Asphaltenes and resins have been separated from four mexican oils suffering from the deposition of asphaltenic material during recovery operations. A SARA separation of the oils was performed and the resins and asphaltenes further analyzed. Characterization methods employed were FTIR, elemental composition both of CHNSO, and trace metals and molecular weight determination using size exclusion chromatography. NMR techniques were applied to two asphaltene samples. The overall scope of the work was to get a better understanding of the nature of the asphaltene stability at a molecular level in these problematic oils. Separation of resin in two fractions indicates that there is no long alkyl chains in these as given by FTIR, which may be the cause of the lack of stability along with the large difference in bubble point and reservoir pressure. *On sabbatical leave from Dept. Chem. Eng., Technical University of Denmark, DK-2800 Lyngby, Denmark.

Asphaltenes are known to cause problems related to flocculation, precipitation, and plugging, either in the formation, production lines, and processing equipment. Different models have been proposed to predict the thermodynamic conditions under which asphaltenes precipitate over the past years. This work analyses the performance of various models on their capability to match the literature experimental data of precipitated asphaltene mass fractions. Twenty-five different models based on equation-of-state (EoS), polymer solution, and thermodynamic-colloidal theories were identified. The performance/test datasets were collected and classified according to their pressure/temperature conditions, CO₂, n-C₅/n-C₇ gas, and liquid titrations. Statistical analysis, including residuals, parity plots, and average absolute relative deviation (AARD, %), were used to compare the adequacy of selected models. Results confirmed the need for further model development for general applications over wide pressure, temperature, and composition intervals.