2017 Volume 50 Issue 4 Pages 244-253
Numerical simulations of methane seepage at the Joetsu Basin, the Japan Sea, were carried out, and the rising and drifting behavior of methane at a water depth of 940 m was investigated. The seeping methane bubbles and hydrates were numerically tracked by a Lagrangian method. The numerical model of tracking methane gas considered the phase change of methane due to thermophysical conditions. The amount of seeping methane from the seabed and the initial bubble diameter distribution were given based on a field observation, and the ambient flow conditions were given with the JCOPET reanalysis data of the JAMSTEC. Three ambient flow conditions were used to assess the dependency of the numerical algorithm because the JCOPET reanalysis data included an uncertainty in the flow velocity components. The comparison of the numerical results obtained using three different ambient flow conditions showed that the plume structure formed by dispersed methane gas/hydrate strongly depended on the horizontal current flow. For the flow conditions close to those of actual sea current conditions, seeping methane gas changed to hydrate after a few meter rising from the seabed, and it returned to the gas phase at a water depth of about 300 m, corresponding to the limit of hydrate stability. The numerically predicted transition of methane gas in deep water agreed with the field observation at the Joetsu Basin. We then concluded that the present numerical model is useful for predicting the methane seepage phenomena in deep water.