Notes

11

Induction logs

Induction Logs

© Schlumberger 1999

Notes

Drilling in the late 1940s started to use oil based mud, which meant that the traditional resistivity measurements were no longer of any use. The objective of an induction type measurement was to measure in these conditions. The evolution, as with many other tools, increased the accuracy and resolution of the measurement. The problem for the induction measurement as with the deep electrical log is that it has to be able to read through the invaded zone into the virgin zone.

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Induction logs

Induction history

The idea for the tool developed out of mine detector work done by Henri Doll during the Second World War.

The objective was to measure resistivity in fresh or oil-based muds.

The first tools had 5 coils to focus the signal.

The next generation of tools employed 6 coils.

Two measurement curves were eventually developed, a medium and a deep paralleling the Laterolog's shallow and deep readings.

The current Array Induction uses software focussing to obtain 5 depths of investigation.

Notes

The tool frequency is around 20kHz. At this level the measured field is proportional to the conductivity. The signal at the receiver coils is a combination of the formation, R, signal and the direct coupled X signal.\The R signal contains the information on the formation. The X has information on how the signal has been affected in transit. Both are measured in modern tools, with the X signal being used to correct the R.The tool works best when the borehole fluid is an insulator such as oil-based mud.Pairs of coils are combined to obtain improved vertical and radial characteristics, i.e. focusing in both directions.The standard tool uses 6 coils to achieve the maximum effect.

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Induction logs

Induction Principle

An Induction tool uses a high frequency electromagnetic transmitter to induce a current in a ground loop of formation.

This, in turn, induces an electrical field whose magnitude is proportional to the

formation conductivity.

Notes

The total signal for the entire borehole system is given by:Ca = CmGm + CxoGxo + CtGt + CsGsGm + Gxo + Gt + Gs = 1

Gm, Gxo,G t, Gs are the geometrical factors for a given defined region.ΣGi = 1.This analysis is very rigorous and understood using Maxwells equations, any situation can be described precisely by the equations.

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Induction logs

Geometrical Factor

In a simple model, (tool centred, homogeneous formation), the response of the tool can be

calculated as the sum of all the formation loops coaxial with the sonde.

Each signal is proportional to the conductivity and to a Geometrical Factor, Gi which depends only on the loop position with respect to the transmitter and receiver positions.

The sum of all the geometrical factors is equal to 1.

Notes

The distance between the transmitter and the receiver is large and hence there is always a “shoulder bed effect. This is partly eliminated by the focusing.Deconvolution is used to give more weight to the centre bed reading at the expense of the shoulder. In older techniques this is simply a numerical filter. Modern tools use the recorded X signal to find the correction dynamically. The latter method is more accurate and extends the range of the tool.

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Induction logs

Shoulder Bed Effect

To minimise the shoulder bed effect, the tool is focused using multiple coils.

In addition, the shoulder bed response is suppressed to improve the vertical resolution.

Deconvolution gives greater weight to the signal measured at the sonde centre and less weight to the signals from either side.

Modern tools use the X-signal to make a non-linear deconvolution correction.

Notes

Another use of the X signal is to perform this skin effect correction. Once again the result is a more accurate measurement than the traditional booster algorithm.The skin effect and deconvolution are well understood and easily solved because the tool uses electromagnetic principles which are described by Maxwells equations. All modern induction tools have used this to improve the quality of the measurement and extend the range.

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Induction logs

Skin Effect

Caused by ground loops creating their own fields and interfering with the signal being measured.

The net result is a reduction in the measured conductivity.

The correction increases with increasing conductivity.

The traditional solution was to employ a booster algorithm.

The current tool uses the X-signal to make the correction.

Notes

As the tool react to conductivity the formation effects are the opposite to the electrical tools.The best readings of the conductivity of the virgin zone, Ct (or in resistivity Rt) are obtained when the conductivities of the mud, Cm, mud cake, Cmc and the invaded zone Cxo are as low as possible. This means oil based mud is ideal hence the objective of the tool has been met.The mud cake is once again taken as negligible.

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Induction logs

Borehole Effects

Induction tools measure Conductivity.Induction tools measure resistivity in Parallel.Thus Induction tools see the borehole environment as:

Cm - Best readings occur in high resistivity mud, oil-based is better, fresh mud is good, salt-saturated mud is worst.

Cmc - Usually neglected as very small.

Cxo - Depends on Rmf - needs to be known.

Ct - Parameter to be measured, the higher the better.

Notes

The objective of the borehole correction procedure is to eliminate the signal from the borehole. In the case of the induction tool this means computing the volume of conductive fluid in that zone.The surface acquisition units do this is real time using the inputs of hole size (from a caliper log) and mud resistivity either continuously measured or the surface value.

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Induction logs

Corrections

The measurement has to be corrected for borehole effects.

For previous generations of tools the procedure is

1) Compute borehole geometrical factor.2) Find additional signal due to the borehole.3) Convert log resistivity into conductivity.4) Remove borehole signal from total signal.5) Convert result back to resistivity.

This is best done in the field using either the Surface Acquisition units

It is also possible using Chart Books.

Notes

This chart and the next slide show how the borehole geometrical factor is computed. They have inputs of borehole diameter and the physical stand off placed on the tool. This knowledge gives the tool position in the borehole.At very small diameters, less than 8 inches the borehole signal for the deep is very small. The shallow still has a dependency on the stand off. As the hole size increases so does the geometrical factor. Current operation practice increases the stand off fitted to the tool with increasing borehole size.The effects on the medium are always much greater than those on the deep.The chart also shows why the tool has difficulty in caves. A change in hole size changes the borehole geometrical factor, which if not corrected, will cause the log to read incorrectly.

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Induction logs

Corrections

Notes

This chart is similar to the previous slide but this time for a low contrast between the formation and mud resistivities.

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Induction logs

Correction Charts

Notes

Once the borehole geometrical factor has been obtained the effect of changing mud resistivity is computed. It is clear from this chart that if the borehole geometrical factor is near zero the mud resistivity plays a small role. This means for the deep measurement that in small hole sizes (near zero borehole geometrical factor) there is little effect even in very salty (low resistivity) muds.However in larger holes when the borehole geometrical factor is large the borehole signal and hence the correction is large.

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Induction logs

Correction Charts

Procedure:Obtain the Borehole Geometrical factorEnter the value on the axis.Draw a line through the mud resistivity to obtain the hole signal..

Subtract the hole signal from the measured conductivity to obtain the corrected value

Notes

Stand-off is not only a value of the physical stand-off on the tool but also a measurement of variations in the tool's position. In larger holes the stand-off should be increased so that the tool's position is better defined.Use both CME-Z centralizers and stand-offs.Place one pair of stand-offs at 45Þ to each other at the top of the AIT cartridge.Place one pair of stand-offs at 45Þ to each other on the sonde electronics housing (not extending over the sleeve).Place one pair of stand-offs at 45Þ to each other on the transmitter housing.

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Induction logs

AIT Corrections

There are well defined borehole corrections to be applied to the measurement. These are made in real time by the software. The inputs required are:

Borehole cross section.Mud resistivity.Stand-off.

The tool can compute any of these from its measured signal as well as the formation resistivity. However, normal practice is to input at least two of them.

A measurement of the mud can be made with an auxiliary sonde or surface measurement. The former is best as logs made have shown considerable heterogeneities in the mud column with depth.

A caliper tool can give the hole dimensions.

Notes

Bed thickness effects are taken care of in the induction tools by modelling and by deconvolution. This means that there is little effect for beds above six feet thick.

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Induction logs

Bed Thickness

The induction needs to be corrected for the effect of resistive or conductive shoulder beds.

After signal processing this effect is minor except in beds less than 6'.

Notes

The very large vertical resolution can make interpretation difficult when it is used in conjunction with the porosity tools which have a resolution of around 2 feet.Enhanced processing takes advantage of the medium tools ability to see all the beds down to two feet. The resulting log is comparable with the porosity tools.In bad hole, either rugous or large, the medium is badly affected. Any use of this measurement to enhance the deep will give a bad log.

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Induction logs

Enhancement

The standard Deep Induction tool has a vertical resolution of 6' to 8'.

It is impossible to improve the tool's hardware design as the measurement is "blind" at some thickness.The Medium Induction tool can "see" all thickness.

The Medium signal is used to enhance the more accurate Deep reading.

Enhanced resolution of 3'.

Very enhanced resolution of 1.5' to 2'.

A problem - the medium may be adversely affected by borehole conditions (rugosity, caving), resulting in a poor deep reading.

Notes

The deep readings are insensitive to the near borehole environment the shallow readings are high resolution but are affected by the environment. The combination of all the measurements results in a log which is precisely focused and has a high vertical resolution.The highest vertical resolution output is more affected by sharp changes in borehole size or in invasion. If the borehole is expected to be in bad shape (or found to be after logging) the lower resolution four feet outputs are used.

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Induction logs

AIT principle

The tool measures 28 independent signals from 8 arrays. There is one transmitter operating at three frequencies. The in-phase (R) and the quadrature (X) signals are both measured.

The conductivities are combined using radial and depth functions.

These are software focused to give:

5 depths of investigation:10", 20", 30", 60" 90".

3 vertical resolutions: 1', 2' and 4'.

Notes

The plot shows how the induction family of tools behave with invasion. The geometrical factor is an integration of the contributions of each cylinder of the borehole.The depth of investigation is again computed using the same criteria as the electrical devices; where it is assumed 50% of the signal is coming from the invaded zone. From the plot the medium is reading a similar distance to the shallow laterolog and the deep to the deep laterolog.

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Induction logs

Depth of Investigation

This is equivalent to the plot seen for the laterolog.

Once again, the depth of investigation can be obtained from this plot using the same criteria.

Depth of investigation = zone contributing 50% of the signal.

Notes

The standard induction tool has a depth of investigation affected by the conductivity of the invaded and virgin zones. The multiple spacings of the Array Induction tool allows the depth of investigation to be computed independent of the conductivities.The deepest reading of the older tool is around 60”, the new tool at 90 goes far beyond this value.

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Induction logs

AIT depth of investigation

The AIT has set radial depths of investigation which are not affected by changes in conductivity.

The values are taken as the point where half the signal comes from shallower levels.

In comparison to the 10", 20", 30", 60" and 90" of this tool, the medium and deep of the old tool are around 30" and 60" respectively.

Notes

The limits are in many ways the opposite of the laterolog devices as expected from a tool measuring conductivity not resistivity. The tool works best in fresh or oil based muds and the condition Rxo>Rt.It does not do well at high resistivity (low conductivity). The plot of dipping beds shows a bed of 10 ohmm surrounded by two beds of 1 ohmm each. As the well (or bed) is deviated, the difference is smeared until above about 40Þ it disappears. This leads to complex situations in deviated wells.

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Induction logs

LimitsCannot be used in salt-saturated muds unless in small hole sizes.

Cannot be used in high resistivity formations.

Poor in thin beds.

Poor when Rxo < Rt.

Dipping beds will affect the logs.

Notes

This is the tool of preference in many situations. The only places it is not run is the salt muds and high resistivity.

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Induction logs

Uses

Measures Rt.

Ideal in fresh or oil-based environments.

Ideal for low resistivity measurements and when Rxo > Rt.

Notes

Induction measurements are much easier to model that the electrical tools. The Geosteering technique provides the directional drillers with the method of guiding the well on a very precise well track using a computed resistivity profile. This profile is compared with a resistivity recorded while drilling. Any deviations from the projected well trajectory will show as a change from the predicted resistivity and thus a course correction can be made.

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Induction logs

Modelling

As for the laterolog tool, it is useful to model the induction response to a given situation.

The Induction is simple to model in almost any case as it is based on electromagnetic theory.Programs exist for both vertical and deviated wells.

Effects such as the effect of dipping beds can be analyzed and the true resistivity of the

layer obtained.

Horizontal wells are also handled so that the response of an electromagnetic tool to a nearby cap rock or water table can be predicted.

This is important in horizontal wells where the technique called Geosteering is used to accurately position the well trajectory.

Notes

This is the same example as the previous slide this time showing a comparison between the Array Induction Tool and the older Dual Induction tool. The invasion profile as has been noted is very clear on the former tool, all curves separating. The older tool is ambiguous with some separation, but the same separation in the non-porous zone above this as well.

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Induction logs

examples 3

The AIT logs (2' vertical resolution) read correctly in this zone giving a hydrocarbon profile.

The DIL logs are ambiguous as the SFL (electrical log) longer reading shallow because Rxo is less than Rt

90 Inch investigation(ohmm) 2000.2

0.2

0.2

0.2 2000.0

2000.0

2000.0

0.010000.0

(ohmm)

Cable tension (TENS)(LBF)

(ohmm)

SFL unaveraged (SFLU)

Medium resistivity (ILM)

(ohmm)Deep resistivity (ILD)

10 Inch investigation(ohmm) 2000.2

20 Inch investigation(ohmm) 2000.2

30 Inch investigation(ohmm) 2000.2

60 Inch investigation(ohmm) 2000.2

Notes

The step profile assumes that the invasion is piston like and stops there. It needs three measurements to solve for the three unknowns. This can be done with a standard tool set. The five measurements of the Array Induction Tool allows a different profile to be evaluated. Here the assumption is a ramp between the invaded and virgin zones. This gives four unknowns which is solvable.

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Induction logs

As the AIT produces five logs with differing depths of investigation, a more realistic description of the invasion can be made.The old model is:

New model:

This model has four unknowns with the addition of a ramp profiled for the invasion.

AIT Rt-Rxo-invasion

Notes

This image displayed on an AIT log simply shows the measured resistivities in a different way. Here there is the borehole at the centre of the image with changing resistivity away from this point.The usefulness, as with a lot of images, is to focus the attention on a particular zone and then interpret the measurements at that place.

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Induction logs

AIT independent model

The AIT can be displayed as an image.

The simplest image is of resistivity radial profile starting at the borehole and going out into the formation.

This image simply extrapolates the readings of the tool assigning colour classes to the resistivity level.

It is called an "independent model" because it makes no assumptions about the resistivity distribution.

Notes

The example shows a porous zone with a clear invasion profile on the resistivity curves. The invasion shown on the left hand side shows two clear radii indicating a ramp like invasion.The zone lower down, however, produces only one radius, showing a step like invasion here. On the right hand side of the picture is the filtrate invasion volume. This follows the invasion profile in showing more filtrate towards the bottom of the zone.

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Induction logs

examples 2

The invaded volumes computed here show an increase with depth. The results could be used to plan sampling points or a well test.

Notes

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Induction logs

AIT parameters

Radius of investigation:10" (A x 10)20" (A x 20)30" (A x 30)60" (A x 60)90" (A x 90)

Vertical resolution (x):1'2'4'

Resistivity range:0.2 - 1000ohm-m