Geological Aspects of the Drilling of the Buchan Field

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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

..,.



;

*.

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-



.“

~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



,.

,,

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-

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OF THE BUCHAN FIELD INSTALLATION :

. .

Fig. 3. Schematic diagrsm of the Buchsn field hIstWation.

: Copyright SPE 1979

.



SUCHAN FIELD

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LITHOLOGICAL

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Copyright SPE 1979

Geological Aspects of the Drilling of the Buchan Field

Documents