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

Nuclear magnetic resonance (NMR) 
NMR is not very sensitive when the amount of wax precipitated is low (e.g., 
less than 5%). Reliable experimental data on wax phase boundary is of vital 
importance for reducing wax risks as well as developing wax models. 
3.4.3 WAT versus WDT 
Wax research at the Institute of Petroleum Engineering, Heriot-Watt University 
showed that there are two major problems with the existing experimental 
methods for determining the wax phase boundary: 
1. Lack of thermal and/or thermodynamic equilibrium: 
•
Almost all existing experimental methods are based on 
continuous cooling and/or heating, hence the system cannot 
achieve thermal or thermodynamic equilibrium. 
•
Although it is possible to improve this shortcoming by reducing 
the sample size and/or rate of temperature changes, these 
measures often results in other limitations (such as increasing 
the experimental time). 
2. WAT vs WDT: 
•
Most research laboratories measures Wax Appearance 
Temperature (WAT) and this often does not represent a real 
equilibrium temperature. 
•
One should remember that wax formation is a crystallisation 
process where a new phase appears. Therefore, for new phase 
formation, there is generally a need for some degree of 
supersaturation/subcooling. 
•
Instead Wax Disappearance Temperature (WDT,) which can 
represent the real equilibrium point, should be measured. 
•
This leads to the conclusion that the WAT is very unlikely to 
represent a real equilibrium point and its use in tuning and 
validation of wax models is open to serious questions. 

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TOPIC 3: Wax Deposition 
 
 
 
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NIVERSITY B41OA December 2018 v3 
As mentioned above, the cloud point (a WAT measurement), is commonly 
measured in laboratories. The cloud point is then used for tuning the wax 
thermodynamic models and these models, in turn, are used as a basis for 
developing the wax deposition model. 
As stated, the problem is that the cloud point is not necessarily an equilibrium 
point (due to subcooling required for forming solids). The degree of subcooling 
for a given fluid can vary (depending on the crystallisation condition). 
For instance, fast cooling may lead to high degree of subcooling. WAT values, 
measured for a given fluid, will then depend on the test procedure – as a result 
depressed value of WAT can be obtained if the sample is cooled fast. 
Wax deposition can take place at temperatures above the cloud point. As 
measured in the laboratory, it has been reported that wax can be deposited in 
pipelines at temperatures higher than WAT (Hamouda et al.,1993, SPE 
25189).