Cement Evaluation
- Sonic Tools
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4.0 Cement Evaluation - Sonic Tools
4.1 Why Cement Bond Logging?
4.1 Why Cement Bond Logging?
4.2 Tool diagram and theory of operation
4.3 Cement Bonding
4.4 Cement Bond Interpretation
4.5 Log Quality Control
4.6 Advantages/ Disadvantages of CBL/VDL
4.7 Exercises
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4.0 Cement Evaluation - Sonic Tools
4.1 Why Cement Bond Logging?
Evaluate zone to zone isolation
Evaluate cement to casing bond
Evaluate cement to formation bond
Identify cement top
Provide correlation between open and casedhole using GR-CCL
Cement returns to surface does not mean
there is good cement bonding throughout the
well and a decision not to run a cement bond
log as a result is a bad decision.
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4.2 Tool diagram and theory of operation
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CBL Tool Diagram
4.2 Tool diagram and theory of operation
Casing
Cement
Formation
Mud
t
20 kHz
Transmitter
3 ft
Receiver
Bonded cement
CBL amplitude
t
5 ft
Receiver
VDL
0
100
CBL amp
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4.2 Tool diagram and theory of operation
The CBL is similar in operation the open hole
Sonic tool. There is only one transmitter
however and two receivers at distances of 3
and 5 feet from the transmitter. As with the
Sonic open-hole tool the Compressional or P
waves are used to measure the time to travel
from the transmitter to the receiver.
The CBL tool is uncompensated, unlike the
open-hole Sonic tool. Centralization of the
CBL is therefore critical to its operation.
Rigid steel Gemoco centralizer who’s outside
diameter match exactly the casing inside
diameter should always be attached to the
CBL tool. This will ensure good centralization.
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4.2 Tool diagram and theory of operation
CBL (Cement Bond Log) Signal
VDL (Variable Density Log) Signal
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4.2 Tool Diagram and theory of operation
CBL Signal
The 3-foot signal from the transmitter to the
first receiver, will primarily measure the
cement to casing bond. If there is little or no
bond the amplitude of the signal will be very
large. If there is good bond the amplitude will
be very small.
This is commonly known as the TT3 (Travel
Time 3 foot) or CBL (Cement Bond Log)
signal. Here the horizontal line is the threshold
detection. The TT3 travel time is measured
between the transmitter pulse at the start and
the amplitude arrive 'E2'. The CBL signal
amplitude is measured by the height of this
first arrival.
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4.2 Tool Diagram and theory of operation
CBL Signal
Electronic detection has been developed
where the signal arrival can be tracked within
a viewing window. This method can also be
employed on full wave sonic tools to detect
open-hole Compressional arrivals behind
casing.
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Parameters for CBL detection
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4.2 Tool Diagram and theory of operation
VDL Signal
A Similar Compressional wave will be
measured with the 5-foot signal from the
transmitter to the second receiver. The signal
will however read deeper into the formation.
and will predominately measure the cement to
formation bond. Large signal amplitude also
indicates bad cement to formation bond and
small amplitude good cement to formation
bond. This is commonly known as the TT5
(Travel Time 5 foot) or VDL (Variable Density
Log) signal. The 5-foot waveform is used
differently to the 3 foot. Here the horizontal
threshold 'cuts through' the positive peaks of
the received signal. It is this cross section
through the positive peaks that is displayed on
the VDL track as if viewed from the top of the
waveform.
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4.2 Tool diagram and theory of operation
CBL/VDL Wave Train
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4.2 Tool diagram and theory of operation
VDL movie
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4.2 Tool Diagram and theory of operation
GR-CCL Signal
The cased-hole Gamma Ray measurement is
used to correlate the service on depth with the
open-hole services. Although it is calibrated, it
is affected by the casing, the cement and
borehole fluids shielding the formation
Gamma Rays. The cased-hole Gamma Ray
will therefore read lower in amplitude but will
have similar character in order to correlate on
depth with the open-hole Gamma Ray.
The CCL is used to correlate any future
Casing guns on depth for perforation
purposes.
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4.3 Cement Bonding
Early CBL tools were not calibrated. The 3foot amplitudes were measured for various
free pipe signals. As different tools were
made, different free pipe amplitudes were
measured from the original tools. CBL tools
therefore need the signal amplitude to be
calibrated for each sized casing.
The travel time of each signal does not require
calibration, as is the case with the Sonic tool.
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Halliburton Chart for CBL Tool response
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Schlumberger Chart for CBL Tool response
Casing Weight Estimated transit Expected
size
(lb/ft)
time (msec)
free-pipe
reading
5 in.
5.5 in.
7 in.
7.625 in.
9.625 in.
10.75 in.
13.375 in.
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15
18
21
15.5
17
20
23
23
26
29
32
35
38
40
26.4
29.7
33.7
39
40
43.5
47
53.5
40.5
45.5
48
51
54
55.5
48
68
SLS-W
SLS-C
245
243
241
254
253
251
250
278
276
275
273
272
271
269
288
287
285
283
320
318
317
315
340
339
338
337
336
335
385
380
238
236
234
248
247
245
243
271
270
268
267
265
264
262
282
280
278
276
313
312
310
308
333
332
331
330
329
328
378
373
77mV
71 mV
61 mV
59 mV
52 mV
50 mV
47 mV
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CBL Log Presentation
4.3 Cement Bonding
Presentation
Track 1 The GR measurement for correlation
to open hole logs. The CCL magnetic collar
locator spikes opposite every casing collar
The TT3 travel time being a function of the
casing size.
Track 3 CBL (3 foot) amplitude in mV (0100mV). For low amplitudes (better cement
bond) the 0-20mV curves comes into the
display for accurate measurement.
Track 4 TT5 signal is displayed in a Signature
presentation. This displays the entire wave
train. This presentation type is rare.
Track 5 TT5 signal is displayed in the pseudo
standard VDL presentation. This is a 'Bird's
eye view' of the TT5 waveform 'above' the
threshold.
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4.3 Cement Bonding
Mnemonics
CBL – Cement Bond Log
VDL – Variable Density Log
SIG – Signature Waveform
TT3/TT – Travel Time 3 foot
TTSL – Transit Time Sliding Gate
TT5 – Travel Time 5 foot
CCL – Casing Collar Locator
GR – Gamma Ray
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CBL Cemented Casing
4.4 Cement Bond Interpretation
Good Cement Bonding
CBL signal - Good cement to casing bond
exists when the CBL signal amplitude is less
than about 5mV. Values less than 10mV can
be considered to have sufficient bond. Good
zone isolation occurs when there is
continuous good bond amplitude of 10 feet or
more. The interval 3307.5-3310.5ft has very
good bond between cement and casing by
virtue of small CBL amplitude in Track 3 of
approx. 4mV. Often at these low amplitudes
the TT3 travel time will initially stretch and
then will cycle skip as the amplitude drops
further. See figures below.
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4.4 Cement Bond Interpretation
CBL Travel Time Stretching
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4.4 Cement Bond Interpretation
CBL Travel Time Cycle Skipping
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4.4 Cement Bond Interpretation
Good Cement Bonding
VDL signal - There are good formation arrivals
indicated by the VDL display in Track 5. Thick
'wavy' VDL response indicates good cement
to formation bond. The formation arrivals are
depicted by the very thick VDL lines (thick
because the amplitude is very high). These
formation arrivals should also track closely to
the open-hole Sonic transit times (DT). This
indicates good cement to formation bond also.
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4.4 Cement Bond Interpretation
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CBL Free Pipe Signal
4.4 Cement Bond Interpretation
Free Pipe Signal
CBL signal - Free pipe signal takes on specific
values for different casing sizes. Typically
61mV for 7 inch and 71mV for 5.5" casing.
Here you can see that the free pipe signal is
reaching a maximum of approximately 75mV.
There is some cement scattered around the
pipe that is reducing the CBL signal in places
but essentially this is still free pipe. There is no
continuous cement seal to the casing. The
TT3 is reading 275mSec and the free pipe
signal is 71mV both indicating 5.5 inch casing.
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4.4 Cement Bond Interpretation
Free Pipe Signal
VDL signal - At the start of the VDL signal in
track 5 there are straight thin lines
representing casing arrivals. Further along the
wave train, there are thicker but relatively
straight arrivals but these are not
representative of the formation. Across casing
collars there is a distinct “Chevron” pattern or
“W” shape on the VDL signal.
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4.4 Cement Bond Interpretation
Minimum Cemented Interval vs Casing Size
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4.4 Cement Bond Interpretation
CBL INTERPRETATION
100
CBL AMPLITUDE IN MV
FREE PIPE
10
100% CEMENT
1
0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
% CEMENT
Percentage of Cement vs CBL Amplitude
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Cement Compressive Strength from Chart Book
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CBL Casing Data
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4.4 Cement Bond Interpretation
Micro Annulus
Micro annulus is often a problem when
performing a casing cement job. Once the
cement has been pumped into the casing
annulus, micro annulus can occur when the
wellhead pump pressure is held past the
setting time of the cement. When the pressure
is released the casing retracts and a thin
break occurs between the casing and the
cement. Typically cement sets in 4-5 hours
and casing pressure is often held for 3-6
hours. Holding of wellhead pressure after
pumping should be kept to a minimum.
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4.4 Cement Bond Interpretation
Micro Annulus
Micro annulus can also be caused by drilling
inside a cemented casing – ie vibrations and
mud weight decreases causing casing to
expand and retract.
Another cause of micro annulus occurs if there
are any residual coatings left on the outside of
the casing during manufacture. When the
cement job is run these coatings can inhibit
the cement to bond to the casing again
leaving a thin micro fracture or break between
the casing and cement. The cement job
requires a pre-flush chemical fluid to remove
any coatings immediately before the cement is
pumped outside the casing.
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4.4 Cement Bond Interpretation
Micro Annulus
Similarly a pre-flush needs to be run to
remove any borehole wall mud cake. This will
help ensure good cement to formation bond
also.
Micro annulus is noticeable when the CBL
signal is approximately 10-20mV. If micro
annulus is suspected, the casing should be
pressured to 1000psi well head and the CBL
survey run again under the 1000psi pressure.
If the CBL signal reduces to below 10mV then
micro annulus exists. If Micro annulus does
occur this is not usually a problem for zone
isolation.
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4.4 Cement Bond Interpretation
Summary
CBL show free pipe under these conditions
a) CBL=71mV (5.5 in) and 61mV (7 in) casing
b) TT is:- steady and correct value for casing
c) No formation arrivals on VDL
d) Strong casing arrivals on VDL
e) Chevron pattern opposite collars on VDL
CBL show good cement bonding under these
conditions
a) CBL < 5mV
b) TT is steady or stretched/cycle skipping
c) Strong formation arrivals (P&S) on VDL
d) No casing arrivals on VDL
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4.4 Cement Bond Interpretation
Effect
TT 2
CBL mv
(T0 mode)
VDL display
FP is High
Strong casing arrivals
No formation arrivals
Chevrons at collars
No cement
100% CSG circumf.
Coverage
Free Pipe
stable
Bonded
pipe
100% B.I
stable …
or Stretch
/ skip
BP is low
Very weak/no casing
arrivals, strong formation
arrivals
Partial
bond
stable
BP < X< FP
Mediumcasing & formation
arrivals
Eccentered
sonde
Decrease >
± 4 µs
VDL display
(Pictorial)
Interpretation
Blurred arrivals
B.I not valid. If formation
arrivals "some cement is
present"
TT stretch
increase
up to
13.7µs
No/weak casing arrivals
Strong formation arrivals
Good cement. Use T0 (or
Tx) amplitude for
calculations
TT skip
increase
step 33 to
55 µs
No/weak casing arrivals
Strong formation arrivals
Good cement. Use only T0
amplitude for calculations
Concentric
casings
possible
increase
20<X<55µs
High
amplitude
Doubling of casing arrivals
frequency
Some cement. BI not valid
if high mV
Fast
Formation
arrivals
Decrease
or stable
Anything
Formation arrivals before
or at ± same time as
casing arrivals
Some cement. B.I not valid.
Micro
annulus
stable
High (reduced
with casing
pressure)
Casing & formation
arrivals
Some cement is present (B.I
not valid)
Channeling
stable
High
amplitude
Casing & formation
arrivals
Some cement is present
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Anything
4.5 Log Quality Control
The Cement Bond Log must be calibrated to a
free pipe signal. This should always be
available from the service company and it
should be insisted upon that it is displayed on
the log. It is preferable that a free pipe log be
performed in the well. In the event that this is
not possible, an alternative shop calibration in
free pipe signal should be made available and
displayed on the field print.
The TT travel time and the CBL amplitude
should read values appropriate for the size of
casing used in the well.
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4.5 Log Quality Control
It is critical that the CBL is well centralized.
Centralisation can be easily performed. If the 3foot travel time is not primarily a straight line,
then the CBL tool is poorly centralized and the
cement bond will not be accurate.
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4.5 Log Quality Control
Check List:
1. Transit times are correct for casing size.
2. Amplitude correct in free pipe.
3. Transit time is steady indicating there is good
tool centralisation.
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4.5 Log Quality Control
CBL Shop Calibration Equipment
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4.6 Advantages/ Disadvantages of CBL/VDL
Advantages:
 CBL/VDL measures the cement bond quality
to casing and also to the formation.
 It is a very cheap, robust tool that has been
successfully used for more than 30 years.
Disadvantages:
 The CBL/VDL tool cannot determine
channeling. If channeling is occurring the CBL
amplitude will read slightly high since it is an
average measurement around the borehole.
 Gas detection will also cause the CBL
amplitude to read slightly high.
 Micro annulus can only be confirmed with a
second higher pressure logging pass.
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4.7 Exercises
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4.7 Exercises
Q1: What is the value of the free pipe CBL
signal in 7 and 5.5 inch casing?
Ans: 71mV (5.5 in) and 61mV (7 in) casing
Q2: The TT travel time appears to be
reasonably steady at 280mSec. What does
this indicate?
Ans: 7inch and that the tool is properly
centralised.
Q3: If the CBL was calibrated in non-free pipe
in 7 inch casing, what do you think it will read
in true free pipe and in true good cement
bond?
Ans: > 71 inches and a bad cement bond
respectfully.
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4.7 Exercises
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4.7 Exercises
Q4: Over which interval(s) do you think there
is good bonding, good zone isolation and
why?
Ans:
Good bonding:
3278-79, 3283-86, 3294-96, 3297-3304,
3307-11, 3314-15, 3317-19m since amplitude
is <= 10mV.
Good Zone isolation:
Bonging intervals with approx 10mV are
probably micro annulus. If a separate
pressured CBL log confirmed micro annulus,
then there is sufficient zone isolation over
these intervals since in 7” casing minimum for
isolation is 10ft(or 3m).
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4.7 Exercises
Q5: Over which interval(s) do you think there
is bad bonding and why?
Ans:
Intervals where amplitude is approaching
20mV and higher:
3275-77, 3280-82, 3287-93, 3296-97,
3304-07, 3311-14, 3315-17m.
A pressure CBL logging run is unlikely to yield
a better cement bond over these intervals.
Therefore this is bad bonding, possibly
channeling.
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CBL-VDL run 1 – RHOM @ 1.35g/cc
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CBL-VDL run 1 – RHOM @ 1.35g/cc
Q6: Based on TT readings can we conclude
that tool is properly centralised?
Ans: Tool is centralised (TT=308us/ft is
theoretical value for 9 5/8” casing)
Q7: Based on CBL readings what can we
conclude from cement job quality?
Ans: CBL reads more than 52mV which is
free pipe in 9 5/8” casing.
Q8: Based VDL what can we conclude from
cement job quality?
Ans: VDL displays both casing and
formation arrivals indicating bad bonding
Q9: How can we confirm this is not micro
annulus?
Ans: Run CBL under pressure or CET/USIT
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CBL-VDL run 1 – RHOM @ 1.35g/cc
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CET run 1 – RHOM @ 1.35g/cc
Q10: What can we conclude about cement
job quality?
Ans: CBL between 3&10mV, no casing
arrivals only formation arrivals on VDL,
therefore perfect cement bond.
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4.7 Exercises
Q11: Name the different causes of micro
annulus.
Ans:
1. Extended time for holding casing
pressure after cement job.
2. Drilling inside cemented casing.
3. Insufficient cleaning of casing surface to
enable good bonding.
Q12: How can you confirm micro annulus?
Ans:
1. Run a CBL log under 1000psi wellhead
pressure. If CBL amplitude reduces to
good cement bond values then it is micro
annulus.
2. Run Ultra Sonic tool – USIT,CET,PET
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