Effect of Long-Term Storage and Thermal History on the Gas Content of Natural Gas Hydrate Pellets under Ambient Pressure

The use of natural gas hydrate (NGH) as a natural gas transportation and storage medium is expected to meet further energy demand in the near future. To exploit NGH for industrial porposes, it is necessary to establish NGH storage systems using their self-preservation properties. In this report, NGH pellets containing CH4, C2H6, and C3H8 were produced by continuous production using a bench-scale unit and successfully stored for 3 months at 253 K under atmospheric pressure. Qualitative and quantitative analyses [phase-contrast X-ray computed tomography (CT) by means of diffraction-enhanced imaging, cryogenic scanning electron microscopy, powder X-ray diffraction, and gas chromatography] were conducted to investigate the progression of dissociation. Some of the NGH pellets were also examined for their thermal history dependence between approximately 85 and 253 K after storage for 1 day or less to assess their stability under a rapid temperature change. The internal texture of the original NGH pellets was dense with hydrate even after 3 months of storage. On the other hand, thermal cracks covered with ice had formed in the hydrate pellet after the temperature rise (from 85 to 253 K). These cracks were assumed to be dissociation sites, but the dissociation was stopped only near the cracks. The mass fractions of NGH were larger than 70% after 3 months of storage and undergoing thermal history. These results demonstrate the excellent characteristics of NGH pellets produced by continuous production, improving their suitability for use as natural gas storage media.