Multi-finger caliper tools are widely used for inspection of casing inner wall condition, particularly for determining wear, corrosion, deformation and scaling. The maximum pipe wall penetration, and hence metal loss, inferred from this method is greatly dependent on the casing inner diameter value (provided by casing manufacturer) used as a reference. Evidence of overestimated casing wear, including new completions, has raised the question around the validity of such interpretations.

Most of the casing is manufactured to API 5CT (ISO 11960) standard. The manufacturer‘s specification has an associated tolerance on casing outer diameter and thickness that is then used in the multi-finger caliper logs interpretation. The tolerance for casing with outer diameter (OD) greater or equal than 4.5″, is between -0.5% and +1% of OD. The pipe wall thickness tolerance ranges between manufacturer's nominal thickness and 12.5% metal loss.

The calculation of casing internal diameter (ID) tolerance is a difficult task as this is controlled by the casing mass tolerance which in practice is not easy to measure. To overcome the challenge, we implemented a new approach of calculating the minimum and maximum casing inner diameter and then using the ID tolerance as a reference for multi-finger caliper logs interpretation.

This approach has been tested with field data, and shows good correlation to actual casing inner diameter measurements performed at surface and is used as a baseline for new casing strings.

This paper will describe in detail the approach taken and compare the difference between interpretation outcomes with and without using the casing inner diameter tolerance. The benefits of calculating casing inner diameter tolerance and applying it to multi-finger caliper log interpretation results will be backed up with evidence from field log data acquired in ACG.

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