Geomechanical and Thermal Responses of Hydrate-Bearing Sediments Subjected to Thermal Stimulation: Physical Modeling Using a Geotechnical Centrifuge

摘要

The geomechanical and thermal responses of sediments can be significantly affected by the dissociation of gas hydrates via various emergent phenomena such as fluid volume expansion, free gas generation, gas migration, and sediment softening. This study explores the geomechanical and thermal responses of hydrate-bearing sediments subjected to thermal stimulation, using physical modeling with a geotechnical centrifuge that enables the simulation of near-seafloor sediment conditions. A water-saturated and CO_2 hydrate-bearing sand column was prepared in a large cylindrical pressure vessel, and a linear stress gradient for a near-seafloor condition was created by increasing centrifugal acceleration. The hydrate-bearing sand column was subjected to thermal stimulation, and changes in temperature, pressure, compressional wave velocity (V_P), shear wave velocity (V_s), and electrical resistance were monitored at various locations across the column. It was found that V_P and electrical resistance were good indicators of the presence of free gas, while V_S reflected the reduction in shear stiffness, caused by decementation resulting from hydrate dissociation. The thermal diffusivity of hydrate-bearing sediments significantly decreased as the gas hydrate dissociated and free gas saturation increased. Such a process is expected to retard gas production from hydrate deposits. Temporary accumulation of excess pore pressures ＞200 kPa was observed even in the fine sandy sediment; this excess pressure resulted from the increased capillary pressure exerted by the hydrate formed at the grain contacts, coupled with the continuous hydrate dissociation against pressure diffusion and resulting gas migration. This suggests a possible sediment volume expansion, an uplifting deformation at the seafloor, or a fracture generation in sediments. By contrast, the vanishing of solid hydrate crystals by hydrate dissociation led to decementation and softening of sediments, indicating a possible postdissociation subsidence at the seafloor and at dissociated regions during gas production from hydrate-bearing sediments.