Numerical Simulation And Experimental Study On Supercritical Carbon Dioxide Pyrolysis Of Oil Shale

Different from traditional fossil energy such as coal,oil and natural gas,oil shale belongs to unconventional oil and gas resources.Kerogen rich in oil shale is immature hydrocarbon generating material,and only after high temperature pyrolysis can oil and gas be produced.China is rich in oil shale resources,especially widely distributed in Songliao basin.It is of great significance to explore the new technology of in-situ pyrolysis of oil shale for improving the effective production efficiency of shale oil and reducing the production cost.In the process of in-situ production of oil shale,displacement pressure and extraction technology are important factors affecting oil recovery.Too high displacement pressure will lead to stress destruction of surrounding strata,further lead to outflow of oil and gas products,and reduce oil and gas recovery efficiency.While the displacement pressure is too low to lift the fractured formation,which leads to the limitation of heat transfer process and also affects the recovery efficiency.The pyrolysis of oil shale by heat carrying supercritical carbon dioxide is an innovative method of extraction assisted high temperature pyrolysis.Its displacement pressure can be adjusted independently with the depth of the ground layer,and its diffusion ability in dense porous media and extraction effect on organic matter are obvious.Therefore,this process is proposed as an innovative idea of oil shale exploitation.On the basis of reading a large number of references,this paper has mastered the basic oil shale in-situ production methods,dynamics and thermodynamics knowledge.First,TG-DSC was used to study the effect of nitrogen and carbon dioxide atmosphere on oil shale pyrolysis,and the control mechanism and kinetic parameters of oil shale pyrolysis were obtained.The results show that the pyrolysis of oil shale accords with the control mechanism of Johnson Mehl averami interface reaction.Compared with319.97 kJ?mol^-1 in nitrogen atmosphere,the activation energy of the second stage of oil shale pyrolysis in carbon dioxide atmosphere is lower,only 249.83 kJ?mol^-1.In the second stage of pyrolysis of oil shale in nitrogen or carbon dioxide atmosphere,there is a linear compensation effect.In addition,the characteristic parameters of oil shale pyrolysis are calibrated by extrapolation method.The results show that the ignition point,the temperature corresponding to the maximum product release rate,and the burnout temperature are similar under nitrogen atmosphere and carbon dioxide atmosphere.However,the thermodynamic analysis shows that the Gibbs free energy?G^?331.26 kJ?mol^-1,activation enthalpy?H^?76.35 kJ?mol^-1 and activation entropy?S^?-338.92J?mol^-1?K^-1of oil shale pyrolysis in carbon dioxide atmosphere are higher than?G^?326.86 kJ?mol^-1??H^?28.98 kJ?mol^-1??S^?-399.54J?mol^-1?K^-1in nitrogen atmosphere,which also verifies the reliability of the kinetic calculation results.The distribution of temperature field,flow field and pressure field in the process of thermal supercritical carbon dioxide pyrolysis of oil shale is studied by the numerical simulation method of thermal fluid solid coupling analysis.The results show that with the increase of injection rate of supercritical carbon dioxide,eddy current is formed at the entrance of core holder,which strengthens the convective heat transfer of multiphase interface,and the entrance temperature increases rapidly.The advance of pyrolysis process leads to the increase of porosity of oil shale,and there is an obvious"entrance effect"in the distribution of axial pressure gradient.In addition,the increase of oil shale porosity weakens the percolation of marginal fluid,and the"horns"on the peak surface of the displacement front gradually wear away,and obvious displacement peak appears.The effect of injection flow,displacement pressure and acting time on the axial displacement rate and front expansion rate is obvious.Finally,in the self-designed high temperature and high pressure pyrolysis oil shale test device,the experiment of high temperature and high pressure nitrogen and supercritical carbon dioxide pyrolysis of Huadian oil shale was carried out,and the gas samples and oil samples collected in the experiment were analyzed by means of gas chromatography and mass spectrometry.The results show that the effective recovery of shale oil increases with the increase of temperature and time.The recovery efficiency of CO2 atmosphere is higher than that of nitrogen at low temperature,but the trend decreases with the increase of temperature.It shows that the extraction efficiency of supercritical carbon dioxide is obvious at low temperature,but with the increase of temperature,the effect of atmosphere on oil shale pyrolysis is lower than that of temperature.The components of shale oil are complex,including n-alkanes,isoalkanes,cycloalkanes,alkenes,aromatics,branched alkanes,acids and ketones.With the increase of temperature and time,the secondary cracking is more serious in nitrogen atmosphere.Alkenes and hydrogen will decrease in the long-term pyrolysis of oil shale at high temperature,which indicates that the C=C double bond breaks and the addition reaction with free methyl and hydrogen radicals occurs.Under the condition of high temperature and high-pressure nitrogen,the composition of low molecular weight aromatic hydrocarbon increased obviously,and the content of alkane C10^C15 was also higher than that of supercritical carbon dioxide.