349
Structure of Petroleum
petroleum has received less attention. And yet, such reactivity cannot be ignored. It is for this reason
that the reactivity of the resin and nonvolatile oils fractions is considered here.
The chemical reactions of the resins have received much less attention than the reactions of the
asphaltenes, and usually only fragmentary reports are available. Like the chemical reactions of
the asphaltene constituents, the chemical reactions of the resin constituents give indications of the
functional types within these constituents. The reactions also indicate the effects that change in the
hydrocarbon structure or functionality will have on the properties of the resin constituents.
Although much work has been reported on the thermal decomposition of asphaltenes, a lesser
amount of work is reported for the resins. The data that are available indicates that, like the asphaltenes,
the resins can be decomposed thermally to produce, on the one hand, a hydrocarbon-type of distillate
and, on the other hand, coke-like material. The prevalent conditions determine the relative proportions
of two products.
One area of resin chemistry that has received attention is the interaction with oxygen. Thus, the
oxidation of resins in benzene solution with air, in the presence or absence of salts of various metals,
proceeds readily to yield asphaltene products (Moschopedis and Speight, 1978b). Substantial uptake
of oxygen occurs, and from the atomic heteroatom–carbon ratios in the starting material and products, it appears that preferential reaction of the more polar entities occurs. Resins also undergo
condensation with formaldehyde, which is especially rapid after introduction of oxygen functions
by oxidation. Resins can also be sulfonated to yield water-soluble or oil-soluble sulfonates. Resins
react with nitric acid to yield complex mixtures of oxidation and nitration products. Reactions with
sulfur cause dehydrogenation as well as the formation of complex sulfides. The overall result is the
production of higher molecular weight material with low hydrogen content.
Resins also react with diazo compounds, which not only produce asphaltene-type products but
also can be used to introduce various functional groups into the molecule. Resins react with acetic
anhydride to afford a variety of acetylated products.
13.2.3.3 Molecular Weight
The molecular weights of resin fractions are substantially lower than those of asphaltenes and do not
usually vary, except for the limits of experimental error, with the nature of the solvent or the temperature of the determination. It may, therefore, be presumed that the molecular weights of resins,
as determined by various methods, are in fact true molecular weights and that forces that result in
intermolecular association contribute very little, if anything, to their magnitude.
13.3 PETROLEUM SYSTEM
Thus, chemical models for asphaltenes that might be more in keeping with maturation pathways
and with behavioral characteristics might well be selected from the two extremes of the molecular weight–polarity diagram. Such models would have smaller polynuclear aromatic systems and
also span the range of functional types as well as molecular sizes (Speight, 1994). It must also
be recognized that such models can have the large size dimensions that have been proposed for
asphaltene constituents but will also be molecular chameleons insofar as they can vary in dimensions depending upon the angle of rotation about an axis and/or the freedom of rotation about one,
or more, of the bonds. The ability of the asphaltene molecules to adapt to a physical environment
is very important in recovery operations. Adsorption of the asphaltene on to the reservoir rock in
parallel to the plane of the rock surface (with two or more points of contact between the asphaltene and the rock) or at a right angle to the rock surface (Speight, 1994) with the remainder of
the molecule suspended in the oil phase can make a difference to the behavioral characteristics
of the oil–rock system.
An early hypothesis of the physical structure of petroleum (Pfeiffer and Saal, 1940) suggested that
asphaltenes are the centers of micelles formed by adsorption, or even by absorption of part of the maltene fraction, that is, resin material, on to the surfaces or into the interiors of the asphaltene particles.
Structure of Petroleum
petroleum has received less attention. And yet, such reactivity cannot be ignored. It is for this reason
that the reactivity of the resin and nonvolatile oils fractions is considered here.
The chemical reactions of the resins have received much less attention than the reactions of the
asphaltenes, and usually only fragmentary reports are available. Like the chemical reactions of
the asphaltene constituents, the chemical reactions of the resin constituents give indications of the
functional types within these constituents. The reactions also indicate the effects that change in the
hydrocarbon structure or functionality will have on the properties of the resin constituents.
Although much work has been reported on the thermal decomposition of asphaltenes, a lesser
amount of work is reported for the resins. The data that are available indicates that, like the asphaltenes,
the resins can be decomposed thermally to produce, on the one hand, a hydrocarbon-type of distillate
and, on the other hand, coke-like material. The prevalent conditions determine the relative proportions
of two products.
One area of resin chemistry that has received attention is the interaction with oxygen. Thus, the
oxidation of resins in benzene solution with air, in the presence or absence of salts of various metals,
proceeds readily to yield asphaltene products (Moschopedis and Speight, 1978b). Substantial uptake
of oxygen occurs, and from the atomic heteroatom–carbon ratios in the starting material and products, it appears that preferential reaction of the more polar entities occurs. Resins also undergo
condensation with formaldehyde, which is especially rapid after introduction of oxygen functions
by oxidation. Resins can also be sulfonated to yield water-soluble or oil-soluble sulfonates. Resins
react with nitric acid to yield complex mixtures of oxidation and nitration products. Reactions with
sulfur cause dehydrogenation as well as the formation of complex sulfides. The overall result is the
production of higher molecular weight material with low hydrogen content.
Resins also react with diazo compounds, which not only produce asphaltene-type products but
also can be used to introduce various functional groups into the molecule. Resins react with acetic
anhydride to afford a variety of acetylated products.
13.2.3.3 Molecular Weight
The molecular weights of resin fractions are substantially lower than those of asphaltenes and do not
usually vary, except for the limits of experimental error, with the nature of the solvent or the temperature of the determination. It may, therefore, be presumed that the molecular weights of resins,
as determined by various methods, are in fact true molecular weights and that forces that result in
intermolecular association contribute very little, if anything, to their magnitude.
13.3 PETROLEUM SYSTEM
Thus, chemical models for asphaltenes that might be more in keeping with maturation pathways
and with behavioral characteristics might well be selected from the two extremes of the molecular weight–polarity diagram. Such models would have smaller polynuclear aromatic systems and
also span the range of functional types as well as molecular sizes (Speight, 1994). It must also
be recognized that such models can have the large size dimensions that have been proposed for
asphaltene constituents but will also be molecular chameleons insofar as they can vary in dimensions depending upon the angle of rotation about an axis and/or the freedom of rotation about one,
or more, of the bonds. The ability of the asphaltene molecules to adapt to a physical environment
is very important in recovery operations. Adsorption of the asphaltene on to the reservoir rock in
parallel to the plane of the rock surface (with two or more points of contact between the asphaltene and the rock) or at a right angle to the rock surface (Speight, 1994) with the remainder of
the molecule suspended in the oil phase can make a difference to the behavioral characteristics
of the oil–rock system.
An early hypothesis of the physical structure of petroleum (Pfeiffer and Saal, 1940) suggested that
asphaltenes are the centers of micelles formed by adsorption, or even by absorption of part of the maltene fraction, that is, resin material, on to the surfaces or into the interiors of the asphaltene particles.