.-

SOCIETY OF PETROLEUM ENGINEERS OF AIME PAPER6200 North Central Expressway NUMBER SPE 4S4Dallas, Texas 75206

THIS IS A PREPRINT - SUBJECT TO CORRECTION

PREDICTION OF OVE WRESSURES IN THE NORTH SEA

by

C. van Baaren, Member AIME, .Shell U.K. Exploration and Production Ltd

@ Copyright 1974American Institute of Mining,Mettdlurktictd, and Petroleum Engineem; Inc.

Thib paper was prcpnrbd for the SPE-Europe:m Siming Meeting 1974 of the Society of PetroleumEngincc?rs of AIME, held in Ansterdam, the Netherlands, May 29-30, 1974. Permission to COPYisre&ictc.f to an abhract of not more than 300 words. IHustrations mav not be couied. The abstractshould cont~ co~~spicuous acknowledgement of where and by whom the-paper is p~esented. Publicationelsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OFPETIMXJWM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriatejournal provided agreement to give proper credit is made.. Discussion of this paper is invfted. Three copies of any discussion should be sent to theNethcrlsnd Section of the Society of Petroleum Engineers, P. O. BOX 228, The Hague, the Netherlands.Such discussion mav be mesented at the almve meeting and, with the paper, may be considered forpublication in one ;f the two SPE rn~gazines.

SUMMARY

Overpressures are encounteredon the NorthSea in rocks of Tertimy to Permian aga. Thedistribution of overpressures varies from basinto basin and examples are disoussed.

The mud log&ng techniques such a; d-exponent, temperature records and wire linelogging techniquessuch as the sonic transittime - depth plot and the slxile resistivity -&epth plot may indioate overprescures inspeoificcases.

lIoweverthe adjustmentof mud weights andcasing settingdepths in explorationandappraisalwells in the North Sea, based on theoverpressurepredic%ioxtechniques,is notjustified gecerallyat presentdue to theunreliability of the mothods.

INTRODUCTIONThe term overpressureis used for

formationfluid pressureexceedingthe nornwl~droetatio fluid pressure. The formationpressuregradientsvary from 0.433 psi/ft forfresh and brackish water to apcmox. 0.465 psi/P

for ealt waterwith a salinityof 80000 ppmNaCl eq.

Appendix1 shews the nomenclatureused.Overburdenpressuregradientscalculated

from bulkdensitylogs vary from O.M psi/ftatshallowdepthsto 0.95 - 1.0 psi/ft at greaterdepths. Thesevalues are in agreementwiththe formationstrengthdata obtainedfrom testscarriedout below a cementedcasing of whi~hresultsare presentedin Appendix2. .

Wowledge of the distributionof %eti-pressureswill make drilling safer and willattributeto more economicalcasing schemes.

OVERPRESSUREPREDICTIONMETIK)DSA sumary of the overpressuredetection

methodsis shown in Appendix3. One group ofmethodsI_asto be applied during the actualdrillingpkse, while the methods in theevaluationphasecan be applied after the holeis drilled.

All these methodsmust be applied and some

may indicatean increasein formationfluidpreeeure.

A popularmetkad is the !d~ exponent,which iB a functionof the penetrationrateendis definedas follows:d= 10g (R/60N) / 10g (12/w106D) eq. 1,

where:R= penetration rate inft/hrN= rotary speed in r.p.m.H= weight on bit in 1000 lbsD = bit diameter in inohesIt canbe correctedfor ihe mud weight in useby the empiricalrelationship;d+i . d.N/M , eq. 2,

where:

M = mud weight in useN= ~noml ~a weighttd*= modifiedd-exponent

0VWRPRESSUR2DISTRIBUTIONIN THE NORTHSEAOverpressuresare encounteredin rocks

from TertiarytoPaleozoic age in sandstoneand carbonates. The distributionof thepressures varies from basin to basin and inthis paper examplesfrom the SouthernNorthSea=-sin, the Central Graben,Firthof Forth Basinand the Viking t3rabenwill be discussed. Amap of thu areas of interestis presentedasAppendix4, while Appendix5 shows measuredformationpressuresversus depth.

.SOUTHZRNN3RTH SEA BASIN

Overpressureshave been locallyencounteredin porous sectionsof the Plattemand ~upt dolomitein the Leman field, whichare sandwichedbetween thick salt packets.The actual formationpressuresare not knownbut formationpressuregradientsover O.$1psi~ft were encountered. The overlfingpost-Zochsteinrocks and underlyingRozliegendessandstonesare normallypressured.

The overpressuresare found,lccallyonly,and @e cutting of the mud isso far the onlyusable techhiqueto know t~at overpressuresare present.

CENTRALGRABEN- FT.RTHOFFORTH BASIN

The Central Graben and Firth of ForthBasin have overpressuresin a Tertia~ clayston/eiltstoneseqtienceand in Mesozoic andPaleozaiccarbonatesand sandstones,withvirtuallyno shale presentin some instances.

In the Tertiaryclaystone/siltstonesequenceoverpressuresof * 0.520 psi\ftoccuraround~ 45OO ft wtdle in one case even O,&Xlpsi/ftwas encourrtere~,while the deeper Eoceneand Paleobenesands are normallypressured.Overpreneurepredictiont.eckiquesare notconclusivein this ease as a nnormal~!treixlis

J..- - . ..- -. . . . . .- .. . . - -..

not availablein the present explorationphase.Appendix6 and 7 show the Pressur~tlog and thfsoni~ transit time - depth plot ruspectivily.The.sonic transittime - depth plot is probablystronglyaffeotedby bore hole diameterand alscpossiblyty variationsin sonic transittime inclaystonesand shales in the zone surroundingthe bore hole.

In the older rocks in the Firthof ForthBasin overpressures UP to 0.520 psi/ftcccur insendstonesend carbonatesbelow a thick chalkylimestoneand marl section. None of thepresentoverpressurepredictiontechniquescanbe appliedin this case.

In the Mesozoicmarl/claystone/shalesequencein the CentralGraben overpressuresupto 0.9 psi~ft have been encountered.

EXPERIENCEIN WELL 22/21-2Bcthd and d+ were plottedon linearand

semilogpaper respectively followingprocedurerecommendedby various proponentsof the method,The plot of d* versus depth is shown in Appendiz8.

.

As a startingpoint it was assumedthat t~depositionalstructuresof the North PermianBasinuere similarto those of the Gulf Coast,at least in the !l?erkiarysequences,and a norskLmud gradientof 0.468 psi/ftwas u8ed in determiningd+. The +heory .s developedfornormal rock bits, but for most of the plottedsectiona J44 insertbit was in use. However,it was felt that absolutevalues of d* are ofno interest,and the trends slwuld still beapparerrc. From the d*-depthplot, it can be .seen th%t no waning of the impendingkick wasgiven, even thmghthe fc~mationwae over-pressuredbyi 3000 psi. It may be said thatthe slight drift of d* to the right over thepreceding three hundredfeet indicatedthepresenceof cap rock, but that is inadmissibleas warming of overpressures. To give moretnsightinto use of d exponentin the North Se&approximated* exponentswere calculatedfrombit recordsfor three mere northerlywells bythe same procedure. These indicateda normaltrend to the right of the Gulf Coast trend(lineBon the plot),and allowedtheinterpretationthat overpressuredrocks werepenetrated. Even so, the overpressurespredictedwere only of the order of 100 psi.

The sonic transittime - depth presentedas Appendix9 indicatesdeviationsfrom thenormal.pressureat even shallowerdepths,althoughthe rockswere apparentlynormally~;ressuredbased on drillinginformation.,

The temperature- depth plot presentedas.Appendix10 shows a reductionin temperatureaLove the overpressuredsandstonewhich may bean indicationof overpressure. Howevermoreresearchis requiredto uee the outcomingmud

I5 4041 u. VAA1 .urM-A.LaJJAv 4

emperaturevariation for reliableprediction sble. In fact the olaystcmevariedIfoverpreasurea. ooneiderablyfrom marl to non caloareous

siltston~, which emphasises the difficulty ofIKIWJ GRABEN selection of such points in practice and caste

overpreseuree in the Viking Graben,north doubt on the validityof any conclusion.lastof the Shetlands.~are fo~d in-the Jurassiu m~- s~wysRrata with formationfluid pressuregradientslpto 0.685 psi/ft. In a few oases the eve- This tcol is believedto bee. potentiallypressuresare also found in the layers over- valuabletool.. I@wevermore researohin geo-lyingthe Juraseic. thermalgradientsand in temperaturemeasure-

Atypioal preesurelog is presentedasments techniquesat seabottomis required.

!ppendix11. The mud weightwas increasedinpresentlythe mud temperatureis measuredon thedrillingrig up to 600 ft above the eeabed.

mtioipation of the overpressured zone. The Considerableamountsof best are transferredtopenetrationrate decreasedas a coriequence the cool North Seawater, which reducesthethereof. A tnor~l~ trend of the ~dt exponent~edifficultto establishdue to the variations

outcomingmud temperaturesi~ificantly.TherefoFethe facilityof measuringmud

in the lithologywhich consistsof silty cley- temperatureat the seabedwill contributetoStoneswith more or less calcareouematter.l?hedeviationof the dt exponentfrom the

improvedpredict~onof overpressures.

!Uow.l! line is believedto be due to SONICTRANSITTIME - DEPTH PIOTSrariat.ionein the lithology. The temperature Of a number of welle the sonic transit. depth graphis not indicatingthe toP of the time versus depth have been plottedin i.ppendix~verpressures,while the sonic - transittimeiepth~lot presentedas Appendix12 indicates

13. From theresulting cloud of points it ie

the top of the overpressuresat the top of ttidifficult to establish a %ormal* trend line.Most of the sonic loge in the top section of the

~arbonaoeoue @hle. hole.have been taken in 17&hole which isCONCLUKCONS washed out considerably in meet cases, reducing

the reliability of the readings.t~f ~~~~

There are severalpossiblereasonsw~thXn the deeper parts of the lmle it is ofter

difficultto select points of identicaltd? e~nent did not give @ clear warningbefore enteringhigh pressureintervals:

Iitlmlogy,which further reducesthe valj.dityofsonic transit time oroseplot as a tool for the

1. The most likelyis that the deeper predictionof overpressures.formationsare no longerpart of a simpledepositionalregime. ..It is questionable

This methd is an after the event-method

whether the d++exponen~will give advanceand its severaldrawbaoksreducesits validity

warningof depopressuresor migratedas a tool for the detectim of overpressures.

pressure. Afewmethodsof overpressureprediction? Becauseof the unpredictablenature of the

havebeen discussed. Generallyspeakingthere-e is littleevidencethat at the moment the

utratigraphyin the North Sea and thepossiblenecessityto move off locationin

methmiscan be used effectivelyin adjustingbad weatherwhile still maintaining

mudweightand casing depxh in the North Sea.

pressurecontrol,.wells &ve been drilled Aa~te pressuredata are availablefromwith mud weights ensuringlarge preesure productionend wireline tests to preparea safe!overbalances. In practicethe over~le,noe economicaldrilliilgprogrammewith optimummudas derivedfrom FIT data is often of the weightsend an optimumcasing scheme.order of 1500 psi at depths of 10000f%.The effectof this is to desensitizetheresponseof penetrationrate (and henced)to changesil~formationpressure. This isolearlyseen in work publishedby U.C.Maurer (JPl?December1965). It is alsoquestionablewhetherthe empiricalcorrectionof the d~ exponentfor mudweight can be appliedon the North Sea~m~dified.

3. The stipulationto choosepoints ofoonstantdrillabilityreoognisesthestrong dependenceof penetrationrate onlithology. Usuallyclaystonewas taken_ the rock of f!constantdrillabilitymbeoapseoF the infrequentOccurrenceof

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.

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SEPAI1 PETROPHYSICS SECTION Appendix 9

SONIC TRANSIT TIME OF SHALES VS. DEPTH

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