Theoretical And Experimental Study On The Autothermic Pyrolysis In-situ Conversion Process (ATS) For Oil Shale Exploitation

Energy is an indispensable basic resource for the development of human society and the construction of material civilization."Poor oil,less gas and relatively rich coal" are the basic national conditions of China's energy resources.Energy security has become the strategic focus of China's economic and social development.Oil shale resources in China are characterized by large amounts of geological resources,great technical difficulty and great development potential.Underground in-situ conversion technology is the main development direction of oil shale development in the future,but there is still a certain distance from commercial application.Efficient pyrolysis of oil shale is one of the key technologies for in-situ conversion of oil shale.Due to the thorny problems of single heating mode,large energy consumption and low conversion efficiency in the process of in-situ conversion of oil shale,in this study,a more efficient compound heating method called the autothermic pyrolysis in situ conversion process(ATS)for oil shale exploitation has been proposed via the effective integration of the conventional physical heating and the heat generated by the chemical reaction of the formation itself,in order to provide new technical ideas for the development and industrial application of oil shale in-situ conversion technology.The main research of this paper is as follows:(1)A theoretical model of autothermic pyrolysis of oil shale was established and its feasibility was proved by laboratory experiments.The theoretical model of autothermic pyrolysis of oil shale with residual carbon oxidation heat release as the core was established.And three main factors affecting the efficiency of in-situ pyrolysis of oil shale by autogenous heating method are put forward.Through the indoor pyrolysis experiment of oil shale,the transformation characteristics of autogenous pyrolysis of oil shale under atmospheric pressure and low temperature(< 400 ?)were studied.The conversion mechanism of oil shale autothermic pyrolysis was further clarified via the comprehensive characterization and systematic analysis of the yield,composition and physicochemical characteristics of residual semicoke and shale oil and gas under different pyrolysis parameters.The recovery factor of cracked oil in the conversion process was more than 90%,which proved the feasibility and efficiency of the theoretical model of autothermic pyrolysis of oil shale.It is further proposed that heat generation donor,high temperature and high pressure in-situ formation conditions,oxygen content and gas flow rate are the main factors affecting the efficiency of in-situ pyrolysis of oil shale.(2)The formation process and main control mechanism of the heat generating donor were clarified,and the effects of factors such as pyrolysis temperature on the heating capacity of the heat generating donor were clarified.It was pointed out that the latent heat of the heat generating donor could fully meet the huge demand for heat for the decomposition of organic matter.It was concluded that residual carbon and intermediate products are the main heat generating donors in the process of oil shale autothermic cracking through the theoretical analysis.However,in the actual autothermic conversion process,it was the residual carbon,as the main component of heat donors,played a vital role in the autothermic conversion process due to the intermediate products will continue to decompose into cracked oil and gas,residual carbon and other products.Activation energy,frequency factor and stoichiometric coefficient of reaction model are the main control mechanisms of heat donor.The quantitative analysis of latent heat in residual carbon shows that the heat of it after complete conversion of organic matter is 2.89 times that required for complete thermal decomposition of oil shale,which can still meet the complete conversion of oil shale.(3)Under the condition of in-situ conversion of high-temperature and highpressure,the in-situ autothermic pyrolysis experiment of oil shale was carried out,which proved the feasibility and efficiency of the method in laboratory scale,revealed the in-situ conversion mechanism of oil shale,and expounded the regional reaction characteristics and product migration process of in-situ conversion of oil shale.Using oil shale samples with a diameter of 100 mm and a length of 500 mm,the laboratory experiment of autogenous pyrolysis under the condition of in-situ formation temperature of 500 ? and pressure of 20 MPa was carried out.The experimental research showed that the excitation temperature of 300 ? can successfully trigger the in-situ autothermic pyrolysis of oil shale,and the pyrolysis reaction peak can be steadily promoted.The energy efficiency of in-situ autothermic pyrolysis was 2.78,which was5.45 times higher than that by injecting high-temperature nitrogen,which proved that the in-situ pyrolysis of oil shale by authigenic heating has a very significant high efficiency.According to the apparent optical characteristics and the evolution characteristics of geochemical properties of each characteristic reaction region after insitu pyrolysis,the whole in-situ conversion process of autothermic pyrolysis can be divided into five typical reaction zones: residue zone,autothermic zone,cracking zone,preheating zone and virgin zone.Meanwhile,with the advance of in-situ conversion reaction,the cracked oil obtained from in-situ pyrolysis showed the regular evolution characteristics of gradual transformation from light to medium and heavy.(4)The in-situ autothermic pyrolysis experiment with oxygen content and injected gas flow rate as experimental variables was carried out,which clarified the mechanism of oxygen injection amount on the whole in-situ conversion process,and verified the in-situ autothermic pyrolysis has a relatively stable product migration process and quality improvement effect.Within the range of experimental parameters involved in this study(oxygen content of 12,16,21 % and injected gas flow of 3,5,7 L / min),the autothermic in situ pyrolysis of oil shale can be successfully triggered at the excitation temperature of 300 ?.However,it is pointed out that a good effect of autothermic in situ pyrolysis of oil shale cannot be achieved with too low or too high oxygen injection amount.Through experimental comparison and analysis,the oxygen content has an obvious effect on the oil yield and energy efficiency.Meanwhile,on the premise of ensuring the stable advancement of the autogenous heat reaction peak surface,the change of oxygen injection in the reaction system had no obvious effect on the migration process and component distribution characteristics of cracked oil and gas products.The effective recovery factor of cracked oil in the autothermic in situ pyrolysis experiment of oil shale with injected gas flow of 5 L / min and oxygen content of 16 % was 67.1 %,which was close to the effective recovery factor of 69.1 % in the in-situ conversion process with high temperature nitrogen injection,and its energy efficiency was as high as 3.46,which is 6.78 times that of the latter's value of 0.51.The autothermic pyrolysis in situ conversion process(ATS)for oil shale exploitation realizes the efficient conversion of oil shale by utilizing the potential heat of the formation itself without the injection of large amounts of external heat,provides a new idea to solve the problem of strong dependence on external heat in the traditional in situ conversion technology.At the same time,the successful implementation of autothermic in situ pyrolysis of oil shale in indoor experimental research also provides a practical and reliable theoretical basis and data support for the practical application of this method in the in-situ conversion of oil shale at the reservoir scale.