Advances in Cased-Hole Compressional and Shear Slowness Sonic Acquisition and Processing
- Douglas R. Murray (Schlumberger S.A.) | Takeshi Endo (Schlumberger) | Shinichi Sunaga (Schlumberger) | Yanhua Li (Schlumberger) | Janarne Voskamp (Schlumberger) | Olivier Desport (Schlumberger)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 19-22 September, Florence, Italy
- Publication Date
- Document Type
- Conference Paper
- 2010. Society of Petroleum Engineers
- 3 Production and Well Operations, 1.2.2 Geomechanics, 5.6.1 Open hole/cased hole log analysis, 1.14 Casing and Cementing, 4.1.2 Separation and Treating, 1.6 Drilling Operations, 4.1.5 Processing Equipment, 5.1.8 Seismic Modelling, 5.1 Reservoir Characterisation
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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.
Acoustic measurements are used throughout the lifetime of a well, from seismic to production. The extraction of compressional and shear slowness in open holes is relatively straightforward; however, in cased holes, the acquisition and interpretation are more difficult because the waveforms are more complex. In the case of poor cement, acoustic coupling to the formation is reduced, which results in attenuated formation signals, strong casing arrivals, and thus sonic measurements with a poor signal-to-noise ratio (SNR).
Cased-Hole Sonic Logging
There are multiple issues associated with acquiring good-quality cased-hole sonic logs.
For monopole waveforms, the key issue is cement bonding. Generally, with an array sonic tool in well-bonded cemented casing in fast to slow formations, one can usually acquire accurate formation compressional slownesses. Under these well bonded conditions, there is an efficient transfer of energy from the formation, and only weak casing modes are present. Poor cement bonding will equate to reduced compressional coherence, and an environment of no casing cement will almost certainly result in an inability to acquire formation compressional slownesses behind the casing.
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