Magnetic flux leakage (MFL) logging tools work on the principle of inducing a magnetic flux within the casing wall and observing the amount of MFL due to variations in the casing thickness. Commonly, these devices utilize coil type sensors to measure the flux leakage and an electromagnet to induce the magnetic field within the pipe wall. Coil type sensors have an inherent limitation requiring the logging tool to move at a fixed logging speed in order to provide a meaningful measurement of flux leakage. Furthermore, the use of electromagnets to induce the magnetic field requires that the magnet be powered from surface which also has intrinsic problems with power limitations and variation in the field strength.

A new generation of logging tool has been developed that utilizes a powerful samarium cobalt (SmCo5) permanent magnet to induce a high magnetic flux within the casing wall and Hall-effect sensors to detect perturbations in the internal surface flux caused by outer or inner defects in the pipe. Hall-effect sensors do not require a consistent logging speed in order to accurately measure flux leakage. The combination of these two developments now enables defects to be located in areas such as near surface where previous logging speed dependant methods would not work.

An advanced interpretation method has also been developed that uses 3-dimensional images generated from high resolution MFL measurements. This type of measurement provides a clearer picture of the defects within the joint and also the ability to visually analyze and classify the MFL associated with collars, and other hardware. Collars and hardware are commonly ignored using traditional interpretation methods.

Several examples are presented which demonstrate the improvements in logging tool functionality and in the use of 3-dimensional visualization techniques, most notably close to surface and near casing collars.

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