Cement Bond Logging - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 202

If the transit-time measurement is triggered by E 1 , both systems give the same
result. If there is cycle skipping, the two gating systems give different results since
the fi xed gate will read E 1 , which will be small, and the fl oating gate will read E 3 ,
which may be larger than E 1 . The fl oating gate relieves the operator of the task of
fi ne tuning the position of the fi xed gate for changes in casing size and weight or
borehole fl uid, but at the same time it causes erroneously high values of amplitude
in well-bonded pipe.

Deviated Holes and Eccentered Tools

The centering of the tool is very critical for the response of the CBL once the tool is
off center, essentially two paths become available for the transmitted compressional
wave (see Fig. 12.16a ). As a result, the waveform at the receiver becomes “smeared”
and this results in two effects on the log:

1. The measured t will be shortened

2. The measured amplitude will be too low, giving a false indication of good bond-
ing (see Fig. 12.16b )

∆

Fig. 12.16 ( a ) Eccentered tool and ( b ) waveforms both centered and eccentered. Courtesy
Schlumberger