Molecular Asphaltene Transformations during Aquathermolysis of Heavy Crude Oil: Analysis of the Literature Data

Abstract

Despite the remarkable effort performed to understand the reaction mechanism of asphaltenes during aquathermolysis reaction, the knowledge level about their molecular structures and reaction pathways is still limited. This work thoroughly reviews and discusses experimental results reported from different samples of asphaltenes during the aquathermolysis reaction to obtain information about the elemental composition, functional groups, and average structural parameters using analytical characterization techniques, including elemental analysis, Fourier transform infrared spectroscopy, synchronous fluorescence spectroscopy, X-ray diffraction analysis, electron paramagnetic resonance spectroscopy, and nuclear magnetic resonance spectroscopy. By studying the structural changes of asphaltenes, theoretical support is provided for future research on the aquathermolysis of heavy crude oils in the presence and absence of various catalysts such as water-soluble, ionic liquids-based, mineral, and heterogeneous. Characterization analyses show that the changes in asphaltenes are strongly influenced by the operating conditions, catalyst type, and asphaltene structure. Certain catalysts and hydrogen donors provide a higher effect on asphaltenes and their functional groups that have been observed through changes in the molecular weight, aromaticity, and condensation as well as representative parameters of the aggregation state of asphaltene molecules.

Experimental Considerations for Proper Development of Aquathermolysis Tests in Batch Reactor Systems

“…The study of the structural changes of various asphaltene samples under aquathermolysis conditions showed the influence of the molecular architectures, operating conditions, and catalyst type, causing different changes in the molecular weight, aromaticity, and condensation, as well as in the state of aggregation of the molecules from oil samples with similar origins . The chemical interaction of oil compounds, steam, and minerals during steam injection processes produces certain gases such as H 2 S, CO, CO 2 , H 2 , and CH 4 .…”

Kinetics of Non-Catalytic Aquathermolysis of Heavy Crude Oil in the Presence of a Hydrogen Donor

“…3,4 However, the efficiency of water conversion into hydrogen is limited for the accomplishment of high degrees of hydrogenation of organic substances, and condensed structure compounds with free radicals lead to the formation of secondary resins and asphaltene molecules by polyaddition reactions, resulting in viscosity regression. 5,6 In order to increase the performance in these processes and provide additional active hydrogen in the reaction system capable of saturating the formed free radicals, some solvents with hydrogen donation capacity via dehydrogenation/hydrogenation mechanisms (tetralin, decalin, and cyclohexane) have been studied to favor the production of light compounds and inhibit polyaddition reactions. 7−10 Consequently, the concentration of free radicals in the reaction system decreases, inhibiting the recombination of generated molecule radicals and thus the viscosity rebounding of heavy crude oil.…”

“…Due to the depletion of conventional oil reserves and increasing energy consumption worldwide, the recovery and upgrading of heavy crude oil resources have received more attention from oil companies and research centers. Steam injection techniques (cyclic steam stimulation, steam flooding, steam-assisted gravity drainage) are the most applied methods for heavy crude oil production, where the application of some catalytic precursors and solvents has allowed for increasing significantly the recovery factor by reducing the content of macromolecules, such as asphaltenes and resins, and increasing the yield of light fractions. , During these processes, the viscosity of heavy crude oil is reduced by composition and structure changes of oil compounds due to a series of reactions in the presence of steam (aquathermolysis), involving cleavage of alkyl chains, carbon-heteroatom bonds, hydrogenation, and heteroatom removal, where active hydrogen is produced in situ by water–gas shift (WGS) reaction and water dissociation. , However, the efficiency of water conversion into hydrogen is limited for the accomplishment of high degrees of hydrogenation of organic substances, and condensed structure compounds with free radicals lead to the formation of secondary resins and asphaltene molecules by polyaddition reactions, resulting in viscosity regression. , In order to increase the performance in these processes and provide additional active hydrogen in the reaction system capable of saturating the formed free radicals, some solvents with hydrogen donation capacity via dehydrogenation/hydrogenation mechanisms (tetralin, decalin, and cyclohexane) have been studied to favor the production of light compounds and inhibit polyaddition reactions. − Consequently, the concentration of free radicals in the reaction system decreases, inhibiting the recombination of generated molecule radicals and thus the viscosity rebounding of heavy crude oil. , …”