One of the most cited and early references in petroleum research is that by Pfeiffer and Saal (J. Phys. Chem. 1940, 44, 139).  These pioneers made a plea regarding the development of a standard method for separating bitumen into fractions (see p. 143 for their quote).

“The quantities of the soluble part (the maltenes) and the insoluble part (the asphaltenes) vary for the same bitumen with the nature and the quantity of solvent used, with the temperature of mixing, and with the time elapsing between mixing and separation of the precipitate.  Therefore the procedure has to be thoroughly standardized.”

In some respects, it seems our world has changed little since 1940.  Then again, the gradual development of petroleum research to the present may have taken some fortuitous avenues.  Researchers often follow hunches.  It is these hunches, these instincts, that have driven much of our understanding by developing numerous methods to classify these inherently complex and diverse fluids.

By using diverse methods, one sees the problem (for now, let’s say all the problems associated with petroleum instead of just asphaltene problems) through different lenses.  Each lens acts as a filter that provides a different view of the material or problem under investigation.  Astronomers (or space physicists) face a similar dilemma.  Those designing telescopes using the UV-visible portion of the electromagnetic spectrum see a very different sky than those using IR, X-rays, gamma-rays, etc.  Each view, when coupled together, gives a clearer perspective of our diverse universe.  However, astronomers often also tend to look at the same object or group of objects.  In time, their cumulative views provide insight that any one technique could not achieve.

The relationship between studying the heavens and studying crude oils has some strong analogies.  For example, both systems are extremely complex and constantly changing.  However, there is also a subtle difference between the two fields.  Multiple petroleum labs do not generally study the same crude oil sample (nor the same batch).  Generally, each lab studies oils from sites that are either readily available say by ownership of a particular field or where a strong producer-researcher partnership exists.  That’s not a bad thing as petroleum research has made tremendous progress.  Even so, the current system does not seem to allow potential synergies from different labs to grow.  It also does not prevent the confusion that can arise when different labs investigate different crude oils with different methods.  By studying the same crude oil from several different perspectives, a very complete picture might result.

To use another relevant analogy in describing the dilemma, if a synthetic chemist wants to reproduce a certain chemical cited in the literature, they use the same starting materials in the reference.  Petroleum chemists and engineers do not have that convenience.  Our raw materials can change readily (for example by merely varying the mixture of wells that are commingled to generate a sample).  To put different labs on similar footing, it is the dissemination of crude oils for study that should be considered as much as the methods used to characterize them.  Our industry is confronted as much by each lab looking at a different crude as by the methods used to study them.  In fact, it is likely that the number of crude oils being studied around the world is greater than the different methods used to study them.  In matters of confirmation or disagreement of results, there simply is not a standard set of crude oils.  Resolving this issue is probably easier said than done as there are certainly legitimate special interests for why one lab wants or needs to study a particular crude.

In essence, an alternative approach to standardizing methods of crude oil characterization is to ensure or coordinate the distribution of select crude oils to different labs using different techniques or methods of characterization.  This approach embraces the old adage “divide and conquer.”  Labs that use the same crude in effect have the same starting point.  This might enhance further dialog when discussing or presenting data while keeping multiple research views alive and well.  Then after some period of review and comparison maybe some method can be deemed “best” or “most appropriate” that can be then be a standard as eluded to by Pfeiffer and Saal.  At this stage, it seems not entirely clear what method (or methods) are best without having different groups compare and contrast a given crude oil or better yet set of crude oils.

Regarding the specific study (and often isolation) of asphaltenes, consideration for characterizing the remainder, the maltenes, should be reviewed as well.  As shown by Andersen and Speight, four fairly common separation schemes were performed on Boscan crude (see Table 1 in Petr. Sci. Tech. 2001, 19, 1).  The resin numbers had more variance than the asphaltene numbers.  For those using resin to asphaltene ratios to substantiate a point, the separation methodology becomes a big issue.  If an analytical procedure is eventually adopted for asphaltene separation, then the remaining maltene fraction is in part defined as well.  However, the further fractionation of maltenes is as open to standardization as methods used in asphaltene isolation.

Michael K. Poindexter
Nalco Company
Sugar Land, Texas