Page 274 - Formation Damage during Improved Oil Recovery Fundamentals and Applications

Formation Damage by Organic Deposition 245

relevant research is briefly expounded on. Finally, some field examples
are provided as practical scenarios of asphaltene problems facing the
industry.

6.1.2 Definition
Asphaltenes are known to be susceptible to changing pressure, temperature,
and oil composition leading to their precipitation and subsequent deposition.
J.B. Boussinggault, the man to first describe asphaltenes in 1837 defined
asphaltenes as “alcohol insoluble, turpentine-soluble solid found in the residue
of a distillation” (Creek, 2005; Buckley, 2012). Speight and Long (1996)
define asphaltenes in terms of their solubility, thus, as being soluble in benzene
or toluene, and insoluble in pentane or heptane (Speight and Long, 1996).
Crude oil has several components, namely, resins, saturates, and aromatics.
Another component of crude oil, the malteneses consist of the portion in
which the asphaltenes are dispersed (Seifried, 2016). Seifried (2016) states that
the fraction of crude containing asphaltene typically varies from 0% to nearly
20%. It is composed mainly of carbon and hydrogen elements and contains
about 80.42% 88.65% carbon, 7.44%B11.10% hydrogen, plus 0.3% 4.9%
oxygen, 0.3% 10.3% sulfur, and 0.6% 3.3% nitrogen (Koots and Speight,
1975; Speight, 1996). Speight explains that there isn’t a good grasp of the
nature of the asphaltene molecules. He however notes that spectroscopic stud-
ies give credence to the idea that asphaltene contains aromatic ring systems
with elements such as nitrogen, oxygen, and sulfur, scattered throughout its
molecular structure (Speight, 1996). Its molecular structure depends on tem-
perature and pressure, making its molecular weight difficult to measure
(Speight, 1996). That notwithstanding, Mullins et al. (2012) estimated it to be
about 750 g/mol using mass spectral methods, with most of the population
being 500 and 1000 g/mol. Yarranton et al. (2013) on the other hand approx-
imated the value to 850 g/mol, using vapor pressure osmometry (VPO).
Another technique, the small angle X-ray scattering (SAXS) method gives
values 10 times larger, and Yarranton explained that the discrepancy between
SAXS and VPO is yet to be solved. Speight stated that a technique that pre-
vents aggregation will result in accurate asphaltene molecular weight (Speight,
1996; Speight, 1999).

6.1.3 Molecular structure of asphaltene
Asphaltenes exhibit complex structures which comprise one or more poly-
aromatic units, sulfur, and nitrogen linked by alkyl chains (Gray et al., 2011).
The “Island” and “Archipelago” structure are the two ways recognized as