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

 
 
Quartz Crystal Microbalance (QCM) 
QCM is an innovative technique used for detecting the WDT. This technique is 
based on the change in the resonant frequency and conductance of QCM. A 
quartz crystal is generally used, as it is not very sensitive to the changes in the 
system temperature. 
Each QCM has a resonant frequency which is a function of the mass of the 
quartz, as well as some other factors like the system’s density and viscosity. It 
is possible to measure one hertz change in the resonant frequency. This is 
equivalent to 1 nano gram (hence the term micro-balance). The procedure is 
as follows: 
•
In a typical experiment, the fluid system is cooled to form wax. 
•
The system is then heated, using step-heating and leaving sufficient 
time, at each temperature step, for the system to achieve equilibrium. 
•
During cooling and wax formation, the wax crystals stick to the surface 
of the QCM, reducing its resonant frequency. 
•
Upon heating, the wax crystals are dissolved and the resonant 
frequency is increased. 
•
A plot of resonant frequency vs temperature is used to determine WAT 
and WDT, as shown in Figure 2. 
•
The QCM is very sensitive, requiring masses in the order of 
micrograms to cause a significant change in the resonance frequency. 
•
The test needs only small quantity of fluid sample. 
•
Thus, after each heating step, it is quick for the QCM testing system to 
reach equilibrium. 
•
The QCM can be reliably applied to reservoir fluids when the visual 
technique is not suitable. 

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TOPIC 3: Wax Deposition 
 
 
 
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NIVERSITY B41OA December 2018 v3 
An example of a WDT test result is shown below: 
 
Figure 2: Wax Appearance WAT and Wax Disappearance WDT 
The above temperatures were measured for a stabilised gas condensate 
sample using a Quartz Crystal Microbalance apparatus. 
Modified ASTM 
The ASTM test method (ASTM D 2500-86) is modified by using a calibrated 
thermocouple placed alongside the thermometer bulb at the base of the 
vessel. This modification leads to an accurate reading of temperatures that 
may not be uniform in the fluids. 
In addition, the test in this laboratory is carried out with step-heating for small 
volumes of sample. The aim of the latter modifications is to improve the 
temperature uniformity in the test sample.