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

Seawater injection 
Where seawater (SW) is injected into an aquifer (or water leg below the oil leg) 
to maintain pressure: 
•
The initial produced water will be formation water (FW). 
•
This will be followed by mixed formation water/seawater (FW/SW), 
once the seawater breaks through at the production well. 
•
Water breakthrough, under this production scenario, is progressive 
possibly taking years to move from initial seawater breakthrough
(< 10% SW) to high seawater (> 90%) in the produced water. 
Where seawater is injected into the oil leg to maintain pressure: 
•
The initial water produced is connate or formation water (FW). 
•
This is followed by a shorter period of mixed formation water/seawater 
(FW/SW). 
•
Eventually production will approach 100% injected seawater (SW). 
•
Although the period of mixed production (FW/SW) may result in a very 
severe scaling problem, this region does not last long (months rather 
than years in several practical field cases). 
•
The short duration of mixed production (FW/SW) is due to piston-like 
displacement of the formation water by the seawater – this will result in 
the production of almost pure seawater after a few months. 
•
Several examples of this scenario exist in the North Sea, most 
noticeably in the Scott field (operators Amerada Hess) where a number 
of wells are supported by seawater injection into the oil leg. 
Partial desulphated seawater injection 
If the barium level in a reservoir formation water is very high (greater than 
1000 ppm), then it may be difficult or impossible to prevent barium sulphate 
scale formation when pure seawater is injected into the system (seawater has 
an 
−
2
4
SO
concentration of around 2900 ppm). 
There is no chemical scale inhibitor, at a practical Minimum Inhibitor 
Concentration (MIC), which will prevent scale formation under all conditions. 
If the concentration of sulphate anion can be reduced in some way, for 
example by using modern membrane technology, then this will alleviate the 
problem and will lead to a reduced scaling tendency of the mixed 
FW/desulphated SW system. The injection of partially desulphated seawater 
into reservoirs is only recommended whenever the FW/SW incompatibility is 
very severe. 
Using desulphation membrane technology, the sulphate content of injection 
brines may be reduced from the 2900 ppm to about 40-120 ppm. This 
significantly reduces the supersaturation of the produced fluid such that scale 
inhibitor work at lower levels (10s of ppm rather than 100s of ppm). 

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TOPIC 2: Oilfield Scale 
 
 
 
18 
©H
ERIOT
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ATT
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NIVERSITY B41OA December 2018 v3 
The use of sulphate reduced seawater does not eliminate the need to squeeze 
wells since it only reduces the sulphate level and does not affect the carbonate 
scaling potential. 
The use of sulphate reduction must be carefully considered as it is an 
expensive capital expenditure
 
item and does not entirely eliminate the 
operating expenditure
 
of squeezing inhibitor or using continual inhibitor 
injection.