CO2-Responsive Surfactants for Enhancing Heavy Oil Recovery: from Fundamentals to Bench-Scale Demonstrations in Canadian Oil Sands Extraction

  • Author / Creator
    Lu, Yi
  • Interfacial properties at the oil-water interface are of key importance to various operations in the petroleum industry, especially in the aqueous-based heavy oil recovery process. However, different operation stages often require different interfacial properties, which could be conflicted with each other at specific circumstances. Reducing oil-water interfacial tension (IFT) by adding surfactants benefits the heavy oil liberation from host rocks/solids, but soon becomes detrimental to the subsequent stage where the separation of heavy oil product from the aqueous phase prefers high IFT values. In this study, the CO2-responsive surfactant series was designed to solve such interfacial problems and enhance heavy oil recovery. Responsive surfactants feature interfacial activity that can be switched on/off by external stimuli. Hence, they permit the modulation of interfacial property in a controllable manner, such that multiple requirements could be accomplished by one single chemical addition. In this study, CO2 gas was selected as the stimuli because it is inexpensive, abundant in nature, and usually available onsite in the flue gas.
    One of the major challenges in applying CO2-responsive surfactants to large-scale petroleum industry concerns their CO2 switchability and robustness under operating conditions. Our strategy is to design a series of CO2-responsive surfactants with facile preparation and tunable switching pH, which allows a suitable reagent to be readily selected based on requirements. These CO2-responsive surfactants were formed by combining monoethanolamine (MEA) with long-chain fatty acids (LCFAs) of variable chain lengths through electrostatic attraction. The tunability of switching pH for this group of surfactants was demonstrated by in situ probing of the CO2-responsive characteristics at the oil/water interface using dynamic IFT measurements. It is shown that the switching pH of the CO2-responsive surfactants was controlled by the hydrocarbon chain length of LCFAs. More importantly, their switching behavior was found to be different at the interface and in the bulk solution, which is attributed to the enhanced molecular interactions at the interface. Since most applications require surfactants to be switched at the interface, the switching pH of CO2-responsive surfactants is thereby most appropriate to be determined through their interfacial responses.
    With comprehensive understandings of the interfacial activity and interfacial response, CO2-responsive surfactants were assessed by their performance in heavy oil recovery, where the process was considered in two stages, heavy oil liberation from solids and heavy oil harvest from emulsions. It is demonstrated that the addition of CO2-responsive surfactants facilitated the release of heavy oil from solid substrates by decreasing oil/water IFT. Meantime, surfactants also helped the aqueous medium carrying out more heavy oil by forming stable heavy oil-in-water (HO/W) emulsions. In a subsequent stage, however, it was difficult to separate the heavy oil from these surfactant-stabilized emulsions, resulting in severe production loss. In contrast, fast demulsification was achieved by activating the CO2-responsiveness of responsive surfactants, and thereby, harvesting more effectively after phase separation. The sustainability of CO2-responsive surfactants was also investigated in the recycled process water.
    Finally, CO2-responsive surfactants were applied to water-based Canadian oil sands extraction process and successfully enhanced ultra-heavy oil (bitumen) recovery in bench-scale demonstrations. Compared with conventional heavy oil recovery where the heavy oil product was harvested after spontaneous phase separation, the Canadian oil sands extraction collects the bitumen by flotation where air bubbles are used to capture the liberated bitumen in the aqueous phase. Nevertheless, the ability to switch off interfacial activity at the interface is also beneficial to the bitumen-air bubble attachment. Bench-scale bitumen flotation experiments were conducted at ambient temperature using real Canadian oil sands ores. Bitumen recovery was significantly improved from 15.0 % to 50.4 % with the addition of CO2-responsive surfactants and the activation of CO2 switching.
    This study demonstrates that the interfacial properties at the oil-water interface are essential to the aqueous-based heavy oil recovery process, and the ability to manipulate these interfacial properties precisely is a promising direction to develop the next generation processing aid for the petroleum industry.

  • Subjects / Keywords
  • Graduation date
    Spring 2020
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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