91
Reservoirs and Reservoir Fluids
chemical changes and/or emulsion formation. These advantages may be of little consequence when
it is not, for various reasons, the intention to recover the various product fractions in toto or in the
original state, but in terms of the compositional evaluation of different feedstocks the disadvantages
are very real.
Methods used for the separation of petroleum and heavy oil into various fractions are often
identified by the acronyms for the names: PONA (paraffins, olefins, naphthene constituents, and
aromatic constituents), PIONA (paraffins, isoparaffins, olefins, naphthene constituents, and aromatic constituents), polynuclear aromatic hydrocarbon (paraffins, naphthene constituents, and
aromatic constituents), PINA (paraffins, isoparaffins, naphthene constituents, and aromatic
constituents), or SARA (saturates, aromatics, resin constituents, and asphaltene constituents).
However, it must be recognized that the fractions produced by the use of different adsorbents will
differ in content and will also be different from fractions produced by solvent separation techniques. However, for heavy oil fractions, the absence of paraffins in the sample usually precludes
many of these acronyms and the most common method is the SARA method.
High-performance liquid chromatography (HPLC) has found great utility in separating different hydrocarbon group types and identifying specific constituent types. Of particular interest is
the application of the HPLC technique to the identification of the molecular types in the heavy oil,
especially the molecular constituents of the asphaltene fraction. The general advantages of highperformance liquid chromatography are (1) each sample may be analyzed as received even though
the boiling range may vary over a considerable range and (2) the total time per analysis is usually
on the order of minutes.
4.5.3 moleCulAr WeIgHt
Even though recovery processes, in general, do not affect the quality of the oil, there is still the need
to determine the molecular weight of the original constituents as well as the molecular weights of
the products as a means of understanding the process. For those original constituents and products,
for example, resins and asphaltenes, that have little or no volatility, vapor pressure osmometry
(VPO) has been proven to be of considerable value.
There have been numerous attempts made to measure the molecular weight of heavy oil, particularly the asphaltene fraction, using a variety of different methods but there appears to be a noticeable lack of consensus on the value obtained for a specific sample. This has been irrevocably traced
to structural aspects of heavy oil and to the behavior of the asphaltene constituents in molecular
dispersion in their own maltene fraction (Koots and Speight, 1975). In particular, the reliability of
the method, the meaning of the term average molecular weight, and the interpretation or usefulness
of the data has received considerable attention.
Currently, of the methods available, several standard methods are recognized as being useful for
determining of the molecular weight of petroleum fractions (ASTM D2224; ASTM D2502; ASTM
D2503; ASTM D2878). Methods for molecular weight measurement are also included in other more
comprehensive standards (ASTM D128, ASTM D3712) and several indirect methods have been
proposed for the estimation of molecular weight by correlation with other, more readily measured
physical properties. They are satisfactory when dealing with the conventional type of crude oils or
their fractions and products and when approximate values are desired.
Vapor pressure osmometry (ASTM D2503), also called vapor phase osmometry, is a relatively
simple and cheap method for the determination of molecular weight. Most osmometers can operate
over a range of temperature through the use of probes that cover specific temperature ranges. This
gives the number average molecular weight and not the molecular weight distribution.
The measuring elements are two temperature-sensitive thermistors placed in a closed, heat-insulated chamber. By means of two syringes, a drop of solvent is placed on one and a drop of the sample
in solution in the same solvent is placed on the other. The chamber is saturated with solvent vapor
and carefully temperature controlled. Because the solution has a lower vapor pressure than the
Reservoirs and Reservoir Fluids
chemical changes and/or emulsion formation. These advantages may be of little consequence when
it is not, for various reasons, the intention to recover the various product fractions in toto or in the
original state, but in terms of the compositional evaluation of different feedstocks the disadvantages
are very real.
Methods used for the separation of petroleum and heavy oil into various fractions are often
identified by the acronyms for the names: PONA (paraffins, olefins, naphthene constituents, and
aromatic constituents), PIONA (paraffins, isoparaffins, olefins, naphthene constituents, and aromatic constituents), polynuclear aromatic hydrocarbon (paraffins, naphthene constituents, and
aromatic constituents), PINA (paraffins, isoparaffins, naphthene constituents, and aromatic
constituents), or SARA (saturates, aromatics, resin constituents, and asphaltene constituents).
However, it must be recognized that the fractions produced by the use of different adsorbents will
differ in content and will also be different from fractions produced by solvent separation techniques. However, for heavy oil fractions, the absence of paraffins in the sample usually precludes
many of these acronyms and the most common method is the SARA method.
High-performance liquid chromatography (HPLC) has found great utility in separating different hydrocarbon group types and identifying specific constituent types. Of particular interest is
the application of the HPLC technique to the identification of the molecular types in the heavy oil,
especially the molecular constituents of the asphaltene fraction. The general advantages of highperformance liquid chromatography are (1) each sample may be analyzed as received even though
the boiling range may vary over a considerable range and (2) the total time per analysis is usually
on the order of minutes.
4.5.3 moleCulAr WeIgHt
Even though recovery processes, in general, do not affect the quality of the oil, there is still the need
to determine the molecular weight of the original constituents as well as the molecular weights of
the products as a means of understanding the process. For those original constituents and products,
for example, resins and asphaltenes, that have little or no volatility, vapor pressure osmometry
(VPO) has been proven to be of considerable value.
There have been numerous attempts made to measure the molecular weight of heavy oil, particularly the asphaltene fraction, using a variety of different methods but there appears to be a noticeable lack of consensus on the value obtained for a specific sample. This has been irrevocably traced
to structural aspects of heavy oil and to the behavior of the asphaltene constituents in molecular
dispersion in their own maltene fraction (Koots and Speight, 1975). In particular, the reliability of
the method, the meaning of the term average molecular weight, and the interpretation or usefulness
of the data has received considerable attention.
Currently, of the methods available, several standard methods are recognized as being useful for
determining of the molecular weight of petroleum fractions (ASTM D2224; ASTM D2502; ASTM
D2503; ASTM D2878). Methods for molecular weight measurement are also included in other more
comprehensive standards (ASTM D128, ASTM D3712) and several indirect methods have been
proposed for the estimation of molecular weight by correlation with other, more readily measured
physical properties. They are satisfactory when dealing with the conventional type of crude oils or
their fractions and products and when approximate values are desired.
Vapor pressure osmometry (ASTM D2503), also called vapor phase osmometry, is a relatively
simple and cheap method for the determination of molecular weight. Most osmometers can operate
over a range of temperature through the use of probes that cover specific temperature ranges. This
gives the number average molecular weight and not the molecular weight distribution.
The measuring elements are two temperature-sensitive thermistors placed in a closed, heat-insulated chamber. By means of two syringes, a drop of solvent is placed on one and a drop of the sample
in solution in the same solvent is placed on the other. The chamber is saturated with solvent vapor
and carefully temperature controlled. Because the solution has a lower vapor pressure than the