Detecting and characterizing asphaltene instability

Methods of detecting and characterizing asphaltenes abound in the literature, but most can raise significant levels of controversy. Because of real differences between samples, it can be difficult for one lab to validate the results of another. In this work, a suite of seven crude oil samples, including two with recognized asphaltene problems and a sequence of three hydrotreater product fluids, have been selected for comparative testing by relatively simple techniques at ambient conditions. The onset of flocculation in response to addition of heptane was identified in batch mode by microscopic observation with the onset solubility conditions calculated from refractive index measurements. These were compared to continuous heptane titrations of toluene solutions of oil with an optical detection system. At essentially infinite dilution, solubility parameter can be calculated from the known properties of toluene and heptane. Further characterization of some asphaltene fractions was provided by measurements of the amount of precipitate, by FTIR, and by elemental analysis. The results demonstrate similarities and differences in the properties measured by these different techniques and help to highlight some of the difficulties of such comparisons of asphaltenes and their stability.