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PLUS: HOW HYBRID SOLUTIONS AND COMPOSITES ARE OPENING NEW DEEPWATER RISER DESIGN HORIZONS

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OFFSHORE ENGINEER

Deep thinking on the Latin American beatFloating production

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Securing the future of decom

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accumulation locations. Offshore portions of the basins represent interesting exploration targets because migration pathways would have been shorter and several major northwest-southeast fault systems exist that could connect the source rocks with younger reservoir intervals. The study used a variety of modeling methodologies using existing and new types of exploration data to build knowledge about the development of the basins aimed at reducing future exploration risk.

Building a modelIn 1999, WesternGeco acquired 14,000km of 2D multiclient seismic data in the deepest parts of the offshore Potiguar and Ceará basins. The dataset was processed using prestack time migration with high

supercontinent. Onshore, the Potiguar basin formed as a result of a rift that failed to completely form. Offshore, it includes a Cretaceous rift overlain by an Upper Aptian to Tertiary sag basin. The offshore portions of Potiguar and Ceará basins are predominantly filled with clastic sediments including turbidite sequences. Basin fill also includes intrusive and extrusive rocks associated with igneous activity. The thick salt deposits common to the southern Brazilian basins are absent. The mature source rocks for most of the known onshore fields are offshore. With burial, heat and time, these deep rocks produced hydrocarbons from kerogen contained within them. The oil and gas traversed a long migration pathway from the source kitchen to the current onshore

The study area included parts of the Potiguar and Ceará basins offshore northeast Brazil. These basins

extend both on- and offshore. Onshore, the Potiguar basin is the third most productive basin in Brazil, with over 100 oil and gas fields, most discovered in the early 1980s. To date there has been sparse development in the nearshore area and none in deepwater. The adjacent Ceará basin has only four offshore fields. The geological history of the region suggests that the offshore areas of both basins are prospective targets for new hydrocarbon exploration. Sedimentary basins with rift, sag and drift sequences formed on both sides of the South Atlantic during the Cretaceous Period when South America and Africa split apart from the Gondwana

Integrating exploration toolsIntegrating several imaging and modeling technologies can reduce exploration risk. Ian Bryant (Schlumberger) and Paul Xu (formerly of Ipex) present an integrated basin and petroleum system study that predicted many oil and gas accumulations in a relatively unexplored region offshore Brazil.

28-layer 3D petroleum systems model. Colors indicate various sedimentary lithologies in the basin-fill. Red indicates igneous intrusions. 2D seismic line locations are also indicated.

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quality results. Seismic processing technologies are continually evolving, taking advantage of increasing computer power and new iterative interpretation and modeling workflows. Consequently, in 2008, the dataset was reprocessed using improved noise suppression, better multiple removal and prestack depth imaging techniques. A complete 3D velocity model was developed to enable

Right: Offshore Potiguar and Ceará basin seismic lines, leased acreage, producing fields, and key wells.

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shallower parts of the basins. The close correspondence between the predicted API gravity of the oils in these forward modeled accumulations to oil gravity measured in the produced oils enhanced confidence in the model predictions.

resource volumes generated. Each run generated a number of modeled accumulations that evolve through geological time. Some of these accumulations corresponded to known oil fields that are producing in the

efficient and consistent depth migration of the 2D lines. The newly conditioned dataset showed improved frequency content and resolution in the shallow section and better fault and reflector definition and continuity in the deeper section compared with the original processing. This latter improvement was particularly important for exploration analysis because the deep, prerift section contains most of the source rocks in the basin. Seven wells from the shallow shelf were tied to the seismic to enable depth conversion and interpretation of eight key horizons to build a 3D geological model of both basins in Schlumberger’s Petrel E&P software platform. The resulting earth model incorporated basin geometry from the seismic and lithologic information from the well data. Basin and petroleum systems modeling requires reconstruction of the geological history of the basin to evaluate the maturation, migration, entrapment and preservation of hydrocarbons through geological time. Data from onshore and shelf wells provided critical information about the ability of the source rocks to generate hydrocarbons, the differences between oil families from these source rocks, and the way the mixtures of these oils can enhance the quality of the accumulations. The geological model of the offshore portions of the Potiguar and Ceará basins was combined with geochemical data and heat flux data to build a 3D petroleum systems model. The depth-converted horizons were exported to Schlumberger’s PetroMod petroleum systems modeling application. Interpolation between the eight mapped horizons was used to generate a 28-layer model. Interpretation of the sparse well log data, together with published geological studies, enabled assignment of laterally varying, appropriate rock properties to the basin-fill. Multiple source rocks occur in both basins, with the main source rocks occurring in the Early Cretaceous rift-filling sequences. The modeling indicated over 1700 billion barrels of oil have been generated from the source rocks present in these basins. Although the Potiguar and Ceará basins contain effective seals, they lack the thick salt deposits that characterize the Atlantic-margin basins further to the south, so trapping efficiency is lower than in the southern basins. The models suggested that less than 40 billion barrels remain in known and yet-to-be discovered accumulations. Multiple model runs were made to assess ranges of uncertainty in the

Predicted hydrocarbon accumulation modeled through geological time.

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TheCSEMsurveyalsoindicatedstructuraldetailsmissingfromthe2Dseismicdata,whichcouldallowfurtherinterpretationofthepotentialaccumulation.

ConclusionsThemulticlientstudybroughttogetheradiversesetofdataintoonepicturethatismoreinformativethantheindividualpieces.EffectiveintegrationofthevarioussourcesofinformationwasenabledbythePetrelenvironmentandsoftwareplug-ins,combinedwiththeknowledgeandskillsofspecialistgeoscientists. Theintegrationofseismicdata,remotesensingsurveysandmodelingofthepetroleumsystemprovidedguidancefortheacquisitionofseveralCSEMsurveys.Asaresultoftheintegratedapproach,themulticlientstudyidentifiedseveralpotentialprospectsinthisrelativelyunexploredarea.

Ian Bryant is currently vice president and worldwide geoscience advisor for Schlumberger Information Solutions. He has held a variety of management positions in Schlumberger, and previously worked for

Shell Exploration & production Laboratory and Shell New Zealand. He has BSc and phd degrees in earth sciences from University of Reading, England. Bryant is a past chair of the SpE development Geology & Geophysics committee and an active member of SpE, SEG, aapG and EaGE.

Paul Xu holds degrees in both analytical chemistry and computer science. He worked as a software development director at Infologic (now a part of Weatherford) for seven years. In 2004, Xu joined HRt as an It and project

manager and was named cEO of Ipex, successor of HRt & petroleum from 2009-11. Xu currently works with HRt Oil & Gas.

acoustically.Whendataacquisitioniscomplete,thereceiversarerecoveredandthedatadownloadedandanalyzed.Thedataaretheninterpreted,firstintermsofelectricalunitsandthenasgeologicformationstakingintoaccountandintegratingothergeophysicaldata. Unlikenaturalsourceplanewaveelectricalmethodssuchasmagnetotelluricswherethinresistivelayersareeffectivelytransparent,theCSEMgenerateddipolefieldinteractswithsuchlayers,andtheirpresence,thickness,andlateralextentmaybedetermined.Aswithanygeophysicalmethod,therearelimitationsonthedepthofburial,layerthickness,andresistivitycontraststhatcanviablybemeasured. CustomdesignofEMsourcecharacteristicsandthegeometryoftheseafloorreceiverarraywilloptimizetheeffectivenessofthetechniquetomeetproject-specificobjectives. TheOceanEM-Connectplug-inforPetrelsoftwarewasusedtodesignCSEMstudiesoverfiveareasofthePotiguarandCearábasins. Thefirststepwastobuildaresistivitymodelofeachofthemajorsequencesbetweenthemappedhorizonsinthebasin.Eachoftheintervalswasassignedasinglelaterallyhomogenousresistivity,andthenresistiveanomalieswereembeddedatvariouslocationswithinthemodel. ThemodeledresultsfromvariousCSEMacquisitionparameterswerethenusedtoevaluatethedetectabilityoftheresistivebodiesandtotuneparameterstoobtainoptimalilluminationwhileminimizingacquisitioncosts.

CSEM resultsThefiveCSEMsurveysacquiredinJune2009detectedsubsurfaceresistivityanomaliesinthebasin-fill;howeverthesecouldberelatedtohydrocarbonaccumulationsorigneousrocks.Tohelpdistinguishbetweenthesealternatives,theresultsfromresistivityinversionandpetroleumsystemsmodelingwerebroughtintothereferenceearthmodel.Theexample(shownleft)showsresultsfromoneofthefivesurveyswheretheforward-modeledhydrocarbonaccumulation(green)iscollocatedwitharesistivityanomaly(yellow)detectedbyinversionofaCSEMsurveyatapointwherethreesurfaceoilslicks(blackdots)hadbeeninterpretedfromsatellitedata.Adjacentindicationsofaccumulationsfromthebasinmodeldidnotcoincidewithresistivityanomaliesofthesamemagnitude,highlightingthevalueofcombiningthebasinandpetroleumsystemmodelingwithassessmentofresistivity.

Adata-exchangeplug-inforPetrelwasusedtobringthepresent-daymodeledhydrocarbonaccumulationsbackintothereferenceearthmodel,wheretheycouldbeviewedinthepresenceoftheseismicandwelldata.Theprojectteamthenconsideredwhatmorecouldbedonetovalidatethisforwardmodelofthedistributionofyet-to-be-discoveredhydrocarbons.

Locating accumulationsThebasinmodelingstudyindicatedtheprobablepresenceofmaturesourcerocksandachargingmechanismfortrapsintheoffshorePotiguarandCearábasins. InterpretedsurfaceseismichorizonsinthePetrelmodelindicatedpotentiallyinterestingstructuresbutnotwhetherthesestructuresarechargedwithhydrocarbons.Overoceans,remotesensingprovidesanindependentindicatorofgeographicareasthatmaycontainaccumulations:naturallyoccurringoilseepscanbemappedusingsatelliteimagesoftheoceansurface.Oilonthewatersurfacesuppresseswaves,andtheseareasreflectlightdifferentlythanthetypicaloceansurface.Bylookingatresultsfromseveralpassesofasatellite,interpreterscaneliminateshort-termeffectsofweatherorthepassageofvessels,leavingpersistentevidenceofnaturalseeps.Thelocationsoftheseseeps,combinedwithstructuralgeologicalinformationfromtheseismicdata,identifiedfivetargetareasoftheoffshorebasinsscheduledformarinecontrolledsourceelectromagnetic(CSEM)surveysthatmightdetectresistivityanomaliesinthebasin-fillsassociatedwithhydrocarbonaccumulations.

Surface EM surveys Electromagnetic(EM)techniquescandiscriminateresistivefromnonresistivebodies.AlthoughtheresistivitydifferencebetweenhydrocarbonsandbrineisnottheonlycontrastsensedbyEMmethods,alackofresistivitycontrastcanbeagoodindicatorthathydrocarbonsarenotpresent.EMacquisitionandinterpretationtechnologieshavematuredsignificantlyoverthepastdecade,andtheCSEMmethodhasbeenwidelyusedtoevaluateprospectswhenthepositionanddepthofthetargetareknownoratleastpostulated. IntheCSEMmethod,alow-frequencyEMfieldistransmittedusingahigh-powerlow-frequencyelectricdipoleantenna(source)towedbehindasurveyvesselwithin50mto100moftheseafloor.AnarrayofsensitivereceiversisdeployedontheseafloortomeasureandrecordtheinducedEMfield.Transmitterandreceiversaretrackedandlocated

A resistive anomaly (yellow) detected by CSEM coincides with a predicted accumulation (green) and surface oil slicks (black dots in inset map).

AcknowledgmentsThe seismic data in this study is from the WesternGeco/TGS Brazil 2D Alliance survey. WesternGeco owns the CSEM data. Ipex provided the geochemical and well data together with interpretation expertise in Brazilian basins.

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