The foundation technology for well logging calipers can be seen in early patents for boiler tube measurement devices designed to measure scale buildup. Believe it or not, dozens of patents have been issued for well calipers. In 1868, a caliper patent was issued to John D. Dale for "discovering fissures in the sides of wells." Dale's device was run in the well on a pole, and said patent specifically mentions oil wells. In 1914, a patent entitled "Tester for Blast-Cavities" was issued to Joseph A. Houston. Houston's caliper was suspended in the well with threaded joints of pipe. In 1920, a patent was issued to Jean Amedée Hardel for a device to line oil wells with cement. Part of the 1920 patent was a caliper to be used to determine the amount of cement required for the lining operation, perhaps the first caliper to be run on flexible cable. This early caliper contained an internal chart recorder, and spring loaded arms with wheels rolling against the borehole wall. The arms were not retractable (unnecessary with wheeled arms), and used an interesting cable arrangement to transmit the arm movement to the recording mechanism.|
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In 1929, a patent was issued to James J. Cabot entitled "Cave Finder", perhaps the first caliper to be run on electric wireline. Mr. Cabot wished to provide a device whereby drillers or operators could determine the actual depths of caves in a borehole (washouts in rotary holes or simple caving areas in cable tool holes). This device used a spring loaded articulated arm comprised of three separate elements. The arms were not retractable, and the articulated design allowed logging up or down. The arms were connected to a switch that opened when the arms extended past a set point. An incandescent indicator lamp would extinguish at the surface when the Cabot device found a cave.
Myron Macy Kinley was a pioneer in wild well control, fighting blowouts around the world for over 50 years (he trained "Red" Adair, "Boots" Hansen, and "Coots" Matthews). Among Kinley's many inventions was an open hole caliper patented in 1937; it was sold to E. P. Halliburton, and became known worldwide as the "Halliburton Open Hole Caliper". The original Kinley caliper used spring loaded arms and an internal recording scheme. The arms were released when the device hit the bottom of the hole (it may have been made in an x-y axis configuration as well as a simple single axis version). Halliburton began commercial caliper logging in 1938; winning a lawsuit with Schlumberger put Halliburton in the logging business. In 1941, Kinley was issued a patent for an improved "Surveying Caliper." The 1941 version used spring loaded two piece articulated arms, and was run on electric wireline. The arms were connected to variable resistors, and thus was born continuous surface caliper recording. In 1954, Myron's son Jack was issued the first patent on a multifinger mechanical caliper for casing and tubing inspection (electrical tubing wear calipers date to at least the Arthur H. Brandon patent of 1943). Kinley Corporation still offers slickline conveyed calipers, and claims to have surveyed over 250,000,000 feet of tubing, casing, and flowlines with their calipers since 1954.
Caliper development continued through the 1940's, driven in part by the new dipmeter log concept. Halliburton also acquired the 1944 Finley T. Robidoux caliper patent. The Robidoux device more closely resembled a modern caliper tool with conventional spring loaded arms that could be held closed while entering the well, then released at will. The original design used an explosive squib to break a band that retained the arms. Later an electromechanical latch was used (solenoid), a design that survived into the 1980's in the Dowell / Worth Well / Bell caliper (said Bell caliper is the large caliper with a bulbous extension shown in the background on this page). The Robidoux caliper used a single conductor logging cable (the patent even details a cable head) and was a resistive device.
A 1947 patent issued to Everett A. Johnson, and assigned to Standard Oil Company, describes the use of a caliper to correct electric logs. The Johnson device is interesting in that the arms were released by jarring the tool. All conventional arm calipers with release mechanisms (predating the motorized arm calipers) suffer from having to be withdrawn from the well for arm closure before deeper logging can be performed. An interesting 1950 patent was issued to Huber and Cannon, and assigned to Standard Oil, for a mud cake caliper. This gizmo used sharp wheels on one set of arms, and pads on the other set in an attempt to determine mud filter cake thickness (to infer the locations of permeable zones).
Much research and a few patents were issued for various pulse telemetry systems for logging tools through the 1940's. In 1953, two patents were issued to Ralph W. Goble, both assigned to Eastman Oil Well Survey Company, for pulse calipers. The electronics in these tools was of course vacuum tube based, and used a magnet to alter the inductance of a coil in an oscillator circuit in one case, and a movable cathode tube (RCA transducer triode, C798-B) in the second case. A number of additional schemes were devised to create a pulse train with a frequency proportional to the arm extension.
By the middle 1950's trial and error resulted in calipers mechanically similar to those we use today. During the late 1950's and into the early 1960's, several patents were issued for mechanisms to both open and close caliper arms downhole. Gasses released by burning powder charges were proposed, as well as complicated hydraulic systems complete with downhole motors and pumps. Of course motorized calipers eventually became the standard, with virtually every manufacturer of logging tools offering versions over the course of the last few decades.
Famous calipers I have known and loved include the old Dowell / Worth Well / Bell solenoid actuated unit made in both 2-1/2 inch and 3-1/2 inch sizes (the latter rated to 60 inch hole measurement). Bell literature from 1978 claims 50,000,000 feet logged with these tools to depths of 23,000 feet. AnaLog Services, Inc. has many of these vintage tools as well as a stock of parts for these old workhorses. The 1-1/4 inch Well Reconnaissance, Inc. caliper remains probably the most common three arm caliper in use by independent loggers, being manufactured for decades. It was made in a resistive version originally, then in a unijunction transistor pulse version, and finally in several more sophisticated solid state pulse versions. It was marketed by both Well Recon and Gearhart-Owen, until GO bought Well Recon in 1979. There are many SIE calipers still in daily use, as well as many Comprobe calipers, especially the one arm Comprobe caliper seen in many open hole / mineral logging tool strings.
Caliper Logging Overview for more information on caliper surveys.