Ultrasonic radial scanner tools provide data to evaluate well cement and casing condition. Specific measurements of interest include internal and external casing diameter, casing thickness, and the acoustic impedance of the material behind casing. Often, the main challenge for ultrasonic radial scanner tool is to distinguish between different materials behind the casing with the same impedance reading; foam cement, light cement, or heavy drilling fluids. Also, since all measurements taken by ultrasonic radial scanner tools are interrelated it is important to view all the individual measurements and gel a complete picture before stating problems with casing or cement. Physical changes inside or outside casing diameter, the thickness of the casing, and even the method of casing manufacturing will cause the cement impedance to read incorrectly.
Described here is the development of three new fast Fourier trails form algorithms that accurately evaluate the acoustic impedance of all cement slurries. The first transform is pan of a unique automatic-calibration process performed by the ultrasonic radial scanner tool to reduce the affect of the casing on the cement acoustic impedance readings. The second transform is used to distinguish between foam cement and other cement types or heavy drilling fluids. The third transform is used in the calculation of the casing thickness., which when used in combination with the internal radius information, results in the identification of the external pipe condition. In one log example, careful external machining of the casing was performed and documented before being run in the well. After the casing was set, the well was cemented and the ultrasonic radial scanner log was run the casing thickness was calculated resulting in the identification of the location and magnitude, both in length and depth, of the machined sections. An additional positive attribute of the developed algorithms is that they are not limited to steel pipes. Another example is presented where the cement bond evaluation was performed on fiberglass casing. The fast Fourier transforms were applied resulting in the evaluation of the fiberglass casing thickness, internal and external pipe measurements, and cement conditions.
Excellent results have been obtained using data acquired during logging operations for cement acoustic impedance evaluation. In this paper we present a series of cased-hole log examples displaying; pipe thickness, foam cement evaluation, and the novel tool self-calibration, all using algorithms with particular re-incarnations of the fast Fourier transform.
Despite the development of sector bond tools (Tello et. al., 1995), segmented bond tools (Lester, 1989) and new ultrasonic radial scanners cement evaluation remains one of the most difficult cased-hole surveys in the industry. Millions of dollars are spent in cement evaluation and remedial work with a hit and miss record at best. Acoustic impedance, the main measurement of the ultrasonic tool, has a linear relationship to the casing thickness (Sheives et. al., 1986). In thin pipe the calculated acoustic impedance of the cement behind casing appears lower than it would be in thick pipe.
