Numerical Analysis of the Dissociation Experiment of Naturally Occurring Gas Hydrate in Sediment Cores Obtained at the Eastern Nankai Trough, Japan

摘要

Oceanic gas hydrate deposits at high saturations have been found within continuous thick sands in areas such as the Eastern Nankai Trough and the Gulf of Mexico. The recent discovery of these deposits has stimulated research and development programs exploring the use of gas hydrates as energy resources. Because the permeability of hydrate-bearing sediments is a crucial factor for successful gas production from oceanic hydrate reservoirs, the permeability of these sediments and the dissociation process of hydrates should be investigated using hydrate cores obtained at these oceanic hydrate reservoirs. In this study, to investigate the permeability of actual hydrate-bearing sediments and the dissociation process of hydrates by a depressuriza-tion method, a numerical simulation was conducted using a state-of-the-art hydrate reservoir simulator. A dissociation experiment of hydrate-bearing sandy cores obtained from turbidite sediments at the Eastern Nankai Trough was analyzed. By choosing appropriate model parameters, the simulator precisely reproduces the dissociation behavior such as cumulative gas production, cumulative water production, and pressure change. The model parameters associated with permeability indicate a pore-filling tendency rather than a coating tendency of the hydrate in the pore space. Although the permeability of the hydrate-bearing cores obtained at hydrate reservoirs in nature was relatively low, the effective water permeability obtained in this study seems promising for achieving depressurization-induced hydrate dissociation. It has been found that the pressure reduction propagates deeply into the hydrate-bearing zone and the hydrate is spatially dissociated. Also, the permeability is beyond the lower limit of threshold permeability, which is absolutely necessary for successful gas production by depressurization. This study confirms the advantage of employing depressuriza-tion as a gas production method, using the hydrate in sandy turbidite sediments at the Eastern Nankai Trough as our test sample. The numerical analysis method used is effective to analyze the dissociation behavior of hydrate-bearing cores obtained at natural hydrate reservoirs, and it enables evaluation of gas productivity in those reservoirs.