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

2.7.2 Formation Water and Seawater (FW/SW) Produced 
We now consider the case when the produced brine composition is between 
that of the original formation brine and the injected brine; that is, it is a co-
mingled brine. 
When formation water and injected seawater are co-produced, the scaling 
potential of the water must again be assessed using scale prediction software. 
By assessing the percentage of seawater in the produced fluids, and using this 
software, the scaling problem can be assessed under various conditions 
throughout the entire downhole and topside production facilities. 
The typical conditions that are considered are: downhole T/P; wellhead T/P; 
flow line T/P; and topside T/P. The most severe type of problem that may 
occur is due to the formation of very insoluble barium sulphate scale. 
A quantitative measure of the BaSO
4
scaling severity is found by applying the 
scaling software as described above. However, a qualitative description of the 
scaling severity is as follows: 
•
A “minimal problem” is expected when the mixed brine water contains 
< 80 ppm Ba (Sr < 100 ppm) 
before 
this precipitates (i.e. we refer to 
the unprecipitated brine mixture composition here and below). 
•
A “moderate” problem is encountered when the mixed brine 
composition contains < 80-300 ppm Ba (Sr > 100 - 500 ppm). 
•
A “severe” problem is encountered when the mixed brine composition 
contains 300 to 1500 ppm Ba (Sr > 500 - 1000 ppm). 
The volume of water being produced and its predicted rise with time along with 
the appropriate production stream condition (T/P) must be assessed exactly as 

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TOPIC 2: Oilfield Scale 
 
 
 
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described above. From this data, it is possible to assess the amount of scale 
forming potential and its location. 
In most cases of barium sulphate scale formation (severe to moderate scale 
problems), the scale will form downhole and in subsea safety valves. The 
sulphate scale problem will tend to move uphole in moderate to minimal 
scaling systems. In this case, the scale will tend accumulate on and in the 
subsea safety valves and uphole tubulars. 
In most typical North Sea formation water (FW) compositions, the largest mass 
of scale is predicted to form during initial production of seawater (SW) – that is 
seawater cuts of around 3% to 15%.