1,720,980 research outputs found
Model-based estimation of multi-phase flows in horizontal wells
No full textThe growing demand for hydrocarbon production has resulted into improved oilfield management with various control and optimization strategies. These strategies in turn strongly rely on the efficiency of downhole equipment which is used to obtain real-time oil and gas production rates with sufficient spatial and temporal resolution. In particular, multiphase flowmeters installed downhole can improve the production of horizontal wells by allocating the zones of oil, gas and water inflow. However, existing multiphase meters are expensive, inaccurate or accurate only within a limited operating range and therefore such monitoring is unrealistic. To overcome these problems one can use so-called multiphase soft-sensors, i.e. to estimate flow rates from conventional meters, such as downhole pressure gauges, in combination with a dynamic multiphase flow model. The soft-sensors can also be used together with hardware sensors to improve their overall performance, e.g. by substituting missing data of the hardware sensor with output of the soft-senor. The research presented in this thesis discusses possibilities and limitations of such multiphase soft-sensors. The groeiende vraag naar koolwaterstofproducten heeft geresulteerd in een verbeterd management van olievelden met verscheidene regel- en optimalisatiestrategien. Deze strategieën vertrouwen sterk op de efficiency van ondergrondse apparatuur die gebruikt wordt voor het verkrijgen van real-time olie- and gasproductiesnelheidsmetingen met voldoende resolutie in plaats en tijd. Ondergronds geïnstalleerde meerfasestromingsmeters in het bijzonder kunnen the productie van horizontale putten verbeteren door het identificeren van de zones waar olie, gas en water instroomt. Bestaande meerfasestromingsmeters zijn echter duur, onnauwkeurig of alleen nauwkeurig binnen een beperkt werkgebied en daarom is een dergelijke manier van monitoren onrealistisch. Om deze problemen te overwinnen kan men gebruikmaken van zogenaamde meerfasen soft(ware)sensoren, waarmee productiesnelheden worden afgeschat met conventionele meetapparatuur zoals drukmeters in combinatie met een dynamisch meerfasestromingsmodel. De soft-sensoren kunnen ook worden gebruikt in combinatie met hardware sensoren om hun prestaties te verbeteren, bijvoorbeeld door het vervangen van weggevallen data met outputs van de soft-sensor. Via een dergelijke constructie kunnen ze ook gebruikt worden voor het diagnosticeren van afwijkende situaties of een defect van een hardware sensor. Het onderzoek dat gepresenteerd wordt in deze thesis bediscussieert mogelijkheden en beperkingen van dergelijke meerfasenstromings-softsensoren.Multi-Scale PhysicsApplied Science
Severe slugging in gas-liquid two-phase pipe flow
No full texttransportation facilities. In an offshore oil and gas production facility, pipeline-riser systems are required to transport two-phase hydrocarbons from subsurface oil and gas wells to a central production platform. Severe slugs reaching several thousands pipe diameters may occur when transporting gas and liquid in these pipeline-riser systems. Severe slugging creates potential problems in the platform facilities, e.g. separators, pumps, and compressors. Severe slugging may cause flooding and overpressurization of the separator, rupture of the pipe, and an increased back pressure at the wellhead. All of these might lead to the complete shutdown of the production facility. Therefore, the accurate predictions of severe slugging characteristics, e.g. slug length, oscillatory period, are essential for the proper design and operation of two-phase flow in the pipeline-riser systems. Pipelines used for the transportation of hydrocarbons in an offshore production facility, are laid out over the seafloor. The uneven seafloor topography forms different pipeline-riser configurations. In this dissertation, we described the severe slugging characteristics in a long downward inclined pipeline-riser system. We carried out experiments in a relatively long pipeline-riser configuration, and also performed numerical simulations using a one-dimensional two-fluid model. It was found experimentally, as also reproduced numerically, that transient slugs were generated in the pipeline upstream of the riser base. These transient slugs effectively contributed to the initial blockage of the riser base. Furthermore, an existing analytical model for the prediction of the flow behaviour in the pipeline-riser system was modified. The modified model, which was tested against our experimental results, showed a better performance than previously published models. We developed a transient drift flux model to simulate the severe slugging characteristics in a pipeline-riser system. The model was tested against experimental data and interestingly, could predict the occurrence of severe slugging in a horizontal pipeline-riser system, which is a subject of debate in the open literature. That motivated us to conduct experiments in a horizontal pipeline-riser configuration. It was observed that severe slugging can develop even in the horizontal pipeline-riser configuration. Moreover, a new class of severe slugging was found and referred to as dual-frequency severe slugging, which corresponds to dual-frequency pressure and flow rate fluctuations. It was found that dual-frequency severe slugging evolves when the pipeline length exceeds a certain threshold. In this dissertation, we also described the severe slugging characteristics in a hilly-terrain pipeline-riser configuration. A hilly-terrain pipeline consists of interconnected horizontal, downhill, and uphill sections. It was observed that, the existence of a hilly-terrain unit in a pipeline-riser system induces a more severe type of slugging, which exhibits longer slugs than that of a horizontal pipeline-riser system. So far we have summarized our work on the characteristics of severe slugging in a pipeline-riser system. In this dissertation, we also discuss the occurrence of severe slugging in an extended reach well. In response to meet the world energy demand, the oil and gas industry has also moved towards development of resources in scattered, isolated oil and gas pockets. Snake wells and fish-hook wells are extended reach wells, which have been used to develop these small hydrocarbon deposits more efficiently than conventional vertical or horizontal wells. The extended reach well resembles the pipeline-riser configuration. The flow conditions, e.g. pressure, and the pipe specifications, e.g. diameter, at the bottom of a well are generally different than the pipeline laid out over the seafloor. It is expected that severe slugging at the bottom of the well is less likely to occur. In this dissertation, we performed numerical simulations to study the possible formation of severe slugging at the bottom of an extended reach well. It was found that severe slugs were initiated at the bottom of the extended reach well. This teaches one to study the well hydrodynamics more carefully when designing an extended reach well.Multi-Scale PhysicsApplied Science
Long liquid slugs in stratified gas/liquid flow in horizontal and slightly inclined pipes
No full textLong liquid slugs reaching several hundreds pipe diameter may appear when transporting gas and liquid in horizontal and near horizontal pipes. The long slugs cause system vibration and separation difficulties that may lead to operational failures. Identifying and predicting the time and length scales of slugging is important for gas and oil production technologies (e.g. for the design of offshore gas and oil pipelines and process facilities). Although mainly short hydrodynamic slugs (40 pipe diameters) have been observed in offshore production fields, the appearance of the long slugs becomes more likely as the field becomes older the operation pressure drops. Therefore, predicting the transition between the different slug types and the flow conditions at which the long slugs appear may be crucial preventing or reducing the negative effects of slugging. The approach adopted in this study is the construction of simplified theoretical models that successively approximate the flow conditions and the corresponding time and length scales of slugging. Experiments and numerical modelling have been performed for validation and comparison matters. The first part of the research deals with identifying the long slug region and sub–regions. Experiments carried out by Zoeteweij (2007) present a detailed flow map for the long slug region and the transition to hydrodynamic slugs or stratified wavy flow. For the prediction of the long slug region a simplified predictive model was constructed. The model calculates the average slug length based on the volumetric liquid rates adjoining the slug, and derives the change in the liquid level, at the tail of the slug, by linear kinematic relation between the tail and the following upstream wave. The model predicts the transition from hydrodynamic to long slugs with a satisfactory agreement. In the second part of the research the emphasis is put on predicting the transition from stratified flow to slug flow or roll–waves. Slugs formed by coalescence between roll–waves are hydrodynamic. Hence, only the flow conditions that lead to a direct transition from strat ified flow to slug flow (i.e. not via roll–waves) may lead to long slugs. For the prediction of the transition to slug flow or roll–waves a theoretical model was developed. The model tracks the displacements of the crest of a long wavelength wave in axial and radial directions. If the wave crest reaches the top of the pipe a slug is formed, whereas if it approaches the downstream end of the wave a roll–wave is produced. Besides to the predictive tool provided, the model sheds some light on the stage prior to forming a slug. The third part of the research considers the effect of the operation pressure on the slug length, and the effect of the liquid excess between the slug front and tail at the formation time. Measurements by Kristiansen (2004) for two–phase air–oil and SF6 gas–oilwere investigated. The measurements were carried out at P = 1–8 barA with high density SF6 gas simulating a pressure up to 65 bar. Three different types of slugs were categorized based on the liquid excess. Slugs with larger liquid excess at formation can grow to become longer. Even a small difference in the liquid excess may lead to a large difference in the slug length. However, at high operating pressures there is no liquid excess and only hydrodynamic slugs are observed. In the final part of the research we investigated and derived the slug frequency by the frequency of vortices due to turbulence in the gas and liquid. We found that the slug frequency and the frequency of oscillation at the interface behave similarly to the frequency of oscillations in the gas phase. However, the intensity of the oscillation at the interface is dominated by the liquid phase. The proposed mechanism for the formation of slugs covers a large range of pipe diameters and flow conditions. Moreover, it reveals the significance of the small–scale initial turbulence on the final development of the large–scale slug flow.Multi-Scale PhysicsApplied Science
Long liquid slugs in horizontal tubes: Development study and characterisation with electrical conductance techniques
No full textApplied Science
On the reconstruction of given objects observed by electrical capacitance tomography
No full textElectrical capacitance tomography (ECT) is a non-intrusive measurement technique for estimating the spatial distribution of different phases in a cross-section. Between twelve electrodes, positioned around circumference of the cross-section, an electric field is created, resulting in 66 capacitances between the electrodes. The 66 capacitances between the electrodes are dependent on the composition of the material within the cross-section, because the electric permittivity of the materials used differs. The aim of this research is to contribute to the further development of ECT. The reconstruction technique is optimised for the measurement system used. It is an iterative technique based on linear back projection. The reconstruction technique is tested by using given objects, that are put into the sensor. Simulations are performed and reconstructions are made. The position of the reconstructed point of gravity is in good agreement with the input. It is possible to estimate accurately the size and permittivity of the reconstructed object for objects with a size 1/3 of the diameter of the cross-section. For smaller objects, however, the results are not as good. Another topic studied is the "soft-field" effect. This means that the electric field is very sensitive to perturbations. This effect is also present in the third dimension and implies that an object is already measured, before it enters the domain between the measurement electrodes. By performing tests with long rods that are inserted into the measurement system, the measurement volume is found to be much larger than the height of the measurement electrodes. In order to quantify this effect, capacitance measurements with rods at different heights in the sensor are performed. Finally, the reconstruction of a single bubble in a fluidised bed with a diameter of 10cm is investigated. Although the bubble is detected, it cannot be reconstructed by using the reconstruction method, because the size of the bubble is small in comparison with the height of the sensors.Kramers Laboratorium voor Fysische TechnologieApplied Science
Bulk Dynamics of Droplets in Liquid-Liquid Axial Cyclones
No full textSeparation of oil and water is an essential step in the treatment of the production streams from fossil oil wells. Settling by gravity is a robust though voluminous process and therewith expensive method at remote locations, leading to a need for smaller separation equipment. In this thesis, we describe the research performed on the development of an inline axial cyclone for oil/water separation. This work is part of ISPT project OG-00-004 and has an experimental nature: a flowrig has been constructed to test different cyclones at flow rates up to 60 m3/h in a 10 cm diameter tube in which brine and low-viscosity lubricant oil can be mixed in almost any proportion. Results are compared with numerical datasets resulting from the same ISPT project. Three different swirl elements have been developed for this project: a strong swirl element and a weak swirl element with 10 cm diameter, and one element with a 26 cm diameter in combination with a tapered tube section. For all three swirl elements, the velocity profile of water has been measured with Laser Doppler Anemometry (LDA). The strong swirl element has a swirl number of 3.7, the weak of 2.3 and the large diameter element of 3.9. The axial velocity profile normalized with the bulk velocity shows vortex breakdown (upstream flow in the center), where the severeness of the breakdown normalized with the upstream bulk velocity shows proportionality with the swirl number. For the azimuthal velocity, the velocity profile was proportional to the bulk velocity. The non-dimensional azimuthal velocity was similar for all three swirl elements in the region |r/D| < 0.2. Outside that region the relative velocity is strongly influenced by the swirl element. Time series obtained with single phase LDA studies were used to estimate the effect of turbulent dispersion on droplet trajectories. A simplified equation of motion based on centrifugal buoyancy, drag and turbulent dispersion was solved for many fictitious droplet paths. The measured, chaotic axial velocity time series was used to mimic the radial component of the velocity fluctuations. With this model, we can predict the smallest droplet size that can be separated with a certain cyclone and the largest droplet size before it is broken by the flow. Model results show good agreement with overall bulk data obtained in the experimental flow rig. With an intrusive endoscope technique, we measured the droplet size distribution at various positions in the axial cyclone. From this, Hinze’s theory for the droplet size in turbulent pipe flow is confirmed. Furthermore, the inverse correlation between azimuthal velocity and median droplet size is shown and quantified: a lower velocity allows larger droplets to survive. Different designs were tested to understand which parameters have a large influence on the industrially relevant parameter of separation performance. This question is answered by variation of the swirl element, swirl tube length, pickup tube diameter, flow rate and droplet size. Changes that affect the droplet size have a severe effect on separation, these are the swirl element and flow rate. Changes that increase the droplet size lead to better phase separation. The other geometrical changes can be used to optimize performance, but are not identified as parameters leading to breakthrough improvements. Two non-dimensional numbers can be used to explain the behavior of the cyclone: the Weber number (We) based on the droplet size upstream of the swirl element and the maximum velocity obtained in the gaps of the swirl element, and the Reynolds number (Re_?) for the droplets downstream of the swirl element based on their median diameter and the azimuthal liquid velocity. Separation is better for a smaller We number, because droplets are less vulnerable for breakup under that condition. A large Re? number is beneficial since the droplets then experience a large centrifugal acceleration which is larger than turbulent dispersion. Both trends are confirmed with experimental data obtained in this project. We propose that there is a function for the maximum possible separation efficiency based on both non-dimensional numbers. The inverse coupling between We and Re_? via the azimuthal velocity makes optimization of separation efficiency difficult. Application of a large diameter swirl element (low velocity and therefore limited droplet breakup) in combination with a gradual tapering of the tube (increasing the azimuthal velocity) is a possibility to obtain both a large We and Re? number. Another option is to place multiple axial cyclones in series, with a stepwise increase of the swirl strength in each subsequent cyclone. In such a configuration, each step is capable of separating smaller droplets than the previous step, without immediate breakup of large droplets. This method should increase the overall quality of the phase separation.Chemical EngineeringApplied Science
Two-fluid simulation of an airlift loop reactor with fluent
No full textAirlift loop reactors are frequently applied in the chemical and biological industry. They are made of two sections interconnected at the top and bottom, with an ungassed downcomer and a gassed riser. In this type of reactor the density-difference between the sections supplies the needed driving force needed for the liquid circulation. Numerical simulations are recognised as a primary tool for improving the performance of process equipment for scaling up of the airlift loop reactors. Three dimensional simulations of two phase (gas and hquid) bubbly flow in a rectangular airlift loop reactor with two downcomer sections have been obtained using the CFD package Fluent 4.5.6. The simulations are based on a full two fluid model with a modified k - ? model for the turbulence. As the interfacial forces between the two phases, the drag force and virtual mass have been taken into account. The results are compared with an one-dimensional mechanical energy balance and are found to be in good agreement when a 'false' time step of 1 \u95 10^-2 s is used. The mean riser and downcomer gas fractions are too high in the simulations with time step of 1 \u95 10^-3 s, in comparison with this one-dimensional balance. Probably this is caused by the virtual mass. The interface turbulent momentum transfer terms (the turbulent diffusion terms), which can only be calculated for a dilute secondary phase are necessary for reahstic results. Outlets with a gas disengagement zone at the top can not be calculated well due to this limitation. An outlet type with fixed velocities on top has to be chosen, which works well but misses the flexibility for the gas throughput and velocities. After refining the grid the flow fleld changes and gives better results, but the calculation time increases. The minimal grid size has to be 30 x 10 x 80 (width x depth X height). Furthermore, two symmetry axis can used namely, one vertical plane through the riser and one vertical between the front and back wall through the riser and downcomer. With the use of both symmetry axis the calculation time is reduced signiflcantly.Kramers Laboratorium voor Fysische TechnologieApplied Science
Globale en lokale hydrodynamica van een air-lift-loop
No full textAir-lift-loops zijn reactoren die veel gebruikt worden in de biotechnologie. De reactoren worden gebruikt om reacties met zuurstof te laten plaatsvinden. De meest belangrijke eigenschap van een dergelijke reactor, is het circulerende karakter op gang gebracht en gehouden door de gastoevoer. Als de stroming eenmaal ingesteld is, zijn de versnellingen verdwenen en blijven dichtheidsverschillen en energiedissipatie in de vorm van frictie over. Met de overgebleven energiedissipatie en dichtheidsverchillen in een mechanische energiebalans en een empirisch model van Richardson & Zaki kunnen de globale grootheden, namelijk de gemiddelde snelheid en gasfractie, in de gehele opstelling goed in kaart worden gebracht, waarbij de enige in te stellen parameter, het gasdebiet, een belangrijke rol speelt. De lokale grootheden in de opstelling, zoals de radiële profielen van gasfractie en belsnelheid, zijn bepaald met glasvezelprobes. Dergelijke probes bestaan uit vier glazen tips, die op grond van breking onderscheid kunnen maken tussen gas en fluïdum, zijn in staat, naast het determineren van gasfractie, ook snelheden en helgroottes waar te nemen. De snelheden en gasfractie zijn langs een lijn in de kolom en op twee hoogtes bepaald, namelijk vlakbij en ver van de begasser vandaan. De begasser van de opstelling bepaalt de mate van uniformiteit in de gasverdeling in de opstelling. Niet-uniformiteit is onmiddellijk terug te zien in de profielen vlakbij de begasser, terwijl de verstoringen verdwijnen in hoger gelegen delen, hoewel daar de verdeling dan niet symmetrisch wordt. Lokale variaties in begassing veroorzaken extra interne vloeistofcirculaties, maar de globale grootheden, bepaald uit gemeten gemiddelde vloeistof snelheid, hebben hier geen last van, zodat voor de globale metingen van een ééndimensionale stroming kan worden uitgegaan. Ook de vloeistofcirculatie in het hoger gelegen gedeelte, heeft geen invloed op de globale metingen. De gemiddelden van de lokale grootheden zijn in overeenstemming met de globale grootheden, mits de lokale grootheden op voldoende plaatsen bepaald kunnen worden. Vanwege de grotere stabiliteit in het hoger gelegen deel van de opstelling is het gemiddelde van de lokale grootheden in overeenstemming met de globale metingen. Dit geldt minder voor de gemiddelde lokale grootheden in het gedeelte van de opstelling vlak bij de begassing.Kramers Laboratorium voor Fysische TechnologieApplied Science
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