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

3.5 Wax Disappearance Temperature (WDT) 
Wax disappearance temperature (WDT), instead of WAT, represents the true 
solid-liquid equilibrium (SLE) point, provided that adequate time is provided for 
the system to reach equilibrium. 
The difference between WDT and WAT, measured for a given fluid, can be 
very significant. For instance, for a North Sea crude oil the WDT, detected 
using microscopy, is 28
o
C higher than the WAT – as measured using the 
same technique (Ronningsen et al., 1991). 
Therefore, the experimental WDT data should be used for developing the wax 
thermodynamic model and this, in turn, should improve the reliability when it 
comes to estimating the wax phase boundary. 
 
3.5.1 Experimental Measurements 
The techniques used for detecting the WDT are listed below 
Visual Approach 
As mentioned the visual technique, sometimes aided by using microscopy, is 
sensitive for detecting the appearance and disappearance of wax crystals in 
clear fluids. It is recommended to combine this technique with step-heating for 
reliable detection of WDT. 

---

TOPIC 3: Wax Deposition 
 
 
 
11 
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3 
In this method the system is cooled down to form wax. Then the temperature 
is increased, in a step-wise fashion, giving enough time for equilibrium at each 
temperature step. 
At the end of each temperature step, the fluid system is visually examined for 
the presence of wax. The lowest temperature at which no wax is present in the 
system is recorded as WDT. 
The accuracy of the system is a function of the temperature steps used near to 
the wax phase boundary – generally an accuracy of better than 1 K is 
achievable. The method can provide reliable results for synthetic mixtures and 
clear reservoir fluids, and is used for measuring WDT for synthetic mixtures 
and clear reservoir fluids.