Oil & Gas From exploration to distribution Week 2 V11 Production Mechanisms and Hydrocarbon Recovery Jean-Marie Voirin W2V11 - Production Mechanisms p. 1 IFPEN - IFP School 2015 / TOTAL SA 2015 / IFP Training 2015 Introduction Inthissession thepart oftheReservoirEngineeringdescribedisdevotedtorecoveryofoil andgasfieldsandassociatedproductionmechanisms,ashighlightedonthischart.Wewill mainly talk about oil fields, however most of the comments apply to gas reservoirs too. Firstwewilltrytounderstandwhyawellisproducingandwhatwecallrecoveryfactor. Thenwewillreviewthedifferentrecoverystages,Primary,SecondaryandEnhancedOil Recovery. Production Intheprevioussections,itwasexplainedhowanoilreservoirisformedduringgeological times and you have seen the properties of the rock and fluids. This drawing of a simplified geological cross-section shows a series of porous and permeable rock layers extending from the surface to the location of the oil reservoir. Theyareoverlaidwithasealingimpermeablelayercalledthecaprockandunderlaidwith other rock layers of no interest here. Most of the porous and permeable layers are saturated with water, this is called an aquifer,exceptintheoilreservoirwhereoilistrapped.Inthis part,waterwasreplacedbyoilovergeologicaltimes.Onlyunmovablewaterremains.The aquiferis,orwas,connectedinsomewaytothesurface,asindicatedbythebluearrow, over a distance which may be large. Duetothislinktothesurfacetheporepressureina reservoirisusuallyclosetohydrostaticmeaningthatitisequivalenttothepressureofa column of water extending from the surface to the reservoir depth. Oilislighterthanwater.Whenawellisdrilledtotheoilreservoirthepressureoftheoil column is less than the reservoir pore pressure. This excess pressure at the wellhead allows the oil to flow naturally. W2V11 - Production Mechanisms p. 2 IFPEN - IFP School 2015 / TOTAL SA 2015 / IFP Training 2015 Thewellproductionrateisafunctionofthepressuredrawdownwhichisthedifference between the pressure at the bottom of the well and the reservoir pressure further from the well. It is also a function of the fluid ability to move through the reservoir to the well. As oil is removedthereservoirpressurewillstarttodropandprogressivelytheratewilldecrease due to the change in drawdown. The oil quantity that can be produced from a reservoir is called recovery. Initially it depends on the natural energy present in the reservoir and on how efficiently this energy is passed on to the producing wells. If natural energy is not sufficient to yield an economical combination of rate and recovery, external energy will have to be provided. This is the subject of this lecture. W2V11 - Production Mechanisms p. 3 IFPEN - IFP School 2015 / TOTAL SA 2015 / IFP Training 2015 Recovery Factor Toeasilycomparetheeffectivenessofthedifferentrecoverymethodswithinagivenfield andbetweenfields,adimensionlessratioisused.ItiscalledtheRecoveryFactorand corresponds to the percentage of the volume of oil effectively produced (or recovered) from a reservoir, as measured at the surface to the volume of original oil in place calculated in the same surface conditions. The Recovery Factor depends on a number of parameters including1.the field rock and fluid characteristics,2.the production mechanisms and the associated recovery stages: natural energy only or supplemented by external energy,3.the number, type and location of wells, producers and injectors4.and the production economic limit related to production level and associated costs. These different items are always interrelated. W2V11 - Production Mechanisms p. 4 IFPEN - IFP School 2015 / TOTAL SA 2015 / IFP Training 2015 Primary Recovery The oil reservoir is located at a depth that can reach a few kilometers. It usually has a length and width of several kilometers and athickness of 10s to 100s of meters. Primaryrecoverycorresponds toproductionmechanisms whichonlyusethenatural energypresentinthereservoir. Itusuallyhappensfirstduring thelifeofafield,henceits name.Thisstageisalsocalled natural drainage. Recoverycanbeincreasedby pumpingtheproducingwells whenneeded.Evensorecovery atthisstageisusuallylowand anoilreservoirwilloftenbe producedbymeansofseveral producingmechanismsoverits lifetime. Themostsimplecase,whichis alsothemostunfavorable, happenswhenthelinkwiththe aquiferisextremelyweakor nonexistentandwhenthereis no free gas in the reservoir.Theonlyenergyavailableisthe energyassociatedwiththe compressionoftheoil,the associatedunmovablewater and the rock. Prior to any production, the reservoir pore volume is filled by oil and unmovable water. Thebarsontherightrepresent therelativevolumesoffluidsin place at the initial pressure. W2V11 - Production Mechanisms p. 5 IFPEN - IFP School 2015 / TOTAL SA 2015 / IFP Training 2015 Whenoilisremovedfromthe reservoirbytheproducingwells, pressuredecreasesandenergy associatedwiththerockand fluidsisreleased.Severalthings take place: thereislessoilbutthe remainingoilhas expanded due to the lower pressure the space left is filled by: thewaterwhichexpandsdueto thedecompressionandoccupies a larger volume in the pores, thearrangementoftherock grainswhichismodified, shrinkingtheporevolume available between them. Productionhasbeenmade possiblebythefluidexpansion andporevolumeshrinkage replacing the produced oil.Thisproductionmechanism alwaysexistsassoonasthe pressure drops in a reservoir.Dependingonrockandfluid characteristics,theamountof producibleoilwillvary.To estimatehowmuchoilcanbe recovered. A material balance of the reservoir system before and after production can be done. In this particular case it results in the equation shown here. The oil volume originally in place attheinitialpressureisontheleft-handsideoftheequationwhereastheright-handside W2V11 - Production Mechanisms p. 6 IFPEN - IFP School 2015 / TOTAL SA 2015 / IFP Training 2015 reflects the oil volume remaining at a later time at a lower pressure P as well as the changes associated with water expansion and pore volume shrinkage.

This material balance equation can be arranged to show the Recovery Factor, the ratio of the oileffectivelyproduced,calledNp,tothevolumeoforiginaloilinplace,expressedin surface conditions, called N. In this primary recovery case, with no aquifer and no free gas, the natural energy available is limited, as compressibilities of the oil, water and rock are very small. Thecorrespondingrecoveryfactorislow.Wheninsertingtypicalvaluesinthematerial balance formula, the calculation gives a result of around 5%. It means that 95% of the oil will be left in the ground. In a gas reservoir, however, as gas compressibility is much larger than that of the water and rock, primary recovery is the main production mechanism and may lead to high recoveries, around 80%.

W2V11 - Production Mechanisms p. 7 IFPEN - IFP School 2015 / TOTAL SA 2015 / IFP Training 2015 Inoilreservoirs too,favorablecases existforwhichadditionalnatural energyispresentleadingto significantly higher recoveries: asyouknow,belowacertain pressure,solutiongaswillbe released from the oil and, due to its highcompressibility,willprovide additional energy by expansion. This primarymechanismiscalled solutiongasdrive.Therecovery factor may rise up to 15 / 20%. Agascapmayalsobepresent,as pressuredropsduetoproduction, thegascapwillprovideadditional energyanddisplacetheoilasit expands downwards due to the high compressibilityandlowdensityof thegas.Thisprimarymechanismis called gas cap drive. Recoverybythismechanismdependsontheratiobetweenthe gas cap and oil volumes. A gas cap of largevolumecanleadtorecoveries up to 40%. Often,thepresenceofanaquifer connectedtotheoilreservoirwill provideenergyandreducethe pressuredropas waterreplacesthe producedoil.Thismechanismis calledwaterdrive.Theaquifercan belocatedatthebottomoftheoil reservoirorattheedge.Anaquifer issaidtobeactiveifitprovides enoughenergytoefficientlyreplacethe produced oil volume. To achieve that, the connection with theoilreservoirhastobegood, especially in terms of permeability,and the size of the aquifer has to be largewhencomparedtotheoil reservoirvolume.Theactivewater driveisthemostfavorablecaseof primaryrecoveryandmayleadtoa recoveryfactorofmorethan50%, sometimes as high as 70%. Combinationsofthesemechanisms mayalsotakeplaceinthesame reservoir. W2V11 - Production Mechanisms p. 8 IFPEN - IFP School 2015 / TOTAL SA 2015 / IFP Training 2015 Secondary Recovery However,mostoften,naturalenergyprovidedbythedifferentprimarymechanismsis limited and recovery remains low. When the natural energy contained in the reservoir is not sufficient to provide a reasonable andeconomicrecovery,additionalenergywillhavetobebroughtinbyinjectingfluids. TheseproductionmethodsarecalledSecondaryRecoverymethodsastheytakeplace following Primary Recovery. Objectives of the secondary recovery methods are to increase recovery by: 1.maintaining the reservoir pressure (or at least limiting its decline) 2.sweeping the oil towards the producers. The secondary recovery methods try to mimic nature and use specific wells to inject either gas(thesameideaasgascapdrive)orwater.Thistechniqueiscalledwaterfloodingand theideacomesfromthenaturalwaterdrive.Itisthemostcommonmethodaswateris easily available at low cost. We will look at this in more detail in the following slides. In some fields, both fluids can be injected in different parts of the reservoir. Waterflooding uses specific wells in addition to the producers to inject the water.Whenawaterfloodisinitiated,watercanbeinjectedtosupplementanexistingaquiferat theperipheryoftheoilreservoir,thisiscalledperipheralinjection.Insomecasesitis sufficient.Ifnot,watercanalsobeinjectedintotheoilzoneeitheratthestartor progressively following peripheral injection. This is called infill injection. Inbothcasesthepurposeofinjectingwateristosweeptheoiltowardsthelowpressure areascreatedbytheproduction.Waterwillreplacetheproducedoilandlimitfurther pressure drop. Nowwewilllookatthelevelofrecoverythatcanbeachievedbythistechnique.This3D drawing represents a segment of a regular pattern of wells in an oil field under a waterflood, 3 producers and 1 injector. It shows what happens as water is injected into the oil reservoir. Apartofthevolumebetweentheinjectorandtheproducersisnotcontactedbythe injectedwater,itiscoloredingreen.Sweepingoftheoilislimitedbothhorizontallyand vertically.W2V11 - Production Mechanisms p. 9 IFPEN - IFP School 2015 / TOTAL SA 2015 / IFP Training 2015 The efficiency of the sweep varies based on: the level of heterogeneities and layering, high permeability layers being invaded by water more quickly, thewaterandoilrelativemobilities,ifwaterissignificantlymoremobilethanthe oil (oil viscosity larger than water viscosity), the volume swept is likely to be small as water will channel through the oil, the well location, density and the types of well drains (vertical, horizontal),and finally the economic criteria which will control the cumulative injected quantities and final oil rate. The most accurate way to estimate sweep efficiency and assess the associated uncertainties is by using a numerical simulation model of the reservoir. Typically70to80%oftheporevolumeiseffectivelysweptwhencombiningallthese elements.Inaddition,inareaseffectivelysweptbywaterthereisstilloiltrappedinthe pores due to strong capillary forces. Trapped oil is measured in the lab on rock core samples. This has a significant degree of uncertainty as the sampling is limited and the reservoir rocks heterogeneous. On average only 60% to 70% of the oil can be mobilized and displaced, the remaining 30 to 40%isdefinitivelylostforwaterfloodrecovery.Whencombiningtheseeffectsandtaking into account the fact that some compartments of the field may not be drained by any well, W2V11 - Production Mechanisms p. 10 IFPEN - IFP School 2015 / TOTAL SA 2015 / IFP Training 2015 therecoveryfactorachievedbywaterfloodingrangesfrom35to 50%. Itmeans thatmore than 50% of the oil in place will still remain in the ground. Enhanced Oil Recovery OnlytheimplementationofspecialprocessescalledEnhancedOilRecoveryorEORwill,in some instances, be applicable to significantly increase the recovery factor by another step. WhenEORfollowsSecondaryRecoveryitcanbecalledTertiaryRecovery.HoweverEOR mayhavetobeimplementedinitially,asinbitumenreservoirs,orafterprimaryrecovery whenwaterorgasinjectionsarenotanoption.Thesetechniquesaremoreexpensiveand moredifficulttoimplementandtheirapplicabilityisreallyfieldspecificastheydepend strongly on rock and fluid properties. As a consequence their contribution to the total world oil production is less than 5% today. Toachievetheirobjectivestheseprocessesmaymodifythefluidmobilitiestoimprovethe sweep efficiency by addingpolymers to the injected water in order to increase its viscosity and make it closer to the oil viscosity or by heating the oil, usually using steam or they may changethecapillarypropertiestomobilizetheoiltrappedinporesafterwaterfloodby addingsurfaceactiveagents(surfactants)totheinjectedwatertostronglydecrease capillaryforcesorinjectingsolventsmisciblewiththeoil,ashydrocarbongases,CO2or N2. Conclusion Tosummarizethislecturewewilllookatatypicalfieldproductionprofilewithtime, production in ordinate, years in abscissa. 1.ItstartsbyPrimaryRecoveryornaturaldrainage.Thenaturalenergypresentinthe reservoirisusedforproduction.Asavailableenergydeclines,sodoestheoilrate. Theareabelowthecurveshowswhatwouldbetherecoveryifnothingelsewas done. W2V11 - Production Mechanisms p. 11 IFPEN - IFP School 2015 / TOTAL SA 2015 / IFP Training 2015 2.Toincreaserecoveryandmaintaintheoilproductionatagivenplateau,Secondary Recoverymethodshavetobeimplementedatacertaintime.Waterhastobe injected to maintain the pressure and sweep the oil towards the producers. 3.As water starts to be produced, the oil rate will decline and other techniques may be needed,atleasttolimitthedecline.Thesetechniquesmaycorrespondto improvements of theSecondaryphase(forexamplehorizontalversusverticalwells) or enhanced methods, the Enhanced Oil Recovery techniques discussed earlier. W2V11 - Production Mechanisms p. 12 IFPEN - IFP School 2015 / TOTAL SA 2015 / IFP Training 2015