And gas hydrates

Let us explicitly consider the two important cases of hydro-quinone clathrates and gas hydrates. 

Furthermore, vast reserves, such as tar sand and gas hydrates, await technology to enable their economically and environmentally sound exploitation. Large coal reserves can also be exploited, for example through gasification and Fischer-Tropsch synthesis. 

Fig. 1. Log data showing the variation of porosity, permeability, and gas hydrate saturation for the Mallik Gas Hydrate Research Program Well-5L-38 (Uddin et al. 2008b).

Just as oil, natural gas is also categorised as conventional and unconventional. Unlike crude oil, however, natural gas deposits are normally classified according to the economic or technical approach, i.e., all occurrences that are currently extract-able under economic conditions are considered conventional, whereas the rest are termed unconventional. Conventional natural gas includes non-associated gas from gas reservoirs in which there is little or no crude oil, as well as associated gas , which is produced from oil wells the latter can exist separately from oil in the formation (free gas, also known as cap gas, as it lies above the oil), or dissolved in the crude oil (dissolved gas). Unconventional gas is the same substance as conventional natural gas, and only the reservoir characteristics are different and make it usually more difficult to produce. Unconventional gas comprises natural gas from coal (also known as coal-bed methane), tight gas, gas in aquifers and gas hydrates (see Fig. 3.17). It is important to mention in this context so-called stranded gas , a term which is applied to occurrences whose extraction would be technically feasible, but which are located in remote areas that at the moment cannot (yet) be economically developed (see Section 3.4.3.1). 

Hydrate nucleation is the process during which small clusters of water and gas (hydrate nuclei) grow and disperse in an attempt to achieve critical size for continued growth. The nucleation step is a microscopic phenomenon involving tens to thousands of molecules (Mullin, 1993, p. 173) and is difficult to observe experimentally. Current hypotheses for hydrate nucleation are based upon the better-known phenomena of water freezing, the dissolution of hydrocarbons in water, and computer simulations of both phenomena. Evidence from experiments shows that nucleation is a statistically probable (not deterministically certain see Section 3.1.3) process. 

In Figure 7.34 the following initial points are used (with C,D,E,F corresponding to letters on Well No. 109 in the reservoir diagram of Figure 7.30) AB = hydrate equilibrium line C = temperature at the top of the pay zone D = temperature at a level of gas and water contact E average gas-hydrate temperature F = temperature at boundary surface between gas and gas-hydrate reserves H = beginning dissociation pressure for gas hydrates. 

Data were obtained to calibrate well logs and gas hydrate production simulators. 

We emphasize that it is not anhydrous salts but salt hydrates (and gas hydrates) that have this important thermal insulating property. Cool climates, which tend to support high hydration states, favor this type of process. Cold evaporitic basins are ideal, and so we find that Mars is where this phenomenology is most likely to have widespread relevance. 

Fig. 6.17 Carbonate isotopic excursions during Neoproterozoic glaciations predicted by the Snowball Earth and gas-hydrate models (after Jacobsen 2001). Zero on the time scale corresponds to onset of cap-carbonate deposition.

Response of cold seeps and gas hydrates to global warming... 

Here, and Pj are the densities of fresh water and gas hydrate respectively, and m is the number of moles of fresh water contained in 1 mole of gas hydrate. and 1S represent the molecular weights of water and gas hydrate, respectively. The value of Mj depends on the degree of occupancy of the hydrate structure. When the structure is fully occupied, 1 mole of gas hydrate contains 5.9 moles of water, its density is 910 kg m and its molecular weight is 122.2 g mol (Ussier and Pauli 2001). 

Fig. 14.22 Schematic illustration of gas hydrate deposits and biogeochemical reactions in near-surface sediments on southern Hydrate Ridge. High gradients in pore water sulfate and methane are typical of methane hydrate-rich environment close to sulfate-rich seawater. At the sulfate-methane interface (also named sulphate-methane transition in earlier chapters of the book) a microbial consortium of methanothrophic archaea and sulfate-reducing bacteria (Boetius et al. 2000) perform anaerobic oxidation of methane (AOM) leading to carbonate precipitation. AOM rates influence hydrogen sulfide fluxes and gradients, which are reflected on the seafloor by the distribution of vent communities around active gas seeps and gas hydrate exposures (Sahling et al. 2002).

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