Compressional Slowness Determination Behind Poorly-Cemented Casing (Case History)

This paper presents an important application of cased-hole acoustic dipole waveform logging to determine the formation compressional-wave slowness (DTC) profile in the presence of poor cement bonding (e.g., unbounded casing, free pipe). In Southern Italy, the formations above the reservoir are often characterized by strong borehole instability and the drilled interval is usually cased immediately after drilling to avoid collapse. For this reason, open log data are often unavailable over this interval even though an acoustic-slowness log (i.e., the DTC profile) is desired for seismic correlation purposes. Conventional monopole acoustic logging can determine DTC through casing, if casing and formation are well bonded. However, the evaluation of the cementation around the liner over this interval indicates that most of the logged interval consists of free pipe. This situation allows the casing to ring and this strong casing-signal and resulting poor acoustic coupling to the formation does not permit the standard DTC determination. In this field example, a decision was made to obtain dipole and monopole waveform data over a 900-meter section inside the liner. The goal was to obtain a reliable DTC by combining the results from processing both the monopole and dipole waveforms. The processing has been carried out following these steps: Detailed evaluation of the cementation and broad subdivision of the interval into three zones: free pipe, poor to medium cementation and good cement bond. Dipole and monopole waveform component analysis in terms of slowness coherence quality determination by processing using selected frequency bands. Final DTC determination using optimized procedures and parameters by combining DTC computed from monopole acquisition (good cement bond zone) with the DTC from the dipole acquisition when passing through a poor cemented interval. The main result of this processing is a reliable DTC curve over the entire well including intervals with poor cement bond by combining results of the processing of both monopole and dipole waves. The important result is the demonstration of the ability to obtain an accurate compressional slowness even through poorly bonded casing.