copy (2) of chapter-04

Phase Behaviour of Hydrocarbon Systems

CONTENTS

1 DEFINITIONS

2 PHASEBEHAVIOUROFPURESUBSTANCES 2.1 ThePhaseDiagram

3 TWOCOMPONENTSYSTEMS 3.1 Pressure-TemperatureDiagrams 3.2 PressureVolumeDiagram

4 MULTI-COMPONENTHYDROCARBON 4.1 PressureVolumeDiagram 4.2 PressureTemperatureDiagram 4.3 CriticalPoint 4.4 RetrogradeCondensation5 MULTI-COMPONENTHYDROCARBON 5.1 OilSystems(BlackOilsandVolatileOils) 5.2 RetrogradeCondensateGas 5.3 WetGas 5.4 DryGas

6 COMPARISONOFTHEPHASEDIAGRAMSOFRESERVOIRFLUIDS

7 RESERVOIRSWITHAGASCAP

8 CRITICALPOINTDRYING

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LEARNING OBJECTIVES

Having worked through this chapter the Student will be able to:

General• Define;system,components,phases,equilibrium,intensiveandextensive

properties.

PureComponents• Sketchapressure-temperature(PT)diagramforapurecomponentandillustrate

onit;thevapour-pressureline,criticalpoint,triplepoint,sublimation-pressureline,themeltingpointline,theliquid,gasandsolidphasezones.

• Definethecriticalpressureandcriticaltemperatureforapurecomponent.• DescribebrieflywiththeaidofaPTdiagramthebehaviorofapurecomponent

systembelow(left|)andabove(right)ofthecriticalpoint.• Sketchthepressure-volume(PV)diagramforapurecomponentillustratingthe

behaviorabovethebubblepoint,betweenthebubbleanddewpointandbelowthedewpoint.

• SketchaseriesofPVlinesforapurecomponentwithatemperaturebelow,atandabovethecriticaltemperature.

• Sketchthethreedimensionalphasediagramforpurecomponentsystems.TwoComponents• PlotaPVdiagramfora2componentsystemandidentifykeyparameters.• PlotaPVdiagramfora2componentsystemandidentifykeyparametersand

therelationshiptothevapourpressurelinesforthetwopurecomponents.• Sketchthecriticalpointlociforaseriesofbinarymixturesincludingmethane

andindicatehowamixtureamixtureofmethaneandanothercomponentcanexistas2phasesatpressuresmuchgreaterthanthe2phaselimitforthetwocontributingcomponents.

• DrawaPTdiagramforatwocomponentsystem,toillustratethecricondentherm,cricondenbarandtheregionofretrogradecondensation.

• Definethetermscricondenthermandcricindenbar.• Explainbrieflywhatretrogradecondensationis.MulticomponentSystems• SketchaPTandPVdiagramstoillustratethebehaviouratconstanttemperature

forafluidinaPVTcell.Identifykeyfeatures.• DrawaPTdiagram foraheavyoil,volatileoil, retrogradecondensategas,

wetgasanddrygas.Illustrateandexplainthebehaviourofdepletionfromtheundersaturatedconditiontotheconditionwithinthephasediagram.

• Describebrieflywiththeaidofasketch,thereasonsforandtheprocessofgascycling,forretrogradegascondensatereservoirs.

• PlotaPTdiagramforareservoirwithagascaptoillustratethegasatdewpointandoilatbubblepoint.

Miscellaneous• Withtheaidofsketchexplaintheprocessofcriticalpointdrying.


Institute of Petroleum Engineering, Heriot-Watt University �

Oilandgasreservoirfluidsaremixturesofalargenumberofcomponentswhichwhensubjectedtodifferentpressureandtemperaturesenvironmentsmayexistindifferentforms,whichwecallphases.Phasebehaviourisakeyaspectinunderstandingthenatureandbehaviourofthesefluidsbothinrelationtotheirstateinthereservoirandthechangeswhichtheyexperienceduringvariousaspectsoftheproductionprocess.Inthischapterwewillreviewthequalitativeaspectsofthebehaviourofreservoirfluidswhensubjectedtochangesinpressureandtemperature.

1 DEFINITIONS

Beforeweconsidertheeffectoftemperatureandpressureonhydrocarbonsystemswewilldefinesometerms:

• System-amountofsubstanceswithingivenboundariesunderspecificconditionscomposedofanumberofcomponents.Everythingwithintheseboundariesarepartofthesystemandthatexistingoutsideoftheboundariesarenotpartofthesystem.Ifanythingmovesacrosstheseboundariesthenthesystemwillhavechanged.

• Components - those pure substances which produce the system under allconditions.

Forexample,inthecontextofreservoirengineering,methane,ethane,carbondioxideandwaterareexamplesofpurecomponents.

• Phases-Thistermdescribesseparate,physicallyhomogenouspartswhichareseparatedbydefiniteboundaries.1Examplesinthecontextofwaterarethethreephases,ice,liquidwaterandwatervapour.

• Equilibrium-Whenasystemisinequilibriumthennochangestakeplacewithrespecttotimeinthemeasurablephysicalpropertiesoftheseparatephases.

• Intensive and extensive properties - physical properties are termed eitherintensiveorextensive. Intensive propertiesareindependentof thequantityofmaterialpresent.Forexampledensity,specificvolumeandcompressibilityfactorareintensivepropertieswhereaspropertiessuchasvolumeandmassaretermedextensive properties;theirvaluesbeingdeterminedbythetotalquantityofmatterpresent.

Thephysicalbehaviourofhydrocarbonswhenpressureand temperaturechangescanbeexplainedinrelationtothebehaviouroftheindividualmoleculesmakingupthesystem.Temperature,pressureandintermolecularforcesareimportantaspectsofphysicalbehaviour.

The temperatureisanindicationofthekineticenergyofthemolecules.Itisaphysicalmeasureoftheaveragekineticenergyofthemolecules.Thekineticenergyincreasesasheatisadded.Thisincreaseinkineticenergycausesanincreaseinthemotionofthemoleculeswhichalsoresultsinthemoleculesmovingfurtherapart.

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Thepressurereflectsthefrequencyofthecollisionofthemoleculesonthewallsofitscontainer.Asmoremoleculesareforcedclosertogetherthepressureincreases.

Intramolecularforcesaretheattractiveandrepulsiveforcesbetweenmolecules.Theyareaffectedbythedistancebetweenthemolecules.Theattractiveforcesincreasesasthedistancebetweenthemoleculesdecreasesuntilhowevertheelectronicfieldofthemoleculesoverlapandthenfurtherdecreaseindistancecausesarepulsiveforce,whichincreasesasthemoleculesareforcedclosertogether.

Themoleculesingasesarewidelyspacedandattractiveforcesexistbetweenthemoleculeswhereasforliquidswherethemoleculesareclosertogetherthereisarepellingforcewhichcausestheliquidtoresistfurthercompression.

Thehydrocarbonfluidsofinterestinreservoirsystemsarecomposedofmanycompo-nents howeverinunderstandingthephasebehaviourofthesesystemsitisconvenienttoreflectonthebehaviourofsingleandtwocomponentsystems.

2 PHASE BEHAVIOUR OF PURE SUBSTANCES

2.1 The Phase DiagramItisbeneficialtostudythebehaviourofapurehydrocarbonundervaryingpressureandtemperaturetogainaninsightintothebehaviourofmorecomplexhydrocarbonsystems.

Phasediagramsareusefulwaysofpresentingthebehaviourofsystems.Theyaregenerallyplotsofpressureversustemperatureandshowthephasesthatexistunderthesevaryingconditions.

Figure1givesapressure-temperaturephasediagramforasingle-componentsystemonapressuretemperaturediagramandthefollowingpointsaretobenoted.



Pres

sure

Temperature

Mel

ting

Poin

t

Sublimation

Vapour Pressure

Triple Point

Critical PointC

1 2

3

Vapour

LiquidSolid

Gas

Figure 1 Pressuretemperaturediagramforasinglecomponentsystem

• Definetheblackoilmodeldescriptionofthecompositionofareservoirfluid.

• ExplainbrieflywhatPNAanalysisisanditsapplication.

Vapour Pressure LineThevapourpressurelinedividesregionswherethesubstanceisaliquid,2,fromregionswhereitisagas,3.Abovethelineindicatesconditionsforwhichasubstanceisaliquid,whereasbelowthelinerepresentconditionsunderwhichitisagas.Con-ditionsonthelineindicatewherebothliquidandgasphasescoexist.

Critical PointThecriticalpointC.isthelimitofthevapourpressurelineanddefinesthecriticaltemperature, Tc and critical pressure, Pcofthepuresubstance.Forapuresubstancethecriticaltemperatureandcriticalpressurerepresentsthelimitingstateforliquidandgastocoexist.Amoregeneraldefinitionofthecriticalpointwhichisbothapplicabletomulticomponentaswellassinglecomponentsystemsis;thecriticalpointisthepointatwhichalltheintensivepropertiesofthegasandliquidareequal.

Triple PointThetriplepointrepresentsthepressureandtemperatureatwhichsolid,liquidandvapour co-exist under equilibrium conditions. Petroleum engineers seldomdealwithhydrocarbonsinthesolidstate,however,morerecentlysolidstateissuesareaconcernwithrespecttowax,asphaltenesandhydrates.

Sublimitation-Pressure LineTheextensionofthevapour-pressurelinebelowthetriplepointrepresentsthecon-ditionswhichdividestheareawheresolidexistsfromtheareawherevapourexistsandisalsocalledthesublimation-pressureline.

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Melting Point LineThemeltinglinedividessolidfromliquid.Forpurehydrocarbonsthemeltingpointgenerallyincreaseswithpressuresotheslopeofthelineispositive.(Waterisex-ceptionalinthatitsmeltingpointdecreaseswithpressure).

3 USE OF PHASE DIAGRAMS

3.1 Pressure -Temperature Diagrams (PT)Considerthebehaviourofacellchargedwithapuresubstanceandthevolumevariedbythefrictionlessdisplacementofapistonasshowninfigure2,below.

P1 Pb P Pd P2

Liquid

Gas

Figure 2 PhaseChangesWithPressureatConstantTemperature

Forexample,followingthepath1-2infigure3onthepressure-temperaturediagram,ieholdingtemperatureconstantandvaryingpressurebyexpansionofthecylinder.



cPc

Tc

Pres

sure

Temperature

Solid Liquid

Mel

ting

- Poi

nt L

ine

Vapour - pressure line

T

Gas

E

A B G

F

1

2

3

4

Figure 3 Pressure-TemperatureDiagramforaSingle-ComponentSystem

Asthepressureisreduced,thepressurefallsrapidlyuntilapressureisreachedlyingonthevapourpressureline.Agasphasewillbegintoformandmoleculesleavetheliquid.Atfurtherattemptstoreducethepressurethevolumeofgasphaseincreases,whileliquidphasevolumedecreasesbutthepressureremainsconstant.Oncetheliquidphasedisappearsfurtherattemptstoreducepressurewillbesuccessfulasthegasexpands.

Abovethecriticaltemperature,followingthepath3-4,adecreaseinpressurewillcauseasteadychangeinthephysicalproperties,forexampleadecreaseindensitybuttherewillnotbeanabruptdensitychangeasthevapourpressurelineisnotcrossed.Nophasechangetakesplace.

Considerthebehaviourofthesystemaroundthecriticalpoint.IfwegofrompointAtopointB,byincreasingthetemperature,wegothoughadistinctivephasechangeonthevapourpressurelinewheretwophases,liquidandgasco-exist.IfwenowgoadifferentroutetoB,startingwiththeliquidstateat‘A’increasethepressureiso-thermally(constanttemperature)toavaluegreaterthanPcatE.ThenkeepingthepressureconstantincreasethetemperaturetoavaluegreaterthanTcatpointF.NowdecreasethepressuretoitsoriginalvalueatG.Finally,decreasethetemperaturekeepingthepressureconstantuntilBisreached.Thesystemisnowinthevapourstateandthisstatehasbeenachieved withoutanabruptphasechange.Thevapourstatesareonlymeaningfulinthetwophaseregions.Inareasfarremovedfromthetwophaseregionparticularlywherepressureandtemperatureareabovethecriticalvalues,definitionoftheliquidorgaseousstateisimpossibleandthesystemisbestdescribedasinthefluidstate.

Thepressure-temperaturediagramforethaneisgiveninFigure4.

�

400

500

600

700

800

40 60 80 100 120

Liquid

Vapor

c

Temperature - º F

Pres

sure

- PS

IA

Figure 4 Pressure-TemperaturediagramofEthane

3.2 Pressure Volume Diagram (PV)Theprocessjustdescribedin3.1canalsoberepresentedonapressure-volumedia-gramatconstanttemperature(Figure5).Asthepressureisreducedfrom1,alargechangeinpressureoccurswithsmallchangeinvolumedueto therelativelylowcompressibilityoftheliquid.Whenthevapourpressureisreachedgasbeginstoform.Thispointiscalledthebubblepoint,iethepointatwhichthefirstfewmol-eculesleavetheliquidandformsmallbubblesofgas.Asthesystemexpandsmoreliquidisvaporisedatconstantpressure.Thepointatwhichonlyaminutedropofliquidremainsiscalledthedewpoint.Sharpbreaksinthelinedenotethebubblepointanddewpoint.



4

PVT CELL PV DIAGRAM

All Liquid

All Gas

First Gas Bubble

Last Drop of Liquid

1

2

Liquid state-rapid change of pressure with small volume change

Pressure remains constant while both gas and liquid are present

Dew Point

GasBubble Point

VolumePr

essu

re

TWO PHASE REGION

SINGLE PHASE T > Tc

T < Tc

T2 > Tc

Figure 5 Pressure-VolumediagramforaSingle-ComponentSystem

Forapuresubstancevapourpressuresatbubblepointanddewpointareequaltothevapourpressureofthesubstanceatthattemperature.Abovethecriticalpoint,ie3-4,thePVbehaviourlineshowsnoabruptchangeandsimplyshowsanexpansionofthesubstanceandnophasechange.Thisfluidiscalledasupercriticalfluid.

A series of expansions canbeperformedat various constant temperatures and apressurevolumediagrambuiltupandthelocusofthebubblepointanddewpointvaluesgivesthebubblepointanddewpointlineswhichmeetatthecriticalpoint.Conditions under the bubble point and dew point lines represent the conditionswheretwophasescoexistwhereasthoseabovethesecurvesrepresenttheconditionswhereonlyonephaseexists.AtthecriticaltemperaturetheP,Tcurvegoesthroughthecriticalpoint.Figure6

10

Bubb

le P

oint

Cur

ve Dew Point Curve

4

3Liquid state rapidchange of temperaturewith small volume change

Critical Point

1

2

Volume

Pres

sure

TWO PHASE REGION

SINGLE PHASE

T = Tc

T < Tc

T > Tc

Pressure remains constant whileboth gas and liquid are present

Figure 6 SeriesofPVlinesforapurecomponent

Thepressurevolumecurveforpurecomponentethaneisgiveninfigure7

Thelocusofthebubblepointsanddewpointsformathree-dimensionaldiagramwhenprojectedintoaP-Tdiagramgivethevapourpressureline(Figure8).

400

500

600

700

800

900

0 0.05 0.10 0.15 0.20 0.25

Liquid Vapor

C

D BA

Specific Volume - Cu. Ft. per lb.

Pres

sure

- PS

IA

Two Phase Region

110 º F90 º F

60 º F

Figure 7 Pressure-VolumeDiagramofEthane


Institute of Petroleum Engineering, Heriot-Watt University 11

Volume

Temperature

Temperature

Liquid

Gas

LiquidGas

and

Liqu

id

Gas

Critical Point

Critical Point

Vapor Pressure Curve

Dew Point Line

Bubble Point Line

Pres

sure

Pres

sure

Figure 8 ThreeDimensionalPhaseDiagramforaPureComponentSystem

4 TWO COMPONENT SYSTEMSReservoirfluidscontainmanycomponentsbutwewillfirstconsiderasystemcon-tainingtwocomponents,suchasystemiscalledabinary.

4.1 Pressure Volume DiagramThebehaviourofamixtureoftwocomponentsisnotassimpleasforapuresub-stance.Figure9showstheP-Vdiagramofatwo-componentmixtureforaconstanttemperaturesystem.

Pres

sure

Volume

Liquid

Gas

Liquid and GasBubble Point

Dew Point

Figure 9 Pressure-VolumeLineforaTwo-ComponentSystematConstantTemperature

1�

Theisothermisverysimilartothepurecomponentbutthepressureincreasesasthesystempassesfromthedewpointtothebubblepoint.Thisisbecausethecomposi-tionoftheliquidandvapourchangesasitpassesthroughthetwo-phaseregion.Atthebubble pointthecompositionoftheliquidisessentiallyequaltothecomposi-tionofthemixturebuttheinfinitesimalamountofgasisricherinthemorevolatilecomponent.Atthedew pointthecompositionofvapourisessentiallythemixturecompositionwhereastheinfinitesimalamountofliquidisricherinthelessvolatilecomponent.Breaksinthelinearenotassharpasforpuresubstances.

Thepressure-volumediagramforaspecificn-pentaneandn-heptanemixtureisgiveninFigure10.Clearlyadifferentcompositionofthetwocomponentswouldresultinadifferentshapeofthediagram.

100

200

300

400

500

600

0 0.1 0.2 0.3 0.4 0.5

Critical point

Specific Volume - Cu. Ft. per lb.

Pres

sure

- PS

IA

454 º F450 º

425 º

400 º

350 º

300 º

Dew Point Line

Bubb

le P

oint

Lin

e

Figure 10 Pressure-VolumeDiagramforN-PentaneandN-Heptane(52.4mole%Heptane)ref.4

4.2 Pressure Temperature DiagramComparedtothesinglelinerepresentingthevapourpressurecurveforpuresubstancesthereisabroadregioninwhichthetwophasesco-exist.Thetwo-phaseregionofthediagramisboundedbythebubble point lineandthedew point line,andthetwolinesmeetatthe criticalpoint. Pointswithinalooprepresenttwo-phasesystems(Figure11).

Considertheconstanttemperatureexpansionofaparticularmixturecomposition.At1thesubstanceisliquidandaspressureisreducedliquidexpandsuntilthebubblepointisreached.Thepressureatwhichthefirstbubblesofgasappearistermedthebubblepointpressure.Aspressureisdecreasedliquidandgasco-existuntilaminuteamountofliquidremainsatthedewpointpressure.Furtherreductionofpressurecausesexpansionofthegas.


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Bycarryingoutaseriesofconstanttemperatureexpansionsthephaseenvelopeisdefinedandwithintheenvelopecontoursofliquidtogasratiosobtained.Thesearecalledqualitylinesanddescribethepressureandtemperatureconditionsforequalvolumesofliquid.Thequalitylinesconvergeatthecriticalpoint.

4.3 Critical PointInthesamewayaspurecomponents,whenmorethanonecomponentispresentliquidandgasescannotcoexist,atpressuresandtemperatureshigherthanthecriti-calpoint.Thecriticalpointforamorethanonecomponentmixtureisdefinedasapointatwhichthebubblepointlineanddewpointlinejoin,ie.itisalsothepointatwhichalltheintensivepropertiesoftheliquidareidentical.Thisaspectisaveryseveretestforphysicalpropertypredictionmethods.

IfthevapourpressurelinesforthepurecomponentsaredrawnontheP-Tdiagramthenthetwo-phaseregionforthemixtureliesbetweenthevapourpressurelines.Inthefigure11thecriticaltemperatureofthemixtureTcABliesbetweenTcAandTcBwhereasthecriticalpressurePcABliesabovePcAandPcB.ItisimportanttonotethatthePcABandTcABofthemixturedoesnotnecessarilyliebetweenthePc&Tcofthetwopurecomponents.

CA

CB

PCAB

TCA

Pres

sure

Temperature

Liquid

Gas

1

2

TCAB TCB

PCA

PCB

Bubble - Point Line

Dew Point

% Liq.

100

75

50

25

0

Critical Point

Figure 11 Pressure-TemperatureDiagramforaTwoComponentSystem

Aspecificmixturecompositionwillgiveaspecificphaseenvelopelyingbetweenthevapourpressurelines.Amixturewithdifferentproportionsofthesamecomponentswillgiveadifferentphasediagram.Thelocusofthecriticalpointofdifferentmix-turecompositionsisshowninFigure12fortheethaneandn-heptanesystem,andinFigure13foraseriesofbinaryhydrocarbonmixtures.Figure13demonstratesthatforbinarymixturee.g.Methaneandn-decanetwophasescancoexistatconditionsofpressureconsiderablygreaterthanthetwophaselimit,criticalconditionsfortheseparatepurecomponents.Methaneisasignificantcomponentofreservoirfluids.

1�

0 100 200 300 400

1400

1200

1000

800

600

400

200

0500 600

Temperature º F

C2

C1

A1

A2

A3B1

B2B3

B

A

C C3

C7

Dew Point li

ne

N-Heptane

Etha

ne

Bubble Point Line

CompositionNo Wt % EthaneC 100.00C1 90.22C2 50.25C3 9.78C7 N-Heptane

Pres

sure

, lbs

./Sq.

In. A

BS

Figure 12 Pressure-TemperatureDiagramfortheEthane-HeptaneSystem2



0

1000

2000

3000

4000

5000

6000

0 -100 0 100 200 300 400 500 600 700

Temperature º F

Pres

sure

Lbs

. (ps

ia)

M

ethan

e

Ethane

Propane N-Butane

N- Pentane

N-Hexane N-Heptane

N-Decane

Two Phases

Single Phase

Figure 13 CriticalPointLociforaSeriesofBinaryHydrocarbonMixtures2

4.4 Retrograde CondensationWithinthetwophaseregionourtwocomponentsystemtherecanbetemperaturesandpressureshigherthanthecriticaltemperaturewheretwophasesexistandsimilarlypressures.Theselimitingtemperaturesandpressuresarethecricondenthermandcricondenbar .Thecricondenthermcanbedefinedasthetemperatureabovewhichliquidcannotbeformedregardlessofpressure,orexpresseddifferently,asthemaxi-mumtemperatureatwhichtwophasescanexistinequilibrium.Thecricondenbarcanbedefinedasthepressureabovewhichnogascanbeformedregardlessoftem-peratureorasthemaximumpressureatwhichtwophasescanexistinequilibrium.(Figure14).

Theselimitsareofparticularsignificanceinrelationtotheshapeofthediagraminfigure14.

ConsiderasingleisothermonFigure14.Forapuresubstanceadecreaseinpressurecausesachangeofphasefromliquidtogas.Foratwo-componentsystembelowTcadecreaseinpressurecausesachangefromliquidtogas.Wenowconsidertheconstanttemperaturedecreaseinpressure,1-2-3,infigure14atatemperaturebetweenthecriticaltemperatureandthecricondentherm.Aspressureisdecreasedfrom1thedewpointisreachedandliquidforms,i.e.,at2thesystemissuchthat5%liquidand95%vapourexists,i.e.adecreaseinpressurehascausedachangefromgastoliquid,oppositetothebehaviouronewouldexpect.Thephenom-enaistermed Retrograde Condensation.From2-3,theamountofliquiddecreases

1�

andvaporisationoccursandthedewpointisagainreachedwherethesystemisgas.Retrogradecondensationoccursattemperaturesbetweenthecriticaltemperatureandcricondentherm.Theretrograderegionisshownshadedinthefigure.

Bubble

Point L

ine

Dew Point Line

% Liq.

100

75

50

25

510

0

Pres

sure

Temperature

Liquid

Gas

1

2

3

Cricondenbar

Cric

onde

nthe

rm

Region of retrograde condensation

Figure 14 PhaseDiagramShowingConditionsforRetrogradeConsiderations5. MULTI-COMPONENT HYDROCARBON

Usingtwocomponentsystemswehaveexaminedvariousaspectsofphasebehaviour.Reservoirfluidscontainhundredsofcomponentsandthereforearemulticomponentsystems.Thephasebehaviourofmulticomponenthydrocarbonsystemsintheliq-uid-vapourregionhowever isverysimilar to thatofbinarysystemshowever themathematicalandexperimentalanalysisofthephasebehaviourismorecomplex.Figure15givesaschematicPT&PVdiagramforareservoirfluidsystem.Systemswhichincludecrudeoilsalsocontainappreciableamountsofrelativelynon-volatileconstituentssuchthatdewpointsarepracticallyunattainable.



PVT CELL PHASE DIAGRAM

All Liquid

Gas / 40% Liquid

All Gas

First Gas Bubble

Last Drop of Liquid

"a"Critical Point

Dew Point

Bubble PointBubble Point

Temperature

Pres

sure

Pres

sure

Volume

Liquid

Bubb

le Po

int Li

ne

Dew Point Line

80%

Liqu

id

60%

40%

20%

Dew Point

Figure 15 PhaseDiagramsforMulticomponentSystems

Wewillconsiderthebehaviourofseveralexamplesoftypicalcrudeoilsandnaturalgases:

Low-shrinkageoil(heavyoil-blackoil) High-shrinkageoil(volatileoil) Retrogradecondensategas Wetgas DryGas

Figure16isausefuldiagramtoillustratethebehaviouroftherespectivefluidtypesabove.Howeveritshouldbeemphasisedthatforeachfluidtypetherewillbedifferentscales.Theverticallineshelptodistinguishthedifferentreservoirfluidtypes.

Isothermalbehaviourbelowthecriticalpointdesignatesthebehaviourofoilsystemsandthefluidisliquidinthereservoir,whereasbehaviourtotherightofthecriticalpointillustratesthebehaviourofsystemswhicharegasinthereservoir.

1�

X5

Pres

sure

Temperature

% Liquid

Gas

(Gas)Black

Oil Volatile

Oil Gas

Condensate Gas

TM2

75

100

50

25201510

50 Single Phase Region

Single Phase Region(Liquid)Single Phase Region

Two Phase Region

CP

Where:

Pb = Bubble point pressure at indicated temperature

Pm = Maximum pressure at which two phases can coexist

Tm = Maximum temperature at which two phases can coexist

C = Critical conditions

X5 = Cricondentherm

Bubble Point Line

Dew Point Line

PmPb

Figure 16 Phasediagramforreservoirfluids

5.1 Oil Systems ( Black Oils and Volatile Oils) Figures17&18 illustratethePTphasediagramsforblackandvolatileoils.

Thetwo-phaseregioncoversawiderangeofpressureandtemperature.Tcishigherthanthereservoirtemperature.Infigure17theline1-2-3representstheconstantreservoirtemperaturepressurereductionthatoccursinthereservoirascrudeoilisproducedforablack oil.Theseoilsareacommonoiltype.Thedottedlineshowstheconditionsencounteredasthefluidleavesthereservoirandflowsthroughthetubingtotheseparator.

If theinitialreservoirpressureandtemperatureareat2, theoil isat itsreservoirbubble pointandissaidtobesaturated,thatis,theoilcontainsasmuchdissolvedgasasitcanandafurtherreductioninpressurewillcauseformationofgas.Iftheinitialreservoirpressureandtemperatureareat1,theoilissaidtobeundersaturated,i.e.ThepressureinthereservoircanbereducedtoPbbeforegasisreleasedintotheformation.Foranoilsystemthesaturation pressure is the bubble point pressure.



Sep.

Pres

sure

Temperature

Liquid

Gas

1 Undersaturated

2 Saturated

3

100

75

50

25

0

Critical Point

Dew

Point

line

Bubb

le Po

int Li

ne

Mole % Liq.

Pb

Figure 17 PhaseDiagramforaBlackOil

Asthepressureisdroppedfromtheinitialconditionasaresultofproductionofflu-ids,thefluidsremaininsinglephaseinthereservoiruntilthebubblepointpressurecorrespondingtothereservoirtemperatureisreached.Atthispointthefirstbubblesofgasarereleasedandtheircompositionwillbedifferentfromtheoilbeingmoreconcentratedinthelighter(morevolatile)components.Whenthefluidsarebroughttothesurfacetheycomeintotheseparatorandasshownonthediagram,thesepara-torconditionsliewellwithinthetwophaseregionandthereforethefluidpresentsitselfasbothliquidandgas.Thepressureandtemperatureconditionsexistingintheseparatorindicatethataround85%liquidisproduced,thatisahighpercentageandasaresultthevolumeofliquidatthesurfacehasnotreducedagreatamountcomparedtoitsvolumeatreservoirconditions.Hencethetermlow-shrinkageoil.

Asthepressureisfurtherreducedasoilisremovedfromthereservoir,point3willbereachedand75%liquidand25%gaswillbeexistinginthereservoir.Strictlyspeakingoncethereservoirpressurehasdroppedtothebubblepoint,beyondthatthephasediagramdoesnottrulyrepresentthebehaviourofthereservoirfluid.Aswewillseeinthechapterondrivemechanisms,belowthebubblepointgasproducedflowsmorereadilythantheassociatedoilandthereforethecompositionofthereservoirfluiddoesnotremainconstant.Thesystemiscontinuallychanginginthereservoirandthereforetherelatedphasediagramchanges.Thesummarycharacteristicsforablackoilsometimestermedaheavyoilorlowshrinkageoilareasfollows.

Broad-phaseenvelope Highpercentageofliquid Highproportionofheavierhydrocarbons GOR<500SCF/STB

�0

Oilgravity30˚APIorheavier Liquid-blackordeepcolour

Volatile oilcontainsamuchhigherproportionoflighterandintermediatehydocar-bonsthanheavierblackoilandthereforetheyliberaterelativelylargevolumesofgasleavingsmalleramountsofliquidcomparedtoblackoils.Forthisreasontheyusedtobecalledhighshrinkageoils.Thediagraminfigure18showssimilarbehaviourtotheblackoilexceptthatthelinesofconstantliquidtogasaremorecloselyspaced.

Points1and2havethesamemeaningasfortheblackoil.Asthepressureisreducedbelow2alargeamountofgasisproducedsuchthatat3thereservoircontains40%liquidand60%gas.

Atseparatorconditions65%ofthefluidisliquid,i.e.lessthanpreviousmixture.Thesummarycharacteristicsforavolatilesometimestermedaheavyoilorhighshrinkageoilwhencomparedtoblackoilsareasfollows.

Notsobroadphaseenvelopeasblackoil Fewerheavierhydrocarbons Deepcoloured API<50˚ GOR<8000SCF/STB

Pres

sure

Temperature

Liquid

Gas

1

2

3

100

75

50

250

Critical PointMole % Liq.

Sep.

Bubb

le po

int lin

e

Dew po

int lin

e

40

Figure 18 PhaseDiagramforaVolatileOil

Clearly,forthesefluids,itisthecompositionofthefluidthatdeterminesthenatureofthephasebehaviourandtherelativepositionofthesaturationlines,(bubblepointanddewpointlines),thelinesofconstantproportionofgas/liquidandthecriticalpoint.


Institute of Petroleum Engineering, Heriot-Watt University �1

Forbothofthesefluidstypesonecanpreventthereservoirfluidgoingtwophasebymaintainingthereservoirpressureaboveitssaturationpressurebyinjectingflu-idsintothereservoir.Themostcommonpractiseistheuseofwaterasapressuremaintenancefluid.5.2 Retrograde Condensate GasIfthereservoirtemperatureliesbetweenthecriticalpointandthecricondenthermaretrograde gas condensatefieldexistsandFigure19givesthePTdiagramforsuchafluid.Abovethephaseenvelopeasinglephasefluidexists.Asthepressurede-clinesto2adewpointoccursandliquidbeginstoforminthereservoir.Theliquidisricherinheaviercomponentsthantheassociatedgas.Asthepressurereducesto3theamountofliquidincreases.Furtherpressurereductioncausesthereductionofliquidinthereservoirbyre-vaporisation.Itisimportanttorecognisethatthephasediagrambelowforaretrogradecondensatefluidrepresentsthediagramforaconstantcompositionsystem.

Beforeproductionthefluidinthereservoirexistsasasinglephaseandisgenerallycalledagas.Itisprobablymoreaccuratetocallitadense phase fluid.Ifthereservoirdropsbelowthesaturationpressurethedewpoint,thenretrogradecondensationoc-curswithintheformation.Thenatureofthiscondensingfluidisonlyinrecentyearsbeingunderstood.Itwaspreviouslyconsideredthatthecondensingfluidwouldbeimmobilesinceitsmaximumproportionwasbelowthevalueforittohavemobil-ity.Itwasconsideredthereforethatsuchvaluablecondensedfluidswouldbelosttoproductionandtheviabilityoftheprojectwouldbethatfromthe‘wet’gas.

Bubble

Point Line

Dew Point Line

Pres

sure

Temperature

Liquid

Gas

1

2

3

100

75

5025

1050

Critical Point

Mole % Liq.

Sep.

Figure 19 PhaseDiagramforaRetrogradeCondensateGas

Oneofthedevelopmentoptionsforsuchafieldthereforeistosetinplaceapressuremaintenance procedure whereby the reservoir pressure does not fall below thesaturationpressure.Watercouldbeusedasforoilsbutgasmightbetrappedbehindthewateras thewateradvances through the reservoir. Gas injection,calledgas

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cycling (Figure20),isthepreferredyetveryexpensiveoption.Inthisprocesstheproducedfluidsareseparatedatthesurfaceandtheliquidcondensates,highvalueproductrelativetoheavyoil,aresentforexport,inanoffshoresituationprobablybytanker.The‘dry’gasisthencompressedandreinjectedintothereservoirtomaintainthepressureabovethedewpoint.Clearlywiththisprocessthepressurewillstilldeclinebecausethevolumeoccupiedbythegasvolumeoftheexportedliquidisnotbeing replaced. Full pressuremaintenance isobtainedby importingdrygasequivalenttothisexportedvolumefromanearbysource.Eventuallytheinjecteddrygasdisplacesthe‘wet’gasandthenthefieldcanbeblowndownasaconventionaldrygasreservoir,ifasuitableexportrouteforthegasistheninplace.Theprocessdescribedisverycostlyandcarrieswithitanumberofrisksnotleastthepossibilityofearlydrygasbreakthrough.

Imported Gas

Gas

Surface Separation

Gas Water Contact

Dry Gas Reinjection

Injection Well

Production Well

Condensate Sales

Figure 20 Gascyclingprocess

Recentresearchhasshownthatthenatureofoilforminginporousmediabythisret-rogradeprocessmaynotbeasfirstconsidered.Theisolationofcondensingliquidsinporousrockisdependantontherelativestrengthoftheinterfacialtensionandviscousforcesworkingintherock.Iftherelativemagnitudeoftheseishighthenthefluidwillbetrappedhoweveriftheyarelowasaresultoflowinterfacialtension,whichisthecasenearerthecriticalpoint,thenthecondensingliquidsmaybemobileandmoveasaresultofviscousandgravityforces.Condensateliquidshavebeenabletoflowatsaturationswellbelowthepreviouslyconsideredirreduciblesaturationproportion.Establishedrelativepermeabilitythinkingishavingtobereconsideredinthecontextofgascondensates.Thephenomenajustdescribedmaygiveexplanationtotheobservationsometimesmadeofanoilrimbelowagascondensatefield.

LookingatthePTphasediagramonemightconsiderthat"blowingthereservoirdown"


Institute of Petroleum Engineering, Heriot-Watt University ��

quicklymightbeanoptionandasaresultvaporisethecondensedliquidsinthefor-mation.Thisisnotaseriousoptionsinceoncethereservoirpressurefallsbelowthedewpointtheimpactoftheincreasingliquidproportionremaininginthereservoircausesthephasediagramtomovetotherightrelativetoreservoirconditions,andanyvaporisingwillbeofthelightestcomponentswhicharelikelytobeingoodsupplyandthereforenotofsignificantvalue.Thesummarycharacteristicsforaretrogradegascondensatefluidareasfollows.

ContainsmorelighterHC’sandfewerheavierHC’sthanhigh-shrinkageoil APIupto60˚API GORupto70,000SCF/STB Stocktankoiliswater-whiteorslightlycoloured5.3 Wet GasThephasediagram for amixture containing smaller hydrocarbonmolecules lieswellbelowthereservoirtemperature.Figure21.Thereservoirconditionsalwaysremainoutsidethetwo-phaseenvelopegoingfrom1to2andthereforethefluidex-istsasagasthroughoutthereductioninreservoirpressure.Forawetgassystem,theseparatorconditionsliewithinthetwo-phaseregion,thereforeatsurfaceheavycomponentspresentinthereservoirfluidcondenseunderseparatorconditionsandthisliquidisnormallycalledcondensate.Theseliquidcondensateshaveahighpropor-tionoflightendsandsellatapremium.Theproportionofcondensatesdependonthecompositionalmixofthereservoirfluidasrepresentedbytheiso-volumelinesonthePTdiagram.

Pres

sure

Temperature

Liquid

Gas

1

2 100

75502550

Critical Point

Mole % Liq.

Sep.

Figure 21 PhaseDiagramforaWetGas

Thereferencewetgas,clearlydoesnotrefertothesystembeingwetduetothepres-enceofwaterbutduetotheproductioncondensateliquids.

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Insomelocationswheretherearenaturalpetroleumleakagesatthesurface,whencondensatesareproducedtheyaresometimescalledwhiteoil.Thesummarycharacteristicsforwetgasareasfollows. GOR<100,000SCF/STB Condensateliquid>50˚API

5.5 Dry GasThephaseenvelopeofthedrygas,whichcontainsasmallerfractionoftheC2-C6components,issimilartothewetgassystembutwiththedistinctionthattheseparatoralsoliesoutsidetheenvelopeinthegasregion(Figure22).Thetermdryindicatestherefore that thefluiddoesnotcontainenoughheavierHC’s to forma liquidatsurfaceconditions.

Thesummarycharacteristicsforadrygasareasfollows.

GOR>100,000SCF/STB

Pres

sure

Temperature

Liquid

Gas

1

2 755025

Critical Point

Sep.

Figure 22 PhaseDiagramforaDryGas

6 COMPARISON OF THE PHASE DIAGRAMS OF RESERVOIR FLU-IDS

Figure16gavearathersimplisticrepresentationofthevarioustypesoffluidswithrespect to therelativepositionofreservoir temperaturewithrespect to thephasediagram.Inrealityitisthephasediagramwhichchangesaccordingtocompositionandtherelativepositionofthereservoirtemperatureandseparatorconditions,andthesedeterminethecharacterofthefluidbehaviour.Figure23givesabetterindica-tionofthevariousreservoirtypeswithrespecttoaspecificpressureandtemperature



scales.Astheproportionofheaviercomponentsintherespectivefluidsincreasesthephaseenvelopemovestotheright.

Dry Gas Wet Gas Gas

Condensate

Separator

Critical Point

VolatileOil

BlackOil

Temperature (ºC)

Pres

sure

Figure 23 Relativepositionsofphasesenvelopes

7 RESERVOIRS WITH A GAS CAP

Figure24illustratesasimplificationofthephasediagramsassociatedwithanoilreservoirwithagascap.Thephasediagramforthegascapfluid,theoilreservoirfluidandforafluidrepresentingthecombinationfluidofamixtureofgasandliquidinthesameproportionsastheyexistinthereservoirarepresented.

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Pres

sure

Temperature

CG

CL

Reservoir Liquid

Total Reservoir Fluid

Reservoir TemperatureReservoir Gas

C

Separator

Initial Reservoir Pressure

Pd=Pb

Figure 24 PhaseDiagramforanOilReservoirwithaGasCap

Thediagramillustratesthatatthegas-oilcontactthegasisatitsdewpressure,theoilisatitsbubblepointpressureandthecombinationfluidliesontheconstantpropor-tionqualitylinerepresentingtheratioofthegasandoilastheyexistinthereservoirsystem.Thegascapmaybedry,wetorcondensatedependingonthecompositionandphasediagramofthegas.

8 CRITICAL POINT DRYING

Althoughnotpartofthetopicofphasebehaviourinthecontextofreservoirfluidsitisusefultoillustratetheapplicationinaverypracticalapplicationinthecontextoftheevaluationofrockproperties.Criticalpointdryinghasbeenusedbyanumberofsciencestopreparespecimensofdelicatematerialsforsubsequentmicrovisualanalysis where conventional preparation techniques will destroy delicate fabric.Criticalpointdryingtakesadvantageofthebehaviouroffluidsaroundthecriticalpointwhereonecangofromonephasetype,likeliquidtogaswithoutavisuallyobservedphasechange.Inthe1980’sitwasobservedinaUKoffshorefieldthattheinterpretedpermeabilityforawellsandinthezonewherewaterinjectionwasproposedwasdifferentfromwellinjectivitytestswhencomparedtothecoreanalysisvaluewherethevaluewasmanytimesmore.Theextentofthisdifferencewassuchthatpermeabilitiesfromthewelltestgavevalueswhichwouldpreventinjectiontotakeplacewhereasthosefromthecoretestswouldresultinpracticalinjectivities.Clearlythedifferencewasimportant.



Thecompanyconcernedembarkedonamoresophisticatedcorerecoveryandanaly-sisprocesssuspiciousthatperhapsthefabricoftherockwasbeingaffectedbycorepreparationmethods.Theyresortedtocritical point drying.

Thecorerecoveredfromthewaterzoneofthereservoirfromasubsequentnewwellwasimmersedandtransferredtothetestlaboratorysubmergedin‘formationwater’.Atthelaboratoryacoreplugsamplewasextracted,cuttosizeandloadedintoacoreholderstillsubmergedinthewater.Thecorewasthenmountedinaflowrig(figure25)andanalcoholwhichismisciblewithwaterdisplacedthewaterinthecore.Carbondioxideatapressureandtemperaturewhereitisintheliquidstatewasthenintroducedwhichmiscibledisplacedthealcohol.ThetemperatureandpressurewasthenadjustedtakingthemaroundthecriticalpointratherthanacrossthevapourpressurelineofthePTphasediagram(figure26)endingupwithatemperatureandpressurebelowthevapourpressurelinewiththefluidnowinagaseousstate.Afterthisprocessthepermeabilitywasmeasuredtobeofthesameorderasthatinterpretedfromthewellinjectivitytest.

Thereasonforthisdifferencewassubsequentlydemonstratedtobeaveryfragileclaywhichduringconventionalcorerecoveryandcleaningwasdamagedtoanextentthatitsporeblockingstructurewasdestroyed.

PT

Core In Holder

Figure 25 Criticalpointdryingsystem

Temperature

Pres

sure

Vapour Pressure Line

GAS

LIQUID

Critical Point

Critical Point Drying Route

Figure 26 Criticalpointdrying

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REFERENCES

1.Fig1Daniels,FFarrington:“OutlinesofPhysicalChemistry,”JohnWiley&Sons,IncNewYork,1948

2.Fig 2 Brown,GG et al. “ Natural Gasoline and Volatile Hydrocarbons,”NaturalGasolineAssociationofAmerica,Tulsa,Okl.,1948.

Fig10Sage,S.G.,Lacy,W.N.VolumetricandPhaseBehaviourofHydrocarbons,GulfPublishingCo.Houston1949

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