Field Results of the Electromagnetic Casing Inspection Log
Authors: Edwards J.M. , Stroud S.G. 
Published: December 2007

The casing inspection tool was introduced to the oil industry in 1960, and since that time a number of logs have been recorded in various areas of operation. The purpose of this paper is to report results thus far obtained with this corrosion-detecting tool. To properly evaluate results of interpretation procedures as developed through field use, interpretation principles as a result of laboratory and field experience, actual field results in various areas and recommended operating procedures. Examples of various casing inspection logs are presented along with follow-up well information necessary to properly evaluate results of interpretation techniques.

Principle of Operation
The casing inspection log uses a method of measuring the effect of eddy currents on a magnetic field. The instrument consists of two radial coils, an exciter coil and a pick-up coil. Fig. 1 is a block diagram of the instrument. The exciter coil is fed from an AC source at the surface, and the resulting magnetic field sets up eddy currents in the casing wails. These eddy currents cause the magnetic field to be attenuated and shifted in phase, and the resulting magnetic field is detected by the pick-up coil. The pickup signal is amplified and transmitted to the surface by modulating a 3 kc carrier. In the surface instrument the carrier is separated from the exciter voltage, amplified and demodulated. At this point the signal is a true reproduction of the bottom-hole signal from the pick-up coil. The phase of this signal is compared with the phase of the exciter voltage, and the resulting phase shift is recorded on a strip-chart recorder. The theory of eddy currents indicates that phase shift is determined by four factors: casing wall thickness, frequency, magnetic permeability and resistivity of the metal. The basic phase-shift equation which follows shows that the phase shift is directly proportional to casing wall thickness.
(1)
where phi = phase shift, D = depth, mu = magnetic permeability,
p = resistivity in micro-ohmcm, andF = frequency in cycles/sec.
The factors of magnetic permeability and resistivity of the metal are practically always unknown for any joint of casing, and logs run thus far indicate that these factors vary considerably among types of casing and manufacturers. There are also indications that stresses placed upon casing as set in wells seem to affect magnetic permeability of the metal. This problem is minimized by using a reference joint to arrive at a metal thickness scale, and using that reference joint for all subsequent logs recorded in the same well. To detect small corroded areas, a differential pick-up coil is used. The log obtained is linear with respect to wall thickness, with an exaggerated indication of small non-conformities.

Equipment
The down-hole tool is illustrated in Fig. 2. Centralizing springs are used at the top and bottom of the tool to minimize wear on the two coil housings. The tool has been designed to withstand normal well temperatures of 350F and pressures of 20,000 psi. It has been operated in wells to 13,500 ft. The construction and operation of the tool are such that satisfactory results can be obtained with any type of well fluid. In the surface control panel the phase angle is measured by a transistorized phase detector. A linear output voltage which is proportional to the phase angle is produced to drive a strip-chart recorder pen in the surface equipment.
P. 377ˆ