Page 253 - Formation Damage during Improved Oil Recovery Fundamentals and Applications

224 Xingru Wu

5.3 PROCESSES OF SCALE FORMATION

Scale forming and deposition is a complex crystallization and
kinetic process. When conditions change or multiple streams of different
waters mix, some compounds that have low solubility in the new condi-
tions would form. When concentrations of these compounds exceed their
solubility, they could precipitate as solids. Three conditions must be
reached simultaneously for these compounds to precipitate: (1) supersatu-
ration, (2) nucleation; (3) adequate contact time for crystal growth. The
supersaturation state is thermodynamically determined, although the
nucleation and crystal growth are affected by kinetic factors.

5.3.1 Solubility and supersaturation
A solution is saturated if the dissolved compounds reach their equilibrium
with their solute. As the name suggests, supersaturation occurs when a
solution contains higher concentrations of dissolved compounds than the
equilibrium concentration of solute. Supersaturation can occur for a
number of reasons such as changes in pressure, temperature, pH values, or
mixing of incompatible waters.
When the concentration of a mineral exceeds the equilibrium con-
centration, precipitation starts to take place. Scaling tendency is used as a
measure of the thermodynamic driving force for the scale formation. The
scaling tendency is measured by the scaling index or the saturation ratio
(Oddo and Tomson, 1994, Langelier, 1936) that is defined as the ratio of
the ion activity product to the solubility product as follows:

activity product
SI log (5.1)
K eq
If SI , 0, the solution is undersaturated, which means that the solu-
tion can dissolve more solute. If SI 5 0, the saturation is at equlirbium;
and when SI . 0, it indicates the solution is supersaturated (or oversatu-
rated). In the literature, there are many simplified and empirical correla-
tions to estimate the scale index (Frenier and Ziauddin, 2008).
For example, calcite formation is based on the following reaction:

Ca 21 22
1 CO -CaCO 3
3