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

2.3 Sulphate Scale Formation 
Sulphate scales result mainly from the mixing of different incompatible waters. 
Two waters are incompatible if they interact chemically and precipitate 
minerals when mixed. 
A typical example of incompatible waters are sea water, with high 
concentrations of 
-
2
4
SO
and low concentrations of 
+
2
Ba
/
+
2
Sr
, and reservoir 
formation waters, with very low concentrations of 
-
2
4
SO
but high concentrations 
of 
+
2
Ca
, 
+
2
Ba
and 
+
2
Sr
(Kaasa). 
In normal oilfield conditions these two types of brine are chemically stable 
when they are kept apart. However, sea water is almost universally used in 
offshore systems in order to maintain reservoir pressure during production. 

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TOPIC 2: Oilfield Scale 
 
 
 
14 
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-W
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NIVERSITY B41OA December 2018 v3 
When seawater is injected into the reservoir during a waterflooding exercise, it 
mixes with the formation water in the reservoir and production systems If the 
supersaturation of a particular salt, 
e.g. 
CaSO
4
, BaSO
4
and/or SrSO
4
is 
reached during mixing, (as the percentage of sea water increases in the 
aqueous phase), crystallisation becomes possible and the formation of scale 
may become a serious problem (Cowan, 1976; Tanner, 1986; Mazzolini, 1990; 
Todd, 1990; Todd, 1992, Vetter, 1972, Mitchell, 1980). 
In linear flow situations, where sea water is injected into a matrix containing 
formation waters, brine incompatibility and sulphate scale formation does not 
generally represent a very significant problem (Weintritt, 1967). 
In linear flow through porous media, frontal dispersive mixing is relatively low 
and, where it does occur, it tends to only precipitate out the small quantity of 
barium sulphate in that mixed zone that forms a “spacer” region between the 
displacing and displaced brines. 
However, in the case of radial flow though heterogeneous or layered systems, 
formation water regions may be partially bypassed by injected sea water. 
Thus, the injected and formation brine mixing is now by co-production from 
different zones in the near-wellbore region and this is where precipitation, due 
to brine incompatibility, becomes more likely. 
To summarise, the areas with the greatest propensity for the mixing of 
incompatible waters are the production and injection near-wellbore areas: 
•
Since inhibitor is often added initially into the injection sea water as a 
precaution, it is believed that the relative risk of precipitation is small in 
the near-wellbore areas around the injection wells. 
•
In the near-wellbore area around producing wells, however, water 
coning and flow convergence increase the potential for brine mixing (as 
waters from different zones enter the near wellbore area at different 
rate). This leads to the possibility of severe formation damage 
problems and associated scaling problems in the production tubing and 
equipment (Payne, 1987; Mitchell, 1980).