Agreement about definitions and reduction of the uncertainties inherent in comparisons of crude oils and their deposits is definately needed.  In the past, comparisons have focused on the amount of material that might form deposits.  ASTM and other standards have directed our thinking toward standardized ways to isolate and quantify those materials, but after many years of testing it should be clear that what is important is not the amount of material, but its stability as a function of temperature, pressure, and composition.  The continuing focus on separations illustrates the principal danger of standards:  a poorly chosen standard can direct attention in the wrong direction. 

Abandoning the term asphaltene altogether may be too extreme a reaction.  There is material that flocculates under well-defined conditons that are distinct from other organics such as wax.  I continue to use the term "asphaltene" as a generic reference to material that is not soluble in the oil's lightest components, but otherwise I agree with Per Fotland that further refining the definition and standardizing isolation techniques are not the best approachs to solving real asphaltene problems. 

There are many methods available for assessing asphaltene stability in reservoir fluids, in stock tank oils, in dilute oil or bitumen solutions, and in fuel oils.  Each of these techniques detects asphaltene flocculation at a slightly different stage because of differences in sensitivity of detection and due to the effects of kinetics of asphaltene flocculation.  Additional confusion occurs because the effects of dilution are not properly taken into account.  Nevertheless, it should be possible to rationalize the results of all these different measures of onset of asphaltene instability.  What has been missing so far is a quantitative description of the oil. 

What do I mean by a quantitative description of the oil?  For the purpose of interpreting asphaltene stability experiments, I mean that we need to know the oil's solubility parameter.

If we want to standardize, the first step is to agree on an accurate measure of the solubility parameter of the starting material.  In our lab, we have been using a simple correlation with refractive index, but there may be other measures.  The next step is to define the onset condition.  I suggest that we choose as a reference point the first aggregates that can be seen with visible light; optical microscopy has the advantage of wide availability that is needed for a standard that can be readily applied in laboratories around the world. 

There are some other practical issues.  It is necessary to account for the effects of kinetics; a standard must define how long we should wait before making an observation and at what temperature.  Dilution with good solvents may be necessary with opaque samples, but it is essential to recognize that dilution affects the onset solubility parameter.  Finally, the standard should address the obvious problem of avoiding local instability.  Attention to these experimental details is important, but the keys are (1) defining a standard onset condition and (2) quantifying all stability observations with respect to solubility.  With those two pieces in place, we can all continue to explore by different experimental methods and in a wide range of pressure, temperature, and composition regimes and yet have a basis for discussing results sensibly with other labs and other researchers.

Jill Buckley
PRRC, New Mexico Tech