In-situ observation of gas hydrate formation and dissociation in hydrate-bearing sediments, and water-permeability measurements for the sediments have been performed by the apparatus combined proton nuclear magnetic resonance (1H NMR) and permeability measurement system, which we had originally designed. As a result of NMR measurement and analysis, it was found that hydrate growth make apparent pore size gradually smaller. On the other hand, hydrate dissociation is supposed to advance from water-hydrate interfaces, where apparent pore size becomes larger gradually. All the sediments used here have the same tendency of decreasing pore-size with changing water saturation due to hydrate formation and growth. NMR measurement for gas hydrate sediments has a great possibility in understanding the structure of pore spaces and the change in porosity and permeability during exploration and production stages.
The remarkable technology of nuclear magnetic resonance (NMR) logging has improved continually. Many cases of Geophysical exploration in oil and gas production have adopted NMR logging tools for evaluating reservoir parameters and for assessing formation productivity. The application of NMR technique to the evaluation for gas hydrate-bearing sediments was reported by Kleinberg (2003a, 2003b). In this study, the possibility of the estimation of permeability and porosity in the formation process of gas hydrates (GH) was firstly examined, and obtained a good correlation between experiment and NMR analysis. This paper presents a NMR methodology for the estimation of permeability change in both processes of GH formation and dissociation in sandy sediments. The NMR measurements were performed in the experiments using high pressure cells and sediments, in order to make clear the hydrate behavior in porous media and pore space. The improvement of permeability and porosity estimation in core samples of gas hydrates was also investigated to be able to introduce for practical use in gas hydrate exploration.