天然堆積物中でのメタンハイドレート形成における堆積物粒子径，粘土鉱物等の影響

Abstract

Accurately estimating natural gas hydrate resources is one of the most important issues when assessing the energy potential of natural gas hydrate, which relies largely on the precision of data on the hydrate saturation level in sediments of a reservoir. The primary mode of occurrence of gas hydrate recognized in the eastern Nankai Trough area is in sediment pores. It is suggested that the distribution of coarse-grained sands is one of the most important factors controlling the occurrence of natural gas hydrates. This research aims to elucidate the particle size and clay mineral effects on hydrate saturation in sediments through an experimental approach. The specimens tested include sand, silty sand, and silt, representing the main sediment types recovered from the gas hydrate distribution region of the eastern Nankai Trough. The results obtained from the experiments clearly indicate a particle size and clay content-dependent trend of low saturation in fine sediment but high saturation in coarse sediment. These results are generally consistent with NMR logging results for high-saturation samples, but are somewhat different for samples with low or medium saturation levels. To obtain a better understanding of the mechanism of these two factors, studies were carried out to investigate the saturation level of methane hydrate in a series of silica powders and clay. The results indicate that particle size and clay contents are the two key factors determining the saturation level of gas hydrate in sediments—the finer the particle size and/or the higher the clay content, the lower the hydrate saturation.