The case study presented describes how successive leak detection logging surveys defined, first the remedial workover plan, and subsequently evaluated the results of the different cementing stages in the workover operation, in a well with gas bubbling at surface.

The subject well evidenced gas bubbling in the cellar, right behind the conductor. None of the annuli exhibited sustained pressure. Therefore, the source and path of the leak was unknown. A tool string including passive spectral acoustic and high precision temperature modules was utilised to define both the source and path of the leak. This diagnostics was performed for remedial workover planning. Subsequently, during the remedial workover, additional logging surveys were utilised to evaluate the isolation operations.

As a result, all reservoir contributing zones were located and the leak path was defined. This information enabled a targeted workover plan pinpointing the most suitable zones for cementing stages, which also included three confirmation surveys at the end of each cementing stage to evaluate the isolation achieved. The first two surveys indicated the success of cementing operations. After the completion of all cementing stages, before the last logging run, gas bubbling at surface re-started. The final survey evidenced an acoustic response compatible with the flow of residual gas trapped behind the casing, which was deemed as the possible cause of this bubbling. This scenario of flow of residual gas was confirmed after one month, when gas bubbling at surface ceased, which proved the success of the workover operation.

This diagnostics, conveyed through tubing in a well filled with gas, allowed for the identification of the source and path of a leak taking place entirely behind casing strings, with no communication with the wellbore. It enabled an effective and efficient remedial workover, allowing for targeted workover planning, as well as online evaluation of remedial operations for decision making, which ultimately led to workover objectives being successfully achieved at minimum cost and time.

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