Assessing Casing Wear in the Absence of a Baseline Caliper Log

Sawaryn, Steven James (BP Exploration) | Pattillo, Phillip D (Clover Global Solutions, LP) | Brown, Chris J. (BP) | Schoepf, Virginie (BP)



Caliper logs provide valuable information on the shape and wear of casing and tubing strings at various times throughout their operational life. In turn, this information is used to determine the remaining design strength. To clearly distinguish deformation and wear from deviations caused by manufacturing tolerance, the caliper measurements can be compared with a baseline log run soon after a tubular string has been run, or with surface-inspection data. However, a baseline log may not always be available. This paper addresses these situations and provides an assessment of the useful information that one can obtain. A mathematical model, based on the properties of the discrete Fourier transform, is presented to determine the caliper offset center and underlying tubular ovality from six or more equi-angularspaced caliper readings. The series-expansion approximation enables these parameters to be determined as a best fit from raw, uncentered data to a numerical accuracy of approximately 0.01% in a single pass. This is consistent with the accuracy and resolution of the currently available calipers. Complete numerical results from test cases based on exact geometric shapes, such as an offset circle and centered ellipse, plus field examples, are also included along with implementation notes. The same calculations can also be used to determine the underlying elliptic shape and orientation of an openhole caliper. In the casing specification API 5CT (2011), internal dimensions are indirectly described in relation to the unloaded casing or tubing outer diameter and wall thickness at surface conditions. The manufacturing tolerances and resulting uncertainties may be significant compared with the wear, but in some cases one can obtain useful information with corrections for downhole tension, temperature, and pressure effects. Details of these corrections and a discussion of other sensitivities are also provided. Such algorithms are usually considered by the service provider to be proprietary, and little quantitative material has been published on them or their interpretation. Also, data are often presented to the customer in only center-corrected form, which greatly restricts future reprocessing. This emphasizes the importance of acquiring and retaining the raw data.

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