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..9
a CopyrightSPE 1979
OE-79. SPE 8153.1.
S 8153
Geological aspects of the drilling ~f the
Buchan field
P, J. HILL
BP Petroleum Development L@. UK
G. SMITH
The Analysts inland Services. UK.
Dr. PA HJll
is the develop’xant
geologistfor tbo Fonies a:.d
Buobsn fields A sraduscef thO
University
of Soutbempton h? joined
BPin 1972andWSS kdt iy
seconded to tbo
BP ReeeszohCentre
in
Sunbury-on-Thames. In
1974
he
wes posted to New Zeelaadbefore
comingo BP PetroleumDevelop
mentin Aberdeen in 1977.Since
tbsttimehohes been reqmeible for
the geologist input into the
development of the Buchan field
and Mteriy the Fonics fiel~
,
INTRODUCTION AND HISTORICAL
*
c
BACKGROUND
The 3uchenfiel~ located some 150 km nonh east of
Aberdeenin 120m ofwater,is situatedlargelyinthe UK
licence block 21/1 (P.241) that was granted m Trans-
world Petroleum(UK) Ltd in July 1972, (Fig, 1). The
discoverywell21/ -1 was drilledin June 1974. andwas
followedduring1975-76 bythreeappraisalwells,twoof
which eventually penetrated’end tested the reservoir.
However, the appraisal wells 21/1-2 and 21/14 ex-
perienced serious drilling problems at rhe top of the
reset40ir section end were eventually los~ These wells
setweesillustrationsc4theextremedifhdth encounteredn
drilling this structure and proved the existence of a
complex, fault-controlledstructurewith a maximumoil
columnof 582 m inoverprmsured,highlyfracturm low
porosity resewoir sandstone of Devonian-LowerCar-
boniferousage(Fig 2).
In July 1977 as a result of a farm-inagreementBP
Petroleum Development Ltd, became the operator of
Grebem
J%
Smith
is the North See
diankt
mensserwith
The Anaiysrs
Inland Services.
A greduateromthe
University of Aberdeemwith a B.SC,
(Hone) degree in Geology. He joined
The Analysts in
1973,
and since
then has hen closely sssoaiated
withThe Analysts weUlogging
divisin k cheNorth Sea m“&
IicenceP.241 and twk a 54 1/6 per cemtinterestin the
Buchenfield.In planninganydevl~lopmentof the fieldit
becemeevident that a conventiold Nonh Sea drilliny
prediction platformwouldbe, e~onomiceily,a veryhigh
riskpro~itiom Tiaesize and complicatednature of the
structureand reservoirsendstonesposedmanyquestions
regarding the reliable calculation of oil in place and
recovery factor, and make the field wot%hyof the title
‘marginal’.
As a result of feasibility studies it was decided to
aevelop the fieldfromeightwellsprcducingtoa convened
semi-submersible ladiontt.Transportationof the stabiljsed
crudewillbebyarankershuttleservicevie.sncffshoreoading
bwy locatedclose to theplatform(Fig. 3). Reduction is
plannedto start in the autumnof 1979.In a timesensitive
pisn of this nature, and inviewofthe pastdrillinghistory.
the successfulcompletion of the drillingphase was an
importantstep in”he developmentof the field
Developmentdrillin~commencedin September 1977
andwascompletedin early
November1978.
Duringthis
..,.
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;
*.
OE-79. SPE 8153.2.
14 monthperiodfourdeviatedwellsweredrilledand the
previouslysuspendedweii
21/1-2ST weem-enterd In
additiontwosatellitewellsweredrilledfromthesouthern
fhinkofthe fieldandwiilbetiedintotheproductionystem
by four subseaflowiines(P@ 4).
Thispaperhighlightshe haztudouapmbiemamscwiated
with the developmentdrillingof this field and howthey
weresuccsssfii[yovercome.
STRUCTURE AND STRATIGWHY OF THE
BUCHAN FIEFD
Tiie Buchansmcture is so east-westorientatedhorst
thatliescioeet~,dteaxisofa regionaiMesozoicsynciittein
whichthem i, an almost completesucwssion of Upper
Paleozoic sEilMesozoicsediments(Fs 2).This suggests
uqji~ of the horatof at least 2 00 m and fromseismic
evidenceat least 1,000 m of this upliftoccurredduring
LowerCretaceoustimes.ThroughoutmuchofthiFperiod
however,thehorstremainedbelowsea leve~althoughhe
Lower Cretaceousshales
Wt dramaticallyover the
highestpartsof thehors~
Thepreciseageof pre-Cretaceous
movementsof the
horst are extremely difFicultto define. However,the
overallstructure
f the
Buchanhorst and itscontinuation
as a linear highto theeaa~ issimilarto theaxialhorstsof
major transcurrentault zones. It seemsmost likelythat
the fomtation of the Buchan
structure
resulted from
nonnai movementson a reactivatedfault systemwhich
was initiated during late Paleozoic times, conterm
pxatteous withthe mqjorepisodeoftranscurrentfaulting
recognisedin highlandScotland 1:is interestingm note
that the Buchan horst lies close to the northeasterly
extension of the Highland Boundary Fault C(onshore
Scotland a m~or transcurrentfauk
Faultmovemcmtppearsto haveceasedbyMaasaichtian
times(LateCretaceous):however,closureon theBuchan
stmcture isapparentintothePaleocene, probablydueto
the effectsof compactionaldrape.
A
generalisedstratigraphyoftheBuchanfieldisshown
in
Fig 5 and the Iithostratigraphicnomenclaturefollows
that proposedbyDeegsn and Sdl A Recent to Lower
Cretaceous succession lies unconformably on the
Devonianhwer Carboniferous reservoir sandstones.
However. bee-we of movement of the Buchan horst
during the Cretaceous, non sequencesand variationsin
thicknessoccur in theCretaceousstrata acrossthe field.
The reservoirsequenceis similar in Iithologyto the Old
Red Sandatones of mainland Scotland and the co-
mbinationf abundantcross-baddin%iningupwardcycles,
and palaaosoldevelopmentindicates that the sediments
arc of fluviatileorigin.Palymologica.1atinggivesan age
rangeof Fatrunenianto Visean(UpperDevonianLower
Carboniferous)forthe upperpart of the reservoirandail
wells drilled in tie field terminate
in this sandstone
sequence.
DRILLING PROBLEMS IN THE WCHAN
FIELD
Duringthediscoveryandappraisaiof theBuchattfield
seriousdrilling problemswere
encountered.These arose
forfourmain reaswns(Fig. 6).
Geophysieai datx
Mappingthe topof thereservoiracrosstheBuchanfield
isa di&tcuhexercis~ QvcrthehighlyfaultedBuchanhorst
Hiii and SmitlE
the quality of the soistnicdata is poor and the reflector
- dle toptUSetVOushowsmanycharactervathticm
This,
togetherwith complexvariationsin velocitygave
errorsofup to 133m inthe predictionsofthedepthofthe
top reservoir.Hencegeophysicscouldnot be reiii upon
to accuratelydetlne thedepth to the top reservoir
atwell
locations.
OverpreestIredmeervoir
Pressure data obtained from the appraisal wells itt-
dicatada reservoirpreasureof7,500 paigat2926 mTVD
subsez an ovorpresauroof some 3,200 psi. The Iowcr
Cretaceousshales,immediatelybelowthechalk markthe
transition zonefrom the ‘normal’pressure regimeof the
upper patt of the section to the highlyovmpressurcd
regimeof the reservoir(Fi 6).
Pore pressure in the sealing Shales
The pore pressurebuildsup veryrapidly inthe Lower
Cretaceousshc s sectionincreasingfroma 1.18SG mud
equivalent(9.8 ppg)to a 1.95 SGmudquiv~ent ( 16.3
ppg) overa 20-30 m interval.Unlessdrilledwitha high
mud weighttheoverpressuredshalesgiveseriousheaving
hole conditions, In addition a comparisonof the pore
pressureandfracturepressuregradientsthroughthechalk
indicatesthat this intervalis not capableof withstanding
thehighmudwcightsneededtocontrolhesloughingofthe
LowerCrotaceouahales.
The hickness of tho,sesIingstales
acrosathe 13uchan
strucwe variesfromlessthan30 mon thecrestto scverai
huudredmctreson the flanks.Hence,it is notpossibleto
predictthedepth to thetopof the reservoirhorn the rapid
build-upofporepressurein theshalesimmediatelybelow
thechalk
Loss Zonos
Twomajorlosszonesoccurin the stratigraphicsection
overlying the Buchan hors~ The uppermost loss zone
wcurs in sandstonesandfracturedlimestonesat thebase
ofthe Danian.Duringthedrillingofwell21/1-2 this zone
wasuncasedwhenthe topC:‘dersservoirwaspenetrated
at a shallower depth than had been anticipated. The
increaseinmudweightrequiredto balancethe reservoir
pressureresultedinconsiderablemudlossestotheDanian
wnds and fracturedlimestones.Eventuallythe wellwas
pluggedbackto the 13%in.casingshoe inthe Eoceneand
then sidetracked
The second loss zone occurs in fractured Turonian
limestonesat the baseof the chaik sectiorLWell 21/1-4
also penetrated the top of the resemoir higher than
expected aqd mud losses, this time to the fracturca
‘Wonien limestones,resultedin the eventwd10ssof the
hole.
T2 SU~d~ thc h~ards in drillingon the Buchan
flcl. stemprimarilyfromthe reservoirbeinghighlyover- ‘
pressurecLand the overlying scalingshalesbeingas little
as 30 m thick The presenceof two potentiai major lost
circulation zones and the uncertainty in predictingthe
depthto thetopof the resemoiradd furthercomplication.,
SUCCESSFUL COMPLETION OF THE BUCHAN
DRILLING PROGIMMME
In drillingan economicallymarginalfield it is vital to
meet the deadlines set by the overalldevelopmentpm
grtunme. The successful completion of the Buchan
developmentdriilingphasedependedonathoroughundcr-
8/9/2019 Geological Aspects of the Drilling of the Buchan Field
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.“
~k%icd sSP8CUoftin+ oftbe Buchan fiOU
staadingcfthe geologyofthe tleldareaand theintegradon
of this data.with soud drillingpracdce.
The nine.slottemplatewasposidotwdontheseabedin
August 1977 and developmentdrilling comtnenced in
September1977 andweecatpleted in eariy November
1978.Duringthis 14monthperiod4
deviatedwellswere
drilledthroughhetemplateanddtepreviouslysuspended
well21/ 1-2ST was recentered In addhiomtwo,satellite
wejlsweredrilledon the southernflackof thefield In all
cases the drilling progmmme was the same end was
specificallydesignedto overcomethe problemsdescribed
shove.Much dependedon accurategeologicaldecisions.
~~hrly in picking the 13% im and 9% in. casing
.
IAlcatios of 13%b casingShoe
TM casing stringwas set below them@orlostcir-
cuiadonzoneinthesendstmwendfractured
halkat the
basecfthe Denim. The shoeweeset some30 m intothe
Cretaceouschu belowthe chertsof the Meaetrichtian.
With the 13%im shoe set in the Maastrichdanchalk the
Danian 10SSzone is cased off.
Location of9% iaOC8singshe
TM casingshoemustbe setbelowthe losszonein the
bsmturedTuronianchalkaadabovetheshaIestnarkingthe
topofthepressureransidonzonaThiscasesoffthechalk
andtheTuronian losszoneendelllowsthemudweightto
be raisedm drill the overpessuredtransidoazone end
safelyenterthe
reservoir.
In thewell21/1-4 theLowerCretaceousshale interval
was only 30 m thick and the possibilityexistedthat this
interval might be even thinner at structurally higher
positionson the Buchsnhors~The optimumpositionat .
whichto setthe shoeisclosetothe topoftheCenomsnien
mudstones.In the absenceof reliabieseismicprediction
threeon-sitecriteriaweredevelopedtoaidinthepickingof
thecritical9% in casingpoints.
(0
(?9
Lhhology -The Upper Cretaceous is composedof
whitelimestones(chalk)interbeddedwithcaicartime
mudatu.les. Within the Turonian-Maastrichtian
interval, three distinct red/brown mudstones and a
cherty section were found to consistently cwcur
acrr,M the Buchan structure. Below this are
Cer.~maniansndpossibly LowerCretaceousshales.
However,as ~itaiestringersoccurintheTuronienthe
first apparance of shtdes is not a conclusive
indictadonof theCenomaniamCarefttl analysisof
~nc~ings bythe geologistgavegoodstradgraphic
Palaeontolow- Ithadbeenestablishedbydetailed
study of earlier wells that seven palaeontoiogicel
meJkelsddbereccacitedntheupperrataeeoua
secdott,These
merkefiwere
distinct-hd idendfiable
andditchcuttingswerecontinuouslydated at the rig
site to ensure that the 9 6 im shoe was set in the
uppermost part of the Cenomaniatt shale intmal
(Fl& 7).
. .
The criticalrecognidonof theCenomanianat the baseof
theUpperCretaceouswasdependenton the identificadon
of speciilcmicrofossils.These pianktonic foraminifertd
:
copyright S?Z
1979
.
OE-79* sPl%81s3.3.
assemblageswere characterisedby en abundanceof
Hedbw gel l aOnt t onens andHedbergel adelrionensti
togetherw it hthefmt downholeecordof Rotal ipw
cwhmani andRotal ipomgrw t homensis(Fi& 7).
(iii)
Pore pressure - The shales below the Turonian
chalkmark the tranaidon fromthe normeIpressure
regime to the highly ovarpreesuredregimeof the
resemoirsecdomTo obtain a measure of the pore
pressure whilst drillinfi the Analysts Inc, Ittstan-
taneouaDrillingEveluadon&g(IDEL) systemwas
usedthroughoutthe 12%imaccdon(Fi~ 8),Thison-
site, computerised logging system continuously
samplesthe formation drilling response end cal-
culates the wre pressure as an equivalent mud
url
weigh everyheifmetroof holedrilletL
As an
aid to
teiy detemtining~re pressurethe important
g
parameters -
bit wear, weighton bit and
rotarys~ed - werestabilisedas muchas possible
during the drilliig of the 12 4 in. hole. The pore
pressure analysisby the IDEL systemis baaed on
establisheddrillingresponse theory integratedwith
basic bit perfornmncetheory.
The pore pressure log plotted every half metre
pinpointedthe pressuretransidon zonein the shales
beneadt theTuronianchalk(Fig 8).
CONCLUSIONS
By combinadonof detailed analysis of the probletm
tight stratisgraphic control sound
drilling practice
and
carefblmonitoringofporepressure,the9%h casingshoe
wasset in itsoptimumpoaidon.
Accurate omsite geology
and pelaeontology ensures identification of the Cen-
manien mudstones and ensured that the loss zone of
i%actumdTuronianchalk was fullypenetratedin 12%im
hole. Detailed and continual analysis of the drilling
response enabled pore pressure to be calculated and
ensured that the pressure transition zone was not pen~
trated. This avoided the serious sloughingof the over-
pressuredshales and the drastic increase in mud weight
with resultant losses to theTuronian chalk.The method
alsoensuredthat the reservoirwas not ;.-rated and so
avoidedWtentialblowoutsituations.
With the 12%in.hole@illedno furtherthan 10m into
the Cenomanianmw stones,and with a mud weightof
around 1.18SG equivalen&he fracturedTuronian chalk
did not giveproblems.Oncethe 9% in.wasset at a depth
still in the normalpressure regimethe 842
in. hole was
drilledwitha mudweightaround1.95 SG equivalentand
the overpressuredshalesectionand reservoirwerepene-
trated safely.
Figure 8 shows the situation in the first development
well21/ 1-6.In this case althoughthe thicknesstlom the
baseof theTuronianchaikto toprsservoiris 70 mdriiledi
the thicknessfromthe base of the chalk to the top of the
transition zone is only 30 m drilled. However,the top
reservoircamein at 2626 mTVD b% some60mhighon
thegeophysicalforecaa~Had the problemsat the9% in.
casing depth not been recognised and the above pro-
cechres establishedand applied thetiwell21/1-6 would
almost certainly have given serious hole problems and
much valuabletime in the developmentdrillingproblem
IOSL
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,.
,,
OE-79. SPi 8153.4.
.
ACKNOWLEDGEMENTS
This paparis theresultofsmdiosundertakenbystcffct
BP Petroieum DevelopmentU Dyce as part of the
&velopment pmgrammeof the Buchan field We cm,
therefore, indebted to a large numberof colleaguesfor
theirhelR canmenraandsuggestionsinthe preparationof
this paper.In particulartheworkofT. Bumhiilcnd I. R
Vann is acknowledged.We thank the Chairman and
BoardofDirectorsof theBritishPetroleumCa U and
the other partners in theBuchanMd for permissionto
Hiiiand Smitix” “
REFERENCES
1. KENNEDY,W.Q.,‘TheGraatGien Fault?,Quun
J Geoi Sot., London, 102,41-72.1946.
2. DEEGAN, C.E. and SCU B,J., ‘A St8Xldd
Iithostmtigraphicomenclatureorthe Centrcicnd
Nodtem NonhSea’,IGSRepmt77/25, BuUedn1,
1977,
3. BUTMR IL, PHELAN,M ad WIGHT,AW.,
‘The Buchanfield An evsluadon of a
fi’actumd
sandstonereservoir’. SPWLA4th Eumoean
SVnL
pubiishthk paper.
Trans.,
Paper &
1976.
.. .
0 copyright 1979
AmoricawInstitute of Mlnin Metaiiurgicai cad Petroioum Eugineem In&
This papar was pmpamd for the Offshore Europa 79 Confomnce 10 conjuncdon with the Aberdsan
Petroieum Section of the Society of Petroieum Engineara of AIME heid
in AberdeemScotiaa4
3-7
Septambar, 1979.
?ermissim @ tzypyis metrfctedto anabatmt
ofnot mom than 300 wortk
Illustrations
may
not be copied The abstrmt sbtmid contain conspicuous
knowiedgementof wlwraand by whomthe
paper is pmaanted Publication oiaewhem
after
publication in thoJournai of Petroiaurn Tacimoi~
or the
Society of
PetroiaumEnginaers
Journai is usuaiiy granted upon request to the Editor of the appropriate
journai Provided asmementto give
proper
credit ismad-
( /
NINIAN~
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8/9/2019 Geological Aspects of the Drilling of the Buchan Field
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... .
GaolOgioalaspaots ofthodriUing c4rho BuohM field”
OE-79. SPE 81S3 5
OCPTH
(m)
2000
2800
1
3000
3200
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1
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FRACTURED OEVONIAN SANDSTONES
F~ 2. Dia~dc SCCtiOIl sofas tie Buchut UUCtUM.
.
TION
TANKER LOAOING AT
SINQL,E FOINT UOORINQ
/4?
ALMLOADIN13UOY
t [
1
,,
WELL 21/1-2ST
.“
12” LOAOINO LINE
TEMPLATg WITH>
4 WELLS
\\ ~
FOUR4“ LINES
(2
FLOWLINES
a 2
ANNULUS LiNes)
A-
(L, /
AND CONTROL HOSE SUNOLES
WV
OFF NORSTSATELLITS ANO
Fig.3 SCHEMATIC DIAGRAM
vERTICAL SATELLITC WELLS
u
OF THE BUCHAN FIELD INSTALLATION :
. .
Fig. 3. Schematic diagrsm of the Buchsn field hIstWation.
: Copyright SPE 1979
.
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SUCHAN FIELD
00”00’
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Fi~ 6. Asummeryfthedrillingroblems in thO Buohenfield.
LITHOLOGICAL
MARKER
UPPER
RSD MuDSTIMR
MlltOLE
RED MUOSTONC
LOWKR
RIO
MUOSTONR
CENOMANiAIA
MUOSTONC
OLOSO
SAtAOSTONC
LAEONTOLOGICAL
MARKER
UID07EXTULARIA
Cloem,
A1&NoANT
RUOC6L9ttOKRlNA
SPP
‘W OLOBOTRUNCANA
Pamwwtoaaa
rOP OSAMULARIA
“
Buahmmlc’
tiCOSSROELLA
- Wlttofmwis
- Od twimm
ROTALI?ORA
- Cwhmeni
- QwmhOfn,nsls
Fig 5. Genereiised etratigrephyof the Buchsn tield,
Fig, 7. Kc~retaceous stradgrephic msrkere over the Buohen
8/9/2019 Geological Aspects of the Drilling of the Buchan Field
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.,-
Gcdogical aspectsf tho driIIhw
of
the Buoban fiald
OE-79. SPE 81$
Wi’i
T’w R{
)EPTH
292s
9s
297S
Uvoi?
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Fig 8. The AIMJyStSnc. Instamancous drilling“evaluationlog - Buchan field.
Copyright SPE 1979