Chemical–physical analysis of rheologically different samples of a heavy crude oil

Heavy crude oil samples extracted from the same well showed some rheological differences mainly in terms of viscosity and response to the addition of a small amount of dodecyl benzen sulfonate. Also slight differences in the activation energy and in the viscosity dependence on a solvent concentration were observed. The crude oil samples, also separated in maltenic and asphaltenic fractions, were fully characterized by elemental and proximate analysis, solvent extraction, thermogravimetry, mass spectrometric and spectroscopic analyses. No significant differences were found showing that the samples have the same composition and molecular structure and demonstrating that the different rheological behavior has to be related only to a different microstructural organization that is not sensed by the chemical–physical tests. This result helps to complete the picture of the complex correlation between rheological and chemical–physical properties of heavy crude oils. While it is accepted that chemical-physically different samples may, or may not, have a different rheology, we show for the first time that unperturbed samples with the same chemical–physical properties may exhibit differences in the rheological behavior. •We consider three samples of a heavy crude oil extracted from a single well.•The three samples show a measurably different rheological behavior.•Compositional, thermal and spectroscopic analyses don't show relevant differences.•The different rheology is only ascribed to a different microstructural organization.•Rheology catches weak structural interactions not sensed by chemical–physical tests.