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the permittivity of the measured material, image reconstruction for ECT is complicated. However, ECT offers some advantages over othertomography modalities, such as no radiation, rapid response, low cost, being non-intrusive and non-invasive, and the ability to withstand high

temperature and high pressure. In principle, ECT can deal with the complexity of MPF measurement by explicitly deriving the componentdistributions at two adjacent planes along a pipeline. Images of the component distributions can be cross-correlated to obtain the velocityprofile of the flow. Multiplying the component concentration and velocity profiles yields a measure of volumetric flow rate for each phaseaccurately. This paper covers the development of ECT for MPF metering and oil separator in the oil industry. The principal strategies andtechnologies that may be used to measure three-phase flows will be discussed, and the status of currently available tomography solutions willbe reviewed. 2005 Elsevier Ltd. All rights reserved.Keywords: Multi-phase flow meter; Electrical capacitance tomography; Cross correlation; Oil separator

1. Introduction

It is important to measure the fluids produced fromoil wells accurately for efficient oil exploitation andproduction [1]. Typically, field wells produce a complexmixture of gas, oil, water and other components, suchas sand, and it is difficult to measure the multi-phaseflows (MPFs). The conventional approach is to separatethe mixture into individual components, and then measurethose separately using single-phase flow (SPF) meters, e.g.,orifice plates for gas and turbine meters for oil. There aresome problems with the required three-phase separators:(1) their bulk, (2) high installation cost and (3) considerablemaintenance. Therefore, it is highly attractive to have

separation. A recent review article written by Falconereported some new developments in MPF metering [50].

During the last decade, considerable efforts have beenmade to develop MPF meters. For example, under theUK National Flow Programme, the National EngineeringLaboratory (NEL) has assessed the performance ofvarious MPF meters under different flow (in particulargasoilwater flow) conditions [51,52]. Currently, thereare several commercially available meters, based ondifferent measurement principles [2,3]. However, all ofthem have some limitations. Almost all of them are flow-regime dependent and most of them can only deal withhomogeneous flows in order to achieve an acceptableaccuracy. Flow-mixing devices are often used to palliate thisTomography for multi-phase floI. Ismaila,, J.C. Gamiob,

aSchool of Electrical and Electronics Engineering, The UnbInstituto Mexicano del Petroleo, Eje Central L

Received 7 October 2004; received in revised

Abstract

Electrical capacitance tomography (ECT) is regarded as a succemulti-phase flows (MPFs). Because of the soft-field nature of ECrelatively simple MPF meters, which are capable ofmeasuring the flow rate of each component directly, without

Corresponding author.E-mail addresses: idrisismail99@yahoo.com (I. Ismail),

jgamio@imp.mx (J.C. Gamio), w.yang@manchester.ac.uk (W.Q. Yang).

0955-5986/$ - see front matter 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.flowmeasinst.2005.02.017ion 16 (2005) 145155www.elsevier.com/locate/flowmeasinst

measurement in the oil industry.A. Bukharia, W.Q. Yangaity of Manchester, PO Box 88, Manchester, M60 1QD, UKenas Nte #152, Mexico, D.F., CP07730, Mexico

26 January 2005; accepted 14 February 2005

l method for visualising cross-sectional distribution and measuringd the non-linear relationship between electrical measurements andproblem because in most cases MPFs are inhomogeneous,especially in horizontal or inclined pipes. A problemwith these devices is that they interfere with the flow,causing pressure drops, which are ultimately reflected as anincrease in the required pumping power. Some devices haveinternal moving parts, which reduce reliability and increase

t and

(a) Conventional MPF metering using separators [43].

maintenance coremote locationIt will also preneeds to be pigdesigned for uphomogenisationin the case ofof homogenisatconcentrate in thpattern.

In the past,of taking MPseparators. Thito be measuredSPF meters. Inimeasuring the lof densitometerfor the produc

phase separators

were developedal viscosity andr high viscosity.and resulted inoverall oilfield

s were developedoilfields in the

earch into MPFs were installedpanies primarily

PF meters hadd developments.o approaches toentional method

ent [43]. It has(b) On-line MPF metering [43].

Fig. 1. MPF metering.

sts, in particular if the meter is used in asuch as the seabed or an unmanned platform.sent operational difficulties if the pipelineged. Others (e.g., some Venturi meters) areward vertical flows and take advantage of the

occurring naturally in the flows. However,high gas fractions (above 80%) the degreeion is limited, because gas bubbles tend toe centre of the pipe, forming an annular flow

oil companies tried to avoid the problemsF measurements by using gravity-baseds allows the liquid and gas components

individually using proven gas and liquidtially, two-phase meters were developed foriquid components together by a combination

separators offers an advantage over three-in terms of cost and size.

From 1940s to 1970s turbine metersand used for measuring crude oil of normorifice and positive displacement meters foImprovements were manufacturer-drivenonly a marginal impact of the meters ondevelopment. Coriolis and ultrasonic meterin the early 1980s and were installed inearly 1990s. In the same time period, resmeters was carried out and MPF meterin oilfields. During this period, oil comdrove the research initiatives, and the Msubstantial impact on the overall oilfielFig. 1 shows typical arrangements of twmeasuring gasoilwater flows: the convusing test separators and on-line measurem146 I. Ismail et al. / Flow Measuremenand Venturi meter. This method is suitabletion fields, where the use of two-phaseInstrumentation 16 (2005) 145155recently been reported that installation of MPF meters in theworld has risen from 200 in 1998 to about 1000 in 2002 [5].

d Ins

cut from these measurements. Cross-correlation techniqueI. Ismail et al. / Flow Measurement an

True three-phase meters are designed to measure fullwell streams of gas, oil and water without any separation.They offer significant saving in cost to off-shore operators(especially in the North Sea), where the economic effectof deep water production makes standard installations oftest separators, return pipelines and platforms uneconomical,to the point that reservoirs would not be produced withoutsome less costly alternative method of measurement.The initial development was aimed at sub-sea wellheadcompletion with metering to handle any percentage of gas,oil and water from 0% to 100%, and any combination inpercentage of the three components. However, while theoverall economics of the production areas has a priority, theuse of MPF meters has been expanded to the measurementsin the range of 5%10% accuracy.

2. Overview of current MPF meters

One of the main problems faced by currently availableMPF meters is that they are flow-regime dependent [3,4,6]. Fig. 2 shows three cases of gasoil flows with an oilvolume fraction of 25% in all cases but with three differentflow regimes: homogeneous, stratified and annular. Thereadings in Fig. 2 would be obtained by a conventionalvolume fraction meter (like those used in commercial MPFmeters) which had been calibrated for homogeneous flows.Thus if the flow regime changes, considerable errors willoccur because the sensing path is localised. Therefore,they can only be used with a rather limited range of flowpatterns (preferably homogeneous or quasi-homogeneousflow). Flow-regime independence is of particular importancefor down-hole flow measurement in inclined, horizontal ormultilateral wells (which are increasingly common), becausethe flow regimes tend to be stratified or other types thatare particularly difficult to measure with the current MPFmeters [4].

In general, commercial MPF meters can be categorisedinto two types:

(1) Measurement by separation techniques. Due to thedifficulty in measuring three-phase components directly,separation techniques are used to segregate gas, oil andwater, and then each stream is measured separately.Further development has introduced partial separation,which typically only separates liquid and gas, toimprove the accuracy for high gas volume fraction andto reduce the size of MPF meters. An example of anMPF meter using partial flow separation is the AgarcorpMPFM 400 (see Fig. 3(a)).

(2) On-line measurement. The new generation of MPFmeters use direct measurement to reduce the expensivespace requirements in an offshore oil platform. They arecompact and have non-intrusive sensors. An example ofthe desirable location of these MPF meters is shownin Fig. 1(b). The ESMER MFM is an example of an

MPF meter without using partial flow separation (seetrumentation 16 (2005) 145155 147

(a) Homogeneous(measured concentration=25%).

(b) Stratified (measuredconcentration=30%).

(c) Annular (measuredconcentration=13.4%).

Fig. 2. Effect of flow regimes on measurement (real concentration = 25%).

Fig. 3(b)). The latest generation has the flexibility forsub-sea installation. Some systems use homogenisers orflow conditioners to ensure a homogenous regime, thusremoving the problem of flow regime dependency.

In terms of the technology employed, commercial MPFmeters vary. Table 1 lists typical techniques used. Most ofthe meters use combinations of component fraction andcomponent velocity measurement techniques. Componentfraction measurement methods can be categorised into twogroups: radioactive attenuation (e.g., -ray) and impedance-based. Methods of determining component velocities canbe grouped into cross-correlation techniques and Venturi-based measurement. Some of the meters adopt signalprocessing measurements techniques to improve the overallperformance. For example, as shown in Table 1, the ESMERmeter of Fig. 3(b) uses a neural network approach tointerpret signals from capacitance/conductance and pressuresensors.

Examples of MPF meters with and without homogenizerare shown in Fig. 4. Fig. 4(a) shows an MPF meterfrom JiskootMixmeter with a static homogenizer. Thehomogenizer is designed to generate differential pressurefor total volume flow rate calculation. A dual-energy -raydensitometer is used to derive the phase fractions of gas andliquid. Fig. 4(b) shows a Roxar-Fluenta (Norway) meterMPFM 1900VI without homogenizer. It uses a combinationof capacitance, inductance, -ray, and Venturi transducers tomeasure bulk electrical properties of the flowing mixture inoil and water continuous flow respectively, and derive waterand Venturi meter are used to measure phase velocity.

t an

rcial3

[9

a new type of MPF meter, which is conceptually simpleand inherently insensitive to variations in the flow regime.(a) With partial flow separation [8].

(b) Without flow separation [9].

Fig. 3. Examples of MPF meters handling various fluid phases.

a. 1. Accuflow (AF series), USA. 2. Agar (MPFM-400),USA. 3. ESMER (T series), UK. 4. Flowsys (Topflow),Norway. 5. Framo (PhaseWatcher Vx), Norway. 6.

Tomography can also be used as a tool to determine theflow regime in order to compensate the non-linearity ofcurrently available MPF meters (caused by their flow-regimedependency).

3. Industrial process tomography

3.1. Concept

The term industrial process tomography refers to awhole range of non-invasive visualization techniques, whichis relatively new (since the late 1980s) and still developing.The aim of industrial process tomography is to obtaincross-sectional images of dynamic industrial processes [6,18]. Tomography techniques provide a novel means ofvisualising the internal behaviour of industrial processes.The cross-sectional images produced by tomographysystems provide valuable information on the process, whichcan be used for visualisation, monitoring, mathematicalmodel verification and possibly for intelligent control. Thereare many types of tomography systems, such as electrical,148 I. Ismail et al. / Flow Measuremen

Table 1Commercial three-phase flow metering systems

MFM comme1 2

Component fraction measurement methodSingle energy -ray absorptionDual energy -ray absorptionImpedance (capacitance, conductance and/or resistance) Component velocity measurement methodCross-correlationVenturi

Other measurement methodsNeural Networks and Signal ProcessingPD flow metermixture volumetric flow rate -ray densitometermixture densitySingle-phase gas meter Single or dual phase liquid meter

Partial or full flow separation required Homogenized flow requiredSub-sea applicationReference [7] [8]

Key reference for commercial meters.Haimo, China. 7. ISA (Dualstream II), UK. 8. Jiskoot(Mixmeter), Netherlands. 9. Kvaerner (DUET), Norway.d Instrumentation 16 (2005) 145155

meters4 5 6 7 8 9 10 11

] [10] [11] [12] [13] [14] [15] [16] [17]

10. Roxar (Fluenta MPFM 1900VI), Norway. 11. TEA(LYRA), Italy.

b. Some of the commercial meters, such as CSIRO, Daniel(MEGRA), MFI and Fluenta, are not listed as they havebeen acquired by other companies.

In contrast, the use of tomography (providing an imageof the whole flow) allows the possibility of developingultrasonic, radiation, nuclear magnetic resonance (NMR),microwave and optical.

d InI. Ismail et al. / Flow Measurement an

(a) JiskootMixmeter with homoge-nizer [14].

(b) Roxar-Fluenta meterMPFM 1900VI, without homogenizer [16].

Fig. 4. Examples of MPF meters handling variation in flow regimes.

Fig. 5. Tomographic MPF measurement.

In order to explain the tomographic method for MPFmeasurement, we suppose that the cross-section of a pipeis divided into N elements of equal area a. Then, theinstantaneous volume flow of phase x is given by

Qx = aN

i=1fi(x)vi (1)

where fi(x) and vi are the concentration (or volume fraction)of phase x and the flow velocity, respectively, in the i -thelement.

In a tomographic flow meter as shown in Fig. 5, twoseries of images of the flow are obtained simultaneouslyin two contiguous cross-sections of the pipe. Eq. (1) canbe applied after the volume fraction distributions fi(x) aredetermined directly by the tomographic images and thevelocity profile vi by processing the two series of imagesusing cross-correlation techniques.

Because the tomographic methods are still a developingtechnology, some challenges still need to be addressed: Improvement of sensor spatial resolution of measurementbetter than 5%.strumentation 16 (2005) 145155 149

Development of more accurate image reconstructionmethods (obviously, inaccurate images will result ininaccurate volume flow estimations).

Improvement of data processing efficiency by using high-performance computing (accurate image reconstructionand the subsequent cross-correlation process are bothcomputation intensive).

Design of mechanical and electronic hardware suitablefor safe and reliable use in harsh industrial environments(not just in the laboratory).

3.2. Tomography sensors

There are many types of tomography sensors, includingionising radiation (e.g., x-ray and -ray), optical, positronemission (PET), nuclear magnetic resonance (NMR),acoustic (including ultrasound), electrical (i.e., capacitive,conductive and inductive) and microwave. Each of thesetechniques has its own advantages and disadvantages.Therefore, the choice depends on the subject underinvestigation.

Several tomographic techniques involving the measure-ment of electrical properties have received significant at-tention: electrical capacitance tomography (ECT), electro-magnetic tomography (EMT) and electrical resistance to-mography (ERT). The main disadvantage of electrical tech-niques is their moderate spatial resolution of the resultantimage, because unlike x-rays, electric fields cannot be con-fined to a direct narrow path between a transmitter and areceiver.

3.2.1. ECTECT has been developed for imaging industrial processes

containing dielectric materials. It is based on measuring thechanges in capacitance that are caused by the change in di-electric material distribution. The capacitance measurementsare taken from a multi-electrode sensor (typically 8 or 12;see Fig. 6(a)) surrounding an industrial process vessel orpipeline. The cross-sectional distribution of permittivity isreconstructed from these capacitance measurements math-ematically using some algorithm. Most ECT systems em-ploy a dedicated design of capacitance measuring circuit,such as the charge/discharge circuit [19] and the AC-basedcircuit [20,21], which have been used successfully to im-age two-component flows, e.g., gas/oil flows in oil pipelines,gas/solids flows in pneumatic conveyors and gas/solids dis-tribution in fluidised beds [2224]. ECT is regarded as soft-field tomography and requires complicated image recon-struction due to the non-linear relationship between the mea-surements and the permittivity distribution. Compared withother tomography modalities, ECT offers some advantages,such as no radiation, rapid response, relatively low cost, be-ing non-intrusive and non-invasive, and withstanding hightemperature and pressure [31].

If the electrical field inside the measurement plane does

not enclose free electrical charge, the relationship between

an150 I. Ismail et al. / Flow Measurement

(a) 8-electrode ECT sensor.

(b) ERT sensor.

(c) EMT sensor.

Fig. 6. ECT, ERT and EMT sensors.

the capacitance and the permittivity distribution is governedby [25] [(x, y)(x, y)] = 0 (2)C = Q

V= 1

V

(x, y)(x, y)d (3)where is the gradient operator, (x, y) is the permittivitydistribution in the sensing field, (x, y) is the electricalpotential distribution, V is the potential difference betweentwo electrodes forming the capacitance and is theelectrode surface. The boundary conditions when oneelectrode is excited with a fixed voltage Vo and all otherelectrodes are kept at zero potential, as occurs during themeasurement procedure, are defined by

= Vo (for the excited electrode) and = 0 (for others). (4)

3.2.2. ERTERT is used to image mixtures where the continuous

phase is conductive [6,18] and the dispersed phase insulatingor conducting to a lesser degree [26]. In this case, the

electrodes are mounted flush with the inside surface of apipe (or vessel) wall and directly in contact with the fluids.d Instrumentation 16 (2005) 145155

Alternatively, a conductive ring (also in contact with thefluids) can be used. A number of different excitation currentpatterns are applied and the resultant voltages are measured.They are then used to construct a conductivity distributioninside the sensor, which reflects the physical distributionof the mixture components. It has been used for imaginghydro-cyclones [27], mixing processes [28] and MPF [29].The operation of ERT systems is basically the same as ECTsystems except that a high-impedance measurement front-end is needed for conductive loads. Thus ERT uses currentinjection techniques with voltage measurement circuits, asshown in Fig. 6(b).

Similarly to ECT, the conductive ring sensor of ERTshown in Fig. 6(b) can be treated as an electrostatic fieldproblem and can be characterized by [30] [(x, y)(x, y)] = 0 (5)where is the gradient operator, (x, y) is the conductivitydistribution in the sensing field and (x, y) is the electricalpotential distribution. The boundary conditions are given by1

nds = I,

2

nds = I, (6)

/n = const (n = 1, 2, . . . , N) (7)

n

= 0 (8)

where is the potential distributions in response to thepresence of currents I , 1 and 2 are the electricalcontact domain for current injecting presenting Neumannconditions, n are electrical contact domains and represents other domains on the external boundary of theconductive ring.

3.2.3. EMTEMT, as shown in Fig. 6(c), is a technique offering a

number of advantages, such as flexibility in sensor designand no contact with the sensing zone. An EMT sensorconsists of a set of excitation coils, which produce amagnetic field within a cross section of a pipe. A set ofdetection coils is used to detect the changes in the field due tochanges in permeability and conductivity inside the vessel.To achieve a high sensitivity, a high excitation frequency isneeded. So far, the image resolution of EMT is poor [32,33].EMT sensors are governed by [33] E = jH (9) H = 0 (10) H = ( + j)E (11)where is the gradient operator, E is the electric fieldstrength, H is the magnetic field strength, is the appliedangular frequency, is the magnetic permeability, is theelectrical conductivity and is the permittivity distributionin the sensing field. The induced voltage, V is given by

j

V =

I coilA J dv (12)

d InI. Ismail et al. / Flow Measurement an

where I is the total current through the coil, A is themagnetic vector potential and J is the current density.

In addition to ECT, ERT and EMT, there areother emerging techniques, such as electrical impedancetomography (EIT), which measures both real and imaginaryparts of impedance, and multi-modality tomography. Amulti-modality system makes use of two or more differentsensing entities to locate or measure different constituentsin the object space. In order to employ tomographyfor MPF measurement, tomography systems capable ofacquiring and processing images with sufficient accuracyand speed (hundreds of frames per second) must bedeveloped. Similarly, due to the need for distinguishingthree components, dual-modality tomography systems arerequired, which can measure two distinct physical propertiesof the fluid. For example, for fluids whose continuous phaseis an electrical insulator, a combination of capacitance and -ray tomography could be used. When the continuousphase is conducting, resistance and -ray tomographycould be employed. Other options include the combinationof capacitance and resistance, and the application ofelectrical impedance spectroscopy (i.e., the use of differentfrequencies).

3.3. Data acquisition and data processing for ECT

During a measurement period, each electrode in an ECTsensor is energised in turn by applying an excitation voltagesignal, and the induced charge/current is detected from allthe other electrodes while their electric potential is keptat zero. Take an 8-electrode sensor as an example. First,electrode 1 is used as the excitation electrode and electrodes28 as the detection electrodes. Next, electrode 2 is used asthe excitation electrode and electrodes 38 as the detectionelectrodes, and so on, up to electrode 7 as the excitationelectrode and electrode 8 as the detection electrode. Foran ECT sensor with N electrodes, there are N(N 1)/2electrode-pair combinations, i.e., N(N 1)/2 independentcapacitance measurements for an image.

There are two approaches to image reconstruction:single-step calculation, e.g., linear back projection (LBP),and iterative processing. The LBP algorithm (also called thesensitivity coefficient method) is the most popular image re-construction for electrical tomography. It relies on the sen-sitivity maps, which can be obtained in advance using finiteelement analysis. An image is obtained by superimposing allcapacitance measurements together using the sensitivity co-efficients as weighting factors. This algorithm is simple andfast, but offers qualitative images only.

To enhance the image quality, various iterative imagereconstruction algorithms have been developed. The basicprinciple is described briefly below. An initial estimate isobtained by a simple algorithm, e.g., LBP. The capacitanceis estimated from the current image and then comparedwith the real measurements. The difference between them

is used to modify the image so that the differencesstrumentation 16 (2005) 145155 151

decrease. When the differences are sufficiently small, theimage is supposed to be the true representation of materialdistribution. This is, in some ways, similar to a feedbackcontrol system. In choosing a reconstruction algorithm,the main considerations are expense, speed and accuracy.Iterative algorithms can increase the accuracy, but theyreduce the speed and cost more. For fast on-line imaging,the LBP algorithm is probably the best if only a reasonableaccuracy is required. When high accuracy is needed, morecomplicated algorithms are preferable [34]. Iterative imagereconstruction is time consuming because the estimation ofcapacitance from an image usually involves finite elementanalysis. Recently a new iterative algorithm has beendeveloped at The University of Manchester. Instead of usingfinite element analysis, a linear forward projection method isused to estimate capacitance, which is much faster, but lessaccurate than finite element methods. This method has beentested experimentally and shown promising results.

4. Current status

4.1. At the University of Manchester

The University of Manchester (formerly known asUMIST) has been leading the research area in industrialprocess tomography since the late 1980s. Until 1996,research had focused on electrical tomography, especiallyECT. In 1991 the first real-time ECT system wasdeveloped in collaboration with the University of Leeds andSchlumberger Cambridge Research Ltd. The system wasused successfully to generate images of gasoil flows in oilpipelines and was also used for other applications [3537]. Afollow-up project explored gasoilwater three-componentflow measurement by a combination of ECT, ultrasonictomography and cross-correlation techniques (see Fig. 7).Further discussions on this work are beyond the scope ofthis paper as we are not in the position to provide furthercomments.

The group has been able to produce new circuitsand techniques, aiming to measure change in capacitancedown to 30 aF (aF = 1018 F), and novel ECTsystems, which have been demonstrated on a widerange of challenging investigations. Recently, a new ECTsystem has been developed, which is based on high-frequency (up to 1 MHz in comparison with 10 kHznormally used for AC measurement) sine-wave excitationand phase-sensitive demodulation [39]. This design hasbeen used to image circulating fluidised beds, pneumaticconveyors and gas/water droplet flows. Compared with thecharge/discharge ECT system, the AC-based system hasseveral advantages:

Both capacitance and loss-conductance measurements,providing the possibility of a dual-modality system.

Improved performance in terms of signal-to-noise ratio

(SNR) and data acquisition rates.

t and Instrumentation 16 (2005) 145155152 I. Ismail et al. / Flow Measuremen

(a) Test flow loop with tomography sensors.

(b) Block diagram of the multi-modality tomography system.

Fig. 7. Test rig for imaging gas/oil/water flow.

Performing frequency sweeping to implement spec-troscopy.

Flexibility in excitation electrode combination to gener-ate the optimal sensing field.

An impedance analyser based ECT system has been de-veloped to quantify low concentration MPF in wet gas sepa-ration processes. It comprises a multi-electrode capacitancesensor, a purpose-built multiplexer, an impedance analyser(HP4284A) and a host PC, as shown in Fig. 8 [40]. Theimpedance analyser based system hardware provides highaccuracy (0.05%) and high resolution (1017 F). The sen-sor was calibrated in an environmental chamber with solidsamples of known permittivity over a range of temperatureand humidity. The results of tests carried out over a rangeof operating conditions (20%95% humidity) demonstratedthat the ECT system is able to reconstruct clear images ofthe liquid droplets distribution inside the separator.

The group has also built an oil separator test rig todevelop separator control and monitoring strategy based onECT. Because an oil separator may contain many differentmaterials, such as gas, foam, oil, emulsion, water and sand,it is difficult to measure the levels of the interfaces. Althoughsome multi-interface level sensors have been developed formeasuring the interfaces in oil separators, there are someproblems with radiation, intrusive and invasive, difficult toclean, high maintenance request, etc. A knowledge-basedtechnique is considered for generating control signals for

different conditions [38]. Fig. 9 shows the test rig and designdiagram.Fig. 8. Impedance analyser based ECT system.

(a) Test rig with horizontal separator and verticalmixer.

(b) Block diagram of the test rig operation.Fig. 9. Test rig for gas/oil/water separator.

d In

are

T f

atioof m

lineog

se fl

two

rinctwo

men

have been developed based on heuristic optimisationpumping power, and extending the field of applicationof MPF measurement into new areas, for example sea-techniques, such as genetic algorithms and simulatedannealing [41]. Compared with other methods, this approachoffers considerable improvement in the reconstruction ofquantitative images. However, it is slow because a verylarge number of iterations are required. Another area ofinvestigation at IMP is sensor design. A high-pressure(1400 psi) sensor capable of operating with actual gasoilmixtures on industrial pressurised pipelines has beendeveloped and tested successfully on a 3 in. MPF testloop [42] (see Fig. 10).

4.3. At other institutions

Some other research activities in electrical tomographyfor the oil industry are listed in Table 2. Some used multi-

floor installations and down-hole measurement in horizontal,inclined, and multi-lateral wells. The advantages of ECTare that the sensing systems are non-intrusive and non-invasive, flow-regime independent, robust and need minimalmaintenance.

Tomographic methods could be applied to MPFmeasurement in two ways [4]: (1) as an instrument todetermine the flow regime in order to correct or compensatethe readings from currently available MPF meters, whichare flow-regime dependent, and (2) as a radically new flow-regime-independent method of MPF measurement in its ownright, without having to resort to any other principles.

ECT can potentially be used within a dual-modalitysystem for simultaneously measuring the volume flow rateof oil, water and gas in oil well flows. Its capability of flowI. Ismail et al. / Flow Measurement an

Table 2Multi-phase flow measurement systems at other institutions

Institution Research

University of Huddersfield & University of Leeds, UK Using ER

NEL, UK Determinfraction

University of Bergen, Norway Oil pipe -ray tomMultipha -ray

Tianjin University, China ERT forTsinghua University, China Using p

measure

Zhejiang University, China Measure

Fig. 10. Industrial ECT sensor mounted in a pressurised flow loop at IMP.

4.2. At IMP

Mexican Petroleum Institute (IMP for its initials inSpanish) has been involved in ECT research since2000, with a focus on gasoil two-phase flow imagingand measurement. New image reconstruction methodsmodality by combining electrical with -ray tomography,expecting that electrical tomography can provide goodstrumentation 16 (2005) 145155 153

a References

or multiphase flow monitoring Ma et al. [44]n of flow patterns and voidultiphase flows using ECT

NEL report [45]

measurement using ECT andraphy

Johansen et al. [46]

ow regime identification using Tjugum et al. [53]

-phase flow and void fraction measurement Dong et al. [47]ipal component analysis to-phase flow concentration

Zou et al. [48]

t of two-phase flow using ECT and Venturi Xie et al. [49]

temporal resolution and -ray tomography can providegood spatial resolution. In the UK, the NEL has carriedout evaluation of ECT for real-time visualisation ofgasliquid distribution under the DTI Flow Programme(19992002). It was found that ECT was able to visualisegasliquid flows and to identify flow regimes. In Norway,ECT was used to visualise an oil separator of 1 m indiameter. Using ECT, it has been demonstrated that theinterface levels could be measured using a model-basediterative image reconstruction algorithm [34]. In China,several universities are reporting improvement in electricaltomography especially in image reconstruction.

5. Conclusions

ECT is a valuable tool in MPF measurement. It wouldsolve the problems with flow-regime dependency afflictingcurrently available MPF meters, eliminating the need formixing devices that have a negative effect on the requiredimaging would also be useful for process control becausethe cross-sectional image can give important additional

and154 I. Ismail et al. / Flow Measurement

information, such as phase distribution. Recent research hasdemonstrated that a highly sensitive ECT system can bedeveloped. The latest sensor is very compact and can easilybe modified for different measurement requirements. Theadditional radial inserts, which are grounded for reducingdirect coupling between adjacent electrodes, has resultedin an electrically stable system, giving confidence that themeasurements are reproducible.

Acknowledgements

Idris Ismail would like to take this opportunity to thankUniversiti Teknologi Petronas for supporting his PhD studyat UMIST and Carlos Gamio would like to thank IMP forsupporting the research project D00046.

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Tomography for multi-phase flow measurement in the oil industryIntroductionOverview of current MPF metersIndustrial process tomographyConceptTomography sensorsECTERTEMT

Data acquisition and data processing for ECT

Current statusAt the University of ManchesterAt IMPAt other institutions

ConclusionsAcknowledgementsReferences