Numerical analyses of dynamic behavior of seabed ground during methane hydrate production

Abstract

Oceanic methane hydrates are now viewed as energy resource because they contain rich methane. However, they generally exist in relatively shallow and uncemented seabed ground layer, thus large deformation and degradation of seabed sediments may occur during gas production from methane hydrate bearing sediments. Moreover, because they are often found in seismically active regions, including the Nankai Trough area of Japan, there is a risk of large earthquakes during gas production. Seabed sediments damaged by gas production may become trigger of disasteres like seabed slides. Furthermore, effects of pore pressure change induced by earthquake on methane hydrate dissociation behavior are also curious problem. In the present study, we have shown a numerical method which can simulate the seismic and chemo-thermo-mechanical coupled behaviors of seabed grounds during gas production, such as phase changes from hydrates to water and gas, temperature changes, ground deformation and the flow of pore fluids. Numerical analyses are performed for the hydrate-bearing sediments at the Daini-Atsumi knoll, Eastern Nankai Trough, Japan, where the world's first offshore production test of methane hydrates was conducted using a predicted Nankai Trough Earthquake for investigating the earthquake-induced dynamic behavior during gas production. From the results, effects of gas production on mechanical behavior of seabed grounds during earthquake is small for this ground conditions. Small increase of the pore pressure due to methane hydrate dissociation during earthquake results in temporal stability of methane hydrates.