The acquisition of high-quality borehole sonic formation slowness data in cased-hole environments is dependent on cement bond quality. It has generally been considered that when cement bond is poor, cased-hole sonic slowness data quality will also be poor. Additionally, in fast formations, when cement bond is good, casing arrivals can interfere with formation compressional arrivals, and in slow formations the acquisition of high-quality cased-hole dipole shear slowness data can be problematic.
The advent of the latest sonic acquisition technology and related interpretation techniques has reduced these formidable challenges. More powerful sonic transmitters, improved receivers, altered transmitter-receiver spacings, and fundamental changes in tool design have meaningfully improved the acoustic signal-to-noise ratio. An improved understanding of cased-hole borehole acoustic modes, the ability to transmit acoustic energies at more optimal frequencies, and the capability to simultaneously acquire cement bond log information have all contributed to improved cased-hole sonic slowness logs.
With the latest technology, accurate formation compressional slownesses can now be extracted in previously problematic cased-hole environments that contained either poor cement bond or fast formation slownesses near to the casing arrival. Additionally, dramatic improvements are evident in formation shear slowness acquisition in very slow formations where previous cased-hole technology had been unsuccessful.
These developments have a profound impact on drilling operations, seismic velocity modeling, geomechanics, and reservoir characterization. In existing cased wells, the latest sonic technology presents a viable option to acquire critical compressional and shear slowness information. In newly drilled wells, the feasibility of acquiring accurate cased-hole slowness information can reduce the necessity and the inherent risk of an openhole logging run.