Study On Constitutive Models For Gas Hydrate-Bearing Sediments

Natural gas hydrate in marine sediments and permafrost area is regarded as a potential energy source of great importance for future.As the hydrate dissociation will reduce the stability of gas hydrate-bearing sediments(GHBS)and may lead to the wellbore failure and geological hazards during gas production.it is of momentous significance to precisely reveal and describe the mechanical behavior of GHBS for safe exploitation.Instead of a simple mixture,GHBS is an intricate compound in which the gas hydrate interacts complicatedly with soil particles.On the other hand,the mechanical behavior of marine sediments is greatly changed due to the presence of the gas hydrate.Thus,in order to analyze both the strength and deformation characteristics of GHBS.and carrv out the numerical simulations,two constitutive models are proposed in this paper,namely,the state-dependent multishear bounding surface model and the state-dependent subloading elastoplastic constitutive model,followed by the comparison between experiment results and model predictions.The specific research works and results are as follows:(1)This paper presents a state-dependent multishear bounding surface model for GHBS.Following the slip theory of plasticity,a constitutive formulation is obtained by splitting the macro constitutive responses of sediments into a macro volume response and a series of micro shear responses in spatial distribution related to virtual microshear structures.In addition,by incorporating the evolution law of hydrate bonding and debonding,this model takes the impact of hydrate on GHBS into consideration.Last but not least,a simple method to calculate the collapse deformation caused by the hydrate dissociation is proposed.The validity of this model is verified by comparing the simulated and experimental results on isotropic consolidation tests,hydrate dissociation tests and drained triaxial compression tests on various GHBS samples,including synthetic and natural GHBS.The result confirms that the model can not only reflect the impact of hydrate and its morphology,as well as the experiment conditions on the mechanical behavior of GHBS,but also predict the non-coaxiality of GHBS during the pure rotation of principal stress directions without additional parameters.(2)This paper also presents a state-dependent subloading elastoplastic constitutive model for GHBS.On the one hand,by incorporating the equivalent skeleton void ratio and modified unified hardening parameter,this model can adequately describe the strain-softening of GHBS.On the other hand,the introduction of the subloading theory makes the model capable of reflecting the impact of loading history on the mechanical behavior of GHBS.Therefore,factors accounting for the strain-softening are comprehensively considered in this model,making this model more applicable.In addition to all the foregoing,a simple method to calculate the collapse deformation caused by the hydrate dissociation is added to this model,correspondingly.This model is validated by comparing the simulated and experimental results on isotropic consolidation tests,hydrate dissociation tests and drained triaxial compression tests on different GHBS samples,including synthetic and natural GHBS.The result shows that this model is capable of reflecting the impact of hydrate and its morphology,as well as the experiment conditions on the mechanical behavior of GHBS.