Over the last five years repeated ruptures of borehole casings connecting underground salt caverns to the surface have occurred.The ruptures pose a high risk to field operation of the caverns used as storage facilities.Apparently,the casing failures may result from lengthening of the borehole casing.At the moment there is no practical method for predicting this kind of casing lengthening and potential final failure.Logging based distance measurements between borehole casing shoe and well head are not sufficiently precise and reference measurements were not carried out.The installation of permanent optical fibre sensors was not considered a feasible option some ten years ago.The idea in this study was to analyse ultrasonic signals from standard cased borehole imager logs-backscattered from the borehole casing-for their frequency content which is possibly influenced by longitudinal borehole casing strain resulting from lengthening of the borehole casing.Standard ultrasonic imagers are usually used for the evaluation of casing-ID,casing wall thickness,and casing-cement connection.The suggested effects of varying frequencies resulting from longitudinal casing strains were expected to be relatively small,but the high data density of the powerful sonic imagers allowed a statistically based attribute analysis.In some of the S-shape boreholes a shift of the frequency content is in fact observed in sections of minimal radii of the cemented casing,which would be expected by increased longitudinal strain.First results and an outlook to further developments of this method are presented.
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