Sediments methane hydrates

Since methane is almost always a byproduct of organic decay, it is not surprising that vast potential reserves of methane have been found trapped in ocean floor sediments. Methane forms continually by tiny bacteria breaking down the remains of sea life. In the early 197Qs it was discovered that this methane can dissolve under the enormous pressure and cold temperatures found at the ocean bottom. It becomes locked in a cage of water molecules to form a methane hydrate (methane weakly combined chemically with water). This "stored" methane is a resource often extending hundreds of meters down from the sea floor. 

In the CSM laboratory, Rueff et al. (1988) used a Perkin-Elmer differential scanning calorimeter (DSC-2), with sample containers modified for high pressure, to obtain methane hydrate heat capacity (245-259 K) and heat of dissociation (285 K), which were accurate to within 20%. Rueff (1985) was able to analyze his data to account for the portion of the sample that was ice, in an extension of work done earlier (Rueff and Sloan, 1985) to measure the thermal properties of hydrates in sediments. At Rice University, Lievois (1987) developed a twin-cell heat flux calorimeter and made AH measurements at 278.15 and 283.15 K to within 2.6%. More recently, at CSM a method was developed using the Setaram high pressure (heat-flux) micro-DSC VII (Gupta, 2007) to determine the heat capacity and heats of dissociation of methane hydrate at 277-283 K and at pressures of 5-20 MPa to within 2%. See Section 6.3.2 for gas hydrate heat capacity and heats of dissociation data. Figure 6.6 shows a schematic of the heat flux DSC system. In heat flux DSC, the heat flow necessary to achieve a zero temperature difference between the reference and sample cells is measured through the thermocouples linked to each of the cells. For more details on the principles of calorimetry the reader is referred to Hohne et al. (2003) and Brown (1998). 

Ocean sediments with hydrates typically contain low amounts of biogenic methane. 

Sediments with Hydrates Typically Have Low Contents of Biogenic Methane... 

Using innovative experiments, Tohidi and coworkers (2001) and Anderson et al. (2001) have shown that hydrates can be formed in artificial glass pores from saturated water, without a free gas phase. They found that with significant subcooling the amount of hydrate formation was proportional to the gas solubility carbon dioxide formed more hydrates from a saturated solution than did methane. Further, the maximum amount of methane hydrate formation was fairly low— about 3% of the pore volume—a value consistent with the amount of hydrates in sediment. 

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.)...

The storage of methane as hydrates offers a potentially vast natural gas resource. As to the question of how much hydrate there is right now, there is no definitive answer. However, the worldwide amount of carbon bound in gas hydrates has been estimated to total twice the amount of carbon to be found in all known fossil fuels originally on Earth. Additionally, conventional gas resources appear to be trapped beneath methane hydrate layers in ocean sediments.22... 

Klauda, ).B. and Sandler, S.I. (2005) Global distribution of methane hydrate in ocean sediments. Energ. Fuel, 19, 459. 

Revelle R., Methane hydrates in continentd slope sediments and increasing atmospheric carbon dioxide. In Changing Climate . Report of the Carbon Dioxide Assessment Committee. National Research Council, National Academy Press, Washington, D.C., pp. 252-261 (1983a). 

Wood W. T., Gettrust J. F., Chapman N. R., Spence G. D., and Hyndman R. D. (2002) Decreased stability of methane hydrates in marine sediments owing to phase-boundary roughness. Nature 420, 656-660. 

Before the arrival of humanity, the longer-term controls were burial of carbon as carbonate, as methane hydrate in sediment, as gas, as coal, or as reduced organic matter (including charcoal). Before the Devonian, fire would have been impossible, except perhaps on lightning-hit microbial peat bogs after drought. 

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 Hydrate-bearing Sediments. SEM images of methane hydrates in gas-saturated sediments display gas hydrates between the quartz grains like a glue or cement (Figure 4a). This cement should contain almost pure gas hydrate after the complete transformation from water (see Sect. 3.1). 

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... 

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