363
Structure of Petroleum
asphaltenic material) (Magaril and Akensova, 1968; Magaril and Ramazaeva, 1969; Magaril et al.,
1970; Speight, 1994), the deposition of asphaltenic material during recovery operations is less well
understood.
It is the general consensus that it is the polar (heteroatom-containing) constituents (two fractions
that are designated as asphaltenes and the resins) of petroleum that are responsible for the formation
of suspended organic solids during a variety of recovery processes (Islam, 1994; Park et al., 1994).
However, recovery chemistry and physics are areas that remain largely undefined insofar as the
chemistry and physics are still speculative. Thus, precise chemical and physical models are difficult
to describe and the factors leading to the incompatibility of petroleum constituents, as might occur
during such operations, remain unknown.
As a first statement of what is known, petroleum is a delicately balanced system insofar as the
different fractions are compatible provided there are no significant disturbances or changes made
to the system. The changes that can occur, which will upset the balance of the petroleum system,
are (1) alteration of the natural abundances of the different fractions, as might occur when gases are
dissolved in the crude oil under reservoir pressure and/or when these dissolved gases are released
at the time when a reservoir is first penetrated; (2) chemical alteration of the constituents as might
occur during recovery processes, especially changes that might be brought on by thermal recovery
processes; and (3) alteration of the distribution of the polar functional groups as might occur during
oxidation or the elimination of polar functions during recovery operations, as might occur when
exposure of petroleum to air occurs.
When such disturbances occur, it is the higher molecular weight constituents that are most seriously affected. This can lead to incompatibility, which is variously referred to as precipitation,
sludge formation, sediment formation, and deposition of asphaltenic material, depending on the
circumstances. When incompatibility occurs, it is the higher molecular weight polar constituents
(the asphaltenes and, in some instances, the resins) that form a separate phase as a result of the loss
of dispersability. In the reservoir, asphaltene incompatibility can cause blockages of the pores and
channels through which the oil must move during recovery operations.
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 polar constituents of crude oil. By such means, the issue of dispersability, fluid–fluid interactions, oil–rock interactions, and the attending issue of incompatibility can
be understood and perhaps predicted.
13.5.1 eFFeCts on reCovery oPerAtIons
There are several mechanisms by which asphaltene constituents can be discharged from crude oil
as sediment and cause problems during recovery operations. However, because so little is known
about these events that can occur in a reservoir, it is difficult to foresee such events. In fact, there is
no general rule that can be applied to the predictability of the separation of asphaltenic sediments.
Each crude oil will behave differently under reservoir conditions and, therefore, predictability of
behavior is difficult, at best speculative. Any adverse effects on the system can increase the potential
for asphaltene incompatibility in the reservoir.
When it occurs, the deposition of asphaltenic material (sediment) can occur during crude oil production and can lead to serious problems. Deposition can occur in the reservoir and in the well tubing, thence being carried downstream through the lines into the separators and other downstream
equipment. Apart from the loss of production, the cost of removing asphaltenic sediments can be
very expensive and significantly alter the economics of a project. What is perhaps more important is that any one of a number of operating conditions can result in the formation of asphaltenic
sediments.
Thus, when a reservoir at the end of a primary depletion period is brought back to operation
above the bubble point pressure, as may happen during a water-flooding operation, part of the gas
Structure of Petroleum
asphaltenic material) (Magaril and Akensova, 1968; Magaril and Ramazaeva, 1969; Magaril et al.,
1970; Speight, 1994), the deposition of asphaltenic material during recovery operations is less well
understood.
It is the general consensus that it is the polar (heteroatom-containing) constituents (two fractions
that are designated as asphaltenes and the resins) of petroleum that are responsible for the formation
of suspended organic solids during a variety of recovery processes (Islam, 1994; Park et al., 1994).
However, recovery chemistry and physics are areas that remain largely undefined insofar as the
chemistry and physics are still speculative. Thus, precise chemical and physical models are difficult
to describe and the factors leading to the incompatibility of petroleum constituents, as might occur
during such operations, remain unknown.
As a first statement of what is known, petroleum is a delicately balanced system insofar as the
different fractions are compatible provided there are no significant disturbances or changes made
to the system. The changes that can occur, which will upset the balance of the petroleum system,
are (1) alteration of the natural abundances of the different fractions, as might occur when gases are
dissolved in the crude oil under reservoir pressure and/or when these dissolved gases are released
at the time when a reservoir is first penetrated; (2) chemical alteration of the constituents as might
occur during recovery processes, especially changes that might be brought on by thermal recovery
processes; and (3) alteration of the distribution of the polar functional groups as might occur during
oxidation or the elimination of polar functions during recovery operations, as might occur when
exposure of petroleum to air occurs.
When such disturbances occur, it is the higher molecular weight constituents that are most seriously affected. This can lead to incompatibility, which is variously referred to as precipitation,
sludge formation, sediment formation, and deposition of asphaltenic material, depending on the
circumstances. When incompatibility occurs, it is the higher molecular weight polar constituents
(the asphaltenes and, in some instances, the resins) that form a separate phase as a result of the loss
of dispersability. In the reservoir, asphaltene incompatibility can cause blockages of the pores and
channels through which the oil must move during recovery operations.
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 polar constituents of crude oil. By such means, the issue of dispersability, fluid–fluid interactions, oil–rock interactions, and the attending issue of incompatibility can
be understood and perhaps predicted.
13.5.1 eFFeCts on reCovery oPerAtIons
There are several mechanisms by which asphaltene constituents can be discharged from crude oil
as sediment and cause problems during recovery operations. However, because so little is known
about these events that can occur in a reservoir, it is difficult to foresee such events. In fact, there is
no general rule that can be applied to the predictability of the separation of asphaltenic sediments.
Each crude oil will behave differently under reservoir conditions and, therefore, predictability of
behavior is difficult, at best speculative. Any adverse effects on the system can increase the potential
for asphaltene incompatibility in the reservoir.
When it occurs, the deposition of asphaltenic material (sediment) can occur during crude oil production and can lead to serious problems. Deposition can occur in the reservoir and in the well tubing, thence being carried downstream through the lines into the separators and other downstream
equipment. Apart from the loss of production, the cost of removing asphaltenic sediments can be
very expensive and significantly alter the economics of a project. What is perhaps more important is that any one of a number of operating conditions can result in the formation of asphaltenic
sediments.
Thus, when a reservoir at the end of a primary depletion period is brought back to operation
above the bubble point pressure, as may happen during a water-flooding operation, part of the gas