The formation of gas hydrates in porous sediments composed of fine-grained media with a larger specific surface should be imaginatively faster, while why do hydrates in natural conditions tend to accumulate in coarse particles? In view of the controversy over the influence of sediment conditions on hydrate formation kinetics, in this work, a high pressure micro-differential scanning calorimeter (HP μ-DSC) with small reactor volume and high precision was specially used to systematically investigate the effects of quartz sand particle size, initial water content, and sediment medium type on the formation kinetics of CH4 hydrate in porous sediments. The results show that the CH4 hydrate formation rate, on a whole, increases with the decreased quartz sand particle size and initial water content. However, it is not completely monotonous, and the difference and degree of the influence for one factor on the hydrate formation depends on the choice of another factor. Moreover, the addition of kaolin or bentonite even with smaller particle size and larger specific surface, however, relatively hinders the formation of CH4 hydrate. By directly observing the formation process of CH4 hydrate in different porous sediments, it was found that the rapid hydrate growth in porous sediments relies greatly on the continuous diffusion and growing of hydrates towards adjacent media. In addition, some interesting phenomena for bentonite wetting and cracking have also been observed. The discoveries in this work provide a systematic and unique insight into the natural gas hydrates formation in porous sediments.
Keywords CH4 hydrate, formation kinetics, porous sediments, HP μ-DSC