365
Structure of Petroleum
(Moschopedis and Speight, 1976b; Moschopedis and Speight, 1978a). In fact, studies of the oxidation of the constituent fractions of petroleum have also shown that it is the most polar fractions that
are more reactive; the less polar constituents are much slower to oxidize (Moschopedis and Speight,
1978a). And, the analysis of the products indicates that oxygen is incorporated as polar functions.
It must be emphasized here that the resin constituents incorporate oxygen to produce (on the basis
of the separation method by which each fraction is defined) asphaltenes. The molecular weights of
the products are higher than those of the starting materials. This can arise through two phenomena:
(1) it is the high molecular weight polar species that are oxidized principally, and (2) the high
molecular weight can arise from some association between the product species.
Another phenomenon that has been observed is that in the initial stages of the mild oxidation
(without molecular degradation), the molecular weight of the asphaltenes (VPO/benzene) is reduced
from an unoxidized molecular weight of 4920 to a molecular weight in the product asphaltenes
of 1645. This change in molecular weight is not only due to some oxidative degradation but also to
the incorporation of oxygen functions within the asphaltenes that interfere with the intermolecular
association between individual asphaltene molecules that is known to occur in nonpolar solvents
such as benzene. In fact, it is quite reasonable to suggest that the molecular weight of 1645 might be
close to a representative molecular weight of an individual hypothetical asphaltene molecule.
Nevertheless, should a similar phenomenon (i.e., nondegradation during oxidation) occur during recovery operations, the result could well be a phase separation of asphaltene material. This in
turn could lead to loss of recovery efficiency of the crude oil because of the phase separation of the
asphaltenic material leading to blockage of the channel systems of the rock.
There are also other implications for the chemical reactions of the asphaltenic material produced by exposure of crude oil constituents to air. It may not be possible to model such phenomena
unless the chemistry of the oxidation is known. Modeling of the deposition of asphaltenic materials
has been studied with some degree of detail (Islam, 1994; Park et al., 1994). If the deposition of
asphaltenic material is purely a physical effect, the models should allow some degree of predictability. However, the participation of chemical phenomena may make any model based on physical
effects difficult to apply with a high degree of precision.
Those recovery processes that involve thermal systems are particularly susceptible to the deposition of asphaltenic material.
The thermolysis of asphaltene constituents produces a variety of products not the least of which
is paraffin species as well as highly polar aromatic species. The paraffinic products render the
oil medium less polar (more paraffinic) and the increased polarity of the asphaltenic core material can only lead to deposition of aromatic polar species in a manner analogous to the deposition
of sediment during, for example, visbreaking. Again, there is the tendency for such deposits to
hinder production. Although the temperatures in thermal recovery processes (with the exception of
fire flooding processes) rarely reach those (>350°C; >660°F), such temperatures are possible. But
another effect can also occur.
Steam flooding of a reservoir can lead to steam distillation (in the reservoir) of the more volatile
constituents of the crude oil. In many cases, it is quite possible that the lower molecular weight
aromatic species might be more volatile in steam than many of the paraffinic constituents of the oil.
The overall effect of such a phenomenon is an increase in the paraffinic character of the oil and,
hence, precipitation of asphaltenic material. During steam flooding, the precipitated asphaltenic
material can be altered or unaltered material.
It is also necessary in any such discussion of the deposition of asphaltenic material to consider
the electrical nature of asphaltenes.
Other studies (Preckshot et al., 1943; Katz and Beu, 1945; Penzes and Speight, 1974) have
shown that asphaltenes in solution carry an electric charge. One postulate of asphaltene structure invoked the concept of zwitterions to explain the possibility of charged species within the
asphaltenes. If the presence of such species is true, there is also the possibility that the induction
of charges during the flow of crude oil through pipelines will, under some circumstances, lead to
Structure of Petroleum
(Moschopedis and Speight, 1976b; Moschopedis and Speight, 1978a). In fact, studies of the oxidation of the constituent fractions of petroleum have also shown that it is the most polar fractions that
are more reactive; the less polar constituents are much slower to oxidize (Moschopedis and Speight,
1978a). And, the analysis of the products indicates that oxygen is incorporated as polar functions.
It must be emphasized here that the resin constituents incorporate oxygen to produce (on the basis
of the separation method by which each fraction is defined) asphaltenes. The molecular weights of
the products are higher than those of the starting materials. This can arise through two phenomena:
(1) it is the high molecular weight polar species that are oxidized principally, and (2) the high
molecular weight can arise from some association between the product species.
Another phenomenon that has been observed is that in the initial stages of the mild oxidation
(without molecular degradation), the molecular weight of the asphaltenes (VPO/benzene) is reduced
from an unoxidized molecular weight of 4920 to a molecular weight in the product asphaltenes
of 1645. This change in molecular weight is not only due to some oxidative degradation but also to
the incorporation of oxygen functions within the asphaltenes that interfere with the intermolecular
association between individual asphaltene molecules that is known to occur in nonpolar solvents
such as benzene. In fact, it is quite reasonable to suggest that the molecular weight of 1645 might be
close to a representative molecular weight of an individual hypothetical asphaltene molecule.
Nevertheless, should a similar phenomenon (i.e., nondegradation during oxidation) occur during recovery operations, the result could well be a phase separation of asphaltene material. This in
turn could lead to loss of recovery efficiency of the crude oil because of the phase separation of the
asphaltenic material leading to blockage of the channel systems of the rock.
There are also other implications for the chemical reactions of the asphaltenic material produced by exposure of crude oil constituents to air. It may not be possible to model such phenomena
unless the chemistry of the oxidation is known. Modeling of the deposition of asphaltenic materials
has been studied with some degree of detail (Islam, 1994; Park et al., 1994). If the deposition of
asphaltenic material is purely a physical effect, the models should allow some degree of predictability. However, the participation of chemical phenomena may make any model based on physical
effects difficult to apply with a high degree of precision.
Those recovery processes that involve thermal systems are particularly susceptible to the deposition of asphaltenic material.
The thermolysis of asphaltene constituents produces a variety of products not the least of which
is paraffin species as well as highly polar aromatic species. The paraffinic products render the
oil medium less polar (more paraffinic) and the increased polarity of the asphaltenic core material can only lead to deposition of aromatic polar species in a manner analogous to the deposition
of sediment during, for example, visbreaking. Again, there is the tendency for such deposits to
hinder production. Although the temperatures in thermal recovery processes (with the exception of
fire flooding processes) rarely reach those (>350°C; >660°F), such temperatures are possible. But
another effect can also occur.
Steam flooding of a reservoir can lead to steam distillation (in the reservoir) of the more volatile
constituents of the crude oil. In many cases, it is quite possible that the lower molecular weight
aromatic species might be more volatile in steam than many of the paraffinic constituents of the oil.
The overall effect of such a phenomenon is an increase in the paraffinic character of the oil and,
hence, precipitation of asphaltenic material. During steam flooding, the precipitated asphaltenic
material can be altered or unaltered material.
It is also necessary in any such discussion of the deposition of asphaltenic material to consider
the electrical nature of asphaltenes.
Other studies (Preckshot et al., 1943; Katz and Beu, 1945; Penzes and Speight, 1974) have
shown that asphaltenes in solution carry an electric charge. One postulate of asphaltene structure invoked the concept of zwitterions to explain the possibility of charged species within the
asphaltenes. If the presence of such species is true, there is also the possibility that the induction
of charges during the flow of crude oil through pipelines will, under some circumstances, lead to