Page 184 - Handbook Of Multiphase Flow Assurance

180 5. Flow restrictions and blockages in operations

Free government-developed models for ion solubility in water at reservoir conditions are
also available for use.
Scale becomes an issue when water or another solvent can no longer dissolve the ions
when conditions such as temperature, pressure or composition change.
There are two common metrics of brine supersaturation: Saturation ratio (SR) and
Saturation index (SI).
Saturation of salts dissolved in water may be expressed as a saturation ratio which is deter-
mined by the activity coefficients of ions and the solubility product of scale.

SR = (a cation a anion ) K/ solubilityproduct

Thermodynamically scale precipitation is possible when SR > 1 but it takes time for ions to
find each other and combine which is represented as kinetics of the process.
For calcium carbonate scale the SR > 1.2 causes scale deposition when water temperature
T water > 100 °C. Under pressure water boils at >100 °C but loses some of its ability to dissolve
carbonate ions and ions diffuse in hot water more rapidly. At lower temperature the satura-
tion ratio SR must reach 2–3 for calcite scale to appear.
For barium sulphate scale to become an operational issue, both the saturation ratio must
be >3 and the scale precipitation amount must be >50 mg/L (Graham et al., 2005; Simpson
et al., 2005).
Brine instability is also reported as Saturation Index (SI).

SI = log 10 (a cation a anion /K solubility product)

When SI > 0 or SR > 1 then aqueous brine is saturated with ions. Various scales precipitate
at different SI. For example, calcite precipitation is expected when SI > 1, barite when SI > 0.5.
Location of scale deposition usually coincides with the location of precipitation.
Rate of scale deposition is usually measured in a laboratory.
An approximate correlation may be made between the rate of calcite scale deposition and
the saturation index based on results presented by Setta et al. (2012).

CalciteScaledeposition rate inches yr) = 33 ( SaturationIndex − 009)
(
. ×
.
/
This correlation is only applicable to calcite because barite uses a different threshold for its
saturation index.
Remedial actions

Scale remediation methods depend on its composition. Halite scale is water-soluble. Halite
scale blockage formed in a North Sea pipeline downstream of a methanol injection port was
cleared by water circulation.
Carbonate scales are soluble in acids. Mild acids such as citric or acetic as well as strong
acids as hydrochloric can be used to dissolve carbonate scale.
Calcium sulfate (gypsum) is also soluble in acids such as hydrochloric or sulfuric.