Chemo-thermo-mechanically coupled seismic analysis of methane hydrate-bearing sediments during a predicted Nankai Trough Earthquake

In the present study, we have developed a numerical method which can simulate the dynamic behaviour of a seabed ground during gas production from methane hydrate-bearing sediments. The proposed method can describe the chemo-thermo-mechanical-seismic coupled behaviours, such as phase changes from hydrates to water and gas, temperature changes and ground deformation related to the flow of pore fluids during earthquakes. In the first part of the present study, the governing equations for the proposed method and its discretization are presented. Then, numerical analyses are performed for hydrate-bearing sediments in order to investigate the dynamic behaviour during gas production. The geological conditions and the material parameters are determined using the data of the seabed ground at Daini-Atsumi knoll, Eastern Nankai Trough, Japan, where the first offshore production test of methane hydrates was conducted. A predicted earthquake at the site is used in the analyses. Regarding the seismic response to the earthquake which occur during gas production process, the wave profiles of horizontal acceleration and horizontal velocity were not extensively affected by the gas production. Hydrate dissociation behaviour is sensitive to changes in the pore pressure during earthquakes. Methane hydrate dissociation temporarily became active in some areas because of the main motion of the earthquake, then methane hydrate dissociation brought about an increase in the average pressure of the fluids during the earthquake. And, it was this increase in average pore pressure that finally caused the methane hydrate dissociation to cease during the earthquake. Copyright (c) 2016 John Wiley & Sons, Ltd.