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Abstract

Production optimization and reservoir managementrelyinpartonperiodicsurveillanceofproducedoil,gasandwater flowratesof individualgascondensatewells.Flowmeasurementatthewellheadhasproventobechallengingdue toanumberof factors.Conventional (gravity-based)TestSeparators(CTS)andin-lineMulti-PhaseFlowMeters(MPFM)areintrusiveandmaybecostlytomobilize,nottomentiontheassociatedHSErelatedrisks.Forfieldswhereaccess and logistics pose a challenge, the deployment ofeither CTS or MPFM packages may require a significantamount of time. In some cases, the required well testfrequency may not be reached with the availableequipment or due to the increased number of wells inproduction.

This paper describes a convenient and cost-effective

approach to production surveillance of gas condensatewells using the non-intrusive SONAR-based Surveillancesystem. The system integrates the clamp-on sonar flowmeter(SONAR)withaPVTandmultiphaseflowenginetocalculate the properties of the produced fluids, and theindividual phase flow rates. The SONAR is amultiphase-tolerantvolumetricflowdevicethatprovidesthebulkflowrateofthefluidstreamwithintheflowline.ThePVTenginecalculatestheindividualphasepropertiesoftheproducedfluids, at the pressure and temperature conditionsmeasuredwheretheSONARisclamped-on.Themeasuredflowrateistheninterpretedintermsofoil,gasandwaterratesatboththeactualandstandardconditions.

ExamplesoffieldperformanceofSONAR-basedsystem

forthree-phaseproductionsurveillanceofgascondensatewellsarepresented.ThetestresultsshowthattheSONAR-basedsystemrepresentsareliable,safeandcost-effectivesolution for recurring production surveillance wherereservoirconditionsarerelativelystableovertime.

Introduction

Monitoringproducedoil,gas,andwaterfromindividualwells plays an important role in reservoir management.Acquiringtimelyandaccuratewellheadmeasurementscanbechallengingduetoarangeoffactors.Usingtraditionalwelltestseparatorsprovidesonlyperiodicmeasurements

and may result in deferred production due to pressuredrop.

While measuring dry gas flow measurement is well-

servedbyawiderangeofgasflowmeteringtechnologies,accurateandcost-effectivemeasurementofwetgasflowremainsachallengefortheupstreamoilandgasindustry.The paper provides a method to provide minimallyintrusive, clamp-on production surveillance for wet gasflowmeasurement.

Exprodevelopedaclamp-onnon-intrusiveproduction

monitoring system, Total Production Surveillance (TPS),designed to provide rapid and cost-effective wellheadsurveillance forawide rangeofapplications, suchasgasandgascondensatewells.Expro’sclamp-onSONAR-based(Sonar) flow meters, PassiveSONAR™ or ActiveSONAR™,operate effectively in multiphase conditions typical formost oil and gas applications. Therefore, they can beinstalled close to the wellhead, allowing accurate flowmeasurementonanindividualwellbasiswithoutinstallinganinlinedeviceordivertingproduction.

SONAR-basedFlowMeasurementTechnology

History. Sonar flow measurement technology was

introduced to the oil and gas industry in 2000, usingSONAR-basedpassive-listeningtechniquestoprovideflowrate and compositional information for downholeapplications(fiber-opticmeter).

In 2004, a clamp-on version of a strain-based sonar

meter (PassiveSONAR™)was introducedwithpiezo-strainsensors,providingsimilarfunctionalityastheoriginalfiber-optic-strain sonar technology with reduced cost andcomplexity. PassiveSONAR™hasbeenappliedtoawiderangeof flowmeasurementapplications, including singleandmultiphasemixtures,fromgas/liquidmixturestohighsolidscontentslurries.

In2009,ExproMetersintroducedtheActiveSONAR™,

based on the pulsed-array sonar technology, usingexternally generated acoustic pulses to “illuminate” thevortical coherent structures. The pulsed-array sonartechnologyallows themeasurementof lower flowrates

UPM 17050

SONAR-based Wellhead Surveillance for Gas Condensate Fields Gabriel Dragnea, Expro Meters Inc., Siddesh Sridhar, Expro Meters Inc.


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thanstrain-basedsonarflowmeters.TheActiveSONAR™meteriswellsuitedforGasandGasCondensatewellsinheavyschedulepipes.ExproMetersutilizesbothtypesofClamp-on SONAR flow measurement technologies toaddressawiderangeofflowlineconditions.

SONAR-basedFlowMeasurement.Sonar-basedflow

measurementsutilizeanarrayofsensors,alignedaxiallyalong the pipe, to characterize the speed at whichnaturallyoccurring,coherentflowstructuresconvectpastthesensorarrayusingsonarprocessingtechniques.Sinceboth single andmultiphase flows typically exhibit thesecoherent structures, the methodology is suitable for awiderangeofapplications.

Figure1showsthenaturallyoccurring,self-generated,

coherent structures characteristic for turbulent flow ofNewtonianfluids.Theturbulenteddiesaresuperimposedover the time-averagedvelocityprofiles andare carriedalongwiththemeanflow.Theseeddiesremaincoherentfor several pipe diameters and convect at, or near, theaverageflowrateinthepipe.

Figure 1: Sonar-based Flow Meters with Coherent

StructuresSonar flow meters use the convection velocity of

coherentstructures(eddies)todeterminevolumetricflowrate.Thesonar-basedalgorithmsdeterminethespeedofthesestructuresbycharacterizingboththetemporalandspatial frequencycharacteristicsof the flow field. Foraseriesofcoherenteddiesconvectingpastafixedarrayofsensors, the temporal and spatial frequency content ofpressure fluctuations are related through a dispersionrelationship,expressedasfollows:

k

Vconvectw

= (1)

Where, Vconvect is the convection velocity of the

disturbance,ωthetemporalfrequency(rad/sec),kisthe

wave number (defined as k=2π/λ) and λ is the spatialwavelength.

In sonar array processing, the spatial/temporalfrequencycontentofsoundfieldsaredisplayedusing“k-ω" plots in which the power of the sound field isdecomposed into bins corresponding to specific spatialwavenumbersandtemporalfrequencies(Figure2).Onak-ωplot, thepowerassociatedwithcoherentstructuresconvecting with the flow is distributed along “theconvective ridge”. The slope of the convective ridgerepresents the speed of the turbulent eddies, which isthenconvertedtovolumetricflow.

Figure2:k-ωplotwithconvectiveridgeSonarWetGasOver-readingCorrelation.Whenusing

the Sonar forwet gas flowmeasurement, the reportedflow rate is higher than the actual flow rate of the gasphaseofthemixture,similartothedP-basedflowmeters(Venturi,orifice,etc.).ThemagnitudeoftheSonarover-reaadingistypicallylessthanthatofthedP-basedmeters.

The over-reading of a sonarmeter is defined as the

ratiobetweenthereported flowvelocity (Vsonar)andthegassuperficialvelocity(Vsg):

sg

sonar VsonarORV

= (2)

Thefollowingempiricalcorrelationwasdevelopedto

characterize the over reading of pulsed-array sonarmeters for wet gas streams flowing in fully developed,horizontalflowlines:

DK

Dw

Slope– 11.57fps


3

2

111 ÷÷

ø

öççè

æ

++÷÷ø

öççè

æ

++= mmsonar Fr

LVFFrLVFOR jb (3)

Where,LVF is the liquid volume fraction (LVF) and Fr

is the gas densimetric Froude number (Fr); β andϕ arecalibrationconstantsand

îíì

³<<-

=5.1 ,1

5.10.5 ,5.0

r

rr

FFF

m (4)

Total Production Surveillance System (TPS). The

system integrates the Sonar flowmeasurementwith anEquation of State (EoS) model for the Pressure,Temperature and Volumetric (PVT) properties of theproducedfluidsingascondensatewellsasshowninFigure3.

Figure 3: Schematic Total Production Surveillance

MethodologyWell bore composition is input by specifying

molecularcompositionofthewellborefluid.TheEoSPVTmodelcalculatesthepropertiesofthewellborefluidatthe location of the sonar meter using the measuredpressure and temperature. The model also calculatesmixture properties such as liquid volume fraction (LVF)andgasFroudenumber(Fr).Thoseparametersareusedinconjunctionwiththeempiricalcorrelationfortheover-reading of the sonar meter to correct the sonar flowvelocityof themixture in termsof actual gas flow rate.Oncethegasflowrateisdeterminedatactualconditions,theoilandwaterratesarethendeterminedfromthePVTmodel. The total mixture is flashed to standardconditions,andgas,oil(condensate)andwaterratesarereportedatstandardconditions.

AschematicofthethetypicalembodimentoftheTPS

systematthewellheadisshowninFigure4:

Figure 4: Schematic Total Production Surveillance

MethodologyTheover-readingcorrectionenablestheActiveSONAR

metertoreportgasratestowithin+/-3%for0<LVF<0.106and0.5<Fr<5.78.Figure5 illustratestheresultsfora4inSonartestedinthewetgasloopatCEESI,CO.

Figure5:Wetgasover-readingcorrection(4inSonar)

TPSSensitivitywithCompositionalParameters.SincedefiningthewellborecompositionisequivalenttospecifyingtheCGR(condensategasratio)andWGR(watergasratio)ofthegascondensatestream,theaccuracyofthecompositionalinformationwouldhaveadirectimpactontheindividualphaseflowratesinferredbytheTPSsystem.

Typically, the sonar-based surveillance relies on the

knowledge of the well bore composition from eitherhistorical well tests or/and fluid sampling. Therefore, abriefsensitivityanalysiswouldbebeneficialtoquantifythemagnitude of flow errors due to errors in compositionalparameters.

MultiphaseFlow&Compositional

Models

P,T

CompostionalInformation

Qoil

Qgas

Qwater

(ClientProvided)

VSONAR

Input

Measurements

Output

TPS1000SystemDiagram

©CopyrightExpro2011ExproMetersWELLFLOWMANAGEMENT


4

Forsimplicity,theflowconditionsfromoneofthetrialspresentedhereinwereused(Table1).TherearetwoCGRvalues used for the analysis, 30STB/mmscf and70STB/mmscf, which are varied by ±20% each, whilekeepingthewatercut(WC)constant,at5%.Theneteffectof±20%changeinCGRisthatboththeoilandwaterflowratesshiftby±20%.

If the CGR is held constant (30STB/mmscf and

70STB/mmscf), andWC is varied by ±2% (absolute), thewater flowratechangesby±40%,which is similar to therelativechangeinWC.

Table1:TPSsensitivitywithwellcomposition

Theanalysisshowstheimportanceofhavingaccurate

compositional inputs in The TPS system in order for theinferred phase rates to be accurate. Therefore, thecompositional information shouldbeupdatedevery timethereisasignificantchangeinthewetgasparameters.

Fieldperformance

Inmanyplacesaroundtheworld,the localregulatorybodyrequiresthewellstobetestedatacertainfrequency,buttheexistinginfrastructuremaynotallowtheoperatorsto comply easily with those requirements. In mostinstances, it could lead to a significant increase of theoperatingcosts,especiallyfortheoffshoreandremotelandfieldswherephysicalaccessposeschallenges.

Oftentimes,thefieldpipinglayoutdoesnotincludethe

means(testseparators)totesteachindividualwellandtheproduction allocation becomes a serious issue for bothfinancialandtechnicalreasons.

Production surveillance and monitoring of individual

wellsusingtheclamp-onSONAR-basedTPSsystem,allowsoperators tomeasure the production of eachwellmorefrequentlyandwithoutdeferringproduction.Also,duetothe clamp-on design, the well production is measuredunder normal operating conditions, helping to identifyunderperformingwellsortheneedforworkover.

The test data from two gas condensate field trials ispresentedherein,oneoffshoreandtheotheronland.

OffshoreTrial.Theoffshoretrialwasconductedinagas

condensate field with two satellite platforms and oneproductionplatform.Thesatelliteplatformsareequippedwithproductionseparators,butnotestseparators.

The client was experiencing significant production

losseswhendivertingwellsthroughthetestseparatordueto backpressure effects. Additionally, the client waslooking to obtain production surveillance data at normalwell flowconditions,unaffectedby the limitationsof thetest separators. The client wanted to increase both thefrequency and efficiency of productiontesting/surveillance.

ElevenwellsweretestedonplatformsAandB:A1,A2,

A3,A4,A5,B1,B2,B3,B4,B5andB6,overa7dayperiod.

Figure6:Sonarflowmeter(6in)The flow data recorded by the flow meter was post

processed using Expro Meters' proprietary TPS1000software,alongwithwellheadpressureandtemperature(provided by client), a well stream compositionreconstructedwithCGRandWGR(watergasratio)valuesdeterminedfromtheApril2014welltestreport,toreportgas,oilandwaterratesatstandardconditions.

Well A5was tested before and after a facility layout

modification,aspartofanoptimizationscheme.Reviewofthe data for both tests (Pre- and Post- Modification)indicated that the flow measurement increased by 2.4MMscfdinthepost-testflowdata.

Well CGR Water Cut WGR Qgas Qoil Qw ater Qgas Error Qoil Error Qw ater ErrorSTB/MMscf STB/MMscf MMscfd STB/D STB/D % % %

U 30 5% 1.58 79.00 2370.0 124.824 5% 1.26 78.79 1891.4 99.3 -0.27% -20.19% -20.43%36 5% 1.89 79.17 2850.0 149.6 0.22% 20.25% 19.87%

V 70 5% 3.68 80.11 5607.7 294.856 5% 2.94 79.73 4464.6 234.4 -0.47% -20.38% -20.49%84 5% 4.42 80.50 6762.1 354.2 0.49% 20.59% 20.15%

W 30 5% 1.58 79.00 2370.0 124.830 3% 0.92 79.02 2370.6 72.7 0.03% 0.03% -41.75%30 7% 2.26 78.98 2369.4 178.5 -0.03% -0.03% 43.03%

X 70 5% 3.68 80.11 5607.7 294.870 3% 2.16 80.15 5610.6 173.1 0.05% 0.05% -41.28%70 7% 5.26 80.07 5604.7 421.1 -0.05% -0.05% 42.84%

Vsonar 75 ft/sPressure 1400 psig

Temperature 200 degFPipe Size 6in Sch 160

Sonar Wet Gas Sensitivity


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Figure7:SonarflowmeterresultsPlatformA

Figure8:SonarflowmeterresultsPlatformB.

The total3-phase flowrates for theAandBwells,as

reported by Expro Meters, have been compared to theproductionseparatorA(platformA),B1andB2(platformB)flowrates.ExproreportedgasratesfortheAwellswerewithin 3% of the A platform production separator rates.ThereportedgasratesfortheBwellswerewithin-3%ofthe B1 and B2 platform production separator rates. Thecondensaterateswerewithin4.5%,whilethewaterrateswithin12%forSeparatorB,whichmaybeindicativeofanincorrectWGR.

As a result of the trial, the client decided to employ

Exprotoperformthewellsurveillanceonaquarterlybasisandexpandthescopetotheentirefield.

Land Trial. The land trialwas conducted in a remote

highpressuregascondensatefield;thepressuresrecordeddownstream of the choke were between 1700psig and2150psig. The client was looking to increase well testfrequencyduetothelocalregulationsandpossiblyreducethecost.

Attherequestoftheclient,ExproMetersperformeda

wellhead surveillance trial on twowells: C1 and C2. TheTPS1000 softwarewas then used to provide single pointvolumetricflowrates(oil,gasandwater)onsite,basedonaveragevelocitymesurements.

ForwellC1,theSONARmeterwasinstalledonthiswell

forsixdays. Theflowdatawaspostprocessedusinglinepressure and temperature readings along with a

wellstream composition (CGR - 33.84 STB/MMSCF andWGR-0.94STB/MMSCF)providedby theclient.Thegasand condensate rates were within 4% of the separatorrates,whilethewaterwithin10%.

The SONARmeterwas installed onwell C2 for three

days.Theflowdatawaspostprocessedusinglinepressureand temperature readings along with a wellstreamcomposition (CGR - 47.1 STB/MMSCF and WGR - 8.715STB/MMSCF) provided by the client. The reported flowrates (gas, condensate water) were all within 3% of thereference.

Figure9:Sonarflowmeter(10in)wellC1.

Figure10:SonarflowmeterresultswellsC1andC2.Theclientiscurrentlylookingintowhatrequirements

need to be met in order to qualify the Sonar-basedSurveillance forgascondensatewell testingby the localregulatorybody.

Conclusions

Aclamp-onproductionsurveillancesystemdesignedtomonitorgascondensatewellswaspresented.Thesystememploysaclamp-onsonarflowmeterastheflowmeteringelement,integratedwithanEquationofStatePVTmodel.Intheapproachdescribedherein,producedoilandwater


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ratesareinferredfromthemeasuredgasrateusingauser-definedwellborecomposition.

Whencombinedwithaccuratewellborecomposition

data, the clamp-on production surveillance systemprovides a practical, cost-effective, real-time surveillanceofgascondensatewells.

Two field cases are presented illustrating the

applicationofthissystemtogascondensatewells.Thetwocases utilized historical well bore compositional data toprovide three-phase surveillance on a periodic or surveybasis.TheresultsforbothtrialsprovedthatSonar-basedsurveillance(TPS)isaviablealternativeforgascondensatewellheadsurveillance.

While the system measures variations of produced

liquids due to variations in gas production, it will notmeasure variations in CGR and/or WC. To account forchanges in CGR or/and WC, an updated well-borecompositionmustbeenteredintothesystem.

Traditional well testing approaches, such as CTS and

MPFM may require a significant amount of time to bedeployed in the field. The Sonar clamp-on approachrequires a shorter amount of time (about 90min) forinstallationandcommissioningwhichallowsthepossibilitytoperformmulti-ratetestingofthewellsormultiplewelltesting in one day. Therefore, the sonar clamp-onmethodology offers the opportunity to increase thewelltestfrequencyatafield-widelevelthusallowingabetterfield/productionmanagement.

The Sonar-based Surveillance is not to replace

traditional well testing methodologies, but to augmentthembyofferingaquick,reliableandcosteffectivesolutionfor applications requiring recurring productionsurveillance, especially where reservoir conditions arerelativelystableovertime.

AcknowledgementsTheauthorsgratefullyacknowledgeExproGroupforpermissiontopublishthisworkandacknowledgetheeffortsofourcolleagueswhohavecontributedtothispaper.Nomenclature

P CTS = Conventional Test Separator V MPFM = Multiphase Flow Meter T TPS = Total Production Surveillance V Vconvect = Convection velocity of turbulent eddies T ω = Temporal Frequency MPFM = Multiphase Flow Meter

T TPS = Total Production Surveillance V Vconvect = Convection velocity of turbulent eddies T ω = Temporal Frequency k = Wave Number λ = Wave Length T ORsonar= Sonar Wet Gas Over-reading V Vsonar = Sonar Velocity T Vsg = Superficial Gas Velocity T LVF = Liquid Volume Fraction Fr = Gas Froude Number β = Sonar Wet Gas Correction Constant T φ = Sonar Wet Gas Correction Constant V m = Sonar Wet GasCorrection Coefficient T EoS = Equation of State CGR = Condensate Gas Ratio T WGR = Water Gas Ratio WGR WC = Water Cut

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