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

 
Produced water re-injection 
In a number of reservoirs in the North Sea, produced waters are re-injected as 
a method of water disposal. The re-injection of produced water, commonly 
along with injection seawater, can pose problems due to brine incompatibility. 
If the produced water is already being protected by a chemical scale inhibitor 
(from both downhole squeeze application and/or topside continual injection), 
the amount of inhibitor is probably only sufficient to protect the brine until it is 
discharged from the platform. 
In the produced water re-injection scenario, the scale inhibitor must work for a 
longer period of time. During re-injection, the temperature may be reduced due 
to mixing with further seawater or transport along seabed injection lines and 
the inhibitor in this mixture may have to cope with an increased sulphate level 
from the mixing of additional seawater into the produced water stream. 
If insufficient scale inhibitor is present in the re-injected brine, then scale 
(carbonate, sulphate) may form and this may lead to well injectivity impairment 
by scale fines. 
The above problems can all be overcome by increasing the topside dose of 
scale inhibitor, such that it will control scale for extended period, at lower 
temperatures and at higher levels of sulphate. However, to do this and avoid 
the problems that may arise in produced water re-injection, these problems 
must be correctly diagnosed. 
Aquifer water injection 
In a number onshore fields in North Africa and the Middle East this is common 
practice but at present few North Sea operators are using aquifer water to 
maintain reservoir pressure. 
Aquifer water does offer the option of injecting water that is sulphate-free into a 
reservoir and thus eliminating the sulphate scaling problem. The carbonate 
scale problem is however not eliminated by such a support mechanism. 

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TOPIC 2: Oilfield Scale 
 
 
 
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NIVERSITY B41OA December 2018 v3 
If the aquifer water is produced to a topside facility prior to re-injection, then it 
could result in carbonate scale formation. Thus, scale inhibitor would still have 
to be applied in order to prevent fine-generation and well-productivity decline 
on reinjection. 
Whenever producing a reservoir supported by aquifer water injection, the 
carbonate scaling tendency of the formation water brine still exists and it will 
therefore still be essential to evaluate the scale problem – also scale inhibitor 
squeezing may still be required to treat this problem.