This paper will share the findings of time-lapse monitoring from two corrosion surveys conducted four years apart in the subject well; present a new processing methodology that improved metal thickness estimation and yielded better results when applied to data from two legacy wells; and describe a novel surveillance tool of unique design that was also deployed in the subject well. The positive and encouraging results achieved using this tool will also be discussed.

Electromagnetic pulse surveys were conducted in 2016 and 2020 to evaluate independently the metal loss in three casing barriers. The 2016 analysis involved a simplistic data processing method. A more sophisticated processing technique was recently applied to both surveys (2016 and 2020). This new method estimates the thickness for each barrier through forward modelling based on the numerical solution of the Maxwell equations. A newly introduced electromagnetic tool was also run in combination. This is a unique approach because it provides a segmented electromagnetic metal thickness evaluation of the first barrier without the need for pad contacts with the casing wall.

The simplistic processing from 2016 assumed, for each barrier, a baseline for the tool readings that corresponded to the nominal casing thickness. It then translated the deflections from this baseline into a metal loss or gain. These figures were output only when they exceeded the tool's accuracy. The advanced processing that was used in 2020, which is based on forward modelling, estimated less metal loss in general in comparison with the 2016 survey. These results agreed with the segmented tool estimations for the first barrier, which was run in combination in 2020. This confirms the methodology's robustness and accuracy. In addition, this new method outputs metal loss figures at every depth point regardless of the tool accuracy. The new processing was applied to previously acquired data sets in two additional wells in the same field, and the obtained results were very satisfactory.

The new tool, which provides a segmented electromagnetic metal thickness evaluation, also delivered exciting results by providing accurate thickness estimations in eight circumferential sectors of the casing wall without pad contact. This constitutes a substantial improvement over the existing all-round and averaged measurement offered by conventional electromagnetic tools. These segmented results enabled the client to make a better-informed decision about the well and to postpone an expensive workover.

This paper confirms the necessity of time-lapse surveys for monitoring the integrity of downhole tubulars. It also proves that numerical solution of the Maxwell equations through forward modelling of acquired electromagnetic data yields robust and more accurate thickness estimations than the previously used methods. Finally, it demonstrates the effectiveness of the new segmented and contactless electromagnetic tool for assessing the first casing barrier.

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