Seafloor subsidence characteristics and its influencing factors during methane hydrate production by depressurization method

To calculate the amount of seafloor subsidence and analyze its influencing factors during methane hydrate production by depressurization method, a coupled thermo-hydro-mechanical model was established. The coupling of multiple physical fields was considered, which includes the dissociation of gas hydrate, gas-water two phase flow, mass and heat transfer in porous media and deformation of formation. Duncan-Chang model was used to describe the deformation and failure characteristics of gas hydrate bearing sediments. The model was solved by the finite element methods. The results show that the hydrate decomposition rate in the lower part of the hydrate layer is faster under the influence of geothermal gradient. The decomposition range in the lower layer is larger than that in upper layer. The vertical effective stress at the hydrate dissociation front has a sudden change due to the change of the mechanical parameters after the dissociation of hydrates. The amount of seafloor subsidence is mainly due to the decrease of pore pressure, tangent elastic modulus and strength caused by dissociation of methane hydrate. The maximum settlement of the whole formation occurs at the interface between gas hydrate reservoir and overlying layer. The maximum settlement of the seafloor occurs near the wellhead. The amount of seafloor subsidence is closely related with bottom hole pressure, permeability, initial tangent elastic modulus and thickness of gas hydrate layer. It gets larger with the increase of hydrate layer thickness and the decrease of bottom hole pressure. The seafloor settles more quickly as permeability increases due to the rapid dissipation of pore pressure. The amount of seafloor subsidence is getting smaller when the initial tangent elastic modulus increases. © 2022, Central South University Press. All right reserved.