5 Resistivity Logging

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Resistivity Logging


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Introduction

The resistivity of a formation is a key parameterin 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 electricalinsulator.

Moreover, perfectly dry rocks are very seldom

encountered. Therefore, subsurface formations have finite,

measurable resistivities because of the water intheir pores or absorbed in their interstitial clay.


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The resistivity of a substance

is the resistance measuredbetween opposite faces of a

unit cube of that substance

at a specified temperature.

The meter is the unit of

length and the ohm is the

unit of electrical resistance.

The resistivity of a formation depends on:

Resistivity of the formation water.

Amount of water present. Pore structure geometry.


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Resistivity is a basic measurement of a reservoirsfluid saturation and is a function of porosity, type

of fluid, amount of fluid and type of rock. Formation resistivities are usually from 0.2 to

1000 ohm-m.

Resistivities higher than 1000 ohm-m are

uncommon in permeable formations but areobserved in impervious, very low porosity (e.g.,evaporites) formations.

Formation resistivities are measured by eithersending current into the formation andmeasuring the ease of the electrical flow throughit or by inducing an electric current into the

formation and measuring how large it is.


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Factors Affecting Resistivity

Resisitivities are dependent on:

Presence of Formation water / Hydrocarbons

Salinity of Formation water

Temperature of Formation water

Volume of water-saturated pore space

Geometry of the pore space

Morphology and species of clay minerals


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CONVENTIONAL ELECTRICAL LOGS

During the fist quarter-century of well logging,the only resistiviry logs available were theconventional electrical surveys.

Thousands of them were run each year in holesdrilled all over the world.

Since then, more sophisticated resistivity loggingmethods have been developed to measure theresistivity of the flushed zone, Rxo, and the true

resistivity of the uninvaded virgin zone, Rt. The conventional electrical survey (ES) usually

consisted of an SP, 16-in. normal, 64-in. normal,and 18-ft 8-in. lateral devices.


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Principle

Currents were passed through the formation bymeans of current electrodes, and voltages weremeasured between measure electrodes.

These measured voltages provided the resistivitydeterminations for each device.

In a homogeneous, isotropic formation of infiniteextent, the equipotential surfaces surrounding asingle current-emitting electrode (A) are spheres.

The voltage between an electrode (M) situated

on one of these spheres and one at infinity isproportional to the resistivity of thehomogeneous formation, and the measuredvoltage can be scaled in resistivity units.


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Basic Definitions and Ohms Law

Ohms Law states that the current flowing

from point A to point B in a conductor I is

proportional to the difference in electrical

potential E between point A and point B.

The constant of proportionality is called the

electrical conductance c.

Current is measured in amperes (A), potential

difference in volts (V), and conductance in

siemens (S) or mho.


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While electrical resistance r, which is the

inverse of conductance is:

Resistance is measured in ohms (). Hence,


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Thus, if we take a cylindrical rock sample withtwo flat faces A and B, and set a potentialdifference E =EA-EB between its end faces, acurrent I will flow through the rock from face Ato face B.

If we measure the current and the potential

difference, we can calculate the resistance ofthe rock sample.


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If the resistance is high, a given potential

difference E will only give a small current I. If the resistance is low, a given potential

difference E will give a high current I.

The value of resistance is a property of the

material which describes how much the material

resists the passage of a current for a givenapplied potential difference.


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If the size of our rock sample changes.

If the length of the sample is doubled, one can see

that the resistance of the sample to the passage ofa current should also double.

If the area perpendicular to the current flow

doubles (the area of the end face in this example),

there is twice the material for the current to pass

through, the resistance of the sample to the

passage of the current should therefore fall to a

half of what is was before. So the resistance (and therefore conductance)

depend upon the size of the sample.


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The resistance per unit length and area is called the

resistivity R, and can be expressed as

where:

R = the resistivity of the sample (m or ohm.m)

E = the potential difference across the sample (volts, V)

I = the current flowing through the sample (amperes, A)

A = the cross-sectional area of the sample perpendicularto the current flow (m2)

L = the length of the sample (m).


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Or

R = rA/L,

The units of resistivity are ohm-meters

squared per meter, or simply ohm-meters

(ohm-m).

Note that a conductivity C can also be defined

as the reciprocal of the resistivity R, and

therefore


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In petrophysical logging of electrical rock

properties there are two main types of tool.

One type measures resistivity directly, and theresult is given in ohm.m (.m).

The other type measures conductivity directly,

and the result is given in either siemens permetre (S/m), or more often to avoid decimal

fractions conductivity is usually expressed in

millisiemens per metre (mS/m) or milli-mhosper meter (mmho/m), where 1000 mmho/m =

1 mho/m.


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Resistivity of Rocks

Reservoir rocks contain the followingconstituents


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All have a high electrical resistivity (electrical

insulators) except the formation water andwater-basedmud filtrate, which are good

electrical conductors and have a low electrical

resistivity. The resistivity of the reservoir rocks therefore

depends only upon the water or water based

mud filtrate occupying its pore space.


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Uninvaded Formations

For uninvaded formations, the measured bulk

resistivity of the rock depends only upon the amountof the aqueous formation fluids present in the rock,and the resistivity of those aqueous fluids.

Since the amount of formation fluids depends both onporosity and water saturation S

w, we can say that the

resistivity of the formation Rt depends upon porosity ,water saturation Sw, and the resistivity of the formationwater Rw.

This resistivity is called the true resistivity of the

formation. It is the resistivity of the formation in the uninvaded

zone, where the rock contains some saturation of oil So,gas Sg, and water Sw, and where So+Sg+Sw=1.


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The aim is to use knowledge of the resistivity ofthe formation, together with independent

knowledge of the porosity and resistivity of theformation waters, to calculate Sw, and henceenable ourselves to calculate the STOOIP.

The uninvaded zone of formations is commonlyonly measured directly by the most deeplypenetrating electrical logging tools.

The shallower investigating tools measure theinvaded zone.

Hence, if one wants a resistivity reading for use inSTOOIP calculations, one should always chose thedeepest penetrating electrical tool of those thathave been run.


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Invaded Zones

In most cases there is an invaded zone, where the

formation fluids have been disturbed by the

drilling fluid.

The resistivity of the formation in this zonedepends upon the resistivity of the mud filtrate

Rmf, the resistivity of any remaining formation

water Rw, the saturation of the mud filtrate SXO,the saturation of the remaining formation water

Sw (if any), and the porosity of the rock .


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Log Presentation

A problem common to all resistivity and conductivitydevices is providing a scale that can be read accuratelyover the full range of response.

Most laterologs were recorded on linear scales.

Because of the very large range of resistivities oftenencountered, the required scale was relativelyinsensitive.

Very low readings, whether resistivity or conductivity,were virtually unreadable.

Backup curves of increased sensitivity were introduced,but they were difficult to read and cluttered the log informations of high contrast.


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For a while, the hybrid scale, first used on theLL3 tool, was employed.

It presented linear resistivity over the first halfof the grid track (log), and linear conductivityover the last half.

Thus, one galvanometer could record allresistivities from zero to infinity.

Although some what awkward to use because

of the odd scale divisions, the hybrid scale didprovide acceptable sensitivity in both low-resistivity and low-conductivity formations.


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Today, the logarithmic scale is the most

acceptable scale for recording resistivity

curves.

Its standard form is a split four-cycle grid

covering the range from 0.2 to 2000.

Even this range is sometimes not sufficient for

the DLL-Rxo measurements; when needed, a

backup trace is used to cover the range from

2000 to 40,000 ohm-m.


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Depth of Resistivity Log Investigation


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Tools Measuring the Uninvaded Zone

(Rt)

These tools (deep induction and deep

laterolog) essentially measure Rt, and the log

value is normally quite close to true Rt,

providing the tool is used in the correctenvironment.

To obtain a more precise value for Rt, certain

corrections must be applied to the raw values.


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Tools Measuring the Invaded zone (Ri)

The actual quantitative value of thesereadings is not as important as how these

readings relate to Rt and Rxo. By comparing

them, we can obtain: Corrected Rt values

Depth of invasion of the mud filtrate

An idea of the formation's permeability An estimate of movable oil


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Tools Measuring the Flushed Zone

(Rxo)

Four different Rxo tools are available, the ML,

MLL, PL and MSFL.

They are intended for different conditions of

salinity, mud cake thickness and diameters of

invasion.


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Resistivity Devices

In the normal device, a current of constant

intensity is passed between two electrodes, Aand B.

The resultant potential difference is measuredbetween two other electrodes, M and N.

Electrodes A and M are on the sonde. B and Nare, theoretically, located an infinite distanceaway.

The distance AM is called the spacing (16-in.

spacing for the short normal, 64-in. spacing forthe long normal), and the point of inscription forthe measurement is at 0. midway between A andM.


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In the basic lateral device, a constant current

is passed between A and B, and the potentialdifference between M and N is measured.

Thus, the voltage measured is proportional to

the potential gradient between M-and N. Thepoint of inscription is at O, midway between

M and N.

The spacing AO is 18 ft 8 in.


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Generally, the longer the spacing, the deeper

the device investigates into the formation.

Thus, of the ES resistivity logs, the 18-ft 8-in.lateral has the deepest investigation and the

16 in. normal the shallowest.

In practice, however, the apparent resistivity,Ra, recorded by each device is affected by the

resistivities and geometrical dimensions of all

media around the device (borehole, invadedand uncontaminated zones, and adjacent

beds).


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Conventional Resistivity Logs

The Short Normal (SN) measures the

resistivity of the invaded zone (Ri).

This curve has the ability to detect invasion by

comparing the separation between the deep

induction and the short normal.

Invasion will indicate permeability. The SN

curve is recorded in Track #2.

Normal Logs


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Electrical spacing of the electrode is sixteen

inches (short normal) or sixty four inches (longnormal).

Normal logs provide reliable resistivity values

for beds greater than four feet in thickness. The curve will be symmetrical around center

of bed. Using this parameter, bed boundaries

will be at the inflection points on the curve.


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There are two electrodes in the sonde, acurrent electrode and a pick-up electrode,

with two other electrodes located an infinitedistance away (one is the cable armor, theother one is on the surface).

A current of constant intensity is passedbetween two electrodes, one in the sonde andthe one on the cable.

The resultant potential difference is measured

between the second electrode in the sondeand the one on the surface.


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There are several factors affecting normal log

measurements:

The resistivity of the borehole (Rm, Rmc, Rmf).

The depth of invasion (di).

Formation thickness - the greater the spacing of

electrodes, the thicker the formation must be to get

accurate readings.

Resistivity of surrounding beds - when there is a highresistivity contrast, distortion of the curve results.


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Lateral Logs The lateral curve is produced by three effective

electrodes (one current and two pick-up) in the sonde. A constant current is passed between two electrodes,

one on surface, and one in the sonde.

The potential difference between the two electrodes,

located on two concentric spherical equipotentialsurfaces, centered around the current electrode, ismeasured.

The voltage measured is proportional to the potential

gradient between the two pick-up electrodes. Point ofmeasurement is halfway between pick-up electrodes(18 feet, 8 inches), making the radius of investigationapproximately equal to the electrode spacing.


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Lateral curves are asymmetrical, and onlyapparent resistivity (Ra) is measured.

The resistivity values must be corrected for Rt.

For thick beds, the lateral curve will define a bedboundary, depending on type of electrodearrangement.

Several factors affecting lateral measurementsare:

Borehole influences (Rm, Rmc, Rmf) are relativelysmall.

Measurements in thin beds are difficult, if notimpossible.


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Principle of the Laterolog

Conventional electrical logging devices need aconductive mud in the borehole to operateproperly.

However, when the resistivity contrast Rt/Rm istoo high the response of these devices weakenbecause the measure current gets partially ortotally shorted out by the mud column.

We are concerned with the situation whereresistivity measurements must be made inrelatively high resistivity formations using saltymuds.


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Another problem occurs when the formation

consists predominantly of resistive thin beds

because current escapes to adjacent beds andthis produces unwanted signals at the

detectors which obscure the resistivity of the

bed of interest. To overcome these limitations, focused

devices were developed in which the measure

current is contained between two almosthorizontal and parallel surfaces out to a

certain distance from the sonde.


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Use is made of auxiliary electrodes, above and

below the measurement electrodes, that

supply currents of the same polarity and arekept at the same potential of the main

electrode to ensure that no current will flow

between them inside the mud column. Auxiliary electrodes are called guard or

bucking electrodes.


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Laterolog Tool

Arrangement of electrodes to cause a currentto flow horizontally into the formation.

Laterolog response of a porous and permeable

formation depends upon several factors. Invasion causes different zones of resistivities.

So the total resistance measured is the sum of

the resistivities of each zone. The region of formation that has the highest

resistance, has the greatest influence.


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The logging current, Io, can be visualized as asheet of current probing laterally into the

formation. The depth of penetration is related to the

length of guarding system.

There are basically three types of electrodesystem.

3 elongate electrodes system (LL3)

7 or 8 (small) electrode system (LL7 or LL8)

Combination (small and elongate) electrodesystem (DLL)

LL3


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LL3 LL3 tool has two arrangements of elecetrodes.

Single electrode

3 eclectrodes

Single electrode

Entire electrode at same potential.

All current lines leave in a direction perpendicularto the electrode face.

No spherical distribution of current until at agreat distance.

Current near the centre of electrode flows in anearly horizontal direction for a considerabledistance.


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3 l t d


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3 electrodes

same electrode has been divided into three partsseparating the small central section.

All three pieces are connected to power supply atsame potential.

The current distribution remains unchanged.

Only difference is that the current going to thecentre is only measured.

Since this current is directed into the formation ina horizontal beam, so this will give the resisitvityof the formation against the centre.

The function of the upper and lower guardelectrodes is to control the current in a horizontalsheet, which is flowing from the centralmeasuring electrode.

If this divided electrode arrangement is placed in a


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If this divided electrode arrangement is placed in amedium such that the resistivity opposite the centresection is much higher than the resistivity opposite the

two guard sections, the relative amount of current goinginto the guard will increase proportionately, to maintaintotal Io unchanged.

The direction of current flow will remain unchanged.

Therefore as long as the thickness of the resistive streak isgreater than the length of the centre section, themeasurement of the resistivity will be very nearly correct.

The presence of a mud column around the electrode

makes an insignificant difference in its response, except incases of extremely large holes such as result from caving.

The current flowing to the central electrode and thepotential of the electrode are measured, and from thesethe resistivity of the formation is estimated.

LL7


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LL7

This laterolog system is comprised of several

small electrodes. The central electrode Ao is symmetrically

positioned between three pairs of electrodes, M1& M2, M1 & M2 and A1 - A2.

Each electrode pair is maintained at samepotential.

A constant current of known intensity is appliedto the centre electrode A

o.

An auxiliary current of the same polarity sentthrough Ao is applied to electrodes A1A2.


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The intensity of the current applied to electrodesA1 and A2 is automatically and continuouslyregulated to maintain the potential differencebetween the two pairs of electrodes, M and M,essentially at zero.

When this condition is maintained, the potential

of all A and M electrodes is identical. The current emanating from the central electrode

Ao is prevented from flowing upward anddownward in the borehole past the electrodes

M1,M2 and M1 and M2, by the current from A1A2,and is therefore forced to flow laterally.


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This action will produce a sheet of current

whose thickness is approximately equal to the

distance separating the mid points ofelectrodes M1, M1 and M2,M2.

This sheet of current that emanates from Ao is

bounded by two surfaces, and for some radialdistance from the borehole is reasonably close

to two parallel planes perpendicular to the

axis of the instrument and passing through

the mid points of M1, M1 and M2,M2.


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Th l t d i d f d


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The same electrode array is used for deeplaterolog and shallow laterolog, but the currentflows are different.

In the LLd (deep) mode, the surveying current Io ,that flows from the center electrode, A0, isfocused by bucket currents from electrodes A2and A2

' supported by A1 and A1'.

The four "A" electrodes are all connected in thismode.

The total current returns to the surface fish(electrode).

This arrangement provides strong focusing deepinto the formation.

Current and voltage are used to computeresistvity.


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Shallow Laterolog (LLs)

The shallow Laterolog (LLs) has the same verticalresolution as the deep Laterolog (2 feet), but responds

more strongly to the region affected by invasion. In the LLs (shallow) mode the bucking currents flow from

A1 to A2 and A1 to A2, reducing the depth ofinvestigation. #the same electrodes are used for theshallow device although in a different way.

The total constant current it is generated downhole andapplied directly to bucking and measure electrodes.

It is split into two components: Ib going to A1 and Io goingto Ao; both currents return to A2 producing a shallow Io

beam The electrodes are switched several times per second

from one to the other configuration, and the tworesistivity traces are produced simultaneously.

Sphericall Foc sed Log


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Spherically Focused Log The SFL device measures the conductivity of the

formation near the borehole and provides therelatively shallow investigation required to evaluate theeffects of invasion on deeper resistivity measurements.

It is the short-spacing device now used on the DIL-SFLtool-developed to replace the 16-in. normal and LL8

devices. The SFL device is composed of two separate, and more

or less independent, current systems.

The bucking current system serves to plug the

borehole and establish the equipotential spheres. The I, survey current system causes an independent

survey current to flow through the volume ofinvestigation; the intensity of this current isproportional to formation conductivity.


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The SFL device consists of current-emittingelectrodes, current-return electrodes, andmeasure electrodes.

Two equipotential spheres about the toolscurrent source are established.

The first sphere is about 9 in. away from thesurvey current electrode; the other is about 50 in.

away. A constant potential of 2.5 mV is maintained

between these two spherical surfaces.

Since the volume of formation between thesetwo surfaces is constant (electrode spacing isfixed) and the voltage drop is constant (2.5 mV),the conductivity of this volume of formation canbe determined by measuring the current flow.


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INDUCTION LOGGING

The induction logging tool was originally developed tomeasure formation resistivity in boreholes containingoil-base muds and in air-drilled boreholes.

Electrode devices did not work in these nonconductive

muds. Experience soon demonstrated that the induction log

had many advantages over the conventional ES logwhen used for logging wells drilled with water-basemuds.

Designed for deep investigation, induction logs can befocused in order to minimize the influences of theborehole, the surrounding formations, and the invadedzone

Principle


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Principle

Todays induction tools have many transmitterand receiver coils.

However, the principle can be understood byconsidering a sonde with only one transmittercoil and one receiver coil.

A high-frequency alternating current of constantintensity is sent through a transmitter coil.

The alternating magnetic field created inducescurrents in the formation surrounding theborehole.

These currents flow in circular ground loopscoaxial with the transmitter coil and create, inturn, a magnetic field that induces a voltage inthe receiver coil.


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Because the alternating current in the transmitter coil


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Because the alternating current in the transmitter coilis of constant frequency and amplitude, the groundloop currents are directly proportional to the formationconductivity.

The voltage induced in the receiver coil is proportionalto the ground loop currents and, therefore, to theconductivity of the formation.

There is also a direct coupling between the transmitter

and receiver coils. The signal originating from this coupling is eliminated

by using bucking coils.

The induction tool works best when the borehole fluidis an insulator-even air or gas.

The tool also works well when the borehole containsconductive mud unless the mud is too salty, theformations are too resistive, or the borehole diameteris too large.

Equipment


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Equipment

The induction tool has been the basic resistivity

tool used in logging low- to medium-resistivityformations drilled with fresh water or oil.

The 6FF40 induction-electrical survey (IES) toolincluded a six-coil focused induction device of 40-in. nominal spacing (hence, the nomenclature,6FF40), a 16-i% normal, and an SP electrode.

The DIL-LL8 system used a deep-reading

induction device (the ID, which was similar to the6FF40), a medium induction device (the IM), anLL8 device (which replaces the 16-in. normal),and an SP electrode.


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Log Presentation and Scales

The induction conductivity curve is sometimes

recorded over both Tracks 2 and 3.

The linear scale is in millimhos per meter

(mmho/m), increasing to the left.

The DIL-LL8 log introduced the logarithmic

grid; the standard presentation.

Comparing Laterologs and Induction


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Comparing Laterologs and Induction

Logs

Induction logs provide conductivity (that can beconverted to resistivity).

Laterologs provide resistivity (that can be

converted to conductivity). Induction logs work best in wells with low

conductivity fluids.

Laterologs work best in wells with low resistivity

fluids. Both logs provide a range of depths of

penetrations and vertical resolutions.


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MICRORESISTMTY DEVICES

Microresistivity devices are used to measure theresistivity of the flushed zone, Rxo and todelineate permeable beds by detecting thepresence of mudcake.

Measurements of Rxo are important for severalreasons.

When invasion is moderate to deep, a knowledgeof Rxo allows the dxeep resistivity measurement

to be corrected to true formation resistivity. Also, some methods for computing saturation

require the Rxo/Rt, ratio.

T R th t l t h h ll


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To measure Rxo the tool must have a very shallowdepth of investigation because the flushed zonemay extend only a few inches beyond theborehole wall.

Since the reading should not be affected by theborehole, a sidewall-pad tool is used.

The pad, carrying short-spaced electrode devices,is pressed against the formation and reduces theshort circuiting effect of the mud.

Currents from the electrodes on the pad must

pass through the mudcake to reach the flushedzone.

Microresistivity logs are scaled in resistivity units.

Microlog


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Microlog

The microlog (ML) is a rubber pad with three button

electrodes placed in a line with a 1 inch spacing . A known current is emitted from electrode A, and the

potential differences between electrodes M1 and M2 andbetween M2 and a surface electrode are measured.

The two resulting curves are called the 2 normal curve(ML) and the 1 inverse curve (MIV).

The radius of investigation is smaller for the second ofthese two curves, and hence is more affected by mudcake.

The difference between the two curves is an indicator ofmudcake, and hence bed boundaries.

The tool is pad mounted, and the distance across the padsis also recorded, giving an additional caliper measurement

(the micro-caliper log).


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The Microlaterolog


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The microlaterolog (MLL) is the micro-scale version of thelaterolog, and hence incorporates a current focusing system.

The tool is pad mounted, and has a central button electrodethat emits a known measurement current surroundedcoaxially by two ring shaped monitoring electrodes, and aring-shaped guard electrode that produces a bucking currentas in the DLL.

The spacing between electrodes is about 1 inch. The tool operates in the same way as the LL7. The focused

current beam that is produced from the central electrodehas a diameter of about 1 inches and penetrates directly

into the formation. The influence of mudcake is negligible for mudcakes less

than 3/8 thick, and in these conditions RXO can bemeasured.

The depth of investigation of the MLL is about 4 inches.


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The Proximity Log


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The Proximity Log

The proximity log (PL) was developed from the

MLL to overcome problems with mudcakes over3/8 thick, and is used to measure RXO.

The device is similar, except that it is larger than

the MLL and the functions of the centralelectrode and the first monitoring ring electrodeare combined into a central button electrode.

The tool operates in a similar fashion to the LL3.

It has a depth of penetration of 1 ft., and is notaffected by mudcake.

It may, however, be affected by Rt when theinvasion depth is small.


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The Micro Spherically Focussed Log


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The micro spherically focussed log (MSFL) is commonlyrun with the DLL on one of its stabilizing pads for the

purpose of measuring RXO. It is based on the premise that the best resistivity data is

obtained when the current flow is spherical around thecurrent emitting electrode (isotropic conditions).

The tool consists of coaxial oblong electrodes around acentral current emitting button electrode.

The current beam emitted by this device is initially verynarrow (1), but rapidly diverges.

It has a depth of penetration of about 4 (similar to the

MLL). The initial narrowness of the current beam means that its

sensitivity to mudcake is somewhere between the MLLand the PL, and is not significantly affected by mudcakeless than thick.


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Investigation Depth


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Investigation Depth


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Uses

Recognition of Hydrocarbon Zones

Calculation of Water Saturation

Facies Recognition

Correlation

Lithology Recognition

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5 Resistivity Logging