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ELECTRICAL SURVEY (ES Log)

ELECTRICAL SURVEY CONCEPTS
The electrical survey was invented by the Schlumberger brothers in 1927. The tool was replaced by the induction log and laterolog between 1950 and 1960, although it was used sparingly into the mid 1970's. The tool required a conductive mud, but worked poorly in salty mud. It did not work in cased holes.

The Electrical Survey, also known as the ES Log, measures resistivity with direct current (DC) or low frequency alternating current (AC) using the principles of Ohm’s Law. The basic measuring system has two current electrodes, A and B, and two voltage measuring electrodes, M and N. A current is passed between A and B, and the resulting voltage is measured at M and N, as in illustrations shown below.


Long and Short Normal Circuit Diagram. M and N are measure electrodes, A and B are current electrodes. Log spacing is the distance AM, usually 16 inches for the short normal. There is a second M electrode at a 64 inch spacing for the long normal. A mechanical switching device in the logging truck, called a pulsator, switches the measuring circuits so that the 16”, 64”, lateral, and SP can be measured sequentially using only 6 wires to the surface. The 4 measurements are displayed as if they were recorded simultaneously. The pulsator also reverses the polarity of the current between measurements so that there is no buildup of electrical potential (polarization) on any electrode. The N electrode in the actual circuit is placed about 18 feet above the tool to reduce resistance effects from the near surface due to dry or frozen ground.

If the formation is uniform, the formation resistivity, Rt, can be computed from the formula Rt = K * V / I, where V is the voltage between M and N, and I the intensity of the current flowing from A to B. K is a geometric factor that depends upon the relative distance between A, B, M, and N and is a constant for a given electrode arrangement.

In practice, the formula gives a weighted average resistivity of the formation, including a small portion of the borehole. This average is known as the apparent resistivity, Ra. Borehole environment correction charts, available from service company chartbooks, are used to correct Ra to approximate Rt. Modern computer software is available to convert Ra to Rt using sophisticated resistivity inversion mathematics, based on an earth model derived from a short spacing resistivity curve.

Two types of electrode arrangements are used, the Normal device, and the Lateral device.

The electrode arrangement and basic circuitry of the Normal device are illustrated above. Electrodes A and M are on an insulating mandrel, called the probe or sonde or logging tool, which is suspended at the end of the logging cable. Electrodes B and N are placed far from A and M, and are either at the surface of the ground or on the cable at a long distance from A and M. The distance AM is known as the spacing. The depth reference point of the measurement is the midpoint between A and M.

The usual electric log has two Normal devices with spacings of 16 inches (short Normal) and 64 inches (long Normal). The depth of investigation is in the order of the spacing.

For the actual Lateral device, current electrodes A and B are placed on the probe. Voltage electrode M is above the current electrodes, generally on the cable, as shown below. Note that the AB and MN electrodes can be interchanged, with no change in the measured result (the law of reciprocity). Electrode N is at the surface of the ground or on the cable at a large distance above A. The midpoint between A and B is the depth reference point, O. The distance MO, usually referred to as AO on log headings (in honour of the original tool design), is defined as the spacing: it is always several times longer than the span AB. With the usual electric log, the spacing is 18 feet 8 inches, and the span is 32 inches.


Lateral Curve Circuit Diagram. The current electrodes A and B are actually the same electrodes as the A and M for the 64 inch normal and M is the N electrode for the normal curves, switched appropriately for the lateral resistivity measurement by the pulsator. The spacing AO is usually 18’ 8” but other spacings were used. The shape and dimensions of the volume sampled by a Lateral device depend upon the resistivity distribution around the probe. In soft formations, the bulk of this volume is contained in a cylinder with height AB and radius approximately the spacing MO (or AO). The radial depth of investigation is about 19 feet, and the measurement gives the average resistivity of an interval 32 inches thick.

The Lateral curve has strange curve-shape artifacts that reduce its usefulness in formations less than 20 feet thick. Complicated interpretation rules are required for thinner beds. Modern resistivity log inversion software is available, using the 16” Normal for bed thickness control, so that Rt can be calculated from the Lateral curve.

In practice, the Lateral curve, two Normal curves and the Spontaneous Potential are recorded, using a mechanical switch, called a pulsator, to sequentially make the four measurements using only six electrodes (and six wires to the surface).

Electrical Survey (ES) Curve Names
        
 -
Schlumberger and Lane Wells

Notes: * = optional curve.  Abbreviations varied between service companies - common abbreviations are shown as well as the generic abbreviation as used elsewhere in this Handbook.

Curves                                     Units                Abbreviations
 16" normal                              ohm-m             R16, SN, or RESS
 64" normal                              ohm-m             R64, LN, or RESD
 18' 8" lateral                            ohm-m             R18, LT, or RLAT 
* 32" limestone                          ohm-m             R32 or RESM
 spontaneous potential             mv                   SP

OR
 10" normal                              ohm-m             R16, SN, or RESS
 40" normal                              ohm-m             R64, LN, or RESD
 15' 0" lateral                            ohm-m             R18, LT, or RLAT 
 spontaneous potential             mv                   SP



Schlumberger ES Log from 1953. Note neat scale and curve name section (10 inch
and 40 inch normals and 18'8"  lateral)

 

Electrical Survey (ES) Curve Names
        
 -
Halliburton and Welex

Notes: * = optional curve.  Abbreviations varied between service companies - common abbreviations are shown as well as the generic abbreviation as used elsewhere in this Handbook.

* Point Source                           ohm-m               Z, or POINT
* 16" normal                             ohm-m             2Z16", SN, or RESS
* 57" normal                             ohm-m             2Z57", 2Z5', SN, or RESS
* 64" normal                             ohm-m             2Z64", SN, or RESS
* 81" normal                             ohm-m             2Z81", 2Z7', LN, or RESD
* 16' 0" lateral                           ohm-m             3Z16', LT, or RLAT 
*  9' 0" lateral                            ohm-m             3Z9', LT, or RLAT 
* 16' 0" inverse lateral               ohm-m             3iZ16', LT, or RLAT 
*  9' 0" inverse lateral               ohm-m             3iZ9', LT, or RLAT 
* 32" limestone                          ohm-m              4Z32" or RESM
* spontaneous potential           mv                   SP

Note: Halliburton inverse lateral is same electrode configuration as Schlumberger lateral (blind spot at bottom of zone). Lateral and normal spacings could vary. Point resistivity is uncalibrated (even though a scale is shown) and cannot be used quantitatively. The letter "Z" stands for impedance, confirming that these logs were run with AC instead of DC systems.

    
Halliburton ES logs from 1954 (left) with Point, 3Z57, 2Z51, 2Z16 -  and from 1949 (right) with Point, 3iZ9, 3iZ16. Note curve names buried in body of header or in depth track, odd scale on Point Resistivity, and varying curve complement and spacings

 

BED BOUNDARIES FROM ELECTRICAL LOG
Picking bed boundaries on ES Logs requires a bit of thought, as shown below. Resistive beds are too thin and conductive beds are too thick. Beds thinner than the spacing appear conductive, even though they are resistive, and vice versa.


Bed boundary picking on ES log in high resistivity (left) and low resistivity beds (right). Resistive beds on the log appear thinner than true thickness, conductive beds appear thicker, by an amount equal to the tool spacing.


Comparison of ES log with IES log shows two problems that can occur. Note that 64” Normal reads very low resistivity in beds thinner than 64 inches (compare to induction curve in right hand track). In thicker beds, induction may read higher values than 64” Normal in hydrocarbon zones because induction reads deeper (less invasion) than the ES log. There is also less borehole effect on the induction resistivity.
 

A set of rules for picking a value for Rt from ES logs has been available for many years, as reproduced below.
 I have not found it to be terribly useful. Although the lateral curve reads more deeply into the rock than the long normal, it's strange curve shape makes it difficult to use in beds that are less than 20 to 30 feet thick. I use the long normal for beds greater than 5 feet thick and rely on the lateral rules very rarely. Invasion can make the long normal read too low. 

Modern resistivity inversion software can be used to resolve the lateral curve shape problem in many cases.


Rules for estimating RESD (Rt) from long normal (R64) and lateral (R18)

 

ELECTRICAL SURVEY EXAMPLES
EXAMPLE ONE: The illustration below illustrates the standard presentation of ES logs with a gamma ray neutron log of the same era. Curve complement (left to right) is:

      SP – solid 20mv/division
      16” normal – solid 0-100
      64” normal – dashed 0-100
      16” normal (backup) 0-1000
      64” normal (backup) 0-1000
      18’ lateral – solid 0-100
      18’ lateral (backup) 0-1000
       
      Gamma ray – solid 1-11 ugr Ra equiv/ton
      Neutron – solid 120-520 counts/sec (cps)

An amplified short normal was often presented (solid line on 0-10 or 0-5 scale), but is not presented on this example. Electrode spacings were not standard in the early days – normals of 10”, 18” and 60” were common, and various dimensions for lateral curves are found.


Note that the lateral curve has an odd shape and is not very useful for quantitative analysis. There are published rules for obtaining moderately accurate values in thick beds (100+ feet) and less accurate values in thinner beds (20+ feet) but modern resistivity inversion software will do a better job.

The 64” normal, with or without borehole corrections, is often taken as a measure of deep resistivity RESD (or Rt). Resistive beds are thinner on logs than the true thickness, by a distance equal to the tool spacing (16 or 64 inches for normal resistivity curves).


Example of ES log (left) with gamma ray and neutron (GRN) (right). Oil – water contact at 2150 feet is easily seen on short and long normal. Odd curve shape of 18’ lateral makes it difficult to use except with modern resistivity inversion software. Gas – oil contact is inferred from reduced neutron porosity, not from anything on the resistivity log curves. ES logs are obsolete and not run today, but there are 50 years worth in well files waiting for reprocessing by modern inversion software to find new oil and gas. The siblings of ES logs, the micro-resistivity logs and laterologs, are still out there in modern dress, so knowledge of their pedigree is still a part of a log analyst’s education. Colour the oil zone green and the gas zone red. Blue is nice for water and grey for shale seems appropriate.

EXAMPLE TWO: This example shows an ES log compared to the induction conductivity curve (which is more accurate than the ES in high resistivity), contrasted with a microlog.  Shaded intervals are permeable rocks. 


Comparison of ES, IES, and MLC in sand - shale sequence (shaded areas are relatively clean sandstones) - note separation between curves on MLC. Colour the separation bright red and count your net sand. Compare to net sand from SP or resistivity logs.

EXAMPLE THREE: There is no reason to leave ES logs in their original format. When digitized they can be displated on a logarithmic scale to match modern logs or combined with other available curves.

 
Computer presentations of ES logs. SP, 16 inch and 64 inch Normals on linear scale, and GR log from a cased hole run (left) - all the curves available on this well. An alternative presentation of same data with resistivity on a logarithmic scale. Note the high resistivity of coal beds, a nice gas sand near the bottom of the log and a shaly gas sand identified mostly from the GR log in the middle of the interval.

 

Copyright E. R. (Ross) Crain, P.Eng.  email
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