Title: Aging Oil-Water Interfaces with Asphaltene and Demulsifier Adsorption: Interface Rheology and Heterogeneity
Advisor: Todd Squires
Asphaltenes are surface-active molecules which naturally appear in the crude oil. They can adsorb at the water-oil interface and form a viscoelastic film, which stabilizes emulsion drops and makes water-oil separation extremely challenging. These emulsion drops can cause serious problems in the petroleum industry, so the understanding of the asphaltene film and a reliable method to enhance water-oil separation needs to be established.
In this study, we apply the interfacial shear rheology on the water-oil interface to characterize the adsorption of asphaltene. Generally, the evolving asphaltene adsorption results in the increase of interfacial stiffness and brings a transition from viscous-dominated to more viscoelastic response to the interface. However, significant variation in the rheological evolution is observed between the nominally-identical experiments. The direct visualization of the interface reveals the mechanical heterogeneity appearing at the evolving film, from both micron and millimeter scale. This heterogeneity seems to be an inherent feature of the asphaltene film and impact the coalescence of asphaltene-stabilized emulsion drops.
In addition, we also reveal that the asphaltenes interfacial rheology can be impacted by the chemical and physical parameters of the experiment. The oil composition such as aromatic fraction, bulk asphaltene composition and the choice of model oil can all influence the evolving rheology. Furthermore, the water-oil geometry, selection of rheometry probes and the way conducted shear rheology can also vary the measured rheological response. Our results thus reveal a more complex adsorption process and interfacial structure than that reported in the literature and provide a deep understanding of how to control the asphaltene adsorption.
The asphaltene-stabilized emulsions are often demulsified by the chemical demulsifier. The second focus in this dissertation is investigating how the various chemical demulsifiers impact the asphaltene interfacial rheology. We first examine ethycellulose (EC) as a model demulsifier using two different mechanisms. The `premixed' experiment allows asphaltene and EC concurrently adsorb, showing that EC can disrupt asphaltene adsorption and retard the stiffening of interfacial film. The experiment of introducing EC to aged asphaltene layer, on the other hand, characterizes that EC softens the stiff layer formed prior to the addition of demulsifier. From the direct visualization of the heterogeneous structure, we further reveal that EC acts inhomogeneously and soften the relatively soft region at the interfacial film initially. This mechanism might impact the demulsification process and provide a new insight into the emulsion destabilization.
The influence of chemical architecture on the rheological response of demulsifier is also investigated. We utilized a series of EO-PO-EO block copolymers to the two mechanisms described in the EC experiment. The high molecular weight is found to decrease the ability of demulsifier on preventing asphaltene from forming an viscoelastic layer at the low concentration but generates no significant change on softening the aged asphaltene layer. High hydrophilic-lipophilic balance (HLB) can better prevent asphaltene from adsorption, but poor ability to soften the aged asphaltene layer.
These results can provide systematic information to guide the design of demulsifier which can provide a particular rheological response at the interface, facilitating the demulsification process.