Revisiting the methodology for asphaltenes precipitation

Abstract

The world increase in energy demand has directed the petroleum industry to exploit unconventional oil reserves such as the Brazilian Pre-Salt reservoirs. Production conditions verified in those reservoirs enhance the occurrence of asphaltene precipitation as well as the correlated problems. Asphaltenes are well known as a solubility class. Such classification makes tough the determination of consistent chemical properties for these compounds. In this scenario, this study aims to analyze the influence of the standard precipitation methodologies upon the asphaltene characteristics. Three oils were selected, which are hereinafter called BR1, BR2, and BR3. Oil BR1 was produced from a Post-Salt reservoir while oils BR2 and BR3 are originated from a Post-Salt one. The asphaltenes were extracted through precipitation by n-heptane according to the ASTM 2007, ASTM 6560 (IP-143), the so-called ASTM 6560 SOHXLET, and the NUESC methodology. Asphaltene fractions were characterized regarding their chemical composition (FTIR) and thermal stability (DSC). The results showed that endothermic events typically registered for compounds which are strange to the commonly accepted asphaltene composition were verified. Also, the degradation temperature varied according to the precipitation method. In turn, the FTIR spectra detected a peak which is usually associated with naphthenic acids (3500 cm−1). Therefore, the FTIR analysis confirmed what was indicated by the thermograms. In summary, it was concluded that the asphaltene fractions that were extracted through the commonly used ASTM 2007 methodology from the Pre-Salt oils were a precipitated complex composed by asphaltenes and wax of naphthenic acids. Thus, the present work results are suggested as a starting point for the analyses of asphaltene properties and their implications on the crude oil behavior.