Kinetics of asphaltene precipitation: effect of different n-alkane precipitants

Abstract

Asphaltenes are a class of molecules that have a tendency to precipitate from crude oils due to changes in its solubility. Asphaltene precipitation can eventually lead to serious problems in oil production such as plugging well bores and reservoirs. Furthermore, precipitated asphaltenes can also deposit onto oil pipelines and decrease oil production. To induce asphaltene precipitation, n-alkanes are usually added to the crude oils. In a typical precipitation experiment, the effect of time is often neglected and measurements are conducted with a waiting time that varies from a few minutes to 24 hours. Based on this short waiting time, it is conventionally believed that asphaltenes can only precipitate when the precipitant concentration is above a critical value. However, this study demonstrates that kinetic effects are significant during asphaltene precipitation. By using optical microscopy, it was shown that asphaltenes can precipitate below the “critical precipitant concentration”. The time required for precipitation varied from a few minutes to several months, depending on the precipitant concentration. A centrifugation-based separation technique was used to determine the amount of precipitated asphaltenes as a function of time until equilibrium was reached. The precipitation kinetics using different n-alkane precipitants was also investigated. It was found that the type of n-alkane has a profound effect on the precipitation kinetics. Additionally, a geometric population balance model has been applied to simulate the kinetic effects discussed above.