Well Logging Course (1st Ed.)

1. Early Electric Log Interpretation

2. Formation Factor

3. Water saturation

4. The Porosity Exponent, m

5. The Saturation Exponent, n

6. A Thought Experiment For A Logging Application

1. Unfocused Devices:A. The Short Normal

B. Estimating the Borehole Size Effect

2. Focused Devices:A. Laterolog Principle

B. The Dual Laterolog

electrode devices

electrode devices, Is one type of electrical logging tool

so named because the measurement elements are simply metallic electrodes

These devices utilize low-frequency current sources, in most cases below 1,000 Hz.

The historical progression from the normal device to traditional focused dual laterologsThe traditional focused dual laterolog was the main device

used for electrode measurements of resistivity for many years, even though it had several known shortcomings.

• Many of these shortcomings were solved by the introduction of array devices, a development that was made possible partly by the availability of fast inversion software.

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

A schematic representation of the short normalThe earliest

commercial device, the short normal, is illustrated in the figure.

A 16 in. [41cm] (thus the designation “short.”) spacing is indicated between current

electrode A and measure

electrode M (voltage electrode)

Fall 13 H. AlamiNia Well Logging Course (1st Ed.) 6

the short normal problems:1: in a very conductive borehole mud

Two basic problems are associated with the short normal, bothrelated to the presence of the borehole,

which is normally filled with a conductive fluid.

1st: There is a sensitivity of the measurement to the mud resistivity and hole size In a borehole filled with very conductive

mud, the current tends to flow in the mud rather than the formation. In this case,

the apparent resistivity as deduced from the injected current and resultant voltage will not reflect the formation resistivity very accurately.

Idealized current pathsFall 13 H. AlamiNia Well Logging Course (1st Ed.) 7

the short normal problems:2: in front of a thin resistive bed

The second difficulty with this measuring technique:Once again,

the conductive borehole fluid provides an easy current path for the measure current into adjacent shoulder beds of much lower resistivity (Rs ) than the formation (Rt ) directly opposite the current electrode.

In this case, the apparent resistivity (from the measurement of

the voltage of electrode M and the current I, in combination with the tool constant)

will again be representative not of the resistive bed, but, more likely, of the less resistive shoulder bed.

Idealized current pathsFall 13 H. AlamiNia Well Logging Course (1st Ed.) 8

Borehole correction chart for the 16 in. short normal.for

an 8 in. hole size, the short normal does a fairly good job of measuring the correct formation resistivity, except for very large mud/formation resistivity contrasts.

However, for the 16 in. borehole size, this is not the case.

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

A schematic of the short normal in common logging situationNote that in the

upper part of the figure some idea of the actual tool implementation is given.

The electrode B is at the surface, whereas the electrode N, to which the potential measurement is referenced, is actually located down-hole on the measurement sonde.

Adapted from Doll et alFall 13 H. AlamiNia Well Logging Course (1st Ed.) 11

A schematic of the short normal response in two common situations A continuing examination of the

shortcomings of the short normal in the Figure reveals the kind of response problems encountered for large contrasts between the shoulder beds and the bed of interest.

In this particular case, the resistivity contrast between beds is 14, and the borehole diameter [d] is half the spacing between current source and voltage electrode [AM].

Even for a bed 3 [36 in] ft thick, it is seen that the central value of resistivity does not attain the desired value.

If the bed is only 6 in. thick, then the behavior becomes nonintuitive, with the apparent resistivity dipping below the value of the shoulder bed.

Fall 13 H. AlamiNia Well Logging Course (1st Ed.) 12

the lateral device

When attempts were made to improve this bed-boundary resolution, the normal device evolved to the lateral device, illustrated in the Fig.

The lateral sonde is much like the normal sonde except that there are two voltage electrodes, and the potential difference between them is used to indicate the resistivity of the formation layer between them. This will be nearly the case for

beds whose thickness exceeds the spacing between the electrodes marked A and N.

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

the lateral device (Cont.)

The figure shows the response to two beds, whose thickness is given in terms of the electrode spacing. It is clear that there has

been some improvement for bed resolution,

but the response is still quite complicated because of current flow through the mud to zones other than the one directly in front of the measuring points.

Fall 13 H. AlamiNia Well Logging Course (1st Ed.) 14

current flow in the borehole and formation from a central electrode

The next step in the evolution of electrical tools was the implementation of current focusing. Figure illustrates,

on the left half of the diagram, the current paths for the normal

device in the case of a resistive central bed. The current tends to flow around it, through the mud, into the less resistive shoulders.

The desired current path is shown on the right half of the figure, where the measure current is somehow forced through the zone of interest.

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

The principle of focusing (Laterolog-3 (LL3))Idealized

current distribution from the Laterolog-3 device in a homogeneous formation, with current focused into the formation.

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

Laterolog-3 (LL3)

in previous slide, there are now three current emitting electrodes, A0, A1, and A 1’.

This type of array is known as a guard focusing device and is commonly referred to as a Laterolog-3 (LL3), device.

The potential of electrodes A1 and A 1’ is held constant and at the same potential as the central electrode A0. Since current flows only if a potential difference exists, there

should, in principle, be no current flow in the vertical direction.

The sheath of current therefore emanates horizontally from the central measurement electrode.

The current emitted from the focusing, or “guard” electrodes is often referred to as the “bucking” current, as its function is to impede the measure current from flowing in the borehole mud.

Fall 13 H. AlamiNia Well Logging Course (1st Ed.) 19

LL3 difficulties

Despite these good intentions, the LL3 device still showed some difficulty with bed boundaries. The effects of shoulder

bed resistivity on the behavior of an LL3 device. The top sketch indicates

current passing through the mud into a highly conductive shoulder.

The bottom sketch indicates the effect of a thin conductive bed.

Fall 13 H. AlamiNia Well Logging Course (1st Ed.) 20

The electrode configuration of the Laterolog-7

Another approach to focusing the measure current is the seven electrode device, or LL7. The electrode configuration of

one such device is sketched here.

Monitor electrodes drive the bucking current in the guard electrode to maintain a differential voltage of zero.

The array is symmetric with A0 in the center.

Adapted from SerraFall 13 H. AlamiNia Well Logging Course (1st Ed.) 21

dual laterologs

The most common traditional electrode devices use a dual focusing system. Those known as dual laterologs combine the features of

the LL3 and LL7 arrays, in an alternating sequence of measurements.

By rapidly changing the role of various electrodes, a simultaneous measurement of deep and shallow resistivity is achieved.

Fall 13 H. AlamiNia Well Logging Course (1st Ed.) 29

the current paths computed for dual laterologsOn the left side, the

electrodes are in the deep configuration. The length of the

guard electrodes, which use parts of the sonde, is about 28 ft [8.5 m] to achieve deep penetration of a current beam of 2 ft [61cm] nominal thickness.

On the right side, they are in the shallow (or medium) configuration.

Fall 13 H. AlamiNia Well Logging Course (1st Ed.) 30

apparent resistivity

For purposes of comparison of the different electrical measuring devices, it is convenient to think of the signal measured as being the result of the influence of three distinct regions of the measuring environment: the borehole, the invaded zone, and the undisturbed

formation. Each of these zones is attributed its own characteristic

resistivity: Rm, Rxo, and Rt . • Generally the mud resistivity Rm is much less than either Rxo or

Rt .

In this model, the response of an electrode device can be conveniently thought of as an approximately linear combination of the invaded zone and the true resistivity.

Fall 13 H. AlamiNia Well Logging Course (1st Ed.) 32

apparent resistivity

This is expressed as:

Ra is the apparent resistivity.

The pseudogeometric factor J is a normalized weighting factor which gives the relative contributions of the invaded zone (of diameter, di ) and virgin zone, to the final answer. It is referred to as the pseudogeometric factor

(as opposed to a pure geometric factor, as will be seen later with the induction tool) since the weighting function will actually be influenced by the contrast between Rxo and Rt.

Fall 13 H. AlamiNia Well Logging Course (1st Ed.) 33

the pseudogeometric factor for several devicesThe comparison of

calculated pseudogeometric factors for a number of common electrode devices.

LLd and LLs refer respectively to the deep and shallow arrays of a dual laterolog device.

Courtesy of SchlumbergerFall 13 H. AlamiNia Well Logging Course (1st Ed.) 34

laterolog’s sensitivity to the borehole

It is important to note the laterolog’s sensitivity to the borehole.

Next slide shows the correction chart for the deep and shallow measurement of a particular dual laterolog device, plotted in a manner similar to chart for the short normal.

This chart is for a centered tool. Other charts are available for an eccentered tool, the

eccentricity being characterized by the standoff between tool and borehole wall.

Fall 13 H. AlamiNia Well Logging Course (1st Ed.) 36

A borehole correction chart for deep & shallow laterolog measurements.It is to be

compared to correction chart for the short normal, to appreciate the improved response due to focusing.

Adapted from SchlumbergerFall 13 H. AlamiNia Well Logging Course (1st Ed.) 37

1. Ellis, Darwin V., and Julian M. Singer, eds. Well logging for earth scientists. Springer, 2007. Chapter 5