Research On Mechanical Properties Of Overlying Soil Layer Of Natural Gas Hydrate Reservoir

Natural gas hydrates are recognized as a new type of energy,which can replace the traditional fossil energy,due to its huge reserves and clean combustion.However,the natural gas hydrate exploitation maybe lead to serious risks such as submarine landslides and platform collapse.Before realizing the commercial exploitation of natural gas hydrate reservoirs,it is necessary to fully understand the mechanical properties of natural gas hydrate reservoirs.Nowadays,most of the researches focused on the mechanical properties of hydrate reservoirs or hydrate crystal itself,the mechanical properties of overlying layer,which refers to the soil layer that covers the gas hydrate reservoir and carries the role of large offshore engineering equipment,was ignored to this day.The disturbances,which generated in the reduction of reservoir structure and strength caused by hydrate decomposition during exploitation or the water pressure recovery after hydrate exploitation,will be directly transmitted to the overlying soil layer and seriously affects its mechanical properties,further lead to serious risks such as platform collapse,submarine landslides and tsunamis.Therefore,this study focused on the mechanical properties of the overlying soil layer in natural gas hydrate reservoirs,and hoped to provide useful data for the security assessment of the natural gas hydrate exploitation in the South China Sea.Firstly,the undisturbed soil on the overlying soil layer of the natural gas hydrate reservoir in the Shenhu area of the South China Sea was obtained through the in-situ drilling,and the detailed measurements of its basic physical parameters were conducted,including its physical and chemical indicators,element content,material composition,particle size gradation and microstructure.Secondly,a series of triaxial undrained shear tests on the overlying soil layer were carried out using the bespoke gas hydrate high-pressure low-temperature triaxial apparatus.It is found that the water saturation of the overlying soil layer is significantly different,due to the analysis of obtained cores and the observation of large amount of associated shallow contained in gas hydrate accumulation area.So,a series of mechanical properties experimental studies on overlying soil samples with different porosities and water saturations were carried out.The results show that the failure strength and elastic modulus of the overlying layer decrease with the increase of water saturation.Under the same water saturation condition,with the porosity increases,the failure strength and elastic modulus also decrease.Meanwhile,the confining stress of overlying soil layer at different buried depths are different,which would affects the mechanical properties of the overlying soil layer.Therefore,in order to explore the effect of confining pressure on the mechanical properties of the overlying soil layer,a series of mechanical properties experimental studies on overlying soil samples with different porosities and water saturations under different confining pressures were carried out.The experimental results show that the failure strength and elastic modulus of the overlying layer increase with the increase of effective confining pressure.Similarly,as the porosity decreases,the internal friction angle and cohesive force of the overlying layer increase.With the same porosity condition,the increase of water saturation leads to the decrease of the internal friction angle and cohesive force of the overlying soil layer.Finally,the overlying soil layer is subjected to different loads due to the historical reasons of seabed stress,and most of them are in a state of over-consolidation.So in order to explore the effect of over-consolidation on the mechanical properties of the overlying layer,a series of mechanical properties experimental study of overlying soil samples with different water saturations under different over-consolidation ratios was carried out.The experimental results show that the failure strength and elastic modulus of the overlying layer increase with the increase of over-consolidation ratio.Under the same over-consolidation ratio,the failure strength and elastic modulus of the overlying layer increase with the decrease of water saturation.