Asphaltene Instability Trends of Light and Heavy Crude Oils

Abstract

In this work, two Iranian crude oils diluted in 1-methylnaphthalene (1-MN) were titrated with selected n-alkanes. Subsequently, samples were observed microscopically to determine the onset of asphaltene precipitation. A series of micrographs from de-asphaltening were used to show visible changes of the asphaltene sizes, shapes, and frequencies by addition the n-heptane to the subsamples after 5, 6, 11, and 24 hour lag times. The refractive indices (RI) of the titrated mixtures at different temperatures below and above the onset conditions were measured aiming to establish the asphaltene instability trend. Results show that for the diluted light and heavy crude oils, the onset of asphaltene precipitation is rather a gradual process with an almost constant slope of RI decrease due to the separation of asphaltene clusters from the mixture. This is a kinetically controlled process. Furthermore, the nature of the precipitant is likely to play a notable role. The rate of RI decreasing with temperature was approximately 0.0004/°C for both tested crude oils.