In oil wells, operators must prove the presence of a barrier behind multiple casings, which is expensive as they need to remove pipes and evaluate a single casing. To reduce cost and carbon emissions, we implemented a solution that involved logging through tubing to evaluate both the condition behind the tubing and the barrier behind the second pipe, while satisfying NORSOK D-10 guidelines for single-casing evaluation.

A combination of ultrasonic and sonic tools was deployed within the tubing. The ultrasonic measurements provide detailed understanding of the annular content behind/around the tubing versus depth along with position of tubing in the casing. The sonic measurements, with lower frequencies, have a greater depth of investigation and thereby evaluate the presence of any solids behind the second pipe. Extensive modeling and laboratory experiments of dual-string configurations in our research facility along with multiple sets of field data helped determination of the presence of solids using both ultrasonic and sonic measurements.

Two wells, a water injector, and an oil producer, were intervened via intervention tower in the Norwegian continental shelf (NCS). For the injector well (7-in. tubing and 9 5/8-in. casing), dual-string evaluation results were compared with the first bond log along with extended leakoff test. For the producer well (5 1/2-in. tubing and 9 5/8-in. casing), dual-string evaluation results were compared against a single-casing validation log. The validation log provides a strong degree of confidence in the dual-string evaluation results. Data from the dual-string evaluation and single-casing log will be used to generate a machine learning model for this well geometry, fluids, cement, and formation and is expected to give another high-resolution azimuthal prediction, which would allow for even higher confidence in the answers in later operations with the possible elimination of the need for a validation log.

This paper discusses an offshore case study where the technology was used in two wells in the NCS. The results and observations with dual-string logging technology will be presented in addition to the validation result after pulling the inner tubing. These dual-string log measurements were then used to streamline the sidetracking program, resulting in cost saving due to offline rig activity and minimizing contingency operation, rig time, and scope while reducing overall carbon emissions.

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