For gas hydrates

Let us consider a clathrate crystal consisting of a cage-forming substance Q and a number of encaged compounds ( solutes ) A, B,. . ., M. The substance Q has two forms a stable modification, which under given conditions may be either crystalline (a) or liquid (L), and a metastable modification (ft) enclosing cavities of different types 1,. . ., n which acts as host lattice ( solvent ) in the clathrate. The number of cavities of type i per molecule of Q is denoted by vt. For hydroquinone v — for gas hydrates of Structure I 1/23 and v2 = 3/23, for those of Structure II vx = 2/17 and v2 = 1/17. 

The value of Ay for gas hydrates of Structure I reported in Table II could thus be derived30 with the aid of Eq. 25 with v — 3/23 from the composition Br2 8.47 H20 of the bromine hydrate following from Miss Mulders accurate study19 of the system Br2-f-H20 cf. Section III.C.(l). It should be possible to derive the value of Ay for hydrates of Structure II in the same way from the equilibrium composition of the SFe hydrate unfortunately the equilibrium composition of this hydrate is not known. The value of Ay for hydrates of Structure II reported in the table has been derived from the vapor pressure of the SF6 hydrate using some further assumptions (cf. Section III.C.(2)(b)). 

At very high pressures (in the GPa range), gas hydrates can undergo structural transitions to hydrate phases and filled ice structures. Figure 2.11 illustrates the structural changes that have been reported for gas hydrates at very high pressures at... 

Westacott, R.E., Rodger, P.M., Direct Free Energy Calculations for Gas Hydrates, in Proc. 213th ACS National Meeting, San Francisco, CA, April 13-17, 42(2) 539 (1997). 

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). 

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. 

Several reaction pathways are built into the FREZCHEM model including (1) temperature change, (2) evaporation, (3) pressure change, (4) equilibrium or fractional crystallization and, for gas hydrates, (5) open or closed carbon systems, and (6) pure or mixed gas hydrates. Under the temperature change option, the user can specify the upper and lower temperature range and a decremental temperature interval (AT) at which equilibrium at a fixed pressure is calculated (e.g., 298.15 to 253.15K with AT = 5 would result in... 

There is an abundance of experimental gas partial pressures for gas hydrate equilibria across a broad range of temperatures (Fig. 3.10 Sloan 1998). The lower temperature limit in our model database for these systems is 180 K (Fig. 3.10) because this is the lower limit of our model s ability to estimate aw (Fig. 3.1, Eq. 3.11), which is needed to calculate the solubility product of gas hydrates (Eq. 3.36). In our model, the upper temperature limit for methane hydrate is at 298 K (25 °C), which is the upper temperature limit for FREZCHEM the upper temperature limit for carbon dioxide hydrate is at 283K (10 °C), which is the temperature where liquid C02(l) becomes the thermodynamically stable phase. 

Given gas partial pressures (Fig. 3.10), a model to calculate solubility product (Eq. 3.36) can be calculated for gas hydrates (Fig. 3.11). The actual solubility product calculations are made at the experimental gas partial pressures, which vary widely (Fig. 3.10). Equation 2.29 was used to adjust all these pressure-dependent estimates (Kp) to a hypothetical 1.0 atm total pressure (iTP0), which is what is presented in Fig. 3.11. 

The analysis of thermo-baric changes in the wet soil samples saturated with CO2 as a function of time under condition of cyclic cooling and heating permits to follow the kinetic and thermo-baric indicators of phase transitions within the pore space of the samples. On cooling of wet gas-saturated soils under gas pressures higher than the three-phase equilibrium line gas - water - CO2 hydrate , conditions for gas hydrates nucleation in pore space of soils are created. Pressure stabilization marks the end of the phase transition of water into hydrate. Upon further cooling below 0°C the remaining, untransformed liquid turns into ice. 

The strong and weak effective H-bond (SWEB) model do not takes into account the energy differences within each H-bond types. An evident advantage of H-bonds form the basis of the simple discrete model. Though decomposition (3) allows to take into account the differences caused by different orientations of the surrounding molecules. The more complex model for gas hydrate frameworks suggested in our companion article. ... 

The phase behavior of NGHs has been extensively researched. The temperature, pressure, the composition of the gas, and the state of the aqueous solution determine the incipient conditions of gas hydrate formation. The most common method for representing phase behavior for gas hydrates is by a temperature versus pressure... 

Of particular interest to those in the natural gas industry is the phase diagram of hydrate systems in the presence of inhibitors. Fig. 3 shows the phase diagram for methane hydrates in the presence of methanol and a NaCl and KCl mixture. The solid line is the three-phase equilibrium curve for methane in pure water. As seen from Fig. 3, forming hydrates in the presence of either an alcohol or salt increases the pressure required for gas hydrate formation, at a given temperature. 

For calculating the chemical potential of water in the liquid solution, or ice phase. Holder, Corbin, Papadopoulos generated chemical potential, enthalpy, and heat capacity functions for gas hydrates at temperatures between 150 and 300 K and derived... 

Liquid hydrate clathrates are formed in conditions similar to those for gas hydrates. The corresponding physicochemical data are given in Table 1. Liquid... 

A., Didyk, B.M., and Lorenson, T.D., 1995. Geochemical evidence for gas hydrate in sediment near the Chile Triple Junction. In Lewis, S.D., Behrmann,... 

Hensen, C., and Wallmann, K., 2005, Methane formation at Costa Rica continental margin - constraints for gas hydrate inventories and cross-decollement fluid flow. Earth and Planetary Science Letters 236 41-60. 

No Defined by its stability field the potential volume for gas hydrate is mueh larger in the deep sea than on the upper slope however, the availability of sufficient methane is a critical requirement for gas hydrate formation. Due to high plankton productivity continental slopes have, in general, higher organic carbon concentrations than deep sea areas. Higher amounts of methane and gas hydrate are therefore restricted to continental margins. 

This table gives measured phase equilibria data of si and sll gas clathrate hydrates (see Table I for gas hydrate structure and physical property data). The temperature and pressure conditions at which gas... 

In addition to the visible gas hydrates recovered in cores, several lines of indirect evidence for gas hydrate occurrences were derived. Downhole geophysical logging shows that the gas... 

Uchida T., Lu H., Tomaru T., Dallimore S. R. and Nankai Trough Scientific Party (2004) Subsurface occurrence of natural gas hydrate in the Nankai Trough area implication for gas hydrate concentration. Resource Geol. 54,35-44. 

Table 2. Experimental data of hydrate formation for gas hydration curing in SDS water.

Figure 2. Sketch of the integrated apparatus for gas hydrate coal syntheses and triaxial test.

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