CBL-VDL

CBL - VDL Cement Bond Logging with DSLT

1

Outline CBL-VDL Introduction to Cementing • •

Role of cementing Mechanics

Sonic as a Cement Evaluation tool Hardware Operations • • • •

Parameters & Setup LQC & Hints Normalization Safety

Factors effecting tool response Log Example 2

CBL-VDL Introduction to Cementing

3

Cementing Objectives

4

Role of primary cementing Typical casing string Conductor Casing (spudded) Isolate loose surface sediments Avoid surface corrosion

Surface Casing Isolate sweet water zones Mounting rig BOP and later CSG strings

Intermediate Casing Isolate loose or high pressure formation

Oil string (Casing/Liner) Isolate production zones Avoid hydrocarbon loss to thief zones Reduce water production 5

Environment Fluid filled casing

b

Cement top. Micro-Annulus due to expansion of casing during cement job. Poor cement to formation bond. (example: due to mud-cake)

Formation Two stage cement job.

Less than perfect cement job.

Double Casing 6

Role of secondary cementing Repair defects in primary cementing job

 Unsuccessful primary cementing  Casing corrosion, leaking, split

 Isolate water production in old well

7

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

 May not isolate over gas zones 8

Good Cement ?

9

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 log to which all subsequent work is depth referenced.

10

Sonic Path in Cased Hole

• CBL measures casing to cement bond • VDL indicates cement to formation bond if casing to cement bond is good 11

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 1 2

We measure E1 amplitude and call it CBL

3 4 Transit Time (TT) CBL: Poor Bond CBL: Good Bond

12

Free Pipe Amplitude If there is no cement bonded to the casing, then the amplitude will not be attenuated. 1

This is called the free pipe amplitude.

2

3 4

CBL: Free Pipe

13

Less Than Perfect Bond The more “free” pipe or “contaminated” cement in an interval , the poorer the bond. 1 2

If the cement job is not perfect, the amplitude will not decrease as much.

3 4

CBL: Poor Bond

14

What is Acoustic Impedance?

15

Sound Transmission 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 Water Steel Cement Sound

16

Sonic Path in Cased Hole

CBL

VDL

17

5ft VDL • 5ft VDL gives deeper depth of investigation than 3ft CBL • 5ft VDL indicates cement to formation bond if casing to cement bond is good

18

Expected Free Pipe CBL Values Casing Weight Estimated transit Expected size (lb/ft) time (msec) free-pipe reading (ETT) 5 in.

5.5 in.

7 in.

7.625 in.

9.625 in.

10.75 in.

13.375 in.

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

These values are for water filled casing Refer to WRM: DSLT table 7 19

Factor effecting tool response • Eccentering • Cycle skipping • Fast Formation (if good cement)

• Fluid Type • Stretch • Casing Size

• Cement quality!

20

Eccentering As in open hole, the amplitude is dramatically decreased when the tool is eccentered. 1 2

3

This will give show an incorrectly good bond. Since the travel distance is decreased, the transit time will also decrease.

4 Transit Time (TT)

21

A bad log which cannot be played back.

Eccentering Effects

• Eccentering by 0.5” reduces amplitude to 30% • It is crucial to have a well-centralized tool for good CBL data 22

Cycle Skipping • Very common in good cement bond (low amplitude E1)

23

Fast Formation

24

Micro Annulus A micro annulus is caused by the expansion of the casing under pressure during cementing. 1 2

E1 amplitude is larger than it should be (poorer bond than actual) A pressure pass can be used to eliminate the micro annulus.

3 4

CBL: Poor Bond

25

Interpretation Hints VDL TYPE OF BOND

26

CBL AMPLITUDE

CASING ARRIVALS

FORMATION ARRIVAL

Free Pipe

High

Large

Very Weak or none

Good Casing to - Cement to - Formation

Low

Weak

Strong

Good Casing Bond - Poor Formation Bond

Low

Moderate to Weak

Weak or none

Microannulus, Channeling, Thin Cement Sheath

High

Moderate

Moderate

Fast Formation Arrivals

High

Absent

Strong

Interpretation Hints

27

Cement to Formation Bond

1 2

3 4

28

Conclusion CBL – VDL Measurement Provides

 Casing to Cement Bond  Cement to Formation Bond  Zonal Isolation Channels / Patchy Cement Identification

Microannulus 29