Article pubs.acs.org/EF
Fate of Organic Liquid-Crystal Domains during Steam-Assisted Gravity Drainage/Cyclic Steam Stimulation Production of Heavy Oils and Bitumen Chuan Qin, M. Becerra, and John M. Shaw* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada ABSTRACT: The fate and impacts of hydrocarbon-based amphotropic liquid-crystal-rich domains (a recently identified material class found in hydrocarbon resources) during production transport and refining are unknown. New materials and process knowledge on this topic will contribute to parsing impacts currently attributed to asphaltenes or other crude oil fractions. In this qualitative work, the fate of liquid-crystal-rich domains in steam-assisted gravity drainage (SAGD) and cyclic steam stimulation (CSS) production environments is surveyed, using a combined laboratory and field study. In the laboratory, a fraction of liquidcrystal-rich domains present in Athabasca bitumen is shown to transfer to the water-rich phase under simulated SAGD and CSS conditions and transfer mechanisms are discussed. In the field study, liquid-crystal-rich domain transfer from Peace River and Athabasca bitumen to process water during SAGD production is demonstrated. Transferred liquid-crystal-rich domains are subsequently captured in surface facilities (primary separation, secondary separation, and water treatment processes) and do not impact steam generator operation, under normal operating conditions. Most of the liquid-crystal-rich domains are returned to the hydrocarbon-rich phase during primary separation. Impacts of liquid-crystal-rich domains on hydrocarbon resource transport and refining and SAGD surface facility optimization comprise foci for future study.
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INTRODUCTION Crystalline solids exhibit three-dimensional long-range order, and liquids are isotropic at comparable length scales. Liquid crystals are an intermediate state of matter between solid and liquid that exhibit one- or two-dimensional order. The natures and properties of organic liquid crystals comprising pure compounds are well-defined. de Gennes won the 1991 Nobel Physics Prize for his pioneering work on this topic. His book, The Physics of Liquid Crystals,1 is an excellent primer. Diverse pure compounds comprising molecular motifs found in hydrocarbon resources2 transition through a liquid crystalline state prior to melting.3 Reacting systems also exhibit this phenomenon. Examples include crystalline sugars, which transition through a liquid crystalline state as they decompose at a high temperature,4 and vacuum residues as they react under hydroconversion conditions.5 Micrometer-scale liquid-crystal-rich domains form and are stable over broad ranges of temperature in unreacted petroleum fractions, including fractions derived from Athabasca bitumen,6 a characteristic thermotropic liquid crystal behavior. For example, in C5 asphaltenes precipitated from Athabasca bitumen, Cold Lake bitumen, Maya oil, and Safaniya vacuum residue, liquidcrystal domains appear at ∼330 K and disappear at ∼430 K.7 Upon exposure to toluene vapor at room temperature, these domains also appear and then disappear over time, a characteristic lyotropic liquid crystal behavior.7 Liquid crystals exhibiting both thermotropic and lyotropic behaviors are deemed amphotropic liquid crystals. From the laboratory experiments performed to date,6,7 liquid-crystal-rich domains in hydrocarbon resources comprise 5−50 μm diameter spheres, with a thin outer shell of liquid crystalline material surrounding an isotropic core. This physical structure explains in part the difficulty of obtaining sufficiently selective sub-samples for definitive chemical analysis © XXXX American Chemical Society
and attribution. Our preliminary attempts to isolate liquidcrystal-rich domains from Athabasca bitumen and to analyze them from a molar mass and molecular structure perspective provided directional information, but the outcomes were not definitive.8,9 These difficulties are being resolved, and compositional analysis of the liquid crystalline material will comprise a forthcoming contribution. Steam-assisted gravity drainage (SAGD) and cyclic steam simulation (CSS) are in situ production processes for highviscosity heavy oils and bitumen. In both processes, the resources are mobilized by heating them with steam, and oil + water emulsions are pumped to the surface for processing. CCS as the name implies is a batch process. One well is used for both steam injection and oil production in an alternating batch mode. SAGD is a continuous process. Two horizontal wells, placed one above the other, are operated continuously in parallel. The upper well is used to inject high-pressure steam, and the lower well is used for production.10 The oil + water emulsions are separated along with dissolved gases at the surface. Water is typically cleaned, reboiled, and re-injected into the reservoir. The oil is typically transported to a remote site and subject to additional processing. Steam-assisted production is commonly applied in California, U.S.A., and Alberta, Canada, where it is applicable to ∼80% of the total recoverable bitumen resource.10−12 During heavy oil and bitumen production and processing, liquid-crystal-rich domains comprise an unknown and uncontrolled class of materials. The fate of these materials and processing problems or potential benefits associated with them are unknown. The potential for liquid-crystal-rich domains to Received: January 31, 2017 Revised: April 10, 2017 Published: April 11, 2017 A
DOI: 10.1021/acs.energyfuels.7b00315 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
approximate sample points in the surface facilities associated with SAGD operation, where known, are noted in Figure 1. Samples A−C are from parallel primary separators at the same plant and are expected to be similar. Samples D and E were stored for more than 10 years. Samples A−C and F were obtained 1 year ahead of this study. Laboratory Sample Preparation: Batch Micro Cell and Mixing Procedures. The cell design and heating system used in this study are identical to those used previously for a laboratory-scale hydroprocessing study.25 The batch micro cells have an internal volume of ∼15 mL and are constructed from Swagelok parts. These cells have an upper operating range of 873 K and 15 MPa, limits well in excess of the conditions explored here. A steady-state temperature of ∼700 K can be reached within the reactor in ∼5 min by placing the cell in a heated sand bath. A total of 2 g of deionized water was loaded into a micro cell with a syringe at room temperature, and then 2 g of bitumen, preheated to 343 K, with a density of ∼0.995 g/mL26 was carefully placed with a spatula on the top of the water surface. After loading, micro cells were sealed and pressurized with nitrogen to 11 MPa and leak-tested. A follow-up depressurization was conducted using a vacuum pump (for about 10 s, at ∼0.1 bar) to remove nitrogen and residual oxygen from the cell. The amount of water evaporated during depressurization is negligible. The micro cells were then placed in the sand bath and either suspended (no external agitation) or shaken vertically at 3 Hz with an amplitude of 3 cm. The cells were removed from the sand bath 30 min after reaching a target temperature and cooled to room temperature. At room temperature, the aqueous phase was on top of the bitumen-rich phase and was collected in vials with a syringe. The dissolved water content in bitumen-rich phases increases with the temperature and can exceed 15 wt % at 645 K.27 To maximize the solubility of water while maintaining the thermal stability of bitumen during high-temperature mixing, 593 K was selected as the highest operating temperature. This value is consistent with the temperature range (573−613 K) used in the cyclic steam stimulation (CSS) process.28 At this temperature, the solubility of water in bitumen is ∼8 mass %.27 At 473 and 523 K, which fall within the conventional operating temperature range for steam generation and re-injection in a typical SAGD process, the solubility of water is ∼0.4 and ∼2.0 mass %, respectively.27 At room temperature, water solubility in Athabasca bitumen is ∼0.01 mass %. As a result of the low volatility of bitumen relative to water, the pressure inside the micro cells is well-approximated by the vapor pressure of water29 as long as liquid water is present. Microscopic Analysis Procedure. An Olympus GX71 inverted microscope equipped with cross polarizers was used to detect liquid crystals. A detailed detection and observation procedure using crosspolarized light was reported previously.4 In brief, samples were placed on glass slides using a 100 μL micro syringe (Hamilton, Ltd.). The observations were made in air at near ambient conditions: the temperature was ∼298 K, and the humidity was ∼33%. Images were captured under both normal and cross-polarized light. If present, liquidcrystal-rich domains appeared as bright, typically red, objects possessing a Maltese cross light pattern under cross-polarized light. Isotropic liquid, if present, appeared dark under cross-polarized light. The contrast of the images was digitally enhanced using ImageJ software. Analysis Repeatability and Artifacts. Duplicate tests were conducted at each condition to verify outcome reproducibility. To avoid the possibility that oil drops form clusters in water, which can appear as anisotropic regions as a result of light reflection from the drop interfaces, all duplicate samples were placed on a vortex mixer to redisperse them prior to observation and the sample slides were rotated 3−4 times manually on the observation stage. The Olympus GX71 system is equipped with a halogen light source possessing a wavelength in the range of 650−950 nm. This wavelength is 1 order of magnitude smaller than the objects of interest (∼5 μm). However, at high magnification, a scattering halo may be seen that can lead to overestimation of domain sizes, especially for smaller objects when polarized light is employed. The effect is amplified when several small objects are close to one another as a result of the impact of multiple scattering events. In this work, the halo effect was only detected at high magnification, as illustrated in Figure 2. Under polarized light (Figure 2a), a liquid-crystal domain has a diameter of 3.9 μm but the halo creates
impact production efficiency, transport and fouling phenomena, oil−water separation and wastewater treatment in surface and sub-surface production environments, and refining catalyst deactivation is unknown and underscores significant materials and process knowledge gaps. For example, processing problems, such as the removal of small (
