Simulation of methane hydrate formation based on the methane-water flow and sediment properties in the Nankai Trough, Japan

The morphology and formation processes of methane hydrate are affected significantly by the characteristics of sediments. In addition, the stability of methane hydrate is constrained by the salt concentration in pore water when methane supply is sufficient, so the distribution of methane hydrate is related to the geophysical/geochemical conditions of subseafloor environment. To examine the geological factors triggering the heterogeneous distribution of gas hydrate, we developed a numerical model using the results of logging and fluid geochemistry for simulating the accumulation process of gas hydrate at IODP Site C0002, Kumano forearc basin in the Nankai Trough; a gas bearing reservoir about 100 meters below the hydrate stability zone is discovered by logging analysis, which is used as methane source responsible for the hydrate preferentially accumulated in intercalated sand layers. The model is developed using the finite difference method and incorporates the mechanism of formation porosity, permeability, and capillary force changes due to hydrate generation. Three hydrate distribution profiles were obtained, and the one calculated from the salt concentration record shows a decreasing trend with decreasing depth. According to the estimated range of methane flow of 0.002 to 1.9 kg/m2·yr, three sets of flow scenarios were designed, and the best fit result was obtained when the flow configurations for methane of 0.03, water of 1.8, and salt of 0.054 kg/m2·yr. Due to the difference in permeability, the hydrate saturation in the sand layer increases significantly while the hydrate saturation in the mud layer decreases. This phenomenon becomes more obvious when the difference in permeability increases from two (10-13 m2 for sand and 10-15 m2 for mud) to three orders (10-12 m2 for sand and 10-15 m2 for mud) of magnitude.