Research On Catalytic Dissociation Mechanism Of S-containing Compounds In Heavy Oil And Viscosity Evolution Of Heavy Oil Upon O-containing Compounds

As the consumption of conventional fossil resources,heavy oil,as a typical unconventional oil reservoir,the exploration and exploitation of which have been paid increasing attention due to its abundant storage and wide distribution.However,the high proportion of complex heavy components in heavy oil leads to elevated viscosity,lower flowability and poorer quality,as well as high viscosity,which have put significant technical challenges to the exploitation,transportation and application.Consequently,the key of efficient utilization of heavy oil lies in decreasing the viscosity as well as improving the flowability and quality.Up to now,many upgrading approaches,including physical recovery?with assistance of external fields including heat,micro wave,sound wave,magnetic force etc.?,chemical exploitation?employing chemical reagents such as catalysts or surfactants?,mixing thin oil and biological upgradation?depending on microorganisms or biomass?,have been developed to reduce the viscosity of heavy oil for efficient recovery.Among these multiply viscosity reduction methods,catalytic aquathermolysis,which represents a very important branch of chemical recovery,has been identified to be one of the most promising approaches,which combines the technical merits of thermal recovery and chemical exploitation.During catalytic aquathermolysis,the complex heavy components can be particially transformed into small molecules irreversibly,leading to decreased viscosity and enhanced quality.Accordingly,the application of transition metal-based catalysts in heavy oil viscosity reduction has been focused in this field,and lots of pertinent researches have been proceeded.Considerable previous reports suggested that the reduction of hetero-containing compounds is beneficial to the decrease of heavy components and increase of light components,which is cognized as one of the most important factors to reduce the viscosity.However,these research efforts mainly focused on the synthesis and application of the catalysts,with respect to the specific mechanism of viscosity reduction,in-depth and systematic research is still insufficient,studies on the catalytic removal of the main high viscosity contributing compounds in heavy oil have hardly been reported at molecular dynamics level.Absence of profound understanding of the heavy oil viscosity reduction mechanism is inferior to the selection and design of well-directed efficient catalysts,which would restrict the industrialization of catalytic aquathermolysis technology.As a result,researching the catalytic decomposition mechanism of the main high viscosity contributors in heavy oil is of significant theoretical and practical significance for seeking efficient heavy oil reduction catalysts and promoting the advancement of catalytic aquathermolysis in oilfield application.In this study,as for S-containig compounds in heavy oil,we combined theoretical calculations and simulative experiments to unravel the catalytic dissociation mechanism.With respect to O-containing compounds,we investigated the viscosity evolution of heavy oil by peroxide oxidation and thermal pyrolysis;in addition,the corresponding decomposition mechanism was discussed preliminarily.The major contents and progress are as follows:1.CH₃SCH₃,thiophane and thiophene were employed as model molecules on behalf of straight-chain saturated sulfides,thiophanes and thiophenes composition in heavy oil,respectively.Simultaneously,common economical-friendly Fe-based M-Fe₂O₄?M=Mn,Fe,Ni,Co,Cu?aquathermolysis catalysts were selected as the catalytic reaction substrate.By theoretical simulation calculation in VASP,the decomposition mechanism of different S-containing compounds in heavy oil on various M-Fe₂O₄ catalysts was investigated systemically at molecular dynamic level.Firstly,we analyzed the catalytic dissociation process and charge variation in CH₃SCH₃ at Fe₃O₄?111?surface with existence of H₂O molecule in different form.Moreover,the decomposition and charge analysis of CH₃SCH₃ with only assistance of Fe₃O₄?111?or H₂O have also been involved.Our research results suggest that,CH₃SCH₃ molecule will encounter a two-steps dissociation to form one H₂S and two CH₃OH with the mediation of dissociative water molecules.In the first step,one C-S bond of CH₃SCH₃,which is anchored at the exposed Fe sites,is attacked by a neighboring dissociative H₂O molecule,generating an attached HSCH₃ and an adsorbed CH₃OH.Analogically,in the second step,the left C-S bond in HSCH₃ is further decomposed by another adjacent free H₂O molecule,producing the final product CH₃OH and HSCH₃,completing the entire decomposition of C-S bonds in CH₃SCH₃.Under the similar simulated reaction condition with CH₃SCH₃,decomposition mechanism of thiophane and thiophene on the surface of Fe₃O₄?111?is discussed by analysis of the variation of energy and charge.The research findings indicate that the dissociation activity of different S-containing compounds follows the trend of straight-chain saturated sulfides>thiophanes>thiophenes.When different transition metal M?Mn,Fe,Ni,Co,Cu?participates the reaction system as M-Fe₂O₄ form,the adsorption strength CH₃SCH₃ of on M-Fe₂O₄ matches the order of Cu-Fe₂O₄?111?>Ni-Fe₂O₄?111?>Co-Fe₂O₄?111??Mn-Fe₂O₄?111?>Fe₃O₄?111?,while the catalytic decomposition activity of C-S bond corresponds to the sequence of Mn-Fe₂O₄?111?>Co-Fe₂O₄?111??Ni-Fe₂O₄?111?>Fe₃O₄?111?>Cu-Fe₂O₄?111?.2.A series M-Fe₂O₄ of nanocatalysts were synthesized for the catalytic decomposition of S-containing model compounds and aquathermolysis of heavy oil.By utilizing C₂H₅SC₂H₅ as model molecule of straight-chain saturated sulfides,we detected the catalytic pyrolysis behavior of straight-chain sulfides on Fe₃O₄ catalyst under different conditions?with H₂O dosage of 0%,1%,5%,10%,30%,50%?.Furthermore,the corresponding catalytic decomposition of thiophanes?thiophane as model?,thiophenes?thiophene as model?and straight-chain saturated thioalcohols compounds?n-butyl mercaptan as model?on Fe₃O₄ were also investigated.In addition,a series of C₂H₅SC₂H₅ and heavy oil hydrothermal pyrolysis experiments upon catalysis of different M-Fe₂O₄ were carried out.The catalytic thermal decomposition experiment results of C₂H₅SC₂H₅ on Fe₃O₄demonstrate that the participation of Fe₃O₄ catalyst is in favor of the breakage of C-S bond and ensure an elevated decomposition rate of C₂H₅SC₂H₅,while the existence of H₂O is capable of capturing the radical intermediate during reaction,stimulating the thorough decomposition of C-S bonds in C₂H₅SC₂H₅.With assistance of 30%addition of H₂O,20.64%of C₂H₅SC₂H₅ can be catalytic decomposed into small molecules.Catalytic removal rate of different S-containing compounds on Fe₃O₄ is in the order of straight chain mercaptan>straight-chain saturated sulfides>thiophanes>thiophenes.The removal rate of C₂H₅SC₂H₅ decomposition on different M-Fe₂O₄ follows the trend of Cu-Fe₂O₄>Ni-Fe₂O₄>Mn-Fe₂O₄>Co-Fe₂O₄?Fe₃O₄,in which,the experiment results of Ni/Mn/Co/Fe-Fe₂O₄ catalysts are in accordance with the calculation results,while the high removal rate of C₂H₅SC₂H₅ in experiment on Cu-Fe₂O₄ mainly originate from its strong absorption to C₂H₅SC₂H₅,and the corresponding conversion of small molecule compounds is lowest.With respect to the application of M-Fe₂O₄ in heavy oil catalytic aquathermolysis,the S element removal rate is in accordance to the sequence of Ni-Fe₂O₄>Mn-Fe₂O₄>Fe₃O₄>Co-Fe₂O₄?Cu-Fe₂O₄,which is coincident with the experiment results of C₂H₅SC₂H₅ catalytic decomposition,except Cu-Fe₂O₄.Simultaneously,the corresponding catalytic viscosity reduction efficiency matches the regulation of Cu-Fe₂O₄?Fe₃O₄?Ni-Fe₂O₄>Co-Fe₂O₄>Mn-Fe₂O₄.3.As regarding to O-containing compounds in heavy oil,we investigated the viscosity evolution mechanism of heavy oil by moderate peroxide oxidation?100?C/12h?and successive thermal pyrolysis?200?C/12 h?,which confirmed the fact that the main O-containing compound contributing to the high viscosity in heavy oil is alkyl carboxylic acids.In corporation with theoretical calculation and practical experiment on decomposition of carboxylic compounds,the dissociation mechanism of O-containing compounds in heavy oil was investigated preliminarily.Heavy oil peroxide oxidation experiment demonstrates that the oxidation process could introduce the of O-containing compounds and elevate organic carboxylic acid content in heavy oil,which accelerated the transformation of aromatic hydrocarbons to resins even asphaltenes,leading to an increased viscosity.By extracting the acidic components from heavy oil and analyzing the structure and composition,results demonstrate that alkyl carboxylic acids compounds are confirmed to be the dominative organic acids in heavy oil.During thermal pyrolysis process,these alkyl carboxylic acids could be decomposed into CO₂ and hydrocarbons,promoting the transformation of heavy components to light components leading to a reduced viscosity.N-butyric is employed as the model molecule of carboxylic acid compounds in heavy oil to investigate the decomposition mechanism by theoretically simulating different free radical dissociation process.The research results indicate that the decomposition of carboxylic acid occurs upon dissociation into propane and CO₂endows the lowest activation energy of 62.88 kcal/mol.When the length of carbon chain increases,the corresponding activation energy of decomposition reaction will raise,which will achieve convergence at 130 kcal/mol after C>12.Model compound?stearic acid?experiment results show that the participation of Cu-based catalyst is beneficial to the decomposition of carboxylic acid.The intervention of o-phenanthrene could decrease the activation energy?117.72 kcal/mol?of stearic acid pyrolysisand enhance the removal rate.