Kuliah-2 Cement Evaluation

B y :

L U K A S U T O J O W I H A R D J O

F R E E L A N C E R

CEMENT EVALUATION

PEN HOLE WELL LOG

Proses Interpretasi Well Logging (Umum

)

OPEN HOLE WELL LOG INTERPRETATI ON : LITHOLOGY, POROSITY & SATURATION Cement Evaluation SATURATION MONITORING PRODUCTION LOGGING WATER FLOW LOGGING

CASED HOLE WELL LOG INTERPRETATION

Objectives of Primary Cementing

• Mixing of unwanted fluids • Fluids escaping to surface • Invading fluids [crossflow] • Casing Corrosion

• Casing Collapse

Shale Zone

Cement Quality Problems

It will potentially

create :

• Mixing of unwanted fluids • Fluids escaping to surface • Invading fluids [crossflow] • Casing Corrosion

• Casing Collapse

Environment Descriptions

Top of Cement

Poor Cement to Formation Bond

Formations Micro-Annulus

Less than perfect cement job. Two stages Cement job

Double Casing

b

CBL-VDL Log Applications

Cement To Evaluate Cement

Job

•Check Integrity of Cement •To Verify Zone Isolation

•To Determine Cement Quality •Is there any Channel ?

•Is it necessary to Repair ? •Will be possible to Repair?

[ by performing a SQUEEZE]

•Where is the Top of the

Cement ? Casing

CBL-VDL

Cement Bond Logging

Physics of Measurement

Basic Sonic Principle

Basic Tool Principle

–

A Transmitter fires an acoustic

signal in all directions

–

Surrounding Media Resonates

–

Receivers record resulting

sound

Basic CBL Principle

Similar to a

Ringing Bell

 When Fluid is behind Casing, pipe is free to vibrate [

loud sound

 When the casing is

bonded to hard cement, casing vibrations are

attenuated propor-tionally to bonded surface Good Bond No Cement

CBL Measurement Principle

Basic Tool Configuration

 1 Transmitter – 2 Receivers

 3 ft Receiver for CBL Measurement

 5 ft Receiver for VDL Analysis

 _{TOOL MUST BE CENTRALIZED}

CBL: CEMENT BOND LOG

VDL: VARIABLE DENSITY LOG 3 ft

5 ft Tx

R₃

CBL-VDL Measurement Principle

Acoustic Signal

- Resulting Sound wave : as recorded at the Receivers

CBL Measurement Principle

CBL Definition

 Amplitude of First Arrival in mV

 Measured at 3 ft Receiver

 It is a function of the Casing-Cement Bond

3 ft Tx

R₃

R₅

Transit Time Definition

• Time elapsed from T₀ to First detected Arrival (above threshold level)

• T T is used as Log Quality Control Indicator

CBL Qualitative Meaning

HIGH CBL signal strength => pipe is free to vibrate

[no cement ]

LOW CBL signal strength => attenuated energy

[cement is present]

Good Bond No

Sound to Signal

• Magnetostrictive transducer (Tx)

A high current is passed through a coil surrounding a magnetic

material introducing a strain and causes a ticking sound (Joule effect)

D

L / L

• Piezoelectric transducer (Rx)

Polarized ceramic crystals in the sonde produce voltage when exposed to strain (Villari effect)

strain

voltage Unstrained

CBL Amplitude Vs. Receivers

Spacing

CBL-VDL Cement Bond

Logging

Gates Settings

NMSG CBLG

WARNING

- The CBL represents one of the most common logs prone to human error

- Incorrect setting of parameters can cause an invalid log - The CBL values are “fluid compensated” if the free pipe

values are adjusted to the expected free pipe value in water , this is done either :

1- If the FCF is known and CBLF is presented

2- If the free pipe check is performed and amplitudes are adjusted (CBAF) to read the expected fee pipe value in water

CBL Measurement T

₀

_Delay Mode

(Fixed Gate Mode)

NMSG: Near Minimum Sliding Gate

NMSG CBLG

CBLG : CBL Gate

View the waveform and check the transit time value

Set NMSG at measured/viewed T T – 10 ms

CBL Measurement T

_X

Mode

(Back-up for the T

₀

_Delay Mode)

SGW: Sliding-Gate Width

SGW

SGCW

SGCW : Sliding-Gate Closing Width

Sliding Gate opens at SGW ms BEFORE previous detected TT and closes

SGCW ms after current TT

TTSL

AMSG

AMSG : Auxiliary Minimum Sliding Gate

Amplitude is CBSL, Transit Time is TTSL

The VDL Signal

VDL: VARIABLE DENSITY LOG

 _{5 ft Receiver for VDL Analysis}  _{Allows easy differentiation}

between casing and forma-tion arrivals

5 ft Tx

R₃

VDL Algorythm Principle

•

Recorded Waveform at

one depth

•

Waveform is cut for only

Positive Peaks

•

Peaks are compared to a

Grey Scale

•

Peaks are shaded and

presented from Top View

•

Final Picture Vs Depth is

obtained

DT Casing = 57 msec/ft

DT Cement = 75 msec/ft

DT Formation ≈ 100 msec/ft

DT Fluid ≈ 189 msec/ft

Slowness

Propagation of the Acoustic Energy

distance

Velocity =

time

1

time

Slowness = =

velocity

distance

The amount of sound trans-mitted between two different materials depends on their acoustic impedance difference

Water Steel Cement Sound Z1 Z2 1. If Z1/Z2 is high ==> low transmittance 2. If Z1/Z2 is low ==> high transmittance

Acoustic impedance

(Z) defined as:

Z =

r

. v

r

: density of material

V: velocity of sound

on that material

Propagation of the Acoustic Energy

cont’d

Waveform Time Analysis

TT_C = FLUID + CASING + FLUID

= 3/12 in x 189 ms/ft + 3 ft x 57 ms/ft + 3/12 in x 189 ms/ft

Waveform Time Analysis

TT_F = FLUID + CEMENT + FORMATION + CEMENT + FLUID

= 2x (3 in / 12 x 189 ms/ft + 2 in / 12 x 75 ms/ft) + 3 ft x 100 ms/ft

Waveform Time Analysis

FLUID ARRIVALS TRAVEL TIME

2” DDT Casing T Cement = 57 = 75 mmsec/ft sec/ft

DT Formation ≈ 100 msec/ft

DT Fluid ≈ 189 msec/ft

TT_f = FLUID

= 3 ft x 189 ms/ft

CBL-VDL Standard Outputs Presentation

• Transit Time TT in micro-seconds [ms]

• CBL Amplitude in millivolts [mV]

• VDL Variable Density Log [waveform visual representation]

0 CBL 100 [mV] 400 TT 200 [ms] 200 VDL 1200 [ms] GR CCL

CBL-VDL Standard Outputs

• Transit Time TT in micro-seconds [ms]

Log Quality Control

• CBL Amplitude in millivolts [mV ]

Quantitative Measurement of waveform energy

• VDL Variable Density Log [wafeform visual representation]  Qualitative Analysis of sound

 Qualitative indicator of the presence of solid materials

between the casing and the formation

CBL-VDL Cement Bond

Logging

Factors affecting the Log

Threshold

E1 T₀

TT

Free Pipe Signal

TT’ DT

Stretching

E1 decreases and TT is detected on a non linear portion of E1

DT STRETCHING is the TT increase from its value in free pipe

In cases of Good Cement

Threshold

E1 T₀

TT

Free Pipe Signal

TT’ DT

TT Cycle Skipping

E1 could not reach Detection Threshold Level T T skips to 3rd _{Peak [E}

3 ]...this is known as CYCLE

SKIPPING

In cases of very Good Cement

Threshold

E1 _E3

E2

T₀

CBL-VDL Cement Bond Logging

Basic Interpretation

CBL Qualitative Interpretation

Free Pipe NORMAL HIGH Casing Arrivals

Usually No Formation Arrivals

Good Bond to Casing & Formation

NORMAL to HIGH / NOISY

LOW No Casing Arrivals Formation Arrivals Good Bond to Casing

Not to Formation

NORMAL to HIGH CAN BE

NOISY

LOW No Casing Arrivals No Formation Arrivals

Poor Bond to Casing NORMAL MEDIUM to HIGH

Strong Casing Arrivals No Formation Arrivals Microannulus NORMAL MEDIUM to

HIGH

Formation Arrivals Casing Arrivals Channeling NORMAL MEDIUM to

HIGH

Formation Arrivals Casing Arrivals Fast Formations LOW HIGH Formation Arrivals

No Casing Arrivals Eccentered Tool LOW LOW DEPENDS

Free Pipe Amplitude

 This is called

FREE PIPE AMPLITUDE

 _{If no Casing-Cement bond,}

amplitude is not attenuated

CBL: Free Pipe

T

5

3

CBL AMPLITUDE VS. CASING SIZE

FREE PIPE CHECK

CBL Interpretation

as expected according to Casing Size

100 100

Cement to Casing Bond

 _{If casing is well bonded,}

soundwave will be attenuated

 _{The received CBL amplitude}

will be low CBL: Free Pipe CBL: Good Bond T 5 3 2

Open-Hole VDL’s (Before Casing)

GR WF1 VDL

(Standard VDL)

Cased-Hole VDL’s (After Casing)

GR _CCL WF1 VDL

(Standard VDL)

GOOD BOND TO CASING

& FORMATION

Irregular Bond

 _{The more “free” pipe or}

“contaminated” cement in an interval, the poorer the bond

 If cement job is not perfect, the amplitude is less

attenuated CBL: Poor Bond T 5 3 2

POOR BOND TO CASING

GOOD BOND CASING

NOT TO FORMATION

Micro Annulus

 _{Very Tiny gap Gap between Casing and}

Cement

Caused for instance by contraction of casing after cement sets if Casing Fluid is changed

 E1 amplitude resembles a poorer bond than actual

 _{Only a pressure pass can be done to}

eliminate the micro annulus

CBL: Poor Bond

T

5

3

Tool Eccentering

Causes for Eccentralization

5

3

2

T

• Improper Equipment selection [ Centralizers ] for Casing Size

• Missing or Broken Centralizer(s)

• Weak Centralizers in deviated wells

• Tool Damaged and/or bent

• Damaged Casing

Consequences

• Unbalanced sound paths

Eccentering Analysis

There will be destructive interference from different sound paths Waveform from close tool side to casing

If the tool is eccentered

Threshold T₀ TT Short Path Waveform Resulting Waveform

Waveform from far tool side to casing

Delayed Waveform

Result is a Bad Log

not recoverable

in Playback

Normal Waveform

Resulting waveform has Dramatic lower amplitude

Resembling a zone of Good Cement

Fast Formation

T

5

3

2

Fast Formation Arrivals

In cases of good cement and

formation slowness < steel slowness formation arrival arrives first

The transit time and CBL amplitude will be affected

DT Dolomite = 43.5 msec/ft

DT Limestone = 47.5 msec/ft

Fast Formation

T

5

3

2

 In the presence of FF arrivals no CBL evaluation is possible, since E1 is due to Formation (Fast) arrivals and not from Casing arrivals (usually with 3 to 5 ft receivers)

 FF arrivals travel longer distance from Tx to RX (casing thickness + cement thickness + formation) than casing arrivals.

 Only in short spacing Tx-Rx (~ 1 ft) the casing arrival will arrive earlier than FF arrival.

 Tools able to measure CBL in FF are: CBT, and QSLT, SSLT-C (short Tx-Rx about 1 ft)

FAST FORMATION High <---CBL Amplitude on areas of fast formation <--- arrivals Transit Time Shorter than Casing arrivals

CBL Quantitative Interpretation



_ATTENUATION

 Logarithm of E1 amplitude [first peak of CBL waveform]



_{BOND INDEX}

Attenuation in zone of interest [dB/ft]

BI =

CBL

Quantitative

Interpretation

M1 Chart

CBL value for 100% Bond [minimum expected amplitude] 70% Bond Index ? CBL value for 70% Bond

CBL Quality Control

 _{Is the tool normalized?}

 _{Are the CBL gate parameters set properly?}  _{Is the measured Transit Time as expected?}

 _{Is the free pipe measured CBL value as expected?}  _{Is the tool properly centered?}

 _{How does the lowest measured amplitude of the log compare}

to the predicted amplitude of the CBL?