An analysis was undertaken to appraise the effectiveness of using the current logging program to evaluate both casing damage and the cement sheath for a major offshore field. The standard program consisted of a Conventional Bond Log (CBL) and rotating ultrasonic tool run in tandem. Four different casing sizes were evaluated, running from 5.5-in., 23 lb. to 9 5/8-in., lb. and including several different alloys ranging from standard steel to chrome alloys. These logs were acquired over several years. The suite of logs involved upgrades to both the acquisition software and to the rotating ultrasonic tools. These changes affected both the casing inspection and cement evaluation modes.

The overall correlation between the rotating ultrasonic log and the CBL log are excellent as shown in exa mples that will be provided. These logs demonstrate how two different cement evaluation tools complement each other to provide a detailed and complete zonal isolation study. The rotating ultrasonic may indicate channels while the CBL may not provide a direct indication of the channel.

Likewise, the rotating ultrasonic shows excellent repeatability in both casing inspection and cement evaluation data. Casing inspection consists of both internal diameter and casing thickness. While the casing size will affect the cement evaluation, the rotating ultrasonic tool requires careful head and frequency selection to optimize the casing inspection mode. The repeatability and accuracy of the casing inspection data is astonishing and exceeds that of any other casing inspection device in use today.

Several logs that will be described have questionable features, but some of these features appear to be caused by well conditions. In particular, it is believed that casing in contact with either the formation and/or outer casing string caused erroneous readings in both the cement and casing inspection data. This perceived wellbore artifact also appears to create errors in the CBL for cement evaluation. Casing centralizers and/or lack of centralizers are easily recognized on both the casing inspection data and cement evaluation.

In another case, improper head selection for the ultrasonic device generated an erroneous log. Changes were made to the real time quality control process on the device to prevent erroneous logging from happening thereafter.

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