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

2.5.2 Scale Inhibitor Chemistry 
As noted above, the main chemicals used to prevent the formation of mineral 
scales are known as scale inhibitors and the main commercially available 
products are shown in Figures 7 and 8. 
The most common scale inhibitors used for topside and downhole application 
in the oil industry may be classified as follows: 
•
Phosphonates
: These are small molecular organic species having a
C-PO
3
H
2
functional unit. 
•
Polymeric scale inhibitors: 
These are small polymeric species e.g. 
polyacrylate, phosphino carboxylates, poly vinyl sulphonates etc. 
The molecular structures of several common scale inhibitors are shown in 
Figures 7 and 8 and the mechanism through which these scale inhibitors 
operate are discussed below. 

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TOPIC 2: Oilfield Scale 
 
 
 
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The type of chemical required for a particular scaling problem, in either a 
vertical or a horizontal well, must be selected based on the following: 
The specific mineral scale to be inhibited 
 
The type of scale inhibitors which will give effective control of sulphate and 
carbonates scales include phosphonates, phosphino carboxylic acids (PPCA), 
carboxylic acids, vinyl sulphonate co-polymers and poly vinyl sulphonate 
(PVS). 
Compatibility with the produced brine chemistry 
 
The scale inhibitor may be incompatible with the produced brine leading to 
precipitation of an inhibitor-cation complex. In particular, the concentration of 
divalent cations has a significant bearing on the compatibility of scale 
inhibitors, when used for squeeze, continuous injection, and topside 
applications; chemicals such as poly vinyl sulphonate (PVS) and sulphonated 
co-polymer have high divalent ion compatibility. 
Low pH phosphonates have moderate compatibility, but partially neutralised 
polymers and phosphonates may have very poor compatibility when a 5% to 
10% scale inhibitor solution in seawater is mixed with the formation water. 
In a vertical well, such problems can be reduced by the application of a 
seawater preflush containing 500 ppm to 1000 ppm scale inhibitor. This 
pushes the formation water away from the near wellbore region and prevents 
the high concentration slug from mixing with the formation water. 
This type of preflush strategy has been used with considerable success in 
vertical and deviated wells. However, it may be very difficult, if not impossible, 
to apply such a preflush in horizontal wells. 
There are significant differences in the types of scaling problem – their 
practical treatment strategy will depend on the type of well which is to be 
treated. A possible way forward when considering the implication of recovery 
methods and well design would be to carry out the following studies: 
1. Evaluate the levels of the selected scale inhibitor in order to determine 
the minimum inhibitor concentration (MIC) required to prevent 
carbonate and/or sulphate scale. 
2. Determine which chemicals have suitable compatibility in the formation 
brine. 
3. Carry out a limited core flood study (using the chosen product from 
steps 1 and 2) in order to better assess issues of formation damage 
and the expected squeeze lifetime. 
4. Carry out a modelling study to design the initial squeeze treatments in 
this formation. From this information, it should be possible to make a 
more informed prediction as to the likely squeeze lifetimes and the 
required frequency of treatment. This information will have significant 
implication for the type of well design. 

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TOPIC 2: Oilfield Scale 
 
 
 
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NIVERSITY B41OA December 2018 v3 
In summary, the last few sections have discussed: 
•
The types of carbonate and sulphate scales which may be expected in 
oil reservoirs, why these occur and how they may be predicted in order 
to make as assessment of the severity of the scaling problem. 
•
Several issues surrounding how the reservoir recovery mechanism and 
the well configuration may influence the scaling problem – in terms of 
the severity of scale formation, the accessibility of the scale deposition 
problem and in the ease of treating the scale problem. 
•
The use of scale inhibitors in the prevention of scale for both downhole 
and topside treatments (which are related). 
In this section, these issues are covered in an introductory manner in order to 
help those who may be unfamiliar with the subject. In later sections, much 
more detail will be given, in terms of both chemical and field operational 
matters, as they relate to scale prevention and removal.