Page 278 - Formation Damage during Improved Oil Recovery Fundamentals and Applications

Formation Damage by Organic Deposition 249

6.2.2 Solubility parameter
A (Hildebrand) solubility parameter is a parameter to estimate the situa-
tion where precipitation changes the solvent properties of the crude oil
(Buckley, 1999). Defined as the square-root of the cohesive energy den-
sity (c) of the solvent, the Hildebrand solubility parameter describes the
degree of solubility for nonpolar or slightly polar substances without
hydrogen bonding. It also describes the amount of energy required to
evaporate one-unit volume of a liquid (Hildebrand and Scott, 1964).
Liquids with a similar solubility parameter are more likely to be miscible.
This parameter is estimated from several techniques, namely, from dilu-
tion experiments, refractive index, and for supercritical fluids, by ideas
based on the principles of thermodynamics.

6.2.3 Asphaltene precipitation models
The thermodynamic and the colloidal theory are the basis for the precipi-
tation models widely described in literature. For the thermodynamic the-
ory asphaltenes are considered as being part of a nonideal mixture which
precipitate at solubility values below certain levels. Two main solubility
theory approaches typically used are the regular solution theory and the
equation of state (EoS) model. The colloidal theory on the other hand,
describes asphaltenes as colloidal particles surrounded by adsorbed resins.
The Flory Huggins solution theory is the solubility model used to
describe asphaltene precipitation. The chemical potential μ of the asphal-
tenes in the crude oil is given by Eq. (6.1) (Hirschberg et al., 1984;
Andersen and Speight, 1999; Wang and Buckley, 2001):

μ 2 μ θ
2 2
a a 5 lnϕ 1 1 2 V m;a ϕ 1 V m;a ðδ a 2δ s Þ ϕ ; (6.1)
RT a V m;s s RT s
Flory Huggins interaction parameter, χ

V m;a 2
χ 5 ðδ a 2δ s Þ (6.2)
RT
θ
where μ 5 chemical potential; μ 5 chemical potential at the reference
state of pure asphaltene phase, ϕ 5 volume fraction; ϕ 5 solubility param-
eter; V m 5 molar volume. The subscripts a and s denote asphaltenes and
solvent containing all nonasphaltene components respectively.