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JANUARY 2011 7

Potential Use of North Dakota Sand and Clay for Natural and

Manufactured ProppantsFred J. Anderson

The overwhelming success of the Bakken/Three Forks oil playinNorthDakota can be greatly, if notwholly, attributed to theadvancesinhydraulicfracturingof“tight”reservoirrock.Aspartofthehydraulicfracturingprocess,proppants(commonlynaturalsandsandmanufacturedceramicspheres)areusedtokeepthefracturesthatarecreatedopenfortheenhancedflowofoilandgas from the fractures into the wellbore. A typical hydraulicfracturing stimulation operation in the Bakken inNorthDakotarequires anywhere from100 to 500 tonsof proppantperwell:roughly150tonsonaverage.TheNorthDakotaGeologicalSurveyhasbeeninvestigatingNorthDakota’ssandandclayresourcesinan effort to better characterize deposits that could potentiallybeusedasproppants. Specifically,weareattemptingto locateuseable deposits of sand (and potentially clays) for hydraulicfracturingandotherindustrialsandapplicationsintheWillistonBasin.

Sand (and gravel) is widespread throughout North Dakota,althoughmost is in the glaciated part of the state,where it is

found inglaciolacustrinebeachdeposits,glaciofluvial (outwash)andmodern stream deposits, and early tomid-Holocene dunefields. Most of the sand and gravel currently mined in NorthDakotaisusedinconstructionandroadbuilding.

Syntheticproppantsaremanufacturedfromavarietyofmaterialsand includealuminosilicateceramicsderived fromthesinteringofbauxite(aluminumoxide)andkaolinite.Aspartofourstudy,therefore,wearealsolookingatsomeofNorthDakota’skaoliniticclaysaspotential rawmaterials for themanufactureofceramicproppants.

ProgressofInvestigationOver 100 sand locations were sampled from across the statefor this study (fig. 1). We have identified the leading sanddepositsbasedupongrainsize,sorting,androundness.Detailedsedimentological and engineering analyses are planned for thebestcandidatesands.

Figure1.ThesandandgravelresourcesofNorthDakota(yellow)andlocationsofsandsamples(browndots)collectedforpossibleuseasproppantsforthehydraulicfracturingofoilwellsintheBakkenandThreeForksplaysintheWillistonBasin(modifiedfromAnderson,inpress).

8 GEO NEWS

MembersofNorthDakota’ssandandgravel-producingindustryarealsoparticipatinginthisevaluation,andseveral producers have submitted sand samplesfor consideration. The results of this study will becommunicatedtoindustrythroughpublicationoftheinvestigationresultsin2011.

We are also characterizing these samples for use in otherindustrial sandapplications includingwell completionmaterials(filter&gravelpacks),filtermedia, and foundrypractices. Theproduction of industrial sands as viable economic depositsgenerally requiresmaterials thatareusuallymoreconsistent incharacter, are available closer to the land surface, and requireminimal processing (screening and washing). Having a betterunderstandingofsomeofthecharacteristicsofourstate’ssandand clay resources is advantageous to both industry and thepeopleofNorthDakota.

ProppantCharacteristicsProppantsarecharacterizedbywhattheyare;thatis,theirphysicalproperties,andhowtheyperform(inbulk)withinthefracturedoil&gasreservoir; inotherwords, theirabilitytoenhanceandfacilitateflow(permeability)inaproppedfracture.Permeabilitymaybethoughtofastherate(distanceperunittime)atwhichafluid(oil,gas,water)canmovethroughaparticularsedimentorrock.Itiscommonlymeasuredincm/sec,feet/dayorthedarcy,after French engineer Henry Darcy, whose pioneering work onfluidflowledtothediscoveryofthelawthatisnamedafterhim(Darcy,1856).Texturallymaturesedimentsgenerallyhavehigherpermeabilities(fig.2)thanlesstexturallymaturesediments.Asa relevant everydayexamplehere inNorthDakota, theBakkenFormationexhibitspermeabilitiesinthemicro(10-6)darcyrange(Nordeng, oral commun., 2010). By comparison, oil and gasreservoirsworldwide typicallyhavepermeabilities ranging from5to500millidarcies(Selley,1998). Theonlywaytoliberateoilandgas froma “tight” reservoir like theBakken isbyhydraulicfracturing(fig.3).

Geologic factors such as reservoir depths, temperatures, andpressuresall relatetoaproppant’sability towithstandfractureclosurewithinthereservoir.Theclosurestrengthistheamountofstrainthatcanbetakenupbytheproppantbefore it failstokeep the induced fractures open during production from thereservoir. Whennaturalproppantsfail,particularitywhenusedin the Bakken Formation, individual sand grains may actuallyfracture under the high stresses and temperatures within theformation/reservoir (Vincent, 2010a& 2010b). These kinds ofgeologic factors are critically important to understand whendesigning fracingoperationswhere significant reservoirdepths,temperatures,andclosurestressesareanticipated.

Figure3.Schematicofanopen“propped”rockfracturewithproppantplacedwithintheapertureofthefracture.Proppantsthataremoreconsistentandwell-roundedasin(a),allowforgreaterinterconnectedporespaceandfacilitatealargeramountof flow (higher permeability) of oil and gas out of the reservoir rock and into thefracture(s) andwellbore. Proppants that are less consistent and have poor shapefactorsasin(b),(higherdegreesofangularity)tendtopackbetterbutresultinlessinterconnectedporespaceandlowerpermeabilities,whichresultsinlessflowofoilandgasintoandthroughthefracture.

Figure 2. Generalized relationships between grainsize and conductivity (permeability) of a sediment(modifiedfromShepherd,1989).

Figure 4. Outcrop of the St. Peter Sandstone near Ottawa,Illinois.Thehigherdegreesofsortingandmineralogicalmaturityof these Ordovician-age white “Ottawa” deposits allow fortheireconomicalproductionforuseasproppantsandinotherindustrialapplications.

JANUARY 2011 9

NaturalProppantsMostnaturalsandproppantsaresourcedastwotypes:“white”and“brown.”“White”orOttawasands(referringtotheirprimarysourceareaaroundOttawa,Illinois)(fig.4)areminedinpartsoftheupperMidwest fromquartz-richCambro-Ordoviciansandstonesthatarecharacterizedbytheirexceptionallyhighdegreesofpurityand textural maturity. These sands were originally depositedin a shallow marine environment where geologic factors suchas weathering, erosion, and repeated refinement in the surfzone have resulted in texturally mature sandstones that arecomposedalmostentirelyofquartz.“Brown”orBradysandsarefoundindepositslocatedinTexas.Becausetheyaresomewhatmineralogically and texturally less mature than Ottawa sands,Bradysandsarewashedduringprocessingtoremoveimpuritiessuchasclaysandfeldspars.Naturalsandproppants(figs.5aandb)areusedmainlyinshallowoperationswherefractureclosurestressesinthereservoirrockarerelativelylow.

BecauseNorthDakota’ssanddepositsaresogeologicallyyoung,theyaremuch lessmature,bothmineralogicallyand texturally,meaning that they consist of compositionallymore varied, lessrounded, less well sorted grains than most sands commonlyusedinhydraulicfracturing.Potentialproppantsandswillmostlikelybefoundinthebetter-sorteddeposits,whichtendtobeofwindblownorigin,liketheDenbighorPembinaDunes(figs.5cand6);orfluvialsedimentswheregeologicprocesseshaveselectivelyremoved someof the coarser and finer grained sediments (forexample,asandbar).

SyntheticorManufacturedProppantsSynthetic, ormanufacturedproppants aredesigned toperformunder specific reservoir conditions (high pressures andtemperatures coupled with high fracture closure stresses) andcanbeengineeredtoperformmorefavorablyinhostilereservoirconditions than natural proppants. Specialty aluminosilicateceramics, made from sintered blends of bauxite and kaoliniticclays, are one example (fig. 7). Several North Dakota clays,includingthosefromtheBearDenMemberoftheGoldenValleyFormation are being evaluated as potential candidates for themanufacture of these types of ceramic proppants. The BearDenMemberof theGoldenValleyFormation isamulti-coloredkaoliniticclaystonethatoutcropsinseverallocalitiesthroughoutDunn,Mercer,Morton,Mountrail,andStarkCountiesinwesternNorth Dakota (fig. 8). The Bear Den Member has been usedfor decades in the manufacture of bricks by the Hebron BrickCompany(Murphy,2009).

Oneofthekeydriversinproppantselectionanduseiscost.Naturalproppants,when appropriate and available, arguably provide amorecost-effectivesolutionthanacustom-designedandshippedsyntheticormanufacturedproppant.However,sincetheremay

Figure5.Photomicrographsof(a)anout-of-stateproppantsandcurrentlyinuseintheWillistonBasin,(b)aNorthDakotasandprocessedforuseasaproppant,and(c)dunesandfromPembinaCounty.Allimagesareat40xmagnification.

Figure6.SanddunefromthePembinaDunesinnortheasternNorthDakota. Small-scalewind ripples are visible on the exposeddunesurface,indicativeofrecentwindactivity.

Figure 7. Photomicrograph of sintered bauxite ceramic proppantspheresusedinthehydraulicfracturingofoilreservoirs.Theuniformityof individual particle shapes combinedwithhighermaterial strengths,specific gravities, and thermogenic resistance have resulted in theextensive use of ceramic proppants (including this one) in Bakkenreservoir-relatedhydraulicfracturingandrefracturingapplications.

(a) (b) (c)

10 GEO NEWS

beabitmorevariability inthecharacterofanaturalproppant,itmayprovedifficulttopredictexpectedwellperformancefromfractofrac,orwithinmultiplefracsinagivenwell.

DevelopmentofProppantsinNorthDakotaWhetherornotaparticulardepositofsandorclaycanbeutilizedasasourceforproppants,dependsonseveraladditionalfactors,includingtheproximityofdepositsto locationswherehydraulicfracturingistakingplace,andthedegreeofnaturalbeneficiationin the deposit; that is, howmany natural cycles of refinementthedeposithasbeenexposedtoovertime.Forexample,ifthedepositistexturallyandmineralogicallymature,therewillbelessprocessing required to produce a given amount of marketablematerial(fig.9).Thisalsoplaysintocosts,asthelesseffortoneneedstoput intotherefiningof thedeposit, the lessmoney isspentbringing thematerial“uptospec”anddowntheroadtomarket. One current market-related aspect that is interesting(andtimely)isthefactthat,acrossthenation,theavailabilityofproppantsands fromtheirprincipalsourceareasappears tobedecliningasdevelopinggasplaysintheeasternU.S.increasetheirdemand.Inaddition,ceramicproppantswillmostlikelycontinuetobe imported into theWillistonBasin fromoverseasmarkets,asmoreanddeeperoilandgasdevelopmentoccursacrossthenation(Besler,oralcommun.,2010).

Since most, if not all, of the currently available natural sandresources are being shipped east for the stimulationof oil andgas reservoirs in Appalachia (especially the Marcellus Shale inPennsylvaniaandNewYork),operatorsintheWillistonBasinmaybewillingtotakewhatismostreadilyavailableandeconomicalas a bit of a frac-design/well production trade-off. Some havesuggestedthatoverallproductionfromBakkenwellsstimulatedwithceramicproppantsmaybeasgreatas30%higherthanthosestimulatedwithnatural(sands)proppants.

Overall, North Dakota’s sand, and possibly clay resources, arelocally abundant, readily available, and contain a sufficientvarietyofmineralogicalcharactertoallowforadiversearrayofpotentialindustrialapplications.Creativityinenduse,facilitated

by producers in industry, will drive the development of NorthDakota’ssandandclayresources. Withanestimatedneedof3million ormore tons* of proppants (around a couple of billiondollars at today’s prices!) to support full development of theBakken,gainingabetterunderstandingofourpossibilitiesfortheproduction of natural and manufactured proppants is prudentandbeneficialforallofNorthDakota.

ReferencesAnderson, F.J., in press, Locations of Sand and Gravel Sites in North

Dakota, North Dakota Geological Survey, Geologic InvestigationsNo.130,1:500,000scalemap.

Babcock, E.J., 1906, The uses and value of North Dakota clays, in theFourthBiennialReportoftheNorthDakotaGeologicalSurvey,

p.191-244.Darcy,H.,1856,LesfontainespubliquesdelavilledeDijon.Paris.,reviewed

inSimmons,C.T.,2008,HenryDarcy(1803-1858):Immortalizedbyhisscientificlegacy,HydrogeologyJournal,v.16,p.1023-1038.

Hansen,M.,1959,ClaysofNorthDakotaasapotentialsourceofalumina:NorthDakotaGeologicalSurveyReportofInvestigation,no.33,

15p.Manz,O.E.,1953,InvestigationofsomeNorthDakotaclaysandshales:

NorthDakotaGeologicalSurveyReportofInvestigation,no.13, 36p.Murphy, E.C., 1995, North Dakota clays, a historical review of clay

utilization in North Dakota: North Dakota Geological SurveyMiscellaneousSeries,no.79,18p.

Murphy, E.C., 2009, The Golden Valley Formation: North DakotaDepartmentofMineralResourcesNewsletter,v.36,no.2,p.1-4.

Selley,R.C.,1998,ElementsofPetroleumGeology(2nded.):SanDiego,CA,AcademicPress,470p.

Shepherd,R.G.,1989,CorrelationsofPermeabilityandGrainSize:GroundWater,v.27,no.5,p.633-638.

Vincent,M.,2010a, ImprovedProduction fromHorizontalWells in theBakken:2010NewHorizonsinOil&GasConference,SouthDakotaSchoolofMines&Technology,RapidCity,SD,Abstract.

Vincent, M., 2010b, A Study of 75 Refracs Performed in the middleBakken:2010NewHorizonsinOil&GasConference,SouthDakotaSchoolofMines&Technology,RapidCity,SD,Abstract.

Figure8.OutcropofthecolorfulBearDenMemberoftheGoldenValleyFormation in Dunn County. The Bear DenMember is found in severalareasintheheartoftheNDportionoftheWillistonBasinandisknowntocontainintervalsrichinkaolinite(Murphy,2009).

Figure9. Asandandgravelprocessingoperation innorthernBurleighCounty. This plant produces materials for a number of industrialapplicationsinNorthDakota.

*Roughly60millioncubicyardsorthevolumetricequivalentoffillingthetoweroftheNorthDakotastatecapitoltwenty-sixtimes!