Page 183 - Reservoir Formation Damage

Crystal Growth and Scale Formation in Porous Media 165

(SrSO 4), magnesium sulfide (MgSO 4) originating from mixing sea water
with brine, and rock and brine interactions (Oddo and Tomson, 1994;
Atkinson and Mecik, 1997); ironhydroxide gel (Fe(OH)^ originating
from the acid dissolution and precipitation of iron minerals such as
pyrhotite (FeS), pyrite (FeS 2), hematite (Fe 2O 3), magnetite (Fe 3O 4), and
siderite (FeCO 3) (Rege and Fogler, 1989); silicium tetra hydroxide gel
(Si(OH) 4] originating from the alkaline dissolution and precipitation of
minerals in shaly sandstones such as quartz and kaolinite (Labrid, 1990); and
polymeric substances produced by in-situ gelation (Todd et al., 1993), alcohol
induced crystallization (Zhu and Tiab, 1993), separation of elemental sulfur
(Roberts, 1997); and surfactant precipitation (Arshad and Harwell, 1985).
Following Oddo and Tomson (1994), precipitation/dissolution reactions
can be symbolically represented by:

An v 3 Pr (9-1)

+2
+2
+2
where Me represents a cation or metal ion such as 5V , Ca , Mg , An
2
2
represents an anion such as CO 3 , SO 4 , and Pr represents a solid
precipitate such as CaCO 3, MgCO 3, BaSO 4, Fe(OH) 3, Si(OH) 4. v p v 2 ,
and v 3 are some stoichiometric coefficients.
Oddo and Tomson (1994) correlated the saturation solubility product,
K sp, empirically as a function of temperature, T, pressure, p, and ionic
strength, 5"., for typical systems. Hence, the saturation ratio given by the
following equation can be used to determine whether the conditions are
favorable for precipitation (Oddo and Tomson, 1994):

(9-2)
F s=[MeV(AnY/K sp
< 1 indicates an undersaturated solution, condition unfavorable for
F s
scaling, if F s = 1, the solution is at equilibrium with the solid scale, and
> 1 indicates a supersaturated solution, condition favorable for scaling.
F s

Organic Precipitation

Typical organic precipitates encountered in petroleum production are
paraffins and asphaltenes. Paraffins are inert and asphaltenes are reactive
substances. They are both sticky, thick, and deformable precipitates
(Chung, 1992; Ring et al., 1994). Therefore, they can seal the pore throats
and reduce the permeability to zero without needing to reduce the porosity
to zero and their deposition at the pore surface and tubing wall is
irreversible unless a solvent treatment is applied (Leontaritis et al., 1992):
The saturation ratio is given by: