| Energy is a crucial guarantee for national security and economic progress,but China’s oil and gas resources are heavily reliant on foreign countries,and the supplydemand imbalance is becoming increasingly apparent.Oil shale,an unconventional oil and gas resource,has vast deposits in China,and its efficient development and usage are vital for alleviating the energy crisis.The in-situ conversion technology is an essential development path toward commercial exploitation of oil shale.The pyrolysis of organic matter in oil shale under in-situ conditions is the basis and prerequisite for optimizing in-situ conversion process parameters and realizing efficient recovery.Firstly,the oil shale in-situ conversion process is carried out in a high-pressure semi-closed system,and the influence of pressure on the pyrolysis of oil shale should not be neglected.Secondly,the temperature of oil shale cracking zone is lower,resulting in a longer heating time for kerogen cracking,which is a process of long-time and slow heating cracking.A significant difference in the oil shale cracking characteristics compared to the high temperature and rapid heating of granular oil shale laboratory conditions.Furthermore,oil shale is pyrolyzed in an aquifer system due to hydraulic fracturing and groundwater intrusion.In general,the in-situ conversion process of oil shale is carried out in a semi-closed system with high pressure,long time and water.However,majority of studies on oil shale pyrolysis characteristics and mechanisms are conducted under laboratory conditions of atmospheric pressure,rapidly heating and anhydrous drying environment,which are different from the actual in situ circumstances and cannot effectively guide the in-situ conversion process of oil shale.Therefore,this paper comprehensively analyzes the oil shale pyrolysis characteristics under the in-situ conditions of pressure,temperature,heating time,and water by combining high-pressure thermogravimetric and semi-closed indoor thermal simulation experiments.The main research contents are as follows:(1)The effects of heating rate and pressure on the thermal mass loss characteristics and pyrolysis kinetics of oil shale were investigated using a highpressure thermogravimetric analyzer.The results showed that the total mass loss and maximum weight loss rate of oil shale decreased significantly when the pressure exceeded 1 MPa.Under pressure conditions,the TG curves of oil shale shifted to the high-temperature zone.Besides,the temperature characteristic parameter also increased while the comprehensive pyrolysis performance declined.The ANOVA results showed that temperature is the most important factor affecting the mass loss of oil shale,followed by pressure,with the heating rate having the least effect.Based on the bi-Gaussian multi-peak fitting method,the reaction rate curve of organic matter(300～700 ℃)was separated into three stages: intrinsic bitumen decomposition and kerogen to pyrolytic bitumen,hydrocarbons generation as well as depolymerization and condensation reactions.The average activation energy of kerogen decomposition to pyrolytic bitumen fluctuated and changed with increasing pressure,with the maximum activation energy at 8 MPa.The average activation energy of oil and gas generation stage was unaltered from 0.1 to 1 MPa before experiencing a steep decrease.The average activation energy of coking reactions stage first increased and then decreased,reaching a maximum at 1 MPa.This was a result of the combined effects of chemical reaction,product diffusion,mineral catalyzing,and secondary reaction.The most likely mechanism function for each stage was determined using the CR and KAS methods.The optimal reaction models were modified with the accommodation function.Finally,the theoretical conversion versus temperature curves were reconstructed and essentially matched the experimental data,proving the correctness of the three-stage model and kinetic parameters.(2)The effects of temperature and pressure on oil shale pyrolysis were systematically investigated under an in situ semi-closed system.The yield and composition of products were studied comprehensively and the pyrolysis mechanism for oil shale coupling of temperature and pressure was proposed.The results demonstrated that pressure prevented organic materials decomposition and accelerated the secondary reaction of volatile.At 500 °C,the oil shale yield at 8 MPa dropped by 65.7% when compared to atmospheric pressure,while the gas yield increased by 93.7%.The variation trend of oil shale oil yield with temperature was related to pressure,and the high pressure lowers the temperature of the secondary reaction.The pressure promoted the conversion of heavy to light components and alkanes to alkanes and aromatics in shale oil.The coking reaction of shale oil dominates under high-temperature and high-pressure conditions.The gas release temperature increased with pressure,and the content of alkene gas and hydrogen decreased while promoting the generation of alkane gas,which was related to the addition reaction of hydrogen-free radicals promoted by high pressure.The semi-coke produced under pressure had better combustion characteristics.When in-situ pyrolysis of deep oil shale,the pyrolysis temperature should be decreased to obtain more shale oil and improve oil quality.(3)The high-pressure conversion of organic matter with bitumen as an intermediate and products composition under medium-temperature and long-time conditions was studied.The results showed that the holding time required for the complete cracking of kerogen was significantly shortened with the increase in temperature.Extending the holding period limited oil generation between 350 ℃ and380 ℃.At 380 ℃,the maximum oil generation rate was 5.6 times that of 350 ℃.Kerogen pyrolysis followed an alternative pathway under medium temperatures and long-time cases,whereas several parallel reactions occurred during the early cracking of oil shale.With increasing of holding time,bitumen is the main source of shale oil.At 350 °C,the gases were primarily created directly by kerogen cracking,whereas at higher temperatures(380～400 °C),the gases were produced concurrently from kerogen and bitumen.With the prolongation of the holding time,the alkane content in shale oil first increased and then decreased,while the alkenes and aromatics increased.In addition,increasing the holding time promoted the high molecular weight n-alkane content in shale oil.The bitumen component was dominated by high molecular nalkanes.The longer the heating time,the worse the combustion performance of the semi-coke.After complete pyrolysis of oil shale,the heat provided by the remaining semi-coke was still 3.45 times of the heat required for oil shale pyrolysis.In the process of in-situ ATS conversion,the holding time should not be too long when the semi-coke as heat generation donor triggers the autogenic thermal reactions.Otherwise,the triggering may fail.(4)A comprehensive investigation was conducted to determine the effect of water on oil shale in-situ pyrolysis behavior,product yield and compositions.The results demonstrated that oil shale pyrolysis under 5% water vapor atmosphere had a lower maximum mass loss temperature,a higher initial pyrolysis temperature,and a considerably shorter temperature interval of organic matter when compared to nitrogen atmosphere.Water promoted the decomposition of carbonate minerals and participated in the water-gas reaction,resulting in lower initial pyrolysis temperature and higher total weight loss in the third stage.Compared to the nitrogen atmosphere,the different of water content and liquid water injection settings(temperature,flow rate,and pressure)had negligible effects on the product yields of oil shale in semiclosed system.Water could participate in chemical reactions as a hydrogen donor resulting in a higher the alkanes in shale oil,while high water injection flow rates would reduce the content of alkanes.With the increase of water content,the low molecules of n-alkanes in shale oil increased,while the medium and large molecules of n-alkanes decreased,indicating that water mainly provides hydrogen free radicals in the oil shale pyrolysis process.However,the opposite trend appeared under different liquid water injection cases.The injection of liquid water had a driving effect on heavy components such as bitumen and resin.However,high pressure leads to the weakening of water displacement.Temperature was the main factor affecting the conversion of organic matter,and water slightly reduced the crackable hydrocarbons in semi-coke.This paper thoroughly explored the pyrolysis characteristics and product compositions under in-situ pyrolysis conditions,which can provide a theoretical basis for the in-situ conversion of oil shale and optimize the process parameters. |
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