Hydrate formation in the two-phase region

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. 

In addition the extensive hydrate exploration experience of Collett (Personal Communication, May 25,2006) suggests that the container can control the amount of hydrate present. For example, when the container is relatively porous, grain-supported sand, the hydrate content can be high (as much as 70-80% of the pore volume). However, for low porosity shales the content may be much lower (e.g., 3% of the pore volume). 

Hydrate formation from free gas will likely initiate at a gas-liquid interface, as observed in the laboratory experiments of Chapter 6. As indicated in Chapter 3, either initial hydrate formation or a solid phase can serve as nucleation sites for additional formation from the gas and aqueous liquid phases. However, most geochemists (Claypool and Kaplan, 1974 Finley et al., 1987, etc.) suggest hydrates form from gas (either at equilibrium or supersaturated) dissolved in the liquid phase, without a free gas. 

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. 

While hydrate formation from a saturated water phase may be possible, substantial accumulation will require geologic-like times because the concentration of dissolved gas in water is extremely low—only 0.5-2.5 x 10-3 methane mole fraction in water and brines at depths from 300 to 5000 m (Hunt, 1979). The concentration of gas in the hydrate (one molecule of gas per six molecules of water) 

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