Oil and Gas Produced Water_Treatment Technologies

7/28/2019 Oil and Gas Produced Water_Treatment Technologies

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Technical BriefOil and Gas Produced WaterTreatment Technologies

the

water sustainability solution


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PRODUCED AND HYDRAULIC FRACTURING FLOWBACK WATERS

Producedwaters,co-producedduringtheextractionofoilandnaturalgasreserves,inaddition

to theflow backwater from hydraulic fracturingactivities (i.e., fracwater)must beproperly

managedinordertomitigateanyenvironmentalimpactsandimpactstoexistingwatersupplies

by energy development activities [1]. Hydraulic fracturing is typically used to open up tightgeologicformationsorreservoirrock(e.g.,shaleformations)sothatthenaturalgasmaybe

moreeasilyextracted.Recentestimatesfortheamountofproducedwaterthatisgeneratedin

the United States (US) range from 1.6 to 2.1million gallons per day (mgd) [1]. As energy

exploration and extraction continue to increase (e.g., oil shale and coal bed methane

development) these volumesofwater willlikely continue to increase [2]. Thechemistry and

compositionofproducedandhydraulicfracturingflowbackwatersishighlyvariableandinmany

cases quite complex. The most significant concern for developing effective management

strategies for these waters is removing, or reducing, the total dissolved solids (TDS)

concentrationpriortoreuse.Thistechnicalbriefprovidesanintroductiontosomeofthemore

commonlyemployed treatment strategies forproduced/hydraulic fracturing flowbackwaters.

Emphasisisplacedonthecurrentmaturationstateofthesetechnologiesandaddressessomeof

theassociatedadvantagesanddisadvantageswiththeiruseformanagingproducedwaters.

OilandGasproducedwatersare commonlycharacterizedbyhighsaltconcentrationswhich

requirestheirdisposalinevaporationponds.

ProducedWaterChemistryandComposition. ProducedwatersProducedwatersaregenerally

characterizedasbrackishwatersolutionscontaininghighconcentrationsofdissolvedminerals,

metals, and salts [1, 3-5] (Table 1). Waters that are characterized by relatively high TDS

concentrations (> 1,000 mg/L) require some form of treatment prior to their discharge or

beneficial reuse [6-8].For comparison thesecondarydrinking water standardforTDS is 500

mg/L as established by the United States Environmental Protection Agency (USEPA).

Additionally,producedwaterscancontainhighlevelsoforganicslikeoils,greases,andbenzene,

toluene, ethylbenzene, and xylene (BTEX) compounds [1]. The specific composition and

chemistryofproducedwatersissitespecificandinfactvarydependentonthelocationandtype

of geologic formation from which the producedwater is extracted [1, 2]. Furthermore, the

chemistryandcompositionofproducedwaterfromasinglesourcemayfluctuategreatlyduring

theoperationofthewell.Thisfactrequiresthattheassociatedtreatmentsystembeflexibleso

that it can accommodate changes in the feedwater quality. Despite the variation inwater


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qualities producedwaters tend to have relatively high TDS concentrations that make them

unsuitable formostpotablewater applicationswithout treatment. Indeed,produced waters

mayhaveTDSconcentrationsthatapproach,orareinexcessof,170,000mg/L,whichisnearly

fivetimesthatofseawater(TDS~36,000mg/L).

Table1.Concentrationsofcommoninorganicandorganicconstituentsinproducedwaters

(adaptedfrom[1,2]).

Constituent Low Medium High

TDS,mg/L 1,000 32,300 400,000

Sodium,mg/L nd 9,400 150,000

Chloride,mg/L nd 29,000 250,000

Barium,mg/L nd n/a 850

Strontium,mg/L nd n/a 6,250

Sulfate,mg/L nd 500 15,000

Bicarbonate,mg/L nd 400 15,000

Calcium,mg/L nd 1,500 74,000

Totalorganiccarbon,mg/L nd n/a 1,700

Totalvolatileorganics,mg/L 0.39 n/a 35

Totalrecoverableoilandgrease,mg/L 6.90 39.8 210

ndvalueisbelowthedetectionlimitoftheanalyticalequipmentused

n/adatanotavailable

HydraulicFracturing(Fracking).Flow-BackWaterChemistryandComposition.Unlikeproduced

watersthechemistryandcompositionoffracwaterispoorlycharacterized.Thereasonforthis

isthefactthatdifferententitiesmayaddproprietarychemicalsandotheradditivesthatarenot

disclosedtothepublic.Generallyspeakinghowever,fracwaterisbrackish(TDS>10,000mg/L)

and contains various organic additives and volatile organic compounds. Example chemical

additivestofracwaterincludepotassiumchloride,guargum,ethyleneglycol,sodiumcarbonate,

potassiumcarbonate,sodiumchloride,boratesalts,citricacid,glutaraldehyde,acid,petroleum

distillate,andisopropanol[9].Frackingrequireslargequantitiesofwatertodegreeofroughly2

to5milliongallonsoffracwaterperwell[10]Notethatasinglewellmaybefrackedovera

dozentimesduringitslifetime.Approximately15%to80%oftheinjectedfracwaterreturnsto

thesurfaceasflowbackwater.

TreatmentCosts.Thecostsassociatedwithmanagingandtreatingproducedand/orfracwaters

can is highly dependent on the chemistry/composition of the raw water and the requiredfinishedwaterquality.Therefore,estimatingthecostsformanagingthesewatersiscomplexat

bestgiventhewidevariabilityinthechemistryofproduced/fracwaters.Insomecasesthecost

of treating the produced water can be prohibitive to energy development ventures.

Furthermore, as clean water is a scarce resource, treating and reusing these waters for

beneficialapplications(i.e.,forirrigation,industrialprocesses,fracwatermakeup,orothernon-

potablepurposes)mayhavesignificanteconomicincentives(ProducedWaterUtilizationActof

2008,H.R.2339).Forhydraulicfracturingrecoveringandreusingtheflowbackwatercanreduce


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costsassociatedwithdisposingofthewastewaterandtheacquisition/transportofnewmakeup

water.Essentialtotherealizationofthesebeneficialreuseapplicationsisthedevelopmentand

implementation of effective produced water treatment systems; however, the complex

chemistriesthatcharacterizethesewatersmakestreatmentbyexistingdesaltingtechnologies

difficultatbest.

RemovingTDSfromanywater isan energyintensiveendeavor.Generallyspeakingtreatment

costswillincreaseratherrapidlyastheTDSconcentrationincreases.Formembraneprocesses,

such as reverse osmosis (RO) this relationship between cost and TDS is attributed to the

relationshipbetweensaltconcentrationandosmoticpressure(i.e.,assalinityincreasessotoo

doestheosmoticpressureof thesolution).Moresalinesolutionswillrequire largerandmore

energyintensivefeedpumpsinorderto overcome theosmoticpressureof thefeedsolution.

Thetypeofdesalinationtechnologyusedwillvarydependingontheioniccompositionofthe

water.Forexample, ionexchangeorpHadjustmentmaybeusedwhenthewaterisprimarily

composed of carbonate species,whilemembrane processes or distillation processes will be

required formore complexwaters.Uniqueconsiderationsassociatedwith producedand frac

watertreatmentsystemsareoutlinedbelow:

Treatmentsystemmobilitytoaccountforthevariablelifetimesofproducingwellsaswellasthedevelopmentofnewones.

High source water recovery to mitigate the further treatment and/or disposal ofwastewatersresultingfromthetreatmentoftheproduced/fracwater.

Variability in source water quality requires that systems be flexible and robust toaccount for changes inwaterquality during thematurationofawell,as well as the

differentwaterqualitiesfromnewlydevelopedwells.

Treatment / finished water quality requirements, together with the chemistry /compositionof theproduced / fracwater,dictatethetypeoftreatmentthatwillberequired.Assuch,theleveloftreatment,andthusthecostoftreatment,mayvaryfrom

onelocationtothenext.

TREATMENTREQUIREMENTSANDCHALLENGES

Theleveloftreatmentthatisrequiredisdictatedbytheintendedapplicationorenduseforthe

treated produced water. Regardless of the intended application however, some form of

treatmentwilllikelyberequiredinordertomeettheregulatorycriteriaforthetargetedend

use.Ascleanwaterisascarceresource,treatingproducedwatermayhavesignificanteconomic

incentives,suchasitsexpandeduseasirrigationwater,processmake-upwater,orevenasa

drinkingsource(ProducedWaterUtilizationActof2008,H.R.2339).WhiletheexactchemistryandcompositionofproducedwatersisvariableitgenerallycontainshighconcentrationsofTDS

andvolatileorganiccompounds(VOCs).HydrocarbonproductsandVOCsmayberemovedusing

a range of conventional treatment systems, such as oil water separators, aeration systems,

dissolvedairflotation,andoxidation processes.Effluents fromtheseconventional treatments

usuallymeettherequirementsforsurfaceholdingpondsandsubsurfaceinjection;however,the

exceptionally high TDS concentrations in produced water present unique and substantial

challenges.The need forreducingTDS concentrations isespecially important forareaswhere


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salinity management is critical, such as for the Colorado River Basin [11]. High TDS

concentrationsarein fact,problematiceven forunderground injection,as a resultofmineral

scalingfromcalciumcarbonateandbariumsulfate,whichcanplugsubsurfaceformations.

Advancedseparationprocesses,whicharecollectivelyreferredtoasdesalinationprocessesare

required. Examples of desalination technologies include electrochemical processes, ion

exchange (IX), mechanical evaporation processes [multi-effect distillation (MED), multi-stage

flash (MSF)distillation, andvapor compression (VC)],capacitivedeionization, pressure driven

membrane processes, and non-pressure driven membrane processes (e.g., membrane

distillation, forward osmosis, electrodialysis reversal). Of these different demineralization

techniquesonly IX,mechanicalevaporationordistillation,andthemembraneprocesseshave

receivedwidespreadapplicationinthetreatmentofproducedwaters.Mechanicalevaporation

processeshavebeenusedtotreatproducedwatersfromavarietyof sourcessuchastheFort

McMurray, Alberta tar sands; however, evaporative processes suffer from a number of

drawbacks.Forexample,large-scalemechanicalevaporationsystemsareenergyintensiveand

complex.Nevertheless,evaporativeprocessesareinsomecasesthebestandonlyoptionfor

treatingchallengingwatersources(TDS>>50,000mg/L).Processeslikecapacitivedeionization

areintheearlystagesofdevelopmentandhaveyettobetestedonareasonablescale,thoughearlyresultsarepromising[7].Thefollowingsectionsareintendedtoprovideabriefoverview

ofaselectnumberoftreatmentprocessesthatarecommonlyusedintreatingproducedandto

someextentfracwaters.

PRODUCED WATER TREATMENT SYSTEMS

Pressure DrivenMembrane Processes. Pressure-drivenmembrane processes areperhaps the

mostwell known desalting technology and includeprocesses such asnanofiltration (NF) and

reverseosmosis(RO).NFisdifferentiatedfromROinthatitisprimarilyusedtoremovalmulti-

valentionslikecalciumandmagnesiumandiscommonlyreferredtoasmembranesoftening.In

addition to NF and RO are several design variations that are meant tomitigatemembrane

foulinginanattempttomaximizetheachievablefeedwaterrecoveryratio.BothNFandRO

havelongbeenusedfortreatingsalinewatersources inmunicipalandindustrialapplications

[12-14],includingproducedwaters[6,7].Themodulardesign,smallequipmentfootprint,low

laborrequirements,andsuperiorproductwaterqualityallmakethemanattractivetreatment

option for producedwaters [2]. NF and RO are considered to be high-pressure membrane

processesastheytypicallyrequirefeedpressuresintherangeof100to1,000psig.Suchhigh-

pressure requirements arise fromthe relativelyhigh osmotic pressures that characterize the

feedwaterstotheseprocesses.

Pressure driven membrane processes utilize a semi-permeable membrane to separate

suspended and dissolved contaminants from a feed solution. Here, pressure is applied to a

feedwaterinordertoforcethewaterthroughthesemi-permeablemembrane,whichretainsthesalt(s)whileallowingwatertopassthroughasaresultofdifferencesindiffusivitybetween

thesoluteandwatermolecules.Becauseitisaseparation,andnotatreatment,processtwo

liquidprocessstreamsareproduced:i)acleandemineralizedproductwater(permeate)andii)a

rejector concentrated brine solution (concentrate).The operatingpressure inthemembrane

systemmustbegreaterthanthesolutionsosmoticpressureinorderforwatertoflowfromthe

feedsolutionandacrossthemembrane.Becausetheosmoticpressureincreaseswithincreasing


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salt concentration the pressure and pumping requirements will increase with TDS

concentrations.

The high-pressure feed pump is the largest energy consumer in high-pressure membrane

processes.Secondaryenergyconsumingdevicesincludetheconcentrateandpermeatebooster

pumps(ifrequired).Forsaltrejectingmembranesenergyconsumptionisdirectlyrelatedtothe

TDSconcentrationinthefeedwater,whichalsoultimatelydeterminestheachievablerecovery

ration(Qproduct/Qfeed)forthedesaltingprocess.Saltsimpartanosmoticpressurethatmustbe

overcomeinordertotransportwateracrossthemembrane.Thus,greaterfeedpressures,and

in turn pumping requirements, areneeded for higher salinity waters. Furthermore, practical

recoveryratiosforfeedTDSconcentrationsof36,000mg/Lare50%,withthisratiodecreasing

asTDSincreasesbeyondthisvalue.ThismeansthatforaproducedwatercharacterizedbyaTDS

concentrationof36,000mg/Lhalfofthewaterwillleavethetreatmentsystemascleanwater,

whiletheotherhalf(i.e.,theconcentrate)muststillbedisposedof.Thisisacriticalconcernfor

producedwaterswheretherawwaterTDSconcentrationmaybemanytimesthatofseawater

(TDS ~ 36,000 mg/L). Thus, concentrate disposal is a significant cost and environmental

consideration for desalting membrane processes. Membrane fouling is another important

consideration because it reduces membrane permeability and necessitates higher feedpressuresinordertomaintainadesiredpermeateflux.

Pictureofatypicalreverseosmosis(RO)desalinationtreatmentsystem

Theimportanceoffoulingpointstothesignificanceofimplementinganeffectivepretreatment

scheme inorder tominimizeenergy costs. This isa particularly relevantpoint for produced

watersastheymaycontainrelativelyhighconcentrationsofrecalcitrantfoulantmaterialssuch

as oils, greases, and dissolved metals [1]. Efforts to overcome fouling have led to the


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developmentof uniquedesignapproachesfor pressuredrivenmembraneprocesses,someof

whichareoutlinedbelow:

Vibratory Enhanced Membrane Process (VSEP): VSEP is a proprietary compactmechanicalmembranesystem (NewLogic Research,Oakland,CA) that canprocess

highsalinitywatersanddilutesludges.VSEPhassuccessfullybeenusedinmorethan

200 commercial-scale industrial applications treating extremely challenging source

waters(highTDS,highsolidscontent).Thesystemconsistsofaseriesofdisk-shaped,

flat-sheetmembranesattachedtoacentralshaft.Theshaftrotatesashortdistancein

onedirection,andthenreversesitself,atafrequencyof50to60timespersecond.At

the outer edge of the membrane disks, the amplitude of the oscillation can be

adjustedtobetween0.25and1.25inches.TheoscillatingmotionintheVSEPsystem

allowsNFandROmembranes to treat high TDSsourcewaters (e.g., 300,000mg/L

TDS)suchasthoseproducedbyshalegasactivities.Theoscillationreducesmembrane

foulingbyincreasingtheshearforcesandmixingatthemembranesurface.Thisaction

significantly reduces foulant deposition and the thickness of the concentration

polarizationlayerthatformsatthesurfaceofsaltrejectingmembranes.Bothofthese

actions would, if not controlled, contribute to a significant loss of permeate fluxthrough the membrane. The shear action prevents the formation of a continuous

scaleonthemembranesurface[15].Instead,themineralsnucleateandformcolloids

in the bulk solution. This allows the VSEP process to achieve higher raw water

recoveries, and treat waters having substantially higher TDS concentrations, than

conventionalROsystems.Furthermore,VSEPiscapableofprocessingsourcewaters

that have high concentrations of suspended solids and organic materials, thus

minimizing the amount of pretreatment requirements. Despite its promise the

application full-scale VSEP systems in produced water treatment applications has

been limited, likelyas a resultof comparatively high energyandcapitalequipment

requirements.

HighEfficiencyReverseOsmosis(HERO): AnotherROdesignapproachthathasbeendevelopedforincreasingtheachievablerecoveryratioforhighsalinitysourcewaters

is the high-efficiency reverse osmosis (HERO) process. Here, scale forming

compounds(Ca,Mg,Si)areremovedbeforetheROstepusingasofteningprocess.

Silicaprecipitationin theROprocessismitigatedbyoperatingata high solutionpH

(pH>9).CollectivelytheseeffortsreducemembranefoulingandallowfortheRO

systemtooperateathigherrecoveryratiosthantraditionalRO.Whileitispossibleto

achieve high feedwater recoveries (>90% in some cases), the consumptiveuse of

chemicalsissubstantialandthechallengesassociatedwithhighosmoticpressuresfor

highlysalinewatersremainsanissue.

Non-Pressure DrivenMembraneProcesses. Non-pressure drivenmembraneprocessesutilizemechanisms other than hydraulic pressure to transport water across a membrane barrier.

Examplesofnon-pressuredrivenmembraneprocessesincludemembranedistillation,forward

osmosis,andpervaporation.Whileeachoftheseprocessesaredescribedingreaterdetailbelow

it is prudent to point out that few of these processes are currently used in treating

produced/fracwaters.Forwardosmosishasperhapsreceivedthemostapplicationinfull-scale

settings.Someof theadvantagesand challengesthatareassociatedwith thesenon-pressure

driven processes arehighlighted in Table 2. Little cost data is available for thenon-pressure


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drivenprocessesthatarediscussedinthisreport;however,whereappropriatereferencewillbe

madeastotheuniquedesigncharacteristicsforeachprocessthatcanimproveorhindertheir

costcompetivenesstomoretraditionaldesaltingtechnologies.

Membrane Distillation (MD). Membrane distillation (MD) is a thermally driven separation

processthathasreceivedattentionasapossiblewaterandwastewatertreatmenttechnologyin

applications such as desalination and water reuse [16-18]. In contrast to processes like RO,

which utilize pressure as a driving force for mass transport, MDutilizes the vapor pressure

differenceacrossamembrane[18].InMD,thevaporpressuredifferenceisaffectedbydifferent

parameters [17]; however, the thermal gradient across the membrane is the primary

mechanism for mass transport. Water vapor is transported from the feed, which is at an

elevated temperature relative to the permeate side, across a hydrophobic microporous

membraneandintoacondensingmedium[17].ThereareavarietyofMDconfigurationswhich

maybeused[18];however,auniversalcriticalprocessparameteristhemaintenanceofthe

liquid-vaporinterface(i.e.,liquidwatercannotpenetratethemembranepores,whichrequires

theuseofadurablehydrophobicmembrane[19].

TheprincipleadvantageofMDderivesfromthefactthatitisathermally,andnotpressure,driven separation process. Therefore, MD does not need to overcome the high osmotic

pressures that characterize producedwaters. For this reason,MD is an attractive treatment

technology for produced waters because it is not osmotically limited like pressure driven

membraneprocesses.Additionally,MDrequiressignificantlyloweroperatingtemperaturesand

thus has lower energy requirements relative to mechanical evaporation processes. It is

importanttobearinmindthoughthatawasteheatsourcemustbeavailableinordertoallow

theMDprocesstofunction.Intheabsenceofaheatsourcetheenergyrequirements,andthus

thecosts,associatedwithMDcan increasedramatically. Finally,because non-volatilesolutes

cannotbetransportedacrossthemembranebarrierinaMDsystem,itis capableofachieving

near100%rejectionofdissolvedsaltsandminerals[17].Forthesereasons,MDisa promising

technologythathasprogressivelygainedattentionas a treatmentalternativeforhigh salinity

sourcewaters[17,20].

ForwardOsmosis.Forwardosmosis(FO)operatesontheprocessofnaturalosmosisinwhich

waterflowsfromanareaoflowsaltconcentration,acrossasemi-permeablemembrane,toan

areaofhighsaltconcentration inanattempttoreachanequilibriumstate(balancingoutthe

osmotic pressure difference between the two solutions. FO is sometimes referred to as

engineeredosmosisasanosmoticagentisusedtodrawwaterfromasalinefeedwater,suchas

producedwater,intoadraworcapturesolution.ThetwomostcriticalcomponentsinanFO

systemaretheosmoticagentandthemembrane.Tobesuccessfultheosmoticagentmustbe

highlysolubleinwater,beeasilyrecovered,andimpartahighosmoticpressurewhendissolved

insolution.Themostpromisingosmoticagentsincludevarioustypesofammoniasaltsbecause

they can be relatively easily recovered from solution and reused. There are a few FOmembranesandsystemscurrentlyonthemarket(seee.g.,HydrationTechnologyInnovations);

however,aninherentchallengewithFOprocessesisconcentrationpolarization.Concentration

polarizationcanoccuronboththefeedandpermeatesidesofthemembrane,aswellasinthe

membrane interior (internal concentration polarization). All of these types of concentration

polarization act to reduce the osmotic pressure gradient between the feed and permeate

solutions resulting in a reduction in the permeate flux rate. This is particularly challenging

becauseitisdifficulttoovercomeoperationally.Forexample,inpressuredrivenprocessesitis


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possibletoincreasethehydraulicpressurethatisappliedinordertomaintainaconstant flux

ratewhileaccommodatinglossesinfluxasaresultoffouling.Conversely,increasingtheosmotic

pressuregradientrequirestheadditionofgreaterquantitiesofosmoticagentand/orincreased

mixing at themembrane surfaces. Regardless of theaction taken concentration polarization

posesasignificanthurdletothewidespreadapplicationofFOintheproducedwatersector,

becauseofthealreadylowfluxratesthatcharacterizethisprocess.Nevertheless,advances in

membranematerials,processdesign,andnewtypesofosmoticagentsarepromising.

Table2. Advantages andchallengesassociatedwith different non-pressure drivenmembrane

processesinproducedwatertreatmentapplications.

Process Advantages Challenges

MembraneDistillation

Lowpumpingrequirementsresultinginlowenergy

footprintassumingwaste

heatsourceisavailable Capableoftreatinghigh

salinitysolutions(TDS>

50,000mg/L)

Requireswasteheatsourcetodrivemasstransport

Lackofcommerciallyavailablemembranes

Susceptibilitytoporefloodingfrommembrane

foulingresultinginlackof

ionrejection

Largelyunprovenatfull-scaleinstallations

ForwardOsmosis

Withproperselectionofosmoticagentitiscapable

oftreatinghighsalinity

solutions(TDS>50,000mg/L)

Pumpingrequirementsarelowasmasstransportis

drivenbydifferencesin

osmoticpressure

Recoveryofosmoticagentcanbetechnicallyand

economicallychallenging

Concentrationpolarization(internal,external)

dramaticallyreducespermeatefluxrates

Comparativelylowfluxratestopressuredriven

membraneprocesses

Fewfull-scaleinstallationsandlimitedcommercially

availablemembranes

Ion Exchange. Ion exchange (IX) is a process in which ions are exchanged between an ion

containing solution and a bed of synthetic resin beads (adsorbent) presaturated with

noncontaminantions,suchassodium(Na+),chloride(Cl

-),hydrogen(H

+),orhydroxyls(OH

-)[21].

Using IX it is possible to selectively remove nitrogen compounds, hardness (i.e., water

softening),andmonovalentionslikesodiumandchloridefromaqueousstreamsandhaswidely

been applied inmunicipal, industrial,and residentialapplications.While IXmaybeused ina

widerangeofapplications,watersofteningwithgelresinsremainsasthemostwidespread.In

mostcasesIXisrestrictedtoapplicationswhereultrapurewaterarerequired(industrialmakeup

water) or for water softening (residential applications); however, in some instances it is

applicable to the treatment of more challenging feed streams such as produced waters.


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Specifically,theapplicationofIXfortreatingproducedwaterswillbedependentontheionic

composition of the feed stream as it ismost appropriate for waters composedprimarily of

bicarbonate ions (HCO3-).Thisformof treatment is termed strong base IX, where the OH

-is

exchangedforthebicarbonateion.InthesecasesIXiscapableofeffectivelytreatingproduced

waterstohighstandards.UnlikemembraneprocessesIXdoesnotuseasemi-permeablebarrier

to separate the dissolved salts and minerals from water. Instead, IX is both an adsorption

processandachemicalreaction.Itresemblesadsorptionbecausesolidparticles(resin)areused

and regenerated, while the chemical reaction specifically applies to the regeneration of the

resin.

IXrequiresarelativelyhighqualitysourcewaterthatis freeofparticulates, foulantmaterials

and other competing ions for the exchange sites in the resin. Therefore, its application has

primarilybeenrestrictedtothetreatmentofCBMproducedwaters,whicharerelativelyfreeof

contaminantsoutsideof theaforementioned bicarbonates, andwaters that have undergone

extensivepretreatment.TheprimarycostsassociatedwithIXaretheresin,regenerationofthe

resin, capital equipment (pumps, motors, IX columns), and disposal costs associated with

disposaloftheregeneratingsolutionfortheIXresin.

MechanicalEvaporation.Mechanicalevaporationprocessesareinmanycasestheonlysuitable

optionfordisposingofhighTDS(TDS>50,000mg/L)wastewaters.Theseprocessesinvolvethe

evaporationofwaterthroughavarietyofmeanstoultimatelyproduceasolidscakecontaining

allofthedissolvedsolidsthatwerepresentintheproducedwater.Theseprocessesareenergy

intensiveandthusareassociatedwithsubstantialcapitalandoperationandmaintenancecosts.

However,as previously stated they arein manycases theonly optionwhen disposalof high

salinity produced waters is required. Summaries of some example mechanical evaporation

technologiesthatmaybeusedintreatingproducedwateraregivenbelow.

Multi-EffectDistillation(MED):MEDisanestablishedprocessfordesaltinghighsalinitywaters (TDS > 36,000 mg/L). In a MED system, water is boiled in a sequence of

evaporators,eachheldatalowerpressurethanthelast.Eachevaporatorintheseriesis

calledan"effect".Becausetheboilingpointofwaterdecreasesaspressuredecreases,

the vapor boiled off in one vessel can be used to heat the next, and only the first

evaporator (the one at the highest pressure) requires an external source of heat.A

reduced pressure in the vapor space of the first evaporatormust bemaintained to

account for the difference in the boiling points of pure and saline water. Another

requirementtomaintainreasonableheatexchangebetweenthepipescontainingthe

condensingsteamandthosewiththeboilingproducedwater,thetemperatureof the

producedwatermustbeseveraldegreeslowerthanthatofthecondensingsteam.MED

systemstypically operateata lowtemperatureof 71.1Cand a high temperatureof

110C. Operating at lower temperatures limits corrosion and these systems can be

constructedoutoflessexpensivematerials.Theamountoffreshwaterproducedperunit amount of heating steam increases almost proportionally with the number of

stages.While in theory,evaporatorsmay bebuilt with anarbitrarilylargenumberof

stages,evaporatorswithmorethanfourstagesarerarelypractical.

Multi-Stage Flash (MSF)Distillation:MSFdistillssalinewaterbyflashingaportionofthe feedwater into steam inmultiple stages. InMSF the produced water is heated

underhighpressureinordertopreventboiling,untilitreachesthefirstflashchamber.


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In the flash chamber the pressure is released and sudden evaporation or flashing

takesplace.Flashingcontinuesineachsuccessivestage,becausethepressureislower

asyouprogressfromonestagetothenext.Thewatergainsheatasitpassesthrough

eachstagebycondensingvaporsthataregeneratedbytheflashingprocess.Thesteam

iscondensedontubesofheatexchangersthatrunthrougheachstage.MSFtreatment

systemstypicallyutilizeawasteheatsourceinordertoreducetheenergyconsumption

byone-halftotwo-thirds.

MechanicalVaporCompression(MVC): MVCisgenerallyusedforsmall-andmedium-scale (Q = 0.005 to 0.5 mgd) desalination systems. The heat for evaporating the

producedwatercomesfromthecompressionofvaporratherthanthedirectexchange

ofheatfromsteamproducedinaboiler.Theboilingpointofthewaterisreducedby

reducingthepressurethatisappliedtoit.Twomethodsareusedtocondensethevapor

soas toproduce enough heat toevaporate incomingproducedwater: amechanical

compressororasteamjet.Themechanicalcompressorisusuallyelectricallydriven.

Mechanical zero liquid discharge (ZLD) systems also fall under the category ofmechanical

evaporation systems and include thermal evaporators, crystallizers and spray dryers. ThesetreatmenttechnologiesarecommonlyusedincombinationwithROsystemsinordertoachieve

azeroliquiddischargestatus(i.e.,noliquidwastestreamresultingfromtreatingtheproduced

orfracwater).Thecapitalandoperationalcostsforthesethermalsystemsaretypicallyhigher

thanforthedesalinationmembranefacilityduetotheextensivemechanicalsystemsandexotic

alloy materials required. In addition, the energy costs associated with the evaporation

processingaresignificant.Zero liquiddischargesystemsultimatelyreducetheconcentrateor

producedwatertoasolidproduct(crystallizedsaltsandminerals)forlandfilldisposal.Insome

cases, the water vapor is recovered. A summary comparison of mechanical evaporation

processestoROisgiveninTable3.

Table3.Comparisonofperformancestatisticsformechanicalevaporationandreverseosmosis

(RO)desaltingtechnologies.

ProcessEnergyUse

a

(kWh/1,000gal)Methodof

OperationSystem

Recovery(%)Relative

CapitalCosts

MSF 58 Steam(heat) 1020 High

MED 29 Steam(heat) 2060 MediumtoHigh

MVC 3053Compression

(heat)3599 High

RO 823 Pressure 3555b LowtoMedium

Notes:

aCombinedelectricalandequivalentthermalenergy. bRecoveryratiosareafunctionofthefeedwaterTDSconcentration.Recoveryratiosincrease

beyond50%asthefeedwaterTDSdecreasesbelowapproximately36,000mg/L.


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WORKCITED

1. Benko, K.L. and J.E. Drewes, Produced Water in the Western United States:

Geographical Distribution, Occurrence, and Composition. Environmental Engineering

Science,2008.25(2):p.239-246.

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In an ever changing world with finite resources, it is now more critical

than ever for America to tap into its existing energy resources: oil and

natural gas. Domestic energy development activities are setting us free

from the reliance of foreign imports. But with this freedom comes achallenge: how to manage the vast quantities of water that are

produced and consumed in the process.

Meet the Nexus Group. A team of forward thinking specialists who havededicated the last 10 years to researching cutting edge technologies

and solutions to this very issue.

Our mission: To bridge the gap between a sustainable environment and

a sustainable economy, one drop at a time.

JONATHAN A. BRANT, [email protected]

NexusGroupSolutions.com

office: 307.766.5446

cell: 307.275.2677

The Nexus Group has prepared this report for the sole use of the Client and for the intended purposes as stated in

the agreement between the client and The Nexus Group under which this work was completed. The report may not

be relied upon by any other party without the express written agreement of The Nexus Group.

Any recommendations, opinions or findings stated in this report are based on circumstances and facts as they

existed at the time The Nexus Group performed the work. Any changes in such circumstances and facts upon which

this report is based may adversely affect any recommendations, opinions or findings contained in this report.

The Nexus Group does not make any warranty, express or implied, or assume any liability or responsibility for the

accuracy, completeness, or usefulness of any third party research, information, apparatus, product, or process

disclosed, or represent that its use would not infringe privately owned rights. Reference herein to any specific

commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily

constitute or imply its endorsement, recommendations, or favoring by The Nexus Group.

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