February22 - 23,2047
Marine Science & Technologt Seminar 2007
Study of Scale Formation in Oil Reservoir During Water fnjection-A review
Amer Badr Bin MerdhzhrUniversiti TeknologiMalaysia
amer0227@tahoacom
Abu Azam Mohd Yassin, Universiti TehnologiMaleysia
abuazo@fkhksautm.ny
Scale deposition is one of the most important and serious poblems which water injection systems are generally
engaged in Scqle sometimes limits or hlocks oil cnd gasproduction b pIuSgW the oil-pra&tcingformation matrix
or frac,tures and peforoted intervals. Scale slso deposited in down-hole pump& tubing, casing flow-lines, heater
treaters, tanks and other production equipment andfacilities- Scsle c6m occur at /or dawnstream af arry point in the
production yystem, at which super-sduratian is generated. Srper-saturation cun be generated in single water fu
changing the pressure and temperature conditions or by mixing two incompatible waters. The most coinmon oilfield
scales deposited are calcium carbonate, calcium sulfate, strontium sulfate qnd barium sulfate. This paper describes
overview formation damage, scale deposition mechanisms and source af oil field scale, scale formation along the
iryjection-water poth in water-food operations, scaling problens encotmtered in oil fields, salubility of scale,
oilfield Scale types, scale control chemicols and laboratory bwestigatians of scale in different media and
procedures used to predict scale are presented
Keywords: formation damage, scale deposition mechanisms, scaling problems, solubility of scale
1. Introduction
The injection of seawaterinto oilfield reservoirsto maintainreservoirpressureand improve secondaryrecoveryis a
well-established,mature, operation. Moreover, the degree of risk posed by depositionof mineral scalesto the
injection and production wells during such operationshas beenmuch studied. Scaledeposition is one of the most
seriousoil field problemsthat inflict water injecfion systemsprimarily when fwo incompatiblewatersare involved.
Two waters are called incompatible if they interact chemically and precipitate minerals when mixed. Typiq{
examplesare seawat6r, with high concentrationof SOa-2and formation waters,with high concentrationsof Ca*2,
Ba*2and Sr*2. Mixing of thesewaters,therefore,could causeprecipitationof CaSOa,BaSO+and/orSrSOaThere are
otherreasonswhy scaleforms, and the amountand location of which are influencedby severalfactors. Yet, superis the primary causeof scale
saturationis the most important reasonbehind mineral precipitation. A supersaturated
formation and occurs when a solution contains dissolvedmaterials which are at higber concentrationsthan their
equilibrium concentration. The degreeofsuper-saturation, also known as the scaling index, is the driving force
for the precipitation reaction and a high super-saturation,therefore, implies high possibilities for salt
pH, and CO2III2Spartial pressurecould also conrlbute in forming a
precipitation. Changesin temperature,pressurre,
(Mackay
et
al2003a).
et al,Z}O3,Moghadasi
scale
l.l CommonOil Field Scales
The most common oil field scalesare listed in Table l, along with the primary variablesthat affect their solubility
(Moghadasiet aI,2A03a). Thesescalesare sulfates such as calcium sulfate (anhydrite, glpsum), barium sulfate
(barite),and Strontiumsulfate(celestite)and calcium carbonate.Other lesscommonscaleshavealso beenreported
suchas iron oxides,iron sulfidesand iron carbonate.Lead and zinc sulfide scalehasrecentlybecomea concernin a
numberof North Seaoil andgasfields (CollinsandJordan,2001).
1.2ScaleDeposition
oil and gasproductionequipmenthasbeenrecognized.Scaledeposition
Scaledepositionin surfaceand subsurface
is one of the most importantand seriousproblemsthat inflict oil field water injectionsystems.Scalelimits and
sometimesblocks oil and gas productionby plugging the oil-producing formation matrix or fractures and
could
perforatedintervals. It can alsoplug production lines and equipmentand impair fluid flow. The consequence
failure, emergencyshutdown,increasedmaintenancecost, and overall decreasein
be production-equipment
productionefficiency. The failure of theseequipmentscould result in safetydangers. In caseof water injection
systems,scale could plug the pores of the formation and resultsin injectivity declinewith time (Todd and Yuan,
AsghariandKharrat,1995;Andersenet aL,2000;Pauloet al,200l; Voloshinet a|.,2003).
1992;Bayona,1993;
February22 - 23, 2007
Marine Science& Technologt Seminar 2007
Scaledepositioncan occurfrom onetype of water becauseof super-saturationwith scale-formingsaltsattributable
to changesin the physical conditions under which the rvaterexists. Scalealso can depositu'hen two incompatible
watersare mixed and super-saflrationis reached(Nassiveraand Essel, 1979;Readand Ringen, 1982;Yellet, et al.,
1982;ToddandYuan, 1992;Moghadasiet a1.,2003b;Moghadasiet aL,2A04}
Table 1: Most commonoilfield scales
Name
ChemicalFormula
Primarv Variables
CalciumCarbonate
CaOOI
Partialpressureof CO:,
temp€rature,total dissolvedsalts,
nH
CalciumSulfate:
Gypsum
CaSO4.2H2O
Hemihydrate
CaSOq.U2HzO
Anhydrite
CaS0a
Barium Sulfate
BaS0a
Strontium Sulfate
SrSOq
Temperature,total dissolved
salts,pressure
Temnerafure.Dressure
prcssure,total
Ternperature,
dissolvedsalts
Iron Cornpounds:
FerrousCarbonate
FeCq
FerrousSulfide
FeS
FerrorxH1'droxide
F{OID,
FerrousHydroxide
F{OH)3
Corrosion,dissolvedgases,pH
1.3 Sourceof Oil Field Scate
The chief sourceof oil field scaleis mixing of incompatible waters. Two waters are called incompatible if they
interact chemically and precipitate minerals when mixed. A t]"jcal exampleof incompatiblewatersare seawater
with hish concentrationof SO.r-'and low concentrationsof Ca", Ba-"/Sr", and formation waters with very lorv
of Ca*2,Ba*2 and Sr*2. Miring of thesewaters,therefore,causes
of SOa-2but high concentrations
"orr."otiutiorrs
precipitation
of CaSOa,BaSO+,and/or SrSOq. Field producedwater (disposalwater) can also be incompatible
with seawater. In caseswhere disposal *ater is mixed with seawaterfor re-injection,scaledepositionis possible
@ayona,1993;Andersenet al,20O0 Bedrikovistskyet a|.,2001;'Stalkeret al.,20[3;' Pauloet al,2A0\.
During the productiorL the rvater is drained to the surface and suffers from significant pressuredrop and
temperaturevariations. The successi\€pressuredrops lead to releaseof the carbondioxide with an increasein pH
value of the producedwater and precipitationof calcium carbonate(Mackay,2003).
Zinc sulfide scaleis more likely that the zinc ion sourcemixes with the hydrogen sulfide-rich sourcewithin the
near well-bore or the production tubing during fluid extraction. Lead and zinc sulfide scales have recently
becomea concernin a numberoil and gasfields. Thesedepositshaveoccurredwithin the productiontubing and
topsideprocessfacilities (Collins and Jordan,2003).
2. Scale Formation along the Injection-Water Path in Water-Flood Operations
is us:ally muchlower than reservoirtemperature.When it travels
At theinjectionwellheadinjectionwat€rternperature
down the rnjectionwell-string,the water coolsthe surroundingformations-and its temperanreand pressureincrease.
with increasingtemperatures(e.g.
If the water is saturatedat surfaceconditiorsrriith saftswhosesolubilitydecreases
anhydrite),scalemay form alongthe well-string.
of temperatureand
Scaleprecipitationfrom the injection ryatermay happenbehindthe mixing zoneasa consequence
with increasing
containing
salts
w'hose
decreases
particularly
rvaters
solubility
of
This
is
true
prs$re dranges
(rvith
present
in the rock
zone
resen'oir
is
mixing
only
brine
oil)
pressure.
of
the
Fonvard
temperatueand decreasing
pores.Betundthe mixing zone,onl.vinjectedwaler in equilibrium at local temperanreand pressure(with residualoil)
Marine Science & Technologt Seminor 2007
February 22 - X, 2407
of insohrblesaftsmay ocqn due to the interactio4 at local temperatureand
exists.In the mixing zone,precipiuation
pressure,of chemicalspecies
containedin the injectionwaterwith chemicalspeciespresentin the reservoirbrine.
Nevertheless,at a differentpressurqthe remainingclearwatermovesaheadmix againwith reseryoirbrine and scale
precipitationmay againake place. This cycle is repeateduntil the remainingclear water reachesa produclion well.
alongthe flow suing up to drc surfacein the prodtrctionwell, andfirrdrerchangesin
Pressureandtemperaturedecrease
thermodynamicconditionsoccw in the surfrce equipment. This may againresult in scale formation. Nonrnlly, these
scalesdo the most damagein the well-borewhentherearemajo'rftlls in pressurebut hardly any temperaturechanges
(Khelil et al,1979).
Therearethreeprincipal mechanismsby which scalesform in both offshore and onshoreoil field system(Mackay,
2005;Jordm andMackay,2005):
a) Decreasein pressue and/or incre'asein temperatweof a b,rine,goesto a reduction in the solubility of the
salt (mostcommonlytheseleadto precipitationof cartronatescales,suchas CaCOr).
Ca (HCO,), <+ CaCO3+ CO2+ H2O
(l )
b) Mixing of two incompatible brines (most commonly formation water rich in cations such as bariunr,
calcium and/orstrontium,mixing with sulftte rich seawater,goesto the precipitation of sulfatescales,such
asBaSOa).
Ba2"(or Sr2* or Ca2*) + SO42- <+ BaSOo (or SrSOn or CaSOo)
Q)
Other fluid incompatibilitiesinclude sulfide scalewhere hydrogensulfide gasmixes with iron, zinc or lead
rich formation \vaters:
(3)
7.n2*+HrS <+ ZnS+2gzn
c) brine evaporation,resulting in salt concentrationincreasingabove the solubility limit and goes to salt
precipitation (as may occur in HP/HT gas wells where a dry gas streammay mix with a low rate brine
streamresulting in dehydrationand most commonlythe precipitationofNaCl).
3. The ScalingProblem in Oil Fields
Scaling depositionis one of the most important and seriousproblemswhich water injection systemsare generally
engagedin. Oil field scalescostsare high due to intenseoil and gasproductiondecline,frequentlypulling of downhole equipment for replacement,re-perforation of the producing intervals, re-drilling of plugged oil wells,
stimulation of plugged oil-bearing formations, and otherremedial workovers through production and injection
wells. As scaledepositsaround the well-bore, the porous media of formation becomesplugged and may be
renderedimpermeableto any fluids.
Many casehistoriesof oil well scaling by calcium carbonate,calcium sulfate, strontium sulfate and barium
sulfatehave beenreported(Mitchell et al, 1980;Lindlof and Stoffer 1983;Vetter et al,1987; Shuleret aI,l99l).
Problems in connectionto oil well scaling in the Russia where scale has seriously plugged wells and are
similar to casesin North Seafields have beenreported(Mitcheli et aL,7980).
Oilfields scaleproblemshave occurredbecauseof water flooding in Saudi oil fields, Algeria, Indonesiain
south Sumatraoilfields, and Egypt in el-Morgan oilfield where calcium *6 su'safinmsulfate scaleshave been
found in surfaceand subsurfaceproduction equipment(El-Hattab,l982).The following is a brief explain of
scalingcasesreportedin the literature.
Mitchell et al. (1980) describedscaleproblems occurring in the forties field could be attributedtwo major
factors:
a) Commingling of forties formation and injection waters could precipitateboth barium and strontium
sulfates.
b) Precipitation of calcium carbonate scale from formation water due to variations in pressure and
temperaturein production systems.
ToddandYuan(199) describedbarium sulfate scaleoccurrencewas a severeproductionproblem in North Seaoil
operations. Barium sulfate is often accompaniedby shontium sulfateto form a completelymixed scalecalled @4
Sr) SOr solid solution. Sulfate-anion-richseawaterinjected into the reservoir formation subsequentlymixed with
formation water,which containsexcessivebarium and strontium.
Bayona(1993) reportedtwo major problemswith seawaterinjection in the norttr Uthmaniyahsectionof the
Ghawarfield in SaudiArabia. The first is maintenanceof acceptablewater quality to preventexcessivelossesof
well injectivity and the secondis control of plugging in the pores and corrosion at a reasonablelevel in the
eguipmentdue to which excessivelossesof well injectivity occur. The only cause of these losses is the
depositionof scalesdue to the presenceof saltsin the injectionwater.
February22 - 23,2007
Marine Science& TechnologtSeminar2007
Salmanet al. 11999)conductedto predict the possibility of scaleformation when seawaterwas injected into the
northemKuwaiti oilfields for reservoir pressuremaintenance.Resultsindicatedthat the seawaterwas likely to be
self-scalingwith respectto calcium carbonateunder productionreservoirconditionscould becomea problem when
the systemunderwenttemperatureandpressurechanges.
Paulo et al. QO0l'l described Sulfate scaledepositionis a common problem in the Alba field in the North Sea
resulted from injected seawatermixing with aquifer brines. The problem is most severein and aroundthe
injection and production well bores and can causeconsiderabledisruption to hydrocarbonproduction after water
breakthrough.
Moghadasiet al. Q003a) described scale formation in the Iranian oilfields has been recognizedto be a major
operational problem causing formation damage either at injection or producing wells. Scale contributes to
equipmentwear and corrosionand flow restrictions,thus rtsulting in a decreasein oil and gasproduction.
4. Solubility of Scales
"solubility" is definedas the limiting amountof solutethat can dissolvein a solventundera given set of physical
conditions. Factors that affect scale precipitation, deposition and crystal growth can be summarizedas: supersaturation,temperature,pressurq ionic strength,evaporation,contacttime and pH. Eflective scalecontrol shouldbe
one of the primary objectivesof any efficient water injection and normal production operationin oil and gas fields.
Therearetwo solubilities of scales:
4.1 Calcium, Strontium, Barium Sulfatesand Calcium Carbonate Solubilities
The solubility of calcium sulfate is an order of magnitudelarger than that of strontium sulfate,with in turn is about
one and one- half ordersof magnitudelarger than that of barium sulfate Figure l For example,Figure I indicates,
that the solubility of strontium sulfate can be larger than 950 mg/I. This solubility, however,is true only when the
solution is stoichiometricallybalanceGi.e., when the numberof strontiumions equalsthe numberof sulfate ions. If
an excessof either ion is introduce4 the solubility is depressedmarkedly. This is known as the "common ion
effecf' (Lindlof and Stoffer 1983). The solubility reachesa maximumin highly concentratedbrinesAccording to Oddo et al, (1991), calcium carbonatesolubility has an inverse relationship with temperatureor
statedmore simply, CaCO:scalebecomesmore insoluble with increasingtemperatureand a solution at equilibrium
wifh CaCO3will precipitatethe solid as the temperatureis increased. The tendencyto form CaCOgalso increases
with increasingpH (as the solution becomesless acid). The decreasein total pressurearoundthe purnpsallows
dissolvedcarbondioxide to escapefrom solution as a gas causingan increasein pH with a subsequentincreasein
the tendencyto form solid.
4.2 Zinc Sulfide,Lead Sulfide and fron Sulfide Solubilities
Lead and zinc sulfide solubility is much lower even than iron sulfide, which is the common sulfide in oil field
environments. For example in a lM (mole/dm3)NaCl brine solution are presentedin Figure 2 at pH: 5 the
solubility of iron sulfide is 65 ppm, whereaslead and zinc sulfides are 0.002 ppm and 0.063 ppm respectively.
Dependingon the exact brine conditions,the solubility of zinc sulfide is between30 and 100 times more soluble
than lead sulfide. As with iron sulfide, the solubility of both lead and zinc sulfide increaseswith increasingsolution
pH (CollinsandJordan,200l).
5. ScaleControl Chemicals
In oil and gaswell operations,water- insoluble scaleis formed in tubing casings,and associatedequipment,as well
as in the well bore and the formation itselt which carq/, at least in part, water or brine waters. Thesewaters can
containinsolublecalcium,barium, strontium,magnesium,and iron salts.
Scaleinhibitors are chemicalswhich delay, reduce or prevent scale fonnation when addedin small amountsto
normallyscalingwater. Accordingto Bezemerand Bauer(1969),the most commonclassesof inhibitor chemicals
compormdsand organicpolymers.
are inorganicphosphates,organophosphorous
acid) @ETPMP) are
Polyphosphonocarboxylicacid @PCA) and Diethylenetriaminepenta(methylenephosphonic
two common commercialscale inhibitors used in the oil and gas industry. Normally, PPCA is regardedas
nucleationinhibitor and DETPMP as a growth inhibitor (Chenel al,2004).
Severalcombined scale removal and inhibition systemscould be consideredin order to meet thesechallenges.
Hydrochloricacid (HCl) may be the most cost effectivetreatmentto remove calcium carbonate,but corrosion
control, systemcompatibility and inhibitor adsorptionmay all be difficult in a combinedtreatrnent. Conversely,
scaledissolversmay offer better corrosion control and scaleinhibitor compatibility when spent,but will be higher
to be met.
cost. Organicacidscould offer a compromisewhich allowsmostof the systemrequirements
February22 - 23,2007
Marine Science& TechnologtSeminar2007
|J
a
r,
(\l
q0l
tJ
!
G
lr,
d-
g ro e
t
F
7
s
3
J
8 rsl
itsCl
SOLUTtOfi
loHE STHEFTGTH
Figure 1: Relative solubilities of three sulfates in brine
itcEs0
lRr&
E
3
E
;
E
E
rtrn
rug
(4
5! - i :
t
E
zls
=.
N,
s
\
I
$
tr6
5
o
at?
il.tl
0,001
Ailr. pH
Figure 2: Comparisonof zinc, leadandiron sulfidesolubilityin lM NaCl brineat 25"C.
6. Laboratory Evaluation
Scaleformation can be predictedby laboratory experiments. Severalexperimentalstudieshave beenconductedto
determinethe scalingpotentialin different oil-fields. ln experimentalstudies,the extentof permeabilitydamage
February22 - 23, 2007
Marine Science & Technologt Seminar 2M7
causedby flowing precipitatesin coresand sandpackswas investigated. Severalinvestigatorstestedincompatible
watersfrom oilfields in the North Sea(Mitchell et al.,1980),the Middle EastandEl-Margon(El-Hattab,1982)that
producedbarium, calcium and strontium precipitates. They reportedthat l5-20-pm-sized crystalsblocked tlre pare
throatsin glassbeadpacksand in alumina cores,by size exclusionand bridging mechanisms-Crystalsoften grew
perpendicularto the pore walls, and that somecrystal aggregatesalso had the form of "books" and rosettes. Todd
and Yuan (1992) also conductedlaboratory investigationsusing North Seareseryoir brines that producedbarirun
and strontium scales.Crystals depositingalong and growing perpendicularto the pore surfacescausedmost of the
ratio of both barium and strontium
reductionin core permeability. They observedthat doubling the super-saturation
producedan increasein the quantity of scaleformed insidethe poresand a changein the morphologyof the crystals.
Both changesincreasethe rate of permeabilitydecline.
7. Modeling Development
Along with laboratoryexperiments,scaleformationcan be predictedby severalmodelsto determinethe scaling
potentialin differentoil-fields. Ceochemicalflow modelsweredeveloped(Shenand Corsby,1983;Yeboaher c/.,
1993;Khatib,1994) for both mineral dissolutionand precipitationprocessesduring convectivetransport. These
modelswere basedon the assumptionof local thermodynamicequilibrium and negligible dispersion. They did not
include solids migration, an assumption, which was severely limiting because many experimental studies
demonstratedthat precipitatedsolids could migratewithin the porousmatrix-
8. Summary
Fromthe abovesurveyofthe literatune,permeabilitydeclinecausedby scaleformation in the porousbed rangedfrom
lessthan 30Votomore than 90% of the initial permeability,dependingon solution composition,initial permeability,
temperature,flow rate and solution injection period. The pattem of permeabilitydecline in a porousmedium due to
scaling injection was characterizedby a steep initial decline which gradually slowed down to a lower. Several
factors influencing scaleformation had beenexamined.Increasingtemperature,super-saturationand flow rate had a
derimental effect on the permeabilityreduction. Pressurehad a slight effecton scaleformation precipitation.
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