02 resistivity measurement

Schlumberger

(05/96)

Contents

B1.0 RESISTIVITY OF THE FORMATION.....................................................................................1B1.1 INTRODUCTION .........................................................................................................1B1.2 FORMATION WATER RESISTIVITY RW......................................................................3B1.3 FORMATION RESISTIVITY MEASUREMENTS .........................................................3

Chart Gen-9: Resistivity of NaCl Solutions..................................................................4B1.4 TO SUMMARIZE ........................................................................................................6B1.5 THE DRILLING PROCESS AND PERMEABLE BEDS.................................................5

Invasion Profiles ........................................................................................................5Chart Gen-3: Symbols Used in Log Interpretation......................................................7

B1.6 SPONTANEOUS POTENTIAL (SP) CURVE ................................................................8Chart SP-1: Rweq Determination from ESSP (Clean Formations)..................................13Chart SP-2: Rw versus Rweq and Formation Temperature..........................................14

B2.0 MEASUREMENT OF Rt BY INDUCTION PRINCIPLES........................................................15B2.1 INTRODUCTION .......................................................................................................15B2.2 INDUCTION LOGGING PRINCIPLES........................................................................15B2.3 SPHERICALLY FOCUSED LOG PRINCIPLES..........................................................16B2.4 DUAL INDUCTION - SPHERICALLY FOCUSED LOG................................................17B2.5 PHASOR-INDUCTION SFL TOOL .............................................................................23

B3.0 MEASUREMENT OF Rt BY LATEROLOG PRINCIPLES....................................................29B3.1 DUAL LATEROLOG.................................................................................................29

B4.0 MEASUREMENT OF RXO BY MICRO-RESISTIVITY PRINCIPLES .....................................35B4.1 INTRODUCTION ......................................................................................................35B4.2 MICROLOG.............................................................................................................36B4.3 MICRO-SPHERICALLY FOCUSED LOG..................................................................38

B5.0 WORK SESSION ..............................................................................................................41

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Introduction to Openhole Logging

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B1.0 Resistivity of theFormation

B1.1 INTRODUCTIONThe resistivity of a formation is a key pa-

rameter in determining hydrocarbon saturation.Electricity can pass through a formation onlybecause of the conductive water it contains.With a few rare exceptions, such as metallicsulfide and graphite, dry rock is a good electri-cal insulator. Moreover, perfectly dry rocks areseldom found. Therefore, subsurface forma-tions have finite, measurable resistivities be-cause of the water in their pores or absorbed intheir interstitial clay.

For the purposes of our discussions we willdivide substances into two general categories,conductors or insulators.

Conductors are substances that pass electricalcurrent (e.g., water, shales, mud). Insulatorsare substances that do not allow electrical cur-rent flow (e.g., hydrocarbons or rock matrix).

The measured resistivity of a formation de-pends on

- resistivity of the formation water- amount of water present- pore structure geometry.

The resistivity (specific resistance) of a sub-stance is the resistance measured between

opposite faces of a unit cube of that substance ata specified temperature. The meter is the unitof length and the ohm is the unit of electricalresistance. In abbreviated form, resistivity is

R = r A/L, where

R is resistivity in ohm-metres,r is resistance in ohms,A is area in square metres,and L is length in metres.(See Figure B1)

The units of resistivity are ohm-metressquared per meter, or simply ohm-metres(ohm-m).

Conductivity is the reciprocal of resistivityand is expressed in mhos per meter. To avoiddecimal fractions, conductivity is usually ex-pressed in millimhos per meter (mmho / m),where 1000 mmho/m = 1 mho/m

C = 1000/R.

Formation resistivities are usually from 0.2 to1000 ohm-m. Resistivities higher than 1000ohm-m are uncommon in permeable forma-tions but are observed in impervious, very lowporosity formations (e.g., evaporites).

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R = raL

OHM-METERS2

METER

R = resistivitya = areaL = lengthr = resistance

Figure B1: Principles of Resistance and Resistivity

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B1.2 FORMATION WATERRESISTIVITY R

W

As previously indicated, formation matricesare insulators; thus a formations ability to con-duct electricity is a function of the connate waterin the formation. Several factors must be con-sidered:

- volume of the water (porosity)- pore space arrangement (type of poros-

ity)- temperature of the formation- salinity of the water.

a) Water SalinityAs salinity increases, more ions are available

to conduct electricity, so Rw (water resistivity)

decreases.

b) Water TemperatureAs water temperature is raised, ionic mobility

increases and resistivity decreases. Chart Gen-9(Figure B2) in the Log Interpretation Chartbook illustrates these relationships.

c) Water VolumeAs water-filled pore space in a rock is in-creased, resistivity decreases. If some water isdisplaced by hydrocarbons (insulators), watersaturation decreases; resistivity increases.

B1.3 FORMATION RESISTIVITYMEASUREMENTS

If we consider a formation with pore spacethat contains only water, its true resistivity iscalled R

o. We know that an important relation-

ship exists between formation resistivity andthe resistivity of the saturating water, R

w. The

ratio of these two values, F, is called formationresistivity factor, or more commonly formationfactor, which is a constant, where:

F = Ro / R

w

For example, if the salinity of the connatewater increases, R

w will decrease. This will in

turn allow current to flow more easily throughthe formation, thus lowering R

o and maintain-

ing F at a constant value. This is what weshould expect as F is an inherent formationcharacteristic.

Formation factor can be related to formationporosity by the general formula

F = a / m where

a = constantm = cementation factor

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Resistivity of NaCl Solutions

F 50 75 100 125 150 200 250 300 350 400

C 10 20 30 40 50 60 70 80 90 100 120 140 160 180 200

Temperature (F or C)

Res

istiv

ity o

f sol

utio

n (o

hm-m

)

ppm

10

8

65

4

3

2

1

0.8

0.60.5

0.4

0.3

0.2

0.1

0.08

0.060.05

0.04

0.03

0.02

0.01

200

300

400

500600700800

10001200140017002000

3000

4000500060007000800010,00012,00014,00017,00020,000

30,000

40,00050,00060,00070,00080,000100,000120,000140,000170,000200,000250,000280,000

Conversion approximated by R2 = R1 [(T1 + 6.77)/(T2 + 6.77)]F or R2 = R1 [(T1 + 21.5)/(T2 + 21.5)]C

300,000

NaC

l con

cent

ratio

n (p

pm o

r gr

ains

/gal

)

Gra

ins/

gal a

t 75

F

10

15

20

25

30

40

50

100

150

200

250

300

400

500

1000

1500

2000

25003000

4000

5000

10,000

15,000

20,000

Chart GEN-9

Figure B2

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B1.4 SUMMARY1. Dry rock formations are an insulator.2. Formations conduct current because of

water in the pore spaces.3. Knowledge of water resistivity (R

w) is

essential for log interpretation.4. Resistivity used rather than resistance.5. Formation resistivity factor (F) is a po-

rosity-related formation characteristic.6. Relationships

a. F = (Rt / R

w) = (R

o / R

w)

100% water saturated porous rockb. F = a / m

7. SymbolsR

w - resistivity of connate water

Rt - true formation resistivity

Rxo

- resistivity of flushed zonea - constantm - cementation factor.

B1.5 DRILLING PROCESS ANDPERMEABLE BEDS

Before proceeding to a discussion of meth-ods of obtaining formation resistivity, let usexamine what happens to a permeable forma-tion when it is penetrated by the drill bit.(Refer to Chart Gen-3 [Figure B3] in this sec-tion or the Log Interpretation Chart book.)

Under normal conditions, the hydrostatichead of the mud column is greater than forma-tion pressure. This differential pressure forcesfiltrate from the mud system into the forma-tion pore spaces, leaving solid particles ormudcake buildup on the borehole wall.Eventually this impervious mudcake will sealoff further invasion (unless it is removed bysome mechanical process; e.g., removing thedrill bit).

Mudcake thickness is symbolized by hm c

.

Invasion Profiles:1. Flushed Zone. Adjacent to the bore-

hole the invasion process flushes outthe original water and some of the hy-drocarbons (if any were present). Theresistivity of this zone is termed R

x o;

the water saturation is called Sx o

where

FRmf

Sxo

2 =R

xo

(for clean formations only)

Plotting Rxo

as a function of radialdepth into the formation yields (FigureB4).

2. Transition Zone. Further from theborehole the flushing action of themud filtrate may create a variety ofsituations. If the flushing proceeds asa uniform front, we call this a stepprofile of invasion (Figure B5[a]). Ifthe intermingling of formation fluidsis gradual, we call this a transitionzone (Figure B5[b]). Sometimes inoil- or gas-bearing formations, wherethe mobility of hydrocarbons is greaterthan the connate water, the oil or gasmove out leaving an annular zonefilled with connate water (Figure B5c).If R

mf > R

w, then the annular zone will

have a resistivity lower than Rxo and R

t

and may cause a pessimistic saturationcalculation.

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Symbols Used in Log Interpretation

dhHole

diameter

didj

h

rj

(Invasion diameters)

Adjacent bed

Zone of transition

or annulus

Flushed zone

Adjacent bed

(Bedthickness)

Mud

hmc

dh

Rm

Rs

Rs

Resistivity of the zone

Resistivity of the water in the zoneWater saturation in the zone

Rmc

Mudcake

Rmf

Sxo

Rxo

Rw

Sw

Rt

Uninvadedzone

Chart GEN-3

Figure B3

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3. True Unaffected Zone. This is the zonethat we want to analyzeit is the for-mation undisturbed by the drillingprocess. Its resistivity is termed R

t,

water resistivity Rw and water satura-

tion Sw. Plotting R

xo, R

i and R

t as a

function of invasion gives us FigureB4.

Rxo

Di

Figure B4: Invasion Process

Rxo

Ri RtR R R

RxoRi

Rt

Rxo

RiRt

Di Di D2 Di

(a) (b) (c)

Figure B5

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B1.6 SPONTANEOUS POTENTIAL (SP) CURVE

a) IntroductionThe SP curve is a continuous recording

(versus depth) of the difference in potentialbetween a moveable electrode in the boreholeand a fixed (zero) potential surface electrode.Units used are millivolts.

The SP was discovered quite by accident inthe early days of electrical logging. In some ofthe first test wells logged by Schlumbergerusing the point-by-point technique, it wasnoted that a small natural potential was presentin the well even when the current source wasturned off. This spontaneous potential is due toa combination of two phenomena: an elec-trokinetic potential is usually negligible and anelectrochemical potential is composed of amembrane potential and a liquid-junction po-tential. The membrane potential is about 5times bigger than the liquid-junction potential.

b) Electrokinetic PotentialIf a solution is forced by differential pressure

to flow through a membrane, an electrical po-tential will appear across the membrane(Figure B6). A similar situation occurs whenthe mud filtrate flows through the mudcakebecause of the differential pressure between themud column and the formation. This elec-trokinetic potential (E

kmc) is generally small.

In a low-permeability formation, where themudcake is only partially built up, this elec-trokinetic potential may be as high as 20 mV.This situation is, however, rare and in generalthe total electrokinetic potential can be ne-glected.

c) Electrochemical PotentialThis potential is created by the contact of two

solutions of different salinity, either by a directcontact or through a semipermeable membranesuch as shales.

Figure B6: Electrokinetic Potential of SP Figure B7: Electrochemical membranepotential of SP

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1) Membrane Potential An ideal cationic membrane because of its

physico-chemical composition is permeable topositive ions (cations) only. Shales are idealmembranes as long as they are not too sandy ortoo limy. In a borehole, a shale section usuallyseparates salty water (generally the connatewater of the virgin zone) from a less salty liquid(generally the mud) (Figure B7). There is mi-gration of the positive ions (Na+) from the saltywater (formation) to the less salty water (mud).

When an equilibrium is reached:- Positive ions that have already crossed

the shale membrane exert a repellingforce on the positive ions in the mud.

- Negative ions left behind in the forma-tion exert an attractive force on thepositive ions which cannot travel anymore into the shale.

The difference of potential appearing betweenthe two solutions is given by the formula:

amfE

m = K ;og

aw

Figure B8: Electrochemical Liquid- Junction Potential of SP

where amf and aw are the electro-chemical ac-

tivities of mud filtrate and connate water,respectively.

2) Liquid Junction PotentialThe liquid junction potential takes place at theboundary between the flushed zone and the vir-gin zone. There is no shale separating the twosolutions. Anions as well as cations can transferfrom one solution to the other (Figure B8) be-cause of the higher salinity of the formationwater and both Na+ cations and Cl anions willmigrate toward the mud filtrate. The Na+ ion iscomparatively large and drags 4.5 molecules ofwater. The Cl ion is smaller and drags only 2.5molecules of water. Hence, the anion Cl willmigrate more easily than the Na+ ions.

Figure B9: SP Circuit Path

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The result is an increase of positive charges leftbehind in the formation water. These positivecharges restrict Cl migration toward theflushed zone. A difference of potential appearsat the boundary between the two solutions:

amfE

j = K log

aw

d) Spontaneous Potential (SP)The total potential of the whole chain is thus

the algebraic sum Em + E

j, which is also called

the Static Spontaneous Potential (SSP). Elec-trokinetic potential is neglected. The SP is the

drop of potential measured across the currentlines in the borehole. Along its path the SSPcurrent has to force its way through a series ofresistances, both in the formation and in themud (Figure B9). This means that the total po-tential drop (which is equal to the SSP) is di-vided between the different formations andmud in proportion to the resistances met by thecurrent in each respective medium. The SP,which is the measure of the potential drop in themud of the borehole, is only part of the SSP.In general, it is a large portion because the elec-trical resistance offered by the borehole is, ingeneral, much greater than that offered by theformations.

SSP = -K logRmfeRwe

Rmf = Rw Rmf RwFRESH MUD

Figure B10: The SP Deflection and its Rmf-Rw Dependency

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So, we can write:

amfSP SSP = (K + K) log

aw

The SP curve is generally presented in track1, and usually recorded with resistivity sur-veys, assuming a conductive mud is in theborehole.

Opposite a permeable formation, the SPcurve shows excursions from the shale base-line. In thick, clean beds the SP deflectiontends to reach an essentially constant deflectiondefining a clean line.

The deflection may be either to the left(negative) or to the right (positive) dependingmostly on relative resistivity of the formationwater and of the mud filtrate (Figure B10).

The magnitude of SP deflections is alwaysmeasured from the shale line and for a clean,water-bearing formation containing a dilutesodium chloride solution is given by

SSP = K log(Rmfe

/ Rwe

)

The constant K depends on the temperatureand salt types in formation water (K = 71 at25C for NaCl).

In practice, the SP is affected by a number offactors, all of which tend to reduce its magni-tude.

The maximum available SP in a thick, clean,water-bearing zone is called the SSP (FigureB10).

The SP is reduced by the shale in a shalyzone, and the deflection is called the pseudo-static spontaneous potential (PSP).

The ratio of these two values, termed =PSP/SSP, can be used as a shale indicator insands. An approximation of the SSP in ashaly sand is SSP = PSP / (1 V

sh) where the

volume of shale (Vsh

) is estimated from thegamma ray deflection, which is discussedlater.

e) Uses of SPThe SP can be used to

- detect permeable beds (a qualitative in-dication only)

- determine Rw, formation water resis-

tivity- give an indication of zone shale content- indicate depositional environment.

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f) Factors Affecting the SP- Bed thickness: SP decreases when

bed thickness decreases.- Invasion: Reduces SP.- Shaliness: Shale reduces SP.- Hydrocarbons: Hydrocarbons in

slightly shaly formations reduce theSSP.

- Mud filtrate: The magnitude and direc-tion of SP deflection from the shalebaseline depends on relative resistivi-ties of the mud filtrate and the forma-tion water.

- Fresh mud: negative SP (Figure B8).R

mf > R

w

- Saline mud: positive SP (Figure B8).R

w > R

mf

Rw = R

mf : zero SP (Figure B8).

g) Solution of Rw from SP

Because of its dependence on Rmf and R

w, the

magnitude of SP deflection enables us to solvefor the R

w of the formation when R

mf is known.

This method, when applied in clean matrix, isgenerally accurate.

1. From the log heading, get Rmf at sur-

face temperature.

2. Convert Rmf to formation temperature

using chart Gen-9 (Figure B2).

3. Convert Rmf at formation temperature

to Rmfe

using:

Rmfe

= 0.85 Rmf (approximation)

If Rmf is below .03 ohm-meter or above

1.5 ohm-meter at formation tempera-ture, use chart SP-2m (Figure B12) toget R

mfe.

4. Calculate static SP from log at zone ofinterest.

5. Enter chart SP-1 (Figure B11) withstatic SP, formation temperature andR

mfe to get R

we at formation tempera-

ture.

6. Enter chart SP-2m (Figure B12) withR

we and formation temperature to get

Rw.

corrosion charts are available to correct for these factors. Pyrite in the formation produces a positive SP.

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Rweq Determination from ESSP(CLEAN FORMATIONS)

0.01

0.02

0.040.06

0.1

0.2

0.4

0.6

1

2

4

6

10

20

40

60

100

0.001

0.005

0.01

0.02

0.05

0.1

0.2

0.5

1.0

2.0

Rmfeq(ohm-m)

Rmfeq /Rweq

aw/a

mfor

Rm

fe/R

we

Rweq(ohm-m)

+50 0 50 100 150 200

ESSP, static spontaneous potential (mV)

250C200C150C

100C

50C0C

500F400F300F

200F

100F

Formationtemperature

0.3

0.4

0.6

0.8

1

2

4

6

8

10

20

40

0.3

0.4

0.50.6

0.8

1

2

3

4

6

8

10

20

30

40

50

5

Schlumberger

This chart and nomograph calculate the equivalent forma-tion water resistivity, Rweq, from the static spontaneouspotential, ESSP, measurement in clean formations.

Enter the nomograph with ESSPin mV, turning throughthe reservoir temperature inF or C to define theRmfeq/Rweq ratio. From this value, pass through theRmfeqvalue to define Rweq.

For predominantly NaCl muds, determine Rmfeq asfollows:

a. If Rmf at 75F (24C) is greater than 0.1 ohm-m,correct Rmf to formation temperature using ChartGen-9, and use Rmfeq = 0.85 Rmf.

b. If Rmf at 75F (24C) is less than 0.1 ohm-m, useChart SP-2 to derive a value of Rmfeq at formationtemperature.

Example: SSP = 100 mV at 250FRmf = 0.70 ohm-m at 100For 0.33 ohm-m at 250F

Therefore, Rmfeq = 0.85 0.33= 0.28 ohm-m at 250F

Rweq = 0.025 ohm-m at 250FESSP= Kclog(Rmfeq/Rweq)

KC = 61 + 0.133 TF

KC = 65 + 0.24 TC

SP-1

Figure B11

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Rw versus Rweq and Formation Temperature

0.005 0.01 0.02 0.03 0.05 0.1 0.2 0.3 0.5 1.0 2 3 4 5

0.001

0.002

0.005

0.01

0.02

0.05

0.1

0.2

0.5

1.0

2.0

Rw or Rmf (ohm-m)

Rw

eq o

r R

mfe

q(o

hm-m

)

250C200C

150C

100C

75C

50C

25C

Saturation

200C150C100C75C50C25C

250C

NaCl at 25C

Gyp-base mud filtrates

EXAMPLE: Rweq = 0.025 m at 120oC. From chart, Rw = 0.031 m at 120oCSpecial procedures for muds containing Ca or Mg in solution are discussed in Reference 3. Lime base mudsusually have a negligible amount of Ca in solution; they may be treated as regular mud types.

SP-2m

Figure B12

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B2.0 Measurement of Rt byInduction Principles

B2.1 INTRODUCTIONWe have two different types or classes of

tools designed for the two most common bore-hole environments:

1. Nonconductive boreholes- including fresh mud systems, invert

mud systems and air-filled holes.a. Dual-Induction SFL tool (no

longer in service)b. Phasor-dual Induction SFL

toolc. Array Induction Imager tool

(AIT)

2. Conductive boreholes- including saline to salt saturated mud

systemsDual laterolog.

B2.2 INDUCTION LOGGINGPRINCIPLES

The induction logging tool was originally de-veloped to measure formation resistivity inboreholes containing oil-base muds and in air-drilled boreholes. Electrode devices did notwork in these nonconductive muds, and at-tempts to use wall-scratcher electrodes wereunsatisfactory.

Experience soon demonstrated that the induc-tion log had many advantages when used forlogging wells drilled with water-base muds.Designed for deep investigation, induction logscan be focused to minimize the influences ofthe borehole, surrounding formations and in-vaded zone.

PrincipleTodays induction tools have many transmit-

ter and receiver coils. However, the principlecan be understood by considering a sonde withonly one transmitter coil and one receiver coil(see Figure B13).

A high-frequency alternating current of con-stant intensity is sent through a transmitter coil.The alternating magnetic field created inducescurrents in the formation surrounding the bore-hole. These currents flow in circular groundloops coaxial with the transmitter coil and cre-ate, in turn, a magnetic field that induces a volt-age in the receiver coil.

Because the alternating current in the trans-mitter coil is of constant frequency and ampli-tude, the ground loop currents are directly pro-portional to the formation conductivity. Thevoltage induced in the receiver coil is propor-tional to the ground loop currents and, there-fore, to the conductivity of the formation.


(05/96) B-16

There is also a direct coupling between thetransmitter and receiver coils. The signaloriginating from this coupling is eliminatedelectronically.

The induction tool works best when theborehole fluid is an insulatoreven air or gas.The tool also works well when the boreholecontains conductive mud unless the mud is toosalty, formations are too resistive or boreholediameter is too large.

B2.3 SPHERICALLY FOCUSED LOG PRINCIPLES

The SFL device measures the resistivity ofthe formation near the borehole and providesthe relatively shallow investigation required toevaluate the effects of invasion on deeper re-sistivity measurements. It is the short-spacingdevice used in the Phasor induction SFL tool.

The SFL system differs from previous fo-cused electrode devices. Whereas those sys-tems attempt to focus the current into planardiscs, the SFL system establishes essentiallyconstant potential shells around the currentelectrode.

Figure B13: Basic two-coil induction log system

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(05/96) B-17

The SFL device is able to preserve the spheri-cal potential distribution in the formation overa wide range of wellbore variables, even whena conductive borehole is present. To accom-plish this, the SFL device is composed of twoseparate, and generally independent, currentsystems (Figure B14). The bucking currentsystem serves to plug the borehole and estab-lish the equipotential spheres. The i

o survey

current system causes an independent surveycurrent to flow through the volume of investi-gation; the intensity of this current is propor-tional to the formation conductivity.

Figure B14: Electrode array of SFL tooland schematic representation of surveying

current (io) lines (dashed) and focusing current (io) lines (solid).

The SFL device consists of current-emittingelectrodes, current-return electrodes and meas-ure electrodes. Two equipotential spheresabout the tools current source are established.

The first sphere is about 9 in. away from thesurvey current electrode; the other is about 50in. away. A constant potential of 2.5 mV ismaintained between these two spherical sur-faces. Because the volume of formation be-tween these two surfaces is constant (electrodespacing is fixed) and the voltage drop is con-stant (2.5 mV), the resistivity of this volumeof formation can be determined by measuringthe current flow.

B2.4 DUAL INDUCTIONSPHERICALLY FOCUSED LOG

This is the most basic of induction devicesand was the reference resistivity induction de-vice for more than 20 years until its retirementin 1990. The tool supplies three focused resis-tivity curves: two induction and a shallow in-vestigating spherically focused curve plus thespontaneous potential (SP). Each curve has adifferent depth of investigation (Figure B15).

Spherically focused loga shallowreading device affected mainly by theflushed (R

xo) zone (radial distance

30 cm).

Medium induction (ILM)depending on the invasion diameterand profile the ILM may be influ-enced by the R

xo or R

t zones or both.

(radial distance 60 80 cm).

Deep induction (ILD) mostly af-fected by R

t, unless invasion is very

deep. Either or both induction curvesmay be influenced if an annulus ispresent (radial distance 1.2 1.5m).


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600

SP

0.0000-150.0000 (MV)

SFLU

2000.00000.2000 (OHMM)

ILD

2000.00000.2000 (OHMM)

ILM

2000.00000.2000 (OHMM)

FILE 2

ILM

DUAL INDUCTION - SP/SFL

Figure B15

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(05/96) B-19

a) Log Presentationa. Logarithmic: A 1:240 scale is pre-

sented with the resistivity curves on alogarithmic scale. This is the pre-ferred presentation for log analysis(Figure B15).

b. Log-lin: The 1:600 scale presents tworesistivity curves, the SFL (averaged)and the ILD on the linear resistivityscale. Also included is the equivalentILD conductivity curve. This presen-tation is primarily for correlation pur-poses. Both presentations are re-corded simultaneously.

b) Tool Characteristics and Applications1. The Dual-Induction SFL tool is most

effective when used in holes drilledwith moderately conductive mud(e.g., where R

mf / R

w > 2.5).

2. Vertical focusing is good, and reliablevalues of R

t may be obtained where

bed thickness is > 4.0 m.3. Because this tool actually measures

formation conductivity and convertsthe values to resistivity, results aremost accurate in zones of low resis-tivity.

4. The recording of three curves that in-vestigate different amounts of forma-tion volume enable us to study inva-sion profiles and where invasion isdeep, make the correction to obtain R

t.

5. Because the two induction devicesproduce their signals by inducing amagnetic field in the formation, theycan be run in air-drilled wells or wellsdrilled with nonconductive mud. (TheSFL tool requires a conductive mudpath to the formation and cannot bepresented.) A gamma ray curve isusually recorded in place of the SP.

Correction charts are available for theinfluence of:

- borehole (diameter and mudresistivity)

- bed thickness- invasion.

c) Limitations1. The logging of large diameter holes

drilled with saline mud should beavoided, particularly in high-resistivityformations. Large borehole signalswill add to the formation signals, pro-ducing anomalously low apparent re-sistivities.

2. In zones of high resistivity (low con-ductivity), e.g. in excess of 250 ohm-m, errors in measurement can occur.

These problems may be minimized by a sys-tem of downhole calibration checks. A thickzero-porosity zone (e.g., limestone or anhy-drite) is used for this purpose. Thus, if diffi-culties in producing a good DIL are expected, itis often advantageous to run a porosity-caliperlog before the DIL. (Note that these changeswere only made to the DIL logs in the remarkssection of the log heading.)

d) Log Responses (Figure B16)For wells drilled with fresh muds (R

mf/R

w >

2.5, Rxo

/Rt

> 2.5) the following general conclu-sions can be reached by log inspection:

- When SFL = ILM = ILD; Rt = ILD,

this indicates zero or very shallow in-vasion.

- When SFL > ILM = ILD; Rt = ILD

this indicates moderate invasion.- When SFL > ILM > ILD; and if R

xo =

SFL, then Rt < ILD, which indicates

deep invasion.


(05/96) B-20

When SFL = ILM > ILD and if Rxo

= SFLchart Rint-2c must be used (Figure B17) to ob-tain R

t. This response indicates very deep inva-

sion.

In general, the closer the medium curve is tothe SFL, the deeper the invasion. The result ofcorrecting for invasion is to obtain an R

t that is

lower than the ILD. Hence, by using ILDwithout correction, you will obtain an optimisticS

w.

e) SummaryBenefits:

1. Dual-Induction SFL tool can most ef-fectively be used in holes filled withmoderately conductive mud, noncon-ductive mud, and air-drilled holes.

2. Vertical focusing is good and givesreliable values of R

t for beds thicker

than 3 m.

3. It measures low resistivities (less than10 ohm-m) accurately.

4. Recording of three focused resistivitylogs, which investigate different vol-umes of formation, enables us tostudy invasion profile and good R

t

values in the case of deep invasion.

Correction charts are available for- borehole- bed thickness- invasion.

Disadvantages:1. Not reliable for resistivities > 250

ohm-m (use a dual laterolog)2. Large hole and saline mud results in

large borehole signals give an unusu-ally low apparent resistivity. (useDLL in this case).

Schlumberger

(05/96) B-21

MODERATEINVASION

VERY DEEPINVASION

SP

20.0000-80.0000 (MV)

SFL

2000.00000.2000 (OHMM)

ILD

2000.00000.2000 (OHMM)

ILM

2000.00000.2000 (OHMM)

DUAL INDUCTION INVASION PROFILES

NO INVASION

SHALLOWINVASION

Figure B16


(05/96) B-22

DIL* Dual-Induction - SFL* Spherically Focused LogID - IM - SFL

1

2

3

4

5

6

7

8

9

10

20

30

40

1.0 1.1 1.2 1.3 1.4 1.5 1.7 1.9

RSFL/RID

RIM /RID

Thick beds, 8-in. [203-mm] hole, skin-effect corrected,DIS-EA or equivalent

90

25

20

25

20

15

3030

40

80

6050

RtRID

RxoRt

10

1.0

0.95 0.90 0.80

1.01

Rxo /Rm 100

di (in.)

70

2

3

5

7

15

di (m)

2.031.521.27

0.50

0.63

0.75

0.38

Rint-2c

Figure B17

Schlumberger

(05/96) B-23

B2.5 PHASOR-INDUCTION SFL TOOLThe Phasor-Induction SFL tool (Figure B18)

uses a conventional dual induction-SFL arrayto record resistivity data at three depths of in-vestigation (see Chart B1). In addition to theusual in-phase (R-signal) induction measure-ments, the tool makes a high-quality meas-urement of the induction quadrature signal (X-signals). These measurements are combinedwith new advances in signal processing toprovide an induction log with thin-bed resolu-tion down to 2 ft [60 cm]. Full correction forsuch environmental distortions such as shoul-der effect and borehole effect are also per-formed.

Since its introduction in the early 1960s, thedual induction tool has evolved into the pri-mary logging service for openhole formationevaluation in fresh and oil-base muds. Previ-ous tools have, however, produced logs withresponse limitations. These limitations haveusually required tedious hand correction. Inextreme cases tool response limitations haveproduced features on logs that were mistakenfor geological features. Although distortionsof the formation resistivity caused by resolu-tion effect and shoulder effect are fully predict-able from electromagnetic theory, automaticcorrection algorithms were not successful be-fore now because of the nonlinearity of the R-signal measurement, which was the onlymeasurement made in the older tools.

New developments in electronics technology,work on computing the response of the induc-tion tool in realistic formation models andmodern signal processing theory have com-bined to allow the development of a newer toolthat is able to overcome the limitations of pre-vious tools.

Central to this development is a nonlineardeconvolution technique that corrects the in-duction log in real time for shoulder effect andimproves the thin-bed resolution over the fullrange of formation conductivities. This algo-rithm, called Phasor Processing, requires theuse of the induction quadrature signals, or X-signals, which measure the nonlinearity di-rectly. Phasor Processing corrects for shoul-der effect and provides thin-bed resolutionthrough Enhanced Processing down to 60 cmin many cases.

Figure B18: Schematic of the Phasor-Induction SFL tool


(05/96) B-24

By adding borehole geometry measurementsin the same tool string, borehole effect can alsobe corrected in real time. With these environ-mental effects removed, a real-time inversionof the data into a three-parameter invasionmodel can be done at the wellsite.

The Phasor induction design provides sev-eral additional advantages over existing tools.These include improvements in the calibrationsystem, sonde error stability, SFL responseand a reduction of signal and cable noise.Each of these improvements contributes to-ward providing more accurate formation resis-tivity measurements over a wider range of re-sistivity and borehole conditions.

a) Phasor Tool Description and FeaturesThe Phasor-Induction SFL tool can be com-

bined with other cable telemetry tools. Meas-urements returned to the surface include deep(ID) and medium (IM) R-signals, ID and IMX-signals, SFL voltage and current, SFL focuscurrent, spontaneous potential (SP), SP-to-Armor voltage and array temperature. Allmeasurements except SP are digitized down-hole with high-resolution analog-to-digitalconverters, and all measure channels are re-calibrated every 6 in. [15 cm] during logging.

The operating frequency of the induction ar-rays is selectable at 10, 20, or 40 kHz, with adefault frequency of 20 kHz. The tool alsoprovides measurements of important analogsignals and continuous monitoring of digitalsignals as an aid to failure detection and analy-sis. Depths of investigation and vertical reso-lution of the measurements are listed.

b) Log PresentationThe same presentation format is used for

both generations of induction tools. The twologs can be identified by the following differ-ences (Figure B19):

1. Deep induction (IDPH)the log in-serts use the IDPH acronym to iden-tify Phasor Processing.

2. Medium induction (IMPH)the loginserts use the IMPH acronym toidentify Phasor Processing.

3. There is a hash mark up the right sideof the depth track.

c) Tool Characteristics, Improvements, and Applications

1. The Phasor-Induction SFL tool canbe most effectively used in holesfilled with moderately conductivemud, nonconductive mud and air-drilled holes.

2. Vertical focusing is good and givesreliable values of R

t for beds thicker

than 2.5 m with no shoulder bed cor-rections required.

3. Low resistivities are measured accu-rately.

4. The recording of three focused resis-tivity logs investigates different vol-umes of formation.

5. It is reliable for resistivities up to1000 ohm-m versus 250 ohm-mwith the normal induction tool.

6. Accurate readings are obtained inboreholes up to 66 cm in diameter(R

t/R

m < 1000).

7. Varying transmitter frequencies im-prove the signal-to-noise ratios.

8. Digital transmission techniques areused to improve accuracy of calibra-tion and measurement.

Schlumberger

(05/96) B-25

Correction charts are available for- borehole- bed thickness- invasion (chart Rint-11a).

Phasor-Induction SFL toolMedian Depth of Investigation

1. Tool Depth

Above 100 ohm-m,homogeneous forma-tion

IDIMSFL

62 in.31 in.16 in.

[1.58 m][0.79 m][0.41 m]

2.

At 0.1 ohm-m, homo-geneous formation

IDIMSFL

48 in.26 in.16 in.

[1.22 m][0.66 m][0.41 m]

Phasor-Induction SFL toolVertical Resolution

Vertical resolution bedthickness for full R

t

determinationno in-vasion

IDPHIMPHIDERIMERIDVRIMVRSFL

8 ft6 ft3 ft3 ft2 ft2 ft2 ft

[2.46 m][1.85 m][0.92 m][0.92 m][0.61 m][0.61 m][0.61 m]

ERenhanced resolution phasor toolVRvery enhanced resolution phasor tool

Chart B1


(05/96) B-26

1450

1/240

1 18-MAY-1992 10:33 INPUT FILE(S) CREATION DATE

CP 32.6 FILE 8 08-JUN-1992 17:03

.20000 2000.0

IDPH(OHMM)

.20000 2000.0

IMPH(OHMM)

.20000 2000.0

-80.00 20.000

SP(MV )

0.0 20000.

TENS(N )

PHASOR PROC.

SFLU QUALITY

IMPH QUALITY

IDPH QUALITY

.20000 2000.0

SFLU(OHMM)

0.0 10.000

IDQF

0.0 10.000

IMQF

0.0 10.000

SFQF PHASOR INDUCTION - SFL

1475

---IDQF

---IMQF

---SFQF

---IDPH

---IMPH

SP---

---SFLU

---TENS

Figure B19

Schlumberger

(05/96) B-27

Phasor* Dual Induction-SFL Spherically Focused LogID Phasor - IM Phasor - SFL

1 2 3 4 5

RIMPH/RIDPH

200

100

50

20

10

5

2

1

Thick beds, 8-in. [203-mm] hole, skin-effect and borehole correctedRxo/Rm 100, DIT-E or equivalent, frequency = 20 kHz

15

RSFL/RIDPH

200

90

120

80

100

70

200

140160

50

100

4030

25

20

15

70

5040

30

20

1075

32

RxoRt

0.95 0.90.8

RtRIDPH

0.7 0.60.5

0.40.3

di (in.)60

1

These charts (Rint-11) apply to the Phasor induction tool when operated at a frequency of 20 kHz. Similarcharts (not presented here) are available for tool operation at 10 kHz and 40 kHz. The 20 kHz charts do provide, however, reasonable approximations of Rxo/Rt and Rt /RIDPH for tool operationat 10 kHz and 40 kHz when only moderately deep invasion exists (less than 100 inches). All Phasor* Induction invasion correction charts are applicable to Enhanced Resolution Logging (ERL*) andEnhanced Resolution Analysis (ERA*) presentation.

Rint-11a

Figure B20


(05/96) B-28

Schlumberger

(05/96) B-29

B3.0 Measurement of Rtby Laterolog Principles

B3.1 DUAL LATEROLOGBroadly speaking, borehole fluids used dur-

ing drilling operations are broken into conduc-tive and nonconductive categories. Each posesparticular challenges in measuring formationresistivities. The dual laterolog is a currentemitting electrode device that performs best insaline muds (i.e., where R

t/R

m >>> 100, R

mf /R

w

< 2.5). It is designed to extract Rt by measur-

ing resistivity with several arrays with differentdepths of investigation.

Measurements responding to three appropri-ately chosen depths of investigation usuallyapproximate the invasion profile sufficientlywell to determine R

t.

For best interpretation accuracy, a combina-tion system should have certain desirable fea-tures:

- Borehole effects should be smalland/or correctable.

- Vertical resolutions should be simi-lar.

- Radial investigations should be welldistributed (i.e., one reading as deepas practical, one reading very shallowand the third reading in between).

a) Description and FeaturesThese requirements resulted in the develop-ment of the dual laterolog MicroSFL tool withsimultaneous recordings. Figure B21 illus-trates the focusing used by the deep laterologdevice (LLD, left) and by the shallow laterologdevice (LLS, right). Both use the same elec-trodes and have the same current-beam thick-ness, but have different focusing to providetheir different depth-of-investigation charac-teristics.

Figure B21: Dual LaterologDeep and Shallow Current Patterns


(05/96) B-30

The DLL tool has a response range of 0.2 to40,000 ohm-m, which is a much wider rangethan covered by previous laterolog devices.

To achieve accuracy at both high and low re-sistivities a constant-power measuring systemis employed. In this system both measure cur-rent (i

o) and measure voltage (V

o) are varied

and measured, but the product of the two Voi

o

(i.e., power) is held constant.

The deep laterolog measurement (LLD) ofthe DLL tool has a deeper depth of investiga-tion than previous laterolog tools and extendsthe range of formation conditions in which re-liable determinations of R

t are possible.

To achieve this, long guard electrodes areneeded; the distance between the extreme endsof the guard electrodes of the DLL-R

xo tool is

approximately 28 ft [8.5 m]. The nominalbeam thickness of 2 ft [60 cm], however, in-sures good vertical resolution. Radial investi-gation is 45 ft [1.21.5 m].

The shallow laterolog measurement (LLS)has the same vertical resolution as the deeplaterolog device at 2 ft [60 cm], but it respondsmore strongly to that region around the bore-hole normally affected by invasion. It uses atype of focusing called the pseudolaterolog,wherein the focusing current is returned tonearby electrodes instead of to a remote elec-trode. This causes the measure current to di-verge more quickly once it has entered theformations, thus producing a relatively shallowdepth of investigation of 20 to 24 in. [50 to 60cm].

b) Log PresentationThe DLL MicroSFL log presentationis similar to that of the Phasor Induc-tion. Differences include an expandedresistivity scale (0.2200,000 ohm-m) and the addition of gamma rayand caliper (if MicroSFL is used).See the log in Figure B23.

c) Tool Characteristics andApplications1. The dual laterolog performs most ef-

fectively in saline mud (high Rt /R

m

ratios) or where Rmf

/Rw < 2.5 (Figure

B22).2. The tool has an excellent resistivity

range; by utilizing a unique design,resistivity resolution from 0.2 to40,000 ohm-m is possible.

Figure B22: Preferred Ranges of Applications ofInduction Logs and Laterologs

Schlumberger

(05/96) B-31

2600

2550

GR

1500 (GAPI)

LLS

20000.2 (OHMM)

FILE 16

CALS

375125 (MM)

TENS

050000 (N)

BS

375125 (MM)

LLD

20000.2 (OHMM)

MSFL

20000.2 (OHMM)

LLS

2000002000 (OHMM)

LLD

2000002000 (OHMM)

DUAL LATEROLOG - MSFL

Figure B23


(05/96) B-32

3. Vertical resolution is excellent. Rt can

be obtained in beds as thin as 2 ft [60cm].

4. The LLD has very little borehole ef-fect in large holes.

5. When combined with an Rxo

meas-urement, the LLD and LLS curvesmay be used to study invasion pro-files and compute a more accurate R

t.

See Chart Rint-9 (Figure B24).6. Assuming borehole conditions are

suitable, the separation of the LLSand LLD curves may be used to givequicklook indications of hydrocar-bons; particularly in salt mud. In saltmuds R

xo/ R

t will be less than 1 so the

better the zone, the greater the separa-tion between the LLS and LLD.

d) Limitations1. The tools should not be used in fresh

muds (Rmf

/Rw > 2.5).

2. The tools requires good centralizationto minimize borehole influence onthe LLD.

3. If invasion is deep, a good value ofR

xo (e.g., from a microspherically fo-

cused log) is required to correct LLdfor invasion influence to obtain anaccurate value of R

t.

Correction Charts are available for the influ-ence of

- borehole (diameter and mud resistiv-ity)

- invasion. (Chart Rint-9b, FigureB24)

- bed thickness.

Schlumberger

(05/96) B-33

Dual Laterolog -Rxo DeviceDLT-D/E LLD - LLS - Rxo Device

1.11.2

1.3 1.41.6

1.8

100

80

60

40

30

20

15

10

8

6

4

3

2

1.5

1

0.8

0.6

0.4

0.3

0.2

RLLD/Rxo

RLLD/RLLS

Thick beds, 8-in. [203-mm] hole,no annulus, no transition zone, Rxo/Rm = 50,

use data corrected for borehole effect

20 30

80

100

120

0.500.75 1.01 1.27

1.522.03

3.04

40 50 60100

70

50

30

20

15

10

7

5

3

1.5

2

0.4

0.2

10060

403020

2.54

1.52

1.010.75

0.50

0.4 0.6 0.8 1.0 1.5 2 3 4 6 8 10 15 20 30 40 50

di (in.)

di (m)

di (in.)

di (m)

RtRxo

RtRxo

RtRLLD

Rint-9b

Figure B24


(05/96) B-34

Schlumberger

(05/96) B-35

B4.0 Measurement of Rxo byMicroresistivity Principles

B4.1 INTRODUCTIONAs has been mentioned, a measurement of

flushed-zone resistivity Rxo

is an important in-put when attempting to define invasion di-ameter. Because the flushed zone may extendonly a few centimetres from the borehole, ashallow-reading device is required. Such toolsare the microlog, microlaterolog, proximity logand the MicroSFL log. All are pad-type de-vices that are pressed against the borehole wallto make their measurements.

Today, the microlog MicroSFL log are com-pletely combinable with all main loggingservices. The microlaterolog and proximitylog have been discontinued because of theirlimitations in design; hence, explanations oftheir measurements are not provided. Anotherservice, the EPT (Electromagnetic PropagationTool), also provides an excellent R

xo measure-

ment. This service is an advanced device andis not discussed in this book. For more infor-mation, refer to Schlumberger Log Interpreta-tion Applications/Principles.

To measure Rxo

, the tool must have a veryshallow depth of investigation. Because thereading should be affected by the borehole aslittle as possible, a sidewall-pad tool is used.

Currents from the electrodes on the pad mustpass through the mudcake to reach the flushedzone. Therefore, microresistivity readings areaffected by mudcake; the effect depends onmudcake resistivity R

mc and thickness h

mc.

Moreover, mudcakes can be anisotropic, withmudcake resistivity parallel to the boreholewall less than that across the mudcake. Mud-cake anisotropy increases the mudcake effecton microresistivity readings so that the effec-tive, or electrical, mudcake thickness is greaterthan that indicated by the caliper.


(05/96) B-36

B4.2 MICROLOGWith the microlog tool, two short-spaced

devices with different depths of investigationprovide resistivity measurements of a smallvolume of mudcake and formation immedi-ately adjoining the borehole.

Comparison of the two curves readily identi-fies mudcake, which indicates invaded and,therefore, permeable formations.

a) PrincipleThe rubber microlog pad is pressed against

the borehole wall by arms and springs (FigureB25). The face of the pad has three small in-line electrodes spaced 1 in. [2.5 cm] apart.With these electrodes a 1- by 1-in. microin-verse (R

1" x1") and a 2-in. [5.1 cm] micronormal

(R2"

) measurement are recorded simultane-ously. The currents emitted from these elec-trodes are totally unfocused and hence flow bythe path of least resistance (Figure B26).

Figure B25: Microlog

As drilling fluid filters into the permeableformations, mud solids accumulate on the holewall and form a mudcake. Usually, the resis-tivity of the mudcake is slightly greater thanthe resistivity of the mud and considerablylower than the resistivity of the invaded zonenear the borehole.

The 2-in. micronormal device has a greaterdepth of investigation than the microinverse. Itis, therefore, less influenced by the mudcakeand reads a higher resistivity, which producespositive curve separation. In the presence oflow-resistivity mudcake, both devices measuremoderate resistivities, usually ranging from 2to 10 times R

m.

In impervious formations, the two curvesread similarly or exhibit some negative separa-tion. Here the resistivities are usually muchgreater than in permeable formations (see Fig-ure B27).

Figure B26: Microlog

Schlumberger

(05/96) B-37

Integrated Hole Volume .100000 M3 LEFT EDGE

BETWEEN PIPS EDGE OUTPUT INTERVAL DEPTH TRACK

EVENT MARK SUMMARY:

Integrated Hole Volume: 2.07418 M3 FROM 2039.87 M TO 1995.07 M

ACCUMULATED INTEGRATION VALUES SUMMARY:

TENS(N )

125.00 375.00

MCAL(MM )

MICROLOG

2025

MCAL---

2000

1/240

61 02-JUN-1992 15:15 INPUT FILE(S) CREATION DATE

CP 32.6 FILE 3 00- -1941 00:39

0.0 40.000

BMIN(OHMM)

0.0 40.000

BMNO(OHMM)

125.00 375.00

BS(MM )

0.0 150.00

SGR(GAPI)

50000. 0.0

---BMNO

---BMIN

TENS---

---SGR

---BS

Figure B27


(05/96) B-38

Under favorable circumstances the micrologcan be used to obtain R

xo but it is generally

considered a good qualitative indicator of per-meability, rather than an R

xo measurement.

b) Microlog Limitations- R

xo/R

mc must be less than about 15.

- Mudcake thickness < 1.2 cm- Depth of flushing > 10 cm, other-

wise the microlog readings are af-fected by R

t.

B4.3 MICROSPHERICALLY FOCUSED LOG

The MicroSFL tool is a pad-mounted,spherically-focused logging device that hasreplaced the microlaterolog and proximitytools. It has two distinct advantages over theother R

xo devices. The first is its combinability

with other logging tools, including the Phasor-Induction SFL, the AIT (Array Induction Im-ager and dual laterolog tools).

Figure B28: Current Distribution of MicroSFL device (left) and Electrode Arrangement (right)

This eliminates the need for a separate loggingrun to obtain R

xo information. See Figure B23

for a log example of the MicroSFL tool withdual laterolog.

The second improvement is in the tools re-sponse to shallow R

xo zones in the presence of

mudcake. The chief limitation of the micro-laterolog measurement was its sensitivity tomudcakes. When mudcake thickness exceededabout 3/8 in., the log readings were severelyinfluenced at high R

xo/R

mc contrasts. The

proximity log, on the other hand, was rela-tively insensitive to mudcake, but it required aninvaded zone diameter of about 100 cm toprovide direct approximations of R

xo.

The solution was found in an adaptation ofthe principle of spherical focusing in a side-wall-pad device. By careful selection of elec-trode spacings and bucking-current controls,the MicroSFL measurement was designed forminimum mudcake effect without any undueincrease in the depth of investigation. FigureB28 illustrates, schematically, the current pat-terns (left) and the electrode arrangement(right) of the MicroSFL tool.

By forcing the measure current to flow di-rectly into the formation, the effect of mudcakeresistivity on the tool response is minimized;yet, the tool still has a shallow depth of inves-tigation.

Synthetic microlog curves can also be com-puted from MicroSFL parameters. Because themeasure current sees mostly the flushed zoneand the bucking current sees primarily themudcake, it is possible to mathematically de-rive micronormal and microinverse curves.

Schlumberger

(05/96) B-41

B5.0 Work Session

1a. Given Rmf

= 2.5 ohm-m at 10oC, find Rmf at 52oC, using Chart Gen-9 (Figure B2).

Rmf

=

b. What is NaCl concentration of the mud filtrate in ppm?

2a. Given a solution salinity of 80,000 ppm, find the solution resistivity at 121oC.

Rm =

b. Given a solution salinity of 10,000 ppm at 20oC, find the solution resistivity at 50oC.

Rm =

3. Given Rm = 0.74 at 20oC, what does R

m equal at BHT if the total depth is 2400 m and the

geothermal gradient is 2oC/100 m (surface temperature is 20oC) ?

Rm = __________________________ at __________________ oC


(05/96) B-42

2150

1/240


CP 32.6 FILE 1 01-APR-1941 17:28

-150.0 0.0

SP(MV )

-|---|+15

2175

SP---

4. From the SP in Figure B30,calculate R

w. Formation

temperature is 63oC.R

mf = 0.79 at 20oC.

a) Rmf = at formation temperature

b) SP = mV

c) Rmfe

= at formation temperature

d) Rwe

= at formation temperature

e) Rw = at formation temperature

f) Rw = at25oC

g) Formation NaClconcentration = ppm

Note: Use charts SP-1 and SP-2m(Figures B11 and B12).

Figure B30

Schlumberger

(05/96) B-43

2325

---GR

SP---

1/240


CP 32.6 FILE 3 01-APR-1941 18:05

-150.0 0.0

SP(MV )

30.000 130.00

GR(GAPI)

-|---|+15

2350

5. Calculate Rw for the zone from 2326 to 2340 m in

Figure B31.

Rmf = 0.110 at 20oC

Formation temperature = 58.9oC

Rw = at25oC

6. Using the log examples in Figure B32 calculate

a) Depth of invasion at A and Band

b) Rt (ILD corrected) at A and B

7. Calculate Rw for the example of the dual induction

SFL in Figure B15.Given: R

m = 3.05 at 17oC

Rmf

= 2.60 at 17oC

BHT = 23oC

Figure B31


(05/96) B-44

1/240


CP 32.6 FILE 8 09-JUN-1992 14:42

.20000 2000.0

SFL(OHMM)

.20000 2000.0

ILD(OHMM)

.20000 2000.0

ILM(OHMM)

-150.0 0.0

SP(MV )

0.0 150.00

GR(GAPI)

ILM(OHMM)

1800

---GR

---SP

---ILM

---ILD

---SFL

1700

---SP

---ILM

---ILD

SFL---

A

B

1725

Figure B32

Schlumberger

(05/96) B-45

1675

1700

1 05-JUN-1992 08:41 INPUT FILE(S) CREATION DATE CP 32.6 FILE 4 01-APR-1941 18:13

-80.00 20.000

SP(MV )

1675

-|---|+10

1725

---SP

8. Calculate Rw for both zones in Figure B33

Rm

= 1.18 at 25oCR

mf= 0.93 at 16oC

BHT = 59oC

a. Top zone: 1685 m to 1695 m

Rw = at 59oC

Rw = at 25oC

b. Bottom zone: 1695 m to 1717 m

Rw = at 59oC

Rw = at 25oC

c. What are possible reasons for thedifference?

Figure B33


(05/96) B-46

02 resistivity measurement

Documents

microresistivity principles

induction principles

formation water resistivity

resistivity of theformationb1

introductionthe resistivity

focused log principles

formation onlybecause

formation temperature