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

4.2 Asphaltenes 
Petroleum reservoir fluids may form solid phases during production and 
processing when their pressure, temperature and/or composition are varied. 
The formation and deposition of solid particles can potentially reduce oil 
recovery and may lead to other major operational problems: 
1. The formation of solid particles in gas production and blockage of 
reservoir pores near well bores have been reported. 
2. Wax deposition in pipelines carrying crude oils at low temperatures is 
well known and has been discussed earlier. 
3. Scale formation, both within the formation itself and within downstream 
processing equipment, is a major problem and has been discussed 
earlier. 
4. Water/gas solid structures, known as gas hydrates, have caused major 
operational problems in transporting water/hydrocarbon mixtures in 
pipelines (and in process equipment), and has also been discussed. 
5. The precipitation of asphaltenes in oil reservoirs under gas injection, 
and under other conditions, will be discussed in this topic. 
We can see that, the prediction of solid formation, and the conditions under 
which solid formation is inhibited, is of major interest in the petroleum industry. 
4.2.1 Characteristics of Asphaltenes 
Petroleum asphaltenes are high molecular weight constituents of crude oil 
comprising polycyclic aromatics or naphthenoaromatics – these have 
hydrocarbon side chains containing nitrogen, sulphur and oxygen compounds. 
A number of asphaltenes have been extracted, from many samples collected 
worldwide, and the elemental composition of these samples have been 
tabulated (Speight, 1980) – the proportion of these elements varies widely 
from 0.3 to 4.9 % for O; 0.3 to 10.3 % for S; 0.6 to 3.3 % for N. 
The structure of asphaltenes has been studied in detail by many investigators 
and an overall summary of these results is available (Yen, 1972). The basic 
units are polyaromatic sheets that are stacked by molecular association to 
form asphaltene particles. 
Several particles may aggregate to form micelles of different sizes. Reported 
data on the molecular weight of asphaltenes, depending on the type of 
analysis used, can range from 500 for individual sheets to 500,000 for 
aggregates. 
Asphaltenes are insoluble in low molecular weight normal paraffins and are 
classified by the precipitating solvent – different solvents precipitate different 
ranges and hence different amounts of asphaltenes. 

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TOPIC 4: Asphaltenes 
 
 
 
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Figure 3 shows the amount precipitated versus carbon number of precipitant. 
Figure 3: Asphaltene Precipitated from Oil Diluted by Different Normal 
Alkanes 
(Andersen and Speight, 1999)
The degree of precipitation (asphaltene yield) increases with increasing 
volume of precipitant and approaches a constant value at high concentration 
of added precipitant (normal pentane is the diluent in Figure 4).
Figure 4: Amount of Precipitated Asphaltenes versus Volume Normal 
Pentane Precipitant 
(Andersen and Speight, 1999)

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Asphaltenes are commonly defined as the normal heptane insoluble fraction of 
crude oil obtained by a standard IP (Institute of Petroleum) test method
(IP 143/90, 1985). The lighter fractions, which are soluble in normal heptane 
but insoluble in ethyl acetate at room temperature, are known as “resins”. 
Reservoir asphaltene precipitation is commonly caused by the addition of light 
hydrocarbon mixtures to oil – typically this may arise during gas flooding, or re-
injection operations; for example, the asphaltenes precipitated by propane 
injection contain a mixture of resins and a heavy aromatic fraction. 
The onset of asphaltene flocculation is defined as the point at which the 
asphaltene sheets begin to aggregate together – this is the precursor to 
emergence of a new solid phase. Several methods are available for the 
detection of the onset of asphaltene flocculation (Jamaluddin, et. al., 2001). 
The most common methods are as follows: 
•
Visual detection using a microscope. 
•
Detection of changes in transmitted light intensity. 
•
Detection of fluid viscosity changes. 
 
Deposition of asphaltenes is generally considered to be a problem with oil 
systems. However, recently such depositions have been reported in tubing 
and surface gas condensate reservoirs surface facilities – Hoflein field is a 
lean gas condensate field located NW of Vienna with a dew point pressure of 
28.5 MPa at 78ºC. 
Figure 5: Asphaltenes precipitated in Surface Facilities 
(Thou, et. al., 
2002)
This black organic material (Figure 5) initially exhibited a high viscosity but, 
when brought to ambient conditions, rapidly solidified (by evaporation of lighter 
components) to a dry powder. 

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Precipitated asphaltenes generally resemble liquid at reservoir conditions. 
However, they appear solid-like at surface conditions in most cases. Since, in 
surface facilities the deposited phase includes other types of heavy 
compounds (such as wax) it may actually behave quite differently to that 
obtained by standard laboratory test methods. 
A review of reported cases of asphaltene deposition and the associated 
problems in oil field is available in the literature (Leontaritis, 1989).