Page 250 - Formation Damage during Improved Oil Recovery Fundamentals and Applications
P. 250
Formation Damage by Inorganic Deposition 221
5.2.1 Carbonate scales
22
Carbonate scales include precipitates containing CO , which are mainly
3
barium carbonate ðBaCO 3 Þ, strontium carbonate ðSrCO 3 Þ, calcium
carbonate ðCaCO 3 Þ, and iron carbonate (FeCO 3 Þ. Of the carbonate
scales, calcium carbonate scale is probably the most commonly encoun-
tered scale in oil and gas production. Many of them can precipitate in
different mineralogical forms. For example, calcium carbonate (CaCO 3 )
can exist as calcite, aragonite, vaterite, or witherite. The equilibrium of
carbonate scales can be summarized by considering the two stoichiometry
equilibria, with calcium used as an example:
C 11 1 2 HCO 3 Þ2C a HCO 3 Þ
ð
ð
a 2
C a HCO 3 Þ -C a CO 3 Þk 1 CO 2 m 1 H 2 O
ð
ð
2
From the above equilibria, we can see that the presence of CO 2 or
increasing CO 2 partial pressure leads to the increase of CaCO 3 solubility
in water. When CO 2 comes out of solution as the pressure decreases,
CaCO 3 solubility in water decreases. As CaCO 3 solubility in water
decreases with the increase of temperature, changing temperature can also
move the equilibrium to the right in the equation and increase the risk of
carbonate scale forming. The solubility of CaCO 3 also increases as the
level of dissolved salts increases (Plummer and Busenberg, 1982).
Many carbonate scales are acid soluble, which generally makes it easier
to remediate than sulfate scales. However, damage in downhole frac-
packs is very difficult to remove without very good treatment diversion
or the use of coiled tubing. Therefore, a proactive approach to downhole
carbonate scale prevention is essential.
5.2.2 Sulfate scales
The concentration of sulfate in formation water is usually low, but it is
high in sea water as shown in Table 5.1. When sulfate-rich water is
injected to flood the barium/strontium-rich formation water, the
tendency of forming sulfate scales increases. The sulfate scales include
barium sulfate ðBaSO 4 Þ, strontium sulfate SrSO 4 Þ like celestite, and
ð
calcium sulfate CaSO 4 Þ. calcium sulfate has three different crystalized
ð
1
forms known as gypsum (CaSO 4 U2H 2 O), hemihydrate (CaSO 4 U H 2 O)
2
and anhydrate (CaSO 4 ). Gypsum is the most common calcium sulfate