Methane hydrate (MH) is one of the potential resources of natural gas. We have developed a numerical model for MH dissociation process in porous media to analyze the physical phenomena in a MH reservoir and predict gas production behavior during MH dissociation. Absolute permeability and relative permeability with MH formation were formulated with our experimental results and introduced into the simulator. Then, MH reservoir model in the adjacent Sea of Japan was constructed on the basis of the results of the preliminary drilling 2003 in Nankai trough area. Using this simulator, some field scale simulation run for depressurization process was carried out to discuss the effect of absolute permeability, relative permeability and MH saturation on MH dissociation and gas production behavior.
Methane hydrate is ice-like solid substance in which water molecule structure contains embedded methane molecules under low-temperature and high-pressure conditions (Sloan, 1998). When 1 m3 of MH is decomposed, about 150m3 of methane gas is produced. MH is one of the potential resources of natural gas in the near future, because the large amount of reservoir exists in marine sediments or in permafrost regions worldwide (Makogon, 1988; Okuda, 1993; Sato et. al, 2001a, 2001b). Some extraction methods of MH from the reservoir in marine sediments have been proposed, such as depressurization, thermal stimulation and inhibitor injection (Makogon, 1981). These are all based on the in-situ dissociation process of MH that is transformed into methane gas and water. Only methane gas can be produced from the reservoirs in marine sediments. To evaluate the productivity of methane gas from the reservoirs, it is necessary to develop the production simulator and carry out parameter study by using the simulator. Fig.1 illustrates the outline of research for MH extraction from marine sediments.