During methane production induced by depressurization, significant mechanical responses of hydrate‐bearing sediment (HBS) such as large volume contraction and subsidence can possibly be generated. Moreover, this phenomenon is further exacerbated by strength and stiffness reduction in the HBSs as hydrate dissociation advances. As a result, the highest compressive strength is concentrated in the vicinity of the production wellbore. Therefore, it is essential to address and evaluate the mechanical responses of HBSs and conduct a stability analysis of the production well during depressurization and hydrate dissociation to prevent the destruction of production facilities and consequent leakage of methane gas. In this study, a reservoir‐scale thermal‐hydraulic‐mechanical coupled simulation using FLAC3D (Fast Lagrangian Analysis of Continua in 3 Dimensions) was conducted to evaluate the mechanical responses of HBSs and the stability of a wellbore during methane production by depressurization at the Ulleung Basin. The structural design and mechanical properties including the dimensions of wellbore components and interface properties related to the interaction behavior between the sediment and wellbore were considered in the three‐dimensional model.

Methane hydrate has attracted attention as a next-generation resource, and many researchers have conducted various studies to estimate its productivity. Numerical simulation is the optimal method for estimating methane gas productivity. Meanwhile, using a reasonable input parameter is essential for obtaining accurate numerical modeling results. Permeability is a geotechnical property that exhibits the greatest impact on productivity. The permeability of hydrate-bearing sediment varies based on the sediment pore structure and hydrate saturation. In this study, an empirical permeability model was derived from experimental data using soil specimens from the Ulleung Basin, and the model was applied in numerical analysis to evaluate the sediment gas productivity and ground stability. The gas productivity and stability of hydrate-bearing sediments were compared by applying a widely used permeability model and the proposed model to a numerical model. Additionally, a parametric study was performed to examine the effects of initial hydrate saturation on the sediment gas productivity and stability. There were significant differences in the productivity and stability analysis results according to the proposed permeability model. Therefore, it was found that for accurate numerical analysis, a regional permeability model should be applied.

Gas hydrates have great potential as future energy resources. Several productivity and stability analyses have been conducted for the Ulleung Basin, and the depressurization method is being considered for production. Under depressurization, ground settlement occurs near the wellbore and axial stress develops. For a safe production test, it is essential to perform a stability analysis for the wellbore and hydrate-bearing sediments. In this study, the development of axial stress on the wellbore was investigated considering the coupling stiffness of the interface between the wellbore and sediment. A coupling stiffness model, which can consider both confining stress and slippage phenomena, was suggested and applied in a numerical simulation. Parametric analyses were conducted to investigate the effects of coupling stiffness and slippage on axial stress development. The results show that shear coupling stiffness has a significant effect on wellbore stability, while normal coupling stiffness has a minor effect. In addition, the maximum axial stress of the well bore has an upper limit depending on the magnitude of the confining stress, and the axial stress converges to this upper limit due to slipping at the interface. The results can be used as fundamental data for the design of wellbore under depressurization-based gas production.