Mechanical Response of Sandy and Clayey-Silty Hydrate-Bearing Sediments in Horizontal Depressurization Production

Natural gas hydrate exists in the loose sediments on the seabed in solid form, and its exploitation involves the multifield coupling of Thermal (T) - Hydraulic (H) - Mechanical (M) - Chemical (C) problems. Hydrate decomposition causes changes in the pore structure; the pore pressures and effective stresses increase; the particle bonding strength weakens; the shear resistance and capacity decrease; and the sediments are at risk of sand production and instability if they are loaded for a long time. To accurately predict the stress–strain change trend and seabed subsidence of the reservoir during the exploitation process, it is urgent to develop a numerical simulation tool that can characterize complex mechanical behavior. Existing thermal–hydraulic–mechanical simulators typically use elastic models for linear stress–strain relationships in sandy sediments. However, South China Sea hydrate reservoirs consist mainly of muddy silt with nonlinear path-dependent behavior. Their elastoplastic properties vary with stress state and loading history, exceeding the simulation capabilities of the current programs. It is necessary to adopt an elastic–plastic-constituted model suitable for clayey-silty sedimentary layers. Therefore, this study constructed a two-way coupling program between TOUGH+HYDRATE and FLAC3D, established the models for sandy and clay-silty layers based on the geological data of the Nankai Trough and the Shenhu area, and compared the gas production and mechanical response in horizontal depressurization production. The conclusions are as follows: Under the production pressure of 4.5 MPa, the maximum seabed settlement of the sandy reservoir in one year is 0.28 m; Under the production pressure of 7 MPa, the maximum seabed settlement of the clay-silty reservoir in one year is 0.56 m. During early production, sandy reservoirs experience rapid sedimentation due to hydrate dissociation and an effective stress increase, causing skeleton instability. Over time, pore compaction reduces permeability and sedimentation slows. In clayey–silty reservoirs, sedimentation remains stable but prolonged, with continuous fine-particle consolidation leading to significant, long-term settlement.