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Cement Bond Logging
with DSLT
Outline CBL-VDL
Introduction to Cementing
•
Role of cementing•
MechanicsSonic as a Cement Evaluation tool
Hardware
Operations
•
Parameters & Setup•
LQC & Hints•
Normalization•
Safety3
CBL-VDL
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Role of primary cementing
Conductor Casing (spudded)
Isolate loose surface sediments Avoid surface corrosion
Surface Casing
Isolate sweet water zones Mounting rig BOP and later CSG strings
Oil string (Casing/Liner)
Isolate production zones
Avoid hydrocarbon loss to thief zones Reduce water production
Intermediate Casing
Isolate loose or high pressure formation
b
Environment
Fluid filled casing
Cement top.
Poor cement to formation bond. (example: due to mud-cake)
Formation
Micro-Annulus due to expansion of casing during cement job.
Less than perfect cement job. Two stage cement job.
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Role of secondary cementing
Repair defects in primary cementing job
Unsuccessful primary cementing
Casing corrosion, leaking, split
Causes of bad cementing
Mechanical problems
Poor pipe centralisation Hole conditions: 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
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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
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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 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 We measure E1 amplitude and call it CBL Transit Time (TT) 2 4 3 1
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Free Pipe Amplitude
If there is no cement bonded to the casing, then the amplitude will not be attenuated.
This is called the free pipe amplitude. CBL: Free Pipe 2 4 3 1
Less Than Perfect Bond
The more “free” pipe or
“contaminated” cement in an interval , the poorer the bond.
If the cement job is not perfect, the amplitude will not decrease as
much.
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4 3 1
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Sound Transmission
Water Steel
Cement
Sound
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
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Sonic Path in Cased Hole
VDL
CBL
5ft VDL
•
5ft VDL gives deeper depth of investigation than 3ft CBL
•
5ft VDL indicates cement to formation bond if casing to
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Expected Free Pipe CBL Values
Casing size Weight (lb/ft) Expected free-pipe reading SLS-W SLS-C 5 in. 15 245 238 77mV 18 243 236 21 241 234 5.5 in. 15.5 254 248 71 mV 17 253 247 20 251 245 23 250 243 7 in. 23 278 271 61 mV 26 276 270 29 275 268 32 273 267 35 272 265 38 271 264 40 269 262 7.625 in. 26.4 288 282 59 mV 29.7 287 280 33.7 285 278 39 283 276 9.625 in. 40 320 313 52 mV 43.5 318 312 47 317 310 53.5 315 308 10.75 in. 40.5 340 333 50 mV 45.5 339 332 48 338 331 51 337 330 54 336 329 55.5 335 328 13.375 in. 48 385 378 47 mV 68 380 373 Estimated transit time (msec) (ETT)
These values are for water filled casing
Factor effecting tool response
•
Eccentering
•
Cycle skipping
•
Fast Formation
(if good cement)
•
Fluid Type
•
Stretch
•
Casing Size
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Eccentering
As in open hole, the amplitude is dramatically decreased when the tool is eccentered.
This will give show an incorrectly good bond.
Since the travel distance is
decreased, the transit time will also decrease.
Transit Time (TT)
A bad log which
cannot be played back.
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4 3 1
Eccentering Effects
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Cycle Skipping
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Micro Annulus
A micro annulus is caused by the expansion of the casing under pressure during cementing.
E1 amplitude is larger than it should be (poorer bond than actual)
A pressure pass can be used to eliminate the micro annulus.
CBL: Poor Bond
2
4 3 1
Interpretation Hints
VDL
TYPE OF BOND CBL CASING FORMATION
AMPLITUDE ARRIVALS ARRIVAL
Free Pipe High Large Very Weak or none
Good Casing -
to - Cement - Low Weak Strong
to - Formation
Good Casing Bond
- Poor Formation Bond Low Moderate to Weak Weak or none
Microannulus, Channeling,
Thin Cement Sheath High Moderate Moderate
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Cement to Formation Bond
2
4 3 1
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