RESERVOIR ISSUE 08SEPTEMBER 201415INTRODUCTION Peter Pond called them tar sands in 1778 and in the early days of the oil business, tar sands were commonly called tar sands with a little bit of pride. The largest oil deposit in the world with a 400 year life span could not besneeredat.Intodayspolitically-correct double-speak, we now call them oil sands, nottobeconfusedwithconventionaloil sands. So oil sands it will be.TheoilsandsofAlbertaappeartobean easytaskforapetrophysicist.Afterall, thesandsareprettyclean,quiteporous, andthefluidpropertiesarereasonably well known. Even a novice geologist should beabletodoit.However,aseriesof forensic log analyses over the last 30 years orsosuggestthattherearesomebasic misunderstandingsabouthowoilsand cores are analyzed and how to calibrate log analysis results to that data. Ineachcase,theforensicanalysiswas undertakenattherequestofaclientwho was unsatisfied with prior work that did not appeartoprovideanadequatedescription of the hydrocarbon potential in an oil sands reservoir.Standardpetrophysicalanalysismodelsare usedforthevolumetricdeterminationof clay,porosity,water,andoil,andfrom thisarealisticpermeabilityestimate. Unfortunately,theDean-Starkcore analysismethod,widelyusedtoassessoil sandcores,doesnotmeasurevolumes. Instead,thetechniquemeasuresoilmass, watermass,andmineralmass.Theseare convertedtomassfractionandthento calculatedporosityandwatersaturation. Rarely,theremaybesomeheliumporosity and permeability data, but this is difficult in unconsolidated oil sands.Itistemptingtocompareloganalysis volumetricstotheDean-Starkcalculated volumetrics,andadjustloganalysis parameterstoobtainagoodmatch.The biggestproblemisthatthisformofcore analysisgivesameasureofporositythat issometimescalledtotalporosity,which includes clay bound water. In real life, some oftheclayboundwaterisnotdrivenoff bytheDean-Starkmethod,sothecore porosity falls somewhere between total and effective porosity. Thecalculatedwatersaturationfrom Dean-Starkalsofallssomewherebetween totalandeffective,whensomeclayis present.Sinceloganalysisgiveseffective porosity and saturation, we are comparing applestoaardvarks.Themessageisthat loganalysiscannotbecalibrateddirectly tothecorevolumetricdatawhenclayis present.Virtuallyalloilsandshavesome claycontentsomewhereintheintervalof interest.ButweCANcalibratetoDean-Stark coredatainthemassfractiondomain,by convertingthevolumetricpetrophysical analysisresultstomassfraction.That allowsustocompareapplestoapples, andlettheaardvarksgoabouttheirown business.Oilsandqualityisjudgedbyits oil mass fraction and net pay is determined by an oil mass fraction cutoff, not porosity and water saturation as in conventional oil. So oil mass fraction is a mandatory output from a petrophysical analysis.There are additional problems to resolve, as will be discussed below.WORKFLOWPetrophysicalanalysisofoilsandsfollows the standard methods that have been in use for more than 40 years:The math for these steps is at www.spec2000.net/01-quickmath.htm,exceptwherenotedinthetest. STEP1:Load,edit,anddepthshiftthe fulllogsuite,includingresistivity,SP,GR, density,neutron,PE,caliper,andsonic, whereavailable.Ifathoriumoruranium correctedGR(CGR)areavailable,load thesetoo.CreateaBadHoleFlagifoneis needed.STEP2:Calculateclayvolume.Because some uranium may cause spikes on the GR, usetheminimumofthegammarayand density-neutronseparationmethods.This eliminatesfalseshalebedsthatwould otherwiseappeartoactasbafflestothe flowofsteamoroil.TheSPisunlikelyto beausefulclayindicatorduetothehigh resistivity of the oil zone.STEP 3: Calculate clay corrected porosity from the complex lithology density-neutron crossplotmodel.Thismodelaccounts forheavymineralsifanyarepresent, compensatesforsmallquantitiesofgasif present, and reduces statistical variations in theporosityvalues.DONOTUSETHE DENSITYPOROSITYLOGALONE. Itwillreadtoolowifheavymineralsare presentandtoohighifgasispresent.The statistical variations at high porosity can give anoisyresult.Someoilsandshaveenough coal or carbonaceous materialto look like a coal bed. Set a coal trigger on the density and neutron and set porosity to zero when thetriggeristurnedon.Thereisnothing complex about the complex lithology model, so use it. See Special Cases below if there is gas crossover in the oil zone.STEP4:Calculateclaycorrectedwater saturationfromtheSimandouxordual-waterequations.Thesedefaulttothe Archiemodelincleansandsbutgivemore oil in shaly sands.STEP 5: Correlate core porosity and core permeability on a semi-logarithmic graph, if APPLES AND AARDVARKS A Tutorial on Petrophysical Analysis in Oil Sands| By E. R. (Ross) Crain, P.Eng., Spectrum 2000 Mindware Ltd, www.spec2000.net, ross@spec2000.netFigure1.Inshalysands,Dean-Starkcoreporosity (blackdots)isoftenlessthantotalporosity(black curve)andhigherthaneffectiveporosity(leftedge of red shading). In clean sands, Dean-Stark matches effective porosity extremely well. (Continued on page 17...) RESERVOIR ISSUE 08SEPTEMBER 201417any data is available. The resulting equation takestheformPerm=10^(A*PHIe+B) where A is the slope and B is the intercept at zero porosity on the graph. See Example in Figure 2.STEP6:Calculatepermeabilityasa continuouscurveversusdepth,usingthe regression analysis in Step 5.Steps1through6covertheconventional volumetric analysis of an oil sand, but we are not finished yet.STEP7:Convertloganalysisvolumetrics to mass fraction values.1: WToil = (1 Sw) * PHIe * DENSHY2: WTshl = Vsh * DENSSH3: WTsnd = (1 - Vsh - PHIe) * DENSMA4: WTwtr = Sw * PHIe * DENSW5:WTrock = WToil + WTshl + WTsnd + WTwtrOil mass fraction:6: Woil = WToil / WTrock7: WT%oil = 100 * WoilTypicaldensitiesareDENSMA=2650, DENSW=DENSHY=1000,DENSSH= 2300 kg/m3. STEP8:Abitumenpayflagiscalculated withaloganalysisoilmassfractioncutoff, usuallybetween0.050and0.085oilmass fraction.Agasflagshouldalsobeshown onthedepthplotswheredensityneutron crossoveroccursontheshalecorrected log data.STEP 9: Oil in place is calculated from the standardvolumetricequation.However, someoperators,especiallysurfacemining people,workintonnesofoilinplace.This equation is:1:OILtonnes = SUM (Woil * DENSoil * THICK) * AREA Thickness is in meters and Area is in square meters.Iftheoilequivalentinbarrelsorcubic metersisneeded,thestandardequation can be used:2:OOIP=KV3*SUM(PHIe*Soil*THICK) * AREA / Bo Where: KV3 = 7758 bbl for English units KV3 = 1.0 m3 for Metric unitsAREA = spacing unit or pool area (acres or square meters) Bo = oil volume factor (unitless)OOIP = oil in place as bitumen (bbl or m3)Recoveryfactorforsurfacemining operationsisveryhigh,maybe0.98or better.ForSAGD,RF=0.35to0.50are used. Since we cant keep the stream away fromtheshalysands,recoverywillvary withtheaveragerockqualityinaSAGD project. Waterhasaveryhighlatentheat,so thevolumeofwatertobesteamedisas importanttotheeconomicsasthevolume ofbitumen.Highwatersaturationisbad newshere,justasinconventionaloil.Top water,topgas,andcaprockintegrityare alsomajorSAGDissues.Thepetrophysical analysisneedstolookattherockswell beyond the bitumen interval.SPECIAL CASE MID-ZONE ANDTOP GASThe conventional equation for porosity in a gas sand is:1: PHIe = ((PHInc^2 +PHIdc^2) / 2) ^ (1 / 2)Thisequationisaccurateenoughformost gaszones,butinveryshallowgassands, itwillunderestimateporosity.Theabove equation must be replaced by:2: PHIe = ((PHInc^X + PHIdc^X) / 2) ^ (1 / X)Where:X is in the range of 2.0 to 4.0, default = 3.0. PHIdcandPHIncareshalecorrectedvalues of density and neutron porosity respectively.Figure 2. Meta/Log AnalysisFigure 3. Calculated tar mass from log analysis (black curve)matchesDean-Starkoilmass(blackdots)in the mid-zone gas as well as in the bitumen interval. (Continued on page 19...)(...Continued from page 15)18 RESERVOIR ISSUE 08SEPTEMBER 2014 Figure4.Oilsandanalysiswithtopwater,bottomwater,topgas,andmidzonegas.Coreandlogdatamatch-butoilmass(thirdtrackfromtheright)isthecritical measure of success. Core porosity is less than total porosity and greater than effective porosity in shaly zones Minor coal streaks occur in this particular area. Oil mass matches core extremely well, even in the mid-zone gas interval (pink shading in porosity track and in the density-neutron track), showing the efficacy of the hydrocarbon partitioning and gas correction models. RESERVOIR ISSUE 08SEPTEMBER 201419TheexponentXisadjustedbytrialand erroruntilagoodmatchtocoreporosityis obtained.Thisporosityisthenusedtofindwater saturation as in Step 4, described earlier. Many,butnotall,gaszonesrelatedtooil sandshavesomeresidualoil.Hydrocarbon saturation needs to be partitioned between bitumen and gas by the following method:3: Vwtr = PHIe * Sw 4: Vhyd = PHIe * (1 Sw) 5:GasTarRatio = Max(0, Min((1 OIL_MIN), (PHIDc PHINc) / MAX_XOVER)) 6: Vgas = GasTarRatio * Vhyd 7: Voil =(1 GasTarRatio) * VhydWhere:OIL_MIN = minimum oil volume in gas zone as seen on core analysis, could be zero.MAX_XOVER=maximumdensityneutron crossover in a gas zone (fractional)Oil weight is calculated in a fashion similar toStep7describedearlier,butusing thepartitionedoilandgasvolumes. CONCLUSIONSAppropriateshale,porosity,andwater saturationmethodsarerequiredforatop qualityoilsandanalysis.Oversimplified methods are not sufficiently accurate.Duetotheincompatibilitybetweenthe Dean-Starktotalporositymodelandthe effectiveporositymodelusedinstandard log analysis, we find that there is no point incomparingthesetwoporosityvalues, exceptinperfectlycleansands.Direct comparison in clean sands offers no insight as to the correctness or otherwise of the porosityinshalysands.Comparingtar massistheonlywaytoputbothmodels onanequalfooting.Oilmassisnot difficult to calculate so there is no excuse to avoid doing the extra step.Mid-zonegasandtopgasmaynotreceive theporositytheydeserveusingthestard density-neutroncrossplotmodel.The modifiedequation,plusthehydrocarbon partitioningmodel,allowbothaccurate porosityandaccurateoilmasstobe calculated. ABOUT THE AUTHORE.R.(Ross)Crain,P.Eng.isaConsulting Petrophysicist and Professional Engineer, with over 50yearsofexperienceinreservoirdescription, petrophysicalanalysis,andmanagement.Heis a specialist in the integration of well log analysis andpetrophysicswithgeophysical,geological, engineering,stimulation,andsimulationphases oftheoilandgasindustry,withwidespread CanadianandOverseasexperience.Hehas authoredmorethan60articlesandtechnical papers.Hisonlinesharewaretextbook,Crains PetrophysicalHandbook,iswidelyusedasa referenceforpracticalpetrophysicalanalysis methods.Mr.CrainisanHonouraryMember and Past President of the Canadian Well Logging Society(CWLS),aMemberofSPWLA,anda Registered Professional Engineer with APEGATITLE START DATE LENGTHINSTRUCTOR Basics of Geomodeling An Overview3-Sep-142 daysDavid Garner An Introduction to Advanced Geostatistics25-Sept-142 days Clayton V Deutsch The Bakken-Three Forks - An Unconventional Petroleum & Reservoir System - A Workshop 08-Oct-141 dayRick Sarg Characterization and Management of SAGD Reservoirs with Geostatistical andOptimization Techniques 17-Nov-144 days Clayton V Deutsch CONTINUING EDUCATION Register Online today at www.cspg.org (...Continued from page 17)