10 electrical

Notes

11

Electrical Resistivity Logs


Schlumberger 1999

Notes

Resistivity is resistance per unit length. We can often employ electrical analogies when dealing with resistivity tools measuring in the formation.This was the first type of measurement ever made and it is still the only way to find and evaluate the hydrocarbons in a reservoir.

22


Resistivity Theory

The resistivity of a substance is a measure of its ability to impede the flow of electrical current.

Resistivity is the key to hydrocarbon saturation determination.

Porosity gives the volume of fluids but does not indicate which fluid is occupying that pore space.

Notes

The flow of current can only be carried by ions in the formation. The ions are only present in the pore space and only in the water. The more ions (more water) the lower the resistivity. The higher the salinity (more ions) the lower the resistivity.The formation water has a resistivity of Rw. The formation containing only water has a resistivity of Ro. This is a definition.

33


Resistivity Theory 2

Current can only pass through the water in the formation, hence the resistivity depends on:

Resistivity of the formation water.Amount of water present.Pore structure.

Notes

Most tools read in the invaded zone, hence only parameters here are required. Resistivity tools have to measure both the invaded and virgin zones. This means that the the parameters for both zones have to be defined. The borehole also contains components which are seen by the tools.These three zones have resistivities, Rm, Rmc, Rmf, Rw of the fluids involved. There are also the resistivities of the formations, Rxo and Rt. The water saturations of both zones also need to be defined as this determines the resistivity, Sxo and Sw. Finally the diameter of the invaded zone, di is needed to compute the contribution from this zone.Some of these parameters are measured, others are calculated.

44


Resistivity Model

Notes

The problem with the surface measurement of the mud resistivities is not with the measurement procedure or equipment, but with the procurement of the samples.The mud sample comes from the mud tanks and is usually good. The mud filtrate and cake come from a sample of mud put through a mud press. This is often done in advance of the logging and the samples left exposed to contamination. The checks given in the Chart Books enable the values to be verified and if necessary redone with fresh samples.These values are important as they are used in corrections and in computations.

55


Mud Resistivities

The first resistivities encountered are those of the mud, mud filtrate and mud cake.

The surface measurements to obtain these values are often erroneous.

Key points:The samples must be identical to the mud used in the logging interval.Check answers using the Chart Book formulae.Rmf < Rm < RmcIdentify the sample source (measured or charts).

Notes

The chart is given in most Chart Books.

66


Salinities chart

This chart is used to compute salinities from resistivities of solution e.g. mud, and vice versa.It is also used to find the resistivities at a given temperature.

0.1

0.2

0.3

0.4

0.5

0.6

0.8

1

2

3

4

56

8

10

0.08

0.060.05

0.04

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0.01

50 75 100 125 150 200 250 300 350 400

10

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2530

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150

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250300

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500

1000

1500

200025003000

40005000

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15,00020,000

280,000

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1200

1400

1700

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70008000

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20,000

250,000 200,000

170,000 14

0,000120,0

00100,0

0080,00

0 70,000

60,000 5

0,00040,00

030,00

0

300,000

10 20 30 40 50 60 70 140 160 20080 90 100 120 180

p

p

m

G

r

a

i

n

s

/

g

a

l

a

t

7

5

F

R

e

s

i

s

t

i

v

i

t

y

o

f

S

o

l

u

t

i

o

n

(

-

m

)

Temperature (F or C)

N

a

C

l

C

o

n

c

e

n

t

r

a

t

i

o

n

(

p

p

m

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r

a

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)

Notes

The simple electrical tool theory forms the basis of all electrical tools. The tool has a current source which creates a series of equipotential spheres centred on the source. The measure electrode measures the voltage at a distance from the source. This voltage is dependent on the spacing and the resistivity of the formation between the two electrodes.

77


Old Tools

The voltage measured at M is proportional to the formation resistivity.

This electrode configuration is the Normal tool.

The distance between the A and M electrodes.The spacing determines the depth of investigation and hence the resistivity being read.

Notes

The configuration of the lateral device is different but the principle is the same with the equipotential surface voltage being read by the measure electrode M.Devices using this type of technique are still in use in Ultra Long Spacing Electrical Log (ULSEL) used for the detection of salt domes and nearby well casings (when drilling relief well for a blow out) also in use in some Russian logging tools.

88


Normal and Lateral Tools

The Lateral device used the same principle. The difference is in electrode configuration and spacing.

Problems came from "thin beds" when the signature of the curve was used to try and find the true resistivity.

Notes

The major problem with these old tools came with thin beds. If the bed was thick enough the tool read the true formation resistivity. If this was not the case the tool read a value which depended on the resistivities of the target bed and the beds on either side and also on the electrode configuration. The curves were distorted.The slide shows a typical distortion of both a normal and a lateral device in a thin resistive bed. These departure curves and a number of electrode spacings were used to compute a true spacing.

99


Old Tools 2

This figure shows some of the "signature curves" for the interpretation of lateral and normal devices in thin beds.A library exists plus the rules to extrapolate the measured value to the true resistivity of the bed.

Notes

The problem of a resistive bed with lower resistivity beds on either side is that in the old tools the current takes the easiest path.The solution is to focus the measure current into the formation. This is done using a current emitted from electrodes above and below the measure electrode.This forces the current to flow in a sheet directly into the formation in front of it with little deviation.

1010


Laterolog Principle

A current-emitting electrode, Ao, has guard electrodes positioned symmetrically on either side.Guard electrodes emit current to keep the potential difference between them and the current electrode at zero.This forces the measuring current to flow into the formation of interest.

Notes

The names of the tools reflect the number of electrodes. Extra electrodes are added to improve the focusing.The current tools use the same electrodes to produce two different depths of investigation, a shallow and a deep measurement. This is achieved using different frequencies for each.

1111


Tool Types

Various configurations have been used:

LL3 to LL7 to LL9 to DLTThese tools added more electrodes and were eventually able to run deep and shallow simultaneously.These tools looked in all directions.

HALS/ARIUsing the same principle, these are azimuthal tools capable of looking in 12 directions.

HRLALatest tool, using modern techniques to eliminate the need for a voltage reference and produce a much more accurate resistivity.

Notes

The objective of this tool is twofold, firstly to better the vertical resolution and secondly to look all around the borehole.This is achieved using a set of twelve electrodes much smaller than the standard ones set in a ring around the tool. This means each electrode looks at a 30 region. As the tool can be run eccentred in the borehole each electrode will have a different borehole correction. To be able to perform this correctly a very shallow measurement is made giving an electrical radius of the hole in front of every electrode. This is used to correct the raw readings. It can also be used to provide a borehole profile.

1212


Azimuthal Laterolog principle

The current emitting electrode is split into twelve separate electrodes.

It has 12 electrodes set equally spaced around the tool giving 12 azimuthal Laterolog readings.

These are focused to give a deep reading and a very shallow reading of the tool stand-off.

Notes

The availability of the passive mode increases the range of the Laterolog measurement. Mud resistivity is best measured using a device in the tool string.

1313


Azimuthal Laterolog principle 2

There are two modes:Active mode: current is emitted from each of the electrodes.12 calibrated resistivities are output in real time.

Passive mode: no current is emitted. This is used if the resistivity is above 2 ohm-m The mud resistivity is needed to compute the resistivities.

Notes

The string of resistivities in series is all measured by the tool. The objective is to minimise the unwanted Rm, Rmc and Rxo and read the best possible Rt. Hence the need for salty muds which would give low Rm, Rmc and Rxo.Rmc is neglected as its a small thickness compared to the beam width of the tool.This type of tool reads best at the highest resistivities.

1414


borehole effects

Laterologs see the borehole environment as:

RLL = Rm + Rmc + Rxo + Rt

Rm Best measurement is in salt-saturated, lowresistivity mud. Worst readings obtained in fresh mud. Measurements cannot be made in oil-based mud.

Rmc Usually neglected as very small.

Rxo Depends on Rmf, needs to be known.

Rt Parameter to be measured, the higher the better.

Notes

The tool can be run either centred or eccentred with stand offs. The volume of mud seen by the tool in the two cases is different hence it is important to know the tool position.The latest tools have small differences between the two m operational modes, the older tools could only be run centred.

1515


Laterolog Corrections

The log should be corrected for the effect of mud resistivity.

There are two possible conditions:Centred.Eccentred.

There is only a small difference between the two in most circumstances.The correction to the shallow is greater than the deep, especially in large hole sizes.However the correction is very small in most circumstances.

Notes

The top chart is for the deep measurement and as expected the borehole effect is minimal as it is designed to read far into the formation. The shallow has increasing corrections with increasing hole size and formation/mud contrast. Eventually the shallow is reading entirely in the borehole.

1616


Laterolog Corrections

Notes

The corrections are very similar to the deep Laterolog in magnitude. This chart shows the centred chart. Once again the chart is entered with the ratio of the resistivity measured divided by the mud resistivity, reading up to the hole size and across to the y-axis gives the correction factor.

1717


Azimuthal Laterolog corrections

The borehole correction is similar to the other Laterolog measurements. It is a function of the borehole diameter and the ratio of formation to mud resistivity.This chart is used to make the correction. It can be done by the surface acquisition system.

Notes

This set of charts is for the eccentred case. The curves are similar for both measurements.

1818


Correction Charts

Notes

In spite of focusing the measurement is still affected by beds above and below. This is the so-called squeeze and anti-squeeze effect. If the shoulder beds are more resistive the reading has to be reduced, if it is less resistive it has to be increased.

1919


Bed Correction

The next correction accounts for the effects of adjacent beds which still occur despite focusing.

If the shoulder bed is highly resistive, the log has to be reduced. (Squeeze.)

If the shoulder bed is of low resistivity, the log has to be increased. (Anti-squeeze.)

Notes

The chart shown is for the deep measurement which has more correction than the shallow device. The chart shows that even with large beds, 10m or so, there is still a correction if there is sufficient contrast between the shoulder beds and the target bed.

2020


Squeeze/Anti-Squeeze

Rs is the resistivity of the bed above and below the formation of interest.

The chart is entered with the bed thickness, moving up the ratio RLLD/RS.

The correction factor is read on the y-axis.

Notes

The shallow Laterolog only has an effect in beds less than 10 feet thick

2121


Squeeze/Anti- Squeeze

The same method is used in this chart for the Shallow Laterolog

Notes

Shoulder-Bed Squeeze Effects

Deep measurement reads too high and results in erroneous invasion profile.

All resistivities read lower than Rt and separation is reduced.

Notes

Current tool strings can be very long. In the past with a single tool this problem was rarely seen. The tool uses frequency to make the deep and shallow measurement. This means that the tool is subject to electromagnetic theory. The effect is that in low resistivity (high conductivity) formations the deep Laterolog reads too high.The effect depends on the length of the tool string and to a certain extent the tools it contains.

2323


String Effect

Laterolog tools have another problem in low resistivity beds due to the frequency of the measurement.

In long combination tools, the LLD reads too high.

The effect has been commonly seen in low resistivity formations.

Notes

Knowing the tool string a correction chart such as the ones shown can be computed. There are two, one for the hole size the other for the mud resistivity. Both have to be used.It is clear that the correct factor is very large below 1 ohm-m.

2424


Correction Example

The correction depends on the hole size, Dh, and the mud resistivity, Rm.

This correction has to be applied before any other borehole corrections.

A new chart is needed for each tool combination.

Notes

The apparent resistivity at large Rt/Rm contrasts is lower because of the shorter length between the tool and the reference electrode.The skin depth of the LLD current, 3mm in steel, is less than the drill pipe thickness.These lead to another problem for Laterolog devices at low resistivities.

2525


TLC effectThere are two effects occurring when a Laterolog tool is run on drill pipe.

1)In TLC operations Laterologs need a special stiff bridle usually made of three sections of tool housing giving a length of 30 feet compared to the normal 80 foot bridle.

2)The total current returns to the pipe which acts as the return electrode.

The relative error is proportional to /Ra (the apparent resistivity).

This can be up to 200% at low Rt/Rm contrasts and low Rm.

Notes

The chart assumes a stiff bridle of 30 feet and computes the apparent resistivity read by the tool to compute the resistivity that would be seen in a vertical well with a normal bridle.Once again the effect is greatest at the lowest resistivities.

2626


Example Chart

The chart is used to transform the TLC reading into the reading theoretically obtained in a vertical well with a bridle.

Notes

After correction for the borehole effects the apparent resistivity, Ra, seen by the tool is composed of some signal coming from the invaded zone and some from the virgin zone. The proportion depends on the diameter of invasion, di.This is taken into account in the fraction J, the pseudo-geometrical factor. This describes how much of the signal comes from the invaded zone. If J=1, the entire signal comes from the invaded zone, this is the case for the very shallow reading tools. If J=0, all the reading comes from the virgin zone. In practice even for the deep Laterolog J is never zero, but a small number.

2727


Pseudo Geometrical Factor

Once corrected, the log can be evaluated to find Rt.

Neglecting the mud and mud cake resistivities (corrected log), the tool response equation is:

Ra = J(di)Rxo + (1-J(di))Rt

Where J(di) is the pseudo-geometrical factor which is a function of the invasion diameter, di. For large di, J(di) is large reflecting the important contribution of the invaded zone to the measurement.

Notes

The plot shows the diameter of invasion plotted against the pseudo-geometrical factor for the common resistivity measurements. The shallowest tools have J=1 for a very small invasion. The deep tool has j reading low even for deep invasion meaning that most of its signal comes from the virgin zone.The depth of investigation of resistivity tools is given as the depth corresponding to J=0.5. This means that 50% of the signal comes from the virgin zone. The example shows the LLS reads about 35 while the deep Laterolog is around 70.

2828


Depth of Investigation

The plot shows the pseudo-geometrical factor versus di for various tools.The relative depth of investigation is defined as the invasion diameter for which the invaded zone contributes to 50% of the signal (J = 0.5).The relative depth of investigation is computed from the chart.

For example, it is 35" for the LLS.

Notes

The example log shows a Laterolog which starts with both the deep and shallow curves together. The deep increases maintaining a separation until finally the shallow increases as well right at the top of the interval. At first glance this look like hydrocarbon but it is actually due to the Groningen effect.The induction log shown as comparison does not increase in the same way as the deep Laterolog.The tool is shown with distance between the measure point and the torpedo noted. The Effect begins when the torpedo enters the high resistivity bed. The other curve shown, labelled LLG, is another deep Laterolog using an electrode on the bridle instead of the torpedo. Hence this curve exhibits the same effect but it starts later. In this way the effect can be recognised as such and distinguished from hydrocarbons.

2929


Groningen Effect

The high and increasing LLD reading, associated with a flat LLS, can be caused by the presence of hydrocarbon in the formation, or by the infamous Groningen effect.

Torpedo

DLT measurepoint

LLSLLD /LLG

Induction

Resistive Bed

GroningenResponse

Laterolog

LLG

LLDincrease

Inductiondoes not react

distance totorpedo =distance

below highresistivity

Groningeneffect seen.

bridleelectrode

Notes

The cause of the effect is the use of the torpedo (the cable) as a voltage reference point for the measurement of the deep Laterolog. If the torpedo enters a highly resistive bed the current is forced into the borehole changing the voltage on the torpedo, it is no longer usable as a reference.

3030


Groningen Effect Physics

This is caused by the voltage reference (cable-torpedo) becoming non-zero.

Caused by highly resistive beds overlying the formation that is being measured.

This forces the deep current into the mud column.

Notes

If the voltage in phase and out-of-phase measurements (DLTD and HALS/ARI) are sent uphole, the problem can be eliminated for the "no-casing" Groningen effect.(If casing is set in the high resistivity, the problem is more complex. The HALS/ARI can solve it by making a

second pass with a different return).Another way is to locate the voltage return under the tool. In this configuration there in no Groningen effect entering a high resistivity bed, but there is on leaving it.

3131


Solutions

The HALS/ARI tool can be corrected for Groningen effect.

There is a curve measurement by the laterologcalled LLG, which gives an indication of the Groningen effect.

LLG is:An LLD using a bridle electrode as return rather than the torpedo.An indicator of the presence of Groningen Effect because:

LLG equals LLD when there is no effect.LLG is affected at a different depth than LLD.

LLG is not an LLD corrected for Groningen.

Notes

3232


Groningen-Affected Log

Notes

3333


Groningen Corrected Log

Notes

No Groningen Effect

Curve separation suggests

invasion but is due to

Groningen effect

HRLA resistivities

clearly show zone is not

invaded

Notes

XX60

XX40

Groningen Separation

2 200LLD

( ohm.m )

2 200LLG

( ohm.m )

2 200LLS

( ohm.m )

2 200Array Resistivity RLA5

( ohm.m )


( ohm.m )


( ohm.m )

2 200Array Resisitivity RLA2

( ohm.m )

2 200( ohm.m )

MD

m5 10

Caliper (CALI)

( in )

0 150Gamma Ray (GR)

( gAPI )

5 10Bit Size (BS)

( in )

0.2 200MSFL (Logarithmic Scale)

( ohm.m )


( ohm.m )

Curve separationshows invasion

High verticalresolution

Curve separation results from Groningen effect

Groningen effectin indicator curve

HRLA tool DLL tool

Groningen Example

Notes

The Laterolog find most of its application in high resistivities where it works best. It will work in fresh muds if the resistivity is high enough.The tools do not measure Rt directly, rather they measure a deep and shallow Laterolog from which Rt can be found.

3636


Laterolog Applications

Measures Rt.

Standard resistivity in high resistivity environments.

Usable in medium-to-high salinity muds.

Good results in high contrast Rt/Rm.

Fair vertical resolution (same as porosity tools).

Notes

Oil based and air (or foam) muds will not allow the current to pass hence no measurement can be made.Modelling is used to predict the log reading in a given formation. It can be used to explain unanswered questions.The condition Rxo > Rt is that of having mud fresher than the formation water in a water zone. Here the Laterolog is trying to read a low resistivity through a higher one.

3737


Laterolog Limits

Cannot be used in oil-based muds.

Cannot be used in air-filled holes.

Affected by the Groningen Effect in some environments.

Difficult to model.

Poor when Rxo > Rt.

Notes

Modelling of resistivity devices means analysing a network of resistances. In the early days this is exactly what was used. Today this technique is replaced by finite element modelling using a computer. The full version of this method is very time consuming hence the field version is much simplified. It is still, within limits, able to predict or explain the log readings.

3838


Modelling

It is useful to model the tool response for different conditions.

The approach of bed boundaries can be seen in deviated wells.

Unusual log responses can be checked with different model formations.

A finite element method has to be used to model Laterologs, and all resistivity tools.

This type of program is heavy on computer time.

Notes

The standard curves are retained and the new curve LLhr is added. This has a vertical resolution of around 8 inches compared with the 24 for the standard tool.The twelve individual curves can be displayed if required. To avoid clutter they are often shifted on the scale.An image around the borehole is made using the twelve curves and extrapolating between them. This display is useful for identifying features seen by the tool.

3939


Azimuthal Laterolog outputs

The standard outputs of the Azimuthal Laterolog are:

Standard LLD and LLS curves.

LLhr - high resolution deep Laterolog.

12 azimuthal resistivity curves.

12 electrical stand-off measurements.

An electrical image of the borehole similar to FMS.

Notes

The superior resolution of this tool is clearly seen in the zone from x660-x680. Here the LLS and LLD show a single bed while the high resolution curves show around 20. The standard measurements are averaging the beds to give a result that is, in this case, incorrect.Note the shift between the 12 curves and the rest of the log for display clarity.

4040


Azimuthal Laterolog usesThe simplest use of the ARI is for deep resistivity in laminated formations. Here the tools high vertical resolution reads the correct value when the LLD averages the beds.

Notes

Fractures will be seen by this tool if they are large enough. The detail seen will not be as fine as with the very high resolution imaging tools but fractures will cause low resistivity to be measured.On the image this will come out as a dark (low resistivity) sine wave. The higher the amplitude of the wave the higher the angle of the fracture.

4141


Azimuthal Laterolog Uses 2

Another use of the ARI is fracture identification.As with any resistivity measurement it reacts to the presence of the conductive fluid (mud) in the fractures. They show up as low resistivity on each of the 12 resistivities at different depths depending on their geometry. The best indication is the image.

Notes

4242


ARI Uses 3

There are a number of other uses for this azimuthal tool:Heterogeneous formation

One or more of the resistivities will react to a heterogeneity while the others read normally. An example could be a shale lens in an oil zone. Here the resistivity will be reduced by the low resistivity shale if a standard LLD is used, however the shale will be "seen" by some of the azimuthal resistivities and the true resistivity of the oil zone can then be understood.

Horizontal wellThe ultimate heterogeneous formation. The azimuthal resistivities will be able to see the overlying and underlying formations, thecap rocks and the water table for example. Knowing where these are will greatly assist in completing the well as well as computing saturations.

Dip computationThis is an extra due to having 12 azimuthal

resistivities and the possibility of adding directional information. The output dips are not as good as a standard Dipmeter as the resolution is not as fine, however, they are sufficient for moststructural interpretations.

Notes

4343


Azimuthal Laterolog parameters

Depth of investigation LLhr close to LLD

Vertical resolution8" (in a 6" hole)

Azimuthal resolution60 for a 1" stand-off

Resistivity range0.2 - 100000 ohm-m

Mud resistivity< 2 ohm-m active mode< 5 ohm-m passive mode

Notes

Standard Laterolog Limits

Limitation in approach

Does not account for coupling between radial and vertical response

Risk of underevaluating reserves

Overestimated Rt in water zones

Underestimated Rt in thin hydrocarbon-bearing zones

Notes

HRLA Solutions Hardware

Mode 1 Mode 2 Mode3 Mode 4 Mode 5

Multiple depth of investigation

Clear indication of invasion

Improved vertical resolution

No need for deep mode or bridle

No Groningen or drillpipe-conveyed logging effects and reduced shoulder-bed effect

Notes

Array Laterolog Principle

24 ft

mode 0 mode 1 mode 2 mode 3 mode 4 mode 5

0 V 0 V 0 V 0 V 0 V 0 V

potential (V)

Return Electrodes

Source Electrodes

Return Electrodes

Mode 2 current lines

Notes

HRLA Software Solutions

2D earth model

More accurate Rtcomputation

Correction for coupling of radial and vertical response

Better inversion with improved formation models

Notes

Tool Radial Response

Notes

4949


Borehole effect

101 100 101 102 103 104 105 1060

0.5

1

1.5

2

2.5

3

Rt/Rm

R

a

/

R

m

Borehole corrections dh=10in, tool centered

RLA1RLA2RLA3RLA4RLA5

dh=10in centered

Notes

Shoulder-Bed Squeeze Effects

Deep measurement reads too high and results in erroneous invasion profile.

All resistivities read lower than Rt and separation is reduced.

Notes

5151


Shoulder-bed Effect- Laterolog

8950

0 2VCL_HILT

( ft3/ft3 )

PEx

0.2 2000RXOZ .HILTC .075 [A7380343]

( ohm.m )

0.2 2000HART Maxis

( ohm.m )

0.2 2000HLLD Maxis

( ohm.m )

0.2 2000HLLS Maxis

( ohm.m )

1 0RSOZ .H

( in )

1 0DSOZ .H

( in )

MD1 : 200

ft

0.45 0.15TNPH .HILTC .0

( ft3/ft3 )

1.95 2.95RHOZ .HILTC .0

( g/cm3 )

9000

Notes

5252


Corrected Dual Laterolog

8950

0 2VCL_HILT

( ft3/ft3 )

PEx

0.45 0.15TNPH .HILTC .0

( ft3/ft3 )

1.95 2.95RHOZ .HILTC .0

( g/cm3 )

0.2 2000RXOZ .HILTC .075 [A7380343]

( ohm.m )

0.2 2000HART .HALS_EC [A7385363]

( ohm.m )

0.2 2000HLLD .HALS_EC [A3157784]

( ohm.m )

0.2 2000HLLS .HALS_EC [A3157794]

( ohm.m )

1 0RSOZ .H

( in )

1 0DSOZ .H

( in )

MD1 : 200

ft

9000

Notes

HRLA Shoulder-bed Effect

HRLA tool HALS tool 1D-Rt comparison

Notes

5454


Full Process

Notes

Except for the Microlog tool all the others were/are focused to pass through the mud cake and read only the invaded zone. They are all resistivity devices. All are pad tools pushed against the wall by a powered caliper device.

5555


Microresistivity Devices

Shallow reading versions of resistivity tools; always pad-mounted.

First was the Microlog which is still in use;Second was the Micro Laterolog (MLL), replaced byProximity (PL) tool, replaced byMicroSpherically Focused Log (MSFL), replaced byMicro Cylindrical Focused Log(MCFL)

Objective is to read Rxo (Invaded Zone Resistivity) only.

Tools are focused to pass through the mud cake.

Notes

This is the very oldest microresistivity device. It has been used for a number of years to measure the sand zones i.e.. the permeable layers. The absolute value of the resistivity is not of interest, only the separation between the two curves.A number of synthetic micrologs have been devised using some of the currents output by the more sophisticated tools.

5656


Microlog Uses

Microlog is used to identify permeable zones.

If the zone of investigation is shale (no invasion), both curves read the same.If the zone is sand (with invasion), Microinverse reads mud cake plus some of the formation and Micronormal reads some mud cake plus the formation (slightly higher).We are only interested in the separation between these curves and so scales are chosen to show this and not the rest of the readings.

2" Micronormal. (A -> M2)1"x1" Microinverse. (A -> M1)(Slightly different depths of investigation).

Notes

A focusing current flowing between the A0 and A1 electrodes passes mainly through the mud cake.In this way, the measuring current is constrained to the formation and importantly to the invaded zone.The exact depth investigated depends on the mud cake thickness and the resistivities of the mud components. However, it is normally around 6 inches.

5757


MSFL Principle

This tool uses a set of 5 electrodes which focus the signal into the invaded zone just beyond the mud cake.

Notes

The mud cake thickness can be estimated from the bit size minus the caliper or from the MSFL caliper minus the Density tool caliper (divided by 2). This assumes that the MSFL caliper floats on top of the mud cake while the other cuts through it. Unfortunately this measurement of a small quantity is a the limit of the tools accuracy. Note the MCFL gives Rxo as output and does not need correction.

5858


MSFL Borehole Corrections

In spite of its focusing, the tool still needs to be corrected for the mud cake thickness and

resistivity.The correction requires an input of mud cake thickness which is not measured directly.

It also needs the mud cake resistivity which is either measured or computed from charts.

The tool focusing has been set assuming there is always some mud cake, hence the tool

always needs some correction.

Notes

The measurement of Rxo is needed to compute the water saturation in the invaded zone, Sxo. Knowing Rxo the deep measurements can be inverted to give the true virgin formation resistivity Rt.As with all pad type tools bad hole conditions will badly affect the measurement quality.

5959


Uses and Limits

Uses:Rxo measurement in water- based muds.Correction for deep resistivity tools.Sxo determination.

Limits:Rugose hole.Oil-based mud.Heavy or thick mud cake.

10 electrical

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