Prediction of the Formation of Water-in-Oil Emulsions - Mathematical Modelling and Numerical Simulation of Oil Pollution Problems - page 64

1000-10,000mPa s, the water droplets may be retained by viscosity alone. This is
felt to be the origin of the entrained water-in-oil type.
3.4.5 The Role of Weathering
Most crude oils and petroleum products require weathering (evaporation) before
they will form emulsions [ 4 ]. Most typical crude oils require weathering to make
the transition from the basic crude to a meso-stable or stable emulsion. Weathering
is necessary to increase the viscosity and the asphaltene/resin content to the point
where the next water-in-oil type is possible. It is also important to note that transitions
cannot occur after a water-in-oil state is created. It should be noted that once a water-
in-oil type is formed, it cannot make a transition to another type even if extensive
weathering or mixing takes place. This is felt to be a result of the exacting conditions
for each type. Further, asphaltenes appear to be tied up in the form of rag in broken
meso-stable emulsions. This rag formation appears to prohibit the formation of other
types of emulsions.
3.5 Modelling the Formation of Water-in-Oil Emulsions
3.5.1 Older Models
The emulsification processes described above were not apparent until about 20years
ago and have since been translated into modelling equations [ 4 ]. The different water-
in-oil states dictate that one simple equation is not adequate to predict formation.
Information on the kinetics of formation at sea and other modelling data was less
abundant in the past. It is now known that emulsion formation is a result of surfactant-
like action of the polar asphaltene and resin compounds. While these are similar com-
pounds that both behave like surfactants when they are not in solution, asphaltenes
formmuch more stable emulsions. Emulsions begin to formwhen the required chem-
ical and viscosity conditions are met and when there is sufficient sea energy. Further,
as pointed out above, three different water-in-oil types are formed, depending on the
oil type and its composition. Some oils do not form any water-in-oil types and this
fact is stated to be a fourth type.
In the distant past, the rate of emulsion formation was assumed to be first-order
with time. This was approximated with a logarithmic curve. The physical assumption
was that all oils uptake water on a first-order basis. This assumption resulted in large
errors in spill models.