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LATEROLOG BASICS
The laterolog was put into service in 1949, predating the induction log by 6 or 7 years, as a replacement for the ES Log in salt mud environments. It was another invention by Henry Doll of Schlumberger. Competitive tool designs were called Guard Logs or Focused Logs. The objective was to focus the current from the tool into the rock better than could be accomplished with the ES Log. Tool designs evolved significantly over the years and modern tools are in widespread use in both salt and fresh mud environments.

Laterologs work best in saltier muds or in normal muds in high resistivity formations. They will not work in air filled or cased holes

The laterolog is a direct current (DC) tool based on Ohm's Law. The tools have been designed to produce reliable resistivity measurements in boreholes containing highly saline drilling fluids and/or when surrounded by highly resistive rocks. The logging current is prevented from flowing up and down within the drilling fluid by placing focusing electrodes (A1 and A2) on both sides of a central measure electrode A0, as illustrated below. The focusing electrodes force measure current to flow only in the lateral direction, perpendicular to the axis of the logging device.

 
Schematic diagrams of laterolog 7 (left), laterolog 3 or Guard log (middle) and spherically focused log (right). Grey shading represents desired current path. The Laterolog 7 electrode arrangements can be likened to two ES logs spliced together, with one tool upside down. The center current electrode A0 is in the middle of the current path. Guard electrodes A1 and A2 keep the current focused. On the LL7, measure electrode pairs M1 and M2 straddle the top and bottom current path boundary. The secret is to keep the current flow constant to get an accurate resistivity

There are two major types of laterologs: three electrode guard systems and multiple electrode systems. Guard systems utilize two elongated focusing (guard) electrodes (A1 and A2 and a small center measure electrode A0. Zero potential difference is maintained between the center and guard electrodes during logging. Resistivity is proportional to the potential (voltage) on the center electrode, as shown mathematically later un this Section.

Seven electrode systems have an additional two pairs of small electrodes placed symmetrically on both sides of the center electrode (M1 – M1’ and M2 – M2’). The zero potential difference is maintained between these additional electrodes. Seven electrode systems include the obsolete LL7 style tool.

Bed resolution of the above tools is 3 feet, considerably better than the 64 inch Normal and most induction logs (except the array induction).

In all guard systems, the zero potential difference between the center electrode and the guard electrodes prevents current emanating from the center electrode from flowing along the borehole even when it contains highly saline mud. Thus, the measure current will assume the shape of a cylindrical disc. 

A Canadian company, Roke Oil Enterprises Ltd, developed a laterolog so stable that it could keep a zero potential difference even in cased holes, allowing the measurement of resistivity through casing.

The thickness of this current disc is approximately equal to the length of the center electrode plus one-half the distances separating it from each of the guard electrodes.

The current density varies inversely with the radial distance and can be calculated from:
        1:  Current density = I / (2 * PI * R * T)

Where,
  I = total current intensity (amperes)
  T = thickness of measure current disc (meters)
  R = radial distance (meters)

Resistivity of the formation is:
      2: Rt = K * V / I (same as ES log except K is different)

Where
  V = potential of measure electrode (volts)
  I = current flow from measure electrode (amperes)
  K = a calibration constant defined by the geometry of the electrode spacing

The path taken by the measure current of a laterolog constitutes a series circuit through the drilling mud, mud cake, flushed and invaded zones, and the undisturbed formation. In a series circuit, the total resistivity is the sum of resistivities along the current path.

The pseudo-geometrical factor concept was developed to estimate the influence of these zones on the measured apparent resistivities, in a manner similar to that described earlier for the induction log. Both borehole and bed thickness correction charts are available in service company chartbooks, based on computer models of the pseudo-geometrical factors for each tool design.

MODERN LATEROLOGS
Dual laterolog tools use 9 electrodes. Additional A1’ and A2’ electrodes provide greater guard electrode coverage than a single upper and lower guard. Different depths of investigation are created by controlling the potential on the outermost guard electrodes. Shaded area at the right shows desired current paths.  

The two depths of investigation are recorded alternately so that the currents do not interfere with each other, but quickly enough that both look like continuous logs. An early version of this tool was called a sequential dual laterolog; here a switch could be set while the tool was downhole to select one or the other electrode set, but it took two passes over the logged interval to obtain both logs.

The spherically focused log is also a 9 electrode system, but the electrodes are arranged to place the guards closer to the center electrode, and the equalizing electrodes further away.

The newest laterologs, called high resolution laterolog and azimuthal resistivity log, have replaced those described above. The high resolution tool has 5 depths of investigation, similar to the array induction log presentation. Typical bed resolution is 2 or 3 feet, and the high resolution curve can resolve 8 inch beds.

The azimuthal tool records resistivity in 8 directions radially around the wellbore. In a vertical well, this may not have great impact, but in a horizontal well, it has serious uses. Looking up, the tool might see the cap rock (low resistivity for shale, high resistivity for anhydrite). Looking down, the tool might see low resistivity for water or high resistivity for more pay. Sideways, the tool should be looking at the pay zone. A composite resistivity is also recorded.

The 8 azimuthal resistivities can be presented as an image log, similar in appearance to a resistivity microscanner image. It has coarse vertical and horizontal resolution compared to the microscanner, but is considerably cheaper to run.

LATEROLOG CURVE NAMES

Laterolog / Guard Log  (LL7, LL3)

Dual Laterolog Simultaneous Type (DLL)

Azimuthal Resistivity Log (ARI)

High Resolution Array Laterolog (HRL)

EXAMPLES OF LATEROLOGS
Like all other logs, laterologs have come in many flavours and styles over the years. Below are examples of older logs that you will find in well files from earlier times.

Sample Laterologs, with gamma ray, showing hybrid resistivity scale (left) and logarithmic scale (right) over same wellbore interval. Many varieties of Laterolog are run today, some with a dozen or more resistivity curves.

The hybrid scale was run from 1950 into the 1970's. It is composed of a linear resistivity scale running from 0.0 on the left to 50 or 100 ohm-m in the middle of the track. From the middle of the track to the right hand margin, the curve is actually a linear conductivity scaled from 20 to 0 or 10 to 0 milli-mhos. These two scales are equivalent to a 50 to infinity or 100 to infinity resistivity scales. These combined curves give the hybrid scale a continuous resistivity range from 0 to infinity across one or two tracks. The conductivity curve was also presented on some logs. The hybrid scale was replaced by the logarithmic scale in the 1970's, which may have backup scales because of the high range of resistivity that can be measured with this tool. 

The SP curve may be present, but it may be pretty flat because laterologs were usually run in salt mud. The SP track may be shifted by splicing the film, as the curve was recorded 28 feet off-depth on some tools. Newer logs usually have a gamma ray curve in Track 1 instead of the SP.

Below is a sample of array induction log and an azimuthal resistivity logs, the latter showing the azimuthal image log presentation.

 
Comparison of array induction log (left) and azimuthal resistivity laterolog (right). Curve complement and presentations vary considerable with age and contractor. The image log on the azimuthal resistivity presentation is "poor man's" resistivity microscanner log, giving a reasonable sand count  regional dip, and some fracture information. A real microscanner image is shown for comparison (left image).

High reso;ution laterolog showing deep invasion and high resolution image.
All curves are focused to about 8 inches.

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