1 basics of geophysical well logs introduction
TRANSCRIPT
Basics of Geophysical Well Logs_Introduction 1
Basicsof
GeophysicalWellLogs:
Introduction
www.spwla.org
www.glossary.oilfield.slb.com
Basics of Geophysical Well Logs_Introduction 2
Well logs: what?
Well logs were developed with the objective of the indirect evaluation of the geological and petrophysical characterization of the subsurface formations. This is achieved by the acquisition, along with the well bore of a drilled well, of a large number of physical measurements (resistivity, density, Hydrogen Index, acoustic waves velocity, etc.) which, by means of a complex interpretation process, are translated into petrophysical properties (Water Saturation, Porosity, Permeability, Volume of shale, etc.), geological characters of the formation (lithology, layer’s dip, depositional environments, sedimentary facies, etc.) and thermodynamic data (temperature, fluid composition and viscosity, etc.).
Basics of Geophysical Well Logs_Introduction 3
Welllogginghistory
Thefirstelectricallogwasrecordedin1927inthewellPechelbronn7intheformofasinglegraphoftheelectricalresistivityoftheformationscutbythewellrecordedwithastationarymethod.Theresistivityprofilewasmainlyused,atthebeginningofthewellloggingtechnology,forcorrelationpurposesandforlocationofpotentialhydrocarbonbearinglevels
Basics of Geophysical Well Logs_Introduction 4
Evolutionofwellloggingtechnology
Sincethisfirstlog,thetechnologyevolvedveryrapidlyand,thankstosophisticateddevelopments,revolutionizedtheoilandgasExplorationandProductionindustry.WellloggingtechnologyisnowusedinallthephasesoftheE&Pprocessfromthedrillingofthefirstwildcatwellinafielduptotheabandonmentofthelastproductivelevelinthesamefield.Duetotheexploitationofalargenumberofphysicalprinciples,welllogscannowmeasurealargenumberofphysicalpropertiesofthegeologicalformationintersectedbyawellandbothinopenandcasedholeconditions.
Basics of Geophysical Well Logs_Introduction 5
WelllogsareacquiredandusedinallphasesoftheE&Pprocess:– duringthedrillingphase(LoggingWhileDrilling);– soonafterthedrillingphase(OpenHoleWireLineLogging);– afterthecompletionofthewellandduringtheexploitationphaseuptotheendof
thereservoirlife(CasedHoleWireLineLoggingandProductionLogging).
Well logs: what?
Basics of Geophysical Well Logs_Introduction 6
GEOPHYSICAL WELL LOGS
Logging While Drilling (LWD)
Open Hole
Wire Line Logging (OH WLL)
Cased Hole
Wire Line Logging (CH WLL)
•Correlations •Formation Evaluation •Geosteering •Pressure Predictions •Seismic interpretation
•Correlations •Formation Evaluation •Geological applications •Rock Mechanics
•Formation Evaluation •Production logs •Auxiliary measurements
Basics of Geophysical Well Logs_Introduction 7
Awelllogistheproductofasurveyoperationconsistingofoneormorecurves,providingapermanentrecordofoneormorephysicalmeasurementsasafunctionofdepthina
wellbore
Modern well logging (Open Hole Wire Line)
Basics of Geophysical Well Logs_Introduction 8
Modern well logging (Open Hole Logging While Drilling)
ModernLoggingWhileDrillingtechnologiesallowtheacquisitionofhighqualityloggingcurves(bothinRealTimeandMemorymodes)forRealTime&/orNearRealTime
FormationEvaluationandGeosteering.
Basics of Geophysical Well Logs_Introduction 9
Scopeofloginterpretation
Loginterpretationistheprocessbywhichthelargenumberofformationpropertiesmeasuredinawellborearetranslatedintoadesiredformationcharacteristicsandpetrophysicalparameterssuchasporosity,hydrocarbonsaturation,permeability,lithology,reservoirgeometryandstructure.
Basics of Geophysical Well Logs_Introduction 10
Petrophysicsisthestudyofthephysicalpropertiesof(sedimentary)rocksandtheirinterstitialfluidsforpurposesofinterpretingdownholemeasurementsintermsofreservoirrockcharacteristics.
Wellloggingapplications
Basics of Geophysical Well Logs_Introduction 11
WellloggingapplicationsFormationEvaluationistheanalysisandinterpretationofwelllogdata,drillstemtests,etc.intermsofthenatureoftheformationsandtheirfluidcontent.Theobjectivesofformationevaluationare:–todeterminethebestmeansfortheirrecovery,and
–toascertainifcommerciallyproduciblehydrocarbonsarepresent,
–toderivelithologyandotherinformationonformationcharacteristicsforuseinfurtherexplorationanddevelopment. Source: SPWLA Glossary
Basics of Geophysical Well Logs_Introduction 12
Masterlog
TheMasterLog(orMudLog)isadocumentshowing(intheformofalog)thevariationofdrillingparametersandwhiledrillinginformationwhichareessentialtothegeologicalandpetrophysicalinterpretationofwelldata(welllogsincluded):– rateofPenetration(ROP),– drillingparameters,– lithologicaldescriptionofcuttings,– chemicalcompositionandcalcimetry,– gascurves,– muddata,– drillingoperation(i.e.coring,etc.)– others.
Basics of Geophysical Well Logs_Introduction 13
KISSANJE-2 LM-16 Pressure Profile
Dep
th (m
TVD
SS)
1800
1810
1820
1830
1840
1850
1860
1870
1880
1890
1900
1910
1920
1930
1940
1950
1960
1970
Formation Pressure (PSIa)2750 2775 2800 2825 2850 2875 2900 2925
y = 0.698x - 64.331
y = 2.6203x - 5408.7
Pressure Profile
Localization of fluid contacts within the reservoir
Pressure Measurements
Basics of Geophysical Well Logs_Introduction 14
ReservoirCharacterizationcorrespondstotheidentificationofamodelforthereservoir,thedynamicbehaviourofwhichmustbeassimilaraspossibletothatofthereservoir.Welllogscontributemostlytothestaticpartofthemodelbygatheringinformationaboutgeological,geochemical,petrophysicalandgeomechanicalcharactersofthereservoir.
Wellloggingapplications
Basics of Geophysical Well Logs_Introduction 15
Themostimportantlogmeasurement:depth!
ThefundamentalmeasurementprovidedbytheServiceCompanyisdepth.Anaccuratedescriptionofthereservoirmaynothaveahighvaluewithoutanaccuratedepthlocationoftheevents.Depthcontrolisofveryhighimportanceforthesuccessofanylogoperationaimedexploration,completionandproductionofhydrocarbons.Incaseofwirelineoperationstheaccuracyofdepthmeasurementisof+/-1foot(0,3m),thankstothetechniquesinusebasedonodometers(calibratedwheels),accuratechecks(magneticmarkers)andwhiledrillingcorrectionsasfunctionofdepth,toolweighttypeofcable,typeofmud,etc..
In case of While Drilling (LWD) operations, depth uncertainty is much higher since absolute depth is based on drill pipe length measurements (Drillers depth).
Main drum for OH Logging (7 conductor cable)
Drum for CH Logging (monocond. cable)
Odometer
Basics of Geophysical Well Logs_Introduction 16
Digital signals
mV
Cal
iper
Physical parameters
Editing and Normalization
Interpretation
Petrophysics
Geology
Geomechanics
Well logs: what?
Field logs + Digital data + Quality Control
Sensors
Electronics
Tool
Acquisition system
Cable
Calibration
Basics of Geophysical Well Logs_Introduction 17
TheFormationEvaluationProcess
Mainstepsoftheprocessare:
• planningofthewelldataacquisition,
• acquisitionphasewithQualityControl,
• preand/orpostprocessing,
• interpretation,
• deliveryoftheresultsandintegration.
Basics of Geophysical Well Logs_Introduction 18
Petrophysicalparameters
Main petrophysical parameters evaluated by means of well log interpretation are: • porosity (Φ), • water saturation (Sw), • permeability (K) By means of well log interpretation, the thickness of productive levels, can be easily evaluated: • gross pay, • net sand, • net reservoir, • net pay and net to gross.
Basics of Geophysical Well Logs_Introduction 19
The petrophysical parameters derived from well log interpretation can, therefore, be used to compute the volume of hydrocarbon (oil and/or gas) originally in place.
7758 • A • h • Φ • (1-Sw) STOOIP = ----------------------------------- (stb) Boi
A • h = Bulk reservoir volume Φ = average effective porosity (%) 1- Sw = initial oil saturation Sw = average Water Saturation Boi = oil volume factor
Well logs: what?
OWC
10
20
30
h
A1
A3
A2
A4
Basics of Geophysical Well Logs_Introduction 20
Oil volume factor
Oil and dissolved gas volume at reservoir conditions divided by oil volume at standard conditions. Since most measurements of oil and gas production are made at the surface, and since the fluid flow takes place in the formation, volume factors are needed to convert measured surface volumes to reservoir conditions. Oil formation volume factors are almost always greater than 1.0 because the oil in the formation usually contains dissolved gas that comes out of solution in the wellbore with dropping pressure.
Well logs: what?
OWC
10
20
30
h
A1
A3
A2
A4
Basics of Geophysical Well Logs_Introduction 21
Porosityistheporevolumeperunitvolumeofformation(ratiobetweenporevolumeandrockvolume). Φt(%)=Vp/Vt*100
Porosityisexpressedinpercentage.Porosityisevaluatedbymeansofthe,socalled,porositylogs:density,neutron,acoustic,dielectricandMagneticResonance.Porositylogsaresensitivetototalporosity(Φt)whiletheeffectiveporosity(Φe)isevaluated,inclasticsequences,bymeansofempiricalrelationshipsbetweenΦt,ΦeandVolumeofshale(Vsh),accordingtothedistributionoftheshales.
Incaseoflaminatedshale:Φe=Φt(1-Vsh)
Petrophysical parameters: porosity
Basics of Geophysical Well Logs_Introduction 22
TotalporosityvseffectiveporosityEffectiveporosity– Coreanalysiscontext:porespacethatisaccessibletohelium(or
water)– Loganalysiscontext:porespacethatisoccupiedbyfreewaterand
hydrocarbons(excludesclayboundwater)Totalporosity:– Coreanalysiscontext:coincideswitheffectiveporosity(totally
inaccessibleporesarerare)– Loganalysiscontext:porositynormallymeasuredbylogs(with
referencetotheporespaceoccupiedbyfreeandboundwater)
Basics of Geophysical Well Logs_Introduction 23
Porosity:primaryvssecondary
FormationPorositycanbeclassifiedas:primaryandsecondary:
• Primaryporosityistheporosityofrockformedatthemomentofthedepositionandmodifiedonlyforthecompaction(thereforenotconsideringthechangesduetochemicaleffects(i.e.fluidmigrationthroughthesediments).
• Secondaryporosityistheadditionalporositygeneratedbypostdepositionaleventsandgenerated(orcanceled)bychemicaldissolution,diagenesis,dolomitizationortectoniceventssuchasthegenerationoffracturesandjoints.
Basics of Geophysical Well Logs_Introduction 24
Primaryporosity
poresformedatthemomentofthedepositionofthesediment:– intergranular(spacesbetweengrains,typicalofclasticformationssuchsandstones)
– intercrystalline(spacesbetweencrystalstypicalofthecarbonates)
Secondaryporosity
poresformedafterthedepositionofthesediment:– duetofracturing(especiallyincompetentrocks),
– duetodissolution(i.e.vuggyporosity),
– duetodiageneticeffects(dolomitization,recrystalli-zation,silicification,etc.)
With respect the origin of the pores, porosity can be classified as:
Petrophysical parameters: porosity
Basics of Geophysical Well Logs_Introduction 27
Theproblemofdifferentscalesofthemeasurements
PORE PLUG STRATUM
BED LOG
Heterogeneous rock fabric
Homogeneous rock fabric
Heterogeneous stratum
Homogeneous stratum
Reservoir scale
Basics of Geophysical Well Logs_Introduction 28
Porosity distribution in sedimentary rocks
Porosities of subsurface formation can vary widely:
• carbonates (limestone/dolomites): – from 0 to 45 %
• evaporites (salt, anhydrite, gypsum, silvite, ecc.): – practically 0 porosity
• consolidated sandstones: – from 5 to 15 %
• unconsolidated sands: – 30% and more
• shales or clays: – often more than 40 %
Basics of Geophysical Well Logs_Introduction 30
Phi core vs Phi log
Poro
sity
from
cor
es (p
u)
0
7.5
15
22.5
30
Porosity from logs (pu)
0 7.5 15 22.5 30
Petrophysical parameters: porosity
An example of correlation between porosity measurements on cores and from log interpretation in sandstones
Basics of Geophysical Well Logs_Introduction 31
WaterSaturationofaformationisthefractionofitsporevolumeoccupiedbyformationwater.
Sw(%)=Vw/Vp*100(Vpporevolume,Vwvolumeofwater)Saturationsareexpressedinpercentage.Thereforeoilorgassaturationisthefractionofporevolumethatcontainsoilorgas.Thesymbolsusedare:
➢Swforwatersaturation;➢Shforgeneralhydrocarbonsaturation;➢Soand/orSgforoiland/orgassaturation.
Thesummationofallsaturations,inagivenformationrock,musttotalto100%andtherefore:
➢Sh=1-Sw
Petrophysical parameters: Water Saturation
Basics of Geophysical Well Logs_Introduction 32
• WaterSaturation(Sw)isgenerallyevaluatedbytherelationshipsamongresistivityandporosityofthereservoirrock.
• Thisrelationship,incleanformations,isexpressedbytheArchieequations.
• Swofaformationcanvaryfrom100%toquitesmallamount(4-5%)alwayspresentinthepores:thisamountisthe,socalled,irreducibleorconnatewatersaturationSwirr.
Petrophysical parameters: Water Saturation
Basics of Geophysical Well Logs_Introduction 33
Irreducible water saturation (Swirr) in typical reservoir rocks
Petrophysical parameters: Water Saturation
Basics of Geophysical Well Logs_Introduction 34
• Permeabilityisameasureoftheeasewithwhichfluidscanflowthroughtheformation.
• TheunitofpermeabilityistheDarcy(D)andthesymbolofpermeabilityisK;thepracticalunitinuseisthemilliDarcy(1mD=1/1000D).
• Thepermeabilityof1Disdefinedasthepermeabilityallowingtoafluidof1cpofviscositytoflowinasectionofrockof1cm2atarateof1cm3/secwithapressuregradientof1atm/cm.
• 1D=0,986910-12m2
Petrophysical parameters: Permeability
Basics of Geophysical Well Logs_Introduction 35
Geologicalcontrolofpermeability• Shalysands
– layering,– grainsizeandsorting,– orientationandshapeoftheclasts,– packing,– cementation,– claycontent.
• Carbonates– degreeofdiagenesis(i.e.dolomitization),– Porositydevelopment,– Fracturepresenceandorientation.
Petrophysical parameters: Permeability
Basics of Geophysical Well Logs_Introduction 37
Classificationofpermeability
AbsolutePermeabilityThepermeabilityofthereservoirrockwhentheporesarefilledbyasinglefluidRelativepermeabilityThepermeabilityofthereservoirrockwhentheporesarefilledbymorethenonefluid;itistheratiobetweentheeffectivepermeabilitytoafluidinpresenceofotherfluidsandabsolutepermeability.
EffectivePermeabilityKw=effectivepermeabilitytowaterKo=effectivepermeabilitytooilKg=effectivepermeabilitytogas
RelativepermeabilityKrw=Kw/K Krel.towaterKro=Ko/K Krel.tooilKrg=Kg/K Krel.togas
Horizontal Permeability (Kh) and vertical permeability (Kv) Permeability is a tensorial property which depends on the direction of the measurements; Kh e Kv in a sedimentary rock may vary as a function of the grain disposition and, in competent rocks, as a function of fracture distribution and orientation.
Basics of Geophysical Well Logs_Introduction 38
ClassificationofpermeabilityEffective permeability
The ability to preferentially flow or transmit a particular fluid when other immiscible fluids are present in the reservoir (e.g., effective permeability of gas in a gas-water reservoir). The relative saturations of the fluids as well as the nature of the reservoir affect the effective permeability. In contrast, absolute permeability is the measurement of the permeability conducted when a single fluid or phase is present in the rock.
Relative permeability
A dimensionless term devised to adapt the Darcy equation to multiphase flow conditions. Relative permeability is the ratio of effective permeability of a particular fluid at a particular saturation to absolute permeability of that fluid at total saturation. If a single fluid is present in a rock, its relative permeability is 1.0. Calculation of relative permeability allows comparison of the different abilities of fluids to flow in the presence of each other, since the presence of more than one fluid generally inhibits flow