B41oa oil and Gas Processing Section a flow Assurance Heriot-Watt University

2.2 Carbonate Scale Formation 
Calcium (and magnesium) carbonate scale formation is caused mainly by a 
reduction in pressure during production. The effect of pressure alone on the 
supersaturation (
P
S
) for carbonate scale (in a real field) is quite complicated.

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TOPIC 2: Oilfield Scale 
 
 
 
12 
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NIVERSITY B41OA December 2018 v3 
The physical and thermodynamic effects of pressure on the solubility of 
CaCO
3
are of only minor significance. More important is the effect of pressure 
on the calcite solubility of carbon dioxide and the related chemical reactions 
that lead to the formation of CaCO
3
precipitate(s). Firstly, the precipitation of 
CaCO
3
is controlled by the following equilibrium: 
2
2
3
3
2
CO
O
H
CaCO
2HCO
Ca
+
+
→
+
−
+
…
.
………
.(2.4) 
Thus, as the pressure decreases, and the systems falls below the carbon 
dioxide bubble point, CO
2
is effectively lost from solution into the gas phase. 
Consequently, the driving force leading to the precipitation of CaCO
3
is 
increased by the above equilibrium. 
Secondly, and in a related manner, the loss of CO
2
from solution results in a 
reduction in the concentration of carbonic acid by the following chemical 
reaction: 
3
2
2
2
CO
H
O
H
CO
→
+
…………………
..
………
....(2.5) 
And carbonic acid dissociates in accordance with the equilibrium reaction 
+
+
→
H
HCO
CO
H
-
3
3
2
…………………
..
……
.
…
...(2.6) 
A reduction in hydrogen ion concentration consequently results in an increase 
in brine pH (Meyers, 1984; Payne, 1987; Meyers, 1985; Wat, 1992; Tanner, 
1986). Increases in brine pH significantly reduce the solubility of CaCO
3
. 
This is illustrated in Figure 4 which presents the predicted CaCO
3
precipitation 
from the above brine system at pH values of 5 and 6. Thus the pressure 
reduction during production leads to the loss of CO
2
from the brine system and 
this in turn leads to CaCO
3
precipitation which is controlled by the 
interconnected chemical equilibrium reactions, equations (2.4), (2.5), (2.6). 
Figure 4: Carbonate Precipitation versus pH and Brine Temperature 

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TOPIC 2: Oilfield Scale 
 
 
 
13 
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NIVERSITY B41OA December 2018 v3 
However, when compared with the sulphate scales, calcium carbonate scale is 
generally a relatively simple problem to deal with, either using chemical scale 
inhibitors or, once formed, by using straightforward dissolution techniques 
(Tanner, 1986; Mazzolini, 1990; Rhudy, 1993; Shaughnessy, 1993). 
For calcium carbonate scale dissolution, mineral acids and organic acids (less 
damaging in terms of corrosion), together with a range of commercial 
sequestrants, are available. Such remedial techniques are discussed in later 
sections of this topic. 
Calcite scale is generally recognised as an early field problem often arising 
prior to sea water breakthrough as a result of the reduction in pressure during 
production. Carbonate scales may also be regarded as a moving problem in 
the sense that, as the pressure reduces, the carbon dioxide bubble point 
gradually migrates lower down the production string and will ultimately move 
into the near wellbore formation. 
This means that treatment of carbonate scale will become more difficult during 
the lifetime of a field, since ultimately scale will begin to form upstream of the 
wellhead injectors. 
At this point, scale “squeeze” treatments (see later) have to be applied in order 
to control the precipitation. However, although the possibility of calcite scale 
continues after sea water breakthrough, sulphate scale (which is generally 
more severe and more difficult to control) becomes a more significant problem 
in most North Sea producers. 
Calcium carbonate scale has been the subject of many investigations (Cowan, 
1976; Meyers, 1984; Meyers, 1985; King, 1989; Hardy, 1992).