PetrGeosc 2000 Hesthammer Fossen Fault Sealing

8/16/2019 PetrGeosc 2000 Hesthammer Fossen Fault Sealing

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Uncertainties associated with fault sealinganalysis

Jonny Hesthammer 1 and Haakon Fossen21Statoil, N-5020 Bergen, Norway (e-mail: [email protected])

2 e!artment o" #eology, $ni%ersity o" Bergen, &llegt. '1, N-500 Bergen,

Norway (e-mail: haaon."ossen@geol.*i+.no)

ABSTRAT! Recent advances in understanding how faultsrestrict uid ow in sandstone reservoirs have led to improvedmodels for reservoir simulation. Thevertheless, there are stillmany uncertainty factors that can render even the mostdetailed simulation model useless. On a detailed scale, theseuncertainties include variations in lateral continuity of faults,

properties and thickness of fault zones, and the inuence of deformation bands within and outside damage zones.Subseismic features such as smallscale relay zones, dragfeatures and fre!uency and distribution of small faults aroundthe fault zone further decrease the con"dence level of simulation modelling results. #etailed analyses of seismic andwell data from the $ullfaks %ield, Thorthern Thorth Sea, havehelped understand the detailed structural reservoircharacteristics. The results from these analyses can, in manycases, be used as input to further enhance models for reservoirsimulation in order to increase the validation of the models.%urthermore, the studies carried out on the $ullfaks %ield

demonstrate that a sound approach to knowledge managementfor increased oil recovery based on fault seal analysis re!uiressharing of gained knowledge from many oil and gas "eldsrather than monopolizing information that cannot be fullyutilized by studies from a single "eld.

"Th#$%R&S! #*ll"as iel, "a*lt !lane, *ncertainty, sealingcharacteristic, reser%oir comm*nication

'(TR%&UT'%(

The last decade has seen rapid growth in our

understanding of how faults a&ect uid ow inoil and gas reservoirs '(llan )*+* -ouvier et al.)*+* -entley -arry )**) (ntonellini (ydin)**/ $ibson )**/, )**+ 0nott et al. )**12opez Smith)**1 3hilds et al. )**4 %ristad et al. )**4%ull5ames et al. )**40nipe )**4 2ia et al. )**4 6ielding et al. )**4,)*** 3rawford )**+ %o7ford et al. )**+ 0nai 0nipe )**+ 8anzocchi et al. )**+, )***Ottesen 9llevset et al. )**+ :alsh et al. )**+a,+ %ossen ;esthammer )**++ ;esthammer)***+


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%ield. This includes information on faultfre!uency and characteristics of drag zones anddamage zones. This integrated approach shouldlead to a fuller and better understanding of how and to what e7tent uid ow is restrictedby faults.

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Several works have discussed how fault sealpotential can be calculated when the lithologicreservoir properties and fault properties areknown '6ielding et al. )**4 0nipe )**4


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Fig, 1, 'a< :ell log correlationdiagram showing the locationof a fault associated with 1 mof missing section in wellA/D)@()C. '-< %racturefre!uency diagram from wellA/D)@()C. ( damage zonewith abundant deformationbands is associated with the 1m fault identi"ed from welllog correlation data. 'c< 3orephotograph from the intervalaround the 1 m large fault.The thin fault zone is clearlyidenti"ed, as are abundantdeformation bands near thefault zone.

lowest capillary entry pressure, the method canbe used to identify which points are likely to leak

uids and which points will represent barriers touid ow. $enerally, a S$R greater than )CB?@Fresults in a membrane seal ':atts )*+4iking $raben


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a*lt sealing A'A


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=n general, lateral discontinuities ')< will alwaysrepresent a serious uncertainty for simulation of uid ow in reservoirs. The uncertaintiesrelated to fault rock properties '?


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Fig, 2, Krincipal sketch of fault structure,consisting of a central fault zone of intensedeformation, and an enveloping zone of microfaults or deformation bands. #eformationbands may also occur outside the damage zone assingle or aggregate structures.

preserved during the core operations. One of these faults is a cemented breccia. (s such, it isnot possible from these data alone to obtainreliable estimates of fault zone thickness as afunction of displacement. %urthermore, thisscarceness of data does not allow for acomparison with onshore "eld data to evaluateif they are comparable. The $ullfaks %ield isone of the most heavily faulted reservoirs in theThorth Sea and clearly demonstrates thediHculty in understanding uncertaintiesrelated to fault zone properties. =n order toresolve this problem, it is necessary to

compile available data from many moreo&shore "elds that have similar lithologies anddeformation history.

&amage /one and the im0ortance of deformation -ands

Studies of faults in sandstones in Gtah and the$ullfaks %ield demonstrate that abundantdeformation bands e7ist in a narrow zone aroundthe fault '%igs ) and ?


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$ullfaks SMr demonstrate that these bands areassociated with abundant !uartz dissolutionwhich seriously lowers the porosity andpermeability within the bands and thus restrictsow across them. The reason for the di&erences

observed between the $ullfaks %ield and$ullfaks SMr is related to the di&erent depths atwhich the reservoir rocks are located. :hereasthe $ullfaks %ield reservoir rocks are generallylocated at )+@@B?C@@ m depth, reservoir rockson $ullfaks SMr are commonly situated belowA@@@ m depth. (t such depths, the temperaturee7ceeds )?@˚3, thus allowing for accelerated!uartz dissolution. Since deformation bands tendto favour localized uid ow along the bands,such zones will e7perience more dissolution of !uartz grains than the surrounding rocks.Obviously, the failure to incorporate deformationbands and their physical properties into faultsealing analysis studies would not reveal anydi&er ences between uid ow across faults onthe $ullfaks %ield and $ullfaks SMr, an error thathas already resulted in serious complicationsassociated with production from the Statf5ord%ormation on $ullfaks SMr.

The width of the deformation bands on the$ullfaks %ield is a function of deformation bandtype, grain size and mineralogy '%ig. A


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Fig, , 'a< :idth of deformation bands versus type.#eformation bands associated with cataclasis arewider than those that are only a&ected bydisaggregation structures. '-< ( plot of grain size

versus width of deformation bands show that thewidth increases with increasing grain size. 'c< Klotof the width of deformation bands with respect tothe main constituent. Nuartzrich clean sandstonesare generally associated with wider deformationbands than sandstones rich in phyllosilicates.

':ibberley et al. )***< may indicate thatdeformation bands in loose sand behave likeordinary faults '%ig. C


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A/D)@3C A))C A)C* P// P?4? QA/D)@3)/ A1C@ A14A ?A ?C +A/D)@3)/ AC/A AC1A P?@ P)CC QSum S S PA/) P?C@

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#, damage


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Fig, 4, #isplacementBlength diagram for faults anddeformation bands. The plot shows thatdeformation bands in consolidated sandstones fromGtah '%ossen ;esthammer )**4< are longer thanfaults with discrete slip surfaces, whereasdeformation bands in unconsolidated sandstonesfrom 8orocco ':ibberley et al. )***< display thesame slope as faults with discrete slip surfaces.The sources for other previously published faultdata are cited in Schlische et al. ')**1


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the faults identi"ed from seismic interpretationon the $ullfaks %ield will consist of more than asingle fault surface 'the true number is likely tobe somewhat higher since not all faults in afault zone are likely to be penetrated by a singlewell


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Fig, +, 'a< %aults at shallowerreservoir levels '-rent $roup<are more abundant than atdeeper stratigraphic levels.'-< The o&set associated withfaults increases with depth. Seemain te7t for discussion.

fre!uency between )@ cm and CB)@ m '%ossen ;esthammer in press


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Several advances in understanding uid owacross faults have led to large improvementsin models for reservoir simulations. There are,however, many important factors governingfault sealing potential that are notincorporated into the e7isting models. Theabove analyses demonstrate how the e7istingmodels for estimating fault seal potential andunderstanding uid ow in sandstone reservoirscan bene"t from detailed analyses of seismicand well data from oil and gas "elds.

The use of core data can help reduceuncertainties related to estimation of fault zonethickness and properties. Similarly, coreanalysis helps understand the e&ect of deformation bands on reservoir simulation,although the e&ect of lateral variations cannotbe revealed by analyses of core data alone.

#ipmeter data help identify the presence andcharacteristics of drag zones, whereas well logcorrelation can reveal if the fault structureconsists of one large or several smallerscalefaults. -y combining seismic data, well logcorrelation data, dipmeter data and core data,detailed information can be obtained about

structural reservoir characteristics. Thisinformation can then be used to enhance themodels for simulating uid ow in reservoirs. (lthough many of the results from the $ullfaks

%ield can be used for general evaluation of reservoir behaviour, e7treme caution must beused when applying the data to individual casestudies 'such as evaluation of fault seal potentialacross a single fault for input to decisions onwhether to drill a well or not


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Fig, 6, Studies of dipmeterdata from the $ullfaks %ieldshow that the di&erencebetween total o&set 'i.e. theo&set outside the zone a&ectedby drag< and missing sectioncan be drastically di&erent, asshown for a fault identi"ed inwell A/D)@3A. See main te7t

for detailed discussion.

obtained from wells. On the $ullfaks %ield,where data from more than ))C km of drilledreservoir from )+@ wells e7ist, such !ualitycontrol is possible, but the large amount of dataand high structural comple7ity re!uire muchfuture work to fully utilize all the availableinformation.

8any of the analyses carried out on the$ullfaks %ield are still associated with largeuncertainties. The results from the analyses maynot be applicable to other "elds. =n order tofurther enhance our understanding of reservoirproperties, it is necessary to compile data fromother "elds in a manner similar to that carriedout on the $ullfaks %ield. This demands an

Fig, 7, 'a< Orientation analysis shows that mainlynorthBsouth trending faults on the $ullfaks %ield areassociated with drag. '-< There is no clearrelationship between missing section associated witha fault and the interval a&ected by drag. This isprobably because drag mainly developed prior to thedevelopment of a distinct slip surface 'see main te7t


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approach to knowledge management whereoil companies are willing to share informationin order to optimize oil recovery. 3ompetition,as such, should not be associated with

Fig, 18, ( plot of formation versus number of faults associated with drag per kilometre shows

that faults at shallower stratigraphic levels '-rent$roup< are more often associated with drag thanfaults at deeper stratigraphic levels. This isprobably because the deeper strata was moreconsolidated at the time of deformation.


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monopolization of routine data 'seismic, core andstandard well log data< but rather how thatshared information is utilized.

The authors would like to thank Statoil, Thorsk

;ydro and Saga Ketroleum for permission topublish the article. The manuscript has bene"ttedfrom comments by $raham 6ielding and ananonymous reviewer.

RThFThRTh(T

hS

(22(Th, G. S. )*+*. 8odel for hydrocarbon migration andentrapment.

&merican &ssociation o" etrole*m #eologists B*lletin,5, +@AB+)).

(ThTOTh922=Th=, (. (6#=Th, (. )**/. 9&ect of faultingon uid ow in porous sandstoneU petrophysicalproperties. &merican &ssociation o" etrole*m#eologists B*lletin, 56, )+)B?@).

-9ThT296, 8. R. -(RR6, . . )**). Representation of fault sealing in a reservoir simulationU 3ormorant block => G0 Thorth Sea. nU roceeings o" the 44th &nn*alechnical 6on"erence 7 8hi+ition o" the Society o" etrole*m 8ngineers, #allas Te7as, ))*B)?1.

-OG>=9R, . #., 0((RSS=K9ST9=Th, 3. ;.,02G9STh9R, #. %.,

OTh6990:9, 3. 3. >(Th #9R K(2, R. 3. )*+*.Threedimensional seismic interpretation and faultsealing investigations, Thun River %ield, Thigeria. &merican &ssociation o" etrole*m #eologists B*lletin,5, )A*4B)/)/.

3(RT:R=$;T, . (., TRG#$=22, -. #. 8(ThS%=92#, 3.S. )**C.

%ault growth by segment linkageU an e7planation forscatter in ma7imum displacement and trace length datafrom the 3anyonlands $rabens of S9 Gtah. /o*rnal o" Str*ct*ral #eology , 15, )A)*B)A?1.

3;=2#S, 3., :(2S;, . . :(TT9RSOTh, . )**4.3omple7ity in fault zones and implications for fault sealprediction. nU 8MllerKedersen, K. 0oestler, (. $. 'eds< yrocar+on seals: m!ortance "or 8!loration an ro*ction. Special Kublication of the ThorwegianKetroleum Society, 5, 1)B4?.

3R(:%OR#, -. R. )**+. 97perimental fault sealingU shearband per meability dependency on cataclastic faultgouge characteristics. nU 3oward,8. K., ohnson, ;. #altaban, T. S. 'eds< Str*ct*ral#eology in 9eser%oir 6haracteri3ation. $eologicalSociety, 2ondon, Special Kublications, 125, ?4B//.

%OSS9Th, ;. ;9ST;(889R, . )**4. #eformationbands and their signi"cance in porous sandstonereservoirs. irst Brea, 1+, ?)B?C.

)**+a. Structural geology of the $ullfaks%ield. nU 3oward,

8. K., ohnson, ;. #altaban, T. S. 'eds< Str*ct*ral#eology in 9eser%oir 6haracteri3ation. $eologicalSociety, 2ondon, Special Kublications, 125, ?A)B?1).

)**++. $eometric analysis and scalingrelations of deformation bands in porous sandstone. /o*rnal o" Str*ct*ral #eology , 17, )/4*B)/*A.

in press. Kossible absence of small faults in the$ullfaks %ield, northern Thorth SeaU implications fordownscaling of faults in some porous sandstones. /o*rnal o" Str*ct*ral #eology , 21.

RVRTh9S, (. )**1. Kroperties of fault populationsin the $ullfaks %ield, northern Thorth Sea. /o*rnal o" Str*ct*ral #eology , 16, )4*B)*@.

%OW%OR#, 0. (., :(2S;, . ., :(TT9RSOTh, .,$(R#9Th, =. R.,

$GS3OTT, S. 3. -GR296, S. #. )**+. Structure andcontent of the 8oan %ault one, Gtah, GS( and its

implications for fault seal predictions. nU ones, $.,%isher, N. . 0nipe, R. . 'eds


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)***a. =mproving seismic data for detailedstructural interpretation. he eaing 8ge, 16, ??1B?/4.

)***+. a*lt geometry an internal "a*lt +loce"ormation in the #*ll"as region, northern NorthSea. Kh# thesis, Gniversity of -ergen, Thorway.

%OSS9Th, ;. )**4a. Seismic attribute analysis instructural inter pretation of the $ullfaks %ield,northern Thorth Sea. etrole*m #eoscience, , )AB?1.

)**4+. The inuence of seismic noise instructural inter pretation of seismic attribute maps. irst Brea, 14, ?@*B?)*.

)**+. The use of dipmeter data to constrainthe structural geology of the $ullfaks %ield, northernThorth Sea. ;arine an etrole*m #eology , 14, C/*BC4A.

;9Th#9Th, . =n press. 3losing the gap betweentheory and practice in seismic interpretation of smallscale faults. etrole*m #eoscience.

;G22, . )*++. Thicknessdisplacement relationships fordeformation zones.

/o*rnal o" Str*ct*ral #eology , 18, /A)B/AC.

OTh9S, $. 0Th=K9, R. . )**1. Seismic attribute mapsapplication to structural interpretations and fault sealanalysis in the Thorth Sea -asin. irst Brea, 13, //*B

/1).0Th(=, T. (. 0Th=K9, R. . )**+. The impacts of faults

on uid ow in the ;eidrun %ield. nU ones, $., %isher,N. . 0nipe, R. . 'eds< a*lting, a*lt Sealing an l*i low in yrocar+on 9eser%oirs. $eologicalSociety, 2ondon, Special Kublications, 135, ?1*B?+?.

0Th=K9, R. . )**4. u7taposition and seal diagrams tohelp analyze fault seals in hydrocarbon reservoirs. &merican &ssociation o" etrole*m #eologists B*lletin,61, )+4B)*C.

0ThOTT, S. #., -9(3;, (., -RO30-(Th#, K. .,-RO:Th, . 2.,

833(22G8, . 9. :92#OTh, (. =. )**1. Spatial andmechanical controls on normal fault populations. /o*rnal o" Str*ct*ral #eology , 16, AC*BA4?.

2=(, O., O8R9, ;., T92892(Th#, ;., ;O2#9Th, 2. 9$92(Th#, T.

)**4. Gncertainties in reservoir production forecasts. &merican &ssociation o" etrole*m #eologists B*lletin,61, 44CB+@).

2OK9, #. 2. S8=T;, 2. )**1. %luid ow in faultzonesU inuence of hydraulic anisotropy andheterogeneity on the uid ow and heat transferregime. 9R2(Th#, . (. -6-9R$, $. )***.3haracterization of fault zones for reservoir modelingU (n e7ample from the $ullfaks %ield, northern ThorthSea. &merican &ssociation o" #eologists B*lletin, 6,*?CB*C).


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Received A@ (pril )*** revised typescript accepted ?4 September )***.

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PetrGeosc 2000 Hesthammer Fossen Fault Sealing