This article presents the key elements for Cement Bond Log Interpretation (free pipe, good bond, hard formation, micro annulus)
Free Pipe
Free Pipe is characterised by the following:
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CBL / VDL |
CET |
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High CBL readings > 50% |
White on log |
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Parallel lines on VDL vs. Depth |
CSMX* & CSMN* are zero |
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Weak formation signals |
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VDL shows clear “w” pattern at collars |
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No change in transit time with depth |
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White on VDL log |
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*CSMX & CSMN are the Maximum and Minimum Cement Strength respectively.
Good Bond
AGood Bond is characterised by the following:
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CBL / VDL |
CET |
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Low CBL readings (m Amp) below 5 |
Black on log |
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Casing signals very weak |
High CSMX & CSMN |
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Strong formation arrivals on VDL provided formation attenuation is not too high |
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Formation arrivals change pattern with depth. Casing arrivals show as parallel lines |
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May be a correlation between GR and VDL in shales |
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Hard Formations
Hard Formations such as anhydrite or dense limestone, behind the casing, are characterised by the following:
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CBL / VDL |
CET |
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Formation arrivals precede the casing arrivals |
May show as free pipe but they will be flagged |
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Can be identified on the VDL |
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Micro-annulus
In the following two cases, the CBL / VDL shows up as poor cement. The CET must be used to differentiate between the cases. This is important since channelling would be considered a failure whereas the existence of a micro-annulus is unlikely to cause problems.
Micro-annulus should not be considered as a failure and is characterised by the following:
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CBL / VDL |
CET |
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Shown as poor cement |
Shown as good cement |
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Decrease in formation signal and increase in casing signal |
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Apply pressure (500-1000 psi) to casing and re-run log. A micro-annulus should now show up as good cement - unless hydraulic seal prevents closing of the gap |
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Channelling
Channelling is characterised by the following:
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CBL / VDL |
CET |
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Shown as poor cement |
Channels are easily seen since whole circumference is scanned |
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Decrease in formation signal and increase in casing signal |
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The CET only determines the cement / casing bond, therefore, channels will be identified only where they extend to contact the casing wall.