[1] Decomposition conditions of methane hydrates in sediments were measured during formation-decomposition cycles. As test sediments, we used silica sand, sandstone, and clays (kaoline and bentonite), which are typical natural materials known as hydrate bearing sediments, and the range of samples cover a range of water saturating abilities. To better understand the results, we also used uniformly sized glass beads. Pore effects on decomposition of these materials were investigated by analyzing the pore-space distributions of the materials and by varying the initial water content of the samples. The results obtained for sand and sandstone samples indicated that the final decomposition temperatures were shifted lower than those for bulk hydrates at the same pressure. Temperature shifts were more negative for smaller initial water contents with the maximum shift being approximately À0.5 K. The results were consistent with those measured for glass beads with nearly the same particle size. For kaoline clays, the shift was at most À1.5 K. We conclude that the decomposition conditions are mainly affected by the pore sizes. The surface textures and mineral components had less influence on the results. We confirmed that glass beads mimic the effect of sediments for sand, sandstone, and kaoline clays, which have little to no swelling when put in contact with water. On the other hand, for bentonite particles, the results indicated that methane hydrates formed not only between the particles but also in the interlayers. A thermodynamic promoting effect was found for dilute bentonite solutions, although the positive decomposition-temperature shift was at most +0.5 K.INDEX TERMS: 1823 Hydrology: Frozen ground; 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; 3339 Meteorology and Atmospheric Dynamics: Ocean/atmosphere interactions (0312, 4504); 4524 Oceanography: Physical: Fine structure and microstructure; KEYWORDS: methane hydrate, decomposition temperature shift, pore space distribution, artificial fine particle, water content Citation: Uchida, T., S. Takeya, E. M. Chuvilin, R. Ohmura, J. Nagao, V. S. Yakushev, V. A. Istomin, H. Minagawa, T. Ebinuma, and H. Narita (2004), Decomposition of methane hydrates in sand, sandstone, clays, and glass beads,

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Gases releasing from shallow permafrost above 150 m may contain methane produced by the dissociation of pore metastable gas hydrates, which can exist in permafrost due to self-preservation. In this study, special experiments were conducted to study the self-preservation kinetics. For this, sandy samples from gas-bearing permafrost horizons in West Siberia were first saturated with methane hydrate and frozen and then exposed to gas pressure drop below the triple-phase equilibrium in the “gas–gas hydrate–ice” system. The experimental results showed that methane hydrate could survive for a long time in frozen soils at temperatures of −5 to −7 °C at below-equilibrium pressures, thus evidencing the self-preservation effect. The self-preservation of gas hydrates in permafrost depends on its temperature, salinity, ice content, and gas pressure. Prolonged preservation of metastable relict hydrates is possible in ice-rich sandy permafrost at −4 to −5 °C or colder, with a salinity of <0.1% at depths below 20–30 m.

Injection of flue gas or CO-N mixtures into gas hydrate reservoirs has been considered as a promising option for geological storage of CO. However, the thermodynamic process in which the CO present in flue gas or a CO-N mixture is captured as hydrate has not been well understood. In this work, a series of experiments were conducted to investigate the dependence of CO capture efficiency on reservoir conditions. The CO capture efficiency was investigated at different injection pressures from 2.6 to 23.8 MPa and hydrate reservoir temperatures from 273.2 to 283.2 K in the presence of two different saturations of methane hydrate. The results showed that more than 60% of the CO in the flue gas was captured and stored as CO hydrate or CO-mixed hydrates, while methane-rich gas was produced. The efficiency of CO capture depends on the reservoir conditions including temperature, pressure, and hydrate saturation. For a certain reservoir temperature, there is an optimum reservoir pressure at which the maximum amount of CO can be captured from the injected flue gas or CO-N mixtures. This finding suggests that it is essential to control the injection pressure to enhance CO capture efficiency by flue gas or CO-N mixtures injection.

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This paper summarizes current understanding of the processes that determine the dynamics of the subsea permafrost-hydrate system existing in the largest, shallowest shelf in the Arctic Ocean; the East Siberian Arctic Shelf (ESAS). We review key environmental factors and mechanisms that determine formation, current dynamics, and thermal state of subsea permafrost, mechanisms of its destabilization, and rates of its thawing; a full section of this paper is devoted to this topic. Another important question regards the possible existence of permafrost-related hydrates at shallow ground depth and in the shallow shelf environment. We review the history of and earlier insights about the topic followed by an extensive review of experimental work to establish the physics of shallow Arctic hydrates. We also provide a principal (simplified) scheme explaining the normal and altered dynamics of the permafrost-hydrate system as glacial-interglacial climate epochs alternate. We also review specific features of methane releases determined by the current state of the subsea-permafrost system and possible future dynamics. This review presents methane results obtained in the ESAS during two periods: 1994-2000 and 2003-2017. A final section is devoted to discussing future work that is required to achieve an improved understanding of the subject.Significant reserves of CH 4 are held in the Arctic seabed [10], but the release of CH 4 to the overlying ocean and, subsequently, to the atmosphere has been believed to be restricted by impermeable subsea permafrost, which has sealed the upper sediment layers for thousands of years [11]. In the regions where permafrost exists, hydrate-bearing sediment deposits can reach a thickness of 400 to 800 m [12,13]. Shallow hydrate deposits are predicted to occupy~57% (1.25 × 10 6 km 2 ) of the East Siberian Arctic Shelf (ESAS) seabed [14]. It has been suggested that destabilization of shelf Arctic hydrates could lead to large-scale enhancement of aqueous CH 4 , but this process was hypothesized to be negligible on a decadal-century time scale [15].Consequently, the continental shelf of the Arctic Ocean (AO) has not been considered as a possible source of CH 4 to the atmosphere until very recently [16][17][18].The key area of the AO for atmospheric venting of CH 4 is the East Siberian Arctic Shelf (ESAS). The ESAS covers greater than two million square kilometers (equal to the areas of Germany, France, Great Britain, Italy, and Japan combined). This vast yet shallow region has recently been shown to be a significant modern source of atmospheric CH 4 , contributing annually no less than terrestrial Arctic ecosystems [19,20]; but unlike terrestrial ecosystems, the ESAS emits CH 4 year-round due to its partial openness during the winter when terrestrial ecosystems are dormant [21]. Emissions are determined by and dependent on the current thermal state of the subsea permafrost and environmental factors controlling permafrost dynamics [21,22]. Releases could potentially increase by 3-5 orders of magnitude,...

Geological activity on icy planets and planetoids includes cryovolcanism. Until recently, most research on terrestrial permafrost has been engineering-oriented, and many related phenomena have received too little attention. Although fast processes in the Earth’s cryosphere were known before, they have never been attributed to cryovolcanism. The discovery of a couple of tens of meters wide crater in the Yamal Peninsula aroused numerous hypotheses of its origin, including a meteorite impact or migration of deep gas as a result of global warming. However, the origin of the Yamal crater can be explained in terms of cryospheric processes. Thus, the Yamal crater appears to result from collapse of a large pingo, which formed within a thaw lake when it shoaled and dried out allowing a large talik (that is layer or body of unfrozen ground in a permafrost area) below it to freeze back. The pingo collapsed under cryogenic hydrostatic pressure built up in the closed system of the freezing talik. This happened before the freezing completed, when a core of wet ground remained unfrozen and stored a huge amount of carbon dioxide dissolved in pore water. This eventually reached gas-phase saturation, and the resulting overpressure came to exceed the lithospheric confining stress and the strength of the overlying ice. As the pingo exploded, the demarcation of the crater followed the cylindrical shape of the remnant talik core.

The climate system is changing globally, and there is substantial evidence that subsea permafrost and gas hydrate reservoirs are melting in high-latitude regions of the Earth, resulting in large volumes of CO2 (from organic carbon deposits) and CH4 (from gas hydrate reserves) venting into the atmosphere. Here, we propose the formation of flue gas hydrates in permafrost regions and marine sediments for both the geological storage of CO2 and the secondary sealing of CH4/CO2 release in one simple process, which could greatly reduce the cost of CO2 capture and storage (CCS). The kinetics of flue gas hydrate formation inside frozen and unfrozen sediments was investigated under realistic conditions using a highly accurate method and a well-characterized system. The results are detailed over a wide range of temperatures and different pressures at in situ time scales. It has been found that more than 92 mol % of the CO2 present in the injected flue gas could be captured under certain conditions. The effect of different relevant parameters on the kinetics of hydrate formation has been discussed, and compelling evidence for crystal-structure changes at high pressures has been observed. It has also been found that temperature rise leads to the release of N2 first, with the retention of CO2 in hydrates, which provides a secondary safety factor for stored CO2 in the event of a sudden temperature increase.