Oil & Gas Science and Technology Rev. IFP, Vol. 54 (1999),
No. 6, pp. 667-678Copyright 1999, ditions TechnipOverpressures:
Causal Mechanisms, Conventionaland Hydromechanical ApproachesD.
Grauls11 Elf Exploration Production, valuation du ptrole et
modlisation du bassin, Avenue Larribau, 64000 Pau - Francee-mail:
[email protected] Surpressions
:origine,approchesconventionnelleethydromcanique
Onrencontresouventdesrgimesanormauxdepressiondanslesbassinssdimentaires.Lesrelationsentrelacontrainteverticaleeffectiveetlaporositonttappliques,depuis1970,danslargiondelaGulfCoast,
afin d'valuer ces surpressions. Des rsultats ont t obtenus en
faisant appel la sismique et lamodlisation de bassin dans les
bassins tertiaires de sable argileux contrainte verticale dominante
et
endsquilibredecompaction.Cependant,lessurpressionsd'originesdiffrenteset/ouadditionnelles(contrainte
tectonique, gnration d'hydrocarbures, contrainte thermique,
transfert li aux failles, fractura-tion hydraulique) ne peuvent pas
tre quantifies en utilisant cette approche.En plus des mthodes
conventionnelles, une approche hydromcanique est propose. Pour
toute profon-deur, la limite suprieure est contrle par les
conditions de fracturation hydraulique ou par la
ractiva-tiondefailles.Lafracturationhydrauliquesupposeunsystmeouvertparpriode,enrgimedecontrainteeffectivemineureprochedezro.Uneconnaissanceapprofondiedesrgimesdecontraintestectoniques
actuels permet une estimation directe de l'volution de la
contrainte minimale. Une valuationquantitative de la pression avec
la profondeur est donc possible, puisque dans les systmes
gologiquescompartiments et/ou non drains, les rgimes de pression,
quelles que soient leurs origines, ont tendance atteindre
rapidement une valeur proche de la contrainte principale mineure.
Ainsi, l'valuation de la sur-pression sera amliore, puisque cette
mthodologie peut tre applique divers environnements golo-giques o
les surpressions ont d'autres origines, les mcanismes tant souvent
combins.Cependant, les tendances de l'volution de pression dans les
zones de transition sont plus difficiles va-luer de faon correcte.
Une recherche complmentaire sur les couvertures et les fermetures
sur faille
estdoncncessairepouramliorerleurprvision.Enplusdel'valuationdelasurpression,leconceptdecontrainte
principale mineure permet de mieux apprhender le systme ptrolier.
En effet, les transfertsd'hydrocarbures lis aux failles, les
domaines de fracturation hydraulique et l'tanchit du
recouvrementdpendent d'une interaction subtile, dans le temps,
entre la surpression et les rgimes de contrainte princi-pale
mineure.Mots-cls : surpressions, contrainte mineure, fracturation
hydraulique, transfert par faille, proprits mcaniques.Abstract
Overpressures:CausalMechanisms,ConventionalandHydromechanicalApproaches
Abnormalfluidpressureregimesarecommonlyencounteredatdepthinmostsedimentarybasins.Relationshipsbetweeneffectiveverticalstressandporosityhavebeenapplied,since1970totheGulfCoast
area, to assess the magnitude of overpressures. Positive results
have been obtained from
seismicandbasin-modelingtechniquesinsand-shale,vertical-stress-dominatedtertiarybasins,whenevercompactiondisequilibriumconditionsapply.However,overpressuresresultingfromotherand/oradditionalcauses(tectonicstress,hydrocarbongeneration,thermalstress,fault-relatedtransfer,hydrofracturing...)
cannot be quantitatively assessed using this approach. Oil &
Gas Science and Technology Rev. IFP, Vol. 54 (1999), No.
6INTRODUCTIONThestudyofoverpressuresinpetroleumexplorationstartedin1970inoffshoreGulfCoastareas.Someconventionalmethods,essentiallybasedonempiricalporosityversuseffectiveverticalstressrelationships,havebeenappliedworldwidewithsomesuccess,insand-shaleTertiarybasinswhenevercompactiondisequilibriumwasthemaincausalmechanism.
The failure of some pressure predictions, and
therecentevolutionofpetroleumexplorationtowardsdeepertargetsintectonicallycomplexbasins(NorthSeaforinstance),
and in frontier areas (deep offshore areas), led us
toreviewoverpressureassessmentandtoproposesomecomplementaryapproaches,tobeusedinadditiontoconventionalapproaches.Arapidreviewofthecausesofoverpressuring
was first carried out in order to rank the
maincausalmechanismsandtoevaluatetheirrelativecontributions. That
was used as background for proposing,
inasecondstage,anotherquantitativeapproachtoassessabnormalpressureregimesatdepth.Thishydromechanicalapproach,calledminimumstressapproach,wasbasedontheknowledgeoftheinsitu
minimumprincipalstress(S3)versusdepthprofile.Theapplicabilityofthisapproachwasillustratedusingcasestudieswherethepresent-dayoverpressureregimesoriginatedfromdifferentcausalmechanisms,veryoftencoupledtogether,andwherethecontribution
of each phenomenon cannot always be correctlyidentified and
assessed.668Ahydromechanicalapproachisthenproposedinadditiontoconventionalmethods.Atanydepth,theupperboundfluidpressureiscontrolledbyinsitu
conditionsrelatedtohydrofracturingorfaultreac-tivation.
Fluid-driven fracturing implies an episodically open system, under
a close to zero minimumeffective stress regime. Sound knowledge of
present-day tectonic stress regimes allows a direct estimationof
minimum stress evolution. A quantitative fluid pressure assessment
at depth is therefore possible, as
inundrainedor/andcompartmentedgeologicalsystems,pressureregimes,whatevertheirorigin,tendtorapidly
reach a value close to the minimum principal stress. Therefore,
overpressure assessment will
beimproved,asthismethodologycanbeappliedtovariousgeologicalsettingsandsituationswherepresent-day
overpressures originated from other causal mechanisms, very often
combined. However, pressure trends in transition zones are more
difficult to assess correctly. Additional research oncap rocks and
fault seals is therefore required to improve their predictability.
In addition to
overpressureassessment,theminimumprincipalstressconceptallowsabetterunderstandingofpetroleumsystem,asfault-relatedhydrocarbondynamictransfers,hydrofractureddomainsandcap-rocksealingefficiencyde-pend
on the subtle interaction, through time, between overpressure and
minimum principal stress regimes.Keywords: overpressure, minimum
stress, hydrofracturing, fault transfer, mechanical
properties.(Chaney, 1950)(Hedberg, 1974)(Meissner, 1976)(Du
Rouchet, 1981)(Barker, 1972)(Powers, 1967)(Burst, 1969)(Master,
1979)(Foster, 1980)v = fluid volume differential por. = porosity =
total stress 1= maximum principal stressv= vertical stress 2=
intermediate principal stress3= minimum principal
stresssOVERPRESSURESCAUSES OF(Jones, 1968)(Gussow, 1938)(Hubbert
and Rubbey,1959)CompressionOil crackinggasgeneration(Dickinson,
1953)(Terzaghi and Peck,
1968)KerogencrackingVincreaseOilgenerationThermalstressesMechanicalstressesOsmosisAquathermalexpansionClaydiagenesisFluid-rockinteractionsPor.-decreaseFault
openandhydofracturing HydraulicfractureregimeReservoirLateralup
dipflowchargingover-vincreaseDarcyflowOthersDynamical
transfersChemicalstressesincreasepor.decreaseThrust-faultsvLateralstrainStrike-slip-faultCompactionv
= 1v =2v=3disequilibriumNormal-faultvincrease-+(Finch, 1969)(Berry,
1973)ArtesianBuoyancyeffectFigure 1Main overpressure causal
mechanisms (Grauls, 1997).D Grauls / Overpressures: Causal
Mechanisms, Conventional and Hydromechanical Approaches1 CAUSES,
CONTRIBUTIONS, AND CONVENTIONALASSESSMENT OF OVERPRESSURESThe
general overview of the different causal mechanisms
andtheirrelativecontributionstothepresentdayoverpressureregimeissupportedbydifferentkeypublications(Fertl,1976;
Magara, 1978), the recent research onto
overpressures(OsborneandSwarbrick,1997)andthein-houseresultsonhydrofracturingandinsitu
minimumstressevaluation.AsshownonFigure
1,andbyorderofimportance:mechanicalstresses,thermaleffect,dynamictransfers,andchemicalstressesarebelievedtobethemainfactorsingeneratingabnormal
pressures in present-day hydrocarbon systems.1.1 Mechanical
StressIn recent sand-shale dominated Tertiary basins, the weight
ofoverburdenorverticalstress(Sv)contributestothedevel-opment of
overpressures in poorly drained, low
permeabilityshale-proneintervals.SinceDickinson(1953),thecompac-tion
disequilibrium phenomena has been considered by
manyauthorsasthemaincauseofoverpressuringinsedimentarybasins(Fertl,1976;Magara,1978).ThemagnitudeofthispressureregimewasquantitativelyassessedfromsoilmechanicsprinciplesdevelopedbyTerzaghi(1968).Empiricalapproachesorporosityversusverticaleffectivestressrelationships(MannandMackensie,1990)havebeendeveloped(Fig.
2).Otherempiricalrelationshipsarestillusedforpredictingabnormalpressuresbeforedrilling,fromlow
frequency, seismic-derived interval velocities (Grauls etal.,
1995).Thesuccessratioobtainedfromseismicpredictions,wascloseto65%consideringarelativeuncertaintyonoverpressureestimate(OVP)lessthanorequalto10%(deltaOVP/OVP
P
S3S3OverchargingTransitionzoneSanddominateddomainShaledominateddomainHP
open-fractured domainAt well locationSv= vertical stress S3=
minimum stress PN= normal hydrostaticpressure trend P = pressure
evolution Figure
8ExamplefromSouthChinaSea:abnormalpressureregimesrelatedtostrike-slipfaulttectonics,deephydrofractureddomainandtothecharging
of the above undrained reservoir level, are controlled at depth
byin situ minimum stress. Close to zero minimum effective stress(S3
P = 0) conditions allow transient fault and fracture related
vertical transfers and charging of the reservoir.Oil & Gas
Science and Technology Rev. IFP, Vol. 54 (1999), No.
6intervalwasnotclearlydefined.Abetterunderstandingofcap rock
sealing properties is required.3.3.2 Central North
SeaAsshownonthelefthandsideofFigure9,atypicalcross-section in
central North Sea area allows three main intervalsto be
successively identified, from top to bottom:
AshalymostdominatedTertiarysection,wheremaxi-mumthicknessreaches3000mandcompactiondisequi-libriumapplies.Maximumpressuregradients,closeto1.55
g/cm3EMW are encountered at 2000-2500 m depth.The Palaeocene sands
act as regional drain and cause thepressure regime to kick back to
subhydrostatic regime.
AnormallycompactedCretaceoussectiondownto4500-5000 m, composed of
shales and chalk, where the pressureregime is usually hydrostatic.
APre-CretaceoussectioncomposedbyJurassicandTriassic overpressured
reservoirs overlain by Late Jurassicand lower Cretaceous shaly
cap-rocks.Thepresent-daypressureregimeintheJurassicplayisessentially
due to the high compartimenting of reservoir dueto faulting and to
rapid burial during Neogene times. In sucha closed system, the
pressure increased to a maximum
valuecorrespondingtotheleakagepressureinthehigheststructural-pointpressurecell.Suchavalue,closetothein
situ minimum stress, cannot be accounted for by
conven-tionalapproaches.Thegeometryofthecompartmentedstructureconditionsthetypeofleakage.Tiltedblockgeometryfavorspreferentialleakagealongthefault.Inthatcase,
excess fluid pressure can contribute to the
propagationofthefaultunderhydrofractureconditions.Thisopenordilatantfaultactedasapreferentialpathwayforepisodichydrocarbon
transfer up to the Palaeocene sands. In
addition,ashighlightedonFigure9,thisopen-faultzone,whenintersectedbyanexploratorywell,canleadtoasuddenpressureincreasetoavalueclosetoS3,withinaveryshortrange
of a few meters. That can lead to unexpected
problemssuchasunresolvedkick,intermediateadditionalcasingsetting,losttime,andextracost.Oppositely,theanticlineshapestructurefavorsaleakagethroughthecap-rock.Extradosfracturesarereactivatedandpropagatedinmode
I, at the apex of structure, forming a pressure
transitionzonethatcanbedetectedfromseismicvelocitiesiftheintervalthicknessexceeds100-150
msTWT.Insummary,theS3approachcontributestoabetterunderstandingofdynamictransfers,andaswelltoabetterassessmentofpresent-dayabnormalpressureregimesdevelopedintectonicallymulti-phasebasins,andresultingfromdifferentorigins:compactiondisequilibrium,compartimenting,fault-related
transfers and buoyancy effect.4 LIMITATIONS As previously
emphasized, the S3approach can be used as
acomplementtomoreconventionalmethods.However,thismethodologypresentssomelimitationsduetothefactthatpetroleumsystemsareneverfullyopenorclosed.Thatintermediatepressureregimesstronglydependonthedynamicsofverticalandlateralchargingandcompartmentsize.
Therefore, assessing overpressures in transition
domains676TertiaryundercompactedshalesPressure and stress Sv
PWellPalaeocenesandCretaceousLPOilwaterJurassiccompartmentedpressure
cell "Open"faultTopreservoir. . . . .. . . . . . . . . . . .. . . .
. . . .. . . . . . . . . . . . .
.S3ShalycaprockTransitionzoneStressDepthP S3P S3Figure 9Example
from North Sea: abnormal pressures through undercompacted Tertiary
section, open fault zone and at top of reservoir compartmentare
controlled by the minimum stress regime. Leakage (LP) and fault
transient behaviour are dependent on in situ minimum effective
stress(S3 P), and account well for vertical hydrocarbon migration
from structural highs to Palaeocene sands.D Grauls / Overpressures:
Causal Mechanisms, Conventional and Hydromechanical
Approachesismorecomplex.Thetransitionzonesaredevelopedbothvertically
and laterally, and their importance in overpressuredsystems has
already been demonstrated by Swarbrick
(1996).Theverticaltransitionzone,aspreviouslyemphasized,ismorefrequentlyobservedattopofanticlinestructures,andcorresponds
to an hydrofractured interval indicating a lack
ofsealingintegrityoftheoverlyingcap-rock.Thelateraltransitionzoneismoreoftenrelatedtocompartmentsize,drainage
area and to the relative efficiency of faults limitingthe different
compartments. These transition zones are clearlydisplayed on Figure
10.Figure 10Sketch showing lateral transition domain linked to
semi-openfaulted compartments and vertical transition domains
inducedby hydraulic fracturing leakage through
cap-rock.Inbothcases,pressureprofileswithintransitionzonescannotbeprovidedwithhighaccuracy,andadditionalresearch
on cap-rock and fault seal is still needed to improvetheir
predictability.
CONCLUSIONSThegeneralreviewofmaincausalmechanismsgeneratingoverpressuresledtotheconclusionthatmechanicalandthermalstressesalongwithdynamictransfersarethemaincausesforgeneratingoverpressuresinpetroleumsystems.Someconventionalapproachesbasedonporosityversuseffective
vertical stress relationships have been applied
withsomesuccesssince1970insand-shaleTertiarybasinstoassesscompaction-disequilibrium-relatedoverpressuring,usingseismicvelocityandbasinmodelingapproaches.However,abnormalpressurescannotbecorrectlyassessedinmultiphasesedimentarybasins(NorthSea),and/orwhenever
additional causal mechanisms are involved
(lateralstrain,thermaleffect,ordynamicprocesses).Inaddition,thepresent-dayfluidpressureprofileobservedisoftentheresultofseveralcausesactingtogether,
eveninsand-shaledominatedbasinswherecompactiondisequilibriumisassumedtobetheonlycontribution.Intheseconditions,assessingthemagnitudeofabnormalpressureregimeisalmost
impossible, as the present-day pressure value
cannotbeprovidedbyasimplesummationofdifferentcontributions.Ahydromechanicapproachbasedontheparameters
controlling the magnitude of abnormal
pressureswasproposed.Thisminimumprincipalstressapproachconsidersthatoverpressuresdevelopedingeologicallyundrainedsystemsatdeptharecontrolledbyrockfailureconditions
whatever the mode of rupture (mode I, I/II, or
II).Suchconditionsdependonfluidpressuremagnitude,rockmechanical
properties and deviatoric stress, and occur
underverylowminimumeffectivestressregimes(S3 Pf#
0),orunderpressureregimesclosetotheminimumprincipalstressS3.TheS3profile,dependentontectonicregime,canbequantitativelyassessedusingapowerlawfunctionofdepth.Atagivendepth,theS3valueincreasesasthetectonicregimeevolvesfromnormal,tostrike-slipandthrustfault.Therefore,soundknowledgeofpresent-daygeologicalcontextsandtectonicregimesshouldallowtheminimumprincipalstressprofiletobeevaluatedand,consequently,
the pressure regime to be well-assessed.
ThisS3-basedapproachiswellsuitedtooverpressuringinducedbylateralstresses,hydrocarbongeneration,reservoircompartmentingandovercharging,fault-relateddynamictransfers,hydrofracturing,andespeciallywhenmultiplecausalmechanismsareinvolvedinthesamearea.Suchanapproach
must be considered as a powerful method that
canbeappliedinadditiontoconventionalones,anddoesnotexcludethemanyway.Somelimitations,however,mustbementioned.Theyareduetothefactthathydrocarbonsystemsarenotfullyclosedoropenatdepth,andthattransitionzonesorintermediatepressureregimesexistwithvalues
in between hydrostatic and minimum principal stress.Better
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