359
Structure of Petroleum
recovery operations to an even greater extent. Both modes of reaction can entrap other species (such
as resins and aromatics) within the space between the rock and the asphaltene.
Another area where incompatibility might play a detrimental role during recovery operations
occurs as a result of aerial oxidation. In general, it is the more polar species that oxidize first
with or without the presence of catalysts leaving oil that is relatively free of heteroatom species
(Moschopedis and Speight, 1978a). Thus, oxidation is a means for the production of highly polar
species in crude oil, leading to the deposition of polar sediments and being analogous to the deasphalting procedure. Thus, after incorporation of oxygen to an oil-dependent limit, significant
changes occur to asphaltenes and resins. These changes are not so much due to oxidative degradation but to the incorporation of oxygen functions that interfere with the natural order of intramolecular and intermolecular structuring, leading to the separation of asphaltenic material (Moschopedis
and Speight, 1976c, 1977).
Thus, with alterations in these parameters, the formation of sediment can occur although the
nature of the sediment will vary and be dependent upon the process of formation. There are also
several process-related destabilizing forces that can cause precipitation of asphaltenic material and
each involves disturbance to the equilibrium that exists within petroleum.
For example, carbon dioxide causes the destabilization of the petroleum equilibrium by lowering pH, by changing oil composition, and by creating turbulence. Usually, asphaltene precipitation increases as the volume of carbon dioxide available to the crude oil increases during the
later stages of carbon dioxide injection or stimulation. The most noticeable primary locations of
asphaltene deposition are the wellbore and the pump regions. In addition, flooding of a rich gas
(miscible flooding) destabilizes the asphaltene–crude oil mixture by lowering the solvent power of
the solution. The hydrocarbon gases used in such applications effectively cause deasphalting (gas
deasphalting, solvent deasphalting) of the crude oil. The negative effect of rich gas is at a maximum
near the bubble point; this effect is alleviated after the bubble point is reached. Similarly, organic
chemicals such as isopropyl alcohol, methyl alcohol, acetone, and even some glycol, alcohol, or
surfactant based solvents, which do not have an aromatic component, may selectively precipitate
asphaltenes and resins.
A decrease in the pressure is another important factor that influences the onset of solids deposition from petroleum. In fact, the effect of pressure particularly noticeable just above the bubble
point for crude oils that is rich in light ends. Depending on the location of the pressure decrease,
deposition may occur in different parts of the reservoir as well as in the wellbore and in the production stream. Furthermore, a decrease in pressure is usually accompanied by a decrease in temperature that can also cause physicochemical instability leading to the separation of asphaltenic material
from the oil. Pressure change alone can also invoke similar asphaltene precipitation.
The asphaltenic material in crude oil is electrically charged through the existence of zwitterions
or polarization within the molecular species (Preckshot et al., 1943; Katz and Beu, 1945; Penzes
and Speight, 1974; Speight and Penzes, 1978; Fotland and Anfindsen, 1996). Therefore, any process
(such as the flow through reservoir channels or through a pipe) that can induce a potential across,
or within, the oil will also result in the electrodeposition of asphaltenic material through the disturbance of the stabilizing electrical forces. In addition, neutralization of the molecular charge will
also result in the formation of sediment.
The mechanism of asphaltene precipitation is very complex (Speight et al., 1982, 1984; Andersen
and Speight, 1993; Buckley, 1996, 1999; Buckley et al., 1998) and controversy as to the nature of
asphaltene solutions persists. However, petroleum consists of a mixture of saturates, aromatics,
resin constituents, and asphaltene constituents which are separated as bulk fractions (Chapters 9
and 10) and it is necessary to consider each of the constituents of this system as a continuous or discrete mixture interacting with each other. The omission of any one fraction or of constituents could
lead to errors in the outcome (Andersen and Speight, 1993).
There are two general approaches to the consideration of the nature of asphaltenes in oil
(Hirschberg et al., 1984; Mansoori et al., 1987; Islam, 1994; Park et al., 1994). The first approach
Structure of Petroleum
recovery operations to an even greater extent. Both modes of reaction can entrap other species (such
as resins and aromatics) within the space between the rock and the asphaltene.
Another area where incompatibility might play a detrimental role during recovery operations
occurs as a result of aerial oxidation. In general, it is the more polar species that oxidize first
with or without the presence of catalysts leaving oil that is relatively free of heteroatom species
(Moschopedis and Speight, 1978a). Thus, oxidation is a means for the production of highly polar
species in crude oil, leading to the deposition of polar sediments and being analogous to the deasphalting procedure. Thus, after incorporation of oxygen to an oil-dependent limit, significant
changes occur to asphaltenes and resins. These changes are not so much due to oxidative degradation but to the incorporation of oxygen functions that interfere with the natural order of intramolecular and intermolecular structuring, leading to the separation of asphaltenic material (Moschopedis
and Speight, 1976c, 1977).
Thus, with alterations in these parameters, the formation of sediment can occur although the
nature of the sediment will vary and be dependent upon the process of formation. There are also
several process-related destabilizing forces that can cause precipitation of asphaltenic material and
each involves disturbance to the equilibrium that exists within petroleum.
For example, carbon dioxide causes the destabilization of the petroleum equilibrium by lowering pH, by changing oil composition, and by creating turbulence. Usually, asphaltene precipitation increases as the volume of carbon dioxide available to the crude oil increases during the
later stages of carbon dioxide injection or stimulation. The most noticeable primary locations of
asphaltene deposition are the wellbore and the pump regions. In addition, flooding of a rich gas
(miscible flooding) destabilizes the asphaltene–crude oil mixture by lowering the solvent power of
the solution. The hydrocarbon gases used in such applications effectively cause deasphalting (gas
deasphalting, solvent deasphalting) of the crude oil. The negative effect of rich gas is at a maximum
near the bubble point; this effect is alleviated after the bubble point is reached. Similarly, organic
chemicals such as isopropyl alcohol, methyl alcohol, acetone, and even some glycol, alcohol, or
surfactant based solvents, which do not have an aromatic component, may selectively precipitate
asphaltenes and resins.
A decrease in the pressure is another important factor that influences the onset of solids deposition from petroleum. In fact, the effect of pressure particularly noticeable just above the bubble
point for crude oils that is rich in light ends. Depending on the location of the pressure decrease,
deposition may occur in different parts of the reservoir as well as in the wellbore and in the production stream. Furthermore, a decrease in pressure is usually accompanied by a decrease in temperature that can also cause physicochemical instability leading to the separation of asphaltenic material
from the oil. Pressure change alone can also invoke similar asphaltene precipitation.
The asphaltenic material in crude oil is electrically charged through the existence of zwitterions
or polarization within the molecular species (Preckshot et al., 1943; Katz and Beu, 1945; Penzes
and Speight, 1974; Speight and Penzes, 1978; Fotland and Anfindsen, 1996). Therefore, any process
(such as the flow through reservoir channels or through a pipe) that can induce a potential across,
or within, the oil will also result in the electrodeposition of asphaltenic material through the disturbance of the stabilizing electrical forces. In addition, neutralization of the molecular charge will
also result in the formation of sediment.
The mechanism of asphaltene precipitation is very complex (Speight et al., 1982, 1984; Andersen
and Speight, 1993; Buckley, 1996, 1999; Buckley et al., 1998) and controversy as to the nature of
asphaltene solutions persists. However, petroleum consists of a mixture of saturates, aromatics,
resin constituents, and asphaltene constituents which are separated as bulk fractions (Chapters 9
and 10) and it is necessary to consider each of the constituents of this system as a continuous or discrete mixture interacting with each other. The omission of any one fraction or of constituents could
lead to errors in the outcome (Andersen and Speight, 1993).
There are two general approaches to the consideration of the nature of asphaltenes in oil
(Hirschberg et al., 1984; Mansoori et al., 1987; Islam, 1994; Park et al., 1994). The first approach