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Data Aqtiltion,Design and b-plementafion:Mature Reservoim

Sodety of Petroleum=gineers

Opportunities and Challenges for Effective Programs in

P.K. Pan@~d ,MB. Clark, Fins Oil and Chemical COrnpamY;T.A. Blasingme and L. Doublet, Twas A&M Univemity.

.-

CoPYri9ht 1994 S-acletf of P.v.1..m En9?.wrs, l.c.

Thk war W- prepared for pr%w!io. S me SPWDOE Ninth Symposium on Impmved bit Rec.avmy held in Tulsa, Okl@homa, U.S.A., %7-20 Aptil 1994,

Thh paper was sdec!ed for pmseneation by an WE Program Commi!lee follwmng rwiw of I. forma!to. contained 1. an ab$tr.ti submitw w the au!ho~s]. ti!ents of the paper,as presented, have nog been rwiewd by the society of Pe!ro!eum Engineers aod are s.binc! 10 Cnrwtron by the author(s). Th@ ma!erial.s Ln8se.1.d. does .’$ .ec-aritY refletiany pmilion of ,he Society of petroleum Engtn&rs, [M oflicws, or members. Papers premnted a4 SPE mwtinqs am subpl w Wblication rwiw by EdNmlal Committals at the SecWy01Petroleum E“@leem, Permisdon to COPy is msl,kled ,. a“ abstract of “al mom ,han X0 words. lIIuWMIO”S may not be copied, ‘ill, abstracl $h.auld .wntiln C.3”SPIC”OUSackrmwledgrmlnt0! where and by whom !ha @sPw LsP,esent@d, W,!(. tib,arkan, SPE, P.O. B,. &X3836, Sbhardsnn, TX 750 B3@36. U.S.A., T@lex 7S3265 SPEW

SuMMAuL .: ... . .=.....-. ~~~~.=

Data acqtiltion ales@ and hnplementation challenges formature reservoirs Which are *ets for Improved OilRecovery (IOR) applications are discussed in this paper.Exampks are provided for Sbaflow Sheff Carbonate (SSC)reservoirs in the Permian M,n of W* T&~.

WfcztAre Wturclteaervoira? Mature reservoim am deilmd

= PrOPefi= titb ufditiond recovery potential byimpknentation of advanced reserroir cko@rimtiOn toolsand tecfmiqws, reserroir management and/or changes inrecovery mecbankm. Attributes of mature reservoirs aredepicted in Pigure 1, which shows the hportaace ofreservoir characterization as a function of tiefd developmentstage. Reservoir cbaraclerhtion and an understanding ofhetemgeoeity becomemoreimporkntformaturiag~moimas these factors have a profound impact on future reservoirdevelopment and management strategies. Mature “hssrvoirsare typically characterized by some type of secundary drivemwhanism. A clnuy$e to a tertiary mode or implementationof other IOR methods may be necessmy to extend theeconomic fiiit and productive life of the tidd.

In recently discovered reservoirs, advances in reservoirmanagement technologies and practices allow operators the

OPPofi~ty ~ ~pkamt timely, judicious and integrateddata acquisition programs (Ref. 1-3). Typicafly, dataacquisition focuses on use of a team approach (Ref. 4, S) asillustrated in Fiiure 2.

A team appmacb is afso important to achieve data acqtiltioztobjectives in mature maervoirs. However, the dataacquisition situation may be very different fmm that of“new” reservoirs. The desire and need for IOR may becritical as the economic finit may be rapidfy appmacbingand data required for IOR may not be available Smafler=ervoir size and lower remaining rssrvea may presenteconomic constraints towards the acqai+tion of e%sentialdutafor the implementation of many IOR methock The lack ofproduction, fluid properties and other data in the earlierstages of field devefopmmt may preseut un~e.r inhistory matching with numerical aimufation methods. Thismsrdts in unreliable reservoir pccfonrrance forecasts for IOR.Often, the impknmntation of data acquidtion programs inmatore reservoirs present opportunities to enbauce near-kmresel-roir performance through effective reservoirmanagement.

Data acqtiltion strategi~ for pmpertics which are beingconsidered for abandonment are not addreased io this papw.Redevelopment of these prapertks is often required to exploitbebind pipe potential and undeveloped zones or horizons.

XNTRODUCTI12N - DATA ACQUISITION~ Y

Tire data acqtiltion process for mature reservoirs ran besegmmted into two @or areaw

1. Res&roir Characte&ation W These data consistof geological, petropbyairai, geophysical and PVT

2 DATA ACQUISITION DESIGN AND IMPLEMENTATION 0PPORTUNITD3S AND SPE 27760CHALLENGES FOR EFFECTIVE PROGRAMS IN MATURE RESERVOIRS

data and am utifized for development of anintegrated reservoir description (IRD).

2. l@es’vOirPerf nrmanceDakReaervoir surveillan~production, wefl teat, pressure, special mre,gsmtatktical and other engineering data. ALSOutilized with rfaervoir characterization data fordevelopment of the IRD.

The IRD may””then be used to assess IOR developmentpotential with the aid of toofs like reservoir sindatots.

Reservoir Cb “ tion ad Het&mgemeitx Figure 1illustrates the importance of ~ervoir heterogeneity as afunction of primary, secondary and tertiary fielddevefopsnent andgeologicaf complexity. The cmt of iqjectantincreases as progression to tertiary recovery methods isimplemented (Figure 1). Generally, defksing reservoirheterogeneity andcompartmentafization is more diftlcrdt withlow porosity and low pmm?abifity carbonate formations tharsfor clastic systems, which may have an order of magnitudeimproved porosity nnd permeability pararnetem.

As a field progresses towards IOR isssplenrentation,reservoirheterogeneity and compartmentalization is generdy the leastunderstood aspect of reservoir characterization. For thisreason, data acqubition programs for maturing reservoimshould focus on detlring reservoir heterogeneity as this factorwilt have a profound impact on the success of IOR projects.Wider application of geophysical data acquisition (2-D tid 3-D surface seismk, cmss-borehole tomography and interweflreflection profifing) have tremendous promise to provideadditionoJ definition for the lateral, interwdl character ofresemoirs.

-Oir Performance and Ea@eer@ Datx Rewrvoirperfomnance data amdysk is criticaf for continuedperformance optimization. fmportant reservoir performancedota acquisition areas include pressure trsmsisnt dataanalysis, Iong-tems production data anafysis, surveillancedata anslysis and data for optimization of completion andproduction procedures.

A relatiomhip similar to reservoir heterogeneity is illustratedin F]gure 3 to emphasize the importance of PvT/PhaseBehavior and SpsciaJ Core data as a function of fielddevelopment stage and gsdogical complexity. The area ofPVT is particularly important because crude oil propertiesare a key ingredient for the application of IOR methodsregardless of gedogiral compleI~ity (e.g. MkMity Pressures

for Hydrocarbon Miscible Processes). Special core data areused to undemtand reck/fluid interaction pararnetem and am

ncsded to forecast future reservuir pcrfonnancc and tomatch past performance

Geostafi.dicaf Metho& Even under the best ofcircumstances, comprehensive data acquisition for reservoimtypicalfy covem onfy a very smsafi fraclion of the reservoir.Geostatistics can be used to quantify and understand theuncertainties inhermt with reservoir characterization (Ref.6). However, comprehensive data acquisition pmgmms as-estill very importantto minimii rmcertaintiea in reservoiranafysea. Obtaining data which provide information over awide range of reservoir scafes, frmss micrnscnpic to cor.+o-Iog scafe and then to interwell scafq are @td in deiltingboundary conditions. This allows for obtaining cfosure in therange of possible outcomes when geostatktical reservoircharacterization methods are used for consideringdevdopment uncertainties.

_ s~$~ Taber and Martin (IW. 7) haveempfm.sud the msportance of making the distinction betweenoil properties and reservoir characteristics. They suggestthat oil viscmity is a good method to classify IOR methodswhich work best for fight oifs, heavy oils and even tar sands.011 viscosity is used as a screening criteria since it af&ts oifremvwy more than oil gravity. For application of numericaltools fike bbxk oil, compositional or thermaf simulation,speciafizsd IOR laboratory fluid data are required.

Important reservoir characteristics for consideration of IORmethods (Ref. 7) incfude current oil saturation, formationtyp% resemoir compartmentalization, heterogeneity andformation pmpertks, such as: gross and nit thickness;porosity and pemneabifity; and depth and temperature

De.veIofxamt Potentiak Assessment of development potentiaffor implementation of IOR methods typically involves the useof reservoir simulators. The quality of reservoir description,tluid and rocldfluid interaction data are key to reducinguncertainty in IOR devefopmesst forecasts with sinusfation.Andinrate of originaf-oil-in-place (OOfP) andremaining-oil-in-piace (ROfP) is rssefuf for screening IOR potentiaf. Use ofreservoir performance analysis methods (Lg. deche curveanalysis, Fetkovitcb anafysis) may be utilized forunderstanding reservoir behavior and optimizing near-tamreservoir performance.

W* ~ Progmms: A sestion of this paper isdevoted to quafity assurance progrnms for data acquiskion.Frequently, in the course of reservoir studies, questions ariseas to the vafidlty of previously acquired data. Design andimplementation of rigorous quafity assurance programs areextrmrely useful in minimking the uncertainties related to

.

, SPE 27760 P.K. PANDE, M.B. CL- T.A. BLASfNGAME, L. DOUBLET 3

data quatity. These programs me af] the more important formatnrc reservoirs since lmdgclsry, tinreand other constraintsmay prevent operators from “reacquiring” certain data tovalidate or invaKdate data quality concerns.

RESER VOIR CHARACTERIZATION/lNT13GRATIjDRI?$$ERVOIRDESCRIPTION. -. . . . . . . . .

Rcaervoir characterization provides inspoctant informationfor assessing reservoir devdopment potential. hrfontmtionobtained from reservoir characterization studies pertains tounderstanding the flow of fluirk in the reservoir. Examplesinclude reservoir heterommeitv and comsrartasentafization.

and/or discontinuous shale and anhydrite beds, assd loss ormhancensent of pm-asity due to diagesrcsis. Rock typingfrom whol,e care and electric log models allows foridentification of flow units and porosity and permeabilityrelationships. This information also helps for selection ofoptimal layecissg and grid cotilgarationa for simulation.Unfortunately, in many ea.% whole care and rock data maynot he available.

In this s@ction the geological, petrophysical, geophysical,phase behavior (PVT) and special core (SCAL) dataacquisition requicenmmta for reservoir characterization anddevelopment of ass integrated reservoir description arediscussed. Aa a focus SSC reservoirs of the Penniars Basinare considered. These res-oira are typically veryheterogeneous, both I.aterally and vertkdy, and are thushighly compartmentalized. Low values of porosity andpermeability typicaDy rcault in low recovery eftlciencies.

In general, reservoir heterogeneities are more complex incarbonates than in clastic reservoirs (l@ure 1). However,notable exceptions am cfastic turbkiite reservoirs which, duein part to camplex reservoir architecture and morphology,can be extremely diftlcuk to dfacribe. Frequently, onlyerosional remnants of the original tnrbidite deposit arepreserwed in the geologic reeord.

Need Far Core The development of a good geologic modelis greatly facilitated hy having rack samples. Preferably themck samples am from whole core Sometim@ only cuttingsor rotary sidewall core are available. ‘l%is data mmt beintegrated with electric log data (gamma-ray, spectral logs,reaistivity, density-neutron or other porosity logs, photo-electric and other log derived data). Unique relationshipsbetween these data will define reservoir rock types andreservoir flow units which can then be extended to aninterwd scale.

Amount of Cor-rz The amount of core required depends

upon the cample.xity of the reservoir. In general, more rockdata will allow for a more detailed reservoir description.The geologist and engineer must work together to determinewhat is needed ta satisfy the simulation rcqnirements withinthe budgetary constraints of the project.

-.$e.~h~. SVWqI~ s~timrdv Pravid= themethodology for interpreting the relationships betweendepositional cnvirmnnents and Iithofacies (Ref. 8-16). Thesequence stcatigraphy approach to interpreting the reservoirfacilitate an integration of gccdogical and engineering data.Reservoir heterogeneities and interwell comectibility can beddineated in the context of flow units based on data framwhole tack anafyaes and petrophysics, including rock-logmodclling.

The seqa?ncs stratigraphy methodology ia applicable tacdhnatcs and cfastics. The facies arcbitccture of the flowunits compri.vhg hetemgcneaus reservoirs is a function ofrapid and very frequent cbangea in sea level over relativelyshort periods of gealogic tisne. These rises and falls incelative sea Ievd produced mrdticycfic shoalhrg upwarddeposits with compkx but predictable stacking patterns andlateral transitions in fithofaciea. The relationships betweenporosity and permahility reflect changes in depositionalenvironment which are rdated to rapid changes in sea levelon a shelf with localized and very low relief topography.Primary depositional porosity types and intercomectivitypatterns have subsequently been overprinted and aftered bydolomitization and/or other rfiagenetic pmcesiea.

Hi@ Eequcnq Euvtacy and De@&tiomf Cycfici@Although the shelf areas where SSC reaervoim weredeposited were essentially topographically featureless,localked highs with a relief of only a foot or less allowed forthe development of shallow water shoals. These shoafingareas may have mnged across several depositionalenvironments extending from the open marine outec shelfinto tidaf flat areas. Wave and carrmt action concentratedand organized peloidal, ooiitic, carbonats sand and bioclasticdebris into gminstone and packstone Iithofaciea whichcomprise the principal mscrvoirs of the Clear Fork Group.A change in ma level of only a few feet may have furtheraccekatsd the formation of these shallow water cycfkdeposits and extended the Iaterd and vertical distributions ofthese high energy Iithofacies.

R-oir Litho&rr+s Tsacis - SbcIfaf Topography andDisgalssis During transgmssiom and sea level rise,accumulations of subtidaf grainstoncs and packstones whichdeveloped on Iodized topographic highs may have beenreworked by wave and tidal current action and subsequentlyredeposited and concentrated in adjacent paleotopographic

.. .

4 DATA ACQUISITION DESIGN AND IMPLEMENTATION: OPPORTUNHTES AND SPE 27760CHALLENGES FOR EFFECTIVE PROGRAMS IN MATURE RESERVOIRS

lows The grainstone and packstone tcxtums, which formedon the highs nsrdwithin the lows, both have the potmtial forgood porosity development and intercormectivity when grainto grain contncts were preaerred. In the absence of a graindominated environment, however, the depositional lows mayhave bean mors mud domiuated reading in a reservoir rocktype with poorer overall reservoir quality.

Tklal flut and supratidnl gminstones and packstones mayhave high porosity from fenestcal developments msd thepreasuce of vrsgs due to the dissolution of minerals andvarious sdlochenss such as bioclastic debris. However, due tothe abuudasrce of mud in rocks dominated hy vugubrporosity, intercossnwtivity is often low and reservoir quaEtypoor.

During periods of sea level fall, subaerial exposure andintermittent inundation could have contributed to theformation of a grain dominated mck as well as theacceleration of diagenetic processes. D@2sresis on highs mayfrave enhanced porosity md pwsneability through refhwdolomitization, whereas diagenesis in sheffnl lows nmy bnvebeen limited due to less overaJ1subaerial exposure.

The overall consequence of three sedimentary and diageneticprocesses is to produce an isochronous reservoir fitbofaciestract comprised of several d[ffercsrt mck type% TfdsIithofariea tract may have porosity and permeabilityenhanccmmnt in grain supported rocks which aftersrate andinterdigitate with mud supported or dominated rock typt%with relatively poor reservoir quafitia. The overafl resuft isreservoir hetemgem?ity and compartnmrtalkation.

Puroaity aud Permahiity - Petraphys&al/Rorf-Lag Modd:Thecomplmity of porosity aud permeability rdatiousfdps canbest be comprehended through the direct examination of mrksamples obtained from conventional whole core adorsidewafl core aassspl~. There is no direct relatiombipbefween porosity and permeability. It does not follow thatreservoir recks with the highest vrdues of porosity have thehighest connectivity.

The interrelatiosssbips between porosity and permeabilitymust first be established through whole core arrafyses. Tfsisdata may he used to develop a petrophysird rock-log modefto define reservoir and non-reservoir rock types andreservoir flow units. Confirmation of the geologic model andfacies architecture involves extension of the mcldlog modelinto walls where okdy log data is availabk. Geological cross-ssctions and various mapping exercises can then be used todefine the spatial and vertiral distributions of Iithofacktitlin their respective depositional environments.

Borehole and ❑ear-borehole information obtained from mrk~P1es, @ dab, and data fmm wefI production and kt&ta cass then be integrated with the geologic reservoircharacterization model through gsostatistirof auslyses andrwervoir simulation.

Wfsule Rork haf~ - “Data Relevant to ResexvuirPerfO— Ass understanding of primary porosity typesand connectivity is criticsd for estabfisbing recovery potentialcud psrforssmnce of reservoir flow unik. P&rophysical arrdpetrologic studies, as wefl as imaging tefhniqssea, will help tofnrthcr delhe the history of diagentic prowwea influencingCosmectitim withh the reservoir.

Whole or sidewofl core.data a!one may be adequate to defineporosity and permeability relatiousbips at the wdlbore.However, it is not practical to obtniu this information fromall wells in the reservoir and this data may not reflectastrophysical properties at the interwdl scale. “Point” dataat the wdlr is also not enough to develop a three dimensionalgenlogic dcwription of flow asritsand the relationship of flowuuita to flow harriers and non-rcwrvoir rorks. Legreaponws must be rafibrated with the whole core todsternsine if a rock type has a sperific log signature Thslog cbasarter cars then be integrated with a rork/Iog modeland followed into the reservoir through recognition in tbenon-cored welfs. Development of a rock-log model may becomplicated iu rcservoim where modem log suites arc notavaiIable.

Rock samples shoafd be systematically obtnined from thereservoir to defineate lateral asrd verticat heterogeneities.The amount of core required and where it should be takenfrom the reservoir will be different for each reservoir and theintended application. Porosity and permeabilitydevdopment is directly related to depositional proces.scswhich caunot be determined fmm log data alone.

Pore to throat sim ratios (the aspect rntio) and the numberof pore throats comecting with a specific pore (coordinationnumber) are meamm of flow potesrtkd and reroveryeftlciesrcy. These dsta can be obtained from thin-section andpore network studies based, for -PI% on liquid resin~P%sa3don of pore systems from rock samples. Threedimensional analyses yieId data that are essential toaccurately depict pore geometries and their interconnectivity.Pore throat data from mercury injection capillary pres.su~”measurements aIong with data fmm SEM image analyses andthin section studks provide information that is rritiral tounderstanding the relationships between pore geometries andpore throats.

.

“~ SPE 27760 ‘ “P.K. P~E, M.B. CLARK, T.A. BLASINGAME, L. DOUBLET

Geophysical Appficatiom: It is feasible and may he costeffective to monitor the movement of fluid interfa~ withseismic field tests. As a waterflood sweeps a reservoir, porefluids are changed from connate water saturations ta watersaturations reflecting residual post sweep conditions. Thechange in water saturations will he evidenced by changes inacoustic impedance which can be monitored by a base surveyand subsequent post flood surveys on time seJected intervals.Carbon dioxide (COJ floods may also be monitored in asimhr reamer.

The monitoring operations can be done with surfacereflection seidc, three dnensionol (3-D) surveys, interwellretlsction profiling and cross-harehole tomography. Inactivefields, surveys can be conducted to sample noise so that itcan later be detected and removed in the data procea.vingOperations. Seiiic fines can dso he strategically positionedaccordkg to engineering and geologic information about thereservoir so as to best monitor the anticipated waterfloodresponses.

Seismic applications in sequence stratigraphy and reservoirmodeiling are well documented in the literature (Ref. 13,17,18). Isochronorsa aurfam or time lima can be tied ta wefl.vand correlated throughout the reservoir. In th~ manner,reck units that have similar electric log signatures andappear to be correlative, but are not actually incommunication, can be identified. 3-D surveys can be usedta help define the internaI and extemat morphology ofdepositional units and their distribution within the reservoir.Infill drilling activities can then be strategically targetedwithin tbe reservoir and the drilling of poor weJ1locationsminimhd.

Recent sub-salt discoveri~ in the Gulf of Mexim reflect thesuccess of state-of-the art sskmic modelling techniques. Atecfuique known as forward modelling of seismic datainvolves using a geologic model to derive synthetic seismicsections based on various velocity and density dataestimations (based on electric log data when available) forassumed mck types, thicknea$ distribution and stratigraphic-strssctud associations. Methods of forward modelling canbe employed (ray tracing, wave equation and other models)depending on the complication of the geologic amlog to besimulated.

Seismic attributes, such as amplitude variation with offset(AVO), aIlow for tbe recognition of gas in clastic andcarbonnts reservoirs. Successful case studies of AVOanalyses are well documented throughout the Gulf of Mexicaregion. Other processing teclmiques involve the conversionof seismic time defkred data to depth defined dati 15yinversion. The wellbore depth information can be tied to the

5

sei.dc and the morphologic surfaces of the geelogic modelwhich may then be spatially defined by depth.

PVT/Pfsaae B&avior Datw The assessment of many IORmethods requires the use of black oil, compositional, mixingparmneter, thermal and other types of simulators. PVT dataare UVmdlynot considered a history match pamsneter sincein most simulation studies, fluid properties can berepresented with single or muftiple PVT region(s).

Tbe orea of greatest uncertainty in the sinndation historymatch process is usually in the geological reservoirdescription. This concern is umafIy the area of most intensefocus during the history match process.

Since PVT data usually are not a history match parameter,it is afl the more important to be certain that the PVTproperti= used for sinmfation are in fact representative ofreservoir conditiom during the entire duration of the historymatch.

Csmmt msd Origimd PVT Da@ PVT data acquisition isoften overlooked since it is sssnally represented with one ortwo PVT regions. Many times only “original” PVT data areavailable. To properly aasrss IOR potential, input requiredfor sixnukitom includes a description of reservoir fluids underthe “existing” or “current” reservoir stat~ If a fiefd hasbeess produced for a long period of time under severaldepktion mechanisms, resampling of wefls with surfacerecombined or bottomhole samples may be required.

IY original PVT data are available and a long period ofra-oduction historv existv. it is necrsarv to “histarv match”~ewly acquired” fiuid da~ with “origin~” fluid da~ with theuse of a PVT simsdator (Ref. 19).

IOR Application Data Reqnkaemtx Many IOR

~PPfi~tiom fOcW on the use of immiscible or misciblem.pxhon gases (e.g. COZ for oil reservoirs, Nz for gascondensate reservoirs) to recover additional hydrocarbons.In these cases, it is critical to understand the fluidinteractions which occur betwems the hydrocarbon andinjection fluid. Equation of state modek (EO.%)am typirdydevdoped to describe the fluid behavior and interaction insinndation studies.

Merrill and Newley (Ref. 20) concluded that EOS requiredfor IOR applications “cannot be reliab[y devefoped from juststanckyd black oil data and a series of slii tube tests”..—.These investigators recommend conducting additionsexperiments which focus on defining injectant-hydrocarbonphase behavior. These tests cost more than what is obtainedfrom routine “black oil” PVT analysis but the cost is reduced

6 DATA ACQUISITION DESIGN AND IMPLEMENTATION OPPORTUNITIES AND SPE 27760 ~CHALLENGES FOR EFFECTIVE PROGFW$LS IN MATURE Reservoirs

significmrtfy when planned and” condurfed as part of arrintegrated PVT sampfii arrdanalysis pmgmrrr.

Oif Flmgrzp_ The use of oil fingerprinting tecfnriquerhave been demonstrated as an effective data acqoiskion andCeservoir Srrcveillance tool (RX 21-24). 011 fingel-printiag isa chromatographic t@mique which can distinguish Mw=different oils. In the case of muftiple PVT regions, oils orzones, the tecfmique can also be used as a method ofproduction allocation for reservoir surveillance.

This tecfnrique plays mr important mle in Pv dataacquisition since it caa be used to estabIiih that maftiple PVfregions exist. Once eatabfiihcd, PVT sampling and t@ingprogmma caa be targeted for each of the zones or PVTregions.

Often, the iafocmation from oil fingerprinting in identifyingdifferent oiIs or PVT regions providea tmmendons insightinto geological and reservoir management iamea. If oifs aresignificantly different, the optimal strategies for developmentand reservoir management may be different for each zone orPVT region. Usually there am underlying geological reasons(i.e. cOmparhner@r.atiOn) for thepreaence. of more thao onePVT region. Tbe fingerprinting data can aid inrmderskmding the fundamental geological tits in a ccservoirwith multiple PVT regions.

011 fingerprinting is also a vafuable rcacrvoir srrrveihrwetool as it can be nsed for production allocation in reservoirswith commingled wells. The technique has advarrtageaoverconventional production logging tools which cannot be usedwhen fluid rates are low. Normal weII operating conditiondo not have to be dkturbed when obtaining productionallOcation data from Oil fingerprinting. This heIps inobtaiaing more representative allocation data. With.production logging, alteration of normal well operatingconditions may affwt the rrqrresentativeneasof the allocationdata obtained.

RoUt& and Special Care Dsda - Rmk/Fhrid Intcractiom:Rixrtine and Special Core Data (SCAL) are essential. forobtaining rock/fluid interaction parameters needed forprdcting future fluid production ratea, as well as in thehistory match of past production or performance. Skopec(Ref. 2.S) bas emphasized that “coring and weflsite corehandfing should follow the beat possible praclicea because tbevalue of core anafysis is fimiied by tbk initiaf operation”.Development of an “integrated” routine and special coreprogram which encompassed considerations fmm the tiefdcore acquisition, preservation, and laboratory anafyses to enduse are important. An example of such an integrated routine

and speckrf cnre program is ihstmted in Figure 4 arrddescribed bdOW.

Cm@ Program, Drif@ Cortvideratioms: One of the mostimpo~t pammeter-s in a coring program is the drillingmud. Some of the concerns which must he addressed indeaigaing a mud pmgrmrr for coring a wefl inckrdewettabtity pammetem, core recovery, lost circrdatiorr,presanre control, maintaining bole integrity, mirrinriingflrrshing, rmd flnid saturation. In Weat Texas cachomteformations, “bkmd” water baaed drilling mud or fluid madefrom celativefy inert materials is usnafly a good choice.Rheologimf and filtration properties shoafd be controlled bythe addition of materiafa or polymers which do not after thenettability nf the whole core. The pH of the coring fluidshordd be neutraf. Good coriog tlnkfs produce qnahty coresand the procedures outlhed above am consistent with otherguidelines recommended in the industcy (Ref. 26).

F14d Cace Handfing Pcocedacea For WettabiityPmswmtion: WettabiIity preservation is irnportmrt as itimpacts the reardts from speciaf core rmafysis laboratorymeavrawments (I@. 27). Thece am mrmy approaches whichcan be used for wettabifity preaermtion. The key is todevelop the most cost-effective stmtegy for particularappficatiorrs. For Weat Texas carbonates, one approachwhich has been found to be effective is to place whole core(typicafly 4 inch diameter) ia a hydrocarbon environment(feaae cmde) immediately upon retcieval in the ticld. Tfdsapproach of utilizing Iease crnde befps to preserve theoriginal wettahility of the reservoir reck.

Sp&af Core Aaafysk Sample Points After placing the corein a hydrocarbon environment, sample points for pluggingthe whole core for speciaf armlysia are sefectcd based ongeologic data, reservoir characterization data and well logcross-sections

Wcttafrility Prcaerration of Care plugs For Sr2AIA Placingthe cut plugs for SCAL in lease crnde has beerrfound to bean effective technique for wettnfility preservation of acquiredcore and core phgs cut for subsequent use in special coreanalysis (Ref. 28-31).

Sumning of PhrgaFor Special Cam Arrafysix The first stepin implementing a SCAL pmgmnr is to make certain that thetesting is performed on the “right” or appropriate rocksarIIPks. TOOoften, expensive labor-ato~ teats are performedon rocks which may not have flow potentiaf. Performingexpensive special core laboratory tests on rocks which do notrepresent the reservoir zones or facies of intere4 not onlywaste vrduable laboratory efforts arrd monetary reaourcrs,

, SPE 27760 P.K. PANDE, M.B. CL- T.A. BLASINGAME, L. DOUBLET 7

but present data which may be analyzed beyond its realsignificance. Thus, it is critical to ascertain that the correctrock types am considered for SCAL. In addition, theappmpfite porosity rmd permeability ranges within aparticular mck type tnust be Utib.ed.

An allowdion of special core tests based on relativeimportance of the rock type or facies, to the reservoirhydrocarbon production ensures that the most importantmck types am included mecitorioudy in the SCAL laboratoryprogcmn.

Imaging Teclumlo@a: Following the step of integratinggeologicaJ facies description to obtain representative samplesfor SCAL, imaging technologies and additionnf screening,surh us effective permeability to oil (k..), can be applied tofurther screm rock samples for SCAL. The use of &easurea no change in oil saturation or wattabiIity for the plugpreserved in lease crude.

The introduction of high-resolution scanning x-ray computedtomography (CT) and nnclear magnetic reaomssce imaging-) Or magnetic rcsonmrce imaging @lRI) to reader 3-dimemioual vissrafhtioms of porous materiafs givs us theability to quantify and quafify features important for fluidflow in tbe cmw Imsgiog technologies allOw us to visualize3-rfimensional distributions of porosity and fluid saturations.

The cmt of imagiog is typicafly a fraction (or severalm%~tud= 1sss) than that of specialized SCAL teata likesteady state and unsteady state displacements, and electricalpropeck data. Besides providing information on selectingthe appropriate sample5 for SW analysis, imagingtechnologies also aid irr the interpretation of SCALIaboratocy teats.

Speciaf Cm-t?Teatiag Conditiom: Often, laboratory data areobtained at non-reservoir conditions. The criticality ofadjusting data at non-reservoir condition to rfservoirconditions must be evaluated. If correlations are used toconvert data to reservoir conditions, their amiability must beaddressed.

Consideration must also be given to the need for conductingthe SCAL tests at reservoir temperature, reservoir pressureand use of synthetic vs. actual reservoir fluids which maybeIive oil or dead oil. The testing conditions of interest are afunction of resecvoir type (m-iddepth) and other conditionsfike the pressure range of interest. It is important to ensurethat representative data are obtained using the most cost-effective testing conditions of temperature, pressure andfluids.

Comidentimr To FiddDevdopmmt Phase Aswith thecaseof PVT Mu, ii may be more cost effective to obtain SCALdata which represent data needs for all phases of fielddevelopment (primary, secondary, tertiary) as one cohesivepmgiam. The alternative is to nbtain SCAL data for eachand every step of reservoir development in differentpmgmms and this may require more time and budgetaryresources than a cahrsive progmm for afl developmentphasea. With une single SCAL pmgmm, duphcation ofSCAL implementation activities ducing the Iatw stagea offiefd development are avoided. In addition, the incrementalcosts to obtain data for a subsequent stage of fieIddevelopment are lower when performed together. One_ple Of this for West Texas carbonates is to conducttectiary displacements of core directfy aftec secondaryrfiaplacements, rather than going back later for the IOR (e.g.COJ displacmcumt at some later point in time.

pETRoPHYsfc4LAND LOGGINGDATA . .

Log Data Acqnkition and Andy& Open hole log dataceprssent property distribrrtiom in the rrservoir that maybecompomd with available core data to focnudate ageological/petrophysicaf modef that can be used throughoutthe fife of the resecvoir. An economically justified cased holemonitoring program can he used to measure cbangea insaturation pmfdes (lT)T), differential depletion (’PDT, andtracer surreys), andproductiotinjection flowregimea (tmcersurreys). In - of highly corrosive fluids, tubularcocrosion surveys can be recorded to chd caaing/tubingintegrity.

when economically feasible, there are several importantfactors to consider in acquisition, stomge, and retrieval ofwell log data. These factors include type and chronologicalage of log “data, log data qrdty control, data acqukitionformat and log mmfysis software packages (l&#. 32-35).

Type and Chmnolngical Age of Log Datz In the matureremcvoirs of the Petmian Basin of West Tmas, there oremany instances in which log data acquisition was neglrctedor kept to a minimum. While it is tcue that complete logsuites are not required to ideatify the obvious pay mm=, inorder to accurately chamcterk the reservoir Iithology andto identify rock and flow units, it is essential to have fairlygood covemge of tbe reservoir. This degrm of detail isdesirable due to the heterogeneous and compartmentaliidnature of many low permeability carbonate reservoirs, andthe fact that core data will usually be limited at best. Ininstancea where only limited, or older open bole log data isavailable, there will usually beat least porosity and resktivitydata along with a gamma ray for correlation. O1dsonic andneutron count cate logs can be easily converted to porosity,

8 DATA ACQUISITION DESIGN AND IMPLEMENTATION OPPORTU?WTIES AND SPE 27760 ,CHALLENGES FOR EFFECTIVE PROGRAMS IN MATURE RESERVOIRS

although they shordd osdy be used qualitatively and forcorrelation betwesn more modern log suites (when available).In cases where them is no open hole log data for a largeportion of the reservoir, cased hole log data fromcompensated neutron, cased hole soniq and thermal decaytools (TDT) rms be used to estimate porosity asrd watersaturation when economically feasible.

L4glhta Qnafity C4mtrok Log data qnafitycn.trol must bepracticed “rsal-tirne” iu the field. AU major logging servicecompanies have their owm quality control programs;however, problems do sometimes occur. Open hole log dataqrsahty must be emmced - leaving location sisrrs this isthe only opportrnrity for data acquisition in an open holeenvironment. A log quality cfmckfiit may help to sliiinatsmany problems, but there is no substitute for constantvigilance.

Data quafity checks are afso important when receiving datafrom log database libraries. An initial scan of the data maysave valuable time later. Much of the data from older logswas usually obtained via hamd-digitizing methods and someof the original curve wsponse charactsristica may have bmlost due to assursstmdy hand. The advent of log data storageon computar hard drives, tap=, and dukettes has eliminatedthis probksrr. fn addition, new digitizing software allowawell logs to be scanned by the computes which helps toefisninate the hormus srror.

Data AcspsMtion Format and I.mg Anafysis Sufhvasw Thedata formats for log data acquisition and storage are widelyvaried. The logging ssrvice company shonld hs ab!e toreproduce the data in a format that is compatible with thesoftware parkage to be used for log analysis. There aremarry robust software packages available for gwdogiral,geophysical, petmphysicd, and reservoir enginseriug usage.The package that beat fita the needs and goak of thecamp~y shoufd km sekcted. Just about all log analysisprograms wiI1 accept LIS, LAS, and ASCII formats,however, there are also a multitude of service company andanalysis software-specific formats that may not becompatible. To–”save “time, a common format for dataacquisition should be choseir prior to the logging job.

RESER VOIR PERFORMANCE .

Irrorder to accurately estimats the performance character ofan oil rewrvoir, we use a team approach. ThK involvesengineering and geoscience personnel to interpret as well asintegrate the geological, petrophysical, and reservoirperformance data. Reservoir performance data (Well tests,production and injsction rates and preamms) can be easily

quantified in an engineering sense. However, theinterpretation of these data, particularly with respect to thegmlogic modsl and the hetemgensitiea premnt in thereservoir, requires camfrd asrd consistent review by ~engineering arrdgmsrience psrsomel.

Such integration allows us ta pmrfict the bshavior of iufillwells asrd otfsec future mbanred oil recovery efforts,eapeciafly when the reservoir heterogeneities that amidentified cars he suhsaqueniJy avoided or exploited. Oneparticsdarly vafusbl~ but typically underused resource, is oiland watw production data. virtually all operators haverecorded monthly ratea on a wdl-by-wdl or a lease-by-leasebaais. These data not ordy provide the mea.rwto estimate in-placs re5erves for a givssr proress (primary, sscon~, ortectiary pmcessrs) but thsse data cars also be used to verifyreservoir properties and to correlate and predict futureperformance us well as iacmrrrentd reaerw due to aparticular process.

The ability ta estimate the expected performance of aressrvoir and contrast this with actual performance data iscritical to mcovsry opttilzation. Inadequate or poorstimulation asrdcompletion techniques may significantly altsrthe producing capacity of a reservoir. Tbe timelyidentification of such cases is imperative to optimizingperformaurs. ISI this section, tools am described to helpcharacterize reservoir heterogeneity, differential depletion,and to establish prefsmntial flow paths in the reasrvoir.Although some reservoir performance data may not beeconomically feasible to obtain, it is rwmnrneuded that as aminimum, it is necessary to acquire frequent and accuraterate and pressure data for reliable performanw analyses.Reservoir performance rms be evaluated through dataacquisition and analyses in the areas of pressure transientdata anaIysia, long-tans production and injection dataanalysis, surveillance data ana!ysis, and completion andproduction pmmdures.

Prexsum Tmrraimt Data Assalysi.x This asralysis inchsdespressure buildup and falloff tests and tbe analysis ofinjectivity tests such as step-rate tests. These analyses mayalso include data fmm both single-well and multiwell tests(ifpecmeabllity and continuity permit) where these data allowestimation of reservoir continuity, which is vital for tbeexploitation of any reservoir, and low permeability reservoirsin particular.

As described in Ref. 36, it is recognized tfrat well testingprocedures in mature reservoirs may often be performedaccurately rising surfacderived pressure data. Thisremedies tbe problem of lost injection and production during

. SPE 2776fI P.It. PANDE, M.B. CLARK T.A. BLASINGAME, L. DOUBLET 9

the pre and post-testing periods. Rsw?rvoir pmpcrtks can beestimated asrdtbe reservoir modef maybe more fufly dflned(i.e. fractured weffs, daaf-porosity systems, boundedreservoir systems, ete).

Criticaf to any pressure transient testing sequessce is thecareful design and implementation of the sequcssce. This isperticufarly true for mature re+ervoim with patterndewsfouments. h“ suds casea interference feahsms can be

assd stirmdation featares sass be inadvertently ignored.

1.4ng-tcrsn Productiadfssjrdio. Data Anafysk ASmcrstioned above the rigorous amdysis of production andiq”rction data has been lorgefy ignored. Tfsie has bsen dueto a concern about data accuracy, but even more so due tothe lack of anafysis and interpretation reaourws within theengineering connmnsity. Refs. 37-40 provide considerableinfonssation towards interpreting production data to the samedepth of accuracy and detail that is rxpected from pressuretransient data.

,,TraditiomIll dscfi”e c-e analysis Which*1= On~pirid

curve fittissg and extrapolation has long bsen the primarymechanism of aaafysis for production data. These anafyscsdo provide good correlation with movable reservoir volomesfor a particular drive process (primary, secondary, ortertiary). However, the empirical methods do not provideinsight into reservoir properties or sustainable productionrstss, although many empirical forecasts have been shown tobe quite accurate ovez short periods.

The most important innovation in production data analysishas been the devefopmeut of type curve methods (Ref. 37,38) where rigorous analytical solutions and correlatedaPirical solutions for enalysk and interpretation are used.In addition, Refs. 39 and 40 provide material balancerelationships thnt can be used to accaunt for variablepressures and cates during production. These type curveanalyses aflow for the identification OR

1. Moveable/recoverable fluids for a particulardevelopment stage.

2. Reservoir pmpertiex psrmeabifity - thkknessproduct, estimate of skin factor (provided transientflow data exists), reswvoir pore volume anddrainage area:

3. Analysis of vsriable-ratehariable pressure dropdata, provided the proper material balance relationsare used for the oil and gas production cases.

4. Rigorous and semi-rigorous productionextrapolations.

With specific regard to water injection and production dsta,assy of the sssafysis and interpretation schemes used for theanafysis of oif production data carsafso be accurately appfiedto water injection and production data. Type curve smofysishas berss attempted and verified for cases of water coning(RX. 41). The most common use of water production datais the coccefation of water-oil ratio v- cnnsufative oilproduction to estimate the cunudative oil recovery at someprescribed fiiiting value of the water-oil ratio.

Reservoir performance contour maps or “bubble maps” ofwater iqjxtion, water production, and oil production can beused to cosrefate movcmesst of water sssd pattern swsepefticiesscy. These maps are nsnslly considered quefitativeinformation. However, monitoring of refative quantiti= offluids can be very nsefsd in determini ng pattern eflicicncy aswell as the need for remedial action.

In most mature reservoirs, production ond injection datamay be the onfy data that are avsifable for performanceanalysis. As the wells in a lease are often connningkxf at asatelfite battery, cace mast hc taken to include the affects ofproduction allocation. One alternative woufd be to simplyenafyz data on a Iea.w-by-lease rather thass well-by-wellbasis.

Ssn-rcWaacc Data Anafysis lo temns of fiefd operations, itis desirable to have as much continuously or regularlymmsured data aa possible. Its particrdsr, analysis of datafrom production logs (where rates permit), continuousmea.muwnent and production dnta are useful. These data arevery relevant, but in afl likelihood, an operator will not beable to justify the expense of such a large colkction of dataunkas the data is taken for a pilot stady.

One of the objectives of thii paper is to encourage at leastsome form of continuous surveifkusce. As mantiossed above,production and injection data mo@oring is a start; however,data from production logs, curs injectivity and pressuretransient tests, and use refativefy sophisticated monitoringtechniques, such es cased hole saturation profiles (ThermalDecay Log), are afso vafuabl.%

For water injection WSIISspecifically, Hall (Jlef. 42) ossdHeam (Ref. 43) plots can be used to detmine =timat~ Offormation properties and identify evidence of pore plugging.In addition, injcstion/pmduction radioactive tracer flowsurveys may help identify plugged zones, as wefl as interwellcommunication via thief zones or hydratilc or naturalfracture connectivity.

—.—

10 DATA ACQUISITION DESIGN AND EWPLEMENTATION. 0PPORTUNITEX3 AND SPE 27760 .CHALX.ENGES FOR EFFECTIVE PROGRAMS IN MATUP.E RESERVOIRS

b311&ddi011and ~ “onRro&durEs: The most importantcIcments for a suecessfuf production operation am a wellplanned and implemented completion coupled with an

aPPrnPfie Sf.inmfation treatmeut. There is simply nosubshtute for a competent completion and stinudationsequence. Complete and accurate data acqui.4tion smdstoreae is fssentiaf for bistorirml data such as the uerforatsdpayinterralv, cnmplelion procedures, stismdstiontrcatmsntv,cud subsequent renrediaf treatrueats. Problrsns arise whenweU files am not updatad on a regufar basis. As this data isrequired to ondyze completion sftlciency, i~ectivity andproduction patteims, aud reservoir pcrformrmce trends, itsiurportancs cannol be ovemnrphiwimd. Although, sm “afterthe fact” eveluatio~ a thorough enafysis of the precedingdata can identify poor stimrdation and completion pracficea.

Often whet is thought to be a complsx producibility/injectivity problem related to reserroir heterogeneity(s), isactaally a sirnpIe problem rslated to completion orstinndation problems such es incomplete removal of near-wellbore damage, partiaf penetration, poor perforationeffkiency, wellbore tiI1, or pore plugging with fimes andcbenrical precipitates.

Remediation of completion and stimtiation problems isafmost always costly, both in terms of treatment costs as wellos reductions in oil production-wbicb are often permanent,regardless of remediation sfforts. But more importantly,these completion and stinndation problems are not onlytumec.essary, they are preventable. Considering matorereservoirs, it can be argued that nothing can be done aboutthe initiaJ completion or stimulation. Thus, the on[yprosperfs left are remedmtion and/Or restimulation.

It is important to make sum that the mistakes of previousoperators or persmm-el are not repeated.

In the course of conducting reservoir studies, situations arisewhere previously acquired data is inadequate or may beinconsistent with respect to othsr known data about thereserroir. To avoid such situations with data acquisitions,the following guidelines are suggested to emure acceptabledata quality.

PVPIPlras? Behavior, Routine and Spcriaf Cm-c QuafityAmmance Precision in labomkwyprofcwoldoes not gamanteeaccumqv in Iabonztory results. Laboratpv.persom@ bavs theability to make very precise measurements in laboratorytests. However, if the material to be tested in the lab is not“properly acquired” then there is a situation where very

prerise rcndts on a lahoratory test are availab!e, but the datais not accurate or representative of reservoir cenditiom orproperties. Examples include execution of proper pmcedmfor bottomhole or surface sampling end procedures forrefrieval and preservation of cers materiel.

Cafiiration uf LabOmtorg Equipmemb if is important toutibe appropriate calibration techniques on laboratoryequipment prior to conducting tests. Frequently; fack ofcalibration of Iabmntory equipment leads to ermmxms data,evq thoogh the laboratory tdmicim may havepainstakingly performed the protocol set for testiug. Takingthese precautions diligently, one can focus on utifizing thevarious laboratory data for the vafkfation of reserroir

perform~~ as opposed tn d@ding whether or not the dataobtained is wo-ithwhile.

Quafity A=mance Guidefiu.s Although, the cmrcspt seemssimplq quafity assurance is one importaut faclor whichreservoir and development engineers frequently do notconsidw as thoroughly as is warranted.

Quality assurance programs shoufd focus on the followingmain areas:

1. Identify problems in data quafity that mu arise duete inadequately precise laboratory procedures.

2. Review raw, rmsmoothed data to track pmbkms totheir fundaruentaf caoses. Engineers or other emdusers of data should review the raw laboratory dataas well as raw data reduction methodologies. Raw,ummoothed data, as well as fiuaf results, should beincluded in data reports from fsboratories.

3. Equipment cahbrations should be checked andmcbecked iuuncdiitely prior to and after conductingLaboratory testing to rule out potential calibrationproblems in analysis of data which may not seemreasonable.

4. Measurement of Iahoratory quantities by mors thanone technique is advisable, especially for criticaldata. This use of cmss-chscks and built inredundancy into laboratory programs makes it easierto quickly identify laboratory quality assuranceproblems as they arise rather than some distimtpoint in the future after the laboratory program iscomplete.

Sprunt and Hmuphreys (Ref. 44) provide an excellentdiscussion on how to obtain ‘reliable laboratory

~ SPE 27760 P.K. PANDE, M.B. CLARf& T.A. BLASINGAME, L. DOUBLET 11

measurements. Gtidefinea have been provided for selectionof Iabomtories for testing. The need for cross-checks, checksamples, and regular reviews with Iahomtories isemphasized. Sprrmt ind Hwmpbreys afso empfrasii theneed for proper catalogukqg of Ma.

CONCHJSIONS

The following cmrcfmione are made with respect to dataacquisition for mature reservoirs:

Data Ac@v.itiors Costs and Wbsea: Costs of individual dataacqtiltion items needed for field development are generallytlti,, (fioWh not ir@@fi~t) when related tOthe COStOf

isrrplenrerrtationof field development and IOR methods.However, the finassanf impacts of these data may betremendous nnd criticaf to a project’s success. For thisreason every attempt must be made to optimize the vafue ofacquired data.

R61umvoir Cbmcbimtion and Reservoir PerformanceAm@& Importance Availability of core is critical toidentifying pay and non-pay rock types and development ofa rock-log model. These data can be used with reservoirperformasw+ petrophysicai, and other data to defiie flowunits. Geophysical methods are an important tool to definennd vafidate interwell features and heterogeneity. UtiIity ofreservoir performance data and surveillance data ore vitat toreservoir performance optimization.

Objectives, Pfams@, Alterziativea and Co~. Thesefactors must be continually evaluated in data acquisitionprograms. Knowing the objectives, planning for them,understanding the akernativa and consequences of takingdata and making meaarsmmenta, and the ss~ativeconsequence of incorrect measurements or data are key tosuccessful data acquisition programs.

Team Approach to Data Acqssi@tion: A multid~~pfiiryapproach will provide for more meaningful data acquisition.The team should include geologists, geophysicists, reservoir,production and drilling engineers, and fabomtory persomel.

Jnitiaf Data Acquisition Operatiom: The initial operationsconducted in acquiring data need to be effestiwdy executedas this will impact all successive measurements.

Qrrafity Asmmnce Design and implementation of qualityassurance program are key to obtaining rtilahle data fordevelopment decisions.

Amo s

The support of the United Stat= Department of Esseryy isappreciated for technology transfer activities for the Class IIOil Program for which the North Robertaon Unit has beessselected. The authors thank the masragemerst of F- Oiland Chmrid Company for their support in disseminatingthe results in this”papw””to tbi petibleum indus@”.

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.

14 DATA ACQUISITION DESIGN ~.. lMPLEiWE&TATION opPoRTi_NITIES ANO SPE 2776aCHALLENGES FOR EFFECTIVE PROGRAMS IN MATURE RESERVOIRS F

Figure 1

SIGNIFICANCE OF RESERVOIEL CHARACTERIZATION AS A FUNCTIONOF FIELD DEVELOPMENT STAGE AND GEOLOGICAL COMPLEXITY

COST OF [NJE.’STANTLOW !4..,.”, “

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FIELD DEVELOPMENT/101IMPLEMENTATIO?4FEA5E

figure 2

TEAM APPROACH TOWARDS DATA AOQUISITION

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Figure 3

81QN1F1CANCE OF PVT/PE&SE BEHAV1OR AS A FUNCTION OFFIELD DEVELOPMENT STAGE AND GEOLOGICAL cOMPLEXITY

COST 08 lNJ ECTANT

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Fiiure 4

ROUTINE AND SPECIAL CORE ANALYSIS(SCALI DATA ACQUISITION PROQW

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SPECL4LCCJB.EANALYSIS

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15