Page 145 - Handbook Of Multiphase Flow Assurance

Asphaltenes 141

Asphaltene deposits can occur in the reservoir, well tubing, flowlines, risers, strainers, sep-
arators, pumps and other locations.
The nature of asphaltenes has been investigated by Katz and Beu (1945). They determined
that the asphaltene particles size is less than 65 Angstroms (0.0065 μm).

Asphaltene chemistry

Asphaltens do not have a specific chemical structure. Instead the asphaltenes are charac-
terized as a fraction of hydrocarbons which is soluble in toluene but insoluble in light hydro-
carbons such as pentane or heptane.
Various structures of asphaltenes have been proposed and substantiated with laboratory
analysis. However these vary with oil deposits around the world. One common characteristic
is multiple fused benzyl rings and presence of heteroatoms such as N, O, S and metals V, Fe,
Ni. The average ratio of carbon to hydrogen atoms in asphaltenes is 1.1. This helps distin-
guish asphaltene deposits from paraffin wax which mainly contain saturated hydrocarbons
with the C:H ratio of 2.
It is thought that asphaltenes flocculate and deposit mainly due to electrostatic and van der
Waals attraction between various parts of the asphaltene molecules and the surfaces. Recent
study (Arsalan, 2015) measured by inverse gas chromatography the Lifshitz-van der Waals
(non-polar interaction with n-alkanes) component of surface energy for asphaltene at 0.313 J/
2
m . The polar component could not be measured due to very strong interaction with the polar
probes (dichloromethane as acid and ethyl acetate as base molecules).
Resin molecules (fraction insoluble in propane and butane but soluble in pentane) help
stabilize asphaltenes in reservoir crude oil.

Reservoir and wellbore plugging

In an undersaturated reservoir all molecules are dissolved in a liquid phase. As reservoir
fluid moves toward wellbore, oil pressure and density decrease, lighter components expand
first due to their higher compressibility. Resins are more soluble in light components and are
stripped away from asphaltenes. Flocculation of asphaltenes occurs at the upper onset pres-
sure (see Fig. 3.9 in Chapter 3). Asphaltenes then can deposit in wellbores or in reservoir pores
(Haskett and Tartera, 1965; Alkafeef et al., 2003; Mansoori et al., 1986; Thaver et al., 1999).
There is a severe shortage of published field data on asphaltene deposition. Many research
works rely on the sole 1965 paper providing a caliper measurement of asphaltene deposit in
one well. Other papers exist which detail asphaltene deposition with fewer detail (Lichaa,
1977; Tuttle, 1983; Von Albrecht et al., 1977).
Asphaltenes get destabilized, flocculate and deposit for the following reasons:
‐ CO 2 flooding—due to change in acidity and strong polar interaction with asphaltene.
‐ light oil flooding—due to solvation of resin molecules away from asphaltene.
‐ gas lift—due to heavy ends from gas dissolving into oil and reducing oil density.
‐ mixing of incompatible fluid streams—due to light ends dissolving in heavier oil.
‐ acidizing—due to change in acidity.
‐ electric charge on surfaces—due to polar attraction of asphaltene.
‐ shear—accelerating deposition rate.