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This paper describes an approach for identifying formations as annular isolation barriers. It also presents details of a novel analysis of cement evaluation and casing integrity data sets as an alternative to casing milling and secondary cementing use in plug and abandonment operations. Recent ultrasonic cement evaluation logs, performed as part of the integrity diagnostics required for plug and abandonment operations, showed an increase in casing ovality when logging through a specific halite formation in the southern North Sea. These findings were used to postulate that the formation was coupled mechanically to the casing and could be used as an abandonment barrier. The circumferential cement evaluation data helped to identify the azimuthal coverage of the formation, and subsequent pressure testing confirmed the integrity of the halite formation as an isolating medium. Multiple offset wells were also analyzed with a focus on identifying additional horizons exhibiting mobility that could be detected using casing logging tools. It became evident that this halite formation showed a consistent and clear correlation between casing ovality and circumferential coverage. Case studies are presented in which the halite formation was identified as an appropriate barrier, based on the combined interpretation of both cement evaluation and casing inspection data. This phenomenon typically occurred when the top of cement was below the halite interval; however, in some cases, the formation movement actually improved the cement bond quality across the zone. By cross-referencing the cement evaluation and casing inspection logging data sets, the mobility of the halite formation can be identified, and subsequent integrity testing confirms its isolating capabilities as an appropriate barrier for plug and abandonment operations. The use of geological formations with suitable mobility as an alternative to a standard milled casing window with a cement plug can reduce rig time and consequently reduce operational costs.

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