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

4.1 Introduction 
An emulsion may form when two liquids that are essentially insoluble in each 
other, for example oil and water, come into contact. However, other 
requirements are that there must be sufficient high shear mixing and that an 
emulsifier must be present – an emulsifier is some sort of surface active agent. 
The high shear mixing breaks up one phase (discontinuous phase) into fine 
cloud of liquid droplets (with very large surface area) into the second phase 
(known as the continuous phase). Depending on the starting conditions, oil 
and water systems form two main types of emulsion. 
Figure 1 shows a Water-in-Oil (W/O) emulsion, where the water phase is 
dispersed as droplets and the oil phase is the continuous phase. 
 
Figure 1: Water-in-Oil Emulsion 
Figure 2 shows an Oil-in-Water (O/W) emulsion, where the oil phase is 
dispersed as droplets and the water phase is the continuous phase; other 
more complex emulsion behaviours are possible – however, W/O and O/W 
emulsions are the two principle types. 
Figure 2: Oil-in-Water Emulsion 

---

TOPIC 4: Asphaltenes 
 
 
 
3 
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3 
Emulsions may also be classified as either micro-emulsions or macro-
emulsions. Micro-emulsions consist of very small (< 20 nm diameter) 
thermodynamically stable droplets. On the other hand, macro-emulsions are 
not thermodynamically stable. In this section we shall only consider macro-
emulsions, since these are the predominant type found in the oil and gas 
industry. 
 
Although macro-emulsions are not thermodynamically stable, the dispersion 
may take a very long time to settle if an emulsifier (surface active agent) is 
present – in this more restricted sense the emulsion is said to be “stabilised” 
by the surface active agent. 
The surface active agents can either be present in the produced oil/water 
system (these are naturally occurring agents) or may have been deliberately 
added to the system. In the case of oil and gas processing for export, the aim 
is to de-emulsify the oil/water system – thus, any surface active agents that 
are present will be naturally occurring compounds, generated within the 
reservoir at the time the oil itself was formed.