New Electronic Casing Caliper Log Introduced For Corrosion Detection
Authors: Edwards J.M. , Stroud S.G. 
Published: December 2007

Abstract
The electronic casing caliper is a new tool that measures the internal diameter of pipe in a well and records the curve at the surface. Rune with the casing inspection tool, the caliper log determines whether metal loss or casing damage has occurred internally or externally. A number of field examples are presented with suggested interpretations.
Introduction
The electronic casing caliper log has been introduced to complement the casing inspection log. Since the casing inspection log measures wall thickness it soon became evident that a companion tool was necessary to run simultaneously with the casing inspection log and record inside pipe diameter. An attempt was made to run a finger-type caliper device but this was found to be unsatisfactory. This paper describes operation of the tool and relates interpretation procedure as determined by laboratory and field examples.
Principle of Operation
The electronic casing caliper log (Fig. 1) uses a non-contacting tool which measures and records the inside diameter of pipe. casing or tubing. The measuring system is extremely sensitive and small variations of inside diameter are capable of being detected. The electronic casing caliper tool can be run separately or simultaneously with the casing inspection tool and is unaffected by borehole fluid. The tool uses an electronic principle to measure the inside diameter of metal pipe and the reading obtained is a measure of the average inside diameter of the pipe over a length of 1 or 2 in., depending on tool size. Successful logs can be recorded through scale, paraffin or cement adhering to the inner surface of the pipe. The log is particularly sensitive in locating vertical splits or cracks because of an interruption of electrical continuity along the inner surface of the pipe. Holes as small as 3/4-in. Diameter for casing sizes and 1/2-in. diameter for tubing sizes are detectable by laboratory and field investigation. Calibration is performed with casing sleeves which are precision bored to exact inside diameters. A two-point calibration insures accurate and repeatable logs. The curve itself is linear and is presented with a scale of 0.025 in./ 1/4-in. division. All sizes of electronic casing caliper tools are approximately 10 ft in length, temperature rated to 350F and pressure rated to 18,000 psi. Construction and operation of the tools are such that satisfactory results can be obtained with any type of well fluid, Table 1 indicates sizes of electronic casing caliper tools and casing inspection tools currently available to the oil and gas industry.
Equipment
The down-hole tool is illustrated in Fig. 1. Centralizing springs are used at the top and bottom of the tool to minimize wear on the tool housing. It may be run individually or simultaneously with the casing inspection tool which measures casing wall thickness. The surface recording equipment includes a conventional strip chart recorder in the logging unit. The log are normally recorded on a scale of 5 in./100 ft.
Interpretation
Fig. 2 is a typical simultaneous electronic casing caliper log and casing inspection log of 7-in., 23-lb casing. The wall thickness as measured by the casing inspection tool is shown on the right. Casing collars are indicated at 3,527 and 3,570 ft.
JPT
P. 933ˆ