Effect of Pore Size Distribution on Dissociation Temperature Depression and Phase Boundary Shift of Gas Hydrate in Various Fine-Grained Sediments

Abstract

Capillarity in small, confined pores has a pronounced effect on the depression of the dissociation temperature of gas hydrates, known as the Gibbs–Thomson effect. However, this effect remains poorly understood in natural fine-grained sediments with wide pore size distributions. This study investigated the effect of pore size distributions of fine-grained sediments on the dissociation temperature of a gas hydrate. A gas hydrate was synthesized under partially water-saturated conditions in nanosized silica gels and in various natural fine-grained sediment samples, including sand, silt, diatoms, a diatom–sand mixture, and clayey sediment. The synthesized hydrate samples were thermally dissociated under isochoric conditions, while the melting temperature depression and the shifted phase boundaries were monitored. We observed a dissociation temperature depression of approximately 0.1–0.3 °C in silt, 0.2–0.4 °C in the diatom sample, and 1.2–1.5 °C in clayey silt, while no temperature depression was observed in sand. In a particular case of diatom–sand mixture, the dual porosity condition with the submicron-scale internal pores of diatoms and the macropores of sands rendered dual phase boundaries, one with an ∼0.4 °C temperature depression and one with no depression, respectively. Despite the wide ranges of pore size, gas hydrates were preferentially formed in smaller pores, which comprise less than 40% of the cumulative pore volumes. This was because the initial water loci exacerbated the Gibbs–Thomson effect in partially water-saturated conditions. Our results provide clear experimental evidence on and novel insights into the effect of pore size distributions of fine-grained sediments on the dissociation behavior and phase boundaries of gas hydrates, both in the presence of free gas and in water-limiting conditions that exhibit a considerable Gibbs–Thomson effect.

Phase behavior and kinetics properties of gas hydrates in confinement and its application

“…The depression of the gas hydrate dissociation temperature with decreasing pore size has been observed by several researchers. 16,17,23 It was observed that there is a shift in gas hydrates dissociation temperature when the pore size is between 3 and 100 nm. Additionally, Nath et al showed using Monte Carlo simulations of methane hydrates in confinement of various pore sizes (using TIP4P and TIP4P/2005 models), a depression in hydrate dissociation temperature of up to around 28 K for 5 nm pore diameter.…”

“…The depression of the gas hydrate dissociation temperature with decreasing pore size has been observed by several researchers. 16,17,23 It was observed that there is a shift in gas hydrates dissociation temperature when the pore size is between 3 and 100 nm. Additionally,…”

Decomposition Characterizations of Methane Hydrate Confined inside Nanoscale Pores of Silica Gel below 273.15 K

“…Significant downward shifts of the dissociation temperature were observed in porous glass [36]. The melting temperature depression and the shifted phase boundaries were monitored [37]. The methane hydrate heat of dissociation into pore water and gas in 7-nm-radius silica gel pores, obtained calorimetrically, was 45.92 kJ/mol [38].…”

“…By studying the mass transfer of these processes, we can explore the control mechanism of hydrate accumulation and mining. Experimental results have shown the relationship between nanoscale pores bearing gas hydrates and dissociation behavior [36][37][38]. The dissociation conditions of methane hydrates in confined small pores were measured.…”

“…The methane hydrate heat of dissociation into pore water and gas in 7-nm-radius silica gel pores, obtained calorimetrically, was 45.92 kJ/mol [38]. Studies [36][37][38] have discussed the effect of pore size distributions of fine-grained sediments on the dissociation temperature, phase boundaries of gas hydrates, and the heat of dissociation.…”