Natural barriers formed by e.g. creeping or plastic shale may provide a solution to plugging and abandonment of old oil and gas wells. In order to qualify a sealing barrier outside the casing, it has to be identified by sonic and ultrasonic logging through casing methods. Rock mechanical laboratory experiments accompanied by ultrasonic velocity measurements on soft overburden shales from North Sea fields have been analyzed in order to provide possible attributes that can be related to shale barriers. Shale failure and plastification is associated with small velocity changes. This means that velocities of plastic and creeping shales may be assessed by e.g. comparison with open hole log data.

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