Cement Bond Logging - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 205

Fig. 12.19 Casing OD weight and thickness

Partial Cementation

The case where a section of pipe shows an amplitude higher than the minimum
amplitude read in well-cemented pipe raises the possibility that a hydraulic seal
does not exist. To quantify the analysis, a bond index is used:

( )

Attenuation in suspectzone dB / ft

Bond index =

( )

Attenuation in well cemented zone dB / ft

Experience shows that if the bond index is at least 0.8 a seal can be expected, pro-
vided the poorly bonded section is suffi ciently long. Figure 12.20 shows that the
larger the casing size, the longer the interval needed at or above the bond index = 0.8
condition for a seal to be assured. A rough rule of thumb is to double the casing size
in inches and subtract 5 to give the number of feet of section required.