Date of Award
asphaltenes, dilatational rheology, adsorption kinetics, optimization, multi-component system, deposition
Asphaltenes, as indigenous components in crude oils, are believed to play an important role in the petroleum production and processing industry. For example, asphaltene molecules can adsorb onto the water / oil interface like the amphiphiles and hinder water droplet coalescence, resulting in the stabilization of water-in-oil emulsions. Also, in the upstream production, depending on the temperature and pressure, asphaltenes can precipitate and deposit onto the reservoir rocks, the wellbores and the pipelines, leading to the change in reservoir rocks’ surface wettability and the blockage of the production facilities. Hence, it is crucial to understand the mechanism of asphaltenes adsorption at the water-oil interface and their deposition onto the solid surfaces, and investigations are reported here on both these adsorption and deposition phenomena.
Defined by their solubility (soluble in aromatic solvents but insoluble in n-alkanes), the asphaltenes form a class of crude oil components with a wide distribution in molecular structures and this brings about polydispersity in surface activity, adsorption behaviors, and aggregate formation. The effects of polydispersity in mixtures of asphaltene molecules affect their dilatational rheology. This aspect of their behavior has often been neglected and is discussed here in the context of multi-component diffusional model simultaneously captures both dynamic interfacial tension and dilatational rheology behavior, with the same parameters, e.g., for composition and interfacial activity. The numerical analysis developed based on the diffusional mixture model reached the similar conclusions as the previous fractionation experiments that a minor fraction of asphaltene mixture (less than 10%) has a much higher surface activity than the bulk of them. This was confirmed by a recent study where the interfacially surface-active asphaltene fraction was separated from the remaining asphaltenes and this fraction was found to be enriched with larger molecules with a higher amount of heteroatoms such as oxygen and sulfur compared to the remaining bulk asphaltenes. The simulation results also supported the hypothesis that the long-term decay of surface tensions observed for asphaltenes-covered interfaces was a diffusion-controlled adsorption process and not a result of molecular reorganization. The applicability of Langmuir isotherm in study of asphaltenes adsorption kinetics was then discussed and justified by considering the reversibility at different scales (pendant drop experiment and emulsions) and introducing the BET model for multilayer adsorption.
Asphaltenes can also precipitate and deposit onto solid surfaces depending on a wide range of factors such as temperature and pressure. A preliminary investigation is presented on solubility effects on asphaltene adsorption from crude oils onto stainless steel surfaces using quartz crystal microbalance with dissipation (QCM-D) techniques. The kinetics of deposition at different concentrations was examined and the sizes of the primarily deposited asphaltene molecules were estimated from the initial adsorption kinetics. The numerical analysis of the experimental data using a theoretical two- step deposition model was attempted, and the optimized mean aggregate size number proved to be quite close to the reported values for some rock types in the earlier adsorption studies. Despite asphaltene precipitation increases with the increasing heptane percentage, the deposition of asphaltenes was found to reach the maximum at the solubility of 70 vol% heptane content. The performance of a commercial model inhibitor was then assessed under different conditions using the developed experiment metrics and the inhibitor was found to be able to reduce the maximum deposition amount at the solubility with 70% heptane fraction, which happened to be the condition that generates the largest amount of deposition. The information related to the solubility effect and the inhibitor performance is essential to help the industry in the assessment of the operating conditions for a better management of asphaltene-related flow assurance problems in crude oil recovery.
Liu, Fang, "Investigation of Asphaltene Adsorption at Liquid-Liquid and Liquid-Solid Interfaces" (2020). CUNY Academic Works.