Basics of Geophysical Well Logs Lithology&Resis7vity 1 · E l =− R θ Fn ln C 1 C 2. Basics...

1BasicsofGeophysicalWellLogs_Lithology&Resis7vity

2BasicsofGeophysicalWellLogs_Lithology&Resis7vity

Whenthewellboreisfilledbyawaterbasedmudandinpresenceofanalterna7onofpermeableandimpermeablelayers,duetoelectrochemicalphenomena,electricalcurrentsarespontaneouslygeneratedattheinterfacesbetweenmudandforma7onandbetweenimpermeableshalesandreservoirsands.TheSPlogisthemeasurementofthepoten7alofadownholeelectrodewithrespecttoasurfacereferenceelectrodewhichispropor7onaltotheintensityofthecurrentsgenerated.

SpontaneousPoten7al

3BasicsofGeophysicalWellLogs_Lithology&Resis7vity

ThemagnitudeoftheSPgeneratedisafunc7onofthesalinitycontrastbetweenmudandforma7onwater.TwoarethemainSPgenera7onmechanisms:• membranepoten7al,• liquid-junc7onpoten7al.

TheSPlogisprimarilyapermeabilitycontrastindicatoraswellas

afundamentallithologylogespeciallyinshalysandsequences

SpontaneousPoten7al

Flushed zone

Virgin zone

+ -

Flushed zone

Virgin zone

+ -

Membranepoten7al

Liquid-junc7onpoten7al €

Em = −RθFn

Ia − IcIa + Ic

ln C1C2

El = −RθFnln C1C2

4BasicsofGeophysicalWellLogs_Lithology&Resis7vity

SpontaneousPoten7al(freshmudandsaltyforma7onwater)

SPsh

SPsd

Vsh = A / B Vsh = (PS-PSsd)/(PSsh-PSsd)

SPlog

A

B

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SPloginterpreta7onproblems

MainproblemsofSPloginterpreta7onaremostlylinkedto:–  lackofpermeabilitycontrast

–  lackofmud/forma7onwatersalinitycontrast

– thinbeds– hydrocarbonoccurrence

Low permeability limestones

Marls

Marls

mV

Lowpermeabilitycontrast

6BasicsofGeophysicalWellLogs_Lithology&Resis7vity 6 mV

Shales

Gas bearing sands

Water bearing sands

Shales

HydrocarboneffectontheSPlog

High permeability sandstones

Shales

Shales

Rm > Rw Rm = Rw Rm < Rw SSP = -K log (Rmf/Rw)

SPlogbehaviorasafunc7onofmud/forma7onwatersalinitycontrast

mV

SPloginterpreta7onproblems

7BasicsofGeophysicalWellLogs_Lithology&Resis7vity 7

Rmf > Rw Rmf = Rw

SP SP

8BasicsofGeophysicalWellLogs_Lithology&Resis7vity

InvertedSP:

Rmf<<Rw

9BasicsofGeophysicalWellLogs_Lithology&Resis7vity 9

High permeability sandstones

Shales

Shales

mV

inflexion point

SP log

Boundaryloca7onbymeansoftheSPlogVer7calresolu7onoftheSPlog

SPloginterpreta7onproblems

inflexion point

10BasicsofGeophysicalWellLogs_Lithology&Resis7vity

Geologicalapplica7onoftheSPlog

InabsenceofSPanomalies,whendealingwithwatersaturatedforma7onsandwithmudsofconstantsalinityinallthewellsunderevalua7on,theSPlogmaybeusedforgeologicalcorrela7onamongwellsandtodefinethedifferentsedimentaryfacies(inordertodefinelateralandver7calevolu7onofthesedimentaryenvironments).

11BasicsofGeophysicalWellLogs_Lithology&Resis7vity

GammaRaylogThenaturalradioac7vityofgeologicalforma7onsisduetothepresenceinrockformingmineralsoftheradioac7veisotopesofelementssuchasUranium(U),Thorium(Th)andPotassium(K).TheseisotopesaremostlyrelatedtoclaymineralswhosecontentinThandKisgenerallyhigherthanassociatedsandandsandstones.Incarbonateforma7onstheradioac7vityismostlyduetothepresenceofUandtheGammaRaylevelisnotdirectlyrelatedtoforma7onshalyness.

12BasicsofGeophysicalWellLogs_Lithology&Resis7vity

Low activity Cement

High activity Cement

Low activity Cement

K = 4% Th = 24 ppm U = 12 ppm

200 GAPI

Typical responses Of GR log

GammaRaylog

Gamma Ray American Institute Test Pit

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(GR-GRsd) Vsh = -----------------

(GRsh-GRsd)

GRsh

GRsd

GR

Caliper - GR Resistivity Density/Neutron

ShalevolumefromGRlogBS, CALI, SP, GR LLD, LLS, MCFL TNPH, RHOB, DT

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GammaRay:environmentalcorrec7onsMain factors affecting GR measurements are: •  hole diameter •  sonde position in the well •  mud loaded with radioactive material

BS

CAL

Measured GR

Corrected GR

Shale

Shale

Sand

Sand

Sand γ

γγ

γ

γ

γ

γ

γ

γ γ

γ

γ

γ

γ

γ γ

Tool eccentered Tool centered

mud

mud

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GammaRay:environmentalcorrec7ons

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GammaRaySpectrometry

Gamma Ray emission spectra

WLLServicesSLB NGSSLB HNGS(PEX)BA SL

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GRincasedhole

GR

Neutron

GRcorrela7onLogincasedhole Forma7onEvalua7onlogginginCH

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Single arm caliper

Two arm caliper

Three arm caliper Four arm caliper

Caliperlog

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Comparisonamongdifferentcalipermeasurements

0

6

6

GR 200

16

16 BS

CAL 0.2 2000 LLD

0.2 2000 MCFL

0.2 2000 LLS RHOZ 1.95 2.95

NPHI 45 -15

0 10 PEF

Mudcakeresul7nginaholediameterrestric7on

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BoreholeGeometryTool(BGT)

21BasicsofGeophysicalWellLogs_Lithology&Resis7vity

“Noisy”logsduetopoorboreholewall

quality.

CALIPER

DRHO

RHOB

TNPH

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Classifica7onofResis7vitylogs

Inrela7ontodepthofinves7ga7on

•  Macro-devicestomeasureRt•  Micro-devicestomeasureRxo

Inrela7ontotoolphysics•  Nonfocused,galvanicdevices(WLL)•  Focused,galvanicdevices(WLLeLWD)

•  Lowfrequencyinduc7ondevices(WLL)•  EMwavepropaga7ondevices(LWD)

23BasicsofGeophysicalWellLogs_Lithology&Resis7vity

Environmentaleffectsaffec7ngresis7vitymeasurements

Rt

Rs

Rs

Rt

Rt

1D VERTICAL (shoulder bed)

Rt

Rxo

1D RADIAL (invasion effect)

Rt2

Rs

Rs

Rt3

Rt1 Rxo1

Rxo3

Rxo2

2D

Rs

Rs

Rt3

Rt1 Rxo1

Rxo3

Rxo2 Rt2v

Rt2HMAX Rt2hmin

2D +dip (dipping beds)

Rm

Rt

Rs

Ro

Rm

Rs

Borehole

3D

Rv

Rh

Anisotropy

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Invasion effects and formation parameters

25BasicsofGeophysicalWellLogs_Lithology&Resis7vity

Sw = Sxo = 100%

Ro= F Rw = (1/Φ^m) Rw Rxo= F Rmf = (1/Φ^m) Rmf

Sw < 100%, Sxo <100%

Rt =(1/Φ^m)(Rw)/Sw^n) Rxo=(1/Φ^m)(Rmf)/Sxo^n)

26BasicsofGeophysicalWellLogs_Lithology&Resis7vity

Invasionandrelatedresis7vityprofiles

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Fresh mud and salty formation water

28BasicsofGeophysicalWellLogs_Lithology&Resis7vity

Caliper - GR Resistivity Density/Neutron

Rmf very close to Rw

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Nonfocusedelectricalresis7vitytools

30BasicsofGeophysicalWellLogs_Lithology&Resis7vity

OldE(electrical)logs

Track 1 Track 2 Track 3

Dep

th

SP (mv) SN (ohmm)

Ampl. SN (ohmm)

LN (ohmm)

IN (ohmm)

Conventional Electrical Log (ES)

SN = Short Normal (spacing 16”) Ampl. SN = Amplified Short Normal

LN = Normal (spacing 64”) IN = Inverse or Lateral (spacing 18’ 8”)

Linear scale

Linear scale

Linear scale

31BasicsofGeophysicalWellLogs_Lithology&Resis7vity

OldE(electrical)logs

Track 1 Track 2 Track 3

Dep

th

SP (mv) SN (ohmm)

Ampl. SN (ohmm)

6FF40 R (ohmm)

6FF40 C (ohmm)

Induction Electrical Log (IES)

SN = Short Normal (spacing 16”) Ampl. SN = Amplified Short Normal

6FF40 R = Induction log deep (40”) resistivity 6FF40 C = Induction log deep (40”) conductivity

Linear scale

Linear scale

Linear scale

32BasicsofGeophysicalWellLogs_Lithology&Resis7vity

Normalandlateralresis7vitylogs

33BasicsofGeophysicalWellLogs_Lithology&Resis7vity

Normalelectricallogresponses:normal

34BasicsofGeophysicalWellLogs_Lithology&Resis7vity

Lateralelectricallogresponses:lateral

35BasicsofGeophysicalWellLogs_Lithology&Resis7vity

WellSecala1SP/ILD/SN/MLLFDC/CNL/BHC

ILD (Rt)

MLL (Rxo)

SN (Ri)

36BasicsofGeophysicalWellLogs_Lithology&Resis7vity

Resis7vitytools:WireLine(WLL)

&WhileDrilling

(LWD)

37BasicsofGeophysicalWellLogs_Lithology&Resis7vity

DualLaterolog0

6

6

GR 200

16

16 BS

CAL 0.2 2000 LLD

0.2 2000 MCFL

0.2 2000 LLS

38BasicsofGeophysicalWellLogs_Lithology&Resis7vity

SfericallyFocusedLog

39BasicsofGeophysicalWellLogs_Lithology&Resis7vity

Environmentaleffectsonfocused

galvanictools

40BasicsofGeophysicalWellLogs_Lithology&Resis7vity

Induc7onlogging

Induction log principle

41BasicsofGeophysicalWellLogs_Lithology&Resis7vity

ILD/SFLexample• blackcurveILD• redcurveSFLU

NorthAdria7cfreshWBM

42BasicsofGeophysicalWellLogs_Lithology&Resis7vity

Rxologging:theMSFLtool

0

6

6

GR 200

16

16 BS

CAL 0.2 2000 LLD

0.2 2000 MCFL

0.2 2000 LLS

43BasicsofGeophysicalWellLogs_Lithology&Resis7vity

Caliper-GR Resis7vity Density/Neutron

44BasicsofGeophysicalWellLogs_Lithology&Resis7vity

WhileDrillingresis7vityloggingWhileDrillingresis7vityloggingareoftwotypes:• galvanic;• EMwavepropaga7on.WhileDrillingGalvaniclogs(AnadrillRABonly)canbeusedonlyinpresenceofWaterBasedconduc7veMuds.EMwavepropaga7onlogs,duetothepresenceofmetallicbodyofthesystem,canbeobtainedonlyusinghigherfrequencieswithrespecttotheWireLineinduc7ononeswithadvantagesanddisadvantages.

45BasicsofGeophysicalWellLogs_Lithology&Resis7vity

LWDPropaga7onResis7vity

T

R1

R2

PS

B

PS (deg.)= f (ε)

Segnale a R1

Segnale a R2

t A

mp

iezz

a

A

AT(dB) = K Log (B/A) = f (σ)

θ

180° 360°

46BasicsofGeophysicalWellLogs_Lithology&Resis7vity

LWDresis7vi7es:Real7mevsmemory

RealTime Memory