354
The Chemistry and Technology of Petroleum
The size of the asphaltene agglomerate varies as a function of the temperature, the asphaltene
concentration, and the identity of the solvent. This establishes that the self-association of the
asphaltenes is reversible and that the molecular size in the solution state results from a true thermodynamic equilibrium between primary particles and reversible aggregates, the equilibrium constant
depending on some property of the solvent. But again, the correlation of the behavior of asphaltenes
in hydrocarbon solvents to their behavior in crude oil is still extremely speculative.
Perhaps the study of asphaltene constituents that have been removed from petroleum is analogous to studying a hard-boiled egg! The system (i.e., the egg protein and the asphaltene phase
relationships) has been denatured by the application of a physical technique.
Perhaps the most appropriate thought when studying asphaltenes constituents vis-à-vis the structure of petroleum is to recall two theories that have some standing in the scientific community:
(1) the Heisenberg uncertainty principle, namely, once a system is studied it changes by virtue
of being studied, and (2) chaos theory, namely, no matter how well you plan, the unexpected is
inevitable. Therefore, it must be recognized that separation of the asphaltene constituents from
petroleum undoubtedly changes these molecular entities in terms of their intermolecular relationship with the other constituents of petroleum and even with their intramolecular relationships
between other asphaltene constituents.
Thus, when we are told that the asphaltene fraction has 20,000 or more constituents and that a
researcher has determined the precise molecular weight and the precise behavior of the fraction, a
modicum of skepticism must arise!
13.4 STABILITY/INSTABILITY OF THE PETROLEUM SYSTEM
Petroleum can be considered to be a delicately balanced system insofar as the different fractions
that contain hydrocarbons (saturates and aromatics) as well as heteroatom constituents (Figure 13.1).
The heteroatom constituents tend to concentrate in the higher molecular weight fractions (the
asphaltenes and resins). The nitrogen, oxygen, and sulfur species that are in near-neutral molecular
locales will also occur in the saturate fraction and in the aromatics fraction, remembering that the
nomenclature is not necessarily precise and that the composition of each fraction is a function of
the separation process.
Instability usually occurs (in heavy oil) with the separation or precipitation of asphaltene
constituents. This may be caused by well stimulation, such as acidizing, which involves a drastic
shift in local chemical equilibria, pH, and liberation of carbon dioxide. It may also increase the
concentration of some ions, such as iron, which will promote the formation of asphaltenic sediment.
Similarly, the separation of wax from paraffinic crude oil will also occur under certain conditions
(some are defined, some are not) and cause problems during recovery by blocking the channels in
the reservoir rock. However, the focus of this section is on the separation of the asphaltene constituents during recovery and refining.
As a recap (see Chapter 12), the asphaltene fraction is a complex mixture of molecular species
of varying molecular weight and polarity (Chapter 12), which causes different reactivity of the
constituents. Thus, carbenes and carboids are lower molecular weight, highly polar species that are
predominantly products of thermal processes and might not occur in the typical recovery process.
However, the application of thermal techniques, such as fire flooding, to petroleum recovery can
produce such species and they will either deposit on the reservoir rock or appear as suspended solids
in the oil. In fact, it is recognized that it is the polar species in the crude oil that govern the oil–rock
interactions in the reservoir from which many sediments can arise.
With the exception of the carbenes and the carboids, the fractions are compatible provided there
are no significant disturbances or changes made to the system. Such changes are (1) the alteration
of the natural abundance of the different fractions; (2) the chemical or physical alteration of the
constituents as might occur during recovery, especially changes that might be brought on by thermal processes; and (3) alteration of the polar group distribution as might occur during oxidation by
The Chemistry and Technology of Petroleum
The size of the asphaltene agglomerate varies as a function of the temperature, the asphaltene
concentration, and the identity of the solvent. This establishes that the self-association of the
asphaltenes is reversible and that the molecular size in the solution state results from a true thermodynamic equilibrium between primary particles and reversible aggregates, the equilibrium constant
depending on some property of the solvent. But again, the correlation of the behavior of asphaltenes
in hydrocarbon solvents to their behavior in crude oil is still extremely speculative.
Perhaps the study of asphaltene constituents that have been removed from petroleum is analogous to studying a hard-boiled egg! The system (i.e., the egg protein and the asphaltene phase
relationships) has been denatured by the application of a physical technique.
Perhaps the most appropriate thought when studying asphaltenes constituents vis-à-vis the structure of petroleum is to recall two theories that have some standing in the scientific community:
(1) the Heisenberg uncertainty principle, namely, once a system is studied it changes by virtue
of being studied, and (2) chaos theory, namely, no matter how well you plan, the unexpected is
inevitable. Therefore, it must be recognized that separation of the asphaltene constituents from
petroleum undoubtedly changes these molecular entities in terms of their intermolecular relationship with the other constituents of petroleum and even with their intramolecular relationships
between other asphaltene constituents.
Thus, when we are told that the asphaltene fraction has 20,000 or more constituents and that a
researcher has determined the precise molecular weight and the precise behavior of the fraction, a
modicum of skepticism must arise!
13.4 STABILITY/INSTABILITY OF THE PETROLEUM SYSTEM
Petroleum can be considered to be a delicately balanced system insofar as the different fractions
that contain hydrocarbons (saturates and aromatics) as well as heteroatom constituents (Figure 13.1).
The heteroatom constituents tend to concentrate in the higher molecular weight fractions (the
asphaltenes and resins). The nitrogen, oxygen, and sulfur species that are in near-neutral molecular
locales will also occur in the saturate fraction and in the aromatics fraction, remembering that the
nomenclature is not necessarily precise and that the composition of each fraction is a function of
the separation process.
Instability usually occurs (in heavy oil) with the separation or precipitation of asphaltene
constituents. This may be caused by well stimulation, such as acidizing, which involves a drastic
shift in local chemical equilibria, pH, and liberation of carbon dioxide. It may also increase the
concentration of some ions, such as iron, which will promote the formation of asphaltenic sediment.
Similarly, the separation of wax from paraffinic crude oil will also occur under certain conditions
(some are defined, some are not) and cause problems during recovery by blocking the channels in
the reservoir rock. However, the focus of this section is on the separation of the asphaltene constituents during recovery and refining.
As a recap (see Chapter 12), the asphaltene fraction is a complex mixture of molecular species
of varying molecular weight and polarity (Chapter 12), which causes different reactivity of the
constituents. Thus, carbenes and carboids are lower molecular weight, highly polar species that are
predominantly products of thermal processes and might not occur in the typical recovery process.
However, the application of thermal techniques, such as fire flooding, to petroleum recovery can
produce such species and they will either deposit on the reservoir rock or appear as suspended solids
in the oil. In fact, it is recognized that it is the polar species in the crude oil that govern the oil–rock
interactions in the reservoir from which many sediments can arise.
With the exception of the carbenes and the carboids, the fractions are compatible provided there
are no significant disturbances or changes made to the system. Such changes are (1) the alteration
of the natural abundance of the different fractions; (2) the chemical or physical alteration of the
constituents as might occur during recovery, especially changes that might be brought on by thermal processes; and (3) alteration of the polar group distribution as might occur during oxidation by