The Study Of Mechanical Properties And Constitutive Model Of Clayey-silty Hydrate-bearing Sediments Of The South China Sea

There are huge reserves of natural gas hydrates in the world,of which nearly 90%of hydrates are accumulated in low-permeability and fine-grained sediments,such as the hydrate reservoir in Shenhu area of the South China Sea.This type of hydrate reservoir has the characteristics of non-diagenesis and weak cementation,and the hydrate exploitation process can easily trigger the subsidence and deformation of the reservoir,and even trigger wellbore collapse,submarine landslide and other disasters.Therefore,it is the key problem to focus on the clayey-silty characteristics of the hydrate reservoir in the South China Sea,explore its mechanical properties and evaluate its stability during drilling and gas production.Based on the contents discussed above,the physical and mechanical properties and the constitutive relationships of the hydrate reservoir in the South China Sea have been studied in detail in this paper,including the physical properties of the marine sediments in hydrate enrichment area and the effects of multiple parameters on the mechanical properties of hydrate-bearing clayey-silty sediments.In addition,a critical state constitutive model for marine hydrate-bearing sediments was established and the mechanical response of the marine hydrate-bearing sediments during gas production by depressurization was analyzed.The specific research contents are as follows:(1)The physical properties of the marine sediments in hydrate enrichment area of the South China Sea were studied.Physical properties such as particle size distribution,specific surface area,pore size distribution,particle shape,water content,liquid and plastic limits,mineral composition,strength and deformation characteristics were obtained,and the results showed that the marine sediments of the South China Sea are well-graded clayey silts,and its specific surface area ranges from 20.38m2/g to 28.48m2/g;The sediment particles are stacked face to face in layered structure,and their mineral composition is mainly quartz,plagioclase,calcite and some clay minerals;The sediments show the properties of strain hardening and shear contraction during the shearing process.Based on the results of mechanical experiments,the strength criterion of the marine sediments was established.(2)Drained triaxial tests of hydrate-bearing clayey-silty sediments of the South China Sea under different consolidation states were carried out to explore the influences of hydrate occurence,effective stress,pore pressure and overconsolidation ratio on strength and deformation characteristics.It is found that the occurence of hydrates can enhance the dilatancy,and the increase of effective confining pressure reduces the strain-softening and dilatancy characteristics;the increase of pore pressure and overconsolidation ratio can improve the initial elastic modulus and failure strength of the hydrate-bearing sediments,and inhibit the volumetric strain at the same time.Based on the experimental results,the interaction mechanism between hydrate and skeleton particles in overconsolidated sediments was explored.(3)The mechanical properties of water-saturated hydrate-bearing clayey-silty sediments of the South China Sea under different drainage conditions were studied.The effects of hydrate saturation,effective stress,saturation environment and drainage conditions on the mechanical properties were studied by carrying out consolidated drained and undrained triaxial tests on the water-saturated hydrate-bearing sediments.The results show that the excess pore water pressure of the hydrate-bearing sediments is positive during the undrained shear process,and the cohesion of the sediments increases linearly with the increase of hydrate saturation,while the internal friction angle almost does not change with hydrate saturation;the sample under undrained condition yields more easily and is closer to the critical state than that under drained condition,and the strength criteria of the hydrate-bearing sediments of the South China Sea under different drainage conditions were established according to the experimental results.At the same time,the internal structure of the hydrate-bearing clayey-silty sediments of the South China Sea was observed by using micro-focus CT visualization system,and it is concluded that the difference of mechanical properties between sandy and clayey-silty hydrate-bearing sediments is caused by the differences of particle size distribution and accumulation habits of hydrates in the pores of the sediments.(4)A critical state constitutive model for marine hydrate-bearing sediments was proposed.By introducing the hardening parameters related to hydrates into the super-loading and subloading surface functions,the model can consider the effects of hydrates on cementation and dilatancy of the sediments,as well as different consolidation states and stress-induced anisotropy properties of the sediments.Based on Comsol Multiphysics coupling software,the constitutive model was embedded into the fully coupled thermal-fluid-mechanical-chemical multiphysics model to simulate the mechanical response of the marine hydrate-bearing sediments during depressurization.The simulation results show that the decomposition rate of hydrates decreases with the decrease of the low-pressure propagation velocity and the stress relaxation occurs near the interface of hydrate dissociation;because the stiffness of clayey layer is lower than that of hydrate-bearing sandy layer,the vertical strain of clayey layer is larger than that of hydrate-bearing sandy layer;during hydrate exploitation,the yield region of the sediments mainly occurs in the hydrate dissociation region and is related to volume compression caused by depressurization.The established coupling model combines the characteristics of the hydrate reservoir in the South China Sea,which is more conducive to the evaluation of the stability of hydrate reservoir in the South China Sea.