ChapterThree

SamplingandLaboratoryAnalysis

3.1SamplingMathematicalmodelsencapsulatedwithinsoftwarepackagesareincreasinglyusedto predict the behavior of hydrocarbon reservoirs and their related productionsystems.Thesemodelsrequire input, initializationandcalibrationdata.For fluidproperty determination this requires to take samples of the fluids of interest,determine their composition, and finally, perform a set of standard tests toproducedatatocalibratethesemodels[19].Before conducting any test, samples of the fluid of interest should be taken aspart of the initial well testing program. There are usually conflicts in thewelltestprogramwiththeneedtoobtainreservoirparametersversusthecollectionofrepresentative samples. Proper design and careful planning is the key tominimizingtheseconflicts[20].

3.1.1WellTestingThe main problems in the well test design for sampling is concerned with theproduction interval and the tubing size. In large hydrocarbon columns, asignificant variation in composition with depth ispossible. In this case, it ispreferable to sample only a limited interval by restricting the perforations. Itsuggested that the intervalsbe restricted to 30-ft column.This then requiresseveral teststobeperformedoverlarge(over300-ft)columns,andaminimumofthreeseparatetests[21].When consideringwell conditioning, the sample collection isbest served by lowflow rates (using small diameter tubing) since low rate production in largediametertubinggivesrisetoanunstableflowregimecalledslugging.However,theratemustbehighenoughtoensurethatliquidsareproducedtosurface[22].Technological advances in recent years have helped here since it may bepossible torunsmalldiametercoiledtubingduringthesamplingphase,returningbacktothelargediametertubingfortheotheraspectsofthewelltest.

3.1.2ConditioningThere are two ways of sampling: (1) down-hole and (2) surface sampling asillustratedinFigure3.1.

Figure3.1:Reservoirfluidandsampling[23]In both methods, proper conditioning of the well prior to taking the sample isnecessary:1. Samplingshouldbedoneassoonaspossibleafterthewelliscompleted.2. The process of drilling and completion usually results in near-well boredamage and contamination,whichmust be cleaned-up before the samplecanbetaken.This isbestachievedbyahighflowrate.However, thismaycause a large pressure draw down that might cause the bottom-holepressure to fall below the saturation pressure. Then, depending on therelative permeability effects, the fluid flowing into the well may beunrepresentativeofthereservoirfluid.Once thebalance is achievedbetweenmaximizing clean-up timeandminimizingpressuredrawdown,themainaimofwhichistoachieve

Uniformflowrate,Uniformgas-to-oilratio,GOR,Stabletopholepressure(PTH)Stablebottomholepressure(PBH)Stablebottomholedensity,( BH)(toensurenoliquidbuildup)Stablewellheadtemperature,(TWH)Thestabilityconditionsaresatisfiedforatleast6hourspriortothesamplebeingtaken[24].3.1.3 TheDown-HoleSamplingInthistechnique,abottleislowereddownholeonawirelineandplacedascloseaspossibletotheopeninterval.Atsomepre-arrangedtimeoronacommandfromthesurface,thebottleisopenedtothefluidflowingaroundituponwhichsomeofthat fluid is allowed to enter the bottle. Unlike surface sampling, the volume offluidthatcanbecollectedisrelativelysmall,typically1literorso.The sample bottle is returned to the laboratory and the fluid is flashed toatmosphericconditions.Thenormalizedmassfractionsofthegas(wgi)andtheoil(woi)inthestocktanksamplearefoundbygaschromatography.Themolarweight(Mo)anddensity( o)oftheoilsamplearethenmeasured.Theflashgas-to-oilratio(GORorRs)inconsistentunits(ft3/ft3orm3/m3)isgivenby (3.1)whereVgm andVomarerespectively thegasandoilmolarvolumes (in fieldunits,ft3/lb-mol)andngandnoarethecorrespondingnumberofmoles.Ifthenumberofmolesof the feed isassumedunity, then .Themolarvolumeof theoil,bydefinition,equalsitsmolarweightdividedbydensity,i.e. (3.2)Combiningtheseresultsallowscalculationofthenumberofmolesofthegasas (3.3)

Meanwhile,theoilandgasmassfractionsareconvertedtomolefractions (xiandyi)usingtheoilandgasmolarweights,MoandMg): (3.4) (3.5) Finally,withthegasandoilsample molefractions(xiandyi)andthenumberofmolesofthegas,thefeedcomposition,zFi,iscalculatedfrom (3.6)Themeasurementofmolarweightsisextremelydifficultandcanbesubjecttoanerror (as large as ±10%);whichwill clearly affect the determination of thewellstreammolarcomposition[25].3.1.4TheSurfaceSamplingThewell is permitted to flow to the surfacewhere a fraction of thewell streamfluid isre-directedtoatestseparatorheldatsomepre-determinedpressureandtemperature. After ensuring the stability conditions being met, samples of theseparator vapor and liquid are collected in a number of bottles,which are thensenttoregionallaboratoriesforanalysis.Themainadvantageofsurfacesamplingoverthedown-holesamplingistheabilityto collect large volumes of fluid. However, there are a number of issues to beconsidered:Liftingalltheproducedfluids,Ensuringarepresentativemixistakenfromtheflowline,Accuratemeteringwiththeconsequentproblemofrecombiningthevaporandliquidstreamstoreconstitutethewellstreamfluid.Most surface samplesare taken througha test separator. Ideally, the inletof thetest separator should be inserted into the main flow line from the well headmanifold. The probe should be preceded by a baffle arrangement to ensure thefluidiswellmixed.

A number of analysis techniques can be employed to ensure any recombinedsample is representative. Firstly, when the liquid bottle is opened back in thelaboratory, the bubble point pressure should be the same as the separatorpressure at which it was sampled and should be corrected for temperature.Secondly, since all the components of the vapor and liquid phases are inequilibrium, then the k-values for each component in the mixture can becalculated. (3.7)StandingsuggestedthatthemeasuredKvaluesshouldobeythefollowingequation[26] (3.8)wherePsepistheseparatorpressure(inpsia)andFiisgivenby (3.9)WhereTbi,Tci,andPciarerespectivelythenormalboilingtemperature,thecriticaltemperature and the critical pressure of component i in the mixture. Pst is thestandard pressure (in consistent units). The constants A0 and A1 are calculatedfrom[26]: (3.10) (3.11)Eq.(3.8) isgenerallyassumedvalid forhydrocarbonmixturesatpressuresupto1000psia(68bar)andtemperaturesupto200°F(366.4K).3.2LaboratoryAnalysisAfter obtaining one ormore representative samples, the next task is to analyzethemtofindwhichcomponentsarepresentandinwhatproportions.Thenasetofstandard experiments should be performed to determine a set of importantparameters.Theparametersmeasureddependonthenatureofthereservoirfluid,i.e.liquidand/orvapor.

3.2.1CompositionDeterminationThe workhorse in this area is the gas chromatograph (see Figure 3.2), whichusuallycomesinoneoftwotypes,packedcolumnorcapillarycolumn.Thepackedcolumnconsistsofaglassorstainlesssteelcoil,typically1-5minlengthand5mminnerdiameter.Thecapillarycolumnsarethin fusedsilica, typically10-100min[27].

Figure3.2:Aschematicofagaschromatographicsystem[28].The sample is injected into the column, which is housed in a temperature-controlled oven. As the temperature is increased on some pre-programmedschedule,thecomponentswillboildependingontheirvolatility.Aninertcarriergassuchasheliumorargonthencarriesthecomponentsalongthetubetoadetector.Theeffluent fromtheGCmixeswith anair/hydrogenmixtureand passes through a flame. The resulting ions are collected between theelectrodestoproduceanelectricalsignal.ThemostpopulartypesofdetectorsaretheFlameIonizationDetector(FID)andtheThermalConductivityDetector(TCD).TheFIDisverysensitivebutitdestroysthe sample. The TCD consists of an electrically-heatedwire whose resistance isaffected by the thermal conductivity of the surrounding gas. The change inresistancecanbecorrelatedtothenatureofthesurroundinggas.TheTCDisnotasaccurateastheFIDbutitisnon-destructive.

3.2.2SaturationPressureDeterminationThebubblepointpressure fora reservoir liquidor thedewpointpressure forareservoir vapor is one of the important measurements performed at saturationconditions.Theexactmechanicsofthemeasurementdependonthefluidtypebutinboth cases itbeginsby loadinga volumeof the reservoir fluid intoaPVTcell(discussedinChapter4).Thiscellisplacedinachamberwhosetemperaturecanbe set as that of the reservoir temperature. Pistons can raise and lower thepressure and valves allow fluid to be injected and removed from the top andbottomofthecell.Somecellscontainawindow,locatednearthebottomofthecelltoallowvisualinspectionofthecontents[29].For a liquid, the pressure is raised to some high pressure, generally slightly inexcessoftheinitialreservoirpressure.Then,thepressureisreducedinaseriesofstagesandthecorrespondingvolumeofthefluidisrecordedateachstage[30].Inthiscase,therelativevolume-pressurebehaviorpresentedinFigure3.3hasbeenobserved.

Figure3.3:ExpectedVrel-PcurvesofcrudeoilaroundthebubblepointforwellA#33.

3.2.3TheConstantMassStudy(CMS)TestTheCMStestisatestperformedwithaknownquantityofrepresentativereservoirfluid sample, which remains constant throughout the test. It is also known asConstant Composition Expansion (CCE) test or Constant Mass Expansion (CME)test. The CMS test is a flash liberation process, since the sample compositionremains constant and as the gas is liberated from solution, it remains in contactwiththeliquidandequilibriumisattainedwithallcomponentsstillpresent[31].ThefollowingcanbedeterminedfromtheCMStest:ReservoirtemperatureatsaturationpressureAmbienttemperatureatsaturationpressureRelativevolume pressurerelationshipCompressibilityfactorThermalExpansionAfixedvolumeofthereservoirfluidischargedintoahighpressurePVTcellwellabove the saturation pressure of the fluid. The cell volume is increased in smallincrements, with the pressure being recorded after each volume increment andafter it reaches equilibrium.When the cell reaches the sample bubble point, thefirstbubbleofthegasevolves.Thecompressibilityofthetwophasespresentinthecell drastically increases due to the gas compressibility beingmuch larger thanthatoftheliquid.ThiscanbeseenwhenthesamplevolumeisplottedagainstthecellpressureasillustratedinFigure3.4.Thetestcanbedescribedstepbystepasfollows[32]:The cell starts at a pressurewell above the bubble pointpressure Figure3.4-A.The pump is backed off to increase the cell volume and the new cellpressure is recorded. In Figure 3.4-B the bubble point pressure (Pb) isreachedand the first bubble of the gas is formed. Lookingat theV-P plotundereachcell,thesingle-phasepartofthecurveisquitelinear.In Figure 3.4-C the cell volume is further increased, such that the cellpressure is now P3, which is less than Pb. More gas has come out of the

solution;asaresultthesamplecompressibilitychangessignificantly.ThisisillustratedintheV-Pplotbythedeviationfromlinearity.In Figure 3.4-D the cell volume is further expanded and the pressurecontinuestodecrease,however, itdecreaseslesswithstepincrease in thesamplevolume.

Figure3.4:SchematicrepresentationoftheConstantMassStudy(CMS)testandthecorrespondingV-Pcurves[31].3.2.4TheSeparatorTest(SEP)Thewell stream fluid arrivingat the surface isusuallyput through twoormorestagesofseparation.Aschematicofa2-stageseparatortestispresentedinFigure3.5.Aseparator iseffectivelya large tankheldatsomepre-determinedpressureand temperature, which allows the fluid to separate into vapor, liquid andoptionallyaqueousphases.Usually,theliquidfromafirststageseparatoristakenasthefeedforthesecondstageseparator,andsoon.Theoreticallyatleast,thelaststage is at standard conditions (Pst = 14.7 psia and Tst = 60 oF) and the liquidarriving here is the stock-tank oil. In practice, especially in an offshoreenvironment,theliquidwillbeputintoasaleslineatsomepressureinexcessofthestandardpressure.Thevaporproducedfromeachstage is collected together

A B C D

andreportedasifithadbeentakentostandardconditions.Again,inpractice,thevaporwillrarelybetakendowntostandardconditionsalthoughthevolumesarecorrectedtotheseconditions[33].

Figure3.5:Aschematicdiagramofa2-stageseparatortest[33].Thesetofseparatorstagesissometimesreferredtoasseparatorsequence,whichrepresents an approximation to the processing plant used in practice. The keyparameterstodetermineareGas-to-oilratio(GOR)ateachstage(=Vgi/Vo2)andhencethetotalGOR=(Vg1+Vg2)/Vo2Oil formationvolumefactor (FVFo)ateachstage(=Voi/Vo2),thetotalFVFo(=Vob/Vo2),whereVobistheoilvolumeatsaturationpressure(Pb)Densitiesofliberatedfluids(oilandgas)ateachstage.TheGORisusuallyreportedasthegasvolumeperoilvolumebothat stocktankstandardconditions(Pst,Tst).Thevolumeofthegasliberatedateachstage,Vgiisatsome elevated pressure and temperature (Pi, Ti) at which its Z-factor, Zi, ismeasured.Thenfromtherealgaslaw

(3.12)Onecancomputethevolumethegaswilloccupyatstandardconditionsfrom (3.13)Bydefinition,thestandardZ-factoratstandardconditions,Zst=1.0.Itissometimespossible to adjust the pressure and temperature of the stages, usually the first-stagepressure,tomaximizeliquidproduction.Inadditiontotheabovementionedlaboratoryanalysisandprocedures,Chapter4is devoted to the determination of the saturation pressure by a recombinationprocess at the reservoir pressure and temperature, and then swelling tests areconductedtoinvestigatehowmuchtheCO2injectionisgoingtoswelltheoil.Thiswillhelpinenhancingtheoilrecovery.

4.3ExperimentalProcedureforSwellingMeasurementA schematic of the swelling test apparatus is shown in Figure 4.5. For volumemeasurementsofbothliveoilandCO2/liveoilmixture,thesyringepumpalongwith theCO2materialbalanceisused.Thesaturationpressureoftheliveoilisfirstmeasuredthenthepressureislowereddowntoabout60barinordertoleavethepistonatitsmaximumpositionat thebacksideof thePVTcell. ValveV5 is thenclosedand thepressureof thesyringe pump is increased up to the previously measured saturation pressure. Thetemperatureofthesyringepumpchamberiskeptconstantat283.15K(onehastowaitupto complete stabilization of the system (zero flow in the syringe pump controller). Theinitial volume of the syringe pump, Vi, is recorded and while keeping the pressureconstant,valveV5isopened,toallowthemovementofthepiston.Thefinalpositionofthepiston keeps the live oil at saturation conditions (constant pressure and temperature).Afterstabilizationthefinalsaturationvolume,Vf,isrecorded.Thedisplacedvolumeinsidethe syringe pump, Vs0, is then computed as Vs0 = Vi Vf. Using the CO2 chart [34], thedensityofCO2atthegiventemperatureandpressureiscalculatedandthedisplacedCO2mass inside the syringe pump,m0, is determined. This mass is the same as the massdisplaced in the cell. Since the cell temperature is known (257 oF), the volume at thebacksideof thepiston,Vc0, canbecomputed.Thevolumeofsaturated liveoil,V0, canbe

obtained by the difference between the overall available cell volume and the displacedvolume inside the syringepump, i.e.V0 =26.22 Vs0ml.AgivenamountofCO2 is theninjectedintothecellandtheproceduredescribedaboveisusedtocomputethedisplacedvolume, Vs01, the displaced CO2 mass,m01, inside the syringe pump, the volume at thebacksideofthepiston,Vc01,andthevolumeofthemixture,V01.Theswelling factor isdefinedastheratiobetweenthevolumeofasaturatedmixtureofCO2/live oil and the volume of saturated live oil at the reservoir temperature [34]. TheamountofswellingexperiencedbyacrudeoilandtheincreaseinthesaturationpressureasaresultofCO2injectionisdeterminedbytheswellingfactorwhichistherelativetotalvolumeorrelativeswollenvolume,Vsw(=newsaturationvolume,V01,ateachincrementaladditionofCO2,dividedbytheoriginalsaturationvolume,V0). (4.1)

Figure4.5:Aschematicdiagramoftheswellingtestprocedure[31].The results and discussion of the above experimental workswill be shown in the nextchapter.

4.4RecombinationoftheReservoirFluidRecombination of the stock-tank oil with the gas from the separator is the first andfundamentalstepforPVTstudyinpetroleumsystems.Sincethegascompositionfromthefirst-stageseparatorisdifferentfromthatobtainedbyflashingthemonophasicwellfluiddirectly to standard conditions, the recombination process using the gas from the first-stage separator can lead to live oil different from the well oil. The surface separationprocesscanbeillustratedintheschematicdiagramshowninFigure4.6.

Figure4.6:Aschematicdiagramofthesurfaceseparatormeteringandsampling[35].