Cement Evaluation - Cement Bond Log Interpretation

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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:

CBL / VDL

CET

High CBL readings > 50%

White on log

Parallel lines on VDL vs. Depth

CSMX* & CSMN* are zero

Weak formation signals

 

VDL shows clear “w” pattern at collars

 

No change in transit time with depth

 

White on VDL log

 

*CSMX & CSMN are the Maximum and Minimum Cement Strength respectively.

Good Bond

AGood Bond is characterised by the following:

CBL / VDL

CET

Low CBL readings (m Amp) below 5

Black on log

Casing signals very weak

High CSMX & CSMN

Strong formation arrivals on VDL provided formation attenuation is not too high

 

Formation arrivals change pattern with depth.

Casing arrivals show as parallel lines

 

May be a correlation between GR and VDL in shales

 

Hard Formations

Hard Formations such as anhydrite or dense limestone, behind the casing, are characterised by the following:

CBL / VDL

CET

Formation arrivals precede the casing arrivals

May show as free pipe but they will be flagged

Can be identified on the VDL

 

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:

CBL / VDL

CET

Shown as poor cement

Shown as good cement

Decrease in formation signal and increase in casing signal

 

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

 

Channelling

Channelling is characterised by the following:

CBL / VDL

CET

Shown as poor cement

Channels are easily seen since whole circumference is scanned

Decrease in formation signal and increase in casing signal

 

The CET only determines the cement / casing bond, therefore, channels will be identified only where they extend to contact the casing wall.

 

Comments  

#1 reela mubark 2014-02-17 07:23
Phenomena of casing wave and
formation wave displayed in
CBL&VDL plot Cement Bond Evaluation
The 1st Interface The 2nd interface
Good Fair Poor Good Poor
High amplitude of casing wave
(Af > 30%) / / √ unknown unknown
Obvious formation wave
(10% < Af ≤ 30%) / √ / √ /
Obvious mud wave (10% < Af ≤ 30%) / √ / / √
Low amplitude of casing wave and low or high amplitude of formation wave (Af ≤ 10%) √ / / √ /
Low amplitude of casing wave or no casing wave (Af ≤ 10%)
Obvious mud wave and no formation wave √ / / / √
Quote