Cement Bond Lo g (CBL) Evaluation
Guidebook QC and Interpretation
Huawen Gai BP EXPLORATION
CBL Evaluation Manual QC and Interpretation
Whetherto run a cement evaluationlog? Whattool to choose? When to run the log? How does the C B L tool work? How do the conditions affect the log? How to carryout Q C operation?
• The system atic way to Interpret the • Historical m Istakes corrected • Squeeze consIderati ons
Cfl
DrHuawen Gai Drilling and CompletionsBranch liP Research Centre Sunbury-on-Thames Middlesex TWJ6 7 LN UK Tel. (+44) (0)932 763495 Fax (+44) (0)932 763352
Acknowledgements Many PEs and DEs in BP Explorationhave directlycontributed to thismanual.I particularly want to thankDavidLaw, DavidMunro,Lee Richardson and Daryl Kellingray of DyceAberdeen and Chris Greavesof WestportLab Houstonfor theirmost valuablecomments and advice. I wantto thankthe followingpeoplein Drilling and CompletionsBranchwho made significant technicalcontributions Dan Ryan, or editorialadvicein preparingthismanual:Chris Lockyear, Ashley Hibbert,GeorgeBrown(Production OperationBranch),JohnMason, JohnBenstedand Nigel Brown. The help from Robin Lewis, Ian Palmerand Andy Gardnerin associated experiments is most appreciated. Severalpeople from loggingservice companies assistedin supplying inf ormation. I’d like to andPitakWangvarangkoon thankSigveMauritzen,AVinceSpinelli ofSchiumberger, and Ruud Henskensof Atlas Wirelinefor the valuable discussions.
H Gai Sunbury, UK June, 1992
L C e m e n t E v a lu a ti o n M a n u al
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1992
C B L E va lua tion M an ua l
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Q C a nd Int erpretati on
Contents: How to Use the Manualand QuickReference Charts ~1.
i-viii
Things you shouldknowabout 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.
Toolprinciples andJargon
4
How the measurement is made —How the tool works downhole — Important features of the tool structure — What a loglooks like — El, £2, E3 etc. — Gates — Transit Time — Stretching — Cycle skipping— Casing arrivals, formation amivals, and mud arrivals— Fastformations— p-annulus— Free pipe
~3. n I form ation Included in the log The log header— The bodyof the log — The log tail
~4.
0.
8 — The
BP questionnaire
Param eters aff ectin g the log results p-annulus — Eccentricity — Channelling— Casing coating —Fast formations — Mud type and conditions— Tem peratur e — Casing diameter andthickness — Casing damages— Casing standoffand open hole geometiy— Double casing strings — WOCtime — Cementparameters and conditions— Computer keyboard operations
9
O perati on Q C In threephases —During logging — After logging
17
Interpretation
21
Before logging
~6.
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.
29
Cem enti ng operation Cementing operation — the CF S
~8. Squeeze con siderati ons
30
Where did the cement go — Whatkind of channelcould it be — Where to squeeze
~9. Log examples
33
Log header/tailand scale — ~i-annulus — Eccentricity — Fastformations— Muddensity— Temperature — Green cement — Double casing string
~1O.Data and Chartsfor Reference
41
Toolperformance comparison — Tool characteristics — Soundvelocities in muds formations — Casing expansion underpressure— Relationship between eccentricity, amplitudeand TTreduction— Interval lengths required for isolation —Amplitude compensation charifor various muds — 3’El readings for 100% cementedand 0% cementedpipes
47
Index
[
C e m e n t E v a lu a tio n M a n u a l - Q C a n d In te rp re ta tio n
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0 U
0) 0)
U
0
Concord.. 99/79901
C B L E va lua ti on M an ua l
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Q C an d Interpr
etati on
How to use the Manual •
Who Isltwrltten for? For P E5 and DEs withinBP who maybeinvolvedin planning cement job evaluation,witnessing the loggingproc ess, interpr eting cementbond logs, or making squeezejob decisions.
• Howto use It ? Muchefforthas been made to ensurethat it is easy to use in fieldapplications. Beginneror expert, you can start whereyou need:
( “ s ” means section)
Job planningand witnessing Log interpretation Undertaking squeeze job
§1, § 5 §4, §6-8
Beginneror requiring basics
§1-4 and then §5-6
§8
Themanual is crossreferenced by
~(~*) and supplied withworking examples.Wherever you start, you should find the neededinformation. If you do not get whatyou want,let us knowand we will sort it out for you and improvethe manual!
•
Updateyourexpertise: The manual has somenew resultsfromrecentresearch.If you are alreadyan expertin interpretation, you are advisedto readthroughat least§ 6 to updateyour expertise.
Structure
What’s included ~fs’~ S K ey knowledge of the tools a nd their selection • Principles a nd jargon used (~2)
(~1)
5Operation
OC in three phases: (~5) Before the job, during the lob a nd after the job contents a nd format (~3)
5L og
O C review (~6) including squeeze consideration reference data (~ 9&10)
• Operation ~‘
IS InterpretatIon 5 Examples
and
(~7)
• Cem ent quali ty, prob abilit y of zonal isolatIon
C onclusions
~
S Squeeze job recommendati
S invalid Comments/queries to DCB RCS
...~
S C om m
‘~ —— — —
• Help o n
The highlighted header at
[
on
log
en ts o n th e manual o~ general interest log interpretation
the top of each page tells y ou where you a re in th e manua~j
Cement Eva luati on M a n u a l- Q C a n d In te rp re ta tio n
1992
The f oll ow ing cha rt s also appear in the ‘Things Y ou Shou ld Know” a nd “Log Interpretation” se cti ons. They a re coll ected here for easy access or qu ick reference. Please refer to the app ropri ate se cti on if a ny detail of the cha rt s is requ ired.
C BL E valua tion M anu al - O C a n d interpretation
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This Charttries to help answerthreekey questionsfor decisionmaking.
Whether t o r u n a cement eva l u ati on 1. Aims
of the cement
3. Cernenhng
~
All ap praisal we lls should b e bond logged a nd so should most production wells
Zonal isol ation o f liners transversing reservoirs should have higher priority ~?n~te~te
service
If experienced personnel with good performance records in a field a re used, th e number of
r~ai~
b e logged ca n
W
In jo b planning
phase these thre e factors m us t b e care full y considered to descide if t h e cem ent j o b should b e bond logged
What t oo l t o cho ose? 1 . M u d weight etc* For OBM >lOppg & W BM >1 3ppg only C BL ty pe o f tool c a n be used
2 . Achieve aim s of t h e logging • Capability o f th e tools (§1.2) • Importance o f isolation • Possible cement conditions
to §constraints 10. l a n d *Please § 10.2 forefer r other factors.
Although
l1I~~ ~’
in m os t cases
3 . Logging service c om pany •experience of personnel •performance o f the ir tool •costs
4
both C B L a n d C E T types of tool c an b e
I used (~1O.1,the m ost important is QC), avoid t h e C B L when I are: 1 ) Intervals con taining a ~s-annulus (~2.12) which logging under I pressure failed t o eliminate(~4.2,~9.3) ~
2 ) Intervals
§9.4)
where isolati on is required
there
con tain fast form ations
When t o run the log?~ To avoid green cem ent (~4.12, §9. 8): do n o t start logging~”)
I within 8hrs after t h e cem ent ha s se t I I 2 ) T o avoid l2-annulus espe ciall y f or CBL: d o n o t reduc e t hel I pressure in cen tra l h ole aft er cem en ting a n d before loggingl U~L1,~9.2)
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The FlowCharton theopposite page offersa systematic way to interpreta CB L log. The actions at eachstepare briefly explainedon this page. See § 6 and thegiven references for detail.
check:sJob planning execution • ‘The 5 data sets’ (see p22) S The logging engineer’s comments •Th e presentation
-
> Mainly in lo g header L o g completeness
Take 1 or 2 minutes to see if theIT and I E.g.:7”, 2 9 lb,ft casing Th e TI should b e in the C B L a re in the expected range and th e I the order of 270~us (see p22) and CBL VD L lo g h as good contrast. from 1 to 65mV. i shouldbe
T he TI curves ar e bound to vary. Y ou m ust k n o w w hy the y d id on th e l o g in handj
E .g .:th e m arked z o n e i s probably d ue to e c c e n tra lis a t io n- N o fast fo rm a tio n w as c o n firm e d b y o th e rlogs i n c lu d in gV D L .
TO C , Tgood o confirm cementand
‘free pipe” is to provide key references for the
II i
BPI
Mainly to substantiate TI an d Am pii tude indications
Concentrate o n zones of interest. T he longer the I interval of hight BPI value, th e b etter chance o f I zonal isolation (see p 26 f or a n example). I
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This Flow Char t provides a common sense approachto Special Investigation. A good understandingof whatcan affect the tool perfomiance and how (in § 2 and §4), is very useful.
involves the following actions
Special investigation
which m a y have to b e iterative
1. Finding information~ 2 . An alysi ng abnormal log behavior 1 3 . Calculating the probability of zonal isoIati~
Proceedl* ~
____________
Ye~
fthl
accounted fo r?
* Either g o to the next action o r resume the m a in interpretation
C BL E valua tion M anu al- Q C a n d In te rp re ta tio n
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N p~
flow chart on previous page
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This Chartand the one on the nextpage are concernedwith decisionof a squeezejob. They ask threecrucial questionsand offer commonsenseanswers.Donot decide to squeezebefore answering these questions!
1 . Where did the cement go ?
An alyse the well a n d ce m en ti ng with t h e l o g conditions together N ot clearly
~cated?~6.2~
CheckfluIf idany loss occured during drilling or cementing(~6)
the difference C a lcu la te from expected value (Note the hofe gauge, washout: caliper log).
N 1 ~~( e s
~
Possible cement Possible heavy loss by lar ge contam inat ion o r b a d slurry leading to quantities. Such cases are green cem e nt. usually easily R e-ru n the C B L If Identifiable. p ossib le .
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Th e cement is likely to b e still in the annulus b ut badly to th e casing b onded a n d maybe to the well. forma tiona s
P age
Possible b a d contam ination at th e cement to p or bad m ud removal. _____________
v ii
6 scenariosof communication channe ls Eccentred casing, mud channel on the narrow side.
Heavily contaminated
cement which may or m ay no t
be solid. Contaminated but solid cement on the wide side with mud channel on th e narrow side. Contaminated but solid cement on the narrow side with mud channel on the wide side.
Ga p between the set cement and the casing
kasy: mud cement
Thick mud cake between the set cement and the formation,
contaminated cement formation
W e ll con dit ion s a n d cem en ti ng op erati on v s po tenti al chann el s Channel type
B a d casingcentralisation
Deviated Di spl ac ing contamination wells
ceme nt ing operation problems
I,
_ _ _ _ _ _ _ _‘.4, _________ Washout section
Severe iii high deviation ~‘o~ei weiis
a, Some delayedcommunications observed In porousreservoirs are believed tobe caused by the disintegrationof the mud cake. This typeof channel Is hardly detectable with today’s technology
og l s vital Bond iin the ati on for eez e jo b design provide partic ularly nform areas: oll f ow ingsqu 1. De pths/l engths of com municati ng channels for positioning rforating gun and bridge plug or packers. the pe municat ing channels for perforating 2. Azimuthof com shot phasearrangem ent:a 45degree channel can be missed! the ventfor the channel fillingsubstances. 3. Identify
C B L E v a lu a ti o interpretation n Manual- O C a n d
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Things Y o u Should
Know
This pa rt of the M anua l provides the sta rting point for proper u se of the t oo l a nd the log. interpretation of
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~1. Thingsyou should know or savings, 1.1 Whetherto run a cement bondlog ornot can resultin substantialexpenditure
2. Existing knowledge ~1 the field Al l appraisal wells I should b e bond logged I an d so should most I pr oduc tion we lls
1. Aims ofthe cement job Zonal isolation o f liners transversing reservoirs should have higher priority
I
______________________
casing than intermediate
logging objectives (e.g. if only TOC and is a decisionwhichrelieslargely on experience, log run at the right time wouldbe the best) and government is requireda temperature
nting service 3. Ceme
company
good performance If experienced with personnel
to choose but aretoolcanbe usually governed sucha (~10. 1, §10.2), a particular dictatedbyby factors th e well conditions s experience, and
legislation cementing operations Criteria
rec ords in a field a re used, the number of wells be reduced to b e logged can probably
emphasisand cost. Expensive
required logging
tools do not necessarilygive the extra
information you really need! (This Manual concentrates on the CBL only.) 1 .2 QC i s the most importantpart of cement bondlogging. This is because the effects of most
cem entplanning considered b should tobedescide phase be these log y factors mjoust carefully bond if th ethree
param m uch ilog s possible are no t while known (~4). as quantitatively logging.perf The best tool hon as the i ts lim even under e.g.:thing to do is to elim inate 1.3 Eve ry eters them as itations ect conditions, The CBL tool can only givean average measurement of the 360°annulus (~2.3). It is impossiblefor the CBLto indicatethe position of a channel. The Variable Density Log (VDL, §2.4) is a qualitative log anddoes not indicatehow muchof the annulus is bonded(~6.5).
I 1. M ud weight etc*
I
For OBM>1 O p p g &
I [~A~hieveaim s ofth e logjI~ijl 3 . Logging service com pany I
I
1
. Capability o fthe tools (§1.2)
•experience
The C E T tool and the US! (~io.i) in their onlyon the casing/cement interface concentrate
of personnel
The ‘cem ent map’ is in fact an interface map. If a channel i s beyond
data measurement.
I *PI~se refer to used I of tool ca •Possibl e cement conditions n be § 10. la n d W BM >l3ppg only C BLt YP~ j~~ Importance o f isolation II factors. I § 10.2 fo r other constraints
•costs 1 •performance of their tool
~Although in m ost cases both C B L a nd C ET types of tool ca n be I used (~1O.1,th e m ost importantis QC), avoid th e C B L when there are: a ~.t-annuIus (~2.12) 1) Intervals containing which logging under pressure failed to eliminate(~4.2,~9. 3) I 2 ) Intervals where isolation i s required contai n fast form ations §9.4) ~ (~4.5,
I I
( ~ i ETo~ avoid green cem ent
I
1.4 Although cem itis not alwaysaccuratebecause of the loggingcan be quantitative, this interface itent is bond not detectable. many factors that affect the log ~4). Therefore always remember t o review the ñillpicture including t h e way th e cement jo b was carriedout (~4, §7 , §8). Don’tlet calculated results
override com m on sense.
1
(~4.12, §9.8): d o n ot start loggin~”~ se t
I within 8hrs after th e cem ent has
I 2) To avoid ~.u-annulus especially fo r CBL: d o n ot reduce t he
ore~~n~ J I pressure i n central hole after cem enting a n d bef §9.2)
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~CBL Evaluation
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Things y o u shoul d know
~2.Tool Principles andJargon used 2.1 Rowthe measurement Is made: The CBL toolhas a sondefor meas urementand an electronics cartridge for signal acquisitionand transmission.The sondeworks on “piezoelectricity” — a physicalpropertyof certainmaterials suchas quartzand piezo-ceramic — ifthe materialis deformed, a voltagewill be generated ifa voltageis appliedto the materialitwill on its surfacesand conversely, deformaccordingly.Mechanicalvibrationsor “waves” approaching such a materialcan thereforebe converted intovoltagesand by measuring thesevoltagesthe mechanicalwaves can be analysed.
2.2 How the toolworks downhole: The sonde (seeFig.2.1) typically has one transmitter and two receivers which arein a metalmandrel and are 3’ and 5 ’ fromthe transmitter.When compressional the transmitteris fired, it will sendout a cylindrical wave train(usuallyabout20kHz). This wavetrainwill travel throughthe mud structures,where differenttypes of into the casing/cement/formation waves such as shear wave wifi be induced by mode conversion phenomenon.Some of the inducedwaveswill travel alongthe cased weilbore,and on their waytheywill sendtheir characteristics backto the mud.The receiversin the mud can thereforepickup thesewaves which carry information aboutthe media.
The earlypart of the receivedwaveformis foundto be indicativeof the qualityof the bond betweenthe casing/cement interface: the betterthe bond, the lowerthe amplitudes.The later partscan tell us, for example, how fast the the sound travelsin the formation(~2. 10, ~4.5). The 3’ receiveris dedicatedto measuringthe first peakof the received waveform, includingitsarrival time and its maximumvalue, conventionally calledEl (~2.5). The arrivaltime is usedto checkif the tool is properly for a validlog (~6.3), centralised and the El valueis usedfor bondquality calculations(~6.6). The 5’ receiverrecords the whole waveformto produceeitherthe VDL or the signaturelog (~6.5) or both.This provides more informationto help detect the bond (~6.5).
2.3 Importantfeatures of toolstructure: are tube-likeandwill respondto mechanical The transmitter and receivers waves withouttellingtheir radial directions.This means that the CBL measurement is an averageof the circumference and is unableto detect the azimuthalposition of an uncemented area in the annulus,knownas a channel. Fig. 2.1 CBL sonde
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To prevent the housing mandrel from “short-circuiting” the transmittedwave, it is cro sssectionally slotted. Consequently the sonde is not rigid and can bendunderits own weight.The tool centralisation should takethis into consideration (~4. 2). 2.4 Whata log lookslike:
A sampleofa commonC BL log is shownin Fig.2.2. The left trackis the TransitTime(Ti’) curve (~2.7). Usuallyin this trackthereare alsoa gamma-ray log and a casing collar locator(CCL) log for depthtie-in. The middletrackis the CBL amplitudecurve whichis a continuousreadingof El (~2.5). The right trackis the VDL log whichis producedby applyinga simpleprocessingto the waveformsreceivedby the 5 ’ receiver.The processingis essentiallythresholdingand positivepeaksare represented by blackline segmentsand negativepeakswhiteones; stacking: theseline segmentsare then stackedalong the well depth and the VDL log is created.If the waveformsare stacked without the thresholdingtreatment,the log createdis called signature log. Note thesecurves maybe namedwithdifferentmnemonics or in differentscale,e.g. extra from those by fixed lettersmay be usedto distinguishcurvesgeneratedby slidinggate (~2.6) gate.
Fig. 2.2 L og sample
The log interpretation is al l aboutmakingsenseof thecurves in thecontext ofthe cementjob and the well.
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Things y o u should know Jargon: 2.5 El, E2, E3 etc.(Fig.2.3) El means theamplitudeand durationof the first peak,conventionallystatedin mV. Similarly E2 is the negativepeak following El, E3 is the nextpositive peak,and so on.
2.6 FIxed gate, Slidingor Floati ng gate (Fig.2.3) For datareduction,the electronics startsmeasuring El onlywhenit is aboutto arrive,and stops measuringwhenit has passed.This measuringperiodis calleda gate.It is vital thatthe gate is openedin the rightposition on the waveformin orderto see El. Therehave beentwo waysof settingthe gate:the fixedgate and the slidinggate. The fixed gate i s set by the tooloperatorto straddleEl. Onceset a fixed gate will openand close irrespective of the waveform.The sliding gate is triggeredopen by the waveformwhenit has first reacheda prese t detectionlevel. Most of the timeboth typesofgate will givethe same value ofEl. However,whenthe El position is causedto changeby certainconditions(e.g. fast formations, §2.11), a fixed gatecouldmiss it but a slidinggate wouldpick it up. On the otherhanda slidinggatecould be triggeredopen both typesof gate by E3 insteadof El if the latteris lowerthan the detection level. Therefore are now commonlyused together. 2.7 TransitTi me (Ti’) (Fig.2.3) The Ti’ is the time spanbetweenwhenthe transmitteris fired and whenthe waveformat the receiverhas reacheda presetdetectionlevel. The slidinggate is openedat the TI’. Note: unless El amplitude coin cides withthe detec tion level, the Ti’ is not the timewhen El reachesits maximumvalue.See also§4.2.
2.8 StretchIng(Fig.2.3) Stretchingmeans the increasein the T due to the decreasein El causedby, for example, ~ increased bond quality. Becausethe Ti’ is relatedto the detectionlevel, a decreasedpeak wifi reachthe detection level later and thus “stretch” the Ti’.
2.9 Cycleskipping(Fig.2.3) Whenthe El valuefor somereason(e.g. very goodbondor severe eccentralisation) becomes lowerthan the detec tionlevel, the first time the waveformreachesthe detection levelcouldbe
part of E3 (or evenE5, E7 etc. if the earlyones all fall below).The TI’ measurementwifi skip a cycle(or two cycles,three cyclesand so on). TheTI’ will be increasedby roughlyan integer numberof the wavelength.
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2.10 CasIng arrivals, Formation arrivals, Mud arrivals(Fig.2.4) It is a combination The receivedwaveformis extremely complicated. of wavetrainswhichhave gone throughdifferentmedia such as casing, formationand mud an d consequently carry information aboutthem. Casing arrivals, formationarrivalsand mud arrivalsare terms to refer to thecorresponding portionsofthe waveform.Becausethe tail of one typeofarrivalswill always be eatenby the head of the next,on a single waveformone cannot clearlysee the joints of two typesof arrivals. However,whenthe VDL log is generated,the featuresof these arrivalsusually standout as shown. The featuresof casing arrivalsand mud arrivalson the VDL are straight stripesstarting at relatively fixedtimes. This is becausethe acousticproperties of the steel casingand that of the mud columnare usuallyhomogeneous. The soundvelocities in the formations, however, can vary substantially along the well, makingthe formationarrivalswanderin time as shownin wiggly stripes. Casing arrivals
2.11 Fast formations
Formation arrivals
Mud arrivals
Fig. 2,4
Formations in whichsoundwavestravel typically fasterthanin steel(57
1.is/ft) are conventionally arrivalswill appearbefore the calledfast formations(~4.5,§10.3). OntheVDL log, fast formation casingarrivalsand overridethem (~9.4).
2.12 Micro-annulus Micro-annulus refers to a minutegap between thecasingand the cement.Such a gap damages acoustic coupling between thecasingand thecement although usuallyit doesnot permit fluid communication. The developmentof a micro-annulus and at what size it will invalidatethe measurementof bond qualityhas not been fully understood (~4.1).
A micro-annulus can make the CB L log look as if the casingwas partiallyor completely unsupported. On the VDL log therewill be strong casing arrivalsas wellas stron g formation arrivals(~9.2). Oncea micro-annulus has occurred,it is not possibleto quantitativelyestimate the bondconditions because the micro-annulus could mask coexistentchannels.
2.13 Free pipe Free pipeis a sectionofpipewhichis not cemented.However, someengineershave been using the termto describea log whichappears as ifthe annuluswerenotcemented.Inthiscase it does not necessarilymean that theannulus is free ofcementor indeedsqueezable.See §9.1 on p 34 for a free pipe example.
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Things y o u shou ld know
~3. Informationincludedin the log It is not imperativethat logs from different servicecompanieshave the same format.But a complete log should have four parts: the header, the body, the summaryor tail and the questionnaire. See exampleshownin §9.1.
3.1 The log headershouldincludethe following: -
GeneralInformatlow log types, well name, log time and date,rig nameand type,location, log measurementbase,log scaleand run number
-
Wellgeometricaldata. deviation,depthsand bit sizesof holesections,depthsand sizes and weightsof casingssections,top and bottom of loggedintervals
data: type, density
-
Wellfluids
-
Cementing data: type, slurrydensities, volumes,additives, retarders, startingand finishing pumpingtimes,labthickeningand setting times, spacertype and densityand volume,fluid loss volume
-
-
Wellpressure andtemperaturedata:pressureappliedafterbumping theplug, pressures appliedat the time of logging,temperatureprofile
data. modules number, Loggingequipment calibratio n status
-
Tool string sketch:centraliser typesand positions
-
Logging engineer’s comments: record theaims of the logging,eventswhichmayhave a bearingon the log and express views on the qualityof the log
3.2 The bodyof the log shouldincludethe followingwhere applicable: -
A “free pipe” reading sectiow recordabouttwo jointsof pipe if available
-
The “main log”: record the maininterval(s) of interest
-
The repeat sectiow recordabout200m
The title ofeach of these sectionsshould also includethe pressure applied evenif itwas zero. All curve scales and legends shouldbe clearlyand corre ctly indicated. Less conventional 1 in the title area. mnemonicsshouldbe explained ‘NotIn wide practiceyet If you witness logs, you can helpspeed up this process!
3.3 The log tall Should includesummariesof tool operational status, softwareinputparameters, and tool calibrationbeforesurvey(~9.1). 3.4 The BP questionnaire (“Log QualityControl Sheet”) Shouldbe completedand signed by the loggingengineer,and includedas partof the hard copy log.
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~4.Parameters affecting the Lo g Results Some key parametersare discussed here.Becausemost of theireffectsare not quantitatively known,it is importantto understand the mechanism by whichthey affect the log, and assess which parameter would carrymore weightthan othersin a givensituation. 4.1 MIcro-annulus
eitherby ersto but a micro-annulus How Notcement fully und canhigh be created casing it occu rs: the contraction after has set,od oryet by casing expansionunder pressuresthat break the cementbond. Casing contractio n can be thermalor mechanical.Casing expansionis usually causedby high pressure such as occursduringa squeezejob. The micro-annulus can therefore be classifiedinto 3 types: thermalcon traction,mechanicalcontractio n and expansion.
The thermalcontraction type is due to the heat releasedfromcementhydration.The casing contractsafter the cementhasset and theheathas dispersed.This typeofmicro-annulus depends on the cementsheaththickness and composition, and the thermalconductivity ofthe formation. type is causedby reductionin pressure,for example,by changing The mechanicalcontraction casingfluid to a lighterone afterthe cementhas set, or holding the pressure insidethe casing before thecement has set and releaseit afterwards.
The expansiontype is usuallycausedby sque eze pressurethat permanentlydamagesthe bond. Whenthe pressureis released,onlythe casingresumesits previoussize but not the cement.
How it affectsthe log: Whenthe cementis bondedto the casing,the acousticenergyis icrotransmittedfromthe casingto the cementeasilyand is thus heavilyattenuated,Whena m annulus hasdeveloped, theenergytransmission is severelyhindered and a largeproportionis is muchworsethana liquidfilledone in terms trappedin the casing. ( A gas filledmicro-annulus of energytransmission). The casingthen rings relatively freely, producingstrong casing arrivals on the VDL log. The El amplitudewill be high, indicatingthat little bond exists. Particular problemswith micro-annulus are: 1) It is not possibleto distinguish a partiallybonded annuluswith a channelfrom a cemented annuluswhichcan provideisolationbut witha 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 musthave led to a goodproportionofthe failed squeezejobs.
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Things y o u shoul d know Becausethe CBL tool measuresthe bond betweenthecementand thecasing,the micro-annulus that destroysthis bond is the most severe factorthat affects the log results. toprevent it: Obviouslytrynot to createthe conditionsmentioned aboveunder which a micro-annu lus may occur. A method practisedby some oil companiesto prevent it from occurringis pump thecementwiperplug with a light fluid and changebackto the weighted the light fluidto coolthe casingwhilethe mud after the cementhave set. Or even circulating cementis setting. Becausea micro-annulus doesnot usuallypermit com municationand only
How
affect the log, it is more importantto eliminateit, if it has occurred,at the time of logging.This the casingusinga wirelinepackoffor sometimesusing a heavymud can be doneby pressurising to increasethe hydrostaticpressure(~5.2). Typeofmlcro-annulus Thermalcontraction Mechanical contraction Expansion
Be preparedto pressu re up to: l000psi Reduced pressure (hydrostatic orwellhead)+l000psi Max squeezeor hydrostatic pressureapplied
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
How it affects the log: Whenthe tool is off the casing centre,the acousticenergyfrom the transmitterwill not reachthe casing circumference simultaneou sly (~2.2). Instead part of the casedweilbore which is closerto the tool formsa shorterpath for someof the energyto go through. Consequently this will causereductionin the CBL amplitudeas well as in the Yr as shown in Fig.4.1. The Yr has been used as a log quality indicator.Traditionallywhen the U 4~.ts the log is reductionis less than accepted with an error of unknown magnitude.Recentresearch results tell us that the amplitude has a unique relationship with the amountofeccen tricity but it is a multi-valuefunctionof theYrreduction(~1O.5). The importance of these resultsare two fold:
4~ts) When occurs the amplitudereductioncaused 1) by “minor” eccentricity (e.g. Ti’ reduction for. eccentricity can be compensated
r
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2) The factthat the amplitudereductionis not uniquelydeterminedby a given TI reduction revealsthe limitation of the U being usedas a qualitycontrolindicator.
How toprevent tool eccentricity:Not only sufficient numberof centralisersare needed,but theymustbe put in the right positions. The followingpointsshouldbe observed: 1) Type ofcentralise?s: The rigid metal type seems to be the best. The rubberfin typeis the can beweak and ineffective, second and the bow springtype is the worst.Worn centralisers but they can be checkedby visual inspection. 2) Number of centralisers: In verticalwellsuse minimumof threecentralisersand in deviated wellsuse minimumof five. Always requestat leasttwo extra ones for the job in caseany faults developin the mountedones(~5.l). 3) Whereto put them: For verticalwellsput centralise rs immed iately above and below the transmitter-receiver sectionand on top oftool assembly(CCL or GR). Notethe casingcollar Withouta centraliser locatoris not an adequatecentraliser! atthe top the CC L and GR section may act as a leverarm to promoteeccentering problem.For deviatedwellsadd a centraliser to the centre of eachsectionwhichdoes not yet havea centraliser.Preferablyalways add an extra one at the near receiverwhich is usedfor CB L amplitudemeasurement. 4.3 Chan nellIng
How it occurs:Whenthe com bined conditions of cementingoperation and down hole of geometries apockets re such of the cement cannot theformmud the inte( nded section thatmud mud from the annulus, mayreside in thedisplace channels Another annulusalland ~7,~8). less recognizedtype of channelis the mud-cakechanne l due to filtration often occurring between the cement sheath and the reservoirformation . A channelmay not be a problemif it doesnot communicate.However,you do not knowuntilit does!
How it affects the log: Ideally we want to detectany channelsand like themto affect the log onlythosechannelswhichare as muchas possibleso that we can identifythem. Unfortunately next to the casing havea strongbearingon the log. Othersare moredifficult to immediately observe.This is becauseof the energy transmissionmechanism, as discussedin ~4.1 and ~2.2. Whena mud channeloccursnextto the casing,a largeportionofthe acousticenergyin the casing to the channelwill not be transmitted to the formation.As a resultmoreenergy corresponding is returnedto the receiverand the El valuebecomeshigher(~6.6). For channelsawayfrom the casing, however,this energytransmissionmechanism is not presented in El but later in time,
and is usuallydrownedin the complicatedwaveform.
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i nings y o u snou ia K n O W A mud-cake channel can sometimesbe identified from the VDL log when the cement/casin g interfaceis wellbonded.Becausethe mud-cakechannelprovidesveryweakcoupling,not much energycouldgo into the formationand the majority wouldbe absorbedby the mud-cakeand the cement.As a resultthe VDL wouldshowweakcasingarrivalswithlittle formationarrivals. ng 4.4 CasIng coati
Some casingshavea layerof coatingsuchas epoxy.If thislayerof epoxyis thick(e.g. >70 mils), it can affect the log in the sameway as a micro-annulus. Butpressurewhenloggingwill not affect the CBL.
4.5 Fastformations Whatformations are they: They are usually strongly packed hard formationssuch as limestoneand dolomite(~10.3). The soundvelocityin the formationsis affectedby the forces they are subjectedto as well as theirmicroscopic structu re. Thereforethe velocitymay vary slightly in the same typeof formationat differentlocations.
How they affectthe log: Whenfast formationsare present,thesound wavein the formations is fasterthanthat in the casing. The latter, however,is the bond qualitymessenger. Thereal El is distorted,or even drownedin the formationarrivals.Whatis measuredhas thereforenothing to do with cementbond quality. Fast formations make itdifficult to evaluatethe cementjob. How to detect them.Fast formationarrivalsare easilyseen on the VDL log (~9.4). The Yr will be shorterand the C B L amplitudesmaybe high.Remember that Ureductioncan alsobecaused by tooleccentricity. It is usuallyeasyto tell fastformationfrom tool eccentricity by examining the VDL log, but it is difficult to see if the log is affectedby the combination of the two. The formationarrivalson the VDL log can be confirmed by the open holesoniclog (~9.4). 4.6 Mudtype and conditions
Howthe mudaffects the log: The mud (or othercasingfluid)is the mediumfor the acoustic signal to go to the casing/cement/formation structureand come back.It does not distortthe shape ofthe signalbut affects the amplitude:any mediumwill attenuate theacousticenergyby scatteringor absorbing.Differentmud will have differentattenu ation rate which affectsEl amplitude.The soundvelocitymay also change with differentmud conditions, thus affecting the U.
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Mud parametersthat affect the acousticattenuation and sound velocityare complicated.The mud densityand the sizes of particlescontainedin it are two majorones. Thegeneral trendis that the denser the mud, the less attenuativeit is (~1o.7).In other words,in densemud the measuredamplitudewill be higherthanthat in lightermud underthesameweilboreconditions. Tiny gas bubblesin the mud can affect the log by increasingthe TI’ and reducing the El unpredictably. 4.7 Temperature
How it affectsthe log: There aretwo aspectsof the temperatureeffect: firstly on physical conditionsof the wellbore suchas casingfluid density, and secondlyon the tool. A good tool ature changes. The responseof sucha toolto the changesin shouldbe insensitiveto temper weilboreconditionsdue tothe well temperature profileshouldbe stableand repeatableon the log. Inmost wellsthis responseis smallbut in high temperature or high temperaturegradient wellsthis can be noticeable. will change,and so will that ofthe Whentemperaturechanges,the sensitivity ofthe transducers includesomeerror.This errorcan be ofsteady electronics. Theoutput of thetool will inevitably state (when the tool is usedto the new temperature)or transient(when the tool is not used to the new temperatur e yet), but usuallyboth. The overalltemperatureeffect cannotbe quantified.In HPHTwellsitis importantto observe the loggingtime and therepeatability of the log sincethe log validitycan be severelyimpaired under thesecircumstances. 4.8 CasIngdiameter and Casing thickn ess
How do they affecttbe log: In casingsofdifferentOD, the attenuation changeis mainlycaused by the differentlengthof the mud path: the largerthe casing, the lowerthe CBL amplitude.The amplitudedecayrates alsodependon the typeof mud in the casing(~l0.7). The mudconditions and the temperatureeffect areclosely linked and shouldbe consideredtogether. A general rule for commonly usedcasingsize is that a millimetreincreasein the thicknessofthe casingwouldcauseabout lmV increasein the “free pipe”C BL amplitude. The reasonis that the thicker the casingthe less acousticenergywouldbe transmitted to the cement,and the energy trapped in the casingwill be more difficult to damp.Joints ofdifferentweightsin one casingstring shouldbe seen from the U curves.
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Things y o u snou la K n O W 4.9 CasIngdamage How does itoccur: Casing can bewornby drill-pipe,or splitby excessivepressure. Perforations of courseblow holesin it. Corrosion can cause seriouspittings in casing. problemand thus How it affects the log: Casing wearor corrosioncan causetooleccentricity reduce the El amplitude.Perforations can damagecementbond, especiallyfor weakcement (compressivestrength<2000psi),by crackingthe cement.Split casingcan causethe tool to becomestuck. Severeirregularityin the casing will be reflectedin the log. For example,casing collars damagethe continuoustransmittingof the acousticenergyand this can be seen from both the Uand the VDL log of light or less well-bondedcement.
4.10 CasIngstandoff and Geometry of the open hole How do theyaffect the log:Whenthe casinghas a 0% standoff(i.e. touchingthe formation), therewill be someformationarrivalscoming throughin the VDL log regardlessof the cement conditionsin the annulus.The contactbetween the formationand the casing may alsodeform the El, givingincorrectbond indications. This can undoubtedlymaskchannels. First, if the annulus The geometryof the open hole wouldaffect the log in two ways(~7). thicknessisless than19mm (0.75”), the El mightbe deformed and increasebecauseofreflection from formation catchingthe first halfof El. (Since this depend s also on the cement and the formation,especiallythe cement/formation interfaceconditions,this thicknessis only a rough guide). Seco removal especiall y in deviate d hole, mud the more washoutarea in conditions much difficult. thermal in swashout sections are differ ent from cemente d Thendly, restofisthe annulus.This may add to the complication of micro-annulus generation and its detection.
Small casingstandoffcan leavevery thin cement in part of the annulus and causethe El problem.Itis reportedby Dowel Schlumberger deformation that casingstandofffrom 100% to 0% could cause 30% increasein the El.
4.11 Double (or concentric)casingstrings How do they affect the log:Whenthe annuluscreatedby one casingstring inside another (eg. the liner overlap)is cemented,the log usuallyshowsthe following features to indicatea goodjob (~9.8):
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1) The Ucurveswill be increased. for example,the increase In a 7” and 9 5/8” combination 20M~~ is typically
2) The first few cyclesof the waveformshown on the VDL log will be narrower,i.e. gaps between thefirst few stripesare smallerthan usual. Also, theapparent casingarrivalscan be so strongthat formationarrivalsare masked. 3) The CBL amplitudes,which arenot usefulin such an interval,can be high. Aroughexplanationfor theseisthat the well-bonded concentric casinghascausedthefrequency ofthe first few cyclesto increase,whilehighattenuationhas causedEl to be skipped,resulting an increasein the U and measurement of E3 as amplitudeoutput(~2.9).
me 4.12 Waiton cement(WOC) ti
How does it affect thelog: Cementslurriestaketime to set and bond to the casing. SinceEl representsthe bondqualityof the cementto the casing, it will changefromfree pipevalue,before the slurrythickens,to bondedvalue, when the cement hasset. WOC time shouldbe the minimumtime to waitbeforelogging.Logs producedat marginalWOC times can be very trick y because the downholeconditions arenot the same as in the laband it may well take a bit longerfor the cementto set; Beside,the slurrycan be contaminated which may stretchnecessary WOC time (~7). For foamedcement,this is oftencomplicatedby the fact that somefoamagents are retardersand are sensitiveto contaminations. It is not possibleto tell greenishcementfroma channel. It is very importantnot to log the welltoo early: one hour saved may cost you ten! It is always good practiceto monitorthe timingof the loggingand the slurrypropertieswhen interpreting a log (~4.l3). 4.13 Cement param eters /conditi ons (~7)
Whatare they: The slurrydensity, cementclass, additivestypes(retardersor accelerators) and weightingagentsare all thingsthat shouldbe known.
How do they affect the log:For neat cementslurry, the heavierit is, the higherthe acoustic attenuationit will havewhen bondedto the casing.Othersolidssuch as bentonite andsilica added in the cementmay increasethe attenuation. ent, resultingin The acousticattenuationof foamedcementis lowerthanthat of the neat cem
higherEl valuesfor 100% bondedcasing. However,the CBL responseto foamedcementhas not been systematically studie d and thereforeis moredifficultto interpret.
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Things y o u should know 4.14 Computer keyboardoperations
Whatarethey: Softwareparametersettingfor communicationcontrol and log resultformatting (~3,~9.1)
How do they affectthe log: It is the engineer’s keyboardoperations that makethe toolcorrectly functionand thedataproperly recorded.Apparently small errorson the keyboardcan spoilall the hard work,and should notbe underestimated. Inthe log presentation,small mistakessuchas incomplete information,wrong legendor scale for curves,can causefrustrationand time loss in the interpretation,or even big mistakesby inexperienced personnel,
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0 I.
a C .)
0
~5. Operation Q C In three phases Oncea decisionis madethat a log is to be run, theoperationQ C shouldbe carriedout in the threephasesas shownin the followingchart:
logging To m eet with engineer to s etht eclear objective & to supply suffici ent informa tion (~5.1)
T o helpoulogging carry t a hi engineer 9 h qu alit y jo b by observing the correct procedure (~5.2)
___________________
5.1 Before logging (Phase 7):
T o accept o r reject the lo g (~5.3)
___________________
Meet withthe loggingengineerand discuss:
1) The aims of the job (zonalisolation, finding TOC or else?) 2) Wellconditions and historyincluding(~3.1): -
-
-
-
Well ambientconditions(deviation,temperature, formationstructure,gas or oil reservoir intervals, intervals of interest,any anomalies) Well fluid characteristics (OBM or WBM, weights,gas/solid contents and sizes, fluid changeaftercementjob?) Casing characteristics (sizes, weights,depths) Pressurehistory(BOP test, casingtest, formationtest, fluid change). Discusswhether pressurewill be needed during loggingif pressurewas appliedto the well after the cement has set or pressurewas maintained during the cementsettingtime.
3) Cementjob (~4. 13 cementtype, density, special additive s, estimatesettingtime, volume, pumpingtime and anyproblemin cementingoperation).Make sure loggingdoesnot start within8hrs after the lab cementsettingtime (~4.12, ~9.8). 4) Choiceof tools(~1), typesnumbers and positions(~4.2), toolstring configuration, centraliser special/back-upequipment needed (such as wireline packoff)
The purpose of thismeeting is to set clear objectivesfor the logging engineer,and supply sufficientinformation for him orher to planand preparefor the job. The meetingshouldbe held at the earliestpossibletime. The loggingengine er should thenproducea job plan whichcan helpyou take appropriateactionsand plan ahead.The loggingengineer’s planshouldinclude: 1) The objectiveof the specificlogging and theapproach to achieveit. 2) Estimatedtime for loggingoperation,includinga detailed plan of wirelinerig-up and rigdown. 3) Indicationof any further information neededin orderto carry out the job successfully and to present the log completely(~3). C B LE valuation
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Q C M atte rs 5.2 Du rIng logging (Phase 2)’
On the wellSite we are to helpthe loggingengineerto carryout a highqualityjob by observing the correctprocedures. The loggingengineershould havea detailed plan of whento do what, but he or she may well alter it as thingschange. Achecklists
1) If the well was pressurisedor the wellfluid was changednot as srcinally planned,discuss with the logging engineerif pressure should be applied during logging to preventthe possiblemicro-annulus effect (~2. 12, ~4.1,~9.2).If wellheadpressureis needed,the following is recommended to determinethe pressure required: -
Run a 0 psi repeat section
-
Identifya potential micro-annulus zone (fom-iation signals behind casingsignals)
-
Stop the logging tool at the micro-annulus depth
-
Switch the panel to time drive i.e. the screendisplay as if the tool was moving
-
Tightenthe wirelinepackoff and start pressure-up(pumpmud slowly intothe casing)
-
Monitor the amplitude untilit no longer drops
-
Stop pressure-up
-
Log underthis pressure
2) The centralisersare the required type/sizeand in good condition.Also check that theyare correctlymountedand securedin therequired position(~4.2).
date has not expired. 3) The tool used shouldbe calibratedand the next calibration 4) The toolstring is correctlyconnected and testedbefore being lowered into the well. Tool calibration beforeand afterthe loggingrun.
5) The parameters/constants Set fromthe keyboardare correct(j3). 6) Do not exceed the maximumloggingspeed. 7) The scales and markingsare correctlyset.
the curveson the screen . Discuss 8) Monitor and recordanysuspectedproblemwiththe logging engineerandsuggestto repeatabnormalsections.If the TI’doesnotrepeat and thedifference problem.Pull out of in amplitudein the repeat sectionis >10%, it could be a centralization hole and change/addcentralisers if necessary.
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5.3 After logging(Phase 3): To accept the log or otherwise.
and in a completemanneris important,not onlyfor producingvalic To present thelog correctly logsfor immediateuse,but alsofor documentation. The logs may be referredto afterthe wel has producedfor severalyears.Missing informationcan be irretrievable der the lo~ and ren meaningless. Achecklist
1) All the signals have been correctlyrecorded.Authoriserig down. 2) The scales and legendsusedare correct(~9. 1). 3) The log header,tail and the questionnaireetc. are fully completedwith no incorrec’ information. inciden ts 4) The loggingengineer’s commentshaveincluded and explainedall quality-related the aims of the loggingand his or her opinionon how well they were achieved.
5 ) The log hard copyis deliveredon the time agreed.Authorise paymentif thereare no Q C problem s, otherwise raise them with the servicecompany.
6) If any eventoccurred during theloggingwhichmay have affectedthe log, preparea repor describingthateventin detail.Attach a copyof the report to the hard copyof the log wher it has arrived.
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0
a 0,
a. 4,
at 0
-J
~6.C B L Interpretation A systematicwayto interpreta CB L log is demonstratedin the interpretation flow charts, Go to the references givenif you areuncertain at any stage. You may have to breakthe main flowchartfor specialinve stigationwhich is shownon page 27, and resumeafterwards.
INTERPRETATION F LOW CHART
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6.1 Q C Review
If the log is pro perlydocumented, it shouldcontain most ofthe information neededfor correct interpretation,The majority of the key parts are includedin the log header (~3). Checkthe followingfouritems. Anythingmissingor incorrectly recordedwill at least causetime loss or an invalidlog.
Item 1: Theplanning and execution ofloggingClear objectives? Effective effortto prevent er’s signatures micro-annulus and eccentricity? Logging actively witnessed— witnessingengine and relevantreports?
1) : Item 2: Thefive data sets are completeand correct(~3. 1) Generalinformation 2) Well geometry 3) Well fluids 4) Cementing operation 5) Well pressureand temperature
Item 3:The logging engineer’scommentsClear and relevantin addressing any problems.The answersto the questionnairecan alsoimplyhis or her competency. tationHeader-bo dy-tail-question naire,signatures and date complete? Item 4: The logpresen ick checkof rangesof the variouscurves 6.2 Qu
77 Curves: The normalranges of Ucurvesdepend mainly on the casingID and mud type/ weight (affecting the sound velocity).Temperatureand the type of tool (the size of the transducers)alsohave someeffect. A rough guidefor calculatingthis rangeis TT(~is)—(casing ID eg. In mm)/(sound velocity in the mud eg. In m n’z.~us §10.3)
+
170(Ms)
The last item is the distanceof T-R spacing(3ft) times the sou nd speed in steel (57~ts/ft).
CBL Curves: The normalrangesofCBL for unbonded pipes depend mainlyon the casingsizes The rangesfor 100% bondedpipesare less reliablebecause (~1O.8) and mud type/weight(~10.7). For foamed cementtherehave not been of the difficulty in controllingthe test conditions. sufficientfield dataor lab resultsfor fully boundedpipesand theinterpretation is thereforemore difficult.
VDL/Signaturelog: Checkwhetherthe time scale/rangeare correct(in the above equation to Sft), the log has goodcontrast,and thereare any fast replade 170 by 285, whichcorresponds formations(~9.4).
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L o g Interpretation 6.3 ExamIne the fl Curves The purposeof examiningthe Ucurvesis to explain the curvevariation,ifany, and investigate the log validity.
The IT curves variation. The Ucurvesfrom a properly centralisedtoolrun in uniformly cementedpipes should ideallybe straightlines in the expected region(~6.2). Parameters (~4.2, affecting theU curvesincludecentralisation §9.3), casingID size/weight(ID)changes(e.g. (~4.5, casingcollars)(S4.8), casingfluids changeoverdifferentdepths(~4.6, §9.5), fastformations
§9.4) and temperature (j4.7, §9.6). The U curveswill also vary to indicatestretching, cycle skipping (j2.9)and wellbonded double-casing §9.7). string (~4.11,
log validity criterion. A widelyquoted log validitycriterion in the literatureand various manualsis that ifthe Ucurvesvaryfor more than±4iis, the log is invalid. Be careful. Variation of the Ucurvesof a reallog is rarelywithinthis limit: investigateif the causewas eccentricity!
A
the Whenthe toolis not properlycentralised,the U can be shorterthannormal.Unfortunately amplitudewill also be reduced.The effects of most other factorsmentionedabove on the amplitudeare likely to be relativelysmall. Traditionally the log is treatedas invalidbecauseof the unknownreductionin the amplitude.This nowhas beenbetterunderstood (~4.2, §10.5) and the amplitude reductiondue to eccentricity can be calculated.However,because the uncertainty that similar criterionbe used with of the effects of other factorsstill exists, it is recommended the Ureductionlimit being5.us (~10.5). Notethe tran sient temperatureeffect can make the log invalid(~4.7, §9.6) and so can fast formations in the intervalof interestarenot quantitatively and micro-annulus — the amplitudes reliable.
the CBL Curves 6.4 ExamIne
CBL curves:Look for To p of Cement(TOC) if applicable(e.g. a non-linerjob), where the CBL curvesswing from the low end of the valuerangeto the high (~6.2, §9.1). Is the TOC in the expected region?Check withthe annulussize and pumped _____ cement volume. A low TOC measuredis often associated with slurry loss anda highone incomplete mud removal.If the TOC is not found the at leastin the top section(~8). slurryhas been contaminated Lookfor good cement sectionwhere the curvesare at the low endof the value range. Note ifa leadand tail slurrysystemwas used, a differenceshouldbe seenin the good cementsectionsfound,where the tail cement shouldgive a lowerreading.Thesewill be usefulin qualitatively evaluatingthecement job. If no good cement sectionis found, the problem could be heavy contaminationbut refer to the SpecialInvestigationChart on page 27. C BL E valaa tion M anu al - Q C a n d In te rp re ta tio n
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Look for “FreePipe” (uncementedpipe) sectionif applicable.Donot be misledby high readingsof the CBL curves: check for the micro-annulus(~4.1)!The CBL ent quality valuesfor free pipe offer references in cem evaluation(~6.6).
log 6.5 ExamIne the VDliSignature The VDL log:must have good black and white contrast.It containsmuchinformation but now only a small portionis extracted,and this is doneby 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 confirmfree pipe (~9.1) (~9.7) 4) to confirmgoodbondin doublecasing strings
5) to confirmgood bondto the casingbut bad bondto the formation(wherethe casingarrivals are extremely low with little or no formationarrivalsand CBL amplitudeindicatesa good bond).
The signature lo& whichis oftensuperimposed on the VDL log, produces the wave amplitude information whichis not availableon the VDL. This information can be usefulin confirmingchangesin bondquality. However, the signaturelog is not as easy to use as the VDL in detecting,for example,the formationarrivalsand that is why it is oftencombinedwith the VDL log. U, of the indications must be The the amplitude and agreement resulting andtheir VDLthere logs could examinations If they do notthe a toolproblem should be in parallel. agree, be in an invalidlog.
L
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6.6 Bond Percentage IndexCalculation
BFlDeflnitiow Thepercentage of the annuluswherethecementis well bondedtothe casing. Therest of the annulus(100% - BPI) is not wellbonded,whichmay be contaminatedcement, localisedsmall gaps betweenthe cementand casing,or a channel. Angle of bonded cement BPI (%)
-
The wholearinulus
lf-
lf-
E
Elm
E
Elc x 100%
Wherethe El’sare fromthe samereceiver(e.g., the 3’ receiver),and thesubscript mrepresents the value measuredin the zoneof interest, f represents freepipevalue,and c the valuefor 100% cementedpipe.
The BPI appliestoany type ofcementsystem(neat, foam, etc.).Note whena leadand tail slurry systemis used,El c shouldbe selectedseparatelyfor the two slurries.WhenElfand El c are available,the correspondingBPI at a givenElm can be foundfrom the above equation , or as shownin Fig.6.l below.
F Ig .6 .1
H ow to findthe
B P I grap hicall y
On the Elm axis, markEl~ and Elfvalues.Mark the BPI (%) axis by equal intervalsfrom 0 to 100. Drawa straightline from(Elc,lOO) to (EljçO). Givena Elm value,the corresponding BPI can be foundas shown.
To providea sealwith high confidence, BPI need to be around95% orhigherfor certainlengths (~l0.6). For gas wells thisrule should be appliedmorestringently.
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L o g Interpretati on BPI CalculationExample Belowis a sectionaround the shoeofa 7’, 2 9 lb/ft linerjob run atthe GulfofMexico. Assuming that the log has passed the QC and quickexaminations, let us see how the BPI is calculated.
Suppose the cementing operation
and other well conditions allowusto believe that at least a section of
perfectcement job can be achieved, we can thenselect the lowest reading, 5mV, as 100% bondedvalue (hopefully!) in a liner job, we therefore look itupfrom §10.8 Elc. The “free pipe” value Elf is nonexistent anduse 62mV (or we can use an available value from a log with close conditions as the job in hand). Between points A and B the average readingof7m V givesthe BPI-(62-7)/(62-5)-96.5%; BetweenB and C the average reading is about9mV. The effect of the slight Ureduction (3—4~.ts) caused by eccentricity can be compensated for (~10.5), in this caseby increasethe amplitude by about10 % 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% bondedintervals for 30ft, the probability of zonal isolation is high. Notemore than50% reductionin Elc(from5mVto 2.4mV) has changedthe BPI value for <5%. Variations in Elf does not affect the B PI much either. This means that for conventional jobs the selection of the reference valuesis important but not critical for a reliable interpretation. Preferably both 0andElfare decidedfrom the log whenever possible. This method applies to all types of slurry design.
CBL
E v a lu a tIo n M a n u a i- Q C a n d In te rp re ta tio n
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SPECIAL INVESTIGATION FLOW CHART~
* Either g o to the n e xt action or r esum e the main interpretation flow
C~L E valuation
M a n u a l - Q C a n d interpretation
chart o n previous page
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ation 6.7 FindingInform
If informationin the log is incomplete,or other information is requiredsuchas open holelogs or somedetail of cementing operation,contactthe right personto obtaini t. Keep an updated list of contactsfor necessaryhelp: their name,specialities, company, base, telephone and fax numbersetc..
6.8 AnalysIng ab norm al log behaviour 1) Review all factorswhich arelikely to affect the log (~4). 2) Eliminateone by one around the key problem in handuntilonly such factors whoseeffects may be significant. 3) Judge whether thesefactorshave invalidatedthe log — a good understanding of all the factors (study §4) maybe veryuseful. 6.9 CalculatIngthe probability ofzonalIsolation Cementingoperationcan offervaluable commonsens e evaluation which should not be If any steps in theoperation werealteredfromplanned, it is suggestedthat discounted(~l.4). the post-jobCPS be run (~7). A startingpointis to judge the likelihoodof at least somesections whichare channelfree. Ifno suchsectionscan be assured andthe log showsa very bad cement job, a remedyjob may have to be considered(~8).
For the BPI calculation, the El valuefor 100% bond(Sl0.8) may have to be found elsewhere if that valuefor the given conditionsis not availablein the literature.The log of a well of close locationwith similar conditionscan proveuseful to providethis reference. The valuecan be at intervalswhere isolationwas requiredand achieved. chosenby findingthe lowest reading Make sure there were no fast formationsor micro-annulus (~4. 1) . 5,~4.
6.10 Keep a record of Interpretation It is recommendedthat the interpretationof a log is recorded,eg, on the backof a log, for ease of later reference. The rubberstamp issued togetherwith this manualis for this purpose.The table fromthe stamp is designedto summarisethe log with minimalamount of information. Stampthe back of the log and fill in the form as you completethe interpretation.
C BL E valuation
M a n u a l - Q C a n d interpretation
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L o g interpretation
~7. Cementing Operation 1. Whatto check~ Donot cometo conclusions on the quality of the cementjob withoutreviewingthe cementing operation.Every stepof the operationis carefullydesignedto ensurea goodjob. Problemsmay occurif anyrecommended stepwas not followedcorrectly.Common sense can oftentell you the nature of the problem. speaking,thereare Cementplacementitself is a largeand complexsubject.But simplistically somerulesofthumb.For example,the morecentralizedthe casingstring, the betterthecement job; Equallyimportantare the displacement ratesof mud, spacer and cement slurry. Usuallythe higher the flow rates, the betterthe displacement.The flow rates are limitedby the formation strengthand the displacement facilities. The rheologyand densityof the fluids are similarly important.The ideais to removethe viscousmud by spaceras completelyas possible,thento displacethe spacerby the cementslurry. The spacer hereis designedto make easy the removal of mud and the placementof cement. Usuallya heavierfluid followinga lighter one can help the displacement.Remem ber, non-vertical wellsare not only difficultto log, but alsodifficult problemand the complicated flowregimesin an to cementbecauseofthe casingcentralisation inclined annulus. Withoutbeing deeplyinvolvedin cemen ting technolo gy, you can always get someideaabout
the qualityof the cement job by checking thefollowing: -
Was thereany lost circulation?
-
Was the casing stringcentralised?
-
Werethe pumpingrates and displacement timing as planned?
-
(density,viscosity, additivesquantitiesetc.)as Werethe well-site-measured fluids properties designed?
Answer“No” to any of thesequestionscouldmeana less thanperfect cement job. If there were lossesto the formationor otheroperationalproblem enco untered duringcement placement, it this into account. could be a bad job and log interpretation must take (CPS) 2. The Cement Placement Simulator
This simulator, developedby the Fluid MechanicsTeam at RCS, can tell you how to carry out the cementjob and whatthe cement job shouldlook like. The CPS is locatedat Dyce, Houston in DEAP. If a bad cementjob is suspectedit is and Sunburyand wifi also be incorporated suggestedthat the CPS be run. Theresultis anotherreferencefor interpretingthe log.
C BL E valuation
M a n u a l - Q C a n d Interpretation
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L o g Interpretati on
~8 Squeeze considerations A bad cementjob indicatedby a bondlog (whichevertype) doesnot meanyou can go ahead ns must be answeredasbestas you can beforeany action: and squeeze.The followingquestio Where didthe cementgo? Whatkind of channel could it be, and whereand how to squeeze? The followingCharts may help you answer these questions.
go?
1 . Where did thecement
Ana lyse the we ll and cem ent ing cond itions together w ith the log N o t cl ea rly ~~d ? ~ 62~
Check if a n y fl uid loss occ u red d ur iriu d~IIing o r cementing(~6) _____________________
__________________________
Possible cement loss b y large quantities. Such cases a re usually ea s ily i den t i fi ab l e.
Possible heavy contamination o r b a d slurry leading t o green cement. Re- run t h e C B L i f poss i bl e.
C B L E valuation M anu al - Q C a n d interpretation
:
Yes
Calculate t h e difference from expected value (Note the hole gauge, w as ho u t: caliper lo g ).
~
Yes
T h e cement i s likely to b e still In t h e annulus b u t badly bonded t o th e casing a n d m ayb e to t h e formation a s w ell.
Possible b a d contamination a t t h e cement top or b a d m u d removal.
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29
6 scenarios of communication channels Eccentred casing,
mud channel on the narrow side.
Heavily contaminated be solid.
cement which may or m ay no t
Contaminated cement but solid mud channel onthe narrow side.on the wide side with Contaminated but soiid cement on the narrow side with m ud channei on the wide side. key:
Ga p between the set cement and the casing
mud cement
Thick mud cake between the set cement and the formation.
contaminated cement formation
W ell co nd it ions a n d ce m en ti ng operation Channel type
Ba d casingcentralisation
Deviated wells
v s potential_channels
Displacing contamination
Cementing operation problems
I,
________________
Severe in hig h deviation
W ashout s e ct io n
Ohorizontal fte n in wail.
I,
~~1
in porousreservoirs Some“delayed” communications observed are believed by the disintegrationof the mud cake. to be caused is hardly detectable This typeof channel with today’s technology
Bond logs providev Ital information for squeeze job design particularly in the following areas:
1. Depths,lengthsof communicatingchannelsfor positioning the perforatinggun and bridge plug o r packers. 2. Azimuthof communicatingchannelsfor perforatin g shot phase arrangement:a 45 degree channel can be missedi 3. Identifythe vent for the channelfilling substances.
L~~Evaiuatlon
Manual~ Q C a n d interpretation
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30
at at U C
at at at
C at at
C. E ‘C
9.1 ALog Example Header[on this page],body and tail [on next page],and questionaire(~3)
~ion~cC~
L o g E xa m p les
C BL E va’ua tion M a n u a l - O C a n d I n te rp re ta tio n
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C B L E valuation
M anu al - Q C a n d In te rp re ta tio n
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L o g E x a m p le s
RUN1
RUN2
~ AMPLITUDE
aEFORE
PRESSURING
AFTER PRESSURIZIFICJ
AMPLITUDE
TO3000FSI& RELEASING *
9.2MIcro.annukis
F~.ashs1om~rho1ding5Op~onthecasing~r3daysRun1wasmnwithOpsiandRun2wsswith7OOpsi.Adequate pxessuxe cen eliminate some miao-annulus. 3OOO~l.Excessivepressuxecanbreakthebondbetween Fig.bwlogsninbefoieandasngwsspiessuzeiisedto the casing and thecement TT
(US
) ~
aUO.00
___L_~ M ~_ __~_.
5~._OOO
V DL
100.00 1200.0Q
(US
1200.0
9.3 Tool E c c e n t r i c i t y Eccentrldty causes reduction inthe Trand inthe CBL amplitude (~1O.5).Ad d an exn~ centmliser aithe3 ’ receiver can alleviate the problem (~6.2).Never run the C BL knowingly off-centred (eg with 7 1 1 rubber centraliser for liner, running in 95/8” casing above the liner).
C B L E v a lu a tio n M anu al - O C a n d In te rp re ta tio n
Page 3 4
TRAVEL TIME 290 •
19 0 T~A/
~
ATTENUATION OS/IT 0 — — PEAK AMPLITUDE— —
20 — — -
API 0
‘LIGNATUITE
VARIABI E
DENSITY
MV 10 0
CC
0
120200
1200 200
‘200
AMI’LIIIED
9. 4 Fast FormationsR The TT fell below the expected value (~5.2). The VDL 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 C B L E v a lu a tio n M anu al - C C a n d In te rp re ta tio n
Page 3 5
L o g E xa m p le s
9 .5 CasIng Fluids Effecton the Log
rr
The mud weight changedfrom 1O.5ppg above 7028ft to 16.6 ppg below. The effect on th e is obvious butnot so on the C B L amplitude. The well temperature profile canmake a difference to the mud (eg in density) if the mud has been static in the hole for somtime, This can cause the iT to vary (~6.2). Quoted from DS Cement Bond Logging Field Reference M anuai .
C B L E v a lu a tIo n M a n u a l
-
C C a n d I n te rp re ta tio n
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9 .6
Tem perat ure Effect onthe Log
The temperaturein the sectionshown was about320°F and 300m belowwas about345°F. The tool was runto TD in 1.5hrs, experien cing high temperature gradiant,and the loggingwas finishedin 2hrs. The toolwas properlycentralised but the C B L amplitudeand the VDL were affectedby transient temperature effect.
9.7 Double CasingString BelowD is 7” (291b/ft) inside9 5/8” (561b/ft) andthe annulus was fully cemented.The TT has increasedby 20its and the VDL showedstrong “pipe ring” withfirst few cyclesbeing narrower. The CBL amplitude here may be E3 (~2.5, ~6.11). C B LE valuation
M anual
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Q C a n d In te rp re ta tio n
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L o g E xa m p les
TIME AFTER CEMENTATION 4 HRS.
a
HRS.
ze
HRS.
9.8 G reenCem ent Rememberthat the downhole condition s are not as idealas in laboratory.Cementslurrycan by mud,spacerorformationfluids. Thethicken easilyget contaminated ing timeand settingtime can be muchlonger than quoted(S4.1, §6.12,13).
C BL E valua tion M anu al - Q C a n d In te rp re ta tio n
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Parameters
CBL
CET
USI
Original design purpose
open h o le
cem ent
bond
bond/corrosion
Calibration conditions C as ing sizes
u p h o le 4.5” -3/8 1 3”
d o w n h o le 4 .5” -95/8”
d o w n h o le 4.5”- 133/8”
O B M
OBM
M ud density
high(eg>l8ppg) u n q u a n fif te d e ffe c t
Micro-annulus
s e v e re
Heavily contaminated cement Cement/formation bond Eccentralisation
Vertical resolution Azimuthal resolution Shape of channel (on casing/cement interf ace ) Cement thickness Compressive strength Double casing string C as ing stand -of f Casing thickness C as ing wall conditions M echa nical complexity Operational robustness Costs *T h is
am plitude
e ffe c t
le s s 9 e ff e c t (liq u id fille d )
a tt e n u a te d
c o u ld in d ic a te a s cha n n e l” n o t m easured less s e n s if iv e
c a n b e d e te c te d s e n s it iv e 3 ft
cha n n e l”
a v o id e d le s s s e n s if iv e 1 .S in s
3ins
n o e ffe c t
v a lu e s
c o u ld
4 5 d e g re e s
non-directonal
de rive d (un ~u sfiflab ly)
s e v e re
1 0 degrees good m eas urem en t
detectable
debatable
e ffe c t
s tr o n g e ff e c t
e ffe c t
le s s
m easurable
som e
e ffe c t
s tr o n g
less e ff e c t
e ffe c t
detectable
is b e c a u s e th e a co u sh c i m p e d a n ce of heavil y contaminated cem
e a sy
g et e a s y t o wrong
relatively low
typically ent is in a sim ila r range
e ffe c t
le s s m easured
co mp le x
OK
abandoned
id e a
e ffe c t
le s s
e ffect
simp le st
less e f fe c t
de rive d le s s e f fe c t
s tr o n g
e ffe c t fil le d ) in d ic a te a s
le s s (li q u id
to d a m a g e good?
typically 4 x C B L
2 xC B L a s that of m u d .
10.1 Performance Comparison of CBL, C E T and US! C BL E valuation
M a n u a l - Q C a n d Interpretati on
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D a ta a n d C harts for R eferen ce
11
_______ I 10.2 CharacterIstics of Commonly Used C BL Tools (quoted from “CBL Field Referenc Manual” DS) C B L E v a lu a tio n M anu al - O C a n d In te rp re ta tio n
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SOUNDVELOCITY MATERIAL TYPE
POROSLfl
(%)
Casing Dolomite Limestone ~ Calsite
~
-
~ Anhydrite
-
Z Gypsum
-
Q ua rts — Halite
-
—
Dolomite Limestone ~ Sandstone ~ Shale —
~
-
5to 20 5 to 20 5 to 20
OBM W BM Water Water + 10% NaC1 Water +20% NaC1 Sea Water Kerosene Air (15 psi, 0°C) Air (3000 psi, 100°C)
-
-
AT (Ms/ft)
57.0
43.5 47.6 49.7 50.0 52.6 52.9 66.6
50.0 to 66.6 20000 to 150 00 54.0 to76.9 18500 to 13 00 0 62.5 to 86.9 16000 to 11500 58.8 to 143 17000 to7000
(MRayl)
(rn/a)
(ft/a)
17,500 23,000 21,000 20,100 20,000 19,000 18,900 15,000
ACOUSTEC IMPEDANCE
5,334 7,010 6,400 6,126 6,096 5,791 5,760 4,572 609 6 56 39 4877 5181
41.6 20.2 17.3 16.6 18.2 13.6 15.2
9.3
to 4752 to 3962 to 3505
to 2133
209 to 222 4785 to 4505 1458 to 1373 198 to 205 5050 to 4878 1539 to 1487 208 4800 1463 192 5200 1585 182 5500 1676 199 5020 1531 230 4340 1324 920 1088 331 780 1280 390
17,0 to 11.5 14.8 to9.4 12.6 to8.2 12.0 to4.3
1.7 to 2.7 1.8 to 3.0 1.5 1.7 1.8 1.6 1.1 0,0004 0.1
of Formation and Mud 10.3 Speed of Sound InVarious Types Schlumberger, (edited from“Well Cementing”,Dowel 1990)
10.4 CasIngExpansion Under Pressure (quotedfrom“CementEvaluations” Atlas, 1990) C BL E valua tion M anu al - Q C a n d interpretation
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41
D a ta a n d C harts for R eference Eccentricity vs
Relative El
IT reduction affectedby cement
Eccentricity vs IT reduction
TI reduction vs
reIativ~ El
Notes: 1) Cementdoesnot affect the relationshipin (a). in (a) (b) and(c) but less in (d). 2) Mud affects relationships
3) Cementaffects amountof Yr reductionas shown in (c), consequentlyYr-El relationshipis not unique. 4) Free pipe U-Elrelationshipin (d) can beused for El conpensation.
10.5 Relationship among Tool Eccentricity,CBL AmplitudeandTr Reduction C BL E valua tion M anu al - O C a n d interpretation
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D ata a n d C h a r t s f o r R e f e r e n c e
F o r BPI95%
10.6Interval Required for Isolation Lengths
C ompensat ion Chartsfor Various Muds, Casing IDand Thicknesses 10.7 Amplitude (quoted from “The Fluid-Compensated Cement Bond Log” Nayfeh et al, S P E Formation Evaluation, pp335-341, 1986) C B L E v a lu a tio n M a n u a l - O C aid In te rp re ta tio n
Page 4 3
Casing s ize
(in )
1/4 2
w e ig h t (Ib )
Free pipe Elf(mV)
100% bonded H) (class E1 0(m V)
9 .5
11.6 13.6
5
15.0 18.0 21.0
51/2
15.5 17.0 20.0 23.0
7
23.0 26.0 29.0 32.0 35.0 38.0 40.0
75/8
26.4 29.7 33.7 39.0
95/8
40.0 43.5 47.0 53.5
1 0 3/4
40.5 45.5 48.0 51.0 54.0 55.5
The dataabove werederivedfroma chartby Pardueet al [Cement BondLogging - A Studyof CementandCasing Variables, JPT May 1963, pp545-555]. Notethey can onlybe used asa general guide.
and 0% Bonded Pipes 10.8 C B L 3’ ReceIverEl Readi ngs for 100% Bonded C B LE valuation
M anual - QC and I n te rp re ta tio n
Page 4 4
Index Amplitude Afte r logg ing Before logging Body, log BPI, Bond Percentage Index Casing, arrivals Casing, coating CC L Cementing Centralisers CE T Channel Check list CPS Criteria, tool selection Cycle skipping Density Diameter, casing Double casing string During logging El, E2, E3, etc Eccentridty,centrallsation Examples, logs Fast formations Features, tool Five data sets Foam cement Formation arrivals Free pipe Gate, fixed and sliding Geometry, openhole Green Cement Header, log Interpretation
4,5,18,22
19 17 8,32 24 7 12 5,11 3,17,11,28 8,18,11 3,39 3,5,7,9,11,24,30 18,19 28 TOC 2,3 6,22 8,13,15,17,28,36 VDL 13 14,37 18 6 5,10,34 31-38 7,23,35 4 21 15,21,24 7 7 6 14 15,38 8,31 20
Isolation Jargon, explained Keyboard operation Manual structure Micro-annulus Mud arrivals OBM Peak
C B L E v a lu a tio n M a n u a l -Q C and in te rp re ta tio n
Pressure Principles Questionnaire Review, QC Signature log Sonde Squeeze job Standoff Stretching Tail, log Temperature Test: BOP, casing, formation Thickness TT, Transit Time USI W BM WOC
2,8,9, 10, 17,18,21 4,5 8,19,21,33 21 4,5,23 4 29 14 6,22 8,31 8,13,17,21,22,37 10,17 10,13,14 17,22,28 6,21,22 39 3,5,7 17,41 15,38
2,9,20,43 5-7 16 2,7,9,18,22,23 7 17,41 6,7
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