Overview of Cement Bond Evaluation

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. 3

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. 6

4.2 Tool diagram and theory of operation

CBL (Cement Bond Log) Signal

VDL (Variable Density Log) Signal 7

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. 8

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. 11

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. 19

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 24

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 29

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 30

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. 34

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. 35

4.4 Cement Bond Interpretation

Summary CBL show free pipe under these conditions a) b) c) d) e)

CBL=71mV (5.5 in) and 61mV (7 in) casing TT is:- steady and correct value for casing No formation arrivals on VDL Strong casing arrivals on VDL Chevron pattern opposite collars on VDL

CBL show good cement bonding under these conditions a) b) c) d) 36

CBL < 5mV TT is steady or stretched/cycle skipping Strong formation arrivals (P&S) on VDL No casing arrivals on VDL

4.4 Cement Bond Interpretation Effect

TT 2

CBL mv (T0 mode)

VDL display

VDL display (Pictorial)

Interpretation

FP is High

Strong casing arrivals No formation arrivals Chevrons at collars

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

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