Theoretical Study On Shear Strength And Constitutive Model Of Gas Hydrate-bearing Sediments

Natural gas hydrates(hereinafter referred to as NGH),commonly called combustible ice,are solid compounds of methane gases and water,which occur in the low temperature-high pressure regimes.It occurs mainly in the unconsolidated loose sediments presented both in the deep oceans and in the porous sediments in permafrost regions.Because of its large storage and pollution-free combustion,it is considered to be the main new energy in the 21 st century now.At present,many countries have been conducting investigation,research and exploitation of hydrates.Therefore,the study programs on how hydrates influence the mechanical characteristics of gas hydrate-bearing deposits have become the concerned highlight in the field.The influence of hydrates on mechanical properties is dual.On one hand,the presentence of hydrates will cause cementation between particles.On the other hand,however,due to the strict conditions of hydrate occurrence,temperature and pressure changes during exploiting processes will lead to hydrate decomposition,thus result in weakened strength.Furthermore,occurrence habits of hydrates in different environments are also different,and their strength of cementation is different,and the impacts on mechanical properties are also distinct,so it is necessary to take this factor into the consideration in the studies.At present,many scholars and researchers at home and overseas have substantiated the laws of mechanical properties such as hydrate strength through abundant experiments.But related theoretical researches are still underdeveloped.Therefore,by analyzing the laws of mechanism how hydrates impose influence on sediments,this paper describes the influence of hydrate on the mechanical properties of gas hydrate-bearing sediments based on the theoretical method and develop an elastic-plastic constitutive model which can better exhibit the mechanical properties of gas hydrate-bearing sediments.The main framework of the paper is as follows:(1)First,hydrate existing between deposits as a kind of cementitious material,the cementation will be strengthened.Secondly,the presence of hydrates increases the contact area between deposits,and improve the ability of skeleton to transmit external load as a medium,that is improves the frictional properties between deposits.The concept of cementing area ratio is introduced and the expression of effective stress is modified based on the principle of effective stress with the consideration of the influence of hydrates.Secondly,the cohesion expressions with consideration of hydrate types and occurrence habits are proposed.Lastly,the effect of high pore water pressure on cohesion after the increase of particle contact area is considered.Based on Moore-Coulomb theory,the expression of shear strength of gas hydrate-bearing sediments is established.Comparing the calculated results with the experimental ones,it exhibits that the expression can well reflect the strength characteristics under different occurrence habits and hydrate content.(2)The decomposition of hydrates will increase pore pressure and result in weakened strength,which will lead to natural disasters in deep oceans probably.Considering the dissolution of methane gas,the volume change caused by hydrate decomposition is measured according to the constructed thermodynamic equation,and the expression of pore pressure is modified in the terms of the laws of elastic deformation.By comparing the experiment results with the simulation results,it shows that the constructed expression of pore pressure in this paper can better present the growth law of pore pressure.(3)Gas hydrate-bearing sediments show structural soil properties due to hydrate cementation.In the framework of the unified hardening model(UH model),with the consideration of the occurrence habits and the damage of the cementation,effective hydrate saturation,damage parameters and the hardening parameter of structural sand are introduced into the yield function.Thus,a new elastoplastic constitutive model of gas hydrate-bearing sediments is developed.The simulated results of the model are compared with the ones of triaxial laboratory experiments,demonstrating that the proposed constitutive model can well present the stress-strain relationship with different hydrate contents under different occurrence habits and confining pressure.It shows that the model is rational and applicable by and large.