Effectiveness of excess gas method for the preparation of hydrate-bearing sediments: Influence and mechanism of moisture content

Excess gas method (EGM) was used to synthesize hydrates in the prepared sediment samples with different moisture contents. During the process of hydrate formation, the inside temperatures and air pressures in the upper and lower part of the samples were monitored. Meanwhile, the content of various types' pore water in the samples with different moisture contents were measured. Based on the growth mechanism of hydrates and the composition feature of pore water, the influence and mechanism of the type and content of pore water on the effectiveness of EGM for the preparation of hydrate-bearing sediment samples (HBSs) were discussed. The results show that the saturation degree of hydrates increases from 19.58% to 29.03% when the moisture content of the samples increases from 5% to 9%, the air pressures in the upper and lower part of samples are similar, and the hydrate distribution is relatively uniform. When the moisture content increases from 9% to 12%, the gas pressure difference between the upper and lower part of the samples increases from 0.25 MPa to 2.55 MPa, the distributions of hydrates become inhomogeneous, implying that the increased water has an important influence on hydrate formation. The bound water content (the quality ratio of bound water to soil particles) is in the range of 0.48%–0.54%, and almost unchanged with the increase of water content. When the unbound water content (the quality ratio of unbound water to soil particles) increases from 4.42% to 11.38%, the hydrate saturation degree difference between the upper and lower part of the samples increase from 1.91% to 19.74%. Therefore, the unbound water content has a significant impact on the process and effectiveness of hydrate formation. The unbound water saturation degree (the volume ratio of unbound water to pores) of 25.66%–33.60% is beneficial to the preparation of HBSs with a higher saturation degree and uniform distribution of hydrates for the studied clayey sand. The results can provide some reference for the effective formation of hydrates and the study of hydrate growth mode in samples.