Research On Shear And Creep Characteristics Of Hydrate-bearing Sediments In The South China Sea

Natural gas hydrate has attracted the attention of governments and researchers around the world due to its abundant reserves and environmental friendliness.However,the process of hydrate mining will damage the cement structure of the reservoir and reduce the bearing capacity of the reservoir,which may induce engineering geological disasters such as wellbore instability,submarine landslides,and methane gas leakage.Clarifying the mechanical properties of hydrate reservoirs is an important prerequisite for evaluating the stability of the reservoir during the mining process.This paper mainly carried out experimental research on the shear and creep characteristics of hydrate sediments in the South China Sea,clarified its strength and deformation laws,and hoped to provide basic data and theoretical support for the safe development of natural gas hydrate resources in the South China Sea.Firstly,the test analyzed the basic physical properties of the marine soil in the Shenhu area of the South China Sea,and obtained multiple parameters such as particle size gradation,material composition,microstructure,liquid-plastic limit and water content.It is found that the marine soil in this area of the South China Sea is a well-graded cohesive soil;there are a variety of clay minerals such as quartz,illite,kaolinite,plagioclase and chlorite,as well as structures such as diatoms;the plasticity index of marine soil is 16.86%.Secondly,conventional single-stage and multi-stage triaxial shear tests of laboratory reshaped specimens were carried out,and it was found that both hydrate-bearing and hydrate-free sediment specimens,when the effective confining pressure is relatively low,its stress-strain curve shows obvious strain softening,and as the effective confining pressure increases,it gradually shows a strain hardening trend;the strength and elastic modulus of the specimens increase with the increase of the effective confining pressure and hydrate saturation,while the volumetric strain increases with the increase of the effective confining pressure and the decrease of the hydrate saturation;the strength and volumetric strain obtained by the multi-stage triaxial shear test are similar to those of the conventional single-stage triaxial shear test,but the elastic modulus is quite different.When the effective confining pressure is 0.5 MPa ? 1 MPa,the elastic modulus of the multi-stage triaxial shear experiment is about 126% ? 212% of the conventional triaxial shear experiment.Finally,a creep test of laboratory remodeled hydrate-bearing specimens was carried out,the effects of stress level,effective confining pressure and hydrate saturation on the creep characteristics of South China Sea hydrate reservoirs are analyzed.The study found that whether it is a hydrate-bearing or hydrate-free sediment specimen,at the same creep time,the axial strain and the strain rate increase with the increase of the stress level and the effective confining pressure;the initial strain of the specimen and the initial rheological strain increase with the increase of the stress level and the effective confining pressure,while the initial rheological time is the opposite;under the same axial load,the presence of hydrate significantly reduces the axial strain and strain rate of the specimen;in the hydrate-bearing sediment specimens,the failure time of the creep experiment with accelerated creep phase is about 80 h,and it decreases with the increase of the effective confining pressure.