Experimental Study And Molecular Simulation Of Shale Gas Adsorption And Diffusion Performance

With abundant resource of shale gas in China,it is of great significance for the country's energy security and the energy structure improvement to develop it.In order to explain the mechanism of its occurrence and migration and enrich the theories about shale gas exploration and development.Taking Longmaxi shale gas formation in the Sichuan Basin as the research object of this thesis,the optimum adsorption model of the shale is selected based on the the shale characteristics,such as mineral composition,pore structure and isothermal adsorption curve,which are obtained through experimental research.And the organic matter skeleton,inorganic mineral skeleton and the mixed component skeleton model of the shale body is constructed.Besides that,by the methods of micro-regular Monte Carlo and molecular dynamics,the microscopic occurrence and migration of methane in shale pores is simulated on the nanometer scale and the effect of the temperature and pore size and gas composition on adsorption or diffusion of shale gas is analyzed.The main conclusions of the thesis are as follows:(1)Experimental results show that Longmaxi shale gas formation has a large buried depth,the content of brittle minerals is more than 60%,and that of average organic matter is more than 2%,which is conducive to the development of natural fractures and the development of induced fractures,and accord with the basic requirements for economic exploitation.And the type of the pore is mainly slit and cylinder in open state,and the size is mainly micropores and mesopores,which is between 1 and 4 nm.(2)The experimental result of Isothermal adsorption is that the absolute adsorption capacity of shale rises rapidly with increasing pressure at first,then the rate of rise slows down,and finally reaches a flat state when saturated.Under the same condition of temperature and pressure,the adsorption amount of different gases are ranked as follows:carbon dioxide>methane>nitrogen.The adsorption and desorption of shale gas in Longmaxi shale gas fonnation is consistent with the characteristics of type I curve of isotherm adsorption.Based on the effect of the fitting between experimental results and the adsorption models,the models are sorted as follow:Langmuir-Freundlich model,Toth model,Langmuir model,Freundlich model.(3)Through molecular dynamics simulation,it is found that the adsorption amount of methane increases rapidly with the rise of pressure and then tends to be gentle;the absolute adsorption amount in different skeletons are ranged as:mixed skeleton>organic matter>inorganic mineral;the larger the pore size,the larger the methane adsorption amount;the adsorption capacity of methane decrease with the increase of temperature;Since the adsorption of methane in shale pores pertains in physical adsorption,and Van der Waals force provides the main energy for methane adsorption in nanopores,after calculation the heat of the methane molecules adsorption is less than 42KJ/mol,and decreases with the increase of the pore size;Under the influence of the potential energy of the pore walls,the methane molecules near the walls aggregate to form an adsorption layer,and the molecules away from the walls are dispersed in the pores.(4)The diffusion coefficient of methane on the surface of shale pores gradually decreases with the increase of pressure,and finally tends to be stable.And the relation between it and pressure is logarithmic.(5)After injecting carbon dioxide into the shale pores,on the one hand,the adsorption amount of methane in the mixed gas is much lower than that of the pure methane under the same condition,which indicates that the replacement effect of carbon dioxide is good;on the other hand,the adsorption amount of carbon dioxide in the mixed gas is only slightly lower than that of the pure carbon dioxide,which means the continuous injection of carbon dioxide in the later stage of mining will not have a great impact on the carbon dioxide stored in the nanotubes,that is,the carbon dioxide can be well deposited in reservoir.Therefore,the injection of carbon dioxide can effectively displace shale gas and achieve the purpose of geological storage of carbon dioxide.