Permafrost methane hydrate

Methane hydrates form when methane molecules become trapped within an ice lattice as water freezes. They can form in very cold conditions or under high-pressure conditions. Both of these conditions are met in deep oceans and in permafrost. In Canada, hydrates have already been found in large quantities in the Canadian Arctic. Methane hydrate has a number of remarkable properties. For example, when brought into an oxygen atmosphere, the methane fumes can be ignited, making it appear that the ice is burning ... 

Natural gas (methane) can be obtained from gas hydrates. Gas hydrates are also called clathrates or methane hydrates. Gas hydrates are potentially one of the most important energy resources for the future. Methane gas hydrates are increasingly considered a potential energy resource. Methane gas hydrates are crystalline solids formed by combination of methane and water at low temperatures and high pressures. Gas hydrates have an iee-hke crystalline lattiee of water molecules with methane molecules trapped inside. Enormous reserves of hydrates can be foimd imder eontinental shelves and on land under permafrost. The amount of organic... 

Ross and Andersson (1982) suggested that this behavior, which was never before reported for crystalline organic materials, was associated with the properties of glassy solids. Waite et al. (2005) measured the temperature dependence of porous methane hydrate thermal conductivity. Early work on this anomalous property led to the development of a thermal conductivity needle probe (Asher et al., 1986) as a possible means of in situ discrimination of hydrates from ice in the permafrost. 

Hydrate dissociation is of key importance in gas production from natural hydrate reservoirs and in pipeline plug remediation. Hydrate dissociation is an endothermic process in which heat must be supplied externally to break the hydrogen bonds between water molecules and the van der Waals interaction forces between the guest and water molecules of the hydrate lattice to decompose the hydrate to water and gas (e.g., the methane hydrate heat of dissociation is 500 J/gm-water). The different methods that can be used to dissociate a hydrate plug (in the pipeline) or hydrate core (in oceanic or permafrost deposits) are depressurization, thermal stimulation, thermodynamic inhibitor injection, or a combination of these methods. Thermal stimulation and depressurization have been well quantified using laboratory measurements and state-of-the-art models. Chapter 7 describes the application of hydrate dissociation to gas evolution from a hydrate reservoir, while Chapter 8 describes the industrial application of hydrate dissociation. Therefore in this section, discussion is limited to a brief review of the conceptual picture, correlations, and laboratory-scale phenomena of hydrate dissociation. 

Because free gas (or gas-saturated water) is less dense than either water or sediments, it will percolate upward into the region of hydrate stability. Kvenvolden suggested that a minimum residual methane concentration of 10 mL/L of wet sediment was necessary for hydrate formation. The upward gas motion may be sealed by a relatively impermeable layer of sediment, such as an upper dolomite layer (Finley and Krason, 1986a) or the upper siltstone sequence, as in the North Slope of Alaska (Collett et al., 1988). Alternatively, permafrost or hydrate itself may act as an upper gas seal. These seals can also provide traps for free gas that has exsolved from solution, and the seals can subsequently act to provide sites for hydrate formation from the free gas. 

Figures 7.11a,b are arbitrary examples of the depths of hydrate phase stability in permafrost and in oceans, respectively. In each figure the dashed lines represent the geothermal gradients as a function of depth. The slopes of the dashed lines are discontinuous both at the base of the permafrost and the water-sediment interface, where changes in thermal conductivity cause new thermal gradients. The solid lines were drawn from the methane hydrate P-T phase equilibrium data, with the pressure converted to depth assuming hydrostatic conditions in both the water and sediment. In each diagram the intersections of the solid (phase boundary) and dashed (geothermal gradient) lines provide the lower depth boundary of the hydrate stability fields.

Figure 7.11 Envelopes of methane hydrate stability (a) in Permafrost and (b) in Ocean sediment. (Reproduced from Kvenvolden, K.A., Chem. Geol., 71, 41 (1988). With permission from Elsevier Science Publishers.)...

In addition to these direct effects, there are also indirect consequences. One consequence is that, as the oceans warm, they absorb less C02 the other is that, as the Canadian, Siberian, and Alaskan permafrost melts, the rotting organic matter will release vast amounts of C02 and methane. The rising C02 concentration of the atmosphere also reduces the pH of the oceans. In addition, about 10 teratons of carbon (tera = 1012) are stored in the frozen methane hydrates of the Arctic regions, which will also be released if the ice melts. 

Methane hydrate is a clathrate compound of water molecules surrounding a methane molecule. Natural methane hydrate is found in permafrost and deep-sea sediment, and has recently attracted much attention as a potential new resource because of the large amount of deposits. Methane hydrate is also expected as new materials for gas storage and transportation due to its unique properties called anomalous preservation, quite slow dissociation from -40 to -10°C at atmospheric pressure, despite of its dissociation over -80"C. ... 

A class of hydrates (compounds containing water) with crystal structures composed of a molecular-water framework that encloses (or enclathrates) other molecules, such as gases. An example is the compound methane hydrate, CH4 nH20 (where n is about 6) that occurs in abundance on Earth in marine sediments and under arctic permafrost. 

Methane hydrates have attracted much attention as future energy resources because of the enormous amounts of those deposits. Because methane has fewer carbon atoms than all other fossil fuels and the amount of exhaust CO2 is relatively small when it bums, hydrates are considered to be cleaner energy resources. Various researchers have reported that they have accumulated extensively in permafrost regions and in sediments beneath the deep ocean floor. ... 

Methane hydrates are mostly found in permafrost (onshore and offshore shelf sediments) in polar regions... 

In the longer term, an oil shortage can be expected in 40 to 50 years, and this will result in increased use of natural gas. The fossil fuel with the longest future is coal, with reserves for more than 500 years. The question whether natural gas reserves in the form of methane hydrate, in which more carbon is stored than in other fossil raw materials, will be recoverable in the future cannot be answered at present, since these lie in geographically unfavorable areas (permafrost regions, continental shelves of the oceans, deep sea). 

Fig. 1 Methane hydrate, which is stable belou- and to the left of the phase boundary line. Also shovra is the geothermal gradient in permafrost as well as marine environments. Where the curves intersect, natural methane hydrate is stable. Natural methane hydrates are found in the lightly shaded region. BSR labels the "bottom-simulating reflector." an unexpected interface found by sonic exploration techniques and usually associated with the interface between sediments with and without hydrate. View this art in color at...

Gas hydrate forms wherever appropriate physical conditions exist—moderately low temperature and moderately high pressure—and the materials are present—gas near saturation and water. These conditions are found in the deep sea commonly at water depths greater than about 500 m or somewhat shallower depths (about 300 m) in the Arctic, where bottom-water temperature is colder. Gas hydrate also occurs beneath permafrost on land in arctic conditions, but, by far, most natural gas hydrate is stored in ocean floor deposits. A simplified phase diagram is shown in Fig. 2A, in which pressure has been converted to water depth in the ocean (thus, pressure increases downward in the diagram). The heavy line in Fig. 2A is the phase boundary, separating conditions in the temperature/pressure field where methane hydrate is stable to the left of the curve (hatched area) from conditions where it is not. In Fig. 2B, some typical conditions of pressure and temperature in the deep ocean were chosen to define the region where methane hydrate is stable. The phase boundary indicated is the same as in Fig. 2A, so methane hydrate is stable... 

Methane hydrate deposits in permafrost regions are not only potential fuel reserves but are also associated with aspects of global warming. It is postulated... 

There are large unconventional gas resources, like methane hydrate or aquifer gas, that could increase the amount of gas resources by a factor of ten or more. Methane hydrate is a clathrate, a crystalline form in which methane molecules are trapped. Hydrates are stable at high pressure and low temperatures (e.g., 100 bar at T< 13 °C), and are found at ocean depths >500m as well as under permafrost conditions. Some experts argue that the quantities of methane hydrates exceed those of all other fossil fuels combined. However, technologies for extracting methane from hydrate deposits have not yet been developed. 

OH radical and neutral H2O (recombination of the H20 ion with electrons leads essentially to dissociation to OH + H). In solid or gaseous form water has been found in a variety of astrophysical sites besides the ISM planets, satellites, comets, circumstellar disks, other galaxies and in our Sun and on our Moon. It also forms a matrix for trapping gases, as clathrates in which guest molecules are trapped within polyhedral water cages the most prominent example is that of methane hydrates which occur on ocean floors and in permafrost. 

A third motivation for studying the behaviour of clathrate hydrates is their environmental implications. There are vast natural deposits of methane hydrates. These are both off-shore deposits, occurring in the continental shoulders of the ocean floor, and land-based deposits in permafrost regions. Some idea of the size of these deposits can be obtained from estimates of the energy reserve contained within these methane... 

There are also large reservoirs of CH4 stored as methane clathrate methane hydrate), a crystalline ice-like structure of water and CH4 that can exist at high pressures and low temperatures, such as may occur in areas of permafrost and beneath certain ocean sediments. Figure 4.51 shows the stability diagram of methane clathrate. Approximately 2 X10 Tg CH4 are estimated to be stored as methane clathrate in the ocean floor (Archer et al., 2009). 

Although atmospheric methane concentrations appear to have stabilized over the past few decades, melting of gas hydrates in permafrost and shallow marine sediments have the potential to rapidly release large quantities of this potent greenhouse gas. As noted in... 

Several hundred to several thousand feet beneath the ocean floor in permafrost and continental edge regions lies a potentially vast source of natural gas in excess of 10 cubic meters of gas hydrates, consisting largely of methane clathrate (53-55). Gas... 

Other vast yet untapped reserves of natural gas (methane) are locked up as hydrates under the permafrost in Siberia. Methane gas hydrates are inclusion... 

Shoji and Langway (1982) described air hydrates found with ice cores off Greenland, while Tailleur and Bowsher (1981) indicated the presence of hydrates associated with coals in permafrost regions. Hondoh (1996) suggested that deep ice hydrates of air in Antarctica can be used to predict the Earth s ancient climate. Such hydrates are formed from air imbedded in snowfall and have been buried at pressure for hundreds of thousands of years. Rose and Pfannkuch (1982) have considered the applicability of the Deep Gas Hypothesis to the origin of methane in hydrates. 

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