364
The Chemistry and Technology of Petroleum
cap formed during blowdown will be redissolved in the oil. In some cases, especially when the gas
contains substantial amounts of propane and butane, phase separation can occur. This is analogous
to the propane and butane deasphalting of a residuum (atmospheric or vacuum) that is employed
in the production of asphalt during refinery operations. It is also analogous to the gas deasphalting
that can occur during maturation of petroleum in a reservoir (before the reservoir is penetrated and
goes into production) when it is believed that dissolution of hydrocarbon gases in the oil can deposit
asphaltenic material on the reservoir rock (Evans et al., 1971).
Following from this, there has also been the suggestion that gravity segregation occurs in connection with gas deasphalting in a reservoir. For example, in reservoirs that have a high vertical
profile, assuming a temperature differential between the top and bottom of the reservoir, the petroleum may be increasingly heavier with depth. The material at the higher points of such reservoirs
can contain more gas in solution than the petroleum in the lower points of the reservoir. Thus, more
asphaltenic material is precipitated at the top of the reservoir than at the bottom of the reservoir.
Some enhanced recovery processes can also prompt the separation of asphaltene sediment. For
example, carbon dioxide flooding and miscible hydrocarbon flooding have been known to produce sediments during operations (Islam, 1994). In carbon dioxide flooding, one or more banks are
injected into the reservoir followed by water as the driving fluid. The major operative mechanism
in carbon dioxide flooding appears to be swelling of the crude oil, miscibility, viscosity reduction,
and most important, reduced interfacial tension. The deposition of asphaltenic material appears to
be a result of displacement of the asphaltenes material by the carbon dioxide that competes with
the asphaltenes for solubility in the crude oil. This is almost the reverse of the situation that occurs
when salt is added to carbonated aqueous solutions and tends to displace the carbon dioxide from
the aqueous system.
A similar effect can be observed in miscible hydrocarbon flooding during which a rich gas
(i.e., gas with a high content of ethane, propane, butane, etc.) is injected into the reservoir (Islam, 1994).
Again, this can approximate the deasphalting process that is used to manufacture asphalt and the
injection of the low-boiling hydrocarbon will discharge asphaltenic material from the crude oil.
The adsorption of asphaltenic material during the separation of petroleum into its component
fractions has become a standard operation for petroleum analysis. It must therefore be presumed
that a similar effect can be operative under certain reservoir conditions, analogous to those prevalent when carbon dioxide and miscible hydrocarbon flooding technologies are applied to crude oil
recovery. Thus, interactions of the polar constituents with the reservoir rock must also be included
in any acknowledgment of the deposition of asphaltenic material. In addition, the deposition of
asphaltenic material on to the reservoir rock can alter the properties of the rock (Islam, 1994) to the
extent that water-wet rock can become oil-wet rock, which ultimately can have an adverse effect on
enhanced oil recovery processes.
Any asphaltenic material adsorbed from the crude oil by the reservoir rock may not be readily
desorbed from the rock (depending upon the nature of the rock) and may even be desorbed from
the rock as insoluble material. If this asphaltene-type material becomes mobile as sediment in the
oil, it can move to the pore channels to the recovery well, thereby blocking the channels, making
recovery difficult if not impossible.
Incompatibility is also possible when asphaltenes interact with reservoir rock, especially acidic
functions of rocks, through the functional groups (e.g., the basic nitrogen species) just as they interact with adsorbents. And, there is the possibility for interaction of the asphaltene with the reservoir
rock through the agency of a single functional group in which the remainder of the asphaltene molecule remains in the liquid phase. On the other hand, there is also the possibility that the asphaltene
constituents can react with the rock at several points of contact thereby enhancing the bonding to the
rock and, in some cases, effecting recovery operations to an even greater extent.
Another area where incompatibility might play a detrimental role during recovery operations
is as a result of oxidation. In general, it is the more polar species that oxidize first. And, after
incorporation of oxygen to a limit, significant changes can occur in asphaltene molecular weight
The Chemistry and Technology of Petroleum
cap formed during blowdown will be redissolved in the oil. In some cases, especially when the gas
contains substantial amounts of propane and butane, phase separation can occur. This is analogous
to the propane and butane deasphalting of a residuum (atmospheric or vacuum) that is employed
in the production of asphalt during refinery operations. It is also analogous to the gas deasphalting
that can occur during maturation of petroleum in a reservoir (before the reservoir is penetrated and
goes into production) when it is believed that dissolution of hydrocarbon gases in the oil can deposit
asphaltenic material on the reservoir rock (Evans et al., 1971).
Following from this, there has also been the suggestion that gravity segregation occurs in connection with gas deasphalting in a reservoir. For example, in reservoirs that have a high vertical
profile, assuming a temperature differential between the top and bottom of the reservoir, the petroleum may be increasingly heavier with depth. The material at the higher points of such reservoirs
can contain more gas in solution than the petroleum in the lower points of the reservoir. Thus, more
asphaltenic material is precipitated at the top of the reservoir than at the bottom of the reservoir.
Some enhanced recovery processes can also prompt the separation of asphaltene sediment. For
example, carbon dioxide flooding and miscible hydrocarbon flooding have been known to produce sediments during operations (Islam, 1994). In carbon dioxide flooding, one or more banks are
injected into the reservoir followed by water as the driving fluid. The major operative mechanism
in carbon dioxide flooding appears to be swelling of the crude oil, miscibility, viscosity reduction,
and most important, reduced interfacial tension. The deposition of asphaltenic material appears to
be a result of displacement of the asphaltenes material by the carbon dioxide that competes with
the asphaltenes for solubility in the crude oil. This is almost the reverse of the situation that occurs
when salt is added to carbonated aqueous solutions and tends to displace the carbon dioxide from
the aqueous system.
A similar effect can be observed in miscible hydrocarbon flooding during which a rich gas
(i.e., gas with a high content of ethane, propane, butane, etc.) is injected into the reservoir (Islam, 1994).
Again, this can approximate the deasphalting process that is used to manufacture asphalt and the
injection of the low-boiling hydrocarbon will discharge asphaltenic material from the crude oil.
The adsorption of asphaltenic material during the separation of petroleum into its component
fractions has become a standard operation for petroleum analysis. It must therefore be presumed
that a similar effect can be operative under certain reservoir conditions, analogous to those prevalent when carbon dioxide and miscible hydrocarbon flooding technologies are applied to crude oil
recovery. Thus, interactions of the polar constituents with the reservoir rock must also be included
in any acknowledgment of the deposition of asphaltenic material. In addition, the deposition of
asphaltenic material on to the reservoir rock can alter the properties of the rock (Islam, 1994) to the
extent that water-wet rock can become oil-wet rock, which ultimately can have an adverse effect on
enhanced oil recovery processes.
Any asphaltenic material adsorbed from the crude oil by the reservoir rock may not be readily
desorbed from the rock (depending upon the nature of the rock) and may even be desorbed from
the rock as insoluble material. If this asphaltene-type material becomes mobile as sediment in the
oil, it can move to the pore channels to the recovery well, thereby blocking the channels, making
recovery difficult if not impossible.
Incompatibility is also possible when asphaltenes interact with reservoir rock, especially acidic
functions of rocks, through the functional groups (e.g., the basic nitrogen species) just as they interact with adsorbents. And, there is the possibility for interaction of the asphaltene with the reservoir
rock through the agency of a single functional group in which the remainder of the asphaltene molecule remains in the liquid phase. On the other hand, there is also the possibility that the asphaltene
constituents can react with the rock at several points of contact thereby enhancing the bonding to the
rock and, in some cases, effecting recovery operations to an even greater extent.
Another area where incompatibility might play a detrimental role during recovery operations
is as a result of oxidation. In general, it is the more polar species that oxidize first. And, after
incorporation of oxygen to a limit, significant changes can occur in asphaltene molecular weight