Molecular Dynamics Study On The Electrodecomposition And Mechanical Properties Of Methane Hydrates

Natural gas hydrate is an emerging energy source.In recent years,natural gas hydrates have been identified in marine and terrestrial tundra around the world.Because natural gas hydrate is a clean energy and has huge reserves,it has caused extensive attention around the world.Although natural gas hydrate has been discovered for half a century,the domestic research on its basic physical properties,particularly for bicrystalline hydrates that contain defective grain boundaries,is still very limited,limiting its commercial recovery.In this regard,using molecular dynamic simulations,this thesis report(1)instability and dissociation behaviors of structural 1,Ⅱ,H methane hydrates and bicrystalline methane hydrates subjected to external electric field;(2)mechanical characteristics of bicrystalline methane hydrate with different symmetrical tilt angle of grain boundary subjected to uniaxial tension perpendicular to grain boundary.The main results are summarized as follows:(1)Structural Ⅰ,Ⅱ and H methane hydrates show different electrical fields for their destabilization and dissociation,with the rank as Ⅰ>Ⅱ>H.For structural Ⅰ methane hydrate,there is no apparent dependence of electrical field for destabilization on crystallographic orientation,whereas for structural Ⅱ and H ones,the destabilization by electrical field depends on crystallographic orientation.(2)Bicrystalline methane hydrates have smaller electrical field for structural destabilization than single-crystal counterpart because of the presence of defective grain boundaries.There is correlation between electrical field of destabilization and tilt angle of grain boundary,with maximum/minimum electric field of destabilization for bicrystal with a Miller index of(150)/(140).(3)Upon uniaxial load perpendicular to grain boundary,tensile strength and failure strain of bicrystalline methane hydrates are smaller than that of single-crystal counterpart,and depend on the tilt angle of grain boundary.Among the bicrystalline structures,methane hydrate with Miller index of(150)/(130)of symmetrical grain boundary show the largest/smallest ultimate tensile strain.The ultimate tension strain is related to the grain boundary energy.