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NTNU Institutt for petroleumsteknologi og anvendt


An Introduction to Scaling
causes, problems and solutions
Margrethe Nergaard

Chriss Grimholt

Trondheim, november 2010

Term paper for the course: TPG 4140 Natural Gas

Scale occurrence, formation and prevention have been investigated. In a natural gas
reservoir, water with dissolved ions will always be present. When parameters as temperature, pressure, concentration or pH are changed, the equilibrium of the system is
shifted. 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. Various
mechanisms can lead to scale formation in the natural gas well, the wellbore and in the
production equipment. In processing systems, especially heated surfaces are targets of
scale. Regeneration of MEG is also connected to scale formation, as the process with
lowest operating costs has higher risk of scale formation and contamination.
After formation, some scales can be removed. This can either be done by physical processes or chemically. Prevention of scale is conventionally done with chemical inhibitors.
This can create large amounts of waste. We claim that increased knowledge on the
chemical background of scale formation, can also contribute to scale prevention.

1 Introduction

2 Chemical background of scale formation

2.2 Chemical potential and supersaturation . . . . . . . . . . . . . . . . . . .
2.3 Nucleation and particle growth . . . . . . . . . . . . . . . . . . . . . . .


3 Scale formation
3.1 Hydrate prevention . . . . . . . . . . . . . .
3.2 Corrosion prevention . . . . . . . . . . . . .
3.3 Combating one problem by creating another
3.4 Formation water . . . . . . . . . . . . . . .
3.5 Calcium carbonate scale mechanism . . . . .


4 Scale in production systems

















5 Scaling in processing systems

6 Removal and prevention of scaling
7 Discussion


8 Conclusion



1 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 subject. The aim of this report is to give an easy
introduction to the scaling phenomena, going though crystallization and equilibrium
theory, the cause of scaling, problems caused by scaling and ending with strategies for
fighting scales.
Scaling is the deposition of a mineral salt on processing equipment. Scaling is a result
of supersaturation of mineral ions in the process fluid. The theory behind scaling is
explained in section 2 with important topics like solubility, nucleation and supersaturation.
This supersaturation of ions are caused by several factors. An important contributor
is the production of high salt content water like formation water from the well. This
increases the ionic concentrations, possibly leading to deposition of scales, as explained
in section 3. This section also discuss how other preventive actions against hydrate
formation and corrosion increase the risk for scale deposition, and how scaling is aected
by changes in physical variables such as concentration (also including pH), pressure and
Scale deposition can cause problems several places in production and processing of natural gas. A summary of the most common problems associated with scaling is given
in section 4 and 5. This includes high risk areas where large temperature and pressure
changes occur, like heat exchangers and pipes.
There are several ways of fighting scales, as discussed in section 6. There are preemptive
methods like chemical inhibitors that hinder the scale growth. These methods are limited
in 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. Chemical
removal is a cheap method for scale removal, but it is eectiveness depends on the
porosity and the type of scale. Mechanical removal is another method for removing scales
after scale deposition, and usually involves scraping, drilling or inducing vibrations to
pipes and equipment. Theres a wast amount of mechanical systems, and only a few are
We summarize the report by discussing how scaling can impact economy, health, security
and environment (HSE) and alter the choice of technical solutions.

2 Chemical background of scale formation

2.1 Solubility product
A salt is a neutral substance formed by combining positive and negative ions. When
a salt is dissolved in a solvent, for most cases water, the salt will separate into its
constituent ions, so-called solute molecules. These ions will interact in various degrees
with the solvent; we say they have dierent ionic activity. The product of the salts ion
activities gives the solubility product, Ksp . If the solution (of solvent and solute) is close
to 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 solvent
there can bein a system before a salt precipitates out. Ksp can vary with solvent, but
water is often used as reference. A low Ksp value means that little salt will be dissolved
in 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 them
have such properties, the result is a neutral solution. This also implies that solvents
pH impacts the solubility product. In the case of calcium carbonate, the carbonate ion
can attract a proton from water, resulting in a basic solution. When reducing the pH
by adding additional acid, the equilibrium will be shifted favoring dissolution of calcium
carbonate 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. More
accurate, it is a measure of how much the free energy, G, of a system changes when
changing the number of moles in this system at constant temperature and pressure. It
can be compared to gravitational potential; like a ball is minimizing its gravitational
potential by rolling down a hill, a system of molecules would try to reach a state of
lower chemical potential to minimize its free energy.
Without going into too many thermodynamic details, we here limit ourselves by stating
that chemical potential is both temperature and pressure dependent. Hence, by changing these parameters, the solubility of a system will be influenced. For most systems,

increasing temperature will increase the solubility. Pressure dependence is usually weak
for systems of dissolved solids, but in oil- and gas wells the pressure can be so high that
this 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 the
solubility 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 product is represented as the solid black line line. This curve is also called the saturation
line, referring to a solution saturated by dissolved ions. Stable region refers to the region where ions are dissolved (meaning: in solution) and labile to where precipitation
will occur spontaneously and solids form. The metastable region is further explained
Behind every chemical process, there must be a thermodynamic driving force. For
precipitation, this is given by the dierence between the chemical potential of a given
substance in the stable and metastable/labile region. To represent this driving force,
the term supersaturation is often used. Supersaturation refers to the dierence in concentration between the bulk concentration (E, C in the figure, but can be anywhere in
the metastable and labile region) and the corresponding concentration at the solubility
curve (B, D) for a given temperature.
At point A, the solution is not supersaturated, there is no thermodynamic driving force
and therefore no precipitation. By changing either the temperature or the concentration
(e.g. by evaporation some solute) one can exceed the solubility concentration (point B
and D) and cross into the supersaturated regime. Now, a thermodynamic driving force
for 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 solid
formation occurring. The short explanation to this is that the supersaturation has to
be suciently high; we have to be in the labile regime of figure 1. A more extensive
explanation requires understanding how particles appear in a solution. This knowledge
is also crucial in order to understand and design scale inhibitors.

In a supersaturated solution, unstable clusters of atoms develop. Local fluctuations in

the equilibrium concentration can trigger these clusters to form seed crystals. Further,
these crystals grow by adsorbing ions, preferably at defects in the surface. After reaching
a critical size, the crystal will exhibit a reduction of free energy when expanding it
surface, favoring growth towards a larger crystal. Large crystals can also redissolve
smaller seed crystals if this lowers the overall free energy of the system. This implies
that formation of seed crystals in regimes where the supersaturation is large, catalyses
the growth of existing crystals[2].
In the metastable region of figure 1, the driving force is not large enough to overcome the
energy amount required to form a surface; a solid particle. Spontaneous precipitation
like the one described above, will therefore not take place. However, when a surface is
present, already existing defects at the surface can act as nucleation sites. By this, the
surface free energy required is lowerd, allowing solid particles to form and grow even in
the metastabile regime. This formation of precipitate at a surface is often referred to as
precipitation fouling or scaling.

3 Scale formation
A natural gas well will, besides producing natural gas, also produce water and carbon
dioxide (CO2 ). The produced water can come from two sources: water vapor in the gas
that condenses into liquid water and formation water containing salts. This water is the
source of hydrate formation and, in combination with CO2 , corrosion. Scaling is caused
by 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 water
molecules[3]. It is formed at high pressures and low temperatures. Hydrates are very
similar to snow and ice and can form plugs in pipelines leading to blockage. To prevent
such hydrates, injecting a substance like mono ethylene glycol (MEG) will lower the
freezing point of water by diluting the system, just like an anti freeze agent. A lower
freezing point ensures that solid structures of water cannot form and hence there will be
no formation of hydrates.

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

3.2 Corrosion prevention

Due to temperature drop, water in the pipeline will at some point condensate. Since
MEG has lowered the freezing point, this water will be in liquid form. Presents of liquid
water leads to a problem with corrosion. The cause of this is the presence of CO2 , which
in contact with water forms a weak carbonic acid (H2 CO3 ), as shown in reaction 1.
CO2 (g) + H2 O (l) H2 CO3 (aq)


Long pipelines are constructed of carbon steel[4]. Carbonic acid will corrode the iron
in the pipeline wall, producing iron carbonate. See reaction 2. This iron carbonate can
precipitate in the production fluid and follow the gas and liquid flow, causing problems
Fe + H2 CO3 FeCO3 + H2


To combat this problem, pH-stabilizing is implemented. Here, pH is increased from the

sour condition of the reservoir into more neutral conditions. This is done by injecting an alkaline chemical, sodium hydroxide (NaOH), together with the MEG. Neutral
conditions decrease the solubility of the corrosion product iron carbonate. When the
solubility is suciently decreased without corresponding decrease in concentration, the
system crosses into the metastable and eventually labile region of figure 1. This results
in a supersaturated system and precipitation of iron carbonate quickly takes place.
The precipitation occur often directly on the pipeline wall, because the energy barrier
for formation of solid particles is lower on solid surfaces. After some time there will be
formed a thin iron carbonate film that covers the iron surface of the pipeline, protecting
it from contact with carbonic acid and water. Corrosion protection is then established
and further corrosion is only possible by eroding away this film.

3.3 Combating one problem by creating another

One problem with pH stabilizing is that it does not only decrease the solubility of iron
carbonate, but also other salt like calcium carbonate. This means that if calcium is
present in the pipeline, which it often is, it will also precipitate on the walls of the
pipeline. Compared to iron carbonate precipitation, scaling of calcium carbonate is
usually not limited to a few millimeters. Because there is a protective film over the iron
surface , 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 discussion
of this reactant supply follows beneath. Precipitation will therefore continue as long as
the reservoir produces water and this result in a thick layer of scale. At some point, the
calcium 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 reservoir
was completely saturated with water before the hydrocarbons appeared, and water is
therefore usually present with hydrocarbons in the reservoirs[5]. This water is called
formation 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 usual

constituents are Na+ , Ca2+ , K+ , Mg2+ , Fe2+ , Cl , SO2

4 and HCO3 .
Formation water which flows out with the hydrocarbon production is called produced
water. The amount depends on the reservoir characteristics and how the wells are
positioned compared to the reservoir phases. Since hydrocarbons are less dense than
water, they will be in the top phase inside the reservoir. As hydrocarbons are drained
from the reservoir, water level in the reservoir will rise. The amount of produced water is
therefore likely to rise during the production lifetime. Produced water can also suddenly
occur 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 will
be treated in details later in this report when scale in production systems is discussed.

For now, a simple equilibrium approach is used to explain how dierent parameters
impact solubility and thus scale formation. To limit the extent of this report, we focus
on 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.



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

H2 CO3 (aq) + H2 O (l) H3 O+ (aq) + HCO

3 (aq)


3 (aq) + H2 O (l) H3 O (aq) + CO3 (aq)


In the water mixture there will be a mixture of the species H2 CO3 , HCO
3 and CO3 .
Finally, in the presence of calcium and carbonic acid, calcium carbonate will precipitate
out as seen by reaction 6.

(aq) CaCO3 (s)
3 (aq) + Ca


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

Ca (HCO3 )2 (aq) CO2 (g) + H2 O (l ) + CaCO3 (s)


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 Chteliers equilibrium principle. This principle states that a chemical system at equilibrium

will always try to counteract any imposed change in pressure, temperature, volume and
composition. All these variables can be changed during gas production and therefore,
aecting the equilibrium and scale formation. A brief discussion of each parameters
follows beneath.
Pressure dependance: When pressure is decreased in a chemical system, the equilibrium
will 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 more
CO2 . A decrease in pressure will then result in more precipitation of calcium carbonate.
Concentration dependance: If the concentration of calcium or carbonic acid is increased
or the partial pressure of CO2 is decreased, then there would be an equilibrium shift
towards the right and more precipitation of calcium carbonate. Increasing the pH by
addition of an alkaline chemical such as NaOH would result in a naturalization of the
H3 O+ complex. This will shift the reactions 4 and 5 to the right, yielding more carbonic
acid. More carbonic acid will shift the equilibrium in reaction 7 towards the right and
more calcium carbonate will precipitate out.
Temperature dependance: The solubility of calcium carbonate will decrease as the temperature increases. This is an interesting phenomena, because most solubilities increase
with increasing temperature and therefor one gets less precipitation. One of the reasons
for this behavior is the fact that the precipitation of calcium carbonate require energy
(endothermic). This can be written into the equilibrium equation as follows:



When the temperature increases, the energy also increases, and the equilibrium will try
to 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 dierent
mechanisms. Common for them are that the system is within, or brought into, a supersaturated regime. Here, precipitation can occur. See figure 1.

Inside the well, water is naturally present. As the water experiences changes in temperature or pressure during production, the system will shift the equilibrium in order to
combat these changes. This shift can bring the system from the stable to the metastable
and labile region of figure 1 and scale can be formed. Species that originally were dissolved precipitate out and scale is formed in perforations in the near wellbore matrix.
Such scale depositions reduce the formation porosity and permeability and thus the
hydrocarbon flow. As scale is formed, pressure drops even further, giving more scale
formation. An illustration of this is found in the right part of figure 2 below. This
process 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 from
entering 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 in
the near-wellbore environment[4, 8]. Incompatible mixing can also occur when seawater
is injected to enhance production. Seawater is often rich in dierent ions than the
production water and upon mixing, sparingly soluble salts precipitate out, as illustrated
to the left in figure 2.
As already mentioned; imperfections in surfaces can act as nucleation sites and initiate
precipitation and scale formation. A high degree of turbulence is found to catalyze
depositions of scale. Turbulence increases mixing and mass transfer in the zone close to
the 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 out
of action due to scale. In production pipes, scale may build up as a thick layer inside
the tubing. These depositions can be so thick that flow is severely restricted as seen in
figure 3.
If the gas produced has acidic gases associated with it, such as H2 S and CO2 , another
mechanism cause problems. As pressure is reduced during production, less acidic gas is
dissolved. This raises the pH, which again lower the solubility and causes precipitation
of carbonates. Scale of this type can extend from the near-wellbore matrix along tubing
and to surface equipment, since the temperature and pressure of the produced water
continuously changes and thus also changes the pH. A similar type of scaling can occur
when 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

pressure drop, CO2 is released from the water and calcium carbonate precipitate out
again in the near-wellbore pores. Like autoscale, these processes are self-generating and
can 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 formation water and thus increased concentration of dissolved species. As enough solvent
is evaporated, the system crosses the solubility curve as seen in figure 1 and scale is

5 Scaling in processing systems

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

5.1 Scale at heated surfaces

Calcium carbonate belongs to a group of salts often called inverse soluble. These salts
do 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 tendency
towards nucleation than a low temperature surface. At this elevated temperature, the
solubility is lowered and a local regime favoring precipitation can occur. If the system
already is within the metastable region, the temperature gradient from the heated surface
may impact the chemical potential and thus the driving force for precipitation. Unit
operations like heat exchangers and reboilers, where surfaces at elevated temperatures
exist, are therefore potential sites for scale formation. Scale at such surfaces will lower the
heat transfer and reduce the units eciency. Also, studies show that prescaled surfaces
have a higher tendency of further scaling, as nucleation sites already is established[10].


Calcium carbonate has three dierent crystal structures, socalled polymorphs. Each
polymorph grows with dierent rates. Also, dierent supersaturations will favor dierent polymorphs. The same applies to temperature regions[11]. To understand which
polymorph that will cause scale at the current processing conditions, is therefore important.
In a current research project, scaling at heated surfaces for a continuous system is
studied. 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 structure
dominates fully over the slow growing cube like structure. In the worst case scenario, the
scaling can be so extensive that that heat transfer is reduced to almost zero. Depending
on 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 occurs
when, can therefore be crucial. It is to be hoped that research will give more knowledge
on how to avoid such scenarios[4].

5.2 MEG regeneration systems

MEG dries the gas by absorbing water and thereby also absorbing dissolved ions. After
removal of the absorbed water, the MEG is said to be regenerated and can again be used
to inhibit hydrates and dry the natural gas. Depending on the regeneration process,
impurities is either carried with the MEG or separated o in the regenerating process.
Since water has a lower boiling point than MEG, water may be removed by simple boiling. A sketch of the reboiler unit from Kollsnes is shown in figure 5. Since calcium
and carbonate cannot carried by the water vapor, these are left in the MEG. As time
proceeds, the concentration of ions in the MEG-loop increases. If concentrations corresponding to supersaturated regimes are reached, scaling can occur. The reboiler is, as
we have seen, especially exposed. But, if concentrations are high enough for spontaneous
nucleation (labile region), crystal growth can occur in the bulk fluid. The whole loop can
be 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 contaminations. Here, both MEG and water are boiled o and later separate into two phases.
This requires more energy and increase operating costs, but allows impurities to be
removed. The illustration in figure 6 is from Snhvit where this technique is in use.


Solid removal is illustrated by a black box in figure 6, but is usually done in a sedimentation tank or in a centrifugation. MEG addition influences both the solubility and
crystal structure of calcium carbonate. As stated earlier, solubility of calcium carbonate
decreases with increasing MEG concentration. It is also found that the particle size
of calcium carbonate precipitate is reduced at high MEG-levels[11]. This can have an
impact on the removal of solids in the reclamation process, as the particles can be too
small 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 after
deposition. 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 consisting
mainly of carbonates (Mx CO3 ) and sulfides (Mx Sy ) and the acid insoluble scales consisting mainly of sulphates (Mx SO4 ). 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 acid
can dissolve the precipitated scales. For calcium carbonate, hydrochloric acid may be
used. Scales that cannot be solved by acids require other treatments. These scales can
removed by using a strong chelating agent. A chelating agent is a complex molecule
which 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 eectiveness of the removal depends on surface to volume ratio of the scale. If the scale has
a large surface area compared to the volume, like porous materials, the scale will be broken down quickly. This is due to the large contact area between the chemical agent and
the scale. If a scale has a low surface to volume ratio, like a non-porous material, only
the exterior surface will be in contact with the chemical agent. Because a non-porous
scale will be broken down only from the exterior surface, while a porous material will be
broken down from the exterior and interior surface, the removal of a non-porous scale
will be much slower.


6.2 Mechanical removal

There are many dierent ways of removing scales mechanically. One can use simple
milling or impact techniques. Scales rarely forms evenly along tubules, and therefore
the energy required to remove the scales vary greatly. If the speed of the milling device
is not set at a suciently high speed, there is a risk of stalling. Impact techniques
works much like a jack hammer, hammering the scales until they break. These impact
techniques work best for brittle scales. Another technique is to use explosives induce
shock vibrations to the pipe, causing brittle scales to break o. This method is eective
against 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. This
works best for soft scales. The jet technique can be combined with a hard particle
substance to remove hard scales. This is very similar to sand blasting, and removes the
scales by erosion. The particles used have to be selected carefully since the jet blasting
with particles can erode pipe wall as easily as scales. Round particles are good for
removing 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 the
formation 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 is
to use a scale inhibitor. Scale inhibitors can hinder scale growth in several ways, where
some of them are: Absorption onto the surface of the scale nuclei to make combination
with other crystals dicult, absorption on a surface defect of a crystal preventing further
growth 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 also
sensitive to equilibrium changes. Absorption inhibitors attack the crystalline structures
and not ions themselves. Therefore, an amount 1000 times lower than the stoichiometric
ratio is needed, resulting in reduced treatment cost.
Another problem with chemical inhibitors is the calcium carbonate itself. As stated previously, calcium carbonate has three dierent crystal structures. The variation is crystal


structures demands dierent inhibition mechanisms for dierent crystal structures. This
can explain why certain inhibitors after some time apparently stop working. Than, scale
formation 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 represents
both a loss of incoming money. For wells completely blocked after a quick autoscale
process, the economic losses can be tremendous as a new wellbore might be needed. Scale
removal by either mechanical processes or chemical additives represents extra expenses in
addition to the time used. And, as already seen, when recycling MEG, a choice between
extra operating costs or potential costs related to scale and polluted MEG must be
taken. For scale at heated surfaces in processing systems, extra energy may be required
to obtain the desired heat transfer.
Health, security and environment (HSE) is important for all industries. Some of the
scales 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 so
much scale that they are put out of action. When looking at environmental impact,
waste formation, use of environmental unfriendly chemicals and energy use must be
taken into the discussion. The scale itself might be treated as a waste after removal. As
stated in section 6, when preventing scale chemically, the result can be large amounts
of created waste. Waste products will also be formed in a MEG-loop, either directly in
the reclamation process or potentially indirectly in the regeneration process.
Some of the chemicals used previously for inhibition of scale have now been banned
due to unwanted environmental impacts. New chemicals are developed, but some of
these are not as ecient and therefore, larger amounts are needed. The environmental
consequences of new chemicals in large amounts are often unclear until after some time
in use[14]. Economical aspects of increased energy use has already been discussed.
However, increased energy use has also environmental impact and minimized energy use
is desirable.
The choice of technical solutions can be altered by scale formation. An example of this
can be when scale in the near wellbore region result in lower permeability. A potential


solution can be to perforate further into the well by using a more eective perforation

8 Conclusion
Scaling is precipitation of sparingly soluble salts at surfaces. This can occur when a
supersaturated regime is established. Formation of scale can occur and cause problems
both down in the well, along the pipelines and in the processing units on the platform or
onshore. If scale is left untreated, the worst case can be blocked wells, blocked pipelines
and blocked processing equipment. Since this is highly unwanted, prevention of scale is
In order to prevent scale in an as environmentally friendly way as possible, a good
understanding of the chemical background for scale formation is necessary. There are
fundamental dierences between dierent scales that can lead to more scaling if the
wrong choices are made.
To avoid all extra costs related to scaling is unrealistic. But the costs can be minimized
by thoroughly risk analyses and a focus on scale prevention.


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[10] Vatneberg S. Scaling of Calcium Carbonate on Metal Surfaces in Mixtures of Mono
Ethylene Glycol and Water [Master Thesis]. NTNU; 2008.
[11] Flaten EM, Seiersten M, Andreassen JP. Polymorphism and morphology of calcium
carbonate precipitated in mixed solvents of ethylene glycol and water. Journal of
Crystal Growth. 2009;331:35333538.
[12] Salami ARA, Monem AA. Downhole and Topside Scale Challenge Removal, Prevention and Inhibition Strategy. Society of Petroleum Engineers. 2010;.


[13] Andreassen JP. Associate professor in Crystallization, Department of Chemical

Engineering, 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 Crystallization]. NTNU; 2010.
[16] http://www.statoil.com/en/technologyinnovation/fielddevelopment/flowassurance
/scale/pages/default.aspx; 2010.
[17] Nergaard M. Master Student, NTNU; 2010.


Figure 1: Solubility diagram[15]

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


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

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


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

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