Page 230 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Formation Evaluation 199

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CEMENT BOND LOG VARUBLE DENSITY LOG 7
API Units Amplitude ___L i
lime

Figure 5-122. CBL-VDL log run in well-bonded casing [220].

very strong attenuation due to cement. Strongest attenuation occurs at "A" with
a very weak formation pattern on the VDL. Comparison with the open-hole VDL
(immediately to the right) shows no unusual attenuations of the formation signal.
This also confirms poor acoustic coupling between cement and formation with
good coupling between casing and cement.
Other possible interpretations for this type of pattern are possible.
1. Gas in the mud can be ruled out by examining long intervals of the log.
Generally, this effect will occur over long rather than short sections in
the well.
2. Eccentered tool in the casing, which causes destructive interference of
compressive-wave first-arrivals, can be confirmed by checking for wiggly
casing arrivals or a slight decrease in the casing-arrival time shown on the
total-transit-time curve or VDL log [220].
3. Thin cement sheaths, caused by excessive mud cake thickness along
a permeable formation, are a problem when cement sheaths are less than
3/4 in. thick (which allows stronger casing arrivals). At times, the cement
and formation have a slight acoustic coupling which gives the VDL a faint
or weak formation signal.