New Electromagnetic Inspection Device Permits Improved Casing Corrosion Evaluation
Authors: Stroud Stanley G. , Fuller Charles A. 
Published: December 2007

Abstract
A casing inspection tool for detecting damage due to corrosion, electrolysis, wear or other causes is described. The tool is the eddy-current type which measures casing-wall thickness. This instrument, operated by an electric-cable service truck, provides a continuous survey recorded on calibrated chart paper. Laboratory tests show the response of the log to holes, pits and casingwall thinning. The results of field logs are shown and discussed with respect to interpretation and various types of casing damage. This tool has been introduced by McCullough Tool Co. as an aid to the solution of the problem of oil-well casing deterioration.

Introduction
The existence of internal and external casing damage due to corrosion and other causes has been a problem in many oil fields. This factor will become increasingly important as secondary-recovery programs are undertaken in older fields. Prior to the development of the casing inspection tool, there has been no method by which to determine the extent of external casing damage. External damage usually became known when a casing leak developed. Many different methods have been used to reduce or eliminate the corrosion rate of oilwell casing, including cathodic protection, casing coatings, full-string cementing and inhibitors. In particular, cathodic protection has been widely used with apparently good results. However, there is a definite need for a method of periodically measuring the extent of external casing damage to evaluate the effectiveness of these protective measures. The casing inspection tool is a down-hole survey instrument developed specifically for the purpose of detecting the extent of casing damage due to electrolysis and corrosion in oil wells. It is an electromagnetic instrument which is operated on a single-conductor electric cable and records the existing condition of the casing in the well. It is felt that the casing inspection service will be of considerable value in the following applications:for detecting the progress of corrosion and determining the effectiveness of cathodic-protection installations;for logging the casing where a re-drill is proposed to determine if the casing has adequate strength to support such an operation;for logging the casing to evaluate its salvage value; andfor locating a leak and determining the general condition of adjacent casing to plan remedial action such as squeezing or patching.

Principle of Operation
The casing inspection log utilizes a method of measuring the effect of eddy-currents on a magnetic field. In practical form, the instrument consists of two radial coils-an exciter coil and a pick-up coil. Fig. 1 is a block diagram of the complete instrument. The exciter coil is fed from an alternating-current source at the surface, and the resulting magnetic field sets up eddy-currents in the casing walls. 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 pick-up 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.
JPT
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