Numerical modeling for the mechanical behavior of marine gas hydrate-bearing sediments during hydrate production by depressurization

Natural gas hydrate, which is called inflammable ice, has drawn global interest. Many countries have carried out the trial production of natural gas from marine gas hydrate formations. However, due to the failure of the bonded structure and the disappearance of the hydrate in the pores of formations, improper production might induce geological hazards such as submarine landslide, non-uniform subsidence and local large deformation of the formations. In this study, a constitutive model based on super-loading and sub-loading surface theory is proposed to investigate the mechanical behavior of gas hydrate-bearing sediments (GHBSs). The mechanical behavior of both GHBSs and the structured clay are modeled by introducing rotational hardening rule, sub-loading surface and super-loading surface. A fully coupled thermo-hydro-chemo-mechanical (THCM) model is applied to study the mutiphysical responses of GHBSs during hydrate production by depressurization. The simulation results of the two-layer model show that the mutiphysical responses are dependent on not only the different material properties of the two layers but also the difference between the hydrate dissociated and undissociated zones. Although depressurization is the driving factor of hydrate dissociation, the heat transfer from the clay layer is also a key factor in the hydrate dissociation. Hydrate dissociation results in the variation in the stiffness and strength of the GHBSs leading to stress and strain and strain redistribution. The yielding and structural failure of sediments mainly occur in the hydrate dissociated zone.