scaling definition, cause and solution

8/11/2019 Scaling Definition, Cause and Solution

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NTNU Institutt for petroleumsteknologi og anvendt

geofysikk

An Introduction to Scaling

causes, problems and solutions

Margrethe Nergaard hriss !rimhol

t

Trondheim, november "#$#

Term paper for the course% T&! '$'# Natural!as


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i

Abstract

Scale occurrence, formation and prevention have been investigated( In a natural gas

reservoir, )ater )ith dissolved ions )ill al)ays be present( *hen parameters as tem+

perature, pressure, concentration or p are changed, the e-uilibrium of the system is

shifted( This can push the system into a state )here the dissolved ionsprecipitate out,

causing a deposition of scale(

orrosion protection of pipelines )ill lead to increased scaling of calcium carbonate(

Mono ethylene glycol .M/!0 used as antifree1e, )ill also impact the scaling( 2arious

mechanisms can lead to scale formation in the natural gas )ell, the )ellbore and in the

production e-uipment( In processing systems, especially heated surfaces are targets of

scale( 3egeneration of M/! is also connec

ted to scale forma

tion, as the process )i

th

lo)est operating costs has higher risk of scale formation and contamination(

After formation, some scales can be removed( This can either be done by physical pro+

cesses or chemically( &reventionof scale is conventionallydone )ithchemical inhibitors(

This can create large amounts of )aste( *e claim that increased kno)ledge on the

chemical background of scale formation,can also contributeto scaleprevention(


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ii

ontents

$ Introduction $

" hemical background of scale formation "

"($ Solubilityproduct ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( "

"(" hemical potentialand supersaturation ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( "

"(4 Nucleationand particlegro)th ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( 4

4

' Scale in productionsystems 5

6 Scaling in processing systems $#

6($ Scale at heated surfaces ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( $#

6(" M/! regenerationsystems ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( $$

7

8 9iscussion $'

5 onclusion $6

Scale formation '

4($ ydrate prevention ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( '

4(" orrosion prevention ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( 6

4(4 ombatingone problem by creatinganother ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( 7

4(' :ormation )ater ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( 74(6 alcium carbonatescale mechanism ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( 7

3emoval and prevention of scaling $"

7($ hemical removal ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( $"

7(" Mechanical removal ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( $4

7(4 &reventionof scale ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( $4


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4

$ Introduction

Scaling can be a serious problem for the oil and gas industry, but it is hard to find a good

comprehensive introduction to the sub;ect( The aim of this report is to give an easy

introduction to the scaling phenomena, going though crystalli1ation and e-uilibrium

theory, the cause of scaling, problems caused by scaling and ending )ith strategies for

fighting scales(

Scaling is the deposition of a mineral salt on processing e-uipment( Scaling is a result

of supersaturation of mineral ions in the process fluid( The theory behind scaling is

e


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" hemical background of scale formation

"($ Solubilityproduct

A salt is a neutral substance formed by combining positive and negative ions(

*hen a salt is dissolved in a solvent, for most cases )ater, the salt )ill separate

into its constituent ions, so+called solute molecules( These ions )ill interact in various

degrees

)ith the solvent= )e say they have different ionic activity( The product of the salt>s

ion

activitiesgives the solubilityproduct, ?sp( If the solution.of solventand solute0is close

to an ideal solution,one can substitute ionic activity)iththe concentrationof each ion(

The solubilityproductis a measure of ho) many moles of ions per unit volume of solvent

there can bein a system before a saltprecipitates out( ?sp can vary )ith solvent,but

)ater is oftenused as reference( A lo) ?sp value means that littlesalt)ill be dissolved

in )aterand thissalt)ill be referred to sparingly soluble salts(

*hen a salt is dissolved in )ater, the resulting solution can be acidic, neutral or basic(

The negative ion can )ork as a base or the positive ion as an acid( If either of

them have such properties, the result is a neutral solution( This also implies that

solvent>s p impacts the solubility product( In the case of calcium carbonate, thecarbonate ion can attract a proton from )ater, resulting in a basic solution( *hen

reducing the p by adding additional acid, the e-uilibrium )ill be shifted favoring

dissolutionof calcium carbonate into carbon dio


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systems,


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increasing temperature)ill increase the solubility( &ressure dependence is usually )eak

for systems of dissolved solids, but in oil+ and gas )ells the pressure can be so high that

this dependency has to be taken into account B$C( Also, for the ease of the discussion,

)e hereby look at an ideal system)here activities can be replaced by concentrations(

Dy )hat )e have stated so far, concentrations greater than the ones dictated by the

solubility product ?sp )ould giveprecipitation( o)ever, this is not al)ays the case(

This is most easily e


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In a supersaturated solution, unstable clusters of atoms develop( Hocal fluctuations in

the e-uilibrium concentration can trigger these clusters to form seed crystals( :urther,

these crystals gro) by adsorbing ions, preferably at defects in the surface( After reaching

a critical si1e, the crystal )ill e


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M/! is transportedin a so+called M/! loop( ere, M/! is in;ectedintothe natural gas

pipeline at the )ellhead and flo)s )iththe natural gas toprocessing siteonshore( M/!

is thenseparatedfrom the )ater,regeneratedand sentback to the offshoreproduction

facilitie( The M/! transportedfrom the reservoir and from theproductionsiteonshore

is called rich M/! and lean M/! respectively(

4(" orrosionprevention

9ue to temperature drop, )ater in the pipeline )ill at some point condensate( Since

M/! has lo)ered the free1ingpoint,this)ater)ill be in li-uid form( &resentsof li-uid

)aterleads to a problem )ith corrosion( The cause of thisis thepresence of @", )hich

in contact)ith)ater forms a )eak carbonic acid ."@4 0, as sho)n in reaction$(

@" .g0 "@ .l0 "@4 .a-0 .$0

Hong pipelines are constructed of carbon steelB'C( arbonic acid )ill corrode the iron

in the pipeline )all, producing iron carbonate( See reaction "( This iron carbonate can

precipitate in the production fluid and follo) the gas and li-uid flo), causing problems

do)nstream(

:e "@4 :e@4 " ."0

To combatthisproblem, p+stabili1ingis implemented( ere, p is increased from the

sour condition of the reservoir into more neutral conditions( This is done by in;ect+

ing an alkaline chemical, sodium hydro


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4(4 ombating one problem by creatinganother

@ne problem )ith p stabili1ing is that it does not only decrease the solubility of iron

carbonate, but also other salt like calcium carbonate( This means that if calcium ispresent in the pipeline, )hich it often is, it )ill also precipitate on the )alls of the

pipeline( ompared to iron carbonate precipitation, scaling of calcium carbonate is

usually notlimited to a fe) millimeters( Decause there is a protective film over the iron

surface , there is no longer a source of iron and noprecipitation can occur(

In contrast,calcium carbonate gets its reactantsfrom the reservoir( A furtherdiscussion

of thisreactant supply follo)sbeneath( &recipitation)ill thereforecontinueas long as

the reservoir produces )ater and this result in a thick layer of scale( At some point, the

calcium carbonate scale can be so thick that the pipeline flo) is completely restricted(

4(' :ormation )ater

In most hydrocarbon reservoirs, )ater is also present( It is believed that the reservoir

)as completely saturated )ith )ater before the hydrocarbons appeared, and )ater is

therefore usually present )ith hydrocarbons in the reservoirsB6C( This )ater is called

formation )ater and usually has a high salt content around three to five percentB7C(

omposition of the formation )ater varies greatly )ith the reservoirs, but the usual

constituents are Na

, a"

, ?, Mg"

, :e"

, lJ

, S@"J

and @J

(' 4

:ormation )ater )hich flo)s out )ith the hydrocarbon production is called produced

)ater( The amount depends on the reservoir characteristics and ho) the )ells are

positioned compared to the reservoir phases( Since hydrocarbons are less dense than

)ater, they)ill be in the top phase inside the reservoir( As hydrocarbons are drained

from the reservoir, )aterlevel in the reservoir )ill rise( The amountof produced )ateris

thereforelikely to rise during theproductionlifetime( &roduced )atercan also suddenlyoccur in fields )here thereused to be littleor no produced )ater(

4(6 alcium carbonate scale mechanism

Scale can form by various reasons depending on a number of factors( Scale formation)ill

be treated in details later in thisreport)hen scale in productionsystems is discussed(


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:or no), a simple e-uilibrium approach is used to es e-uilibrium principle( This principle statesthat a chemical systemat e-uilibrium

4

@"J


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)ill al)ays try to counteract any imposed change in pressure, temperature, volume and

composition( All these variables can be changed during gas production and therefore,

affecting the e-uilibrium and scale formation( A brief discussion of each parameters

follo)sbeneath(

&ressure dependance % *hen pressure is decreased in a chemical system,the e-uilibrium

)ill try compensateby increasing the pressure( Decause @" is the only gaseous specie,

the only )ay to increase pressure is by shifting the e-uilibrium to)ards producing more

@" ( A decrease in pressure )ill thenresultin moreprecipitationof calcium carbonate(

oncentration dependance% If the concentrationof calcium or carbonic acid is increased

or the partial pressure of @" is decreased, then there )ould be an e-uilibrium shift

to)ards the right and more precipitation of calcium carbonate( Increasing the p by

addition of an alkaline chemical such as Na@ )ould result in a naturali1ation of the4@

comple


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Inside the )ell, )ater is naturallypresent( As the )ater e


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pressure drop, @" is released from the )ater and calcium carbonate precipitate out

again in the near+)ellbore pores( Hike autoscale,these processes are self+generating and

can completely seal )ells and boreholes in fe) daysBLC(

In )et gas production, evaporation+induced scaling can also occur( 9uring production,lo)ered pressure e


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alcium carbonate has three different crystal structures, socalled polymorphs( /achpolymorph gro)s )ithdifferentrates( Also, differentsupersaturations)ill favor differ+ent polymorphs( The same applies to temperature regionsB$$C( To understand )hich

polymorph that )ill cause scale at the currentprocessing conditions, is thereforeimpor+

tant(

In a current research pro;ect, scaling at heated surfaces for a continuous system is

studied( A scanning electron microscope picture from thispro;ect is sho)n in figure '(

At the conditionsof thise


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Solid removal is illustrated by a black bo< in figure 7, but is usually done in a sedi+

mentation tank or in a centrifugation( M/! addition influencesboth the solubility and

crystal structure of calcium carbonate( As stated earlier, solubility of calcium carbonate

decreases )ith increasing M/! concentration( It is also found that the particle si1e

of calcium carbonate precipitate is reduced at high M/!+levelsB$$C( This can have animpact on the removal of solids in the reclamation process, as the particles can be too

small and lightfor both conventional separation techni-ues(

7 3emoval and prevention of scaling

As discussed by rabtreeet al(BLC, scale can be removed chemically or mechanically after

deposition( Also, scale formation can be prevented before deposition occurs(

7($ hemical removal

Scales can be divided into t)o ma;or categoriesB$"C% the acid soluble scales consisting

mainly of carbonates .M


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7(" Mechanical removal

There are many different )ays of removing scales mechanically( @ne can use simple

milling or impact techni-ues( Scales rarely forms evenly along tubules, and therefore

the energy re-uired to remove the scales vary greatly( If the speed of the milling deviceis not set at a sufficiently high speed, there is a risk of stalling( Impact techni-ues)orks much like a ;ack hammer, hammering the scales until they break( These impact

techni-ues )ork best for brittle scales( Another techni-ue is to use e


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structuresdemands differentinhibitionmechanisms for differentcrystalstructures( This

can e


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solution can be to perforate further into the )ell by using a more effectiveperforation

gun(

5 onclusion

Scaling is precipitation of sparingly soluble salts at surfaces( This can occur )hen a

supersaturated regime is established( :ormation of scale can occur and cause problems

both do)n in the )ell, along thepipelines and in theprocessing units on theplatform or

onshore( If scale is left untreated, the )orst case can be blocked )ells, blocked pipelines

and blocked processing e-uipment( Since this is highly un)anted, prevention of scale is

important(

In order to prevent scale in an as environmentally friendly )ay as possible, a good

understanding of the chemical background for scale formation is necessary( There are

fundamental differences bet)een different scales that can lead to more scaling if the

)rong choices are made(

To avoid all e


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3eferences

B$C elbOk M, ?;elstrup S( :ysikalsk ?;emi( :agbokforlaget="##7(

B"C Mullin *( rystalli1ation( 'thed( Dutter)orth+einemann="##$(

B4C Sandengen ?( &rediction of Mineral Scale :ormation in *et !as

ondensate &ipelines and in M/! .Mono /thylene !lycol0 3egeneration &lants

B&h9 ThesisC(NTNU= "##7(

B'C Sandengen ?( ydratesand !lycols( http%GG)))(ipt(ntn u(noGP;s gGundervisningGnaturgassG

lysarkGHysarkSandengen"#$#(pdf= november "#$#(

B6C oel @:, Ama;ouoyi A, N)okoye U( haracteri1ationof :ormation*ateron+stituents and the /ffect of :resh *ater 9ilution from Hand 3ig Hocation ofthe

Niger 9elta,Nigeria( Appl Sci /nviron Manage( "#$# une=$'."0%48'$(

B7C ?o;en !&( StatoilASA &rincipal 3esearcher 3Q9 !as &rocessing and HN! Statoil

ASA B&ersonal correspondence by e+mailC= "#$#(

B8C hilingar !2, Mourhatch3, Al+Rahtani!9( :undamentalsof orrosion and Scal+

ing + :or &etroleumand /nvironmental/ngineers( !ulf &ublishing ompany= "##5(

B5C ahn :, ook M, !raham M( ydrocarbon /


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B$4C Andreassen &( Associate professor in rystalli1ation, 9epartment of hemical

/ngineering, NTNU B&ersonalcontact,oral informationC="#$#(

B$'C http%GG)))(uis(noGre s earchGnaturalSs cience sGchemistrySandSenvironmentG

oilfieldproductionchemicalsG= november "#$#(

B$6C Andreassen &( SupersaturationB&resentationgiven in the course T?& '646 rys+

talli1ationC( NTNU= "#$#(

B$7C http%GG)))(statoil (comGenGte chnologyinnovationGfielddevelopmentGflo)assurance

GscaleGpagesGdefault(asp


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:igure $% Solubility diagramB$6C

:igure "% Scaling due to mi


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:igure 4% &ictureof Scale deposition in a pipeB$7C

:igure '% S/M pictureof differenttypesof calcium carbonatecrystalsB$8C


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:igure 6% A sketch of the reboiler unit from ?ollsnesB'C

:igure 7% Sketch of the reclamation unit at SnhvitB'C

scaling definition, cause and solution

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