Multi-step formation of asphaltene colloids in dilute solutions

Essential role of structure, architecture, and intermolecular interactions of asphaltene molecules on properties (self-association and surface activity)

One of the important challenges of the oil industry is the formation of asphaltene deposits and emulsions, which cause many operational and economic problems. Asphaltenes are heavy and polar fractions of petroleum with a mixture of diverse molecules. Their structural complexity makes the understanding of their properties puzzling. The purpose of this review is to understand the self-association and surface activity properties of asphaltenes. There are some popular models for the mechanism of asphaltene aggregation; each alone is not complete and without defects. Experimental studies and molecular dynamics demonstrate that the mechanism of aggregation is influenced by asphaltene' structure, architecture, and intermolecular forces. Factors such as oil composition, temperature, and pressure affect its intensity. In this article, these issues and their impact on the self-assembly of asphaltenes and ways to prevent it, especially chemical inhibitors, have been discussed in detail.

Surface Interaction of Water-in-Oil Emulsion Droplets with Interfacially Active Asphaltenes

Adsorption of interfacially active components at the water/oil interface plays critical roles in determining the properties and behaviors of emulsion droplets. In this study, the droplet probe atomic force microscopy (AFM) technique was applied, for the first time, to quantitatively study the interaction mechanism between water-in-oil (W/O) emulsion droplets with interfacially adsorbed asphaltenes. The behaviors and stability of W/O emulsion droplets were demonstrated to be significantly influenced by the asphaltene concentration of organic solution where the emulsions were aged, aging time, force load, contact time, and solvent type. Bare water droplets could readily coalesce with each other in oil (i.e., toluene), while interfacially adsorbed asphaltenes could sterically inhibit droplet coalescence and induce interfacial adhesion during separation of the water droplets. For low asphaltene concentration cases, the adhesion increased with increasing asphaltene concentration (≤100 mg/L), but it significantly decreased at relatively high asphaltene concentration (e.g., 500 mg/L). Experiments in Heptol (i.e., mixture of toluene and heptane) showed that the addition of a poor solvent for asphaltenes (e.g., heptane) could enhance the interfacial adhesion between emulsion droplets at relatively low asphaltene concentration but could weaken the adhesion at relatively high asphaltene concentration. This work has quantified the interactions between W/O emulsion droplets with interfacially adsorbed asphaltenes, and the results provide useful implications into the stabilization mechanisms of W/O emulsions in oil production. The methodology in this work can be readily extended to other W/O emulsion systems with interfacially active components.