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

2.1.4 Scale Prediction 
When suspicion arises that a mineral scale problem may be encountered in a 
system, either downhole or topside, it is very important to predict the severity 
of this problem. 
Effective prediction of scaling requires a reliable thermodynamic (equilibrium) 
model for the prediction of the scaling tendency, combined with a kinetic model 
for the prediction of the scaling rate (Zhang, 2001). 
From the equilibrium model, it is possible to calculate whether it is 
thermodynamically possible to precipitate minerals and the maximum amount 
of scale that can precipitate under a specified set of conditions. 
The kinetics of a reaction will determine how fast the reaction proceeds in 
order to bring a system to thermodynamic equilibrium and is influenced by 
several factors, with temperature being the most important. 

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TOPIC 2: Oilfield Scale 
 
 
 
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NIVERSITY B41OA December 2018 v3 
It is therefore necessary to include kinetic considerations in an evaluation of 
the scaling potential. Normally, the field operator does not have control over all 
the variables that determine scale deposition, particularly the water 
composition, but the application of thermodynamic and kinetic models could 
help identify some production parameters that could be controlled 
(Zhang,2001). 
Software prediction packages are commercially available which can allow us 
to carry out calculations to answer the following questions (that are of 
increasing difficulty): 
•
Will there in fact be a scale problem in the field? 
(Answer: yes/no?). 
•
What will be the “severity” of this problem? 
(Answer: “insignificant”, “mild”, “moderate”, “quite severe”, “very 
severe” – these terms will be defined later). 
•
Is it possible to calculate the severity of the problem quantitatively? 
(Answer: the amount of scale in mg/l for various brine mixing ratios, 
and the corresponding supersaturations). 
Simple prediction methods include: 
A. The Stiff-Davis method used to predict the formation of calcite. 
B. The Skillman-McDonald-Stiff method used to predict the formation of 
gypsum and barite. 
C. The Oddo and Tomson method is a more complex, but also a more 
accurate way of predicting formation of all the common oilfield scales. 
The basis of each of these methods is discussed in the following paragraphs, 
starting with the prediction of calcite formation: