Evaluation Study On The Stability Of Soil Layer Around The Horizontal Well Of Natural Gas Hydrate Decomposition Phase Change In The South China Sea

Natural gas hydrate is a kind of strategic energy resource,and of great importance for improving the national energy structure and developing potential efficient clean energy.It is estimated that huge reserve of gas hydrate exists in South China Sea,and the highefficient safe exploitation has been included in the strategic research plan.Hydrate exploitation leads to the hydrate dissociation and phase transformation,accompanied by the soil softening and the redistribution of pore fluid pressure,resulting in the deformation and failures of the stratum,the damage of marine facilities,and even serious geological hazards such as marine landslides.Thisthesis focuses on the stratum and well instability during hydrate trial productionin South China Sea,Theexperiments are conducted to study the synthesis and decomposition characteristics of hydrate in porous media by using silty sand and silty clay as the skeleton,and the macroscopic mechanical parameters before and after hydrate dissociation in the institute of Mechanics,Chinese Academy of Sciences.The deformation and failure characteristics of the formation and thehorizontal well are analyzed through the numerical simulation to provide a reference for the practical engineering.First,based on the nuclear magnetic resonance apparatus,the synthesis growth process and decomposition process of THF hydrate in porous media was observed,and the nuclear magnetic resonance images and T2 spectrums were obtained.The hydrate was found in cohesive,coated and suspended types in the porous media.The synthesis process can be divided into the initial phase,induction phase,rapid synthesis phase and stable phase,while there is no obvious phase advance during the dissociation process,just dissociating from outside to inside along the solid-liquid interface.Second,t the integrated apparatus for the synthesis,decomposition,dynamic and static mechanical properties of hydrate-bearing sediments(HBS),and the consolidation apparatus were used to test the mechanical behaviors containing cracks,the dynamic behaviors before and after hydrate decomposition,and the consolidation and deformation under different loading in about 30 days.The results show that: the existence of initial cracks changes the internal structure of HBS,and reduces the bearing capacity;there is a critical relative dynamic load amplitude under dynamic load,leading to the vibration strengthening and vibration damage;the one-dimensional compression consolidation deformation process of HBS can be divided into three stages: the instantaneous deformation,consolidation deformation,and creep deformation,and the relationship between strain and time is presented empirically by using a power function.Finally,based on the mechanical parameters obtained from the experiments,the FLAC 3D software was adopted to carry out the numerical simulation of the instability of the stratum around the horizontal well during hydrate dissociation in the South China Sea.The results show that the deformation of the stratum and the horizontal section of the well increases with the increase of the dissociation length,the settlement “funnel” appears in the overlying layer,and the “free space” appears in the upper boundary of the dissociation zone,and the slope is prone to shear failure with the expansion of dissociation zone.Some key sites are monitored to describe the development of the stratum and well deformation.Thisresult can provide a reference for the safe evaluation of the second hydrate trial production in China.