Cbl Eval Guidebook

Cement

Bond Log

(CBL) Evaluation

Guidebook

QC and Interpretation

Huawen Gai BP EXPLORATION

QC and Interpretation

Whether to run a cement evaluation log?

What tool to choose?

When to

run

the log?

How does the CBL tool work?

How do the conditions affect the log?

How to carry out QC operation?

• The systematic way to

Interpret the Cfl

• Historical mIstakes corrected

• Squeeze consIderations

DrHuawen Gai

Drilling and Completions Branch liP Research Centre

Sunbury-on-Thames Middlesex TWJ6 7LN

UK

Tel. (+44) (0)932 763495 Fax (+44) (0)932 763352

Acknowledgements

Many PEs and DEs in BP Explorationhave directly contributed to this manual. I particularlywant to thank David Law, David Munro, Lee Richardson and Daryl Kellingray of Dyce Aberdeen and Chris Greaves of Westport Lab Houston for their most valuable comments and advice. I want to thank the following people in Drilling and Completions Branch who made significant technical contributions or editorial advice in preparing this manual: Chris Lockyear, Dan Ryan, Ashley Hibbert, George Brown (Production Operation Branch),John Mason, John Bensted and Nigel Brown. The help from Robin Lewis, Ian Palmer and Andy Gardner in associated experiments is most appreciated.

Several people from logging service companies assisted in supplying information. I’d like to thankSigveMauritzen, AVince SpinelliandPitakWangvarangkoon ofSchiumberger, and Ruud Henskens of Atlas Wireline for the valuable discussions.

H Gai Sunbury, UK

June,1992

Contents:

How to Use the Manual and Quick Reference Charts i-viii

~1. Things you should know about the tools 3

Whether to run a log and Criteria to choose a tool—QCis most important— What the tools can do at their best— Do not interpret logs in isolation

~2. Tool principles andJargon 4

How the measurement is made—How the tool works downhole—Important

features ofthe tool structure — What a log looks like—El, £2, E3 etc.—

Gates— Transit Time—Stretching— Cycle skipping— Casing arrivals,

formation amivals, and mud arrivals— Fastformations— p-annulus— Free pipe

~3. Information Included inthe log 8

The log header— The bodyof the log— The log tail—The BP questionnaire

~4. Parameters affecting the log results 9

p-annulus—Eccentricity—Channelling—Casing coating—Fastformations—Mud

type and conditions— Temperature—Casing diameter and thickness— Casing

damages— Casing standoff and open hole geometiy—Double casing strings—

WOCtime—Cementparameters and conditions—Computer keyboard operations

0. OperationQC Inthree phases 17

Before logging—During logging—After logging

~6. Interpretation 21

Intespretation Chart— QC review— Quick checks— Examine the TTcurves— and the

CBL curves and the t’DL log—BPI calculation with example—Special Investigation

Chart

~7. Cementing operation 29

Cementing operation— the CFS

~8. Squeezeconsiderations 30

Where did the cement go— Whatkind of channel could it be— Where to squeeze

~9. Log examples 33

Log header/tail and scale—~i-annulus—Eccentricity—Fastformations—

Muddensity— Temperature—Green cement—Double casing string

~1O.

Data and Charts for Reference 41

Toolperformance comparison— Tool characteristics—Soundvelocities in muds

formations— Casing expansion underpressure—Relationship between eccentricity,

amplitude and TTreduction—Interval lengths required for isolation—Amplitude

compensation charifor various muds—3’El readingsfor 100% cementedand 0%

cementedpipes

Index 47

Concord..

99/79901 0 U 0) 0) U 0

How to use the Manual

• Who Isltwrlttenfor?For PE5 and DEs within BP who maybe involved in planning cement job evaluation,witnessing the logging process, interpreting cement bond logs, or making squeeze job decisions.

• How touseIt?Much efforthas been made to ensure that it is easy to use in field applications. Beginner or expert, you can start where you need:

Job planning and witnessing §1, §5

(“s”

means section) Log interpretation §4, §6-8

Undertaking squeeze job §8

Beginner or requiring basics §1-4 and then §5-6

The manual is cross referenced by~(~*) and supplied with working examples. Wherever you start, you should find the needed information. If you do not get what you want, let us know and we will sort it out for you and improve the manual!

• Updateyour expertise:The manual has some new results from recent research. If you are already an expert in interpretation, you are advised to readthrough at least §6 to update your expertise.

Structure

What’s included

~fs’~SKey knowledge of the tools and their selection (~1) • Principles and jargon used (~2)

5Operation OC in three phases: (~5) Before the job, during the lob and after the job

5Log contents and format (~3) • Operation OC review (~6)

~‘ISInterpretatIon including squeeze consideration (~7)

5Examples and reference data(~9&10) • Cement quality, probability of zonal isolatIon

Conclusions~ S Squeeze job recommendation

Sinvalid log

Comments/queries...~ SComments on the manual o~general interest

to DCB RCS ‘~— — — — •Help on log interpretation

The highlighted header at the top of each page tells you where you are in the manua~j

The following charts also appear in the

‘Things You Should Know” and “Log

Interpretation” sections. They are collected

here for easy access or quick reference.

Please refer to the appropriate section if any

detail of the charts is required.

Whether to run a cement evaluation

1.Aims of the cement ~ 3. Cernenhng service

Zonal isolation of All appraisal wells If experienced personnel

liners transversing should be bond logged with good performance

reservoirs should and so should most records in a field are

have higher priority production wells used, the number of

~?n~te~te Wr~ai~be loggedcan

In job planning phase these three factors must be carefully considered to descide if the cement job should be bond logged

What tool to choose?

1. Mud weight etc* 2. Achieve aims ofthe logging 3. Logging service company For OBM>lOppg & • Capability of the tools (§1.2) • experience of personnel

WBM>1 3ppg only CBL type • Importance of isolation • performance oftheir tool oftool can be used • Possible cement conditions • costs

*Please refer to§10. land §10.2 for other constraints

factors.

4l1I~~~’

Although in most cases both CBL and CET types of tool can be

I

used (~1O.1,the most important is QC), avoidthe CBL when there

I

are:

1) Intervals containing a ~s-annulus(~2.12)which logging under

I

pressure failed to eliminate(~4.2,~9.3)

2) Intervals where isolation is required contain fast formations

~ §9.4)

When to run the

log?~

To avoid green cement (~4.12,§9.8): do not start logging~”)

I

within 8hrs after the cement has set

I

I

2) To avoid l2-annulus especially for CBL: do not reduce thel

I

pressure in central hole after cementing and before loggingl

U~L1,~9.2)

J

The Flow Chart on the opposite page offers a systematic way to interpret a CBL log. The actions at each step are briefly explained on this page. See §6 and the given references for detail.

check:sJob planning execution

• ‘The 5 data sets’ (see p22) >Mainly in log header SThe logging engineer’s comments

• The presentation Log completeness

Take 1 or 2 minutes to seeif theIT and I E.g.:7”, 29lb,ftcasing The TI should be in

the CBL arein the expected range and the I the order of 270~us(see p22) and CBL

-VDL log has good contrast. i should be from 1to 65mV.

The TI curves are bound to vary. You must

know

why they did on the log in handj

E.g.:the marked zone is probably due to

eccentralisation

-

No fast formation was

confirmed by other logs including

VDL.

ToconfirmTOC, I

good cementand I ‘free pipe” is to i provide key references for the

BPI

Mainly to substantiate TI and Ampiitude indications

Concentrate on zones of interest. The longer the I Elf-Elm

interval of hight BPI value, the better chance of I SPI El -El

zonal isolation (see p26 for an example). I f c

involves the following actions

Special investigation

which may have to be iterative

1. Finding information~

2. Analysing abnormal log behavior

1

3. Calculating the probability of zonal isoIati~

Proceedl* ~ Ye~

fthl

Np~ ~.c~yalid io~

____________ accounted for?

* Either go to the next action or resume the main interpretation flow chart on previous page

This Chart and the one on the next page are concerned with decision of a squeeze job. They ask threecrucialquestions and offer common sense answers. Do not decide to squeeze before answeringthese questions!

1. Where did the cement go?

Analyse the well and cementing

conditions together with the log

:

Not clearly ~cated?~6.2~ Yes

CheckIfanyfluid loss Calculate the difference

occured during drilling from expected value

or cementing(~6) (Note the hofe gauge,

washout: caliper log).

~ N1~~(es

Possible cement Possible heavy The cement is likely Possible bad

loss by large contamination or bad to be still in the contamination

quantities. Such slurry leading to annulus but badly at the cement

cases are green cement. bonded tothe casing top or bad mud

usually easily Re-runthe CBLIf andmaybe to the removal.

Identifiable. possible. formation as well. _____________

Heavily contaminated cement which may or may not be solid.

Contaminated but solid cement on the wide side with mud channel on the narrow side.

Contaminated but solid cement on the narrow side with mud channel on the wide side.

kasy:

Gap between the set cement and mud

the casing cement

contaminated

Thick mud cake between the set cement

cement and the formation, formation

Well conditions and cementing operation vs potential channels

Channel Bad casing- Deviated Displacing cementing

type centralisation wells contamination operation problems

I,

‘.4, Severeiii

_________________

high deviation Washoutsection ~‘o~ei weiis

a,

Some delayed communications observed In porous reservoirs are believed to be caused by the disintegration of the mud cake. This type of channel Is hardly detectable with today’s technology

Bond logs provide vital information for squeeze job design particularly in the following areas: 1. Depths/lengths of communicating channels for positioning

the perforating gun and bridge plug or packers.

2. Azimuth of communicating channels for perforating shot phase arrangement: a 45 degree channel can be missed! 3. Identify the vent for the channel filling substances.

interpretation of the log.

~1. Things you should know

1.1 Whether to run a cement bond log or not can result in substantial expenditure or savings,

1. Aimsofthe cement I 2. Existing knowledge~1 3. Cementing service and is a decision which relies largely on experience, logging objectives (e.g. if only TOC

job the field company is requiredatemperaturelogrun at the right time would be the best) and government

Zonal isolation of

I

All appraisal wells

liners transversing should be bond logged If experienced personnelwith good performance legislation Criteria to choose a particular tool can be dictated by the well conditionscementing operations(~10.1,§10.2), but areusually governedby factors suchas experience,and

reservoirs should I and so should most

have higher priority

I

production wells records in a field are

______________________ used, the number of wells required loggingemphasis andcost. Expensive toolsdo not necessarily give the extra

casing be reduced

than intermediate to be logged can probably information you really need! (This Manual concentrates on the CBL only.)

1.2

QC is the most important part of cement bondlogging. This is because the effects of most

cement job should be bond logyplanning phase these three parameters on the log are not quantitatively known (~4).The best thing to do is to eliminatethem as much as is possible while logging. factors must be carefullyconsidered to descideifthe 1.3 Every tool has its limitations even under perfect conditions, e.g.:

The CBL toolcanonly givean average measurement of the 360° annulus (~2.3).It is

impossible forthe CBLto indicatethe position of a channel. TheVariable Density Log (VDL, §2.4)is aqualitative loganddoesnot indicate how much ofthe annulusis bonded (~6.5).

I

1. Mud weight etc* I [~A~hieve aims ofthe logjI~ijl

3.

Logging service company I The

CET

tool and the US!(~io.i)concentrateonly on thecasing/cement interfacein their

I

For OBM>1 Oppg

&

I

. Capability ofthe tools (§1.2)

1

• experience of personnel data measurement. The ‘cement map’ is in fact an interface map. If a channel is beyond

I of tool can be used • Possible cement conditions

I

• costs

I *PI~serefer to

§

10. land

1.4 Although cement bond logging can be quantitative, itis not always accurate because of the

I

WBM>l3ppg only CBLtYP~j~~Importance of isolation 1 • performance oftheir tool

I

this interface it is not detectable.

I

factors.

I §10.2 forotherconstraints many factors that affect the log ~4). Therefore always remember to reviewtheñill picture including the way the cement jobwas carriedout (~4,§7, §8). Don’t let calculated results

~Although in most cases both CBL and CET types of tool can be override common sense.

I used (~1O.1,the most important is QC), avoid the CBL when there

1

I

are:

I

1) Intervals containing a ~.t-annuIus(~2.12)which logging under pressure failed to eliminate(~4.2,~9.3)

I

2)

Intervals where isolation is required contain fast formations

~ (~4.5,§9.4)

(~iE~

To avoid green cement (~4.12,§9.8): do not start loggin~”~

I

within 8hrs after the cement has set

I

2) To avoid ~.u-annulusespecially for CBL: do not reduce the

I

pressure in central hole after cementing and before~~n~J

§9.2)

~2. Tool Principles andJargon used

2.1 Row the measurement Is made:

The CBL tool has a sonde for measurement and an electronics cartridge for signal acquisition and transmission. The sonde works on

“piezo-electricity”— a physical property of certain materials such as quartz and

piezo-ceramic— ifthe material is deformed, a voltage will be generated

on its surfaces and conversely, ifa voltage is applied to the materialitwill deform accordingly. Mechanical vibrations or “waves” approaching such a material can therefore be converted into voltages and by measuring these voltages the mechanical waves can be analysed.

2.2 How the tool works down hole:

The sonde (see Fig.2.1) typically has one transmitter and two receivers which are in a metal mandrel and are 3’ and5’fromthe transmitter. When the transmitter is fired,itwill send out a cylindrical compressional wave train (usually about 20kHz). This wave train will travel through the mud into the casing/cement/formation structures, where different types of waves such as shear wave wifi be induced by mode conversion phenomenon. Some of the induced waves will travel along the cased weilbore, and on their way they will send their characteristics back to the mud. The receivers in the mud can therefore pick up these waves which carry information about the media.

The early part of the received waveform is found to be indicative of the quality of the bond between the casing/cement interface: the better the bond, the lower the amplitudes. The later parts can tell us, for example, how fast the the sound travels in the formation (~2.10, ~4.5).

The 3’ receiver is dedicated to measuring the first peak of the received waveform, including itsarrival time and itsmaximumvalue, conventionally called El (~2.5).The arrival time is used to check if the tool is properly centralised for a valid log (~6.3),and the El value is used for bond quality calculations (~6.6).The 5’ receiver records the whole waveform to produce either the VDL or the signature log (~6.5)or both. This provides more information to help detect the bond (~6.5).

2.3Importantfeatures of toolstructure:

The transmitter and receiversare tube-like andwill respond to mechanical waves without telling their radial directions. This means that the CBL measurement is an average of the circumference and is unable to detect the azimuthal position of an uncemented area in the annulus, known as a channel.

Fig. 2.1 CBL sonde

To prevent the housing mandrel from “short-circuiting” the transmitted wave, it is cross-sectionally slotted. Consequently the sonde is not rigid and can bend under its own weight. The tool centralisation should take this into consideration (~4.2).

2.4 What a log looks like:

A sample ofa common CBL log is shown in Fig.2.2. The left track is the Transit Time (Ti’) curve (~2.7).Usually in this track there are also a gamma-ray log and a casing collar locator (CCL) log for depth tie-in. The middle track is the CBL amplitude curve which is a continuous reading of El(~2.5).The right track is the VDL log which is produced by applying a simple processing to the waveforms received by the 5’ receiver. The processing is essentially thresholding and stacking: positive peaks are represented by black line segments and negative peaks white ones; these line segments are then stacked along the well depth and the VDL log is created. If the waveforms are stacked without the thresholding treatment, the log created is called signature log. Note these curves may be named with different mnemonics or in different scale, e.g. extra letters may be used to distinguish curves generated by sliding gate (~2.6)from those by fixed gate.

Fig. 2.2 Log sample

The log interpretation is all about making sense of the curves in the context ofthe cement job and the well.

Jargon:

2.5 El, E2, E3 etc. (Fig.2.3)

El means the amplitude and duration of the first peak,conventionally stated in mV. Similarly E2 is the negative peak following El, E3 is the next positive peak, and so on.

2.6 FIxed gate, Sliding or Floating gate

(Fig.2.3)

For data reduction, the electronics starts measuring El only when it is about to arrive, and stops measuring when it has passed. This measuring period is called a gate. It is vital that the gate is opened in the right position on the waveform in order to see El. There have been two ways of setting the gate: the fixed gate and the sliding gate. The fixed gate is set by the tool operator to straddle El. Once set a fixed gate will open and close irrespective of the waveform. The sliding gate is triggered open by the waveform when it has first reached a preset detection level. Mostof the time both typesofgate will givethe same value ofEl. However, when the El position is caused to change by certain conditions (e.g. fast formations, §2.11), a fixed gate could miss it but a sliding gate would pick it up. On the other hand a sliding gate could be triggered open by E3 instead of El if the latter is lower than the detection level. Therefore both types of gate are now commonly used together.

2.7 Transit Time (Ti’) (Fig.2.3)

The Ti’ is the time span between when the transmitter is fired and when the waveform at the receiver has reached a preset detection level. The sliding gate is opened at the TI’. Note: unless El amplitude coincides with the detection level, the Ti’ is not the time when El reaches its maximum value. See also §4.2.

2.8 StretchIng (Fig.2.3)

Stretching means the increase in the T~due to the decrease in El caused by, for example, increased bond quality. Because the Ti’ is related to the detection level, a decreased peak wifi reach the detection level later and thus “stretch” the Ti’.

2.9 Cycleskipping(Fig.2.3)

When the El value for some reason (e.g. very good bond or severe eccentralisation) becomes lower than the detection level, the first time the waveform reaches the detection level could be part of E3 (or even E5, E7 etc. if the early ones all fall below). The TI’ measurement wifi skip a cycle (or two cycles, three cycles and so on). The TI’ will be increased by roughly an integer number of the wavelength.

2.10 CasIng arrivals, Formation arrivals, Mud arrivals (Fig.2.4)

The received waveform is extremely complicated. It is a combination of wave trains which have gone through different media such as casing, formation and mud and consequently carry information about them. Casing arrivals, formation arrivals and mud arrivals are terms to refer to the corresponding portions ofthe waveform. Because the tail of one type ofarrivals will always be eaten by the head of the next, on a single waveform one cannot clearly see the joints of two types of arrivals. However, when the VDL log is generated, the features of these arrivals usually stand out as shown.

The features of casing arrivals and mud arrivals on the VDL are straight stripes starting at relatively fixed times. This is because the acoustic properties of the steel casing and that of the mud column are usually homogeneous. The sound velocities in the formations, however, can vary substantially along the well, making the formation arrivals wander in

time as shown in wiggly stripes. Casing arrivals Formation arrivals Mudarrivals

2.11 Fast formations

Fig.2,4

Formations in which sound waves travel typically faster than in steel (571.is/ft) are conventionally called fast formations (~4.5,§10.3). On the VDL log, fast formation arrivals will appear before the casing arrivals and override them (~9.4).

2.12 Micro-annulus

Micro-annulus refers to a minute gap between the casing and the cement. Such a gap damages acoustic coupling between the casing and the cement although usuallyitdoes not permit fluid communication. The development of a micro-annulus and at what sizeitwill invalidate the measurement of bond quality has not been fully understood (~4.1).

A micro-annulus can make the CBL log look as if the casing was partially or completely unsupported. On the VDL log there will be strong casing arrivals as well as strong formation arrivals (~9.2).Once a micro-annulus has occurred,itis not possible to quantitatively estimate the bond conditions because the micro-annulus could mask coexistent channels.

2.13 Free pipe

Free pipe is a section ofpipe which is not cemented. However, some engineers have been using the term to describe a log which appears as ifthe annulus were not cemented. Inthis caseitdoes not necessarily mean that the annulus is free ofcement or indeed squeezable. See §9.1 on p 34 for a free pipe example.

~3. Information included in the log

It is not imperative that logs from different service companies have the same format. But a complete log should have four parts: the header, the body, the summary or tail and the questionnaire. See example shown in §9.1.

3.1 The log header should include the following:

- GeneralInformatlowlog types, well name, log time and date, rig name and type, location,

log measurement base, log scale and run number

- Wellgeometrical data.deviation, depths and bit sizes of hole sections, depths and sizes

and weights of casings sections, top and bottom of logged intervals

- Wellfluidsdata:type, density

- Cementingdata:type, slurry densities, volumes, additives, retarders, starting and finishing

pumping times, lab thickening and setting times, spacer type and density and volume, fluid loss volume

- Wellpressure and temperature data: pressure applied afterbumping the plug, pressures

applied at the time of logging, temperature profile

- Logging equipmentdata. modules number, calibration status - Toolstringsketch: centraliser types and positions

- Loggingengineer’scomments: record the aims of the logging, events which may have a

bearing on the log and express views on the quality of the log

3.2 The body of the log should include the following where applicable:

- A “freepipe”reading sectiowrecord about two joints of pipe if available

- The“main log”:record the main interval(s) of interest - The repeatsectiowrecord about 200m

The title ofeach of these sections should also include the pressure applied even ifitwas zero. All curve scales and legends should be clearly and correctly indicated. Less conventional mnemonics should be explained1 in the title area.

‘Not In wide practiceyet If you witnesslogs,you can help speed up this process!

3.3 The log tall

Should include summaries of tool operational status, software input parameters, and tool calibration before survey (~9.1).

3.4 The BP questionnaire (“Log Quality Control Sheet”)

Should be completed and signed by the logging engineer, and included as part of the hard copy log.

~4.

Parameters affecting the Log Results

Some key parameters are discussed here. Because most of their effects are not quantitatively known, it is important to understand the mechanism by which they affect the log, and assess which parameter would carry more weight than others in a given situation.

4.1 MIcro-annulus

How it occurs: Not fully understood yet but a micro-annulus can be created either by casing

contraction after the cement has set, orby casing expansion under high pressures that break the cement bond. Casing contraction can be thermal or mechanical. Casing expansion is usually caused by high pressure such as occurs during a squeeze job. The micro-annulus can therefore be classified into 3 types: thermal contraction, mechanical contraction and expansion. The thermal contraction type is due to the heat released from cement hydration. The casing contracts after the cement hasset and the heat has dispersed. This type ofmicro-annulus depends on the cement sheath thickness and composition, and the thermal conductivity ofthe formation. The mechanical contraction type is caused by reduction in pressure, for example, by changing casing fluid to a lighter one after the cement has set, or holding the pressure inside the casing before the cement has set and releaseitafterwards.

The expansion type is usually caused by squeeze pressure that permanently damages the bond. When the pressure is released, only the casing resumes itsprevious size but not the cement.

How it affects thelog: When the cement is bonded to the casing, the acoustic energy is transmitted from the casing to the cement easily and is thus heavily attenuated, When a micro-annulus has developed, the energy transmission is severely hindered and a large proportion is trapped in the casing. (A gas filled micro-annulus is much worse than a liquid filled one in terms of energy transmission). The casing then rings relatively freely, producing strong casing arrivals on the VDL log. The El amplitude will be high, indicating that little bond exists. Particular problems with micro-annulus are:

1) It is not possible to distinguish a partially bonded annulus with a channel from a cemented annulus which can provide isolation but with a micro-annulus.

2) The effect of a micro-annulus can be so bad that the log may look like that the pipe is completely unsupported. This must have led to a good proportion ofthe failed squeeze jobs.

Becausethe CBLtool measures the bond between the cement and the casing, the micro-annulus that destroys this bond is the most severe factor that affects the log results.

How toprevent it:Obviously try not to create the conditions mentioned above under which a micro-annulus may occur. A method practised by some oil companies to prevent it from occurring is pump the cement wiper plug with a light fluid and change back to the weighted mud after the cement have set. Or even circulating the light fluid to cool the casing while the cement is setting. Because a micro-annulus does not usually permit communication and only affect the log, it is more important to eliminate it, if it has occurred, at the time of logging. This can be done by pressurising the casingusing a wireline packoffor sometimes using a heavy mud to increase the hydrostatic pressure (~5.2).

Type ofmlcro-annulus Be prepared to pressureupto: Thermal contraction l000psi

Mechanical contraction Reduced pressure (hydrostatic or wellhead) +l000psi Expansion Max squeeze or hydrostatic pressure applied Limited by~burst pressure of casing; Casing pressure test; Liner top test.

*After the recent cementing. See 4.2 for pressure determination during logging. 4.2 Tool eccentridty

Howit affectsthelog: When the tool is off the casing centre, the acoustic energy from the

transmitterwill not reach the casing circumference simultaneously (~2.2).Instead part of the cased weilbore which is closer to the tool forms a shorter path for some of the energy to go through. Consequently this will cause reduction in the CBL amplitude as well as in the

Yr

as shown in Fig.4.1.

The

Yr

has been used as a log quality indicator. Traditionally when the U reduction is less than 4~.tsthe log is accepted with an error of unknown magnitude. Recent research results tell us that the amplitude has a unique relationship with the amount ofeccen-tricity but it is a multi-value function of theYrreduction(~1O.5).The importance of these results are two fold:

1) When “minor” eccentricity (e.g. Ti’ reduction 4~ts)occurs the amplitude reduction caused by eccentricity can be compensated for.

2) The fact that the amplitude reduction is not uniquely determined by a given TI reduction reveals the limitation of the U being used as a quality control indicator.

How to prevent tool eccentricity: Not only sufficient number of centralisers are needed, but

they must be put in the right positions. The following points should be observed:

1) Type ofcentralise?s: The rigid metal type seems to be the best. The rubber fin type is the second and the bow spring type is the worst. Worn centralisers can be weak and ineffective, but they can be checked by visual inspection.

2) Number of centralisers: In vertical wells use minimum of three centralisers and in deviated wells use minimum of five. Always request at least two extra ones for the job in case any faults develop in the mounted ones (~5.l).

3) Where to put them: For vertical wells put centralisers immediately above and below the transmitter-receiver section and on top oftool assembly (CCL or GR). Note the casing collar locator is not an adequate centraliser! Without a centraliser atthe top the CCL and GR section may act as a lever arm to promote eccentering problem. For deviated wells add a centraliser to the centre of each section which does not yet have a centraliser. Preferably always add an extra one at the near receiver which is used for CBL amplitude measurement. 4.3 ChannellIng

How it occurs: When the combined conditions of cementing operation and down hole

geometries are such that the cement cannot displace all the mud from the intended section of the annulus, pockets of mud may reside in the annulus and formmud channels (~7,~8).Another less recognized type of channel is the mud-cake channel due to filtration often occurring between the cement sheath and the reservoir formation. A channel may not be a problem if it does not communicate. However, you do not know untilitdoes!

How it affects the log: Ideally we want to detect any channels and like them to affect the log

as much as possible so that we can identify them. Unfortunately only those channels which are immediately next to the casing have a strong bearing on the log. Others are more difficult to observe. This is because of the energy transmission mechanism, as discussed in ~4.1 and ~2.2. When a mud channel occurs next to the casing,a large portion ofthe acoustic energy in the casing corresponding to the channel will not be transmitted to the formation. As a result more energy is returned to the receiver and the El value becomes higher (~6.6).For channels awayfrom the casing, however, this energy transmission mechanism is not presented in El but later in time, and is usually drowned in the complicated waveform.

A mud-cake channel can sometimes be identified from the VDL log when the cement/casing interface iswell bonded. Because the mud-cake channel provides very weak coupling, not much energy could go into the formation and the majority would be absorbed by the mud-cake and the cement. As a result the VDL would show weak casing arrivals with little formation arrivals. 4.4 CasIng coating

Some casings have a layer of coating such as epoxy. If this layer of epoxy is thick (e.g. >70 mils), it can affect the log in the sameway as a micro-annulus. Butpressurewhen logging will not affect the CBL.

4.5

Fast formations

Whatformations are they: They are usually strongly packed hard formations such as

limestone and dolomite (~10.3).The sound velocity in the formations is affected by the forces they are subjected to as well as their microscopic structure. Therefore the velocity may vary slightly in the same type of formation at different locations.

How theyaffectthe log: When fast formations are present, the sound wave in the formations

is faster than that in the casing. The latter, however, isthe bond quality messenger. The real El is distorted, or even drowned in the formation arrivals. What is measured has therefore nothing to do with cement bond quality. Fast formations make itdifficult to evaluate the cement job.

How to detect them. Fast formation arrivals are easily seen on the VDL log (~9.4).The

Yr

will be shorter and the CBL amplitudes may be high. Remember that U reduction can also becaused by tool eccentricity. It is usually easy to tell fast formation from tool eccentricity by examining the VDL log, but it is difficult to see if the log is affected by the combination of the two. The formation arrivals on the VDL log can be confirmed by the open hole sonic log (~9.4). 4.6 Mudtype and conditions

How the mudaffects the log: The mud (or other casing fluid) is the medium for the acoustic

signal to go to the casing/cement/formation structure and come back. It does not distort the shape ofthe signal but affects the amplitude: any medium will attenuate the acoustic energy by scattering or absorbing. Different mud will have different attenuation rate whichaffects El

amplitude. The sound velocity may also change with different mud conditions, thus affecting the U.

Mud parameters that affect the acoustic attenuation and sound velocity are complicated. The mud density and the sizes of particles contained in it are two major ones. The general trend is that the denser the mud, the less attenuativeitis (~1o.7).In other words, in dense mud the measured amplitude will be higher than that in lightermud under the same weilbore conditions. Tiny gas bubbles in the mud can affect the log by increasing the TI’ and reducing the El unpredictably.

4.7 Temperature

How it affects thelog:There are two aspects of the temperature effect: firstly on physical conditions of the wellbore such as casing fluid density, and secondly on the tool. A good tool should be insensitive to temperature changes. The response of such a tool to the changes in weilbore conditions due to the well temperature profile should be stable and repeatable on the log. In most wells this response is small but in high temperature or high temperature gradient wells this can be noticeable.

When temperature changes, the sensitivity ofthe transducers will change, and so will that ofthe electronics. The output of the tool will inevitably include some error.This error can be ofsteady state (when the tool is used to the new temperature) or transient (when the tool is not used to the new temperature yet), but usually both.

The overall temperature effect cannot be quantified. In HPHT wellsitis important to observe the logging time and the repeatability of the log since the log validity can be severely impaired under these circumstances.

4.8 CasIng diameter and Casing thickness

How do they affecttbe log: In casings ofdifferent OD, the attenuation change is mainly caused

by the different length of the mud path: the larger the casing, the lower the CBL amplitude. The amplitude decay rates also depend on the type of mud in the casing (~l0.7).The mud conditions and the temperature effect are closely linked and should be considered together.

A general rule for commonly used casing size is that a millimetre increase in the thickness ofthe casing would cause about lmV increase in the “free pipe” CBL amplitude. The reason is that the thicker the casing the less acoustic energy would be transmitted to the cement, and the energy trapped in the casing willbe more difficult to damp.Jointsofdifferent weights in one casing string should be seen from the U curves.

4.9CasIng damage

Howdoesitoccur: Casing can beworn by drill-pipe, or splitby excessivepressure. Perforations

of course blow holes in it. Corrosion can cause serious pittings in casing.

How it affects the log: Casing wear or corrosion can cause tool eccentricity problem and thus

reduce the El amplitude. Perforations can damage cement bond, especially for weak cement (compressive strength <2000psi), by cracking the cement. Split casing can cause the tool to become stuck. Severe irregularity in the casing will be reflected in the log. For example, casing collars damage the continuous transmitting of the acoustic energy and this can be seen from both the U and the VDL log of light or less well-bonded cement.

4.10CasIng standoff and Geometryofthe open hole

How do they affect the log: When the casing has a 0% standoff (i.e. touching the formation),

there will be some formation arrivals coming through in the VDL log regardless of the cement conditions in the annulus. The contact between the formation and the casing may also deform the El, giving incorrect bond indications. This can undoubtedly mask channels.

The geometry of the open hole would affect the log in two ways (~7).First, if the annulus thickness isless than 19mm (0.75”), the El might be deformed and increase because ofreflection from formation catching the first half of El. (Since this depends also on the cement and the formation, especially the cement/formation interface conditions, this thickness is only a rough guide). Secondly, in washout areas especially in deviated hole, mud removal is much more difficult. The thermal conditions in washout sections are different from the rest of the cemented annulus. This may add to the complication of micro-annulus generation and its detection. Small casing standoff can leave very thin cement in part of the annulus and cause the El deformation problem. Itis reported by Dowel Schlumberger that casing standoff from 100% to 0% could cause 30% increase in the El.

4.11 Double (or concentric)casing strings

How do they affect the log: When the annulus created by one casing string inside another

(eg. the liner overlap) is cemented, the log usually shows the following features to indicate a good job (~9.8):

1) The U curves will be increased. In a 7” and 9 5/8” combination for example, the increase is typically 20M~~

2) The first few cycles of the waveform shown on theVDLlog will be narrower, i.e. gaps between the first few stripes are smaller than usual. Also, the apparent casing arrivals can be so strong that formation arrivals are masked.

3) The CBL amplitudes, which are not useful in such an interval, can be high.

Arough explanation for these isthat the well-bonded concentric casinghas caused the frequency ofthe first few cycles to increase, while high attenuation has caused El to be skipped, resulting an increase in the U and measurement of E3 as amplitude output (~2.9).

4.12 Wait on cement (WOC) time

How does it affect the log: Cement slurries take time to set and bond to the casing. Since El

represents the bond quality of the cement to the casing, it willchange from free pipe value, before the slurry thickens, to bonded value, when the cement has set.

WOC time should be the minimum time to wait before logging. Logs produced at marginal WOC times can be very tricky because the down hole conditions are not the same as in the lab and itmay well take a bit longer for the cement to set; Beside, the slurry can be contaminated which may stretch necessary WOC time (~7).For foamed cement, this is often complicated by the fact that some foam agents are retarders and are sensitive to contaminations. It is not possible to tell greenish cement from a channel.

It is very important not to log the well too early: one hour saved may cost you ten! It is always good practice to monitor the timing of the logging and the slurry properties when interpreting a log (~4.l3).

4.13 Cement parameters/conditions (~7)

Whatare they: The slurry density, cement class, additives types (retarders or accelerators) and

weighting agents are all things that should be known.

How do they affect the log: For neat cement slurry, the heavier it is, the highertheacoustic attenuation it will have when bonded to the casing. Other solids such as bentonite and silica added in the cement may increase the attenuation.

The acoustic attenuation of foamed cement is lower than that of the neat cement, resulting in higher El values for 100% bonded casing. However, the CBL response to foamed cement has not been systematically studied and therefore is more difficult to interpret.

4.14 Computerkeyboard operations

What are they: Software parameter setting for communicationcontrolandlog result formatting (~3,~9.1)

How do they affect the log: It isthe engineer’s keyboard operations that make the toolcorrectly

function and the data properly recorded. Apparently small errors on the keyboard can spoil all the hard work, and should not be underestimated.

In the log presentation, small mistakes such as incomplete information, wrong legend or scale for curves, can cause frustration and time loss in the interpretation, or even big mistakes by inexperienced personnel,

0

I.

a C.)

To meet with the logging

To help logging engineer

engineer to set clear

carry out a

hi9h

quality

To accept or reject the

objective & to supply

job by observing the

log (~5.3)

sufficient information

correct procedure (~5.2)

(~5.1)

___________________ ___________________

5.1 Before logging (Phase 7): Meet with the logging engineer and discuss:

1) The aims of the job (zonal isolation, finding TOC or else?) 2) Well conditions and history including (~3.1):

- Well ambient conditions (deviation, temperature, formation structure, gas or oil reservoir

intervals, intervals of interest, any anomalies)

- Well fluid characteristics (OBM or WBM, weights, gas/solid contents and sizes, fluid

change after cement job?)

- Casing characteristics (sizes, weights, depths)

- Pressure history (BOP test, casing test, formation test, fluid change). Discuss whether

pressure will be needed during logging if pressure was applied to the well after the cement has set or pressure was maintained during the cement setting time.

3) Cement job (~4.13 cement type, density, special additives, estimate setting time, volume, pumping time and any problem in cementing operation). Make sure logging does not start within 8hrs after the lab cement setting time (~4.12,~9.8).

4) Choice of tools (~1),tool string configuration, centraliser types numbers and positions (~4.2), special/back-up equipment needed (such as wireline packoff)

The purpose of this meeting is to set clear objectives for the logging engineer, and supply

sufficient information for him orher to plan and prepare for the job. The meeting should be held at the earliest possible time. The logging engineer should then produce a job plan which can help you take appropriate actions and plan ahead. The logging engineer’s plan should include: 1) The objective of the specific logging and the approach to achieveit.

2) Estimated time for logging operation, including a detailed plan of wireline up and rig-down.

3) Indication of any further information needed in order to carry out the job successfully and to present the log completely (~3).

CBLEvaluation Manual-QC and Interpretation Page 17

Once a decision is made that a log is to be run, the operation QC should be carried out in the three phases as shown in the following chart:

QC Matters

5.2 DurIng logging (Phase 2)’

On the well Site we are to help the logging engineer to carryout a high quality job by observing the correct procedures. The logging engineer should have a detailed plan of when to do what, but he or she may well alter it as things change.

A check lists

1) If the well was pressurised or the well fluid was changed not as originally planned, discuss with the logging engineer if pressure should be applied during logging to prevent the possible micro-annulus effect (~2.12, ~4.1,~9.2).Ifwell head pressureis needed,the following is recommended to determine the pressure required:

- Run a0psi repeat section

- Identify a potential micro-annuluszone (fom-iationsignalsbehindcasingsignals) - Stop the logging tool at themicro-annulusdepth

- Switch thepanel to time drive i.e. the screen display as if the tool was moving - Tightenthewirelinepackoff andstart pressure-up (pumpmudslowly into the casing) - Monitortheamplitude untilitno longer drops

- Stop pressure-up - Log underthis pressure

2) The centralisers are the required type/size and in good condition. Also check that they are correctly mounted and secured in the required position (~4.2).

3) The tool used should be calibrated and the next calibration date has not expired. 4) The tool string is correctly connected and tested before being lowered into the well. Tool

calibration before and after the logging run.

5) The parameters/constants Set from the keyboard are correct (j3).

6) Do not exceed the maximum logging speed. 7) The scales and markings are correctly set.

8) Monitor the curves on the screen. Discussand record any suspected problemwith the logging

engineerand suggest to repeat abnormal sections. IftheTI’does not repeat and the difference in amplitude in the repeat section is>10%,it could be a centralization problem. Pull out of

hole and change/add centralisersifnecessary.

logs for immediate use, but also for documentation. The logs may be referred to after the wel has produced for several years. Missing information can be irretrievable and render the lo~ meaningless.

A check list

1) All the signals have been correctly recorded. Authoriserigdown. 2) The scales and legends used are correct (~9.1).

3) The log header, tail and the questionnaire etc. are fully completed with no incorrec’ information.

4) The logging engineer’s comments have included and explained all quality-related incidents the aims of the logging and his or her opinion on how well they were achieved.

5) The log hard copy is delivered on the time agreed. Authorise payment if there are no QC problems, otherwise raise them with the service company.

6) If any event occurred during the logging which may have affected the log, prepare a repor describing that event in detail. Attach a copy of the report to the hard copy of the log wher it has arrived.

0 a 0, a. 4, at 0 -J

A systematic way to interpret a CBL log is demonstrated in the interpretation flow charts, Go to the references given if you are uncertain at any stage.

You may have to break the main flow chart for special investigation which is shown on page 27, and resume afterwards.

INTERPRETATION FLOW CHART

6.1 QC Review

If the log is properly documented, it should contain most ofthe information needed for correct interpretation, The majority of the key parts are included in the log header (~3).Check the following four items. Anything missing or incorrectly recorded will at least cause time loss or an invalid log.

Item 1: Theplanning and execution of logging Clear objectives? Effective effort to prevent

micro-annulus and eccentricity? Logging actively witnessed— witnessing engineer’s signatures

and relevant reports?

Item 2: Thefive data sets are complete and correct (~3.1):

1) General information 2) Well geometry 3) Well fluids

4) Cementing operation

5) Well pressure and temperature

Item 3:The logging engineer’scomments Clear and relevant in addressing any problems. The

answers to the questionnaire can also imply his or her competency.

Item 4: The log presentation Header-body-tail-questionnaire, signatures and date complete?

6.2 Quick check ofrangesof the various curves

77 Curves: The normal ranges of U curves depend mainly on the casing ID and mud type/

weight (affecting the sound velocity). Temperature and the type of tool (the size of the transducers) also have some effect. A rough guide for calculating this range is

TT(~is)—(casingIDeg. In mm)/(sound velocity in the mud eg. Inmn’z.~us§10.3)+170(Ms)

The last item is the distance of T-R spacing (3ft) times the sound speed in steel (57~ts/ft).

CBL Curves: The normal ranges ofCBL for unbonded pipes depend mainly on the casing sizes

(~1O.8)and mud type/weight (~10.7).The ranges for 100% bondedpipes are less reliable because of the difficulty in controlling the test conditions. For foamed cement there have not been sufficient field data or lab results for fully bounded pipes and the interpretation is therefore more difficult.

VDL/Signature log: Check whether the time scale/range are correct (in the above equation

replade 170 by 285, which corresponds to Sft), the log has good contrast, and there are any fast formations (~9.4).

6.3 ExamIne the fl Curves

The purpose of examining the U curves is to explain the curve variation, ifany, and investigate the log validity.

The IT curves variation.The U curves from a properly centralised tool runinuniformly cemented pipes should ideally be straight lines in the expected region (~6.2).Parameters affectingtheU curves include centralisation (~4.2,§9.3), casingID size/weight (ID)changes (e.g. casing collars) (S4.8), casing fluids change over different depths (~4.6,§9.5), fast formations (~4.5, §9.4) and temperature (j4.7, §9.6). The U curves will also vary to indicate stretching, cycle skipping (j2.9) and well bonded double-casing string (~4.11,§9.7).

A log validity criterion.A widely quoted log validity criterion in the literature and various manuals is that ifthe U curves vary for more than ±4iis,the log is invalid. Be careful. Variation of the U curves of a real log is rarely within this limit: investigate if the cause was eccentricity! When the tool is not properly centralised, the U can be shorter than normal. Unfortunately the amplitude will also be reduced. The effects of most other factors mentioned above on the amplitude are likely to be relatively small. Traditionally the log is treated as invalid because of the unknownreduction in the amplitude. This nowhas been better understood (~4.2,§10.5) and the amplitude reduction due to eccentricity can be calculated. However, because the uncertainty of the effects of other factors still exists, it is recommended that similar criterion be used with the U reduction limit being 5.us (~10.5).

Note the transient temperature effect can make the log invalid (~4.7,§9.6) and so can fast formations and micro-annulus— the amplitudes in the interval of interest are not quantitatively

reliable.

6.4 ExamIne the CBL Curves

CBL curves: Look for Top of Cement (TOC) if

applicable (e.g. a non-liner job), where the CBL curves swing from the low end of the value range to the high (~6.2,§9.1). Is the TOC in the expected

region? Check with the annulus size and pumped _____

cement volume. A low TOC measured is often

associatedwith slurry loss anda high one incomplete mud removal. Ifthe TOC is not found the slurry has been contaminated at least in the top section (~8).

Look for good cement section where the curves are at the low end of the value range. Note ifa lead and tail slurry system was used, a difference should be

seen in the good cement sections found, where the Leadcement

tail cement should give a lower reading. These will be useful in qualitatively evaluating the cement job.

If no good cement section is found, the problem Tail cement

could be heavy contamination but refer to the Special Investigation Chart on page 27.

Look for “Free Pipe” (uncemented pipe) section if applicable. Do not be misled by high readings of the CBL curves: check for the micro-annulus (~4.1)!The CBL values for free pipe offer references in cement quality evaluation (~6.6).

6.5 ExamIne the VDliSignature log

The VDL log: must have good black and white contrast. It contains much information but now

only a small portion is extracted, and this is done by visual examination. The main uses of the

VDL log are: --___________

1) to detect micro-annulus (~9.2) 2) to detect fast formation (~9.4) 3) to confirm free pipe (~9.1)

4) to confirm good bond in double casing strings (~9.7) 5) to confirm good bond to the casing but bad bond to

the formation (where the casing arrivalsare extremely low with little or no formation arrivals and CBL

amplitude indicates a good bond).

The signature lo& whichis often superimposed on the

VDL log, produces the wave amplitude information which is not available on the VDL. This information can be useful in confirming changes in bond quality. How-ever, the signature log is not as easy to use as the VDL in detecting, for example, the formation arrivals and that is why it is often combined with the VDL log.

The indications of the U, the amplitude and the VDL logs must be in agreement and their examinations should be in parallel. Ifthey do not agree, there could be a tool problem resulting an invalid log.

The rest of the annulus (100%-BPI) is not well bonded, which may be contaminated cement,

localised small gaps between the cement and casing, or a channel.

Angle of bonded cement Elf- Elm

BPI (%) - - x 100%

The whole arinulus Elf- Elc

Where the El’s are from the same receiver (e.g., the 3’ receiver), and the subscript m represents the value measured in the zone of interest,

f

represents free pipe value, and c the value for100%

cemented pipe.

The BPI applies to any type ofcement system (neat, foam, etc.). Note when a lead and tail slurry system is used, El c should be selected separately for the two slurries. When Elfand El c are available, the corresponding BPI at a given Elm can be found from the above equation, or as shown in Fig.6.l below.

FIg.6.1 Howto findthe BPIgraphically

On the Elm axis, mark El~and Elfvalues. Mark the BPI (%) axis by equal intervals from 0 to 100. Draw a straight line from (Elc,lOO) to (EljçO). Given a Elm value, the corresponding BPI can be found as shown.

To provide a sealwith high confidence, BPI need to be around 95% orhigher for certain lengths (~l0.6).For gas wells this rule should be applied more stringently.

Log Interpretation

BPICalculationExample

Below is a section around the shoe ofa 7’, 29 lb/ft linerjob run atthe Gulf ofMexico. Assuming that the log has passed the QC and quick examinations, let us see how the BPI is calculated. Suppose the cementing operation and other well conditions allow us to believe that at least a section of

perfect cement job can be achieved, we can then select the lowest reading, 5mV, as 100% bonded value Elc. The “free pipe” value Elf is nonexistent(hopefully!) in a linerjob, we therefore look itup from §10.8 anduse 62mV (or we can use an available value from a log with close conditions as the job in hand). Between points Aand B the average reading of7mV gives the BPI-(62-7)/(62-5)-96.5%; BetweenB and C the average reading is about9mV. The effect of the slight U reduction (3—4~.ts)caused by eccentricity can be compensated for (~10.5),in this casebyincreasethe amplitude by about 10% to lOmV. The

corresponding BPI here is therefore (62-10)/(62-5)91%.

If El~cannot be clearly def’med on the log, we can also use a reasonable value elsewhere, e.g. 2.4mV from §10.8. The BPI for the two intervals will be (62-7)/(62-2.4)92% and (62-l0)/(62-2.4)—87% respectively. As there exist about90% bonded intervals for30ft, the probability of zonal isolation is high. Notemore than 50%reduction in Elc(from5mVto 2.4mV) haschangedthe BPI valuefor <5%. Variations in Elf does not affect the BPI much either. This means that for conventional jobs the selection of the reference valuesis important but not critical for a reliable interpretation. Preferablyboth 0and Elf are decided from the log whenever possible. This method applies to all types of slurry design.

* Either go to the next action or resume the main interpretation flow chart on previous page

6.7 Finding Information

If information in the log is incomplete, or other information is required such as open hole logs or some detail of cementing operation, contact the right person to obtain it. Keep an updated list of contacts for necessary help: their name, specialities, company, base, telephone and fax numbers etc..

6.8 AnalysIng abnormal log behaviour

1) Review all factors which are likely to affect the log (~4).

2) Eliminate one by one around the key problem in hand until only such factors whose effects may be significant.

3) Judge whether these factors have invalidated the log— a good understanding of all the

factors (study §4) may be very useful.

6.9 CalculatIng the probability ofzonal Isolation

Cementing operation can offer valuable common sense evaluation which should not be discounted (~l.4).If any steps in the operation were altered from planned, itis suggested that the post-job CPS be run (~7).A starting point is to judge the likelihood of at least some sections which are channel free. Ifno such sections can be assured and the log shows a very bad cement job, a remedy job may have to be considered (~8).

For the BPI calculation, the El value for 100% bond (Sl0.8) may have to be found elsewhere if that value for the given conditions is not available in the literature. The log of a well of close location with similar conditions can prove useful to provide this reference. The value can be chosen by finding the lowest reading at intervals where isolation was required and achieved. Make sure there were no fast formations or micro-annulus (~4.5,~4.1).

6.10 Keep a record of Interpretation

It is recommended that the interpretation of a log is recorded, eg, on the back of a log, for ease of later reference. The rubber stamp issued together with this manual is for this purpose. The table from the stamp is designed to summarise the log with minimal amount of information. Stamp the back of the log and fill in the form as you complete the interpretation.

~7.

Cementing Operation

1. What to check~

Do not come to conclusions on the quality of the cement job without reviewing the cementing operation. Every step of the operation is carefully designedto ensure a good job. Problems may occur if any recommended step was not followed correctly. Common sense can often tell you the nature of the problem.

Cement placement itself is a large and complex subject. But simplistically speaking, there are somerules ofthumb. For example, the more centralized the casing string, the better the cement job; Equally important are the displacement rates of mud, spacer and cement slurry. Usually the higher the flow rates, the better the displacement. The flow rates are limited by the formation strength and the displacement facilities. The rheology and density of the fluids are similarly important. The idea is to remove the viscous mud by spacer as completely as possible, then to displace the spacer by the cement slurry. The spacer here is designedto make easy the removal of mud and the placement of cement. Usually a heavier fluid following a lighter one can help the displacement. Remember, non-vertical wells are not only difficult to log, but also difficult to cement because ofthe casing centralisation problem and the complicated flow regimes in an inclined annulus.

Without being deeply involved in cementing technology, you can always get some idea about the quality of the cement job by checking the following:

- Was there any lost circulation? - Was the casing string centralised?

- Were the pumping rates and displacement timing as planned?

- Were the well-site-measuredfluids properties (density, viscosity, additives quantities etc.) as

designed?

Answer “No” to any of these questions could mean a less than perfect cement job. Ifthere were losses to the formation or other operational problem encountered during cement placement, it could be a bad job and log interpretation must take this into account.

2. The Cement Placement Simulator (CPS)

This simulator, developed by the Fluid Mechanics Team at RCS, can tell you how to carry out the cement job and what the cement job should look like. The CPS is located at Dyce, Houston and Sunbury and wifi also be incorporated in DEAP. If a bad cement job is suspected it is suggested that the CPS be run. The result is another reference for interpreting the log.

Log Interpretation

~8 Squeeze considerations

A bad cement job indicated by a bond log (whichever type) does not mean you can go ahead

and squeeze. The following questions must be answered as best as you can before any action: Where did the cement go? What kind of channel could it be, and where and how to squeeze? The following Charts may help you answer these questions.

1. Where did thecement go?

Analyse the well and cementing

conditions together with the log

:

Not clearly~~d?~62~ Yes

Check if any fluid loss Calculate the difference occured duririu d~IIing from expected value or cementing(~6) (Note the hole gauge, _____________________ washout: caliper log).

__________________________ ~ Yes

Possible cement Possible heavy The cement is likely Possible bad loss by large contamination or bad to be still In the contamination quantities. Such slurry leading to annulus but badly at the cement casesare green cement. bonded tothe casing top or bad mud usuallyeasily Re-runthe CBL if andmaybe to the removal. identifiable. possible. formation as well.

Heavily contaminated cement which may or may not be solid.

Contaminated but solid cement on the wide side with mud channel on the narrow side.

Contaminated but soiid cement on the narrow side with mud channei on the wide side.

key:

Gapbetweenthe set cement and mud

the casing cement

contaminated

Thick mud cake between the set cement

cement and the formation. formation

Well conditions and cementing operation vs potential_channels

Channel Bad casing- Deviated Displacing Cementing

type centralisation wells contamination operation problems

I,

I, Severe in

________________

high deviation Washout section Often in_{horizontal wail.}

~~1

Some “delayed” communications observed in porous reservoirs are believed to be caused by the disintegration of the mud cake. This type of channel is hardly detectable with today’s technology

Bond logs provide vItal information for squeeze job design particularly in the following areas: 1. Depths,lengths of communicating channels for positioning

the perforating gun and bridge plug or packers.

2.Azimuth of communicating channels for perforating shot phase arrangement: a 45 degree channel can be missedi 3. Identify the vent for the channel filling substances.

at at U C at at at C at at C. E ‘C

Log Examples

Log Examples

AFTER

PRESSURIZIFICJ AMPLITUDE ~ aEFORE TO3000FSI&

RUN1 RUN2 AMPLITUDE PRESSURING RELEASING

*

9.2MIcro.annukis

F~.ashs1om~rho1ding5Op~onthecasing~r3daysRun1wasmnwithOpsiandRun2wsswith7OOpsi.Adequate pxessuxe cen eliminate some miao-annulus.

Fig.bwlogsninbefoieandasngwsspiessuzeiisedto 3OOO~l.Excessivepressuxecanbreakthebondbetween the casing and thecement

TT (US )~ ___L_~M~___~_. VDL (US

aUO.00 5~._OOO 100.00 1200.0Q 1200.0

9.3ToolEccentricity

Eccentrldty causes reductionintheTrand intheCBLamplitude (~1O.5).Addan exn~centmliser aithe 3’ receivercan alleviate the problem(~6.2).Never run the CBL knowinglyoff-centred (eg with711rubber centraliser for liner, running in95/8” casing above the liner).

9.4Fast FormationsR

The TT fell below the expected value(~5.2).TheVDL and Signature logs also showed that formation arrivals overtook the casing arrivals.

The formation arrivals can be confirmed by the open hole sonic log: the delta-T curve should closely mirror the formation arrivals on the VDL, as shown below.

Quoted from “Cement Evaluation” Bigelow E L, Western Atlas International, Houston, 1990

Log Examples

9.5CasIng Fluids Effecton theLog

The mud weight changed from 1O.5ppg above 7028ft to 16.6 ppg below. The effect on the

rr

is obvious butnot so on the CBL amplitude. The well temperature profilecanmakea difference to the mud (eg in density) if the mud has beenstaticin the hole for somtime, This can cause the iT to vary (~6.2).

Quoted from DS Cement Bond Logging Field Reference Manuai.

tool was run to TD in 1.5hrs, experiencing high temperature gradiant, and the logging was finished in 2hrs. The tool was properly centralised but the CBL amplitude and theVDLwere affected by transient temperature effect.

9.7Double Casing String

Below D is 7” (291b/ft) inside 9 5/8”(561b/ft) and the annulus was fully cemented. The TT has increased by 20its and the VDL showed strong “pipe ring” with first few cycles being narrower. The CBL amplitude here may be E3 (~2.5,~6.11).

Log Examples

TIME AFTER CEMENTATION

4 HRS. a HRS. zeHRS.

9.8 Green Cement

Remember that the down hole conditions are not as ideal as in laboratory. Cement slurry can easily get contaminated by mud, spacer orformation fluids. The thickeningtime and setting time can be much longer than quoted (S4.1, §6.12,13).