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

Homogeneous Nucleation 
For initial homogeneous nucleation, there are two free energy terms that 
control this process (Nancollas, 1985; Naono, 1967). These relate to the 
favourable free energy, related to the release of supersaturation and an 
opposing (unfavourable) free energy, related to the creation of a surface. 

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TOPIC 2: Oilfield Scale 
 
 
 
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In supersaturated systems, agglomeration of ions to a critical size leads 
ultimately to precipitation as the supersaturation is released. 
However, in mildly supersaturated solutions the unfavourable free energy due 
to the creation of a surface can lead to the dissolution of agglomerated 
particles and in this manner supersaturated systems can remain stable for 
long periods of time 
 
Figure 2: Free Energy of Nucleation versus Cluster Radius 
Heterogeneous Nucleation 
In field production scenarios it is recognised that precipitation is more likely to 
occur on surfaces which are already present in the system. Such surfaces may 
be existing scale deposits, metal surfaces offering available sites for 
adsorption of lattice ions (production equipment, pipelines etc.) or the rock 
formation itself. 
Heterogeneous nucleation on such surfaces would be more favourable than 
homogeneous nucleation, since the free energy barrier (due to the creation of 
a surface) has been significantly reduced (Naono, 1967). 
Crystal Growth 
Once nucleation has occurred, either by homogeneous nucleation, or 
heterogeneous nucleation, crystal growth will then continue in order to release 
the supersaturation from the system. 
Crystal growth occurs from so called 
active sites 
on the crystal surface. These 
active sites
 
may be kinks on steps formed on the surface. Such active growth 
sites allow further growth because collision of solution ions at these sites 
leaves the colliding ions effectively in contact with several surface ions 
enabling strong bonds to form. 

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TOPIC 2: Oilfield Scale 
 
 
 
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Scaling ions which land on flat areas of surface have the propensity to bounce 
back into solution (due to weaker/fewer contact points) or move around the 
surface until they are effectively trapped by multiple binding at the active 
growth sites, see Figure 3. 
 
Figure 3 Representation of a Crystal Surface with Defects 
All the factors listed in this section control the possibility of scale formation. 
These factors depend on both the need for the solution to be supersaturated in 
a salt, as well as the kinetics of scale formation.