Post on 01-Jun-2018
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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 cased-
hole 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 bondlog as a result is a bad decision.
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CBL Tool DiagramCBL Tool Diagram
4.2 Tool diagram and theory of operation
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20 kHz
Transmitter
3 ftReceiver
5 ftReceiver
Casing
Formation
t
t
Bonded cement
Mud
Cement
VDL
CBL amplitude
0 100
CBL amp
4.2 Tool diagram and theory of operation
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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 theSonic 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 theCBL is therefore critical to its operation.
Rigid steel Gemoco centralizer who’s outsidediameter match exactly the casing inside
diameter should always be attached to the
CBL tool. This will ensure good centralization.
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CBL (Cement Bond Log) Signal
VDL (Variable Density Log) Signal
4.2 Tool diagram and theory of operation
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4.2 Tool Diagram and theory of operation
CBL SignalThe 3-foot signal from the transmitter to the
first receiver, will primarily measure the
cement to casing bond. If there is little or nobond 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 beemployed on full wave sonic tools to detect
open-hole Compressional arrivals behind
casing.
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VDL movie
4.2 Tool diagram and theory of operation
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4.2 Tool Diagram and theory of operation
GR-CCL SignalThe cased-hole Gamma Ray measurement is
used to correlate the service on depth with the
open-hole services. Although it is calibrated, itis affected by the casing, the cement and
borehole fluids shielding the formation
Gamma Rays. The cased-hole Gamma Raywill 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 3-foot amplitudes were measured for various
free pipe signals. As different tools were
made, different free pipe amplitudes weremeasured 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
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 31247 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)
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18CBL Log Presentation
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4.3 Cement Bonding
PresentationTrack 1 The GR measurement for correlation
to open hole logs. The CCL magnetic collar
locator spikes opposite every casing collarThe TT3 travel time being a function of the
casing size.
Track 3 CBL (3 foot) amplitude in mV (0-100mV). 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
MnemonicsCBL – Cement Bond Log
VDL – Variable Density Log
SIG – Signature WaveformTT3/TT – Travel Time 3 foot
TTSL – Transit Time Sliding Gate
TT5 – Travel Time 5 footCCL – Casing Collar Locator
GR – Gamma Ray
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21CBL Cemented Casing
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4.4 Cement Bond Interpretation
Good Cement Bonding
CBL signal - Good cement to casing bond
exists when the CBL signal amplitude is lessthan about 5mV. Values less than 10mV can
be considered to have sufficient bond. Good
zone isolation occurs when there iscontinuous good bond amplitude of 10 feet or
more. The interval 3307.5-3310.5ft has very
good bond between cement and casing byvirtue 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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CBL Travel Time Stretching
4.4 Cement Bond Interpretation
4 4 C t B d I t t ti
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CBL Travel Time Cycle Skipping
4.4 Cement Bond Interpretation
4 4 C t B d I t t ti
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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 (thickbecause the amplitude is very high). These
formation arrivals should also track closely to
the open-hole Sonic transit times (DT). Thisindicates good cement to formation bond also.
4 4 C t B d I t t ti
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26CBL Free Pipe Signal
4.4 Cement Bond Interpretation
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4 4 Cement Bond Interpretation
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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 linesrepresenting casing arrivals. Further along the
wave train, there are thicker but relatively
straight arrivals but these are notrepresentative of the formation. Across casing
collars there is a distinct “Chevron” pattern or
“W” shape on the VDL signal.
4 4 Cement Bond Interpretation
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Minimum Cemented Interval vs Casing Size
4.4 Cement Bond Interpretation
4 4 Cement Bond Interpretation
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CBL INTERPRETATION
0.1
1
10
100
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
C B L A M P L I T U D E I N
M V
100% CEMENT
FREE PIPE
Percentage of Cement vs CBL Amplitude
4.4 Cement Bond Interpretation
Cement Compressive Strength from Chart Book
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p g
CBL Casing Data
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4.4 Cement Bond Interpretation
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4.4 Cement Bond Interpretation
Micro Annulus
Micro annulus is often a problem when
performing a casing cement job. Once thecement has been pumped into the casing
annulus, micro annulus can occur when the
wellhead pump pressure is held past thesetting 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.
4.4 Cement Bond Interpretation
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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 andmud 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 jobrequires a pre-flush chemical fluid to remove
any coatings immediately before the cement is
pumped outside the casing.
4.4 Cement Bond Interpretation
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p
Micro Annulus
Similarly a pre-flush needs to be run to
remove any borehole wall mud cake. This willhelp 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 doesoccur this is not usually a problem for zone
isolation.
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4.4 Cement Bond Interpretation
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Effect TT 2 CBL mv(T0 mode)
VDL display VDL display(Pictorial)
Interpretation
Free Pipe stable FP is High
Strong casing arrivals
No formation arrivals
Chevrons at collars
No cement
Bonded
pipe
100% B.I
stable …
or Stretch
/ skip
BP is low
Very weak/no casing
arrivals, strong formation
arrivals
100% CSG circumf.Coverage
Partial
bondstable BP < X< FP
Mediumcasing & formation
arrivals
Eccentered
sonde
Decrease >
± 4 µsAnything Blurred arrivals
B.I not valid. If formationarrivals "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
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4.5 Log Quality Control
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g y
It is critical that the CBL is well centralized.Centralisation can be easily performed. If the 3-
foot travel time is not primarily a straight line,
then the CBL tool is poorly centralized and thecement bond will not be accurate.
4.5 Log Quality Control
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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.
4.5 Log Quality Control
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CBL Shop Calibration Equipment
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4.7 Exercises
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Q1: What is the value of the free pipe CBLsignal 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.
4.7 Exercises
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4.7 Exercises
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Q4: Over which interval(s) do you think thereis 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
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Q5: Over which interval(s) do you think thereis 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.
CBL-VDL run 1 – RHOM @ 1.35g/cc
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CBL-VDL run 1 – RHOM @ 1.35g/cc
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Q6: Based on TT readings can we concludethat 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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CET run 1 – RHOM @ 1.35g/cc
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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.
4.7 Exercises
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Q11: Name the different causes of microannulus.
Ans:
1. Extended time for holding casingpressure after cement job.
2. Drilling inside cemented casing.
3. Insufficient cleaning of casing surface toenable 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 microannulus.
2. Run Ultra Sonic tool – USIT,CET,PET