Effluent English

7/30/2019 Effluent English

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Reiniectionof effluentwater from oil dehydratorand desalter

into main pay formation of SouthRumailaoil fieldBy Dr.AyadA. Al-Chalaby

Abstract

Effluentwaterproduced sa resultof crudeoil dehydrationnddesaltingrocesssnormallydisposed ff through njection nto Dammam ormationwith consequentpollutionof the aquiferand he high cost nvolved n the upkeep f injectionwellswhichare subjectedo frequent logging. hepresent aperooks nto theprospects

of reinjecting heeffluentwater,which s composedmainlyof theproducedormationwater,back into the formationafter removingall contaminantsuch as oil droplets,sand, ust, oxygen,Bacteriaand he rest. t also nvestigatesn preliminary orm, theinjectivity of the water nto the formationstructure nd he compatibilityof injectedwaterwith addedportionof river washwaterprovidedat the entrance f the desalter.A suggestionor a pilot plantfor the treatment nd monitoringof the effluentwaterprior o reinjectionsproposed.

1- Introduction

As the water cut pushesup with time, formation water will be produced with thecrude oil in an increasing ate as the production goeson. In most producingwells ofRumaila fields, the formation water is expected o reach20% of the total produced

fluid in the near future. For a net production of one million barrels per day, thisamounts to nearly quarter a million of wastewater every day. Add to this theadditional wash water quantity consumed at the rate of -5Yo of net production, thetotal amount of effluent water to be disposed off is quite large. This present a realenvironmental hazard, both for the Dammam aquifer and the soil surrounding the

production stations,since the disposal s diverted toward a make shift pond in case heinjectionwells areblocked,which occurs requently.

Beside the environmentalproblems, a valuable water resource s wasted which canbe utilized to supplement he water injection requirementof the field, if properly

treatedand managed.At the moment, high quality fresh water for water injection isprovided from Garmat Ali River after it undergoes reatment to reduce the TSS toabout3 ppm with lO-micron maximum particle size n addition o the introductionofpesticides and anti-corrosion additives. The re-use of effluent water for waterinjection purposeswould ultimately reduce he fresh water requirement by some 15oloin southRumaila field.Although the preparation of effluent water is expensivebecauseof the high salinity,

which may exceed 200,000 ppm and the various contaminations, t is apparent hatmaking use of these arge amountsof water is both cost effective and preserving othe environment.



2- Efflent water analysis

For the purposeof this investigation,Central degassing tationof SouthRumaila(CDS) was

consideredbecauset is the largestaniloldest station n the field. Threebanks(A,B andC)are ittedwith electro-static ehydration/desaltingnits with ourpurcapacityof 80.000BBD each. n fig.1, the schematic rrangement f th. dehydratorsshown,wheremost of the effluent water is removed(-g};/i. The remaining raceofwater and salt will be removedat the desalter,where a small amountof fresh water(-5%) is added for this purpose.The effluent water coming out of the desalter srecycledback nto thedehydrator r by-passed irectly to theskimmer.

Wetoil fromfirststagedegasser

RecycledWaterfrom desalter

Oil back o

Testsamplewere aken rom threewet crudeSouth Rumaila field during April 2000. Thevariousocationd re istedn Table .

Fig.l- Schematic iagramme or a dehydrator of CDS

Gas nto flare

ffluent waterto disposal

units in CentralDegassingstations naverage (Tlpical) data obtained at

Table 1- Typical test data for effluent water at different locations orBank A in central Degassing tation of south Rumaila field



Fourmaincharacteristicsanbe drawn romobtainedesults f table :

l- The high salinity of water with TDS in excessof 200,000,and total

hardness f 60,000 lus.2- Too highoi l content,whichexceeds y far the design alueof 50ppm.3- High iron content sa resultof multiplecorrosion.4- Low pH values which can be attributed to the high content of carbon

dioxide generated t the formation as a result of the oxidation of organicmatter with continuoussupply of air from the surfacevia water iniectionsystem.

3- Corrosion tests:

High iron ions are indicative of the high conosion rate of transport system andtreatmentvessels nitiatedby the saline, ow pH water phase n the Crudeoil stream.To investigate the progressof corrosion from the wellhead to the dehydrator,6 wellsand threedifferent bankswere tested or iron content.The resultsare isted in table 2.

Table 2 -Build up of Fe ions or 6 oil wellsand 3 banks nCentral degassing tationof SouthRumaila

Test results have indicated that both banks A and B receive crude oil whichcontains formation water of high salinity and low pH with high degreeof corrosionrate. This is especiallypronounced or bank A, wherethe Fe contentrisessharplyas itgoes hrough the systemfrom 120 at wellhead to 350 ppm at dehydratorouttet. HigtrFe content at wellhead is attributed to corrosion of the poduction tubing and casing.Bank C on the otherhanddoes not show any appreciablecorrosion, mainly due to lowcontent of dissolved solids in the formation water and neary normal pH value of 6.9.The reasonbehind this is believed to be the proable break through oi injection waterinto the producing formation with the obvious dilution.

Further investigation was requiredrate on test specimenssubjected oMild steelcouponswith areaof 17.8period of 72 hours in an oil bathsimulated:

in the laboratory o establish he actual corrosionsimulated environment of the formation water.cm2and densityof 7.8 glcm3were tested over aat 70o C. Four conditions for the water were

;Lepgthqf ,l.rilftowind .

'+Jl(,ni,i,.;

:.Ieat outlet

i$,ffiR 5 5 l , , r , '178,000 12 0 t'.,.,;,:: 1600 .) .+l1'.359R62. ,

; ' ; 178.000 133.,,. i,r,iff.ri3000..l.'; ,r.$ I 185 i'+,.-.r t ' . 3 5 0 ' ' j

B:,r R 52:ii "157.000 218 :. 300 l : 230 i . + , . i ;295R 132 , i . i165.000 i: ;4000, ; ' : ..,, '65 :';: '

295 , . ' . ' ,1'Ct,;,,:i11!

::i:fii?ii.r:_iii*.i$:;l

R 7 4 ' : , iirv27.000 0 . 3 : ' : i . , i tri6300 0.8 ,r.'iL,"l: r : : : : '" :

i r 1 0 : i . ' : i

R 158 :, ; :i25,000 f,-"+3000' :

9 .6 'o ; - r ' . . 1 : l 0 :



1- Effluent water from the dehydratorwithout any further treatment2- Effluent water,neutralized o pH=8.03- Effluent water,neutralizedanddeoxygenatedy injectingHelium gas4- Effluent water, neutralized,deoxygenated, nd treatedwith 100 ppm

SodiumDichromate scorrosionnliibitor

Following est esultswereobtained:

Testmode Number of coupons Average loss W(mg) p PR(1tm/Y)r1;.; 1 29.8, 41.08; ,

':'::2i;tiiii/ : ' . . 20.4 , 28.12

'::

i1'3r.:: ::::)::::: 79.2 26.49, t5.8 8.0

of

Table3 - Averageweight lossandcorrosion or Effluent water in DPR due to generalfour testmodes

Thedepthof penetrationate n pm peryear s calculatedromtheequation;

DPR=0.s43(LW)39.4pAT

where:DPR Depthof penetrationate pm y )

AW: Weightoss mg)A : Areaof coupon17.8 m2T : Time 72hours)p : Density f coupon7.8 glcm3)

Table3 indicateshat the high salinityof water s the main factorgoverninghegeneral orrosion f mild steel.Neutralizinghe pH valueand removingdissolvedoxygenhelps o reducehe corrosionateby nearly35%.Theuseof anti-corrosionagent n heotherhandhelpso reducehemetalossdrastically.

4- Comrratibilitv ests:

Scale endencyof injectedwater s ofmajor importance,since depositionoflarge particles inside the formation mayclog the pouresproressivelyand lead tocessationof flow. For this purpouseeffluent water compatibility was tested,with various ratios of mixtures withfresh wash water. In fig.l, the Index

(K) values for CaCO3 and CaSO4aremeasured and recorded for effluentwater without mixing with freshwater.

Fig.l- Scale ndex of CaCO3 and CaSO4fo r Effluent water of South Rumaila Field

o ^= - zii> - ax '0)

!c - o

-8

-10

-12



Actual K valuesat full ionic strengtharehigher han recordedand for positive valuesof CaCo3 with tendencyo risewith temperature,omescaleat formationconditions

is anticipatedwhich canbe treatedwith the properscale nhibitorNo precipitationsexpected or CaSO4.Dilution with freshwater ndicatea small ncreaie n the indexvalue of CaCO3 at low fresh o effluent waterratio, but reducesappreciablyat higherratioS or all temperaturesfig.2).

Fig.2- Index valuesor CaCO3at3 different temperatures as

function of Effluent waterdiluted with fresh water

5- IniectivitvTests:

Testwasperformedo establishhe formation amage xpectedrom injectionofwaterof varyingquality.Coresof high permeability.o* Zubair ormation n SouthRumaila ield were-selectedor this purpose. 9suth coresrom wellsnumberRg4,R99'R107,Rl10, Rl35 andR141werepreparedn diam. f 3.75cmand7.0cm ongandcleanedwith Toluenehendried n anelectric oven at 70o C for 6 hours.Following procedureswere ollowed:

1- Porosity was measured by usingNitrogen gas in a Core labPorosimeter evice.

2- Effluent water was treatedby 02 andFe reduction, chlorination andpassing hrough 8 micron filter.

3- Tests were carried out at formationconditions with compatibilityestablished t 70oC

4- Initial porosity was measuredafterpassing the water through 0.4micron filter.

5- Formation damageafter actual estswas assessed by reverse flowthrough cores after passinga 0.45micron filter

Fig.3- Core from well R l4l afterinjection rvith Fresh water mixed with107' of effluent water

?

o ^ -

(t

o

= zU)

1 . 5o

E1xc,

E o.s

20 40 60 80

Freshwaterdilution %



Table 4 lists the main test values or tliree coresand threemixtures of effluent-freshwatercontent'The coredarnages a measure f the rateof diminishingpermeability

expressesy the relativewaterpermeability rw definedby ;

k ..,,, Meaured permibilityI \ r 'w:

Initial - permiabilitv

Table 4- Injectivity testparameters or three cores rom SouthRumaila wellswith effluentwater, freshwater and a mixture of both.

On fig'4, the relative permeability s plotted n the range 100-2400 c for normal testflow after passing hrough an 8 micron filter and in the range 100 to 500 cc as areverse low after finishing the normal test flow and by passing hrough0.45 micronfilters to eliminateanyclogging.

120

; 100L

!<

i 80.cl.s 60

t40(,

E20(!

ou0

,r l]r

10o/o*,,,';,',Etflueht

0 1000 2000 30oo o 200 400 600Accumulativenjecteduantity ( cc ) (cc)

a) Filtered8 micron b) Filtered0.45microns(Normal flow) ( Reverse low )

Fig.4- njectivity testresults or three coresof South Rumaila main payFormation with effluent fresh water compositions

Test resultsshow an obvious rend or formationdamagewhen filtered freshwater isinjected reaching a relative porosity of 40o/oat reverie flow. An improvement is

: Effluent :'.,,,watei,.:/o,

, in'mixtuid,.

Core

from

well

Core:depth

( m ) , r l' : : : : : . ' : :, ' . (%\

Porosity :,Pout' ..Volume

:(cc)::,

:Kw-fnitialpermeabilify

(mD)

Volume

injected(cc)

'Co ie

iiut"ig": l : : l ; t : ': :(O/o\ ':"

100 R84 3293 ' . ) . , 18.6 ': 1:73:, 2400",,L6',90 R84 3304 19.3 16.3 2st t , / ) ,QQ, 89

0 . , .Rl07 3316 i ,,19

16.g i 856 4300 ,72',:

\ : ' .

; . . . , , , . r , , i t , i#U

. 'r'.:'3;l

"li:i",



recordedwhen l\Yo effluentwater s added eachingKrw:60%.pure effluent wateron the other hand showedno formationdamagewittr'retativepermeabilityexceeding100%' The reasonsbehind the deterioratingporosity of fresh water and its mixturewith effluentwaterare hought o be dueto ii'"omputiuifity ano presence f bacteria.

5-Fur therAssessments

Injectivity estsperformed ofar are by no meansconclusive nd thereforeahoroughestprogrammes planedo precedehe njection cheme. heprogrammeincludeshe ollowingests:

a-water susceptibility: o fullyevaluatelayswelling,inesmovements ndcriticalvelocityandestablishheprog.u--" foron-siteor. n%ooing.clay swellingcharacteristicsr movement f fines s to be evaluatedy floodingselectedlugswith filteredsimulatednjection "J;.;aher establishingermeabilityto simulatedormationwater.Any finesevolved rorntheplugmatrix*iir u" trappedon filter membraneun in linedownsj...ur:Scanning lectronmicroscopesEM) iso be used o visuallyshow he clayblockirg .rr"."&J.irti.r, their fypeandgrowthstructurewithin thepores.

b- water analysis To includeanalysis nd evaluation f producedwater withrespect o water injection requirements.haracteristicsike solid contentsanddistribution,oil-in water content, urbidity,pH ;;l;;; microbiorogicalype andoncentrationndcorrosively il l be nvestigatedothon-rit.and n th-eaboratory.

c- Permeability eduction on-site estson several oresaken romtheproducingzone o determine ptimumnjectionwaterqualitywithminimumblockingendency.Proceduresike XRD clay.anaiysis,asporosityandpermeability easurementsndvacuum aturate lugswill beappliedo determinerecautionsnd rquir._.n* ,overtblockage.

d- compatibility study :. scale predictions or sulphateand carbonate calesdepending n thermodynamic,ctivitycoefficient,und^ ommon on effects o beerformed'varying effectssuchasPFivalue,coz content,Effluent/wash

water atio,temperaturendpressure il l beconsidered

6- Pilot rrlant :

. fhtintendedproject nvolves he introductionof new systemsor the treatmentandnjection of highly salty water at 20 different productioi'stations in the south onlywith ultimatepumping-capacityof 10 millions tonsper year.The capitalcost will beppreciableand if available, the implementation period

i-rruy"*"..d 3 years. For thiseason t was hought hat a pilot plant of'moderate ire to Le acquiredand nstalledatouth Rumaila Degassing iation to asses he true ..quir.-.nts and mishapsof therocessbeforeentering nto thefull scaleproject.



The pilot plant is sized o produce200008BL lday l million ton/year) f injectionquality waterwith following main features fig.5);

l- Existing system consistingof 12 producing wellheads, low lines,Manifold, Separation essel,Dehydrator,Desalterand freshwatersupply

' line will be dedicatedo tlie plant andprotectedagainst orrosion.2- Introduction of a new treatment unit downstreamthe dehydrator to

produce high quality water with max TSS of 3 ppm, 10 micron maxparticlesize andoil contentof 5 ppm.

3- Provide for sufficient storageund pr.pr to transfer he treatedwatervia3 new lines to the injection welis to be received by a mobile waterinjection PumP,which are allocated or the injectiott auty Juring ttreassessmenteriod.

4- Injection of variouschemicalsat different ocations n the systemsuchascorrosion inhibitors, demulsifiers, descarers, biocides, oxygenscavengers,locculantsandprecipitants srequired

Fig.5 is a flow diagramme showing the pilot plant equipment proposed forimplementation, which wereordered hroughthl memorandumoiunderstanding early200t .

7- References

l- Interim r:p_ort Feasibilitystudy or the treatmentof dehydrator ffluentwater"SOC,1999

2- Technicaleport surveyof SouthRumaila ffluentwater , Nalco-Exxon,April2000.

3- Michaluk, P.G. etal "Recent developmentof produced water fordisposal..."Gas& petrochemicalseminar, lmaty,Kazakhstan,1944.

4- Unidro roposalr62.00.p0l"producedater ilotplant', Jan200l5- GlobalprocessystemsroposalQ-D-00-102iProducedwaterpilotplant,,

N0v.200



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Effluent English