Effect of Asphaltene Dispersants on Aggregate Size Distribution and Growth

A crude oil system can become unstable because of changes in hydrocarbon composition, pressure, or temperature during normal production from the reservoir or during commingling with dissimilar crude oils. These changes can generate asphaltene particles that can result in significant production and refining problems. The generation of these particles is a two-step process: phase separation and asphaltene particle growth. Phase separation occurs when nanosize asphaltene particles from the crude oil precipitate and grow into large aggregates. Our study on asphaltene precipitation shows that large asphaltene particles are aggregates consisting of very small (sub-micrometer) size asphaltene particles. One mechanism to control asphaltenes is to kinetically inhibit the phase separation of asphaltenes by adding a small amount of a chemical that interferes with the phase separation processes. Another mechanism to control asphaltenes is to inhibit growth by stabilizing the colloidal suspension of the sub-micrometer asphaltene particles to significantly slow the flocculation and settling processes. Asphaltene chemical additives of known molecular structures as well as proprietary chemical blends were selected for this study. None of chemicals studied inhibited phase separation; however, some of the dispersants did slow or stop flocculation and growth. Four different analytical techniques have been used to study the effect of chemical additives on asphaltene aggregation and settling and to evaluate the effectiveness of different asphaltene chemicals in keeping asphaltene particles suspended/dispersed in crude oils. From the turbidity measurement, asphaltene dispersants can be classified into three categories based on their performance. The particle size distribution measurement showed three different types of asphaltenes: stable asphaltenes, colloidal asphaltenes, and flocculated asphaltenes, on the basis of aggregate sizes. Asphaltene dispersants can stabilize colloidal asphaltenes and slow the growth and formation of flocculated asphaltenes. These results can be used to select the best chemical treatment plan for preventing/reducing asphaltene settling and deposition.