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

Slight Sulphate Supersaturation/Initial SW Breakthrough 
If the scaling problem is assessed to be small (i.e. slight sulphate 
supersaturation) and low water volume (< 500 bbls) and if there is a low SW 
percentage (0% to 15%), then a scale inhibitor squeeze treatment is 
recommended – given that monitoring has shown scale build-up on safety 
valves, or has shown a restriction in production tubing diameter. 
The duration of the effective protection for such a scale inhibitor treatment 
could be 6 month to 18 months depending on the exact severity of the scaling 
problem and the volumes of produced water. 
Moderate Sulphate Supersaturation/Later SW Breakthrough 
As the amount of seawater increases, the mass of scale that is able to form is 
less but, as has been already described, the supersaturation also increases. 
That is, the supersaturation (driving force for scale to form) is greater closer to 
a seawater cut of 50:50 FW/SW. 
As a consequence, the amount of scale inhibitor required to control scale is 
higher; in fact, the MIC normally rises to 2 to 4 times higher than is required for 
initial seawater breakthrough. This has important consequences for the 
definition of test condition when assessing the problem and selecting a scale 
inhibitor, either for downhole or topside application. 
Severe Scaling: Higher Sulphate Supersaturation and Higher SW 
Volumes 
If the scaling problem is assessed to be more serve (a higher supersaturation 
with respected to sulphate) and the water volume is higher (> 1,000 bbls per 
day), then chemical scale inhibitor injection into the formation is essential to 
give protection. 
For more severe cases, the duration of such a treatment is dependent on 
produced water volumes, although a typical range would be between 3 to 12 
months. 

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TOPIC 2: Oilfield Scale 
 
 
 
35 
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NIVERSITY B41OA December 2018 v3 
As with all scale problems, if physical evidence of scale build-up on subsea 
safety valves is found, the removal of the scale must be carried out and 
followed immediately by a scale inhibitor squeeze treatment. The inhibitor 
scale squeeze process will need to be repeated a number of times over the life 
of the well as the inhibitor level falls below the MIC (Minimum Inhibitor 
Concentration). 
The longevity of the scaling problem is due to the nature of the recovery 
process, in which pressure support is conducted by injection of seawater into 
an aquifer below the oil leg. 
The application of a scale dissolver treatment prior to the application of scale 
inhibitor squeeze has, in some fields, been found to be beneficial. This is 
because perforation tunnels and restriction in pipe work can be controlled by 
application of sulphate scale dissolvers (see above).