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

4.2.2 Asphaltene Precipitation 
The mechanism of asphaltene precipitation is very complex and not well 
understood. It has been suggested that asphaltenes are partially dissolved in 
crude oil but are mainly colloidal forms stabilised by resin molecules 
(Sachanen, 1945). 
The resin-asphaltene attraction may become destabilised when the oil 
composition is changed such as in gas injection processes, alternatively the 
resins may be adsorbed onto rock surfaces, which then results in asphaltene 
deposition. 
As asphaltenes are charged particles, they may also precipitate as a result of 
the streaming potential generated in the flow of oil through porous media 
(Leontaritis and Mansoori, 1987). Carbon dioxide, a commonly used gas for 
enhanced oil recovery, may also precipitate asphaltic materials in contact with 
oil (Monger and Khakoo, 1981). 
It has been suggested that CO
2
reduces the pH of water present in the 
reservoir and causes the precipitation of asphaltenes and the formation of rigid 
emulsions. Miscible oil displacement achieved by injecting a slug of 
hydrocarbon intermediates such as LPG is particularly prone to asphaltene 
precipitation. 
Variations of pressure and temperature are also known to affect asphaltene 
precipitation. Contrary to wax deposition, the effect of temperature on 
asphaltene flocculation is less significant than pressure: 
•
Within a reservoir, if undersaturated oil is reduced in pressure, then this 
may lead to asphaltene precipitation. 
•
Unsaturated reservoir oil is more prone to asphaltene precipitation. 
When the reservoir pressure drops (by pressure depletion) asphaltenes 
appear and become a maximum just around the bubble point of the oil. 
The reduction of pressure below the bubble point, results in the 
liberation of light hydrocarbons, which in turn generally lowers the 
solubility of asphaltene compounds in the liquid phase. 
•
On the other hand, if the reservoir pressure is raised back well above 
its bubble point, then any precipitated asphaltenes will dissolve back 
into the oil phase once again. 

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TOPIC 4: Asphaltenes 
 
 
 
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NIVERSITY B41OA December 2018 v3 
A typical phase envelope for asphaltene, on a temperature-pressure diagram 
at constant composition, is shown in Figure 6. Notice that asphaltenes begin to 
appear as the pressure falls towards the bubble point. 
Figure 6: Pressure-Temperature Asphaltene Phase Behaviour 
The maximum amount of asphaltene deposited occurs around the bubble 
point. Notice that, as the pressure continues to fall, the asphaltene redissolve 
back into the oil phase. The behaviour of asphaltene deposition at constant 
temperature due to reduction of pressure is shown in Figure 7. 
Figure 7: Asphaltene Deposition Behaviour (around bubble point of the 
oil phase) 
This behaviour resembles that of retrograde condensation, where condensate 
is initially formed and then is vaporised back into the vapour phase as 
pressure continues to fall. 

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TOPIC 4: Asphaltenes 
 
 
 
10 
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NIVERSITY B41OA December 2018 v3 
Therefore, an oil sample may have an upper asphaltene (pressure) point, often 
referred to as onset of asphaltene flocculation, and a lower one. Asphaltene 
can only be present at a pressure between these two values at a given 
temperature and constant overall system composition.