TOPIC 2: Oilfield Scale
5
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ERIOT
-W
ATT
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NIVERSITY B41OA December 2018 v3
For example sodium chloride will dissolve in water and then dissociate:
−
+
+
→
Cl
Na
NaCl
Solubility values of the most common scales are shown in Table 1.
Table 1: Comparison of Various Properties
of the Most Common
Oilfield Scales (Lide, D. R. (Editor-in-Chief):
CRC Handbook of
Chemistry and Physics
, 75th Ed., CRC Press Inc., 1995
http://www.minerals.net.)
The scales in Table 1 are ranked in order of solubility and hardness. BaSO
4
is
the least soluble and hardest, and therefore the most difficult to remove once
formed (the properties of sand is provided as a reference).
There are various measures used to gauge how likely an aqueous solution is
to form scales. These are related to how saturated a solution has become and
the kinetics of scale formation:
Ion Product and Solubility Product Constant
Multiplying the concentrations
of the ions together
)
(
−
+
×
Cl
Na
will
give the
Ion Product
IP
(
). If the solution is saturated with respect to that mineral, and
in equilibrium, the ion product will equal the Solubility Product Constant
SP
K
.
The solubility product constant for any
salt will vary with pressure,
temperature,
ionic strength, and possibly pH.
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