Effect of temperature on the appearance of flocculated asphaltene in an unperturbed unstable American shale oil: insights from a colloidal modeling

This work aimed to study the onset of flocculation of asphaltenes using optical microscopy and to evaluate the effect of temperature on the destabilization of an unstable undisturbed American shale oil. After magnetic stirring for 24 h, the flocculated asphaltene was redispersed. Increasing the temperature to 40°C led to asphaltenes flocculating. After heating, the sample was cooled back to room temperature. We calculated the precipitate volume from microscopy data and found that after cooling the system had a larger precipitate volume than in the previously studied scenarios. When comparing the volumes of precipitates before and after cooling, it is possible to observe that the volume of smaller particles does not vary significantly in each class, but the volume of larger particles increases with cooling, indicating an irreversible phenomenon. Statistical analysis of the optical microscopy results allowed a detailed analysis of the phenomenon. To study the colloidal effect, a theoretical analysis was performed based on Hamaker theory and De Gennes scaling theory, presenting compatible results when the parameters of the repulsive term were adjusted. When the parameters of both the attractive term and the repulsive term of the model were kept constant, greater stability was verified with the increase of temperature. Conversely, it was observed that the reduction of the parameter related to the length of the alkyl chains can explain the experimental macroscopic behavior of the shale oil here studied and also some different behaviors reported in the literature, i. e., increasing of flocculated volume with increasing temperature. This size reduction of the length of the alkyl chains can be attributed to a desolvation effect. Hence, the appropriate parameterization of the model showed great potential to mimic the behavior of increasing and also decreasing stability with the increase of temperature. The sets of parameters were related to reaction-limited aggregation (RLA) and diffusion-limited aggregation (DLA), which allowed to interconnect the discussion on experimental stability of asphaltenes, molecular simulation, and the theoretical discussion. Furthermore, the proper use of this theory seems to be a promising way to represent microscopic interactions in asphaltene systems, including previous results from the literature. This work presents (i) an experimental case that shows reversibility of asphaltene flocculation only with magnetic stirring; (ii) the effect of temperature on the stability of asphaltenes, and also (iii) brings a contribution with the volume of precipitate, making it possible to conclude on the irreversibility in the thermal cycle applied. In addition, the proper use of this modeling can predict the stability changes as a function of temperature, being a promising way to represent microscopic interactions in asphaltene systems, including previous results from the literature.