Poor removal of mud from around pipe Channels or missing cement
Pressure problems
Influx of formation fluid Loss of cement into permeable zone: Permeable, weakened or missing cement
Micro-and cement annulus (Casing flexing contraction) Changes in pressure inside casing before setting Over pressuring the casing after cement setting
May not isolate over gas zones 8
Good Cement ?
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Why run CBL-VDL • Primary application is to evaluate the quality of the casing cement job • Secondary but equally important application is to depth match the open hole GR to the cased hole CCL • The CBL-VDL-GR-CCL becomes the cased hole base log to which all subsequent work is depth referenced.
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Sonic Path in Cased Hole
• CBL measures casing to cement bond • VDL indicates cement to formation bond if casing to cement bond is good 11
Cement to Casing Bond If the casing is well bonded, the sound will be attenuated as it travels through the casing The received amplitude will be low 1 2
We measure E1 amplitude and call it CBL
3 4 Transit Time (TT) CBL: Poor Bond CBL: Good Bond
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Free Pipe Amplitude If there is no cement bonded to the casing, then the amplitude will not be attenuated. 1
This is called the free pipe amplitude.
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3 4
CBL: Free Pipe
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Less Than Perfect Bond The more “free” pipe or “contaminated” cement in an interval , the poorer the bond. 1 2
If the cement job is not perfect, the amplitude will not decrease as much.
3 4
CBL: Poor Bond
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What is Acoustic Impedance?
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Sound Transmission The amount of sound transmitted depends on the acoustic impedance (Z) difference between the two materials •If Z1/Z2 is high, low transmittance •If Z1/Z2 is low, high transmittance Z1
Z2 Water Steel Cement Sound
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Sonic Path in Cased Hole
CBL
VDL
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5ft VDL • 5ft VDL gives deeper depth of investigation than 3ft CBL • 5ft VDL indicates cement to formation bond if casing to cement bond is good
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Expected Free Pipe CBL Values Casing Weight Estimated transit Expected size (lb/ft) time (msec) free-pipe reading (ETT) 5 in.
