Resistivity logs, well logging .Mahmoud althini

Resistivity logs

1. Lectures-4 & 5
Resistivity Logs
Well Logging & Subsurface Geology
Dr. Mohammed Gumati

2. •Electricallogsareperhapsthemostimportanttoolsavailabletoapetrophysicist.This
isbecausetheyprovideamethodforcalculatingthewatersaturation,uponwhich
calculationsofSTOOIParebase
1. Introduction
2. Principle Uses of Electrical Logs
•Themostimportantuseofresistivitylogsisthedeterminationofhydrocarbon-bearing
versuswater-bearingzones.Becausetherock’smatrixorgrainsarenonconductive
andanyhydrocarbonsintheporesarealsononconductive,theabilityoftherockto
transmitacurrentisalmostentirelyafunctionofwaterinthepores.
2

3. 3. Resistivity logs are used to:
• Determine hydrocarbon-bearing versus water-bearing zones.
• Indicate permeable zones.
• Determine porosity.
•Facies and electro-facies analysis.
•Correlation.
3

4. •Typical Resistivities
✓0.5 Ω.m to 1000 Ω.m for typical formations.
✓Soft formations i.e. shaly sands range from 0.5 Ω.m to about 50 Ω.m.
✓10 Ω.m to 100 Ω.m for hard formations (carbonates).
✓Evaporites (salt, anhydrites) may have several thousand Ω.m.
✓Formation water will range from 0.015 Ω.m (very salty brines).
✓Several Ω.m, fresh water reservoirs.
✓Sea water has a resistivity of 0.35 Ω.m @ 75°F.
4

5. Induction Logs [coil logs] (measure formation conductivity)
Induction (deep and medium)
Galvanic devices [electrode logs and laterologs] (measure formation resistivity)
Normal Microlaterolog (MLL)
Lateral Microlog (ML)
Laterolog (deep and shallow) Proximity Log (PL)
Spherically Focused Log (SFL) MicroSpherically Focused Log (MSFL)
Resistivity Log Depth of Investigation
Flushed Zone (Rxo) Invaded Zone (Ri) Uninvaded Zone (Rt)
MicroLog (ML) Short Normal (SN) Long Normal (LN)
Microlaterolog (MLL) Laterolog-8 (LL8) Lateral Log
Proximity Log (PL) Spherically Focused Log (SFL)Deep Induction Log (ILd)
MicroSpherically Focused Log (MSFL) Medium Induction Log (ILm) Deep Laterolog (LLd)
Shallow Laterolog (LLs) Laterolog-3 (LL3)
Laterolog-7 (LL7)
Classification of Resistivity Logs
Blue: ObsoleteRed: Currently in use.
5

6. 4. Old Electrical Logs
•These logs will be discussed briefly because data from them may still be
encountered when reanalyzing mature fields.
•Takeanhomogeneousandisotropicmedium
thatextendstoinfinityinalldirections.Now
passacurrentfromanelectrodeAinthe
mediumtoanotherMinfinitelydistant.
•TheresistivityofthematerialbetweenA
andMisthecalculatedas:
??????=??????????????????
(??????
??????−??????
??????)
??????
=??????????????????
∆??????
??????
ΔV = the potential difference between A and M.
4πr is defined by the geometry of the system (spherical symmetry).
I = Current.
The basic principle of resistivity measurement.
6

7. •The normallogging devices.
The distance AM = r is called the spacing. Two spacings were commonly used,
a short normal spacing equal to 16 inches, and a long normal spacing equal to
64 inches.
N.B.Thelongerthespacing,thegreaterthedepthofpenetrationofthecurrent
intotheformation,buttheloweritsverticalresolution.
The standard normal configuration. The standard lateral configuration
•Another arrangement is
possible, where electrodes
A and B are placed close
together with respect to
the distance between A
and M. This is called the
lateralconfiguration.
7

8. 5. Modern Resistivity Logs (Laterologs)
The LL3 tool electrode configuration
5.1.The LL3has 3 current emitting electrodes.
•The middle one, which is 1 foot long emits the main current.
•while the 5 foot long electrodes either side of it emit a current (bucking current) that
is designed to help keep the central current more focussed.
8

9. 5.2.The LL7has 7 electrodes.
•A constant current is emitted from the centre electrode. A bucking current is emitted
from the two far electrodes (80 inches apart).
•The vertical resolution of the LL7 is 3 ft. and the sensitivity is 0.2 to 20,000 Ω.m
•Measures the resistivity of the virgin formation, R
t
The LL7 tool electrode configuration
5.3. The LL8is similar to the LL7, but has
the current return electrode.
•Measures R
XOrather that R
t. closer to the
current emitting electrodes. This gives a
•current disk that does not penetrate as far
into the formation
•The vertical resolution of the LL8 is about
1 ftand the sensitivity is 0.2 to 20,000
Ω.m.
9

10. 5.4. The DualLaterolog
•The dual laterolog (DLL) is the latest version of the laterolog.
•Can be run in a deep penetration (LLd) and shallow penetration (LLs) mode.Both
are now commonly run simultaneously and have a bed resolution of 2 feet, and a
sensitivity of 0.2 to 20,000 Ω.m.
IntheLLdcurrentsareemitted
fromtheA1electrodesalsobeing
emittedfromtheadditionalfarthest
electrodes,A2.
IntheLLstheA1electrodesemita
buckingcurrent,buttheA2electrodes
aresettosinkthiscurrent(i.e.,the
buckingcurrentcomesoutofA1and
intoA2electrodes).
The DLL electrode configuration in both the LLd and LLs modes.
10

11. •Theresistivityreadingsfrom
thistoolcanandshouldbe
correctedforboreholeeffects
andthinbeds.
Dual laterolog-R
xotornado chart for
correcting deep resistivity to R
t.
Given:
LLD = R
LLD= 16.0 ohm-m
LLS = R
LLS= 10.0 ohm-m
MSFL = R
MSFL= 4.5 ohm-m
R
LLD/R
MSFL= R
LLD/R
xo= 3.6
R
LLD/R
LLS= 1.6
1.PlotR
LLD/R
xo(=3.6)ontheverticalaxisandR
LLD/R
LLS(=
1.6)onthehorizontalaxis.Plottheintersectionofthese
valuesonthetornadochart,anddeterminesubsequent
ratiovalues.
2.R
t/R
LLD: The scale for this value is represented by the solid
red lines. The scale values are in red and range from 1.1 to
1.8. Our value =1.32
3.di:Thediameterofinvasionaroundtheboreholeispickedfrom
thechartbyfollowingthedashed,bluelinestothescalesat
thetopofthechart.Thescalefrom20to120givesdiin
inches,andthescalefrom0.50to3.04givesdiinmeters.Our
valuefallsbetweenthescalevaluesof40and50inches,so
weassignavalueof43inches.
11

12. 4.R
t/R
xo: The scale for this ratio value is represented by the heavy, blue, solid lines. The scale values are in black, increase frombottom to
top, and range from 1.5 to 100. Our value falls between the scale values 3 and 5 (much closer to 5), so we assign a value of 4.8
5.Finally, corrected values for true resistivity of the formation (R
t) and resistivity of the flushed zone (R
xo) are determined using these
ratios.
(R
t/R
LLD) ×R
LLD= R
t (corrected)
(ratio) ×log value = R
t(corrected)
1.32 ×16.0 ohm-m = 21.1 ohm-m (= R
t true formation resistivity).
R
t(corrected)/(R
t/R
xo) = R
xo
R
t(corrected)/(ratio value from chart) = R
xo
21.1 ohm-m/4.8 = 4.4 ohm-m (= R
xo, resistivity of flushed zone)
12

13. 5.5. The Spherically Focussed Log(SFL)
Thesphericallyfocussedlog(SFL)hasanelectrodearrangementthatensuresthe
currentisfocussedquasi-spherically.Itisusefulasitissensitiveonlytotheresistivity
oftheinvadedzone.Verticalresolutionis30inches.
The SFL electrode configuration.
13

14. 5.6. Micro-Resistivity Logs
•The microlog is a pad-type resistivity device with three button electrodes placed in a line
with a 1 inchspacing that primarily detects mudcake.
The microlog electrode configuration.
•AknowncurrentisemittedfromelectrodeA,andthe
potentialdifferencesbetweenelectrodesM1andM2and
betweenM2andasurfaceelectrodearemeasured.
•The micro-normal device investigates 3to 4 inches into
the formation (measuring R
xo)
•Thedetectionofmudcakebythemicrologindicatesthat
invasionhasoccurredandtheformationispermeable.
•Themicrologdoesnotworkwellinsaltwatermuds(where
R
mf~R
w)orgypsum-basedmudsbecausethemudcake
maynotbestrongenoughtokeepthepadawayfromthe
formation.
5.6.1. The Microlog (MLL)
14

15. 5.6.2. The Microlaterolog
•The microlaterolog (MLL) is the micro-scale version of the laterolog,
The MLL electrode configuration.
•Thetoolispadmounted,andhasacentralbutton
electrodethatemitsacurrentsurroundedcoaxially
bytworing-shapedmonitoringelectrodes,anda
ring-shapedguardelectrodethatproducesa
buckingcurrent.
•themicrolaterologshouldberunonlywith
saltwatermuds.Itisstronglyinfluencedby
mudcakethicknessesgreaterthan1/4inch.
•The depth of investigation of the MLL is about 4
inches.
15

16. 5.6.3. The Proximity Log(PL)
The PL electrode configuration.
•The proximity log (PL) was developed from the MLL to overcome problems with
mudcakes over 3/8 inch thick, and is used to measure R
XO.
•Ithasadepthofpenetrationof1½ft.,andisnot
affectedbymudcake.Itmay,however,beaffectedby
R
twhentheinvasiondepthissmall.
•It can be used with freshwater muds.
16

17. 5.6.4. The Micro Spherically Focussed Log
The MSFL electrode configuration.
•It has a depth of penetration of about 4 inches
(similar to the MLL).
•Generallyisverygoodatdeterminingflushed-
zoneresistivity(R
xo).
•The tool consists of coaxial oblong electrodes arounda central current emitting
button electrode
17

18. 5.7. Induction Logs
•Theselogswereoriginallydesignedforuseinboreholeswherethedrillingfluid
wasveryresistive(oil-basedmuds).Itcan,however,beusedreasonablyalsoin
water-basedmudsofhighsalinity,buthasfounditsgreatestuseinwellsdrilled
withfreshwater-basedmuds.
•Thesondeconsistsof2wirecoils,a
transmitter(Tx)andareceiver(Rx).
Highfrequencyalternatingcurrent(20
kHz)ofconstantamplitudeisapplied.to
thetransmittercoil.Thisgivesrisetoan
alternatingmagneticfieldaroundthe
sondethatinducessecondarycurrents
(eddycurrents)intheformation.
secondary currents
(eddy currents)
induced in the
formation.
18

19. •Induction logs measure formation conductivity rather than resistivity.
•Formation conductivity is related to formation resistivity through the following
equation: C= 1000/R.
where:
C = conductivity in millimho/m (= milliSiemens)
R = resistivity in ohm-m
•Theintensityofthesecondarycurrentsgeneratedintheformationdepends
uponthelocationintheformationrelativetothetransmitterandreceivercoils.
Hencethereisaspatiallyvaryinggeometricalfactortotakeintoaccount.
•Thisfigureshowsthesensitivity
mapforthisspacefora
homogeneousmedium,showing
that50%ofthetotalsignal
comesfromclosetothetool
(boreholeandinvadedzone)
betweenthetransmitterandthe
receiver.
19

20. Skin Depth = ??????=
??????
??????????????????
δ = Depth (m)
μ = Magnetic permeability (4π x 10
-7
for free space)
ω = 2πf and f = signal frequency (Hz)
C = Conductivity (mmho/m)
Example:
Permeability = 1 (air), Frequency = 20,000 Hz, and resistivity = 100 Ω.m ( C
= 1/100 = .01 mho.m)
Skin depth = 50 meters
•Theskineffectisaproblemthatoccurswithveryconductiveformations,
theelectro-magneticallyinducedsecondarycurrentsbecomelargewhich
resultsinthereductionofthesignalrecordedonthelog.Thisis
automaticallycorrectedforduringtheloggingrun.
•SkinDepthisameasureofthedistancetowhichanelectromagneticwave
willpenetrate.
20

21. •The following tools are in common use today.
5.7.1. The 6FF40 Induction-Electrical Survey Log
5.7.2 The 6FF28 Induction-Electrical Survey Log
The 6FF40 induction-electrical survey log (IES-40) is a 6 coil device with a nominal
40 inch Tx-Rxdistance. The 6FF40 has a resolution widthof 8 ft.
The 6FF28 induction-electrical survey log (IES-28) is a smaller scale version of the
IES-40. It is a 6 coil device with a nominal 28 inch Tx-Rxdistance.
5.7.3 The Dual Induction-Laterolog
The dual induction laterolog (DIL) has several parts:
(i) A deep penetrating induction log (ILd) that is similar to the IES-40.
(ii) A medium penetration induction log (ILm), a shallow investigation laterolog (LLs).
21

22. •The ILmhas a vertical resolutionabout the same as the ILd(and the IES-40),
but about half the penetration depth. ILD and ILM logs have resolution widths
of 8and 5 ft, respectively.
5.7.4 The Induction Spherically Focussed Log
The induction spherically focussed log (ISF) combines a IES-40i, and a SFL. It is
often run in combination with a sonic log
5.7.5 Array Induction Tools
•The newest logs are array induction logs (AIT, HDIL).
•ItconsistsofoneTxandfourRxcoils.Intensivemathematical
reconstructionofthesignalenablestheresistivityatarangeofpenetration
depthstobecalculated,whichallowsthecompleteinvasionprofiletobe
mapped.
•With reducing the vertical resolutionup to 2 ft.
22

23. 5.7.6. Phasor Induction Log (IDPH)
•The Phasor induction tool is basically the same as a dual induction with deep,
medium, and SFL measurements.
•Provides thin-bed resolution down to 2 ft in many cases.
•ThestandardresolutionIDPH(deepphasorresistivity)andIMPH(mediumphasor
resistivity)logshaveresolutionwidthsof8and5ft,respectively,whichare
identicaltotheILDandILMlogsinresolution.
5.8. Log Presentation
•Resistivity logs are presented in Track 2or in Tracks 2and 3combined on a
log scale.
•The units are Ω.m, and sensitivity scales of 0.2-20 Ω.m (3 log cycles) all the
way up to 0.2 to 20,000 Ω.m (6 log cycles) can be used.
23

24. The presentation of electrical logs
24

25. The presentation of electrical logs
25

26. Typical resistivity log responses.
thetypicalresponseofanelectricaltool
inasand/shalesequence.Notethe
lowerresistivityinshales,whichisdue
tothepresenceofboundwaterinclays
thatundergosurfaceconduction.
Thedegreetowhichthesandstones
havehigherresistivitiesdependsupon
(i)theirporosity,
(ii)theirporegeometries,
(iii)theresistivityoftheformationwater,
(iv)thewater,oilandgassaturations
(oilandgasaretakentohaveinfinite
resistivity).
26

27. AnexampleofaDLLlog.Thisshows
goodseparationoftheLLsandLLd
fromeachotherandfromtheMSFL,
indicatingthepresenceofapermeable
formationwithhydrocarbons(gasinthis
caseinaformationofabout15%
porosity).
27

28. The response of resistivity logs in formations with various fluids
(recognition of hydrocarbon zones).
5.9. Uses of Electrical Logs
5.9.1. Recognition of Hydrocarbon Zones
Recognition of oil and gas in reservoir rocks is carried
out by:
•Oil shows in the log.
•Noting a difference in the shallow, medium and deep
resistivity tool responses.
A.Ifallthreecurvesarelowresistivity,andoverlieeach
other,theformationisanimpermeableshale,or,rarely,
theformationispermeableandwater-bearingbutthe
mudfiltratehasthesameresistivityastheformation
water.
B.Ifallthreecurvesarehigherresistivitythanthe
surroundingshales,andoverlieeachother,theformation
isanimpermeablecleanerformation(sandstone,
limestone).
C.Iftheshallowcurvehaslowresistivity,butthemedium
anddeeppenetratingtoolshaveahigherresistivitythatis
thesame(theyoverlieeachother),theformationis
permeableandcontainsonlyformationwater.
A B C D
D.Iftheshallowcurvehaslowresistivity,themediumasahigherresistivity,andthe
deeponehasanevenhigherresistivity(i.e.,thereisseparationofthemedium
anddeeptoolresponses),theformationispermeableandcontainshydrocarbons.
28

29. The behaviour of the resistivity log responses for different
formation water salinities.
•Ifthemudfiltrateresistivityisconstant,theeffectis
greaterforformationswithfreshformationwatersthan
thoseforsalineformationwaters,andinthecaseof
extremelysalineformationwatersthedeepresistivityin
theformationcanbesmallerthanthatoftheadjacent
shalebeds.
•Figuresaregivenasexamplesonly.Thisisbecause
therearetoomanyvariableparameters(mudfiltrate
resistivity,mudcakeresistivity,formationwater
resistivity,waterandhydrocarbonsaturations,lithology
andporosityetc.)tobecomprehensive.Ingeneral,the
bestapproachisto(i)seewhattypeoftoolshavebeen
usedtocreatethelogs,(ii)discovertheirpenetration
depths,(iii)discoverthetypeofdrillingmudandthe
resistivityofthemudfiltrateusedinthedrilling,(iv)
takeaccountofinformationfromotherlogs(caliperfor
mudcake,SPforpermeablezones,gammarayfor
shalevolume,andsonic,neutronanddensitytoolsfor
porosity),andtheninterprettheresistivitycurvesfrom
firstprinciples!
29

30. 5.9.2. Calculation of Water Saturation
•TheresistivitylogvaluesforthedeeptoolsRtinreservoirintervalscanbeused
withareliableporosityΦ,theformationwaterresistivityRw,andmandnvalues
thatarederivedfromlaboratorymeasurementsoncore,tocalculatethewater
saturationinthezone.
5.9.3. Textures and Facies Recognition
•The texture of a rock has a great effect upon its electrical response.
•This is because the electrical flow through the rock depends upon the tortuosity of
the current flow paths, which is described by the formation factor F.
30

31. •Shows small scale deltaic cycles recorded by an IES-40.
•Theshapeselectricallogscanbeusedtodistinguish
faciestypesaswithotherlogs(e.g.,sonicandgamma
ray).Thesearesometimescalledelectrofacies,which
aredefinedbyRider[1996]as“Suitesofwirelinelog
responsesandcharacteristicssufficientlydifferentto
beabletobeseparatedfromotherelectrofacies.”.
Note,ingeneralthewirelinelogscharacteristicsthat
defineaparticularelectrofacieswillalsoincludedata
fromwirelinelogsotherthanelectricallogs.
31

32. 5.9.4. Correlation
•Electricallogsareoftenusedforcorrelation.Thedeeplogs(IES-40,ILd,LLdetc.)
arethebesttouse,BECAUSEtheyaresensitivetogrosschangesintheformations
•However,itmustbenotedthatthe
resistivityofaformationalso
dependsuponformationfluidpressure
andformationwaterresistivity,which
arenon-stratigraphicvariables,and
changesinthemfromwelltowellmay
confusecorrelations.
32

33. 5.9.5. Lithology Recognition
•Electrical logs are dramatically bad at indicating lithologies.
•Sandsshalesandcarbonateshavenocharacteristicresistivityastheir
resistivitiesdependuponmanyfactorsincludingporosity,compaction,fluid
resistivity,textureetc.
•As the electrical logs are very sensitive to texture, they are extremely good at
discriminating between lithologies of different types.
33

34. Characteristic resistivities from various lithologies recorded by resistivity logs.
•Gypsum –1000 Ω.m.
•Anhydrite –10,000 -ꝏΩ.m.
•Halite -10,000 -ꝏΩ.m.
•Coals –10 –106 Ω.m.
•Tight limestones and dolomites –80 –
6000 Ω.m.
•Disseminated pyrite -<1 Ω.m (pyrite has
a resistivity of 0.0001 –0.1 Ω.m.
•Chamosite -<10 Ω.m.
34

35. O/W
35

36. 5.10. Bed Resolution
•Thesmallertheelectrodespacing,thebettertheverticalandbedresolution.
Thisisshowninthefigurebelow.Oneshouldchosethetoolforthepurpose
required,andthisisrelatedalsotoinvestigationdepth.
Differences in bed resolution from different electrical tools (Rider, 1996). 36

37. 5.11. Investigation Depth
•Thefigureattherightsummarizesthedepthsof
investigationofthevarioustools,andtheTable
summarizestheresistivityvaluescommonly
measured.
Summary of the different depths of
investigation for different electrical tools
•Finebedstructurerequiresveryshallowreading
toolsinordertoobtainsufficientbedresolution.
•Changesin formation textureare best seenon
logs that measure the invaded zone. BECAUSE
the texture have a large effect on fluidswith the
mud filtrate
37

38. Electrical tool penetration and resistivity measurements.
38

39. 5.12. Comparing Laterologsand Induction Logs
•Induction logs provide conductivity (that can be converted to resistivity).
•Laterologsprovide resistivity (that can be converted to conductivity).
•Induction logs work best in wells with low conductivity fluids.
•Laterologswork best in wells with low resistivity fluids.
•Both logs provide a range of depths of penetrations and vertical resolutions.
39