Well Logging Course (1st Ed.)

1. Introduction

2. Microelectrode Devices

3. Uses For Rxo

4. Azimuthal Measurements

5. Resistivity Measurements While Drilling

1. Introduction

2. The Two-coil Induction Device

3. Three-coil Focusing And Multi Coil Induction Devices

4. Induction log corrections and calibrations

5. A sample induction log

Advantages and Disadvantages of Conductive mudThe presence of a conductive mud in the borehole

is somewhat of a nuisance for electrode devices. Many improvements have been made in electrode tool

design to compensate for the problems.

However, conductive borehole mud does provide one advantage: it effectively places

the current and voltage measurement electrodes into electrical contact with the formation whose resistivity is to be measured.

Fall 13 H. AlamiNia Well Logging Course (1st Ed.) 5

Induction devices

What about those cases in which the mud is nonconductive (oil-base mud) or nonexistent (air-filled hole), or in which a plastic liner has been inserted into the borehole? It is for these cases that

the induction tool was designed originally, although it has since found widespread use in conductive muds.

Induction devices use medium frequency (several 10s of KHz) alternating current to energize transmitter coils in the sonde; they, in turn, induce eddy currents in the formation whose

strength is proportional to the formation conductivity. The magnitude of the induced currents is measured by

receiver coils in the tool that sense the magnetic field generated by the induced currents.

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The principle of the induction tool

an induction logging device consists of a transmitter coil,

excited by an alternating current of medium frequency (≈20 kHz)

and a receiver coil.

The current loops in the conductive formation produce an alternating magnetic field detected by the receiver coil.

One axially symmetric ring of current-bearing formation is indicated in the figure.

the voltage detected at the receiver coil will vary directly with the conductivity of the formation and with the square of the excitation

frequency.

Fall 13 H. AlamiNia Well Logging Course (1st Ed.) 8

Focusing The Two-coil Sonde

The response of the two-coil device can be altered to minimize the “tail” of sensitivity

to beds above and below the measurement coils or

to decrease the sensitivity to layers closest to the borehole.

For an illustration of how the response is altered or focused, we examine the technique for changing the depth of

investigation of the two-coil sonde.

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Focusing The Two-coil Sonde (Cont.)

The focusing idea is simply to add a second receiver coil which is a bit closer to the

transmitter and to use its response, which will be somewhat shallower than the original receiver, to subtract from the response of the original.

By placing the receivers suitably and selecting the proper number of turns, this subtraction should eliminate much of the signal from

regions close to the borehole.

A similar procedure is used to sharpen the vertical resolution of the tool. This will change the sensitivity of the tool

measurement to layers of different conductivity

above and below the measurement coils.

Fall 13 H. AlamiNia Well Logging Course (1st Ed.) 10

The principle of three-coil focusing

A second coil, wound with reverse polarity, produces a

signal which cancels some of the signal from close to the borehole.

Focusing The Three-coil SondeFall 13 H. AlamiNia Well Logging Course (1st Ed.) 12

Multi coil Induction Devices

The principle of focusing using multiple coil arrays was known early on and exploited in a variety of configurations in the 1950s.

The industry eventually settled on a standard deep reading array known as the 6FF40 because it had six coils

(three transmitter and three receiver) with so-called fixed radial and vertical focusing (FF) and 40 in. [1m] between the main transmitter and receiver.

This remained the standard, with only minor modification,

until the introduction of multi-array devices in the 1990s.

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corrections

The relatively large volume of formation surveyed means that both shoulder and skin effect can be significant, and

therefore corrections are applied before displaying the log.

It was realized early on that shoulder effect could be removed by applying a suitable filter, in other words deconvolving the response.

The problem was to find simple filters that suited a wide range of conditions,

since complex functions could not be handled at the wellsite at that time.

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correction approximation

Originally, different options were offered for different conditions, but for consistency these were soon reduced to one

a three station average that was considered suitable for a shoulder bed resistivity of 1 ohm-m:

σd is the deconvolved reading at depth z,

the σa are the measured readings at z and 78 in. above and below.

This is an approximation that was easy to implement in early analog logging units.

Fall 13 H. AlamiNia Well Logging Course (1st Ed.) 17

an invasion correction for Induction logAlso, if the diameter of invasion is more than about 40 in.,

an invasion correction is needed.

As with laterologs, we assume a step profile model of invasion to solve for Rt, for which we need three logs with different radial response. Since shallow induction devices may have very large borehole

signals, the shallow log is provided by a microresistivity device or a shallow laterolog such as a spherically focused log.

The medium log is provided by a modified 5FF40 induction array (two transmitters and three receivers), to which several small coils have been added. Skin effect and shoulder effect are both less, so that no automatic shoulder

correction is needed but a skin effect correction is made on the same principle as the ILd.

The result is known as the ILm, or IM.

Tornado charts are available to solve for Rt , Rxo, and di from ILd, ILm, and SFL.

Fall 13 H. AlamiNia Well Logging Course (1st Ed.) 18

Borehole correction for ILm and ILd

Although both ILm and ILd have radial focusing that minimizes any contribution from the borehole, there are situations where the borehole signal can be

significant – for example salty, highly conductive muds with low conductivity formations.

Borehole correction charts are available to make the appropriate corrections in smooth boreholes.

Charts also exist to correct the remaining shoulder bed effect for both Ilm and ILd.

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Induction Or Electrode

an induction signal is proportional to conductivity whereas the signal from an electrode tool is

proportional to resistivity.

This means that for measuring Rt the induction is preferred when the conductivities near

the borehole are low, i.e., mud resistivity Rm is high and Rt < Rxo.

On the other hand when Rt is high the induction signal is low and the measurement is less accurate. For laterologs the conditions are the reverse.

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the calibration of induction devices

Two calibrations are made at surface: one in air and one from a single copper loop with a

resistor in series. The loop is placed around the device at a specified position and

with a resistor chosen to give a signal equivalent to a formation of, for example, 0.5 S/m.

In practice, even though traditional sondes are made of nonmetallic materials there are always some metallic parts, for example from pressure bulkheads and cable shields, that give an R signal. • This is known as the sonde error. • It is measured by the calibration in air and canceled during

logging. • Adjustments may be made to allow for change in the sonde error

under temperature and pressure.

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the induction curves corrections

As in the case of the laterolog curves, the induction curves must be checked for any necessary corrections before attempting quantitative interpretation.

In addition to the same general types of borehole corrections, the induction may also require correction for bed thickness and shoulder effect. The magnitude of this correction will depend on an

estimate of the bed thickness and the resistivity of the shoulder beds.

This type of correction will certainly be necessary for thin streaks of zones.

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A sample induction log

Two thick clean zones are indicated as A and B.

Two much thinner clean streaks are shown as C and D.

In typical induction conditions (Rxo > Rt ) the SFL provides reasonable

information on Rxo and, not being pad-based, does not have

the sensitivity to rugosity of microdevices.

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1. Ellis, Darwin V., and Julian M. Singer, eds. Well logging for earth scientists. Springer, 2007.