Abstract: In the exploitation of natural gas hydrate with depressurization method,the instability and failure of hydrate bearing sediment is a key factor restricting its effective development.Taking into account the effect of the simultaneous changes in hydrate saturation and effective stress on the main mechanical parameters of hydrate bearing sediment,a fluid-solid coupled elastoplastic model was established for the reservoir deformation and failure induced by depressurization and decomposition of natural gas hydrate,USDFLD subroutine was programmed,and a experimental method with orthogonal numerical simulation was adopted to study the sensitivity of initial hydrate saturation,downhole pressure and effective principal stress difference to the deformation and failure of hydrate bearing sediment near the well.The results demonstrated that the downhole pressure and the effective principal stress difference were two significant factors affecting the deformation and failure of the near-well reservoir,the effect on the maximum equivalent plastic strain of the near-well reservoir was descending in order of downhole pressure, effective principal stress difference and initial hydrate saturation,and the effect on the plastic range was descending in order of effective principal stress difference,downhole pressure and initial hydrate saturation.It is recommended to optimize the downhole pressure design according to the geological conditions of hydrate reservoir. The study results are of great significance to the safety and controllability of natural gas hydrate exploitation with depressurization method.

Key words: natural gas hydrate, depressurization and decomposition, hydrate bearing sediment, orthogonal numerical simulation experiment, maximum equivalent plastic strain, plastic area range