CementBondLog(CBL)EvaluationGuidebook

QCandInterpretation

Huawen Gai

BP EXPLORATION

CBLEvaluationManualQCandInterpretation

Whetherto run a cementevaluationlog?Whattool to choose?When to run the log?

How doesthe CBL tool work?How dothe conditions affect the log?How to carryout QC operation?

• The systematicwayto Interpret the Cfl• Historical mIstakescorrected• SqueezeconsIderations

DrHuawenGaiDrilling and CompletionsBranchliP ResearchCentreSunbury-on-Thames

MiddlesexTWJ67LNUKTel. (+44) (0)932763495Fax (+44) (0)932763352

Acknowledgements

ManyPEsandDEsin BPExplorationhavedirectlycontributedto thismanual.I particularlywantto thankDavidLaw, DavidMunro,LeeRichardsonandDaryl Kellingrayof DyceAberdeenandChris Greavesof WestportLab Houstonfor their mostvaluablecommentsand advice.

I wantto thankthefollowing peoplein Drilling andCompletionsBranchwhomadesignificanttechnicalcontributionsoreditorialadvicein preparingthismanual:Chris Lockyear,Dan Ryan,AshleyHibbert,GeorgeBrown(ProductionOperationBranch),JohnMason,JohnBenstedandNigel Brown. The help from Robin Lewis, Ian Palmer and Andy Gardnerin associatedexperimentsis mostappreciated.

Severalpeoplefrom logging servicecompaniesassistedin supplyinginformation. I’d like tothankSigveMauritzen, AVinceSpinelliandPitakWangvarangkoonofSchiumberger,andRuudHenskensof Atlas Wireline for thevaluablediscussions.

H GaiSunbury,UKJune, 1992

L Cement Evaluation Manual - QC and Interpretation 1992

CBL Evaluation Manual — QC and Interpretation

Contents:

How to UsetheManualandQuick ReferenceCharts i-viii

~1. Thingsyou should know about thetools 3Whetherto runa logand Criteria to choosea tool—QCis mostimportant—Whatthetoolscan do at their best—Do notinterpretlogsin isolation

~2. Tool principles andJargon 4How themeasurementismade— How thetoolworksdownhole— Importantfeaturesofthetoolstructure — Whata loglookslike — El, £2, E3etc.—Gates— Transit Time— Stretching— Cycleskipping— Casingarrivals,formationamivals,andmudarrivals— Fastformations—p-annulus—Freepipe

~3. Information Includedin the log 8Thelogheader—The bodyof thelog— Thelogtail — TheBPquestionnaire

~4. Parametersaffectingthe log results 9p-annulus— Eccentricity— Channelling— Casingcoating— Fastformations— Mudtypeandconditions— Temperature— Casingdiameterandthickness— Casingdamages— Casingstandoffandopenholegeometiy— Doublecasingstrings—

WOCtime— Cementparametersandconditions— Computerkeyboardoperations

0. OperationQC In threephases 17Beforelogging— During logging— After logging

~6. Interpretation 21IntespretationChart — QCreview—Quickchecks—Examinethe TTcurves—andtheCBLcurvesandthe t’DL log— BPIcalculationwith example— SpecialInvestigationChart

~7. Cementingoperation 29Cementingoperation— theCFS

~8. Squeezeconsiderations 30Wheredidthecementgo— Whatkind of channelcouldit be— Whereto squeeze

~9. Logexamples 33Logheader/tailandscale— ~i-annulus— Eccentricity— Fastformations—Muddensity— Temperature— Greencement— Doublecasingstring

~1O.DataandChartsfor Reference 41Toolperformancecomparison— Tool characteristics— Soundvelocitiesin mudsformations— Casingexpansionunderpressure— Relationshipbetweeneccentricity,amplitudeandTTreduction— Interval lengthsrequiredfor isolation— Amplitudecompensationchariforvariousmuds— 3’El readingsfor 100% cementedand0%cementedpipes

Index 47

[Cement Evaluation Manual - QC and Interpretation 1992

Concord..99/79901

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CBL Evaluation Manual — QC and Interpretation

How to usetheManual

• WhoIsltwrltten for? ForPE5 andDEswithin BPwhomaybeinvolvedin planningcementjob evaluation,witnessingthe loggingprocess,interpretingcementbond logs, or makingsqueezejob decisions.

• HowtouseIt? Muchefforthasbeenmadeto ensurethatit is easyto useinfield applications.Beginnerorexpert,you canstartwhereyou need:

Jobplanningand witnessing §1, §5 (“s” meanssection)Log interpretation §4, §6-8Undertakingsqueezejob §8Beginneror requiringbasics §1-4andthen §5-6

Themanualis crossreferencedby ~(~*) andsuppliedwith working examples.Whereveryou start, you shouldfind theneededinformation.If you do notgetwhatyou want, let usknow andwe will sort it outfor you and improvethe manual!

• Updateyour expertise:Themanualhassomenew resultsfromrecentresearch.If you arealreadyanexpertin interpretation,youareadvisedto readthroughatleast§6 to updateyourexpertise.

Structure What’s included

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

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

5 Log contents and format (~3)

• Operation OC review (~6)~‘ IS InterpretatIon including squeeze consideration (~7)

5 Examples and reference data (~9&10)

• Cement quality, probability of zonal isolatIon

Conclusions ~ S Squeeze job recommendation

S invalid log

Comments/queries ...~ S Comments 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

[ Cement Evaluation Manual - QC and Interpretation 1992

The following charts also appear in the‘Things You Should Know” and “Log

Interpretation” sections. They are collectedhere for easy access or quick reference.

Please refer to the appropriate section if anydetail of the charts is required.

CBL Evaluation Manual - OCand interpretation Page ii

This Charttries to help answerthreekey questionsfor decisionmaking.

Whether to run a cement evaluation1. Aims of the cement ~ 3. Cernenhng service

Zonal isolation of All appraisal wells If experienced personnelliners transversing should be bond logged with good performancereservoirs should and so should most records in a field arehave higher priority production wells used, the number of

~?n~te~te Wr~ai~be logged can

In job planning phase these threefactors must be carefullyconsidered to descide if thecement job should be bond logged

What tool to choose?1. Mud weight etc* 2. Achieve aims of the logging 3. Logging service companyFor OBM>lOppg & • Capability of the tools (§1.2) • experience of personnelWBM>1 3ppg only CBL type • Importance of isolation • performance of their toolof tool can be used • Possible cement conditions • costs*Please refer to § 10. land§ 10.2 forother constraintsfactors. 4l1I~~~’

Although in most cases both CBL and CET types of tool can beI used (~1O.1,the most important is QC), avoid the CBL when thereI are:

1) Intervals containing a ~s-annulus(~2.12)which logging underI 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 II 2) To avoid l2-annulus especially for CBL: do not reduce thelI pressure in central hole after cementing and before logginglU~L1,~9.2) J

CBLEvaluation Manual- QC and Interpretation Page ii

TheFlowCharton theoppositepageoffersasystematicwayto interpreta CBL log. Theactionsateachsteparebriefly explainedon this page. See§6 and thegiven referencesfor detail.

check:sJob planning execution• ‘The 5 data sets’ (see p22) > Mainly in log headerS The logging engineer’s comments• The presentation Log completeness

Take 1 or 2 minutes to see if theIT and I E.g.:7”, 29 lb,ft casing The TI should be inthe CBL are in the expected range and the I the order of 270~us(see p22) and CBL

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

The TI curves are bound to vary. Youmust know why they did on the log in handj

E.g.:the marked zone is probably due toeccentralisation - No fast formation wasconfirmed by other logs including VDL.

To confirm TOC, Igood cementand I‘free pipe” is to iprovide keyreferences for theBPI

Mainly tosubstantiate TIand Ampiitudeindications

Concentrate on zones of interest. The longer the I Elf - Elminterval of hight BPI value, the better chance of I SPI El - Elzonal isolation (see p26 for an example). I f c

CBLEvaluation Manual - QC andInterpretation Page iv 1 CBL Evaluation Manual - OC andInterpretation Page v

This Flow Chart provides a common senseapproachto SpecialInvestigation. A goodunderstandingof whatcan affect thetool perfomianceandhow (in §2 and§4), is very useful.

involves the following actions

Special investigation which may have to be iterative

1. Finding information~2. Analysing abnormal log behavior 13. 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

CBLEvaluation Manual- QC and Interpretation Page vi

This Chartandthe oneon the nextpageareconcernedwith decisionof asqueezejob. Theyaskthreecrucialquestionsandoffer commonsenseanswers.Donotdecideto squeezebeforeansweringthesequestions!

1. Where did the cement go?

Analyse the well and cementingconditions together with the log

:

Not clearly ~cated?~6.2~ Yes

CheckIf any fluid loss Calculate the differenceoccured during drilling from expected valueor cementing(~6) (Note the hofe gauge,

washout: caliper log).

~ N1~~(es

Possible cement Possible heavy The cement is likely Possible badloss by large contamination or bad to be still in the contaminationquantities. Such slurry leading to annulus but badly at the cementcases are green cement. bonded tothe casing top or bad mudusually easily Re-run the CBL If and maybe to the removal.Identifiable. possible. formation as well. _____________

CBLEvaluation Manual- OCand Interpretation Page vii

6 scenarios of communication channels

Eccentred casing, mud channel on the narrow side.

Heavily contaminated cement which may or may notbe solid.

Contaminated but solid cement on the wide side withmud channel on the narrow side.

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

kasy:Gap between the set cement and mudthe casing cement

contaminatedThick mud cake between the set cementcement and the formation, formation

Well conditions and cementing operation vs potential channelsChannel Bad casing- Deviated Displacing cementingtype centralisation wells contamination operation problems

I,

‘.4,Severe iii_________________ high deviation

Washoutsection ~‘o~ei weiis

a,Some delayed communications observed In porous reservoirsare 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 squeezejob design particularly in the following areas:

1. Depths/lengths of communicating channels for positioningthe perforating gun and bridge plug or packers.

2. Azimuth of communicating channels for perforating shotphase arrangement: a 45 degree channel can be missed!

3. Identify the vent for the channel filling substances.

CBL Evaluation Manual- OC and interpretation Page viii

Things You Should Know

This part of the Manual provides the startingpoint for proper use of the tool and

interpretation of the log.

CBLEvaluation Manual - CC and Interpretation Page

~1.Things you should know

1.1 Whetherto run acementbondlog ornotcan resultin substantialexpenditureorsavings,

1. Aims ofthecement I 2. Existing knowledge ~1 3. Cementingservice andis a decisionwhichrelieslargely on experience,logging objectives(e.g. if only TOCjob the field company is requireda temperaturelog run at theright time would bethe best)andgovernmentZonal isolation of I All appraisal wellsliners transversing should be bond logged If experienced personnel legislation Criteria to choose a particular tool canbe dictated by the well conditions andwith good performance cementing operations (~10.1,§10.2), but areusually governedby factors suchas experience,reservoirs should I and so should mosthave higher priority I production wells records in a field are

______________________ used, the number of wells required logging emphasisandcost. Expensive tools do not necessarilygive the extracasing be reducedthan 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 logy parameters on the log are not quantitatively known (~4).The best thing to do is to eliminateplanning phase these three them as much as is possible while logging.factors must be carefullyconsidered to descide if the 1.3 Every tool has its limitations even under perfect conditions, e.g.:

The CBL tool canonly give an average measurement of the 360° annulus (~2.3).It isimpossiblefortheCBLtoindicatethe position of a channel. TheVariable Density Log (VDL,§2.4) is a qualitative log anddoes not indicatehowmuchof the annulus is bonded(~6.5).

I 1. Mud weight etc* I [~A~hieve aims of the logjI~ijl 3. Logging service company I The CET tool and the US! (~io.i)concentrateonly on the casing/cementinterfacein their

I For OBM>1 Oppg & I . Capability of the 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 • Possibl e cement conditions I • costsI *PI~serefer to § 10. land

1.4 Although cementbondlogging canbequantitative,it is notalwaysaccuratebecauseof theI WBM>l3ppg only CBLtYP~j~~Importance of isolation 1 • performanceoftheir tool I this interface it is not detectable.I factors.I § 10.2 for otherconstraints many factors that affect the log ~4). Therefore always remember to review theñill picture

including the way the cement job was 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 1I 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 setI 2) To avoid ~.u-annulusespecially for CBL: do not reduce theI pressure in central hole after cementing and before~~n~J

§9.2)

CBLEvaluation Manual - CC and Interpretation Page 2 ~CBLEvaluation Manu a - CC and Interpretation Page 3

Things you should know

~2.Tool Principles andJargonused

2.1 RowthemeasurementIs made:

TheCBL toolhasa sondefor measurementandanelectronicscartridgefor signal acquisition and transmission.The sondeworks on “piezo-electricity” — a physicalpropertyof certainmaterialssuchasquartzandpiezo-ceramic— if thematerialis deformed,a voltagewill begeneratedon itssurfacesandconversely,if avoltageis appliedto thematerialit willdeformaccordingly.Mechanicalvibrationsor“waves” approachingsucha materialcan thereforebe convertedinto voltagesand by measuringthesevoltagesthemechanicalwavescanbeanalysed.

2.2 How the tool works down hole:

Thesonde(seeFig.2.1) typically hasone transmitterand two receiverswhich arein a metalmandrelandare3’ and5’ fromthetransmitter.Whenthetransmitteris fired, it will sendouta cylindrical compressionalwavetrain (usuallyabout20kHz).This wavetrainwill travel throughthemudinto the casing/cement/formationstructures,wheredifferent types ofwavessuch as shear wave wifi be induced by mode conversionphenomenon.Some of the inducedwaveswill travel alongthe casedweilbore,andon their waytheywill sendtheir characteristicsbackto themud.Thereceiversin themudcan thereforepickup thesewaveswhichcarry informationaboutthemedia.

Theearlypartof the receivedwaveformis foundto be indicativeof thequality of thebond betweenthe casing/cementinterface: thebetterthebond, thelower theamplitudes.Thelater partscan tell us, for example,how fast thethe soundtravelsin theformation(~2.10, ~4.5).

The 3’ receiveris dedicatedto measuringthefirst peakof thereceivedwaveform,includingitsarrivaltimeanditsmaximumvalue,conventionallycalledEl (~2.5).Thearrival time is usedto checkif thetool is properlycentralisedfor avalid log (~6.3),andtheEl valueis usedfor bondqualitycalculations (~6.6).The 5’ receiverrecords the whole waveform toproduceeithertheVDL orthesignaturelog (~6.5)or both.This providesmore informationto help detectthebond (~6.5).

2.3 Importantfeaturesof tool structure:

Thetransmitterandreceiversaretube-likeandwill respondto mechanicalwaveswithout telling their radial directions. This meansthat the CBLmeasurementis anaverageof thecircumferenceandis unableto detectthe azimuthalpositionof anuncementedareain theannulus,knownasa channel.

Fig. 2.1 CBL sonde

CBLEvaluation Manual- QCand interpretation Page 4

To prevent the housingmandrel from “short-circuiting” the transmittedwave, it is cross-sectionallyslotted.Consequentlythesondeis notrigid andcanbendunderitsownweight.Thetool centralisationshouldtakethis into consideration(~4.2).

2.4 Whatalog looks like:

A sampleofa commonCBL log is shownin Fig.2.2.Theleft trackis theTransitTime (Ti’) curve(~2.7).Usuallyin this tracktherearealsoa gamma-raylog anda casingcollar locator(CCL) logfor depthtie-in. Themiddle trackis theCBL amplitudecurvewhich is a continuousreadingofEl (~2.5).The right trackis theVDL log whichis producedby applying a simpleprocessingtothe waveformsreceivedby the 5’ receiver.The processingis essentiallythresholdingandstacking:positivepeaksarerepresentedby blackline segmentsandnegativepeakswhite ones;theseline segmentsarethenstackedalong thewell depthandtheVDL log is created.If thewaveformsarestackedwithout thethresholdingtreatment,thelog createdis called signaturelog. Note thesecurvesmaybenamedwith differentmnemonicsor in differentscale,e.g. extralettersmaybeusedto distinguishcurvesgeneratedby sliding gate(~2.6)from thoseby fixedgate.

Fig. 2.2 Log sample

Thelog interpretationis all aboutmaking senseof thecurvesin thecontextofthe cementjobandthewell.

CBL Evaluation Manual- QC and Interpretation Page 5

Things you should know

Jargon:

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

El meanstheamplitudeand durationof thefirst peak,conventionallystated in mV.Similarly E2 is the negativepeakfollowingEl, E3 is thenextpositive peak,andso on.

2.6 FIxed gate,Slidingor Floatinggate(Fig.2.3)

For datareduction,theelectronicsstartsmeasuringEl only whenit is aboutto arrive,andstopsmeasuringwhenit haspassed.This measuringperiodis calleda gate.It is vital thatthegateisopenedin therightpositionon thewaveformin orderto seeEl. Therehavebeentwo waysofsettingthegate:the fixedgateandtheslidinggate.Thefixedgateis setby thetooloperatortostraddleEl. Onceseta fixedgatewill openandcloseirrespectiveof thewaveform.The slidinggateis triggeredopenby thewaveformwhen it hasfirst reacheda presetdetectionlevel.

Mostof thetimeboth typesofgatewill givethesamevalueofEl. However,whentheEl positionis causedto changeby certainconditions(e.g. fast formations,§2.11), a fixed gatecouldmissit buta sliding gatewould pick it up. On theotherhanda sliding gatecouldbetriggeredopenby E3 insteadof El if the latteris lower thanthedetectionlevel. Thereforeboth typesof gatearenow commonlyusedtogether.

2.7TransitTime(Ti’) (Fig.2.3)

TheTi’ is thetime spanbetweenwhenthetransmitteris fired andwhenthewaveformat thereceiverhasreacheda presetdetectionlevel. Theslidinggateis openedattheTI’. Note:unlessEl amplitudecoincideswith the detectionlevel, theTi’ is not the timewhen El reachesitsmaximumvalue.Seealso§4.2.

2.8 StretchIng(Fig.2.3)

Stretchingmeansthe increasein theT~dueto the decreasein El causedby, for example,increasedbondquality. BecausetheTi’ is relatedto thedetectionlevel, a decreasedpeakwifireachthe detectionlevel laterandthus“stretch” the Ti’.

2.9 Cycleskipping (Fig.2.3)

WhentheEl valuefor somereason(e.g.very goodbondorsevereeccentralisation)becomeslowerthanthe detectionlevel,thefirst time thewaveformreachesthedetectionlevel couldbepartof E3 (or evenE5,E7 etc. if theearlyonesall fall below). The TI’ measurementwifi skipa cycle(or two cycles,threecyclesandso on).TheTI’ will be increasedby roughlyan integernumberof the wavelength.

CBL Evaluation Manual- QCand interpretation PageS

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

Thereceivedwaveformis extremelycomplicated.It is acombinationof wavetrainswhichhavegonethrough different mediasuch as casing, formation and mud and consequentlycarryinformationaboutthem. Casingarrivals, formationarrivalsandmud arrivalsaretermsto referto thecorrespondingportionsofthewaveform.Becausethetailof onetypeofarrivalswill alwaysbeeatenby theheadof thenext,on a singlewaveformonecannotclearlyseethejointsof twotypesof arrivals.However,whentheVDL log is generated,thefeaturesof thesearrivalsusuallystandout asshown.

Thefeaturesof casingarrivalsandmudarrivalsonthe VDL are straight stripesstarting at relativelyfixed times.This is becausetheacousticpropertiesof thesteelcasingandthatof themudcolumnareusuallyhomogeneous.Thesoundvelocitiesin theformations,however,canvary substantiallyalongthewell, making theformationarrivalswanderintime asshownin wiggly stripes. Casing arrivals Formation arrivals Mud arrivals

2.11 FastformationsFig. 2,4

Formationsin which soundwavestraveltypically fasterthaninsteel(571.is/ft)areconventionally

calledfastformations(~4.5,§10.3).OntheVDL log, fast formationarrivalswill appearbeforethecasingarrivalsand overridethem(~9.4).

2.12 Micro-annulus

Micro-annulusrefersto a minutegapbetweenthecasingandthecement.Suchagap damagesacousticcoupling betweenthecasingandthecementalthoughusuallyit doesnot permitfluidcommunication.Thedevelopmentof a micro-annulusand at what size it will invalidatethemeasurementof bondquality hasnot been fully understood(~4.1).

A micro-annuluscan make the CBL log look as if the casingwas partially or completelyunsupported.On theVDL log therewill be strongcasingarrivals aswell as strongformationarrivals(~9.2).Oncea micro-annulushasoccurred,it is not possibleto quantitativelyestimatethe bondconditionsbecausethe micro-annuluscouldmaskcoexistentchannels.

2.13 Freepipe

Freepipeis a sectionofpipewhich is not cemented.However,someengineershavebeenusingthetermto describea log whichappearsasif theannuluswerenotcemented.Inthiscaseit doesnotnecessarilymeanthat theannulusis free ofcementor indeedsqueezable.See§9.1 on p 34for a free pipeexample.

CBL Evaluation Manual- QCand Interpretation Page 7

Things you should know

~3.Information included in the log

It is not imperativethat logs from differentservicecompanieshavethe sameformat.But acomplete log should have four parts: the header,the body, the summaryor tail and thequestionnaire.Seeexampleshownin §9.1.

3.1 The log headershouldincludethefollowing:

- GeneralInformatlowlog types,well name,log timeanddate,rignameandtype,location,log measurementbase,log scaleandrun number

- Wellgeometricaldata. deviation,depthsandbit sizesof holesections,depthsand sizesandweightsof casingssections,top andbottomof loggedintervals

- Wellfluids data: type,density

- Cementingdata: type,slurrydensities,volumes,additives,retarders,startingandfinishingpumpingtimes,labthickeningandsettingtimes,spacertypeanddensityandvolume,fluidlossvolume

- Wellpressureandtemperaturedata: pressureappliedafterbumpingtheplug, pressuresappliedat thetime of logging, temperatureprofile

- Logging equipmentdata. modulesnumber,calibrationstatus

- Tool string sketch:centralisertypesandpositions

- Logging engineer’scomments:recordtheaimsof thelogging,eventswhich mayhaveabearingon thelog andexpressviewson thequality of thelog

3.2 The bodyof the log shouldincludethefollowing whereapplicable:

- A “freepipe” readingsectiowrecordabouttwo jointsof pipeif available

- The “main log”: recordthe maininterval(s)of interest

- Therepeatsectiowrecordabout200m

The title ofeachof thesesectionsshouldalsoincludethepressureappliedevenif it waszero.All curve scalesand legendsshould be clearly and correctly indicated. Lessconventionalmnemonicsshouldbeexplained

1in the title area.‘Not In widepracticeyet If you witnesslogs, you canhelpspeedup this process!

3.3 The log tall

Shouldinclude summariesof tool operationalstatus, softwareinput parameters, andtoolcalibrationbeforesurvey(~9.1).

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

Shouldbecompletedandsignedby theloggingengineer,andincludedaspartof thehardcopylog.

LCBL Evaluation Manual- QC and interpretation PageS

~4.Parametersaffecting the Log Results

Somekey parametersarediscussedhere.Becausemost of theireffectsarenot quantitativelyknown, it is importantto understandthemechanismby whichthey affect thelog, and assesswhich parameterwould carrymoreweightthanothersin a givensituation.

4.1 MIcro-annulus

How it occurs: Not fully understoodyet buta micro-annuluscan becreatedeitherby casingcontractionafterthecementhasset,orby casingexpansionunderhighpressuresthatbreakthecementbond. Casingcontractioncanbethermalor mechanical.Casingexpansionis usuallycausedby high pressuresuchasoccursduringa squeezejob. The micro-annuluscan thereforebeclassifiedinto 3 types:thermal contraction,mechanicalcontractionand expansion.

The thermal contractiontype is due to theheatreleasedfrom cementhydration.The casingcontractsafterthecementhassetandtheheathasdispersed.This typeofmicro-annulusdependson thecementsheaththicknessandcomposition,andthethermalconductivityoftheformation.

Themechanicalcontractiontype is causedby reductionin pressure,for example,by changingcasingfluid to a lighteroneafter thecementhasset,orholding thepressureinsidethe casingbeforethecementhassetand releaseit afterwards.

Theexpansiontypeis usuallycausedby squeezepressurethat permanentlydamagesthebond.Whenthepressureis released,only thecasingresumesits previoussizebut notthe cement.

How it affectsthe log: When the cementis bondedto the casing,the acousticenergyistransmittedfromthecasingto thecementeasilyandis thusheavilyattenuated,Whena micro-annulushasdeveloped,theenergytransmissionis severelyhinderedanda largeproportionistrappedin thecasing.(A gasfilled micro-annulusis muchworsethanaliquid filled onein termsof energytransmission).Thecasingthenringsrelativelyfreely, producingstrongcasingarrivalson the VDL log. TheEl amplitude will behigh, indicating that little bond exists. Particularproblemswith micro-annulusare:

1) It is notpossibleto distinguisha partiallybondedannuluswith a channelfrom a cementedannuluswhich can provideisolationbut with a micro-annulus.

2) Theeffect of a micro-annuluscanbe so badthat thelog may look like that the pipe iscompletelyunsupported.This musthaveledto a goodproportionofthefailedsqueezejobs.

CBL Evaluation Manual- QC and interpretation Page 9

Things you should know

BecausetheCBLtoolmeasuresthebondbetweenthecementandthecasing,themicro-annulusthat destroysthis bond is themostseverefactor that affectsthe log results.

How topreventit: Obviouslytrynot to createtheconditionsmentionedaboveunderwhicha micro-annulusmay occur. A methodpractisedby someoil companiesto preventit fromoccurringis pumpthecementwiperplug with a light fluid andchangeback to theweightedmudafterthecementhaveset.Or evencirculatingthelight fluid to cool thecasingwhile thecementis setting. Becausea micro-annulusdoesnotusuallypermitcommunicationandonlyaffect thelog, it is moreimportantto eliminateit, if it hasoccurred,at thetime of logging.Thiscanbedonebypressurisingthecasingusingawirelinepackofforsometimesusinga heavymudto increasethehydrostaticpressure(~5.2).

Typeofmlcro-annulus Be preparedto pressureup to:Thermalcontraction l000psiMechanicalcontraction Reducedpressure(hydrostaticorwellhead)+l000psiExpansion Maxsqueezeor hydrostaticpressureapplied

Limited by~burstpressureof casing;Casingpressuretest;Liner toptest.* After therecentcementing.See 4.2for pressuredeterminationduring logging.

4.2 Tool eccentridty

How it affectsthelog: Whenthetool is off the casingcentre,theacousticenergyfrom thetransmitterwill not reachthe casingcircumferencesimultaneously(~2.2).Insteadpart of thecasedweilborewhich is closerto thetool formsa shorterpathfor someof theenergyto gothrough.Consequentlythis will causereductionin the CBL amplitudeaswell asin theYr asshownin Fig.4.1.

TheYr hasbeenusedasa log qualityindicator. Traditionally when the Ureduction is less than

4~.tsthe log isacceptedwith an error of unknownmagnitude.Recentresearchresultstellus that the amplitude has a uniquerelationshipwith theamountofeccen-tricity but it is a multi-valuefunctionoftheYrreduction(~1O.5).Theimportanceof theseresultsare two fold:

1) When“minor” eccentricity(e.g.Ti’ reduction� 4~ts)occurstheamplitudereductioncausedby eccentricitycanbe compensatedfor.

r CBL Evaluation Manual- QCandinterpretation Page 10

2) The factthat theamplitude reductionis not uniquelydeterminedby agiven TI reductionrevealsthe limitation of theU beingusedasa qualitycontrol indicator.

How topreventtooleccentricity: Not only sufficientnumberof centralisersareneeded,buttheymustbeput in theright positions.The following pointsshouldbeobserved:

1) Typeofcentralise?s:Therigid metaltype seemsto bethebest.Therubberfin typeis thesecondandthebowspringtypeis theworst. Worncentraliserscanbeweakandineffective,but they canbecheckedby visual inspection.

2) Numberof centralisers:In verticalwellsuseminimum of threecentralisersandin deviatedwellsuseminimumof five. Alwaysrequestatleasttwo extraonesfor thejob in caseanyfaultsdevelopin themountedones(~5.l).

3) Whereto put them:For vertical wells put centralisersimmediately aboveand belowthetransmitter-receiversectionandon topoftool assembly(CCL or GR). Notethecasingcollarlocatoris notanadequatecentraliser!Withouta centraliseratthetoptheCCLandGRsectionmayactasaleverarmto promoteeccenteringproblem.Fordeviatedwellsaddacentraliserto thecentreof eachsectionwhichdoesnot yet havea centraliser.Preferablyalwaysaddanextraone at thenearreceiverwhich is usedfor CBL amplitudemeasurement.

4.3 ChannellIng

How it occurs: When the combinedconditions of cementingoperationand down holegeometriesaresuchthat thecementcannotdisplaceall themudfrom theintendedsectionoftheannulus,pocketsof mud mayresidein theannulusandformmud channels(~7,~8).Anotherless recognizedtype of channelis the mud-cakechanneldue to filtration often occurringbetweenthecementsheathandthereservoirformation.A channelmaynot bea problemif itdoesnot communicate.However,you do not know until it does!

Howit affectsthe log: Ideally wewant to detectanychannelsandlike themto affect thelogasmuchaspossibleso thatwe can identify them.Unfortunatelyonly thosechannelswhichareimmediatelynext to the casinghavea strongbearingon thelog. Othersare moredifficult toobserve.This is becauseof theenergytransmissionmechanism,asdiscussedin ~4.1 and ~2.2.

Whenamudchanneloccursnextto thecasing,alargeportionoftheacousticenergyin thecasingcorrespondingto thechannelwill notbetransmittedto theformation.As a resultmoreenergyis returnedto thereceiverandtheEl valuebecomeshigher(~6.6).Forchannelsawayfrom thecasing,however,this energytransmissionmechanismis notpresentedin El but later in time,and is usuallydrownedin the complicatedwaveform.

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A mud-cakechannelcan sometimesbe identified fromthe VDL log whenthe cement/casinginterfaceiswell bonded.Becausethemud-cakechannelprovidesveryweakcoupling,notmuchenergycouldgo into the formationandthemajority wouldbeabsorbedby themud-cakeandthecement.As a resulttheVDL wouldshowweakcasingarrivalswith little formationarrivals.

4.4CasIngcoating

Somecasingshavea layerof coatingsuchasepoxy.If thislayerof epoxyis thick(e.g.>70mils),it canaffectthelog in thesamewayasamicro-annulus.Butpressurewhenloggingwill notaffecttheCBL.

4.5Fastformations

Whatformationsare they: They are usually strongly packedhard formations such aslimestoneanddolomite(~10.3).Thesoundvelocity in theformationsis affectedby theforcesthey aresubjectedto aswell astheirmicroscopicstructure.Thereforethevelocity mayvaryslightly in the sametypeof formationatdifferent locations.

Howtheyaffectthelog: Whenfast formationsarepresent,thesoundwavein theformationsis fasterthanthat in thecasing.Thelatter,however,isthebond qualitymessenger.TherealElis distorted,orevendrownedin theformationarrivals.Whatis measuredhasthereforenothingto do with cementbond quality. Fastformationsmake it difficult to evaluatethe cementjob.

Howto detectthem.Fastformationarrivalsareeasilyseenon theVDL log (~9.4).TheYr willbeshorterandtheCBL amplitudesmaybehigh.RememberthatU reductioncanalsobecausedby tool eccentricity.It is usuallyeasyto tell fastformationfrom tool eccentricityby examiningthe VDL log, but it is difficult to seeif the log is affectedby thecombinationof thetwo. Theformationarrivals on theVDL log canbe confirmedby theopenholesoniclog (~9.4).

4.6 Mud type andconditions

Howthe mudaffectsthelog: Themud (or othercasingfluid) is themediumfor theacousticsignal to go to the casing/cement/formationstructureand comeback.It doesnot distort theshapeofthesignalbutaffectstheamplitude:anymediumwill attenuatetheacousticenergybyscatteringor absorbing.Different mud will have different attenuationratewhich affects Elamplitude.The soundvelocity mayalsochangewith differentmud conditions,thusaffectingtheU.

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Mud parametersthat affect theacousticattenuationandsoundvelocity arecomplicated.Themud densityandthesizesof particlescontainedin it aretwo majorones.Thegeneraltrendisthat thedenserthe mud, the less attenuativeit is (~1o.7).In otherwords, in densemud themeasuredamplitudewill behigherthanthatin lightermud underthesameweilboreconditions.

Tiny gasbubblesin the mud canaffect the log by increasingthe TI’ and reducingthe Elunpredictably.

4.7Temperature

How it affectsthe log: Therearetwo aspectsof the temperatureeffect: firstly on physicalconditionsof thewellboresuchas casingfluid density, andsecondlyon thetool. A good toolshouldbe insensitiveto temperaturechanges.Theresponseof sucha tool to the changesinweilboreconditionsduetothewell temperatureprofileshouldbestableandrepeatableon thelog. In mostwells this responseis smallbut in hightemperatureorhigh temperaturegradientwells this canbe noticeable.

Whentemperaturechanges,thesensitivityofthetransducerswill change,andsowill that oftheelectronics.Theoutputof thetoolwill inevitablyincludesomeerror.This errorcanbeofsteadystate(whenthetool is usedto thenew temperature)ortransient(whenthetool is notusedtothenew temperatureyet), but usuallyboth.

Theoverall temperatureeffect cannotbequantified. In HPHTwells it is importantto observethelogging time andtherepeatabilityof the log sincethelog validity canbeseverelyimpairedunderthesecircumstances.

4.8 CasIngdiameterandCasingthickness

How dotheyaffecttbelog: In casingsofdifferentOD, theattenuationchangeis mainlycausedby thedifferentlengthof themudpath: thelargerthecasing,thelowertheCBL amplitude.Theamplitudedecayratesalsodependon thetypeof mudin thecasing(~l0.7).Themudconditionsand thetemperatureeffect arecloselylinked and shouldbeconsideredtogether.

A generalrulefor commonlyusedcasingsizeis that amillimetreincreasein thethicknessofthecasingwouldcauseaboutlmV increasein the“free pipe” CBL amplitude.Thereasonis thatthethickerthecasingthelessacousticenergywould betransmittedto thecement,andtheenergytrappedinthecasingwill bemoredifficult to damp.Jointsofdifferentweightsin onecasingstringshouldbe seenfrom theU curves.

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4.9 CasIngdamage

Howdoesit occur: Casingcanbewornbydrill-pipe,orsplitby excessivepressure.Perforationsof courseblow holesin it. Corrosioncancauseseriouspittings in casing.

How it affectsthelog: Casingwearor corrosioncancausetooleccentricityproblemandthusreducetheEl amplitude.Perforationscandamagecementbond,especiallyfor weakcement(compressivestrength<2000psi),by crackingthe cement.Split casingcan causethe tool tobecomestuck.Severeirregularity in thecasingwill bereflectedin thelog. Forexample,casingcollarsdamagethecontinuoustransmitting of theacousticenergyand thiscanbe seenfromboth theU andtheVDL log of light or lesswell-bondedcement.

4.10CasIngstandoffandGeometryofthe openhole

How do theyaffectthelog:Whenthecasinghasa 0%standoff(i.e. touchingthe formation),therewill besomeformationarrivals coming throughin theVDL log regardlessof thecementconditionsin theannulus.ThecontactbetweentheformationandthecasingmayalsodeformtheEl, giving incorrectbond indications.This canundoubtedlymaskchannels.

The geometryof the openholewould affect the log in two ways(~7).First, if theannulusthicknessislessthan19mm(0.75”), theEl mightbedeformedandincreasebecauseofreflectionfrom formationcatching thefirst half of El. (Since this dependsalsoon the cementandtheformation, especiallythecement/formationinterfaceconditions,this thicknessis only a roughguide). Secondly,in washoutareasespeciallyin deviatedhole, mud removal is muchmoredifficult. Thethermalconditionsin washoutsectionsaredifferentfromtherestof thecementedannulus.This mayaddto the complicationof micro-annulusgenerationandits detection.

Small casingstandoffcan leavevery thin cementin part of the annulusand causethe Eldeformationproblem.Itis reportedby Dowel Schlumbergerthat casingstandofffrom 100%to0% couldcause30%increasein the El.

4.11Double (or concentric)casingstrings

How do theyaffectthe log: Whentheannuluscreatedby onecasingstring inside another(eg. theliner overlap)is cemented,thelog usuallyshowsthefollowing featuresto indicateagoodjob (~9.8):

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1) TheU curveswill be increased.In a 7” and9 5/8” combinationfor example,theincreaseis typically

20M~~

2) The first few cyclesof the waveformshown on the VDL log will benarrower, i.e. gapsbetweenthefirst few stripesaresmallerthan usual.Also, theapparentcasingarrivalscanbeso strongthat formationarrivalsaremasked.

3) TheCBL amplitudes,which arenot useful in suchan interval,canbehigh.

Aroughexplanationfor theseisthatthewell-bondedconcentriccasinghascausedthefrequencyofthefirst few cyclesto increase,while highattenuationhascausedEl to beskipped,resultingan increasein the U andmeasurementof E3 as amplitudeoutput(~2.9).

4.12 Waiton cement(WOC) time

How doesit affectthelog: Cementslurriestaketime to setandbond to thecasing.SinceElrepresentsthebondqualityof thecementto thecasing,it will changefromfreepipevalue,beforetheslurry thickens,to bondedvalue,when thecementhasset.

WOCtime shouldbetheminimumtime to waitbeforelogging.Logs producedatmarginalWOCtimes canbevery tricky becausethe downholeconditionsarenot thesameasin the lab anditmaywell takea bit longerfor thecementto set;Beside,theslurrycanbecontaminatedwhichmaystretchnecessaryWOC time (~7).For foamedcement,this is oftencomplicatedby thefactthat somefoamagentsareretardersandaresensitiveto contaminations.It is not possibletotell greenishcementfrom a channel.

It is very importantnot to log thewell too early: onehoursavedmaycostyou ten! It is alwaysgoodpracticeto monitor thetiming of theloggingandtheslurrypropertieswheninterpretinga log (~4.l3).

4.13Cementparameters/conditions(~7)

Whatarethey: Theslurrydensity, cementclass,additivestypes(retardersoraccelerators)andweighting agentsare all thingsthat should beknown.

How dotheyaffect the log: For neatcementslurry, the heavierit is, the higherthe acousticattenuationit will havewhen bondedto thecasing.Othersolidssuchasbentoniteand silicaaddedin thecementmay increasethe attenuation.

Theacousticattenuationof foamedcementis lower thanthat of theneatcement,resultinginhigherEl valuesfor 100% bondedcasing.However,theCBL responseto foamedcementhasnot beensystematicallystudiedandthereforeis moredifficult to interpret.

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Things you should know

4.14 Computer keyboardoperations

Whatarethey: Softwareparametersettingfor communicationcontrolandlog resultformatting(~3,~9.1)

How dotheyaffectthelog: It istheengineer’skeyboardoperationsthatmakethetoolcorrectlyfunctionandthedataproperly recorded.Apparentlysmallerrorson thekeyboardcanspoil allthehardwork, andshouldnot beunderestimated.

In thelog presentation,small mistakessuchas incompleteinformation,wrong legendorscalefor curves,cancausefrustrationandtime loss in the interpretation,or evenbig mistakesbyinexperiencedpersonnel,

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To meet with the logging To help logging engineerengineer to set clear carry out a hi9h quality To accept or reject theobjective & to supply job by observing the log (~5.3)sufficient information correct procedure (~5.2)(~5.1) ___________________ ___________________

5.1 Beforelogging (Phase7): Meet with the loggingengineeranddiscuss:

1) Theaimsof the job (zonalisolation, finding TOC or else?)

2) Well conditionsandhistory including (~3.1):

- Well ambientconditions(deviation,temperature,formationstructure,gasoroil reservoirintervals,intervalsof interest,anyanomalies)

- Well fluid characteristics(OBM or WBM, weights, gas/solidcontentsand sizes,fluidchangeaftercementjob?)

- Casingcharacteristics(sizes,weights,depths)

- Pressurehistory (BOP test,casingtest, formationtest, fluid change).Discusswhetherpressurewill be neededduring logging if pressurewasapplied to thewell after thecementhassetor pressurewasmaintainedduringthecementsettingtime.

3) Cementjob (~4.13 cementtype, density, specialadditives,estimatesettingtime, volume,pumpingtime andanyproblemin cementingoperation).Make sureloggingdoesnotstartwithin 8hrsafter thelab cementsettingtime (~4.12,~9.8).

4) Choiceof tools (~1),tool string configuration,centralisertypesnumbersandpositions(~4.2),special/back-upequipmentneeded(such aswireline packoff)

The purposeof this meetingis to setclearobjectivesfor thelogging engineer,and supplysufficientinformationfor himorherto planandpreparefor thejob.Themeetingshouldbeheldatthe earliestpossibletime. The loggingengineershouldthenproducea job planwhich canhelpyou takeappropriateactionsandplanahead.Theloggingengineer’splanshouldinclude:

1) Theobjectiveof the specificlogging andtheapproachto achieveit.

2) Estimatedtime for logging operation,including a detailedplan of wireline rig-up andrig-down.

3) Indicationof any furtherinformationneededin orderto carry outthejob successfullyandto presentthe log completely(~3).

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~5.Operation QC In three phases

Oncea decisionis madethat a log is to berun, theoperationQC shouldbecarriedout in thethreephasesasshownin the following chart:

QC Matters

5.2 DurIng logging (Phase2)’

On thewell Siteweareto helptheloggingengineerto carryouta highquality job by observingthecorrectprocedures.The loggingengineershouldhavea detailedplanof whento do what,but heor shemaywell alter it asthingschange.

A check lists

1) If thewell waspressurisedorthewell fluid waschangednot asoriginally planned,discusswith the logging engineerif pressureshouldbe applied duringlogging to preventthepossiblemicro-annuluseffect(~2.12,~4.1,~9.2).If well headpressureis needed,thefollowingis recommendedto determinethepressurerequired:

- Runa0 psi repeatsection

- Identifya potentialmicro-annuluszone(fom-iationsignalsbehindcasingsignals)

- Stopthe loggingtool atthe micro-annulusdepth

- Switchthepanelto time drive i.e. the screendisplay asif thetool wasmoving

- Tightenthe wireline packoffandstartpressure-up(pumpmudslowly intothe casing)

- Monitor the amplitudeuntil it no longerdrops

- Stoppressure-up

- Log underthis pressure

2) Thecentralisersaretherequiredtype/sizeand in goodcondition.Also check that theyarecorrectlymountedandsecuredin therequiredposition(~4.2).

3) The tool usedshouldbe calibratedandthenext calibrationdatehasnot expired.

4) The tool string is correctlyconnectedandtestedbeforebeingloweredinto thewell. Toolcalibrationbeforeandafter thelogging run.

5) Theparameters/constantsSet fromthekeyboardare correct(j3).

6) Do not exceedthemaximumlogging speed.

7) Thescalesand markingsarecorrectlyset.

8) Monitorthecurvesonthescreen.Discussandrecordanysuspectedproblemwith theloggingengineerandsuggestto repeatabnormalsections.If theTI’ doesnotrepeatandthedifferencein amplitudein the repeatsectionis >10%, it couldbea centralizationproblem.Pull out ofhole andchange/addcentralisersif necessary.

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5.3After logging (Phase3): To acceptthe log or otherwise.

Topresentthelog correctlyandin a completemanneris important,notonly for producingvaliclogsfor immediateuse,butalsofor documentation.Thelogs maybereferredto afterthewelhasproducedfor severalyears.Missing information can be irretrievableand renderthe lo~meaningless.

A checklist

1) All thesignalshavebeencorrectlyrecorded.Authoriserig down.

2) Thescalesandlegendsusedarecorrect(~9.1).

3) The log header,tail and the questionnaireetc. are fully completedwith no incorrec’information.

4) Theloggingengineer’scommentshaveincludedandexplainedall quality-relatedincidentsthe aimsof the loggingand his or heropinion on how well they wereachieved.

5) The log hardcopy is deliveredon thetime agreed.Authorisepaymentif thereareno QCproblems,otherwiseraisethem with the servicecompany.

6) If anyeventoccurredduringtheloggingwhichmayhaveaffectedthelog, preparea repordescribingthateventin detail.Attacha copyof thereportto thehard copyof thelog wherit hasarrived.

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~6.CBL Interpretation

A systematicwayto interpreta CBL log is demonstratedin theinterpretationflow charts,Go tothereferencesgivenif you areuncertainatanystage.

You mayhaveto breakthemain flow chartfor specialinvestigationwhich is shownon page27, andresumeafterwards.

INTERPRETATION FLOW CHART

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6.1 QC Review

If thelog is properlydocumented,it shouldcontainmostoftheinformationneededfor correctinterpretation,Themajority of the key partsare includedin thelog header(~3).Checkthefollowing four items. Anything missingor incorrectlyrecordedwill atleastcausetime loss oran invalid log.

Item 1: Theplanningandexecutionof logging Clearobjectives?Effective effortto preventmicro-annulusandeccentricity?Logging activelywitnessed— witnessingengineer’ssignaturesand relevantreports?

Item 2: Thefive data sets arecompleteand correct(~3.1):

1) Generalinformation2) Well geometry3) Well fluids4) Cementingoperation5) Well pressureandtemperature

Item3:Thelogging engineer’scommentsClearandrelevantin addressinganyproblems.Theanswersto thequestionnairecanalsoimply his orher competency.

Item 4: ThelogpresentationHeader-body-tail-questionnaire,signaturesanddatecomplete?

6.2 Quick checkof ranges of thevarious curves

77Curves: Thenormalrangesof U curvesdependmainly on thecasingID andmud type/weight (affecting the soundvelocity). Temperatureand the type of tool (the size of thetransducers)alsohavesomeeffect. A roughguidefor calculatingthis rangeis

TT(~is)—(casingID eg. In mm)/(soundvelocity in themud eg. In mn’z.~us§10.3)+170(Ms)

The last item is the distanceof T-R spacing(3ft) timesthe soundspeedin steel(57~ts/ft).

CBLCurves:ThenormalrangesofCBL for unbondedpipesdependmainlyon thecasingsizes(~1O.8)andmudtype/weight(~10.7).The rangesfor 100%bondedpipesarelessreliablebecauseof the difficulty in controlling the test conditions.For foamedcementtherehave not beensufficient field dataor labresultsfor fully boundedpipesandtheinterpretationis thereforemoredifficult.

VDL/Signature log: Checkwhetherthe time scale/rangearecorrect(in theaboveequationreplade170 by 285, which correspondsto Sft), thelog hasgoodcontrast,andthereareanyfastformations(~9.4).

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Log Interpretation

6.3 ExamInethefl Curves

Thepurposeof examiningtheU curvesis to explainthecurvevariation,if any,andinvestigatethelog validity.

The IT curvesvariation. TheU curvesfrom a properly centralisedtool run in uniformlycementedpipesshould ideally be straight lines in the expectedregion(~6.2).ParametersaffectingtheUcurvesincludecentralisation(~4.2,§9.3),casingID size/weight(ID)changes(e.g.casingcollars)(S4.8),casingfluids changeoverdifferentdepths(~4.6,§9.5),fastformations(~4.5,§9.4)andtemperature(j4.7, §9.6). The U curveswill alsovary to indicate stretching,cycleskipping(j2.9) andwell bondeddouble-casingstring (~4.11,§9.7).

A log validity criterion. A widely quotedlog validity criterion in the literatureandvariousmanualsis thatif theU curvesvaryfor morethan±4iis,thelog is invalid. Becareful.Variationof theU curvesof a reallog is rarelywithin this limit: investigateif thecausewaseccentricity!

Whenthetool is not properlycentralised,theU canbeshorterthannormal.Unfortunatelytheamplitude will alsobereduced.The effects of most other factorsmentionedaboveon theamplitudearelikely to berelatively small.Traditionallythelog is treatedasinvalid becauseoftheunknownreductionin theamplitude.This nowhasbeenbetterunderstood(~4.2,§10.5)andtheamplitudereductiondueto eccentricitycanbecalculated.However,becausetheuncertaintyof the effectsof other factorsstill exists, it is recommendedthat similar criterionbeusedwiththeU reductionlimit being5.us (~10.5).

Note the transienttemperatureeffect canmake the log invalid (~4.7,§9.6) and so can fastformationsandmicro-annulus— theamplitudesin theintervalof interestarenot quantitativelyreliable.

6.4 ExamInethe CBL Curves

CBL curves: Look for Top of Cement(TOC) ifapplicable(e.g. a non-liner job), where the CBLcurvesswingfromthelow endof thevaluerangetothe high (~6.2,§9.1). Is the TOC in the expectedregion?Checkwith the annulussize and pumped _____

cement volume. A low TOC measuredis oftenassociatedwith slurrylossanda highoneincompletemud removal.If theTOC is notfoundtheslurry hasbeencontaminatedat leastin thetop section(~8).

Lookfor goodcementsectionwherethecurvesareatthelow endof thevaluerange.Noteifa leadandtail slurrysystemwasused,a differenceshouldbeseenin thegoodcementsectionsfound,wherethe Leadcementtail cementshouldgive a lowerreading.Thesewillbeusefulin qualitativelyevaluatingthecementjob.If no good cementsectionis found, theproblem Tail cement

could be heavy contaminationbut refer to theSpecialInvestigationCharton page27.

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Look for “Free Pipe” (uncementedpipe) section ifapplicable.Donotbemisledby highreadingsof theCBLcurves: check for the micro-annulus (~4.1)!The CBLvaluesfor free pipeoffer referencesin cementqualityevaluation(~6.6).

6.5 ExamInethe VDliSignature log

TheVDL log: musthavegoodblack andwhite contrast.It containsmuchinformationbutnowonly a small portionis extracted,andthis is doneby visual examination.Themain usesof theVDL log are: --___________

1) to detectmicro-annulus(~9.2)

2) to detect fast formation (~9.4)

3) to confirm free pipe(~9.1)

4) to confirm goodbondin doublecasingstrings(~9.7)

5) to confirmgoodbondto thecasingbutbadbondtotheformation(wherethecasingarrivalsareextremelylow with little or no formation arrivals and CBLamplitudeindicatesa good bond).

Thesignaturelo& whichis oftensuperimposedon theVDL log, producesthe wave amplitude informationwhichis not availableon theVDL. This informationcanbeusefulin confirming changesin bondquality. How-ever, thesignaturelog is not aseasyto useastheVDLin detecting,for example,theformationarrivalsandthatis why it is oftencombinedwith theVDL log.

The indications of the U, the amplitude and theVDL logs must be in agreementand theirexaminationsshouldbein parallel. If theydo not agree,therecouldbeatoolproblemresultingan invalid log.

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6.6 Bond PercentageIndex Calculation

BFlDeflnitiow Thepercentageof theannuluswherethecementis well bondedtothecasing.Therestof theannulus(100% - BPI) is not well bonded,whichmaybecontaminatedcement,localisedsmall gapsbetweenthe cementandcasing,ora channel.

Angle of bondedcement Elf- ElmBPI (%) - - x 100%

Thewholearinulus Elf- Elc

WheretheEl’s arefromthesamereceiver(e.g., the3’ receiver),andthesubscriptmrepresentsthevaluemeasuredin thezoneof interest,f representsfreepipevalue,and c thevaluefor 100%

cementedpipe.

TheBPIappliestoanytypeofcementsystem(neat,foam,etc.).Notewhenaleadandtail slurrysystemis used,Elc shouldbeselectedseparatelyfor thetwo slurries.WhenElfandEl c areavailable,the correspondingBPIata givenElm canbefoundfrom theaboveequation,or asshownin Fig.6.l below.

FIg.6.1 How to find the BPI graphically

On theElm axis, markEl~andElfvalues.Mark theBPI (%) axisby equalintervalsfrom 0 to100. Draw a straightline from(Elc,lOO) to (EljçO). GivenaElm value,thecorrespondingBPIcanbefoundasshown.

Toprovidea sealwith high confidence,BPIneedto bearound95%orhigherfor certainlengths(~l0.6).For gaswells thisrule shouldbeappliedmorestringently.

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Log Interpretation

BPI CalculationExample

Belowis a sectionaroundtheshoeofa 7’, 29 lb/ft linerjob run attheGulf ofMexico. Assumingthat the log haspassedthe QC and quickexaminations,let us seehow theBPI is calculated.

Supposethecementingoperationandotherwell conditionsallowusto believethatat leastasectionof

perfectcementjob canbe achieved,we canthenselectthelowestreading,5mV, as100%bondedvalueElc.The“freepipe” valueElf is nonexistent(hopefully!)in alinerjob, wethereforelook itup from §10.8anduse62mV (or we can usean availablevaluefrom a log with closeconditionsasthejob in hand).

BetweenpointsAandB theaveragereadingof7mV givestheBPI-(62-7)/(62-5)-96.5%;BetweenB andC theaveragereadingis about9mV. The effectof theslightU reduction(3—4~.ts)causedby eccentricitycanbe compensatedfor (~10.5),in this caseby increasethe amplitudeby about10% to lOmV. ThecorrespondingBPI hereis therefore(62-10)/(62-5)91%.

If El~cannotbe clearlydef’medon thelog, we can alsouseareasonablevalueelsewhere,e.g. 2.4mVfrom §10.8. The BPI for the two intervals will be (62-7)/(62-2.4)92%and (62-l0)/(62-2.4)—87%respectively.Asthereexistabout90%bondedintervalsfor30ft, theprobabilityof zonalisolationis high.

Notemorethan50%reductionin Elc(from5mVto 2.4mV)haschangedtheBPI valuefor <5%.Variationsin Elf doesnot affect theBPI mucheither.Thismeansthatfor conventionaljobs theselectionof thereferencevaluesis importantbutnotcritical for areliableinterpretation.Preferablyboth

0andElf aredecidedfromthelog wheneverpossible.Thismethodappliesto all typesof slurry design.

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SPECIAL INVESTIGATION FLOW CHART~

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

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6.7Finding Information

If informationin thelog is incomplete,orotherinformationis requiredsuchasopenholelogsor somedetail of cementingoperation,contact theright personto obtain it. Keepanupdatedlist of contactsfor necessaryhelp: their name,specialities,company,base,telephoneandfaxnumbersetc..

6.8AnalysIng abnormallog behaviour

1) Reviewall factorswhich arelikely to affect thelog (~4).2) Eliminateoneby onearoundthekeyproblemin handuntil only suchfactorswhoseeffects

maybesignificant.3) Judgewhetherthesefactorshaveinvalidatedthe log — a good understandingof all the

factors(study §4) maybe veryuseful.

6.9 CalculatIngthe probability ofzonal Isolation

Cementing operationcan offer valuable common senseevaluationwhich should not bediscounted(~l.4).If anystepsin theoperationwerealteredfromplanned,it is suggestedthatthepost-jobCPSberun (~7).A startingpointis to judgethelikelihoodof atleastsomesectionswhich arechannelfree. If no suchsectionscanbeassuredandthelog showsavery badcementjob, a remedyjob mayhaveto be considered(~8).

For theBPI calculation,theEl valuefor 100%bond(Sl0.8) mayhaveto befoundelsewhereifthat valuefor thegiven conditionsis not availablein theliterature.Thelog of a well of closelocationwith similar conditionscan proveuseful to providethis reference.Thevaluecanbechosenby finding the lowestreadingat intervalswhereisolationwasrequiredandachieved.Make suretherewereno fast formationsor micro-annulus(~4.5,~4.1).

6.10 Keep a record of Interpretation

It is recommendedthattheinterpretationof alog is recorded,eg,on thebackof a log, for easeof later reference.Therubberstampissuedtogetherwith this manualis for this purpose.Thetablefrom thestampis designedto summarisethelog with minimal amountof information.Stampthebackof thelog and fill in theform as you completetheinterpretation.

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Log interpretation

~7.CementingOperation

1. Whatto check~

Donot cometo conclusionson thequality of thecementjob without reviewingthecementingoperation.Everystepof theoperationis carefully designedto ensureagoodjob. Problemsmayoccurif anyrecommendedstepwasnot followedcorrectly. Commonsensecanoften tell youthenatureof the problem.

Cementplacementitself is a largeandcomplexsubject.But simplisticallyspeaking,therearesomerulesofthumb.Forexample,themorecentralizedthecasingstring,thebetterthecementjob;Equallyimportantarethedisplacementratesof mud,spacerandcementslurry. Usuallythehighertheflow rates, thebetterthedisplacement.The flow ratesarelimited by theformationstrengthandthe displacementfacilities. Therheologyanddensityof the fluids are similarlyimportant.The ideais to removetheviscousmud by spacerascompletelyaspossible,thentodisplacethespacerby thecementslurry. Thespacerhereis designedto makeeasytheremovalof mud andtheplacementof cement.Usuallya heavierfluid following a lighter onecanhelpthedisplacement.Remember,non-verticalwellsarenot only difficult to log, but alsodifficultto cementbecauseofthecasingcentralisationproblemandthecomplicatedflow regimesin aninclinedannulus.

Without beingdeeplyinvolved in cementingtechnology,you canalwaysgetsomeideaaboutthe qualityof thecementjob by checkingthefollowing:

- Wasthereany lost circulation?

- Wasthecasingstring centralised?

- Werethe pumpingratesand displacementtiming asplanned?

- Werethewell-site-measuredfluids properties(density,viscosity,additivesquantitiesetc.)asdesigned?

Answer“No” to anyof thesequestionscouldmeanalessthanperfectcementjob. If therewerelossesto theformationorotheroperationalproblemencounteredduringcementplacement,itcouldbea badjob andlog interpretationmusttakethis into account.

2. The CementPlacementSimulator(CPS)

This simulator, developedby theFluid MechanicsTeamatRCS, can tell you how to carry outthecementjob andwhatthecementjob shouldlook like. TheCPSis locatedatDyce,Houstonand Sunburyandwifi alsobe incorporatedin DEAP. If a bad cementjob is suspectedit issuggestedthat theCPS berun.Theresultis anotherreferencefor interpretingthe log.

CBL Evaluation Manual - QCand Interpretation Page 28

Log Interpretation

~8Squeezeconsiderations

A bad cementjob indicatedby a bondlog (whichevertype)doesnotmeanyou cango aheadandsqueeze.Thefollowing questionsmustbeansweredasbestasyou canbeforeanyaction:Wheredid the cementgo?Whatkind of channelcouldit be,andwhereandhow to squeeze?The following Chartsmayhelpyou answerthesequestions.

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 differenceoccured duririu d~IIing from expected valueor cementing(~6) (Note the hole gauge,

_____________________ washout: caliper log).

__________________________ ~ Yes

Possible cement Possible heavy The cement is likely Possible badloss by large contamination or bad to be still In the contaminationquantities. Such slurry leading to annulus but badly at the cementcases are green cement. bonded to the casing top or bad mudusually easily Re-run the CBL if and maybe to the removal.identifiable. possible. formation as well.

CBL Evaluation Manual - QC and interpretation Page 29

6 scenarios of communication channelsEccentred casing, mud channel on the narrow side.

Heavily contaminated cement which may or may notbe solid.

Contaminated but solid cement on the wide side withmud channel on the narrow side.

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

key:

Gap between the set cement and mudthe casing cement

contaminatedThick mud cake between the set cementcement and the formation. formation

Well conditions and cementing operation vs potential_channelsChannel Bad casing- Deviated Displacing Cementingtype centralisation wells contamination operation problems

I,

I,

Severe in________________ high deviation

Washout section Often inhorizontal wail.

~~1

Some “delayed” communications observed in porous reservoirsare 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 squeezejob design particularly in the following areas:

1. Depths,lengths of communicating channels for positioningthe perforating gun and bridge plug or packers.

2. Azimuth of communicating channels for perforating shotphase arrangement: a 45 degree channel can be missedi

3. Identify the vent for the channel filling substances.

L~~EvaiuatlonManual~QC and interpretation Page 30

atatUCat

atat

C

atatC.E‘C

9.1 A Log Example

Header[on this page],body andtail [on next page],andquestionaire(~3)

~ion~cC~

Log Examples

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Log Examples

AFTERPRESSURIZIFICJ AMPLITUDE

~ aEFORE TO3000FSI&RUN1 RUN2 AMPLITUDE PRESSURING RELEASING

*

9.2MIcro.annukis

F~.ashs1om~rho1ding5Op~onthecasing~r3daysRun1wasmnwithOpsiandRun2wsswith7OOpsi.Adequatepxessuxeceneliminatesomemiao-annulus.

Fig.bwlogsninbefoieandasngwsspiessuzeiisedto3OOO~l.Excessivepressuxecanbreakthebondbetweenthecasingandthecement

TT (US )~ ___L_~M~_�__~_. VDL (USaUO.00 5~._OOO 100.00 1200.0Q 1200.0

9.3Tool Eccentricity

EccentrldtycausesreductionintheTrandintheCBLamplitude(~1O.5).Addanexn~centmliseraithe3’ receivercanalleviatetheproblem(~6.2).Neverrun theCBL knowinglyoff-centred(egwith 711 rubbercentraliserfor liner, runningin95/8” casingabovetheliner).

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TRAVEL TIME ATTENUATIONOS/IT

290 190 20 0• ~ T~A/ — — - — — PEAK AMPLITUDE — — ‘LIGNATUITE VARIABI E DENSITY

API MV0 100 0 120200 1200 200 ‘200

CC AMI’LIIIED

9.4 Fast Formations R

The TT fell below the expectedvalue (~5.2).The VDL and Signature logs also showed thatformation arrivals overtook the casingarrivals.

The formation arrivals canbe confirmed by the openhole sonic log: the delta-T curve shouldcloselymirror the formation arrivals on the VDL, as shownbelow.

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

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Log Examples

9.5 CasIng Fluids Effecton theLog

The mud weightchangedfrom 1O.5ppgabove 7028ft to 16.6 ppgbelow. The effecton the rris obviousbutnot soon theCBL amplitude.The well temperature profile canmakea differenceto the mud (eg in density) if the mud hasbeenstaticin the hole for somtime, This can causethe iT to vary (~6.2).

Quoted from DSCement BondLoggingField ReferenceManuai.

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9.6 TemperatureEffect onthe Log

Thetemperaturein thesectionshownwasabout320°Fand300mbelowwasabout345°F.Thetool was run to TD in 1.5hrs,experiencinghigh temperaturegradiant,and the loggingwasfinishedin 2hrs.The tool was properlycentralisedbut the CBL amplitudeand theVDL wereaffectedby transienttemperatureeffect.

9.7Double CasingString

BelowD is 7” (291b/ft) inside 9 5/8” (561b/ft) andtheannuluswasfully cemented.TheTT hasincreasedby 20itsandtheVDL showedstrong“pipe ring” with first few cyclesbeingnarrower.TheCBL amplitudeheremaybeE3 (~2.5,~6.11).

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Log Examples

TIME AFTER CEMENTATION4 HRS. a HRS. ze HRS.

9.8 GreenCement

Rememberthat thedownhole conditionsarenot asidealasin laboratory.Cementslurrycaneasilygetcontaminatedby mud,spacerorformationfluids. Thethickeningtimeandsettingtimecanbemuchlongerthanquoted(S4.1, §6.12,13).

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Parameters CBL CET USI

Original design purpose openhole cement bond bond/corrosion

Calibration conditions uphole down hole down hole

Casing sizes 4.5” - 13 3/8” 4.5” -95/8” 4.5”- 133/8”

Mud density high(eg>l8ppg) OBM<loppg OBM<llppgunquanfifted effect WBM<l3ppg WBM<l5ppg

Micro-annulus severe effect less 9effect less effect(liquid filled) (liquid filled)

Heavily contaminated cement amplitudeattenuated could indicate as could indicate aschannel” channel”

Cement/formation bond can be detected not measured avoidedEccentralisation sensitive less sensifive less sensifive

Vertical resolution 3ft 3ins 1.Sins

Azimuthal resolution non-directonal 45 degrees 10 degrees

Shape of channel (on no effect detectable good measurementcasing/cement interface)

Cement thickness values debatable less effect less effect

Compressive strength derived (un~usfiflably) derived idea abandonedDouble casing string severe effect less effect less effect

Casing stand-off strong effect less effect less effect

Casing thickness strong effect measurable measured

Casing wall conditions some effect strong effect detectable

Mechanical complexity simplest complex easy to damage

Operational robustness OK easy to get wrong good?

Costs relatively low typically 2xCBL typically 4xCBL

*This is because the acoushc impedance of heavily contaminated cementis in asimilar range as that of mud.

10.1 Performance Comparisonof CBL, CET andUS!

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Data and Charts for Reference

11

_______ I10.2CharacterIsticsof CommonlyUsedCBL Tools(quoted from “CBL Field ReferencManual” DS)

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SOUNDVELOCITY ACOUSTECMATERIAL POROSLfl AT IMPEDANCE

TYPE (%) (Ms/ft) (ft/a) (rn/a) (MRayl)

Casing - 57.0 17,500 5,334 41.6Dolomite - 43.5 23,000 7,010 20.2Limestone - 47.6 21,000 6,400 17.3

~ Calsite - 49.7 20,100 6,126 16.6~ Anhydrite - 50.0 20,000 6,096 18.2Z Gypsum - 52.6 19,000 5,791 13.6

~ Quarts - 52.9 18,900 5,760 15.2— Halite — - 66.6 15,000 4,572 9.3

Dolomite 5 to 20 50.0 to 66.6 20000 to 15000 6096 to 4752 17,0 to 11.5Limestone 5 to 20 54.0 to 76.9 18500to 13000 5639 to 3962 14.8 to 9.4

~ Sandstone 5 to 20 62.5to 86.9 16000to 11500 4877 to 3505 12.6 to 8.2~ Shale 58.8to 143 17000 to 7000 5181 to 2133 12.0 to 4.3

— OBM - 209to 222 4785 to 4505 1458 to 1373 1.7to 2.7WBM - 198to 205 5050 to 4878 1539 to 1487 1.8to 3.0Water - 208 4800 1463 1.5

~ Water+ 10%NaC1 - 192 5200 1585 1.7Water + 20% NaC1 - 182 5500 1676 1.8SeaWater - 199 5020 1531 1.6Kerosene - 230 4340 1324 1.1Air (15 psi, 0°C) - 920 1088 331 0,0004Air (3000 psi, 100°C) - 780 1280 390 0.1

10.3Speedof SoundIn VariousTypesof FormationandMud(editedfrom “Well Cementing”,Dowel Schlumberger,1990)

10.4 CasIngExpansionUnder Pressure(quotedfrom “CementEvaluations”Atlas, 1990) _______________

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Data and Charts for Reference

Eccentricity vs Relative El Eccentricity vs IT reduction

IT reduction affected by cement TI reduction vs reIativ~ El

Notes: 1) Cementdoesnot affect the relationshipin (a).2) Mud affectsrelationshipsin (a) (b) and(c) but lessin (d).3) Cementaffects amountof Yr reduction as shown in (c), consequentlyYr-El

relationshipis notunique.4) FreepipeU-El relationshipin (d) canbeusedfor El conpensation.

10.5RelationshipamongTool Eccentricity,CBL AmplitudeandTr Reduction

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Data and Charts for Reference

For BPI�95%

10.6IntervalLengthsRequiredfor Isolation

10.7Amplitude CompensationChartsfor VariousMuds, CasingID andThicknesses(quoted from “The Fluid-Compensated Cement Bond Log” Nayfeh et al, SPE FormationEvaluation, pp335-341,1986)

CBLEvaluationManual - OCaid Interpretation Page43

Casing size weight Free pipe 100% bonded(class H)(in) (Ib) Elf(mV) E10(mV)

4 1/2 9.511.613.6

5 15.018.021.0

51/2 15.517.020.023.0

7 23.026.029.032.035.038.040.0

75/8 26.429.733.739.0

95/8 40.043.547.053.5

10 3/4 40.545.548.051.054.055.5

Thedataabove werederivedfroma chartby Pardueetal [CementBondLogging- A StudyofCementandCasingVariables,JPTMay1963, pp545-555].Notetheycanonlybeusedasageneralguide.

10.8CBL 3’ ReceIverEl Readingsfor 100% Bondedand0% BondedPipes

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Index

Amplitude 4,5,18,22 Pressure 2,8,9,10,17,18,21After logging 19 Principles 4,5Beforelogging 17 Questionnaire 8,19,21,33Body, log 8,32 Review,QC 21BPI, Bond PercentageIndex 24 Signaturelog 4,5,23Casing,arrivals 7 Sonde 4Casing,coating 12 Squeezejob 29CCL 5,11 Standoff 14Cementing 3,17,11,28 Stretching 6,22Centralisers 8,18,11 Tail, log 8,31CET 3,39 Temperature 8,13,17,21,22,37Channel 3,5,7,9,11,24,30 Test: BOP,casing,formation 10,17Checklist 18,19 Thickness 10,13,14CPS 28 TOC 17,22,28Criteria, tool selection 2,3 TT, TransitTime 6,21,22Cycle skipping 6,22 USI 39Density 8,13,15,17,28,36 VDL 3,5,7Diameter,casing 13 WBM 17,41Doublecasingstring 14,37 WOC 15,38During logging 18El, E2, E3, etc 6Eccentridty,centrallsation 5,10,34Examples,logs 31-38Fast formations 7,23,35Features,tool 4Five datasets 21Foam cement 15,21,24Formationarrivals 7Freepipe 7Gate,fixedandsliding 6Geometry,openhole 14GreenCement 15,38Header,log 8,31Interpretation 20Isolation 2,9,20,43Jargon,explained 5-7Keyboardoperation 16ManualstructureMicro-annulus 2,7,9,18,22,23Mud arrivals 7OBM 17,41Peak 6,7

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