heat management.
Summary Standard design procedures for two-phase pipelines subject to pigging operation still rely on steady-state methods. Because the flow behavior under a pigging operation is transient, more rigorous methods must be developed for a proper design of such operations. This paper presents an experimental and theoretical study on transient pigging operation. An extensive experimental program was carried out to acquire two-phase transient flow and pigging data on a 420-m- [1,378-ft-] long, 77.9-mm- [3.068-in.-] diameter horizontal pipeline. A computer-based data-acquisition system was used to obtain detailed information of the flow behavior during the experimental runs. The data include measurements of the time variation of the liquid slug size ahead of the pig, flow rates during the slug delivery, pig velocity, and pressure and liquid holdup distributions. The data were acquired at four measurement stations installed along the pipeline. A pigging model was developed for predicting the dynamics of a pigging operation. The model incorporates an improved simplified transient model that has been validated with the experimental data. A mixed Eulerean-Lagrangean approach was used to couple the transient model and the pigging model. The resulting computer simulator can predict the transient two-phase flow in a pipeline with or without pigging. Introduction The passage of spheres, or more recently of foam pigs, which is commonly known as a pigging operation, is performed in wet-gas pipelines to remove periodically liquid accumulation in the lower portions of hilly-terrain pipelines. This operation helps keep the pipeline free of liquid, reducing the overall pressure drop and increasing the pipeline flow efficiency. A frequently pigged two-phase gas pipeline is capable of transporting up to 70% more gas as compared with an operation without pigging. During a pigging operation, several flowing zones are present (see Fig. 1). Far downstream from the location of the pig is the undisturbed two-phase flow zone, in which the effects of the pig are not yet felt. As the pig moves, a liquid slug zone forms and grows by scooping the liquid from the downstream undisturbed flow zone. A very low liquid-holdup zone, or the gas zone, forms behind the pig because the sphere removes most of the liquid phase. Finally, the last zone is the redeveloping two-phase flow zone. The boundaries of the zones all move from upstream to downstream at different velocities. Pipelines subject to pigging operation invariably are operated under transient conditions.
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