The earliest collar locators were whisker like devices comprised of stiff wires attached to the cable with the free ends looking up. This arrangement could go down the hole smoothly, but the wires would catch in the gap between joints of pipe at the collars (couplings) when pulled up. The operator could detect the collar by the increased tension on the cable. This technique is still occasionally used for shallow "poor boy" shooting operations. This technology evolved into the sonic collar locator where sound made by the feeler wires was analyzed. The operator listened with headphones and an audio amplifier for the sound change when the feelers crossed the gap at the casing collars. The modern magnetic casing collar locator (CCL) was developed in the late 1940's and quickly displaced the earlier methods. The old royalty structure for the magnetic CCL was curious; if only a CCL log was run, no royalty fee was due, but if the CCL was run in conjunction with a perforating gun or nuclear log, then there was a royalty charge.
While the modern CCL tool is a relatively simple device, it is one of the most important tools used in cased hole logging. The CCL is a magnetic device, sensitive to the increased metal mass at a casing or tubing collar. It is almost always run with cased hole logs, and is the primary depth control log. The CCL can only be effective as a depth control log if it is accurately tied in to open hole logs, usually using cased hole
gamma ray and/or
neutron logs to facilitate the correlation.
Magnetic fields are visualized as lines of flux around a magnet. When an electrical conductor moves through lines of magnetic flux, a small current is induced in that conductor. If the wire is in the form of a coil so that it crosses the lines of flux many times, the current generated is increased in direct proportion to the number of turns of wire in said coil, a classic electrical generator.
Coils of fine magnet wire with thousands of turns are used in the CCL to obtain a current strong enough to be detected by the surface equipment. If the coil of wire is in a magnetic field and the lines of flux are distorted, current is generated just as if the wire was moved through the lines of flux. In the CCL, the increased metal thickness at collars causes just such a distortion in the lines of flux around the
CCL magnets, and thus creates the small current that ultimately is displayed as the familiar collar "kick" on the log.
The most common current CCL design consists of a
coil located between two magnets. The magnets are cylindrical with like poles facing each other (north to north or south to south) to focus the lines of flux outward in a plane perpendicular to the axis of the CCL tool. This like-pole facing each other arrangement produces a CCL more sensitive to small changes in metal mass, creating a better chance of detecting the small gap in flush joint pipe, or collars in heavily corrosion-encrusted casing. Many other CCL designs exist; two coils arranged on either side of a single magnet is probably the next most common arrangement. In addition, there are many exotic designs that have come and gone over the years. The AnaLog Services, Inc. collection even includes a bizarre beast with 14 magnets and six coils.
As mentioned above, the CCL log is used for depth control and depth correlation. In deeper wells, operators sometimes install one or more short casing "pup joints" as recognizable depth markers. Because perforations in casing or tubing can frequently be detected with a CCL tool, the CCL log is sometimes run for verification or proof of perforating. The CCL log will also sometimes "see" damaged casing if there is enough change in the mass of metal to cause a significant distortion in the magnetic lines of flux. Holes in casing can often be identified, and parted casing shows up beautifully.
Logging CCLs are equipped with downhole amplifiers. Sometimes logging CCLs are built into tools, and sometimes they are used separately as is the case with most cement bond tools. There are good and bad
downhole CCL amplifier designs, and AnaLog frequently replaces and/or upgrades downhole CCL amplifiers for our customers. The CCL log is generally displayed between tracks one and two, sometimes called the depth track, and usually to the right of said depth track, on the suggested American Petroleum Institute (API) log format. However, the CCL record is occasionally displayed elsewhere on certain specialty logs.
Shooting CCLs contain no amplifier and are most commonly run for depth control with perforating guns. Because the voltage generated by a shooting CCL presents a potential, although extremely remote danger when electric blasting caps (detonators) are located below the CCL, it is often said that the diodes used in the shooting CCL circuit are there for safety purposes. In truth, they are used because the old non-resistorized caps would short out the CCL signal. Modern resistorized caps probably do not need said diodes, and it is generally believed that a shooting CCL cannot accidentally fire a cap. Nevertheless, it is still a darn good idea to use the blocking diodes, and have them installed in the proper orientation (they are absolutely necessary when using non-resistorized caps). To be able to shoot with both polarities, a "double diode" is required with the pair in parallel and turned in opposite directions. A few designs use pairs of diodes in series frequently called a quad diode, again often said to be for extra safety, but in actuality probably to prevent clipping of the CCL signal in cases where it exceeds 0.6 volts (the "turn-on" voltage for a typical silicon diode).
Always check your protection diode device before every shot, and always verify that the shooting CCL is oriented with the correct end looking at the perforating gun.