Characterization of the Asphaltene Onset Region by Focused-Beam Laser Reflectance : A Tool for Additives Screening

Deposition problems have more commonly been reported in crude oils with low asphaltene concentrations rather than in crude oils with high contents of asphaltenes, which have been shown to have a very stable behavior. Therefore, it is more important to determine the onset conditions at which asphaltenes start to flocculate rather than the amount of solids determined by standard procedures (i.e., IP-143 standard), because it typically does not provide any data about their stability. This work deals with the study of asphaltenes aggregation kinetics near the onset by using focused-beam laser reflectance measurements (FBRMs). Four crude oils and a 190+ residue have been investigated to determine the onset ratio and the kinetic evolution of the particle size distributions. The results show that there is no relation between the total amount of asphaltenes and their stability. The kinetic study of the asphaltene aggregation near the onset has shown that the time needed to reach the equilibrium is much longer than for greater n-heptane/crude oil ratios. Moreover, information on the existence of a delay and the growth rate is also provided. Concerning the use of asphaltene inhibitors, experimental data show that they act by shifting the onset n-heptane/oil ratio to higher values, whereas the kinetics and particle size distributions are unaffected. This unexpected conclusion should not be generalized to other n-heptane/oil ratios far from the onset region or other additives that may have a different inhibition mechanism. Considering that the results of the onset n-heptane/oil ratios, aggregation kinetics and particle size distributions show no apparent relation to the total asphaltene content. FBRM data constitute a valuable tool for the assessment of asphaltene deposition problems and the screening of efficient additives to control asphaltene stability.