NTNU – Institutt for petroleumsteknologi og anvendtgeofysikk

An Introduction to Scalingcauses, problems and solutions

Margrethe Nergaard Chriss Grimholt

Trondheim, november 2010

Term paper for the course: TPG 4140 – Natural Gas

Abstract

Scale occurrence, formation and prevention have been investigated. In a natural gasreservoir, water with dissolved ions will always be present. When parameters as tem-perature, pressure, concentration or pH are changed, the equilibrium of the system isshifted. This can push the system into a state where the dissolved ions precipitate out,causing a deposition of scale.

Corrosion protection of pipelines will lead to increased scaling of calcium carbonate.Mono ethylene glycol (MEG) used as antifreeze, will also impact the scaling. Variousmechanisms can lead to scale formation in the natural gas well, the wellbore and in theproduction equipment. In processing systems, especially heated surfaces are targets ofscale. Regeneration of MEG is also connected to scale formation, as the process withlowest 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. Prevention of scale is conventionally done with chemical inhibitors.This can create large amounts of waste. We claim that increased knowledge on thechemical background of scale formation, can also contribute to scale prevention.

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Contents

1 Introduction 1

2 Chemical background of scale formation 22.1 Solubility product . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2 Chemical potential and supersaturation . . . . . . . . . . . . . . . . . . . 22.3 Nucleation and particle growth . . . . . . . . . . . . . . . . . . . . . . . 3

3 Scale formation 43.1 Hydrate prevention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.2 Corrosion prevention . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53.3 Combating one problem by creating another . . . . . . . . . . . . . . . . 63.4 Formation water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.5 Calcium carbonate scale mechanism . . . . . . . . . . . . . . . . . . . . . 6

4 Scale in production systems 8

5 Scaling in processing systems 105.1 Scale at heated surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . 105.2 MEG regeneration systems . . . . . . . . . . . . . . . . . . . . . . . . . . 11

6 Removal and prevention of scaling 126.1 Chemical removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126.2 Mechanical removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136.3 Prevention of scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

7 Discussion 14

8 Conclusion 15

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1 Introduction

Scaling can be a serious problem for the oil and gas industry, but it is hard to find a goodcomprehensive introduction to the subject. The aim of this report is to give an easyintroduction to the scaling phenomena, going though crystallization and equilibriumtheory, the cause of scaling, problems caused by scaling and ending with strategies forfighting scales.

Scaling is the deposition of a mineral salt on processing equipment. Scaling is a resultof supersaturation of mineral ions in the process fluid. The theory behind scaling isexplained in section 2 with important topics like solubility, nucleation and supersatura-tion.

This supersaturation of ions are caused by several factors. An important contributoris the production of high salt content water like formation water from the well. Thisincreases the ionic concentrations, possibly leading to deposition of scales, as explainedin section 3. This section also discuss how other preventive actions against hydrateformation and corrosion increase the risk for scale deposition, and how scaling is affectedby changes in physical variables such as concentration (also including pH), pressure andtemperature.

Scale deposition can cause problems several places in production and processing of nat-ural gas. A summary of the most common problems associated with scaling is givenin section 4 and 5. This includes high risk areas where large temperature and pressurechanges occur, like heat exchangers and pipes.

There are several ways of fighting scales, as discussed in section 6. There are preemptivemethods like chemical inhibitors that hinder the scale growth. These methods are limitedin there use, because a inhibitors works best for specific scale types and crystal structure.A more versatile methods for scale fighting are removal after deposition. Chemicalremoval is a cheap method for scale removal, but it is effectiveness depends on theporosity and the type of scale. Mechanical removal is another method for removing scalesafter scale deposition, and usually involves scraping, drilling or inducing vibrations topipes and equipment. Theres a wast amount of mechanical systems, and only a few arediscussed.

We summarize the report by discussing how scaling can impact economy, health, securityand environment (HSE) and alter the choice of technical solutions.

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2 Chemical background of scale formation

2.1 Solubility product

A salt is a neutral substance formed by combining positive and negative ions. Whena salt is dissolved in a solvent, for most cases water, the salt will separate into itsconstituent ions, so-called solute molecules. These ions will interact in various degreeswith the solvent; we say they have different ionic activity. The product of the salt’s ionactivities gives the solubility product, Ksp. If the solution (of solvent and solute) is closeto an ideal solution, one can substitute ionic activity with the concentration of each ion.

The solubility product is a measure of how many moles of ions per unit volume of solventthere can bein a system before a salt precipitates out. Ksp can vary with solvent, butwater is often used as reference. A low Ksp value means that little salt will be dissolvedin water and this salt will be referred to sparingly soluble salts.

When a salt is dissolved in water, the resulting solution can be acidic, neutral or basic.The negative ion can work as a base or the positive ion as an acid. If either of themhave such properties, the result is a neutral solution. This also implies that solvent’spH impacts the solubility product. In the case of calcium carbonate, the carbonate ioncan attract a proton from water, resulting in a basic solution. When reducing the pHby adding additional acid, the equilibrium will be shifted favoring dissolution of calciumcarbonate into carbon dioxide (CO2) and calcium ions.

2.2 Chemical potential and supersaturation

Chemical potential is a measure of the reactivity of a component in a solution. Moreaccurate, it is a measure of how much the free energy, ∆G, of a system changes whenchanging the number of moles in this system at constant temperature and pressure. Itcan be compared to gravitational potential; like a ball is minimizing its gravitationalpotential by rolling down a hill, a system of molecules would try to reach a state oflower chemical potential to minimize its free energy.

Without going into too many thermodynamic details, we here limit ourselves by statingthat chemical potential is both temperature and pressure dependent. Hence, by chang-ing these parameters, the solubility of a system will be influenced. For most systems,

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increasing temperature will increase the solubility. Pressure dependence is usually weakfor systems of dissolved solids, but in oil- and gas wells the pressure can be so high thatthis dependency has to be taken into account [1]. Also, for the ease of the discussion,we hereby look at an ideal system where activities can be replaced by concentrations.

By what we have stated so far, concentrations greater than the ones dictated by thesolubility product Ksp would give precipitation. However, this is not always the case.This is most easily explained by an illustration, see figure 1. Here, the solubility prod-uct is represented as the solid black line line. This curve is also called the saturationline, referring to a solution saturated by dissolved ions. “Stable region” refers to the re-gion where ions are dissolved (meaning: in solution) and “labile” to where precipitationwill occur spontaneously and solids form. The metastable region is further explainedbelow[2].

Behind every chemical process, there must be a thermodynamic driving force. Forprecipitation, this is given by the difference between the chemical potential of a givensubstance in the stable and metastable/labile region. To represent this driving force,the term supersaturation is often used. Supersaturation refers to the difference in con-centration between the bulk concentration (E, C in the figure, but can be anywhere inthe metastable and labile region) and the corresponding concentration at the solubilitycurve (B, D) for a given temperature.

At point A, the solution is not supersaturated, there is no thermodynamic driving forceand therefore no precipitation. By changing either the temperature or the concentration(e.g. by evaporation some solute) one can exceed the solubility concentration (point Band D) and cross into the supersaturated regime. Now, a thermodynamic driving forcefor precipitation is established and formation of solids may take place.

2.3 Nucleation and particle growth

Although a driving force is established, a solution can be supersaturated without solidformation occurring. The short explanation to this is that the supersaturation has tobe sufficiently high; we have to be in the labile regime of figure 1. A more extensiveexplanation requires understanding how particles appear in a solution. This knowledgeis also crucial in order to understand and design scale inhibitors.

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In a supersaturated solution, unstable clusters of atoms develop. Local fluctuations inthe equilibrium concentration can trigger these clusters to form seed crystals. Further,these crystals grow by adsorbing ions, preferably at defects in the surface. After reachinga critical size, the crystal will exhibit a reduction of free energy when expanding itsurface, favoring growth towards a larger crystal. Large crystals can also redissolvesmaller seed crystals if this lowers the overall free energy of the system. This impliesthat formation of seed crystals in regimes where the supersaturation is large, catalysesthe growth of existing crystals[2].

In the metastable region of figure 1, the driving force is not large enough to overcome theenergy amount required to form a surface; a solid particle. Spontaneous precipitationlike the one described above, will therefore not take place. However, when a surface ispresent, already existing defects at the surface can act as nucleation sites. By this, thesurface free energy required is lowerd, allowing solid particles to form and grow even inthe metastabile regime. This formation of precipitate at a surface is often referred to asprecipitation fouling or scaling.

3 Scale formation

A natural gas well will, besides producing natural gas, also produce water and carbondioxide (CO2). The produced water can come from two sources: water vapor in the gasthat condenses into liquid water and formation water containing salts. This water is thesource of hydrate formation and, in combination with CO2, corrosion. Scaling is causedby salts and can occur when the produced water contains formation water.

3.1 Hydrate prevention

A hydrate is a solid structure where a gas molecule is surrounded by a cage of watermolecules[3]. It is formed at high pressures and low temperatures. Hydrates are verysimilar to snow and ice and can form plugs in pipelines leading to blockage. To preventsuch hydrates, injecting a substance like mono ethylene glycol (MEG) will lower thefreezing point of water by diluting the system, just like an anti freeze agent. A lowerfreezing point ensures that solid structures of water cannot form and hence there will beno formation of hydrates.

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MEG is transported in a so-called MEG loop. Here, MEG is injected into the natural gaspipeline at the wellhead and flows with the natural gas to processing site onshore. MEGis then separated from the water, regenerated and sent back to the offshore productionfacilitie. The MEG transported from the reservoir and from the production site onshoreis called rich MEG and lean MEG respectively.

3.2 Corrosion prevention

Due to temperature drop, water in the pipeline will at some point condensate. SinceMEG has lowered the freezing point, this water will be in liquid form. Presents of liquidwater leads to a problem with corrosion. The cause of this is the presence of CO2, whichin contact with water forms a weak carbonic acid (H2CO3), as shown in reaction 1.

CO2 (g) + H2O (l) � H2CO3 (aq) (1)

Long pipelines are constructed of carbon steel[4]. Carbonic acid will corrode the ironin the pipeline wall, producing iron carbonate. See reaction 2. This iron carbonate canprecipitate in the production fluid and follow the gas and liquid flow, causing problemsdownstream.

Fe + H2CO3 � FeCO3 + H2 (2)

To combat this problem, pH-stabilizing is implemented. Here, pH is increased from thesour condition of the reservoir into more neutral conditions. This is done by inject-ing an alkaline chemical, sodium hydroxide (NaOH), together with the MEG. Neutralconditions decrease the solubility of the corrosion product iron carbonate. When thesolubility is sufficiently decreased without corresponding decrease in concentration, thesystem crosses into the metastable and eventually labile region of figure 1. This resultsin a supersaturated system and precipitation of iron carbonate quickly takes place.

The precipitation occur often directly on the pipeline wall, because the energy barrierfor formation of solid particles is lower on solid surfaces. After some time there will beformed a thin iron carbonate film that covers the iron surface of the pipeline, protectingit from contact with carbonic acid and water. Corrosion protection is then establishedand further corrosion is only possible by eroding away this film.

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3.3 Combating one problem by creating another

One problem with pH stabilizing is that it does not only decrease the solubility of ironcarbonate, but also other salt like calcium carbonate. This means that if calcium ispresent in the pipeline, which it often is, it will also precipitate on the walls of thepipeline. Compared to iron carbonate precipitation, scaling of calcium carbonate isusually not limited to a few millimeters. Because there is a protective film over the ironsurface , there is no longer a source of iron and no precipitation can occur.

In contrast, calcium carbonate gets its reactants from the reservoir. A further discussionof this reactant supply follows beneath. Precipitation will therefore continue as long asthe reservoir produces water and this result in a thick layer of scale. At some point, thecalcium carbonate scale can be so thick that the pipeline flow is completely restricted.

3.4 Formation water

In most hydrocarbon reservoirs, water is also present. It is believed that the reservoirwas completely saturated with water before the hydrocarbons appeared, and water istherefore usually present with hydrocarbons in the reservoirs[5]. This water is calledformation water and usually has a high salt content around three to five percent[6].Composition of the formation water varies greatly with the reservoirs, but the usualconstituents are Na+, Ca2+, K+, Mg2+, Fe2+, Cl−, SO2−

4 and HCO−3 .

Formation water which flows out with the hydrocarbon production is called producedwater. The amount depends on the reservoir characteristics and how the wells arepositioned compared to the reservoir phases. Since hydrocarbons are less dense thanwater, they will be in the top phase inside the reservoir. As hydrocarbons are drainedfrom the reservoir, water level in the reservoir will rise. The amount of produced water istherefore likely to rise during the production lifetime. Produced water can also suddenlyoccur in fields where there used to be little or no produced water.

3.5 Calcium carbonate scale mechanism

Scale can form by various reasons depending on a number of factors. Scale formation willbe treated in details later in this report when scale in production systems is discussed.

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For now, a simple equilibrium approach is used to explain how different parametersimpact solubility and thus scale formation. To limit the extent of this report, we focuson the formation of calcium carbonate.

The reactions that lead to formation of solid calcium carbonate are as follows[7]: First,carbon dioxide reacts with water to produce carbonic acid as seen by reaction 3.

CO2 (g) + H2O (l) � H2CO3 (aq) (3)

This carbonic acid will continue to dissociate hydrogen, creating new deprotonatedspecies of carbonic acid, as seen in reaction 4 and 5.

H2CO3 (aq) + H2O (l) � H3O+ (aq) + HCO−3 (aq) (4)

HCO−3 (aq) + H2O (l) � H3O+ (aq) + CO2−

3 (aq) (5)

In the water mixture there will be a mixture of the species H2CO3, HCO−3 and CO2−3 .

Finally, in the presence of calcium and carbonic acid, calcium carbonate will precipitateout as seen by reaction 6.

CO2−3 (aq) + Ca2+ (aq) � CaCO3 (s) (6)

Since produced water usually contains a carbonic acid and calcium ions, a recombinationof these reactions will give a better representation of the situation, as seen by reaction7.

Ca (HCO3)2 (aq) � CO2 (g) + H2O (l) + CaCO3 (s) (7)

Calcium and carbonic acid together in liquid form will be in equilibrium with water,solid calcium carbonate and CO2 gas.

Most behaviors of the calcium carbonate equilibrium can be predicted from Le Châte-lier’s equilibrium principle. This principle states that a chemical system at equilibrium

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will always try to counteract any imposed change in pressure, temperature, volume andcomposition. All these variables can be changed during gas production and therefore,affecting the equilibrium and scale formation. A brief discussion of each parametersfollows beneath.

Pressure dependance: When pressure is decreased in a chemical system, the equilibriumwill try compensate by increasing the pressure. Because CO2 is the only gaseous specie,the only way to increase pressure is by shifting the equilibrium towards producing moreCO2. A decrease in pressure will then result in more precipitation of calcium carbonate.

Concentration dependance: If the concentration of calcium or carbonic acid is increasedor the partial pressure of CO2 is decreased, then there would be an equilibrium shifttowards the right and more precipitation of calcium carbonate. Increasing the pH byaddition of an alkaline chemical such as NaOH would result in a naturalization of theH3O+ complex. This will shift the reactions 4 and 5 to the right, yielding more carbonicacid. More carbonic acid will shift the equilibrium in reaction 7 towards the right andmore calcium carbonate will precipitate out.

Temperature dependance: The solubility of calcium carbonate will decrease as the tem-perature increases. This is an interesting phenomena, because most solubilities increasewith increasing temperature and therefor one gets less precipitation. One of the reasonsfor this behavior is the fact that the precipitation of calcium carbonate require energy(endothermic). This can be written into the equilibrium equation as follows:

energy + Ca (HCO3)2 (aq) � CO2 (g) + H2O (l) + CaCO3 (s) (8)

When the temperature increases, the energy also increases, and the equilibrium will tryto counteract this by consuming energy. The equilibrium is then shifted towards right,favoring precipitation of calcium carbonate.

4 Scale in production systems

Scale can occur in all parts of a gas production system and due to several differentmechanisms. Common for them are that the system is within, or brought into, a super-saturated regime. Here, precipitation can occur. See figure 1.

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Inside the well, water is naturally present. As the water experiences changes in temper-ature or pressure during production, the system will shift the equilibrium in order tocombat these changes. This shift can bring the system from the stable to the metastableand labile region of figure 1 and scale can be formed. Species that originally were dis-solved precipitate out and scale is formed in perforations in the near wellbore matrix.Such scale depositions reduce the formation porosity and permeability and thus thehydrocarbon flow. As scale is formed, pressure drops even further, giving more scaleformation. An illustration of this is found in the right part of figure 2 below. Thisprocess is known as autoscaling and in worst case, pores can be completely blocked.

When the well is drilled and later completed, fluids (mud) are used to, among others,stabilize the drill hole and provide hydrostatic pressure to prevent formation fluids fromentering the well bore. Formation water and drilling fluids are said to be incompatible;meaning, chemical reactions will take place upon mixing. This can also lead to scale inthe near-wellbore environment[4, 8]. Incompatible mixing can also occur when seawateris injected to enhance production. Seawater is often rich in different ions than theproduction water and upon mixing, sparingly soluble salts precipitate out, as illustratedto the left in figure 2.

As already mentioned; imperfections in surfaces can act as nucleation sites and initiateprecipitation and scale formation. A high degree of turbulence is found to catalyzedepositions of scale. Turbulence increases mixing and mass transfer in the zone close tothe surface[9]. When a fluid passes a hinderance, a pressure drop is often experienced.These factors can explain why scale often occurs in downhole completion equipment.This can cause severe problems if devices such as safety valves and gas lifts are put outof action due to scale. In production pipes, scale may build up as a thick layer insidethe tubing. These depositions can be so thick that flow is severely restricted as seen infigure 3.

If the gas produced has acidic gases associated with it, such as H2S and CO2, anothermechanism cause problems. As pressure is reduced during production, less acidic gas isdissolved. This raises the pH, which again lower the solubility and causes precipitationof carbonates. Scale of this type can extend from the near-wellbore matrix along tubingand to surface equipment, since the temperature and pressure of the produced watercontinuously changes and thus also changes the pH. A similar type of scaling can occurwhen using CO2 for secondary recovery. As water turns acidic when CO2 is dissolved,calcium carbonate will dissolve from the limestone well. When production leads to

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pressure drop, CO2 is released from the water and calcium carbonate precipitate outagain in the near-wellbore pores. Like autoscale, these processes are self-generating andcan completely seal wells and boreholes in few days[9].

In wet gas production, evaporation-induced scaling can also occur. During production,lowered pressure expands the hydrocarbon gas. This leads to evaporation of the for-mation water and thus increased concentration of dissolved species. As enough solventis evaporated, the system crosses the solubility curve as seen in figure 1 and scale isformed[9].

5 Scaling in processing systems

Even if little or no formation water is produced, there will still be some ions and impu-rities present in the water. These can origin from corrosion, pH-stabilization, mineralsin the reservoir or from seawater and cause precipitation and scale in the processingsystems. To ease the following discussion, we only look at the processing systems withinthe MEG-loop.

5.1 Scale at heated surfaces

Calcium carbonate belongs to a group of salts often called inverse soluble. These saltsdo not follow the general trend on increasing solubility with increasing temperature.Instead, when temperature increases, the solubility goes down and favors precipitation[2].This can cause trouble in systems where heated surfaces are present.

Since calcium carbonate is inverse soluble, a heated surface will have a greater tendencytowards nucleation than a low temperature surface. At this elevated temperature, thesolubility is lowered and a local regime favoring precipitation can occur. If the systemalready is within the metastable region, the temperature gradient from the heated surfacemay impact the chemical potential and thus the driving force for precipitation. Unitoperations like heat exchangers and reboilers, where surfaces at elevated temperaturesexist, are therefore potential sites for scale formation. Scale at such surfaces will lower theheat transfer and reduce the unit’s efficiency. Also, studies show that prescaled surfaceshave a higher tendency of further scaling, as nucleation sites already is established[10].

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Calcium carbonate has three different crystal structures, socalled polymorphs. Eachpolymorph grows with different rates. Also, different supersaturations will favor differ-ent polymorphs. The same applies to temperature regions[11]. To understand whichpolymorph that will cause scale at the current processing conditions, is therefore impor-tant.

In a current research project, scaling at heated surfaces for a continuous system isstudied. A scanning electron microscope picture from this project is shown in figure 4.At the conditions of this exact experiment, the fast-growing needle-like crystal structuredominates fully over the slow growing cube like structure. In the worst case scenario, thescaling can be so extensive that that heat transfer is reduced to almost zero. Dependingon the dimensions of the unit operation, this can happen in a short time (hours or days)and can put the whole unit out of action. Understanding which crystal structure occurswhen, can therefore be crucial. It is to be hoped that research will give more knowledgeon how to avoid such scenarios[4].

5.2 MEG regeneration systems

MEG dries the gas by absorbing water and thereby also absorbing dissolved ions. Afterremoval of the absorbed water, the MEG is said to be regenerated and can again be usedto inhibit hydrates and dry the natural gas. Depending on the regeneration process,impurities is either carried with the MEG or separated off in the regenerating process.

Since water has a lower boiling point than MEG, water may be removed by simple boil-ing. A sketch of the reboiler unit from Kollsnes is shown in figure 5. Since calciumand carbonate cannot carried by the water vapor, these are left in the MEG. As timeproceeds, the concentration of ions in the MEG-loop increases. If concentrations corre-sponding to supersaturated regimes are reached, scaling can occur. The reboiler is, aswe have seen, especially exposed. But, if concentrations are high enough for spontaneousnucleation (labile region), crystal growth can occur in the bulk fluid. The whole loop canbe contaminated by crystals and in worst case, the whole MEG mass must be changed.

A process known as MEG reclamation solves the problem with upconcentrated contam-inations. Here, both MEG and water are boiled off and later separate into two phases.This requires more energy and increase operating costs, but allows impurities to beremoved. The illustration in figure 6 is from Snøhvit where this technique is in use.

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Solid removal is illustrated by a black box in figure 6, but is usually done in a sedi-mentation tank or in a centrifugation. MEG addition influences both the solubility andcrystal structure of calcium carbonate. As stated earlier, solubility of calcium carbonatedecreases with increasing MEG concentration. It is also found that the particle sizeof calcium carbonate precipitate is reduced at high MEG-levels[11]. This can have animpact on the removal of solids in the reclamation process, as the particles can be toosmall and light for both conventional separation techniques.

6 Removal and prevention of scaling

As discussed by Crabtree et al.[9], scale can be removed chemically or mechanically afterdeposition. Also, scale formation can be prevented before deposition occurs.

6.1 Chemical removal

Scales can be divided into two major categories[12]: the acid soluble scales consistingmainly of carbonates (MxCO3) and sulfides (MxSy) and the acid insoluble scales con-sisting mainly of sulphates (MxSO4). The M refers to a metal ion, for example calcium,and x/y refers to stoichiometric ratios. Acid soluble scales, the use of a strong acidcan dissolve the precipitated scales. For calcium carbonate, hydrochloric acid may beused. Scales that cannot be solved by acids require other treatments. These scales canremoved by using a strong chelating agent. A chelating agent is a complex moleculewhich breaks up the scale by isolating and tying up the metallic ions in the scale[9].

Chemical scale removal is usually a cheap and easy way to remove scales, but the effec-tiveness of the removal depends on surface to volume ratio of the scale. If the scale hasa large surface area compared to the volume, like porous materials, the scale will be bro-ken down quickly. This is due to the large contact area between the chemical agent andthe scale. If a scale has a low surface to volume ratio, like a non-porous material, onlythe exterior surface will be in contact with the chemical agent. Because a non-porousscale will be broken down only from the exterior surface, while a porous material will bebroken down from the exterior and interior surface, the removal of a non-porous scalewill be much slower.

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6.2 Mechanical removal

There are many different ways of removing scales mechanically. One can use simplemilling or impact techniques. Scales rarely forms evenly along tubules, and thereforethe energy required 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 techniquesworks much like a jack hammer, hammering the scales until they break. These impacttechniques work best for brittle scales. Another technique is to use explosives induceshock vibrations to the pipe, causing brittle scales to break off. This method is effectiveagainst thin layers. Thick layers are usually too strong for a safe removal by explosives.

Jet blasting techniques involve using an high velocity jet stream to remove scales. Thisworks best for soft scales. The jet technique can be combined with a hard particlesubstance to remove hard scales. This is very similar to sand blasting, and removes thescales by erosion. The particles used have to be selected carefully since the jet blastingwith particles can erode pipe wall as easily as scales. Round particles are good forremoving scales, and reduce the damage to the pipes.

6.3 Prevention of scale

The cost of scales removal can be quite costly. Therefore it is sensible to prevent theformation of scales. One of the easiest ways is to dilute ion rich waters with fresh water,reducing the concentration of ions and thus the saturation of the system. Another way isto use a scale inhibitor. Scale inhibitors can hinder scale growth in several ways, wheresome of them are: Absorption onto the surface of the scale nuclei to make combinationwith other crystals difficult, absorption on a surface defect of a crystal preventing furthergrowth or use a chelating agent.

One problem with chelating inhibitors is that a stoichiometric ratio often is required.This both creates large amounts of waste and are costly. These kind of inhibitors are alsosensitive to equilibrium changes. Absorption inhibitors attack the crystalline structuresand not ions themselves. Therefore, an amount 1000 times lower than the stoichiometricratio is needed, resulting in reduced treatment cost.

Another problem with chemical inhibitors is the calcium carbonate itself. As stated pre-viously, calcium carbonate has three different crystal structures. The variation is crystal

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structures demands different inhibition mechanisms for different crystal structures. Thiscan explain why certain inhibitors after some time apparently stop working. Than, scaleformation occur and some times even more aggressive than before inhibition[13].

7 Discussion

In all businesses, time is money. Extra time used to deal with scale problems representsboth a loss of incoming money. For wells completely blocked after a quick autoscaleprocess, the economic losses can be tremendous as a new wellbore might be needed. Scaleremoval by either mechanical processes or chemical additives represents extra expenses inaddition to the time used. And, as already seen, when recycling MEG, a choice betweenextra operating costs or potential costs related to scale and polluted MEG must betaken. For scale at heated surfaces in processing systems, extra energy may be requiredto obtain the desired heat transfer.

Health, security and environment (HSE) is important for all industries. Some of thescales that can form (phosphorous scales) can be poisonous and represent a health risk.Scaling at safety valves can cause huge safety problem if these valves are covered by somuch scale that they are put out of action. When looking at environmental impact,waste formation, use of environmental unfriendly chemicals and energy use must betaken into the discussion. The scale itself might be treated as a waste after removal. Asstated in section 6, when preventing scale chemically, the result can be large amountsof created waste. Waste products will also be formed in a MEG-loop, either directly inthe reclamation process or potentially indirectly in the regeneration process.

Some of the chemicals used previously for inhibition of scale have now been banneddue to unwanted environmental impacts. New chemicals are developed, but some ofthese are not as efficient and therefore, larger amounts are needed. The environmentalconsequences of new chemicals in large amounts are often unclear until after some timein use[14]. Economical aspects of increased energy use has already been discussed.However, increased energy use has also environmental impact and minimized energy useis desirable.

The choice of technical solutions can be altered by scale formation. An example of thiscan be when scale in the near wellbore region result in lower permeability. A potential

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solution can be to perforate further into the well by using a more effective perforationgun.

8 Conclusion

Scaling is precipitation of sparingly soluble salts at surfaces. This can occur when asupersaturated regime is established. Formation of scale can occur and cause problemsboth down in the well, along the pipelines and in the processing units on the platform oronshore. If scale is left untreated, the worst case can be blocked wells, blocked pipelinesand blocked processing equipment. Since this is highly unwanted, prevention of scale isimportant.

In order to prevent scale in an as environmentally friendly way as possible, a goodunderstanding of the chemical background for scale formation is necessary. There arefundamental differences between different scales that can lead to more scaling if thewrong choices are made.

To avoid all extra costs related to scaling is unrealistic. But the costs can be minimizedby thoroughly risk analyses and a focus on scale prevention.

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[9] Crabtree M, Eslinger D, Fletcher P, Miller M, Johnson A, King G. Fighting Scale– Removal and Prevention. Oilfield Review. 1999 autumn;.

[10] Vatneberg S. Scaling of Calcium Carbonate on Metal Surfaces in Mixtures of MonoEthylene Glycol and Water [Master Thesis]. NTNU; 2008.

[11] Flaten EM, Seiersten M, Andreassen JP. Polymorphism and morphology of calciumcarbonate precipitated in mixed solvents of ethylene glycol and water. Journal ofCrystal Growth. 2009;331:3533–3538.

[12] Salami ARA, Monem AA. Downhole and Topside Scale Challenge “Removal, Pre-vention and Inhibition Strategy”. Society of Petroleum Engineers. 2010;.

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[13] Andreassen JP. Associate professor in Crystallization, Department of ChemicalEngineering, NTNU [Personal contact, oral information]; 2010.

[14] http://www.uis.no/research/natural_sciences/chemistry_and_environment/oil_field_production_chemicals/; november 2010.

[15] Andreassen JP. Supersaturation [Presentation given in the course TKP 4535 Crys-tallization]. NTNU; 2010.

[16] http://www.statoil.com/en/technologyinnovation/fielddevelopment/flowassurance/scale/pages/default.aspx; 2010.

[17] Nergaard M. Master Student, NTNU; 2010.

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Figure 1: Solubility diagram[15]

Figure 2: Scaling due to mixing of incompatible fluids (left) and autoscaling (right) [9]

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Figure 3: Picture of Scale deposition in a pipe[16]

Figure 4: SEM picture of different types of calcium carbonate crystals[17]

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Figure 5: A sketch of the reboiler unit from Kollsnes[4]

Figure 6: Sketch of the reclamation unit at Snøhvit[4]

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