Mechanism Of Dual-structure Catalysts With Hydrogen Donor And Catalytic Center For Catalytic Aquathermolysis Of Heavy Oil

With the continuous reduction of conventional crude oil, to meet the world’s growing appetite for energy, heavy oil resource development has aroused much concern these years.Heavy oil accounts for more than70%of the global residual petroleum resources which represent a critically important hydrocarbon reserve for matching the increasing demands for fossil fuels. However, the high viscosity and density of heavy oil prevents it from being exploited by conventional recovery techniques. Reducing the viscosity, improving reservoir permeability and increasing production pressure become the applicable ways to recover the heavy oil. Many methods, such as thermal recovery, physical recovery, chemical recovery, microbial recovery, etc. have been proposed and applied. Among which, thermal recovery techniques are most widely used. The development trend of heavy oil recovery techniques are basically based on thermal recovery, and then combined with other technology under the different conditions in various oilfields.Refer to principle of upgrading of petroleum, catalytic aquathermolysis is one of the most promising technology for exploiting heavy oil on the basis of thermal recovery techniques. In this technology, high-temperature steam is injected into the oil layer with catalysts. The oil layer is regarded as a natural reactor. With the energy provided by the steam, the catalyst can accelerate the aquathermolysis of heavy oil, partially change the quality of heavy oil and irreversible reduce the viscosity of heavy oil. Obviously, the catalyst plays a key role in this technology. To date, many studies have shown that, transition metal ions (serve as catalytic center) with suitable ligands could promote the aquathermolysis reaction. The studies also show that, the viscosity reduction rate of some extra-heavy oil is really poor by using the catalyst alone. But it can be apparently improved when some hydrogen donor such as toluene, methanoic acid and tetralin, etc. is coupled together. Hydrogen donor is regarded to give rise to better pyrolysis performance obeying a concerted mechanism with the catalytic center. Nevertheless, the mixed utilization of catalyst and hydrogen donor brings unnecessary complexity, extra cost and inevitable pollution in field operation, which prevents it from being scaled-up. Meanwhile, the study on the synergy mechanism of hydrogen donor is not systematic and in-depth due to the lack of direct experimental evidence, which limits the development of the novel catalysts. Motivated by the concerted effect brought by hydrogen donor, we attempt to design a series of novel catalysts with catalytic center and hydrogen donor, which are expected to play a dual role of both transition metal and hydrogen donor. Guiding significance is expected for preparing more effective catalysts for field application. In this paper, we carry out the following research in response to the above scientific and technical issues.Firstly, motivated by the concerted effect brought by hydrogen donor, the catalyst B which owns the dual structure of ferrum metal ion and hydrogen donor has been prepared and characterized by Fourier transform infrared spectroscopy (FT-IR), elemental analysis (EL) and inductively coupled plasma-mass spectrometry (ICP-MS). And then it has been used in catalytic aquathermolysis of Xinjiang F10223#extra-heavy oil (85000mPa·s at50℃). From the orthogonal experiments of catalytic aquathermolysis, we have found the best viscosity reduction of oil sample by94.7%could be obtained using0.2wt%catalyst with the o/w ratio and pH value of8:2and7, respectively at200℃for24h. Moreover, we have also found that the catalyst B shows more signigicant viscosity reduction effectthan the coupling utilization of catalyst A (Ferrum is employed as catalytic center and macromolecular aromatic sulfonic is utilized as ligand.) and hydrogen donor. And it also has excellent thermal stability and universality. Then, the same and different influences on the aquathermolysis of heavy oil catalyzed by catalyst A and B are in-depth studied and compared. The compositions and structure of oil sample before and after reaction were analyzed by element analysis (EL),1H-nuclear magnetic resonance (1H-NMR) and gas chromatography-mass spectrometry (GC-MS). After reaction catalyzed by catalyst A and catalyst B, the heavy components of oil sample has respectively decreased by9.39%and10.17%. The types and levels of alkanes in saturated hydrocarbons (SH) have increased obviously. Meanwhile, a small amount of compounds such as alkylbenzenes, alcohols and ketones appear in the aromatic hydrocarbons (AH). Upon aquathermolysis, the elemental contents of O, N and S are all decreased, with the increasing of the H/C, the changes of the heteroatom components and H/C of resin and asphaltene after reaction with catalyst B are more obvious than catalyst A. after aquathermolysis, both the aromaticity and aromaticity condensation of heavy components have decreased. The structural parameters of the heavy components have changed more obviously upon catalytic aquathermolysis. the aromaticity and aromaticity condensation of resin and asphaltene after reaction with catalyst B are higher than catalyst A. Moreover, after catalytic aquathermolysis, carbon dioxide, alkanes, naphthenes, olefins and benzene series appear in the pyrolytic gas (the compounds are same to the blank sample). Some oxygen-containing compounds such as cyclopentanone and acetophenone exist in the pyrolytic gas after catalytic aquathermolysis. More compositions such as xylene, trimethylbenzene, etc. have been found in the pyrolytic gas after reaction with catalyst B than catalyst A. The results have revealed that, multiple types of actions happened throughout the catalytic aquathermolysis process, such as depolymerization, pyrolysis, hydrogenation, isomerization decarboxylation and ring-opening, etc. The heavy components of oil sample can be pyrolyzed to the light components more easily upon catalytic aquathermolysis. After reaction, the average molecular structure of the heavy oil becomes smaller, the associative structure becomes more loosened and the cohesion between molecules is weakened, eventually leading to the obvious viscosity reduction of heavy oil. Compared to the catalyst A, the above actions are more significant during the catalytic aquathermolysis catalyzed by catalyst B. The outstanding performance of catalyst B may be related to its special structure. Its ligand is rich in the small molecular structure of hydrogen donor. Due to the small ligand size, the diffusion of the catalyst in the heavy components becomes more readily, which allows higher probability for the catalytic center to attack the heteroatoms. The C-R (R=S, N, O) bonds are consequently activated. These bonds can then react with the neighboring water molecules or dissociate directly to form light contents. Meanwhile, the structure of the hydrogen donor in catalyst B also plays some role in making the increasing of H content in heavy component, which is significantly higher than the catalvst A.Secondly, it is found that, with the same ligand type and cation concentration, Cu2+-catalyst has stronger catalytic activity on the viscosity reduction effect for many heavy oils than Fe3+-catalyst, especially for super-heavy oil. Then, the catalyst C which owns the dual structure of copper metal ion and hydrogen donor has been synthesized and characterized by FT-IR. After that, the catalyst B and C are used in catalytic aquathermolysis of six heavy oils for comparative study. Compared to catalyst B, the catalyst C shows better viscosity reduction effect for the heavy oils, especially for the Shengli extra-heavy oil (1.8×105mPa-s at50℃). The higher asphaltene content of heavy oil is, the more difference between the viscosity reduction rates of the two catalysts is. As a result, Shengli extra-heavy oil was selected as the research object to in-depth study the same and different influences on the aquathermolysis of heavy oil catalyzed by the two catalytic ions. The compositions and structure of oil sample before and after reaction were analyzed by EL,’H-NMR, Gel permeation chromatrography (GPC) and GC-MS. Compared to catalyst B, more heavy components have been pyrolyzed to the light components after reaction with catalyst C. Upon aquathermolysis, the amounts of sulfur element and H/C of resin and asphaltene have all decreased, while the nitrogen content of heavy components have increased. By contrast, the decrease of the H/C and oxygen content of resin and asphaltene after catalytic aquathermolysis with catalyst C are more obvious than that of heavy contents after reaction with catalyst B. With the participation of the catalysts, the aromaticity and aromaticity condensation of asphaltene have decreased, while the branching index of that has increased. The aromaticity, aromaticity condensation and branching index of asphaltene after catalytic aquathermolysis with catalyst C are lower than that of asphaltene after reaction with catalyst B. Compared to asphaltene, there is no significant structural change of the resin before and after reaction. The number average molecular weights of asphaltene have respectively decreased from7021to1860and2092g/mol after catalytic aquathermolysis. After reaction, the average structural parameters such as the number of total carbons (CT), aromatic carbons (CA), non-bridgehead aromatic carbons (CP), total rings (RT), aromatic rings (RA), naphthenic rings (RN) and aromatic ring substitution (n) of asphaltene molecule have decreased obviously. Compared to catalyst B, the changes of all these average structural parameters of asphaltene molecule after reaction with catalyst C are more remarkable and more compositions exist in the pyrolytic gas after reaction with catalyst C. The above results have revealed that, during the catalytic aquathermolysis, the catalytic ions chiefly reacted with some conjugated π-bonds and bridge bonds (C-R, R=S, O, N, C), causing the tight macromolecular ring system of heavy components be depolymerized to fragments of different sizes. Some small fragments with high H/C such as alkanes, naphthenes, olefins and benzene series dissociated into light components and pyrolytic gas under the high-temperature catalytic condition, and the remaining macromolecular fragments with low H/C reassociated together after reaction. Moreover, the comparison results show that the two catalytic centers mainly act on the asphaltene of oil sample. The content and structure of the asphaltene have changed evidently after reaction. Among all the actions, the contribution of the pyrolysis and depolymerization to the viscosity reduction of heavy oil are more than the others. The decrease of the heavy contents and the increase of the light contents could eventually cause the significant viscosity reduction of the heavy oil. The catalyst C has stronger catalytic action on the AH and asphaltene than the catalyst B, while it has weaker catalytic action on the SH and resin. In addition, the catalyst C mainly causes the depolymerization and cleavage of some bridge bonds of the macromolecular ring system, whereas the catalyst B primarily leads to the isomerization of side chains and ring-opening. The catalyst C is more suitable to catalyze the aquathermolysis of the heavy oil with high asphaltene for application.Thirdly, it is found that different metal centers have different catalytic activity on the catalytic aquathermolysis of heavy oil. The catalyst E which owns the dual structure with two metal centers has been synthesized and characterized by FT-IR, and then used in catalytic aquathermolysis of Shengli extra-heavy oil. From the single-factor experiments and orthogonal experiments of catalytic aquathermolysis, we have found the optimum conditions are as follows:the temperature is200℃, the o/w ratio is8:2, the percentage of catalyst is0.3wt%, the pH value is7and the reaction time is24hour. And the viscosity could be reduced by93.9%under the optimum reaction conditions. Meanwhile, we have comparatively studied the viscosity reduction effect of the catalyst A, B, C, D (which owns the dual structure of molybdenum metal center and hydrogen donor) and E. We have found that the order of viscosity reduction effect of different catalysts is E> C> B> D> A. The viscosity reduction effect of dual-structure catalysts (B, C, D, E) is better than single-structure catalyst (A). Among the dual-structure catalysts, catalyst E shows the best viscosity reduction effect, the viscosity reduction rate can hit93.9%, with10.57%in conversion of heavy content to light content. After that, electron spin resonance (ESR) has been used to study the mechanism of the dual-structure catalysts for catalytic aquathermolysis of heavy oil. The free radicals of oil sample and heavy components before and after reaction have been analyzed. The radical concentration of oil sample increases with increasing reaction temperature after reaction with catalyst E. The significant growth of radical concentration occurs when the temperature exceeds180℃. With the increasing o/w ratio, percentage of catalyst and pH value, all the radical concentrations of oil sample show a sharp decline after the first rising trend. The inflection points are respectively in the o/w ratio of6:4, percentage of catalyst of0.3%and pH value of8. With the participation of the catalysts, the radical concentrations of oil sample and heavy components arise obviously. From the perspective of free radicals, it is found that the order of activity of different catalysts on the oil sample is E> C B> D> A, the order of activity of different catalysts on the resin is E> D> B> C> A, the order of activity of different catalysts on the asphaltene is E> C> B> D> A. The catalytic activity of dual-structure catalysts is better than single-structure catalyst. Among the dual-structure catalysts, catalyst E shows the best catalytic activity. From the perspective of heteroatom content of heavy components, the hydrodesulfurization of the heavy components is the most significant after reaction with catalyst E.Lastly, the catalyst B has been used in the field tests of FZ023#and FD320037#heavy oil wells. The results indicate that, the viscosity of FZ023#and FD320037#heavy oil could be reduced by52.6%and55.9%, with17.33%and6.01%in conversion of heavy content to light content, which proves the catalyst B shows good viscosity reduction effect in the field test. During the third steam huff and puff periods with the catalyst B, the working days of the FZ023#well have decreased, the liquid and oil production are improved clearly, the oil production has increased by364t. By contrast, the field test of the FD320037#well has not met the expected growth of oil production.There are two innovative points in this paper:(1) A series of dual-structure catalysts for catalytic aquathermolysis of heavy oil have been designed and prepared, which include the types of single metal center and two metal centers. Then, the novel catalysts have been used in the catalytic aquathermolysis of heavy oil. Moreover, a variety of modern instrumental analysis methods have been used to study the mechanism of the dual-structure catalysts comparatively.(2) Electron spin resonance (ESR) has been firstly used to study the mechanism of the dual-structure catalysts for catalytic aquathermolysis of heavy oil. The free radicals of oil sample and heavy components before and after reaction have been analyzed. We have tried to reveal the mechanism of dual-structure catalysts from the perspective of free radicals.