Matching pursuit algorithm for cement evaluation behind the casing

Abstract

The oil and gas sector makes extensive use of ultrasonic tool technology for casing and cement examination for well integrity. This requires the determination of the thickness of the casing, its internal structure, and the mechanical properties of the cement behind the casing. In essence an ultrasonic instrument analyses the ultrasonic pulse echoes returned off the internal surface of the casing. To determine the acoustic impedance of the material behind the casing, it can be difficult to distinguish between the primary reflection from the interior surface of the casing and the ones due to the ringing inside the casing wall itself. The primary reflection offers details regarding the internal geometry of the casing, and the decay of the energy within the casing wall reveals more about how well the cement is bonded to the casing. The ultrasonic signals that the transducer receives when it functions as a receiver are easily contaminated by noise, or part of the reflection may be blocked, or the main waveform and the many ringing waveforms are difficult to separate. As a result, the echo signals will have frequency and phase drift or be deformed. To address the various energy packets and extract the pertinent component independently, a matching pursuit algorithm was developed with a specific dictionary to deal with the different energy packets and extract the relevant component separately. The algorithm was tested on simulated and field data.