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

2.6.2 Scale Removal 
If mineral scale deposition cannot be prevented (or if the strategy to prevent it 
has been unsuccessful) it may be possible in some cases to remove the scale. 
The removal of mineral scale deposits, or remediation of its effects, is 
generally carried out using either chemical or mechanical means. 
 
Chemical Removal Methods: Carbonate Scale 
Inorganic acids, such as hydrochloric acid (HCl) are commonly used to remove 
carbonate scales. In some cases, the HCl acid slug has corrosion inhibitor 
added to it; this to reduce corrosion damage that is especially rapid on 
chromium steels. 
Nitric acid (HNO3) is somewhat less aggressive to the metal surface - it will 
deposit a protective layer on the metal surface, which will help resist further 
corrosion. For high chromium steels, organic acids may also be used; e.g. 
acetic acid or formic acid. 
For iron carbonate, iron sulphides and iron oxide scales, HCl plus a 
sequestering agent is normally recommended to remove the scale. This 
approach prevents ferric hydroxide from precipitating from the spent acid 
solution. Typical sequestering agents include citric acid, acetic acid, EDTA, 
NTA and erythorbic acid. 
In order to remove mixed carbonate/sulphate scales chemically, some account 
must be taken of the precise composition of the scale deposit. Acids such as 
HCl, acetic acids and speciality chemicals have been used. In addition, 
whereas HCl is known to be ineffective in removing pure (or nearly pure) 
barium sulphate scale, it may act on the CaCO
3
in a mixed carbonate/sulphate 
scale – thus, “wormholing” and breaking up the mixed scale which can then be 
flushed out. 
Chemical Removal Methods: Sulphate Scale 
It is generally the case that sulphate scales are more difficult to remove 
chemically than carbonate scales. The only acid-insoluble scale which is 
chemically reactive is calcium sulphate (gypsum) which can be removed by 
reaction with KOH. This reacts to produce Ca(OH)
2
which is acid soluble. 
Gypsum can also be removed directly by the application of EDTA and or 
DTPA. 
Barium sulphate scale is one of the most difficult mineral deposits to remove 
chemically; the use of chelating agents forms the main chemical approach to 
its removal. Both EDTA and DTPA are the basis of many of the commercial 
sulphate scale dissolver packages (strontium and barium sulphate scales). 

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TOPIC 2: Oilfield Scale 
 
 
 
30 
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NIVERSITY B41OA December 2018 v3 
The application of scale dissolvers is common for cleaning scale build-up on 
production tubing. In addition, the application of scale dissolvers into the near-
well bore has been quite successful in a number of production wells in the UK 
sector of the North Sea.