1 Basics of Geophysical well logs Introduction

Basics of Geophysical Well Logs_Introduction 1

Basicsof

GeophysicalWellLogs:

Introduction

www.spwla.org

www.glossary.oilfield.slb.com


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.).


Welllogginghistory

Thefirstelectricallogwasrecordedin1927inthewellPechelbronn7intheformofasinglegraphoftheelectricalresistivityoftheformationscutbythewellrecordedwithastationarymethod.Theresistivityprofilewasmainlyused,atthebeginningofthewellloggingtechnology,forcorrelationpurposesandforlocationofpotentialhydrocarbonbearinglevels


Evolutionofwellloggingtechnology

Sincethisfirstlog,thetechnologyevolvedveryrapidlyand,thankstosophisticateddevelopments,revolutionizedtheoilandgasExplorationandProductionindustry.WellloggingtechnologyisnowusedinallthephasesoftheE&Pprocessfromthedrillingofthefirstwildcatwellinafielduptotheabandonmentofthelastproductivelevelinthesamefield.Duetotheexploitationofalargenumberofphysicalprinciples,welllogscannowmeasurealargenumberofphysicalpropertiesofthegeologicalformationintersectedbyawellandbothinopenandcasedholeconditions.


WelllogsareacquiredandusedinallphasesoftheE&Pprocess:– duringthedrillingphase(LoggingWhileDrilling);– soonafterthedrillingphase(OpenHoleWireLineLogging);– afterthecompletionofthewellandduringtheexploitationphaseuptotheendof

thereservoirlife(CasedHoleWireLineLoggingandProductionLogging).

Well logs: what?


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


Awelllogistheproductofasurveyoperationconsistingofoneormorecurves,providingapermanentrecordofoneormorephysicalmeasurementsasafunctionofdepthina

wellbore

Modern well logging (Open Hole Wire Line)


Modern well logging (Open Hole Logging While Drilling)

ModernLoggingWhileDrillingtechnologiesallowtheacquisitionofhighqualityloggingcurves(bothinRealTimeandMemorymodes)forRealTime&/orNearRealTime

FormationEvaluationandGeosteering.


Scopeofloginterpretation

Loginterpretationistheprocessbywhichthelargenumberofformationpropertiesmeasuredinawellborearetranslatedintoadesiredformationcharacteristicsandpetrophysicalparameterssuchasporosity,hydrocarbonsaturation,permeability,lithology,reservoirgeometryandstructure.


Petrophysicsisthestudyofthephysicalpropertiesof(sedimentary)rocksandtheirinterstitialfluidsforpurposesofinterpretingdownholemeasurementsintermsofreservoirrockcharacteristics.

Wellloggingapplications


WellloggingapplicationsFormationEvaluationistheanalysisandinterpretationofwelllogdata,drillstemtests,etc.intermsofthenatureoftheformationsandtheirfluidcontent.Theobjectivesofformationevaluationare:–todeterminethebestmeansfortheirrecovery,and

–toascertainifcommerciallyproduciblehydrocarbonsarepresent,

–toderivelithologyandotherinformationonformationcharacteristicsforuseinfurtherexplorationanddevelopment. Source: SPWLA Glossary


Masterlog

TheMasterLog(orMudLog)isadocumentshowing(intheformofalog)thevariationofdrillingparametersandwhiledrillinginformationwhichareessentialtothegeologicalandpetrophysicalinterpretationofwelldata(welllogsincluded):– rateofPenetration(ROP),– drillingparameters,– lithologicaldescriptionofcuttings,– chemicalcompositionandcalcimetry,– gascurves,– muddata,– drillingoperation(i.e.coring,etc.)– others.


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


ReservoirCharacterizationcorrespondstotheidentificationofamodelforthereservoir,thedynamicbehaviourofwhichmustbeassimilaraspossibletothatofthereservoir.Welllogscontributemostlytothestaticpartofthemodelbygatheringinformationaboutgeological,geochemical,petrophysicalandgeomechanicalcharactersofthereservoir.

Wellloggingapplications


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


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


TheFormationEvaluationProcess

Mainstepsoftheprocessare:

• planningofthewelldataacquisition,

• acquisitionphasewithQualityControl,

• preand/orpostprocessing,

• interpretation,

• deliveryoftheresultsandintegration.


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.


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


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


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


TotalporosityvseffectiveporosityEffectiveporosity– Coreanalysiscontext:porespacethatisaccessibletohelium(or

water)– Loganalysiscontext:porespacethatisoccupiedbyfreewaterand

hydrocarbons(excludesclayboundwater)Totalporosity:– Coreanalysiscontext:coincideswitheffectiveporosity(totally

inaccessibleporesarerare)– Loganalysiscontext:porositynormallymeasuredbylogs(with

referencetotheporespaceoccupiedbyfreeandboundwater)


Porosity:primaryvssecondary

FormationPorositycanbeclassifiedas:primaryandsecondary:

• Primaryporosityistheporosityofrockformedatthemomentofthedepositionandmodifiedonlyforthecompaction(thereforenotconsideringthechangesduetochemicaleffects(i.e.fluidmigrationthroughthesediments).

• Secondaryporosityistheadditionalporositygeneratedbypostdepositionaleventsandgenerated(orcanceled)bychemicaldissolution,diagenesis,dolomitizationortectoniceventssuchasthegenerationoffracturesandjoints.


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:



Laboratorypetrophysicalmeasurements


Porositymeasurements


Theproblemofdifferentscalesofthemeasurements

PORE PLUG STRATUM

BED LOG

Heterogeneous rock fabric

Homogeneous rock fabric

Heterogeneous stratum

Homogeneous stratum

Reservoir scale


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 %


Porosity distribution in typical sedimentary rocks


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


An example of correlation between porosity measurements on cores and from log interpretation in sandstones


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


• WaterSaturation(Sw)isgenerallyevaluatedbytherelationshipsamongresistivityandporosityofthereservoirrock.

• Thisrelationship,incleanformations,isexpressedbytheArchieequations.

• Swofaformationcanvaryfrom100%toquitesmallamount(4-5%)alwayspresentinthepores:thisamountisthe,socalled,irreducibleorconnatewatersaturationSwirr.



Irreducible water saturation (Swirr) in typical reservoir rocks



• Permeabilityisameasureoftheeasewithwhichfluidscanflowthroughtheformation.

• TheunitofpermeabilityistheDarcy(D)andthesymbolofpermeabilityisK;thepracticalunitinuseisthemilliDarcy(1mD=1/1000D).

• Thepermeabilityof1Disdefinedasthepermeabilityallowingtoafluidof1cpofviscositytoflowinasectionofrockof1cm2atarateof1cm3/secwithapressuregradientof1atm/cm.

• 1D=0,986910-12m2

Petrophysical parameters: Permeability


Geologicalcontrolofpermeability• Shalysands

– layering,– grainsizeandsorting,– orientationandshapeoftheclasts,– packing,– cementation,– claycontent.

• Carbonates– degreeofdiagenesis(i.e.dolomitization),– Porositydevelopment,– Fracturepresenceandorientation.

Petrophysical parameters: Permeability


Porosity/permeabilityrelationships


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.


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


Horizontalvsverticalpermeability

1 Basics of Geophysical well logs Introduction

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