Shiraz 2009 - First International Petroleum Conference & Exhibition Shiraz, Iran, 4 - 6 May 2009 B40Simulation Study of Conventional Fire Flooding(CFF) in Fractured Combustion Cells: A PromisingTool along ExperimentS. M. Fatemi (Sharif University of Technology), R. Kharrat* (PetroleumUniversity of Technology) & C. Ghotbi (Sharif University of Technology)SUMMARYThe Conventional Fire Flooding (CFF) process application feasibility on fractured carbonated reservoirsremained questionable. In this paper first combustion parameters and reaction kinetics of a naturallyfractured low permeability carbonated heavy oil reservoir in Iran called Kuh-E-Mond applied tosimulation study. After that, simulator has been validated with Kuh-E-Mond combustion tube experiment.Recovery mechanism in single block matrix is different from one in conventional model since oxygen firstflows into the fractures and then diffuses from all sides into the matrix. Combustion of the oil in thefractures produces some water ahead of fracture combustion front which prohibits oxygen from earlybreakthrough through fractures into production well. Water imbibes to the matrix and causes matrix oildrainage to the producer. This oxygen diffusion/water imbibition based recovery mechanism is slower inproduction rates compare to conventional model recovery mechanism, and causes lower produced oilquality since less oxygen is available for matrix. Further, sensitivity analysis on air injection rate,formation thickness, injection well depth of perforation, horizontal fractures and also effect of wateralternating air process on fracture model results have been studied. Shiraz 2009 - First International Petroleum Conference & Exhibition Shiraz, Iran, 4 - 6 May 2009 1.Introduction Astheresourcesavailableforconventionaloilinworldwidecontinuetodecline,further developmentofheavyoil,extraheavyoilandbitumenrecoverytechnologiesiscriticalto meet present and future energy requirements (Nasr and Ayodele , 2005) and (Etherington and McDonald,2004).Fortheproductionofoilfromheavyoilreservoirs,thermalmethodsare applied widely. One of these is in situ combustion (ISC) process. In this process air is injected intothereservoirandtheoxygenintheairburnspartoftheoil,therebygeneratingheat, which reduces the oil viscosity and enhances oil recovery (Prats, 1984), (Thomas, 1988) and (Partha,1998).InsomefieldtrialsofISC,thecombustionprocesscouldnotbesustainedif therewerefracturesinthereservoir(Al-Baharetal.,2004).Itisargued,sincefracturesare much more permeable than the surrounding reservoir rocks, the injected air will flow almost exclusively through the fractures and will contact only oil present in these fractures or in their immediate vicinity. In this case, not only the reaction rate is too low because of the very small contact area between air flow and fracture walls, but also the total amount of fuel available for combustion might be insufficient to sustain the combustion process.AccordingtoSchulteandVries(1982),ifonlythelowreactionrateisresponsiblefor dying out the combustion front, in densely fractured reservoirs contact area between air flow and fracture walls might be sufficiently large to sustain combustion, assuming that sufficient fuelisavailable.They foundthatduetohighpermeabilityoffractures,airfirstspreads through the fractures and then diffuses into the matrix from its surrounding faces. Also, it was found that diffusion is the most important phenomena for burning and recovering oil from the matrix. 2.Methodology 2.1.Combustion Tube Experiment Inadditionahigh-pressurelaboratorycombustiontubesystem(Figure-1)wasdesignedand wasbuilttoevaluatethein-situcombustionprocesswithairandoxygen-enrichedair.The combustionassemblyconsistsofacombustiontube,highpressurejacket,heatingjacket, pressurebackregulator,massflowcontrol,O2/CO/CO2analyzers,anddataacquisition system.Thecombustiontubeiscylindricalstainlesssteeltubeof0.325inches(0.826cm) wall thickness. It has a 3.94 inch (10 cm) internal diameter, and is 40 inches (100cm) long. 2.2.Simulation of Combustion Tube Experiment In numerical simulation of combustion tube, a vertical matrix block which is consisted of 20 grid block (center of grids will be located on the thermocouple locations in experiment) in z direction,onegridblockinx,ydirectionsisconsidered.Inthismodelthefollowingsix components and pseudocomponents were introduced to the simulator: water, heavy oil (HO), light oil (LO), inert gas, oxygen and coke. All noncondensable gases such as CO2, CO and N2 werelumpedtoasingleinertgastominimizethenumberofequationstobesolved.KEM flashcompositionhasbeenlumpedintotwogroupknownaslight(C1-C11)andheavyoil (C12+). Table 2 shows comparison between simulation and experimental data. 2.3.Three-Dimensional Conventional Combustion Cell Simulation Arectangularcombustioncell(3D)measuring1m0.3mrectangleby0.055mdeepwas usedtocarryouttheCFFsimulation.Inordertoachieveanadiabaticconditionforthe3D cell operation, no heat loss to the surrounding has been assumed. Permeability and porosity of thecellsetequalto1270mdand0.414.Drycombustiontestwerecompletedusingthe standardISCwellarrangement,i.e.VIVP(verticalinjectorverticalproducer)andwells completed in the midway thickness of the cell. External heaters used to ignite the combustion in the cell.2.4.Three-Dimensional Fractured Combustion Cell Simulation SinceKEMisalsoafracturedreservoir,theconventionalcombustioncelloftheprevious sectionwasmodifiedtostudytheeffectoffractures.Thematrixblockissurroundedby verticalfractures(twofractureperpendiculartotheflowdirectionandtwointheflow direction).Theoilproductionmechanismisbasedonlytothefracturescommunicationand Shiraz 2009 - First International Petroleum Conference & Exhibition Shiraz, Iran, 4 - 6 May 2009 the oil has to be produced from the fractures, since both injection and production wells have been completed in the fractures layers. 3.Results and Discussion 3.1.Comparison of ISC Process in Conventional and Fractured Systems Recovery mechanism is somewhat different in fractured model compare to conventional case. Infracturedmodeltheinjectedairflowsinsidefracturessincetheyarethehighpermeable, lowresistiveporousmediaandfromthere,itdiffusesintothematrix.Thisdifferent mechanism affects to the process outputs such as: API of the produced oil, front temperature, sweepefficiency,oxygenconsumption,producedgreenhousegases,ultimateoilrecovery, produced water, front shape and air breakthrough time.3.2.Sensitivity Analysis of ISC Process for Fractured Combustion Cell 3.2.1.Air Injection Rate Air injection rate should be optimized since higher ones cause, higher quality of the produced oilandwaterproductiondelay.Butfromanotherpointofviewthereissomereductionon ultimateoilrecoveryafteralimit,higherfronttemperaturewhichshouldnotbeabove carbonatedrockdecompositiontemperature,acceleratedairbreakthroughandhigherOPEX for bigger compressors that can inject air for higher rate to the system. 3.2.2.Formation Thickness Formationthicknessincreasereducesaveragesystemtemperatureandoilrecoveryfactor fromthesystem.Inthecaseofsufficientlythickformation,combustionfrontcaneven extinct. Lower quality (lower API) crude oil produced in the case of thicker formation. Post-mortem analysis of the sand-packs, at the same cross-section located the same distance from injection well, shows lower volumetric sweep efficiency in the case of thicker formation. 3.2.3.Injection Well Depth of Perforation Tostudytheeffectofinjectionwelldepthofperforation,thethickerformationofthelast sectioninwhichcombustionextinct,choseandtheinjectionwellcompletedatthelowest layer instead of the middle one. By this new depth of completion, ISC process was sustainable and as a result oil recovery factor increased significantly. Perforation depth of injection well shouldbeoptimizedinordertohaveasustainablecombustionfrontfromonesideand avoidingextradrillingcostsfordeeperinjectionwell(whichguaranteescombustionfront) from another side. 3.2.4.Water Alternating Air Injection: WAA ThemainobjectofusingwaterinISCprocessisutilizingthegeneratedheatwhichisleft behindthecombustionfrontandgoestowasteaftersomemovementofcombustionfront. Economycanbeimprovedbyheatrecuperationthroughwaterinjectionwhichleadsto reducingtheamountofairinjection.Toinvestigatethebenefitsofin-situsteamgeneration and also auto-ignition after water injection, in this part of paper water alternating air process simulatedforbothconventionalandfracturedmodels.Itwasfoundthattheoilproducedin thecaseofWAA-ISCislowerquality(lowerAPI)thanthatofdrycombustionprocess. Betterairsweepefficiency(accordingtocokedepositioninpost-mortemanalysis)inwater alternativeaircomparetodryISCbecauseofthesteamsweep,isanotherbenefitforthis technology.Thevalueofwaterairratio(WAR)shouldbeoptimizedaccordingtothese benefits in conjunction with higher oil recovery factor. 4.Conclusions Presenceoffracturesreducedcombustionfrontandsystemaveragetemperature.Lower combustiontemperatureinthiscasecanreducethepossibilityofcarbonatedrock decompositionanditsdisastrouseffectonultimateoilrecoveryfactor.Combustionfront profile, in terms of peak temperature zone, is more uniform in fractured model compare to the distinctconcaveshapeinconventionalmodel.Producedoilfromfracturedmodelhadlower quality than conventional model. Shiraz 2009 - First International Petroleum Conference & Exhibition Shiraz, Iran, 4 - 6 May 2009 Accordingtotheoxygenprofilesattheendoftheprocess,higherverticalandarealsweep efficiencyoftheburnedzoneachievedinnon-conventionalmodelcomparetothe conventionalone,althoughtheglobalsweepefficiencyreducedbecauseoflowerprocess performancerateinfracturedmodel.Thesamewastrueforvolumetricsweepefficiency according to post-mortem analysis of the deposited unburned coke. Water bank of combustion process,generatedatthecentralpartofthecellinconventionalmodel,comparetothe fractured zone in the non-conventional model. This generated water in the later case, prevents oxygen to breakthrough from fractures into the production well, as well as, can be considered as an oil depletion mechanism from matrix, since water imbibes into the matrix from fractures and pushes oil into the producer zone.Forcombustionprocesstobesustainableandfeasibleinfracturedmodelinadditiontoair injection rate (which should be optimized according to air breakthrough and oil recovery and avoidingextraproductiontimeandasaresultextraOPEX),alsoperforationdepthof injection well should be optimized in order to have a sustainable combustion front from one side and avoiding extra drilling costs for deeper injection well (which guarantees combustion front stability) from another side. Water alternating air in the fractured system had the same benefits as the conventional model (higher oil recovery factor and better areal and volumetric sweep efficiency). In the fractured modelthesameasconventionalmodel,WARandtheperiodofwaterinjectionshouldbe optimized. References Ahmed T. [1989] Hydrocarbon Phase Behavior. Gulf Publication Company. Al-Bahar, M.A., Merrill, R., Peake, W., Jumaa, M., and Oskui, R. [2004], Evaluation of IOR Potential within Kuwait, SPE Paper 88716, presented at the 11th Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, UAE, October. Danesh A. [1998] PVT and Phase Behavior of Petroleum Reservoir Fluids. Elsevier. Etherington, J.R., and McDonald, I.R. [2004] Is Bitumen a Petroleum Reserve? SPE 90242, presented at the SPE Annual Technical Conference and Exhibition, Houston, Texas, USA, September. FatemiS.M.[2008]FeasibilityStudyofISCforKEMNaturallyFracturedCarbonatedHeavyOil ReservoirinLaboratoryScale,PartI-1DSimulation.InternalDocumentforthefulfillmentofthe MSc. 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VanGolf-RakhtT.D.[1982]FundamentalofFracturedReservoirEngineering.ElsevierScientific Publication Company, Netherlands. Shiraz 2009 - First International Petroleum Conference & Exhibition Shiraz, Iran, 4 - 6 May 2009 Table 1 KEM Reactions Stoichiometry from TGA experiment. Reaction TypeReaction StoichiometryLight Oil Oxidation Heavy Oil Oxidation Heavy Oil Thermal Cracking Coke Oxidation O H CO O CokeCoke LO HOO H CO O HOO H CO O LO2 2 22 2 22 2 255 . 0 11 . 196 . 25 154 . 234 . 28 53 . 51 99 . 5658 . 6 96 . 11 23 . 13+ ++ + ++ + Table 2 Comparison of KEM combustion tube experimental and simulation results. DataExperimental ResultsSimulation Results Front Propagation Rate 0.2 cm/min(0.42 ft/hr)0.42 ft/hr Total Process Time 470 min (7.88 hr)7.92 hr Front Average Temp.587C1090F (587.77C) Front Location @ end 2.25 ft2.3 ft Production Well Temp. @ end 285C550F (287.77C) Figure 1 Schematic of KEM combustion tube experiment set up.