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1BasicsofGeophysicalWellLogs_Porosity
2BasicsofGeophysicalWellLogs_Porosity
Porosity(F),canbedefinedastheratiobetweenthevolumeoftheporesandthetotalrockvolume.Porositydefinestheformationfluidstoragecapabilitiesofthereservoir.Welllogsallowthein-situ,indirectevaluationoftheformationporosity.Themostusedonesare:• Density• Neutron• Sonic• NuclearMagneticResonanceSinceallthesemeasurementsareinfluencedbyformationfluidsandmineralsofthematrix,theselogsarealsousedtoidentifythedifferentlithologies.
PorosityLogs
3BasicsofGeophysicalWellLogs_Porosity
DensitylogTheDensitylogmeasurestheapparentdensity(RHOB)oftheformation,bymeansofthe interactionbetweenthegammaraysemittedbyaradioactivesourceandtherock.
Theapparentdensityisaffectedby:- densityofthe“matrix”oftherock,- porosityofrock,- densityofthefluidsthatsaturatetherock.Applicationsofthedensitylogare:- in-situevaluationoftheporosity,- in-situlithologicalanalysis,- in-situanalysisofthefluids.
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DensitylogTheDensityLogallowsthemeasurement,atinsituconditions,ofthebulkdensity(ρb)subsurfaceformations.ThebulkdensityρbisproportionaltotheTotalPorosityoftherock(Φt)and,takingadvantageofthisrelationship,itispossibletodeterminetheporosityoftheformationwithhighaccuracy.
Theprimarymeasurementofporosityistheelectronicdensity(ρe)oftherockwhichisproportionaltoρb.ThephysicalprincipleusedistheinteractionbetweentheGammaRaysemittedbyaGRsourceandthematter.
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Densitylog
Energyofthedensitylogsource(662KeV)
ThisinteractiondoesnotoccurduetothelowerenergyofGRsemittedbytheDensitylogsource
Predominantinteractionfortheρbmeasurement
PredominantinteractionforthemeasurementofPef
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Gammaraysproducedbyaradioactivesourceinteractwiththesubsurfaceformations.TheGammaraysgeneratedbyaCs137source,becauseoftheirenergy(662KeV),mayonlyinteractwiththeelectronsorbitingaroundatomicnuclei.HeavieristheformationhigherisitselectronicdensityandlesseristhecountrateofgammaraysdetectedbytheSodiumIodide(NaI)scintillationdetectorsplacedatafixeddistancefromthesource.ThisisduetoboththeComptoninteractionforhigherenergygammaraysandbythePhotoelectricAbsorptioneffectforlowerenergygammarays.TheCountRateofgammaraysatahighenergywindowisusedforthemeasurementofthebulkdensity(rb)andtheCountRateofthegammaraysatalowenergywindowisusedforthemeasurementofthePef.
Densitylog
16”
12”
Notatscale
DOI@5”VR@1ft
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Densitylogρb=bulkdensityρe=electronicdensityZ=AtomicnumberA=Atomicweightρe=ρb(2Z/A)foramonoatomicsubstance
ρb=1,0704ρe-0,1883foralargenumberofsubstances
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Density API Test Pit
Austin Limestone Rho =2.21 g/cc
Bedford Limestone Rho = 2.42 g/cc
Vermont Marble Rho = 2.675 g/cc
Short Spaced Detector Count rate
Lon
g S
pace
d D
ete
cto
r co
un
t ra
te
Measured Rhob Low density
High density
Densitylog:thecalibrationprocess
The“densitytransform”istheequationofthisstraightline
9BasicsofGeophysicalWellLogs_Porosity
“Densitytransform”andDRHO
ρmc ρb
DRHOCorrectionisafunctionof:ρmc-ρbcontrast&tmc
tmc
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11BasicsofGeophysicalWellLogs_Porosity
Densitylogporosity
12BasicsofGeophysicalWellLogs_Porosity 12
Densitiesofmineralsandreservoirfluids
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ρmf
ρma
Sxo
Vma
Φ
1-Φ
1
ρmf
ρma Vma
Φ
1-Φ
1
ρhy
Sxo
SRH
Sw = 100 % and Sw = Sxo ρb = (1- Φ) ρma + Φ ρmf
Basic Density log equations
Sw < 100 % and SRH = 1- Sxo ρb = (1- Φ)ρma + Sxo Φ ρmf + [(1 – Sxo) Φ] ρhy
14BasicsofGeophysicalWellLogs_Porosity
Whenthereservoirispartiallygassaturated,theFormationDensityislowerwithrespecttotheonewecanmeasurewhenthesamerockistotallyoilorwatersaturated.Thiseffectisknownas“GASEFFECTONDENSITYLOG”.InfrontofGasBearinglevels,theRHOBcurveneedstobecorrectedforthis“GASEFFECT”.
Densitylog:thegaseffect
15BasicsofGeophysicalWellLogs_Porosity
NeutronlogTheneutronlogismostlyinfluencedbythehydrogencontentoftheformation.In“clean”wateroroilsaturatedformations,theneutronlogmeasuresan“apparent”totalporosityoftherock.Whenthelogisacquiredin“limestonematrix”(i.e.ρma=2,71g/cc),theneutronlogmeasuresdirectlythetotalporosityoftheformation.Ifthelithologyofthematrixisdifferentfrom“limestone”(calcite),thesocalledlithologycorrectionisrequired.
Otherneutronlogapplicationsare:– formationfluidanalysis,– lithologyanalysis.
16BasicsofGeophysicalWellLogs_Porosity
Theneutronsproducedbythechemicalsourceattheaverageenergyof4,5MeV,interactwiththenucleioftheelementspresentintheformationloosingsomeoftheirenergyateachcollision.ThehighestenergyislostwhentheneutronhitsaHydrogenatomofasimilarmass.Withhighformationhydrogencontentthelossofenergybytheneutronsisfastandlesstimeisnecessarytoreducetheenergyoftheneutronto“epithermal”levelfirst(0,6eV)andto“thermal”energylater(0,025eV).Whenaneutronis“thermalized”itcanonlybecapturedbyhighcrosssectionelementspresentintheformationsuchasClorine(Cl),Iron(Fe),Boron(Bo)andGadolinium(Gd).Theexcessofenergyoftheseatomsisreleasedbytheemissionofcaptureγraysofspecificenergy(γraySpectrometry).
Neutronlog(CNL)
17BasicsofGeophysicalWellLogs_Porosity
APS Electronic source
14 MeV
Chemical source 4.5 MeV
(inelastic scattering)
Neutronlog
CNLtypeoftoolDOI@8”-12”VR@2ft
12”16”
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Neutroncalibration&“NeutronPorosityTransform”
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ThischartisusedforthelithologycorrectionofNeutronreadings
Neutronlog
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HImf
HIma
Sxo
Vma
Φ
1-Φ
1
HImf
HIma Vma
Φ
1-Φ
1
HIhy
Sxo
SRH
Sw = 100 % and Sw = Sxo
HIlog = (1- Φ) HIma + Φ HImf
since HIma is negligible HIlog = Φ Himf where HImf = 1
BasicNeutronlogequations
Sw < 100 % and SRH = 1- Sxo
HIlog = (1- Φ) HIma + SxoΦ Himf +
[(1 – Sxo) Φ] HIhy
21BasicsofGeophysicalWellLogs_Porosity
Whentheporosityispartiallysaturatedbylowdensityhydrocarbon,neutronreadingsarelowerwithrespecttotheonesinawaterorwaterandoilsaturatedformation.Thisisthesocalledgaseffect.Ingasbearingzones,a“lighthydrocarboncorrection”isnecessarybeforeaquantitativeuseoftheneutroncurve.
NeutronlogGaseffect
22BasicsofGeophysicalWellLogs_Porosity
Density/Neutronscales
Thesearethesocalled“compatible”scalesforthepresentationofDensityNeutronlogsbasedona“limestone”matrix,i.e.whenNPHI=0,RHOBis2,70g/cc).
45 30 15 -15 0
1.95 2.95 2.20 2.45 2.70
COMPENSATED DENSITY NEUTRON FORMATION DENSITY
RHOB (g/cc)
NPHI (p.u.) limestone matrix
60 45 30 0 15
1.70 2.70 1.95 2.20 2.45
COMPENSATED DENSITY NEUTRON FORMATION DENSITY
RHOB (g/cc)
NPHI (p.u.) limestone matrix
23BasicsofGeophysicalWellLogs_PorosityAnexamplefromaNorthAdriaticSeasand/shalereservoir
24BasicsofGeophysicalWellLogs_Porosity
RHOBvsNPHICrossplotforCNL
Schlumberger
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Shales
Shales
Oilbearingsands
Gasbearingsands
Waterbearingsands
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AtypicalDensity/Neutron
crossplot
27BasicsofGeophysicalWellLogs_Porosity
Acoustic(sonic)logs
Acousticlogsmeasurethevelocityofpropagationofacousticwavesinsubsurfaceformations.
Thisvelocityisafunctionof:- rockmatrix- porositydistribution
Applicationsofacousticlogsare:-porosityevaluation,-lithologyidentification.
28BasicsofGeophysicalWellLogs_Porosity
Acousticwavesinawellbore
T
R
St P S D
P
P
modeconversion
modeconversion
Stoneley
WavetraincomponentsD DirectwavesP CompressionalwavesSShearwavesStStoneleywaves
Standardacoustictoolsarebasedonlyoncompressionalwavemeasurements,whilenewtechnologyacoustictools(ArraySonic)measureallthecomponentofthewavetrain(P,SandStoneley).Directwavesarestronglyattenuatedandnotdetected.
29BasicsofGeophysicalWellLogs_Porosity
Acousticlogs:conventionalBHCtools
T R1 R2
5 ft
2 ft 3 ft
SpaziatureLongSpacing8’-10’/10’-12’
30BasicsofGeophysicalWellLogs_Porosity
Acousticlogmeasurements
Acoustigloggingtoolsmeasurethereciprocaloftheacousticvelocity,theintervalTransitTimeΔt.TransitTimeismeasuredinµsec/ft,i.e.thetimeinµsecrequiredtothewavetocross1footofformation.
31BasicsofGeophysicalWellLogs_Porosity
PorosityfromSonicLogsWyllieequation
32BasicsofGeophysicalWellLogs_Porosity
Porosityfromsoniclogs:RaimerHuntequations
33BasicsofGeophysicalWellLogs_Porosity
Porosityfromsonic:WyllievsRaimerHunt
RaimerHuntequationsaremostlyusedinunconsolidatedformationsbecausetheyallowforanintrinsiccorrectionoftheunder-compactioneffect.
34BasicsofGeophysicalWellLogs_Porosity
CycleSkippingonBHCAcousticLogsinpresenceofgas
Inacoustictransittimeorsonicloggingwhen,duetotheattenuationoftheacousticwavesgeneratedbythepresenceofgas,theamplitudeofthefirstarrivaloftheacousticwavetrainislargeenoughtobedetectedbythenearreceiverofareceiverpairbutnotlargeenoughtobedetectedbythefarreceiver,thenoneormorecycleswillbeskippeduntilalatercyclearriveswhichhasenergyabovethedetectionlevel.Thissituationiscalled"cycleskipping."Itsonsetischaracterizedbyasharpdeflectiononthetransittimecurvecorrespondingtooneormoreaddedcyclesoftimebetweenreceivers."Shortcycleskipping,"wherethenearreceiveristriggeredacycletoolatecanalsooccur,resultinginanabnormallyshorttraveltime(TT).
Gaseffect
GWC
35BasicsofGeophysicalWellLogs_Porosity
CycleSkippingonBHCAcousticLogsinpresenceofgas
Inacoustictransittimeorsonicloggingwhen,duetotheattenuationoftheacousticwavesgeneratedbythepresenceofgas,theamplitudeofthefirstarrivaloftheacousticwavetrainislargeenoughtobedetectedbythenearreceiverofareceiverpairbutnotlargeenoughtobedetectedbythefarreceiver,thenoneormorecycleswillbeskippeduntilalatercyclearriveswhichhasenergyabovethedetectionlevel.Thissituationiscalled"cycleskipping."Itsonsetischaracterizedbyasharpdeflectiononthetransittimecurvecorrespondingtooneormoreaddedcyclesoftimebetweenreceivers."Shortcycleskipping,"wherethenearreceiveristriggeredacycletoolatecanalsooccur,resultinginanabnormallyshorttraveltime(TT).
36BasicsofGeophysicalWellLogs_Porosity 36
Lithologyandporosity
determinationbymeansoftheSonic/Neutroncrossplot.
37BasicsofGeophysicalWellLogs_Porosity
Density Sonic Neutron (CNL)
Neutron (APS)
8”1/2 8”1/2 8”1/2 8”1/2
Comparisonsamongporosity
loggingtools
38BasicsofGeophysicalWellLogs_Porosity
Step0-TheinitialDataSet
39BasicsofGeophysicalWellLogs_Porosity
Step1-CleanintervalsdetectionfromGR,ResistivityandDensity/Neutronreadings
40BasicsofGeophysicalWellLogs_Porosity
Step2-ComputeRwfromRo-porosityrelationships
Ro NPHI RHOB
Ro = 1,85 ohmm RHOB = 2,10 g/cc NPHI = 30%
ΦDN = 34% a = 1, m = 2
Rw = Ro Φm/a Rw = 1,85* 0,342 Rw = 0,21 ohmm
Aquifer inferred from the typical Resistivity invasion profile
41BasicsofGeophysicalWellLogs_Porosity
Step3-SelectthepointsforSwevaluationreadingRt,RHOBandNPHIateachpoint
Rt1
Rt2
Rt3 NPHI3
RHOB1
RHOB2
RHOB3
NPHI2
NPHI1
NPHI4 RHOB4 Rt4
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