Caliper Logging Overview

A dizzying number of variations of well logging calipers are available.  For purposes of this discussion, they are grouped into two major categories:  (1) Multifinger Tubular Inspection Calipers, and (2) Conventional Calipers (acoustic based and other non-mechanical caliper technology is beyond the scope of this effort).  While conventional calipers are capable of gross casing or tubing inspection, the first group includes those multiple feeler calipers designed to generate casing or tubing profile data.  See Caliper History for the story of the development of well logging calipers.

Multifinger Tubular Inspection Calipers

Multifinger mechanical calipers are among the simplest and most accurate measurement tools for the inside condition of pipe.  They are principally used to evaluate wear and corrosion.  The tool is centralized with an array of fingers or feelers that reach out to the inside wall of the tubular goods under investigation.  There are many types of multifinger calipers used to profile tubing or casing, including pump down devices, slick line devices, and electric wireline devices.  Pump down and slick line devices are still available in purely mechanical versions, but computer memory technology is becoming more common.  Multifinger calipers are made with as few as 12 fingers on some tubing calipers, and up to 80 fingers or more on some casing calipers.  Multifinger calipers are run as a specialty service; the balance of this page is devoted to conventional calipers.  See Kinley Corporation's website for more information on mechanical multifinger calipers, and see the Sondex website for some state-of-the-art multifinger caliper technology.

Conventional Calipers

Conventional calipers are used to make a continuous recording of the gauge and rugosity of a wellbore.  Conventional calipers are available in one, two, three, and four arm versions (a few exotics have been made with six or more arms).  As a generalization, one and two arm calipers tend to measure the maximum dimension of a non-round hole, while three arm calipers tend to measure something closer to the minimum dimension.

Since the 1940's, calipers have been an important part of open hole logging.  A caliper may be used to center or eccenter a tool string (sometimes also used in the same fashion with cased hole tools); a caliper-like arm (pad) may carry sensors or a radioactive sealed source.  A caliper is of great importance in the determination of the validity of open hole logs.  Modern technology can use the caliper to correct certain open hole curves.  The savvy petrophysicist may even be able to draw conclusions about where net productive intervals are located since mud filter cake buildup occurs across permeable zones resulting in under gauge caliper readings (depending on arm tip type).

But calipers are the most fun to run in shot holes or open hole completions.  In this application, the caliper can be regarded as a cased hole or production logging tool.  It is interesting to compare a shot hole caliper log to lithology logs since the softer and harder zones within the productive interval are often easily recognized (the softer material tends to cave more after the shot, leaving a bigger hole).

The arm movement of a well logging caliper must be translated into usable data.  Early caliper designs were either purely mechanical or used resistive elements.  There have been dozens of schemes over the years, including elaborate mechanical contrivances, hydraulic pressure transducers, magnetically altered coils, mechanical movement of internal vacuum tube elements, magnetic flux gates, magnorestrictive elements, and on and on.  While there are still some exotic caliper transducer schemes floating around, most modern calipers utilize a variable resistor a/k/a potentiometer or "pot" somehow connected to the movable arms in both dc and pulse tools.  Comprobe offers an interesting magnetically coupled bow spring caliper that can double as a centralizer, as an example of one of the more unusual calipers presently manufactured.

Caliper Applications

Caliper measurements generally give some idea about the mechanical condition of a well or borehole, including dramatic representations of formation washouts and swelling clays.  The following is just a sampling of caliper applications:

  1.  Determination of cement volumes for well construction or plugging / plugback purposes.
  2.  Determination of gravel volumes for gravel pack jobs.
  3.  Determination of optimal locations to seat packers (as in well testing).
  4.  Determination of optimal directional drilling kick-off points.
  5.  Determination of optimal perforation points.
  6.  Evaluation of scale build up.
  7.  Evaluation of well shots (high explosives shots and propellant shots).
  8.  Evaluation of acid jobs in open hole (before and after surveys).
  9.  Evaluation of hole conditions prior to fishing.
10.  Location of parted or collapsed casing.
11.  Location of liners and casing size reductions.
12.  Correction of various flow profile logs (and many open hole logs).
13.  Demonstration of bit gauge compliance.

It is arguable that a caliper should always be run as part of a logging suite.  See this Cardinal Surveys page for more interesting caliper notes.

DC vs. Pulse Calipers

Resistive or "dc calipers" are still in use for open hole / mineral logging, and to a lesser extent for cased hole logging.  The Dowell / Worth Well / Bell solenoid release caliper is a classic example of a resistive dc caliper.  The Dowell calipers were made in 2-1/2 inch and 3-1/2 inch diameter body sizes, and offered interchangeable arms for maximum logging diameters of 12, 24, 36, 48, and 60 inches.  The 1-1/4 inch Well Reconnaissance, Inc caliper was originally manufactured as a resistive caliper, with early pulse circuits for said tool being little more than an afterthought.  Depending on how dc calipers are configured, they can be subject to some drift; the ground path should preferably not be the armor of the cable.  Further, in deep or geothermal hot environments, the resistance change of the logging cable copper conductors can actually become an issue.

Pulse calipers began to appear in the 1950's.  A pulse caliper produces a pulse train with a frequency proportional to arm extension.  Early pulse calipers used several novel schemes to vary the frequency of vacuum tube based oscillators of the time, but most modern pulse calipers simply use a pot actuated by the movable arms to vary the voltage fed to a voltage-to-frequency circuit of some type.  Pulse calipers typically have better resolution than dc calipers, but that is not necessarily true.  Pulse calipers are immune to variations in line resistance, so may offer advantages in geothermal or deep logging, though it should be noted that Dowell dc calipers have been used to depths of 23,000 feet.  All things considered, we prefer pulse calipers for cased hole type work.

X-Y Calipers

Four arm calipers that record separate measurements from each of the two opposing sets of arms are commonly called X-Y calipers.  When used on single conductor electric wireline, one axis is transmitted up the line as positive pulses, while the other axis utilizes negative pulses.  X-Y calipers give a rough idea of hole shape, and can be used to make somewhat more accurate estimations of hole volume than with a single axis caliper.  If a hole were a true ellipse, then an X-Y caliper would give an accurate reading of both the minor and major axes of the ellipse, but real world boreholes are never truly round or truly elliptical.  Borehole volume computer modules have been manufactured for use with X-Y calipers, and modern computer logging systems are capable of making these same calculations.

Geometry / Linearity

A basic problem with armed calipers is that they do not make linear measurements.  The displacement of the measuring element in the tool (usually a pot) is not linearly proportional to hole diameter.  In fact the relationship between hole diameter to transducer displacement is that of the sine / cosine of the angle of the caliper arm to the body of the tool.  The nature of the sine / cosine function is such that the measurement is reasonably linear out to an arm extension of something less than 45 degrees, becoming increasingly non-linear as arm extension approaches straight out, or 90 degrees.  Most designs physically restrain the arms so as not to extend much past 60 degrees.  Many attempts have been made to linearize calipers, both electronically and mechanically.  Elaborate mechanical linearization schemes have included articulated multipart arms with compensation slots roughly "S" shaped.  Attempts to utilize special sine / cosine corrected potentiometers have proven to be economically infeasible.

Most of the more common calipers, like the classic 1-1/4 inch Well Reconnaissance, Inc. tool (also marketed by Gearhart-Owen, until GO bought Well Recon in 1979), have no mechanical linearization.  Further, they use a linear resistance transducer (Linipot is a trade name for these expensive little critters), so non-linearity is inescapable.  The early pulse version of the Well Recon / GO calipers is based on a simple unijunction circuit that is horribly nonlinear and must be upgraded to get decent caliper logs.  The later D5K1 based pulse circuit uses a 100 kilohm Linipot with a 10 kilohm offset trimpot to ground, and a 500 kilohm linearization pot feeding 10 volts to the Linipot.  Because the Linipot acts as a voltage divider, and voltage dividers are inherently non-linear, the series 500 kilohm trimpot allows adjustment of the divider non-linearity.  Unfortunately the divider non-linearity is an exponential function and not the sine / cosine correction needed.  Nevertheless, this approach can render the Well Recon / GO calipers extremely linear out to about 24 inches with the long 14 inch arms and out to about 14 inches with the short 6 inch arms, and pretty decent out to around 60 degrees of arm extension.  Try 300 to 350 kilohm for a trial setting of the linearization trimpot.

AnaLog Services, Inc. has a "trig chip" linearization circuit that produces a perfectly linear caliper from fully closed to maximum arm extension for any caliper.  But the best linearization solution is a computer logging system; they are wonderful for linearizing calipers, and solve the problem so painlessly (the more calibration points the better).

Some Pointers

Caliper logs with pulse type tools are normally run with a common ratemeter.  The time constant (TC) should be kept short, certainly no more than one (1) second for acceptable log detail (we have installed a 0.5 second time constant position in GO / MLS ratemeters for customers doing caliper logging).  Calibration should be done with rings or multipoint winged linear calibrators; stick type or one arm calibrators tend to cause calibration errors.  When running a casing collar locator (CCL) with a caliper, run the largest size practical, since it will tend to centralize and give weak readings.  If you have a temperature panel that is scalable like the SIE temperature panel, you can set it to display hole size (very impressive to customers).  Line audio is handy when opening and closing motorized calipers (we offer inexpensive line audio boxes that are fun to listen to and also impressive to customers).

And finally two tips from Bill Hawkins with Comprobe.  Bill rightly observes that if the springs in a caliper will not lift it off the floor, they are too weak.  Bill also tells of a nifty log presentation trick for caliper logs:  Use two ratemeters and set one to "reverse" and one to "normal", with all other settings identical; using two pens, this arrangement will draw an actual picture of the well!

The foregoing is an oversimplified discussion of the caliper log; most theoretical background and interpretative information has been omitted.  Contact us if you need more detailed information; we have thousands of pages on the subject.

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Last 10-20-10