355
Structure of Petroleum
exposure to aerial oxygen during the recovery process. In the reservoir, asphaltene incompatibility
can cause blockages of the pores and channels through which the oil must move during recovery
operations (Islam, 1994; Park et al., 1994; Leontaritis, 1996).
All of these incidents cause disturbances to the petroleum system. However, when such
disturbances occur, it is the higher molecular weight constituents that are most seriously affected
eventually leading to incompatibility (precipitation, sediment formation, and sludge formation)
depending upon the circumstances. Thus, the dispersability of the higher molecular weight constituents becomes an issue that needs attention. And one of the ways by which this issue can be
understood is to be aware of the chemical and physical character of the higher molecular weight
constituents. By such means, the issue of dispersability, and the attending issue of incompatibility
can be understood and even predicted.
The asphaltene fraction is chemically complex but it can be conveniently represented on the
basis of molecular weight and polarity (Chapter 12; Long, 1979, 1981; Speight, 1994). For different
crude oils, the slope of the line representing the distribution of molecular weight and the variation in polarity will vary (Speight, 1994). Asphaltenes cannot be crystallized in the usual sense of
the word. However, asphaltenes can be subfractionated by the use of a variety of techniques. The
fractions vary in molecular weight and also in terms of the functional group types and functional
group content (Francisco and Speight, 1984). These data lend support to, and reaffirm, the concept
that the asphaltene fraction is a complex mixtures of molecular sizes and various functional types
(Chapter 12).
In addition, any variation of the major parameters (precipitant, precipitant/oil ratio, time, and
temperature) can cause substantial variations in the nature and amount of the separated asphaltenic
material. It can vary from a dark brown amorphous solid to a black tacky deposit, either of which
under the prevalent conditions could be termed an asphaltene fraction (Chapter 12), but is not correct
by the definitions of the standard test methods used for the separation (Chapter 10).
Petroleum is a balanced system insofar as the components of the system exist in harmonious balance with each other. Changes to the system can result in the formation of an insoluble (solid) phase
and, therefore, the effects that cause the separation of this immiscible phase need to be addressed.
Separate, or insoluble, phases are produced when external effects perturb the system. Such
effects might arise during recovery operations, refining operations, or storage.
The presence of asphaltenes is known to have a significant effect on the processability of crude oil.
Indeed, the presence of asphaltenes is particularly felt when circumstances permit their separation
(precipitation, phase separation) from the oil medium leading to the deposition of solids during
recovery operations. Phase separation will cause coke formation during processing as well as failure
of an asphalt pavement by loss of physical structure of the asphalt-aggregate system (Speight, 1992).
Thus, there is the need to understand the structure and stability of crude oil and one way in
which this can be achieved is by an investigation of asphaltene precipitation/flocculation, by titration, using solvent/nonsolvent mixtures. This may give an indication of both asphaltene and maltene
(deasphaltened oil) properties (Heithaus, 1962; Fuhr et al., 1991).
At the point of incipient precipitation, that is, the point at which separation of asphaltenes from
a crude oil becomes apparent, the precipitated material is, presumably, a conglomeration of species based on molecular size and polarity of the type that constitute the asphaltenes (Long, 1979,
1981). This phenomenon has, however, not been addressed in any detail and is certainly worthy of
investigation in order to increase the understanding of crude oil (asphaltene/maltene) relationships.
Thus, in order to investigate the mechanism of asphaltene precipitation, the change in precipitate with changes in the composition of nonsolvent/solvent has been examined (Andersen and
Speight, 1992). Immiscibility is known to occur at solubility differences between solvent and solute
of about 3.5–4 MPa 1/2 . Hence, the solubility parameter of the precipitated material can be estimated
to fall within the range 20.2–20.7 MPa 1/2 , which is in accordance with previous work (Hirschberg
et al., 1984). Using the data further, it is predictable that there will be no precipitation of material
above a flocculation threshold of 0.33 (δ eff = 16.2 MPa 1/2 for the total oil).
Structure of Petroleum
exposure to aerial oxygen during the recovery process. In the reservoir, asphaltene incompatibility
can cause blockages of the pores and channels through which the oil must move during recovery
operations (Islam, 1994; Park et al., 1994; Leontaritis, 1996).
All of these incidents cause disturbances to the petroleum system. However, when such
disturbances occur, it is the higher molecular weight constituents that are most seriously affected
eventually leading to incompatibility (precipitation, sediment formation, and sludge formation)
depending upon the circumstances. Thus, the dispersability of the higher molecular weight constituents becomes an issue that needs attention. And one of the ways by which this issue can be
understood is to be aware of the chemical and physical character of the higher molecular weight
constituents. By such means, the issue of dispersability, and the attending issue of incompatibility
can be understood and even predicted.
The asphaltene fraction is chemically complex but it can be conveniently represented on the
basis of molecular weight and polarity (Chapter 12; Long, 1979, 1981; Speight, 1994). For different
crude oils, the slope of the line representing the distribution of molecular weight and the variation in polarity will vary (Speight, 1994). Asphaltenes cannot be crystallized in the usual sense of
the word. However, asphaltenes can be subfractionated by the use of a variety of techniques. The
fractions vary in molecular weight and also in terms of the functional group types and functional
group content (Francisco and Speight, 1984). These data lend support to, and reaffirm, the concept
that the asphaltene fraction is a complex mixtures of molecular sizes and various functional types
(Chapter 12).
In addition, any variation of the major parameters (precipitant, precipitant/oil ratio, time, and
temperature) can cause substantial variations in the nature and amount of the separated asphaltenic
material. It can vary from a dark brown amorphous solid to a black tacky deposit, either of which
under the prevalent conditions could be termed an asphaltene fraction (Chapter 12), but is not correct
by the definitions of the standard test methods used for the separation (Chapter 10).
Petroleum is a balanced system insofar as the components of the system exist in harmonious balance with each other. Changes to the system can result in the formation of an insoluble (solid) phase
and, therefore, the effects that cause the separation of this immiscible phase need to be addressed.
Separate, or insoluble, phases are produced when external effects perturb the system. Such
effects might arise during recovery operations, refining operations, or storage.
The presence of asphaltenes is known to have a significant effect on the processability of crude oil.
Indeed, the presence of asphaltenes is particularly felt when circumstances permit their separation
(precipitation, phase separation) from the oil medium leading to the deposition of solids during
recovery operations. Phase separation will cause coke formation during processing as well as failure
of an asphalt pavement by loss of physical structure of the asphalt-aggregate system (Speight, 1992).
Thus, there is the need to understand the structure and stability of crude oil and one way in
which this can be achieved is by an investigation of asphaltene precipitation/flocculation, by titration, using solvent/nonsolvent mixtures. This may give an indication of both asphaltene and maltene
(deasphaltened oil) properties (Heithaus, 1962; Fuhr et al., 1991).
At the point of incipient precipitation, that is, the point at which separation of asphaltenes from
a crude oil becomes apparent, the precipitated material is, presumably, a conglomeration of species based on molecular size and polarity of the type that constitute the asphaltenes (Long, 1979,
1981). This phenomenon has, however, not been addressed in any detail and is certainly worthy of
investigation in order to increase the understanding of crude oil (asphaltene/maltene) relationships.
Thus, in order to investigate the mechanism of asphaltene precipitation, the change in precipitate with changes in the composition of nonsolvent/solvent has been examined (Andersen and
Speight, 1992). Immiscibility is known to occur at solubility differences between solvent and solute
of about 3.5–4 MPa 1/2 . Hence, the solubility parameter of the precipitated material can be estimated
to fall within the range 20.2–20.7 MPa 1/2 , which is in accordance with previous work (Hirschberg
et al., 1984). Using the data further, it is predictable that there will be no precipitation of material
above a flocculation threshold of 0.33 (δ eff = 16.2 MPa 1/2 for the total oil).