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

3.2 Paraffin Wax Deposition 
Hydrocarbon reservoir fluids consist of variety of light and heavy hydrocarbon 
species, as high as C
100
. These hydrocarbons are in-solution due to the high 
temperature and pressure prevailing at reservoir depth. The presence of other 
light and intermediate hydrocarbons is also an important factor, as they act as 
a natural solvent, keeping the heavier hydrocarbons in solution. 
Whenever hydrocarbons are produced to the surface, there are three major 
changes: 
1. The fluid system temperature decreases, as heat is lost to the 
surroundings, fluid expansion occurs and whenever light compounds 
evaporate from solution. 
2. The fluid system pressure decreases, due to frictional pressure drop, 
potential energy loss (and conversion to kinetic energy). 
3. There could also be compositional and phase changes, because of 
variations in the system temperature and pressure. 
Table 1 shows the melting points of some of the paraffin molecules found in 
hydrocarbon reservoir fluids. As shown in the table, heavy hydrocarbons have 
very high melting point and one may expect them to be deposited at reservoir 
conditions. 
The fact is that these hydrocarbons can remain in the liquid phase, at 
temperatures well below their melting points, is due to the presence of some 
light and intermediate compounds, which act as natural solvents. 
However below a certain temperature, which depends on the fluid system, this 
natural solvent behaviour is no longer able to keep the heavy-ends in solution; 
as a result, these heavy hydrocarbons will come out of solution as solid wax. 

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TOPIC 3: Wax Deposition 
 
 
 
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NIVERSITY B41OA December 2018 v3 
Clearly, any further reduction in the system temperature will result in more wax 
formation as lighter hydrocarbon join in the wax formation process. Table 1 
below shows the melting point of a range of pure paraffinic species: 
Table 1: Melting Points of Some Paraffins 
In addition to paraffins and iso-paraffins, other components in reservoir fluid 
can also take part in wax formation – these include naphthenes and, to a 
lesser extent, aromatics. 
Wax formation is a strong function of system temperature and, to a lesser 
extent, the presence of light components and system pressure. The wax 
phase boundary can be determined experimentally, or by using 
thermodynamic models. All these experimental techniques, predictive models, 
(and factors affecting their reliability), will be discussed later. 
The range of hydrocarbons found in wax is a strong function of minimum 
system temperature. Reservoir temperatures are generally in the range of 80 
to 150 ºC, while the pipeline temperature could drop as low as 4ºC in subsea 
transfer lines. For some fluid systems, such a huge reduction in pipeline 
temperature can result in significant wax formation.