overview of cement bond evaluation

8/9/2019 Overview of Cement Bond Evaluation

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Cement Evaluation

- Sonic Tools


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4.0 Cement Evaluation - Sonic Tools

4.1 Why Cement Bond Logging?


4.2 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


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



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5

20 kHz

Transmitter

3 ftReceiver

5 ftReceiver

Casing

Formation

t

t

Bonded cement

Mud

Cement

VDL

CBL amplitude

0 100

CBL amp



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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



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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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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



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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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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 C t B d I t t ti


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CBL Travel Time Cycle Skipping




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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.



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26CBL Free Pipe Signal



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4 4 Cement Bond Interpretation


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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.



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Minimum Cemented Interval vs Casing Size




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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


Cement Compressive Strength from Chart Book


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p g

CBL Casing Data


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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.



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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.



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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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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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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.



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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.



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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

overview of cement bond evaluation

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