TOPIC 2: Oilfield Scale
6
©H
ERIOT
-W
ATT
U
NIVERSITY B41OA December 2018 v3
Thus, solutions may have the following range of Saturation Index:
•
A saturated solution (in equilibrium) will have a
0
=
SI
.
•
An undersaturated solution will have an
0
<
SI
.
•
A supersaturated solution will have an
0
>
SI
.
•
The saturation index has a logarithmic scale. For example,
a solution
with an
3
=
SI
is ten times more oversaturated than a solution with an
2
=
SI
.
It is important to remember that a positive
SI
does not necessarily mean that
a scale will form – the kinetics of scale formation may be too slow. As stated
earlier, it is a measure of
possible
scale formation.
Supersaturation
The supersaturation of an aqueous solution of a salt (
P
S
) refers to how much
more of the salt is currently dissolved in the solution above that which would
be present at equilibrium. Some of the salt must precipitate (or crystallise) from
a supersaturated solution in order for it to come to equilibrium.
The supersaturation is an important determining factor that controls the
precipitation of salts from solution.
In a sense, the supersaturation may be
thought of as the thermodynamic “driving force” for precipitation; i.e. it is more
likely that precipitation will occur easily from
a higher than from a lower
supersaturated solution of a salt. The degree of supersaturation, in turn, is
affected
by changes in temperature, pressure and pH – all of these factors
may change during oil production.
For
sulphate scales, the main factor causing precipitation is the mixing of
incompatible injection sea water (high in sulphate anions) with the formation or
connate waters (containing barium, strontium and calcium cations).
For carbonate scales, the controlling factor is the reduction in pressure during
production leading to the loss of carbon dioxide (CO
2
) from aqueous solution
and the deposition of calcium carbonate.
For supersaturated aqueous salt systems there are a number of mechanisms
that determine the kinetics of scale formation – these are as follows:
1. Homogeneous nucleation.
2. Heterogeneous nucleation.
3. Crystal growth.
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