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Metastable states of gas hydrates



V.A. Istomin1, V.G. Kvon2, and V.A. Durov3

1 Moscow Physical and Technical Institute
2 VNIIGAZ
3 Moscow State University

Since recent years, most authors have been attributing the metastable states of gas hydrates to the so-called self-preservation phenomenon. In fact, many different metastability effects in hydrate systems can exist. Apart from the academic interest, research activity in this area can also bring the significant commercial results in the nearest future.

Metastable states of gas hydrates can be classified as metastable state of the hydrate phases (the phases which are intrinsically stable, but metastable with respect to another gas hydrate phase or regarding its possible dissociation into gas and water), and metastable equilibria of gas hydrates (in these cases at least one of the contacting phase is metastable). It should be noted that in thermo-dynamic simulations of metastable equilibria (really existing or hypothetic), slightly modified gas hydrates software for stable equilibria can be used.

GAS HYDRATE METASTABLE PHASES

Metastable state of gas hydrate phase as itself may exist due to surface stabilisation by covering the hydrate surface of thermodynamically stable solid phase. For instance, in gas hydrate self preservation phenomenon, ice film covers the surface of metastable hydrate at T < 271 K, and in special preservation (forced preservation) a more stable gas hydrate film covers the metastable hydrate phase. In the latter case, temperature can be negative or positive (Celsius), and gas pressure could differ from the ambient conditions. The possibility of special (forced) hydrate preservation was theoretically predicted in 19982,3 in fact, this phenomenon was recently observed in experiments (but without explanation), in particular, by J. Ripmeester et al.4 Gas hydrate hysteresis, s i.e. the difference between hydrate formation and dissociation conditions, can be sometimes interpreted as a special preservation phenomenon (hysteresis5 occurs due to formation of "hydrate onion," i.e. a crystalline particle formed by gas hydrates layers with different compositions).

Figure 1. Stable and metastable three-phase
equilibria of methane hydrate:
1 - methane +
CS I hydrate + hexagonal ice Ih equilibrium;
2 - methane + CS I hydrate + water equilibrium;
3 - methane + CS II hydrate + hexagonal
ice equilibrium (hypothetic); 4 - methane +
CS II hydrate + water equilibrium (hypothetic);
5 - methane + CS I hydrate (small cavities are
empty) + hexagonal ice equilibrium (hypothetic);
6 - methane + CS I hydrate (small cavities are
empty) + water equilibrium (hypothetic)


GAS HYDRATE METASTABLE STATE AT HIGHER PRESSURES

At high pressure phase transition of hydrate crystal to amorphous phase may occur, i.e. the loss of thermodynamic stability when the system reaches the spinodal line (the "amorphisation effect"). Such amorphisation phenomenon was discovered in hydrate systems of volatile organic liquids, for example, for tetrahydrofurane hydrate cubic structure II (CSII). The system reaches its metastable state before the spinodal line. When pressure is falling, the crystalline structure of the gas hydrate system is reconstructed at pressure which appears to be much below that for the spinodal-line ("memory effect").

Principally, this amorphisation can take place in naturally-occurring gas hydrates near the spinodal line. Another metastability is also possible in natural conditions. Let us consider the effect at methane hydrate case. The three-phase equilibrium curve in the methane + water + hydrate domain passes its maximum at temperatures of about 45 - 46oC. At positive temperature (Celsius) - for instance, at +10oC - gas hydrate will be formed from methane and liquid water at p1 ≈ 7.0 MPa. Under pressure above the certain level, p2 (~1000 MPa), it should be melt. Therefore, the state of such methane hydrate between p1 and p2 should be considered as metastable. We may also consider such metastability as a special kind of forced preservation effect.

PREPARING METASTABLE HYDRATE PHASES UNDER NON-EQUILIBRIUM CONDITIONS

When producing gas hydrates in essentially overcooling conditions, or essentially exceeding equilibrium pressure, we can see the coexistence of different hydrate structures in the same sample. It is also possible to produce an "unfilled" hydrate structure, i.e. gas hydrate with partially-filled (probably, small) cavities. Indeed, metastable forms of gas hydrates often emerge as a by-product when the required crystalline hydrate forms are produced, either due to system overcooling or in excess of equilibrium pressure, when two hydrate structures - both stable and metastable - can be formed. From the kinetic point of view, presence of metastable hydrate in stable hydrate phase is due to difference in formation rates. For example, under gas hydrates formation from natural gas-condensate gases (when propane and isobutane components are in the gas phase), a mixture of both cubic crystalline structures (CS I and II) will be most likely formed. This was evidenced in our earlier experiments6 We used a differential thermal analysis method to investigate the "methane + ethane + propane + methanol aqueous solution + CS II gas hydrate" system: dissociation thermograms showed presence of a metastable hydrate form. The same situation was detected during studies of gas hydrates under high pressures in clathrate laboratory at the Institute of Inorganic Chemistry (Siberian Branch of the Russian Academy of Sciences). The presence of admixtures of another hydrate forms can be detected using both calorimetry techniques and structural analysis. In addition, both methods can produce experimental data on phase diagrams of metastable gas hydrates.

Table 1
Phase equilibrium: methane + CS I hydrate + hexagonal ice, methane + CS I
hydrate + cubic ice, and methane + CS I hydrate + + supercooled water

Metastable hydrate forms - as admixtures to the basic gas hydrate phase - can be easily produced when temperatures and pressures for various hydrate forms are close. This should be kept in mind when interpreting the phase equilibrium data for such systems - for instance, for methane + ethane (propane or isobutane) hydrate formation. We also expect the presence of several hydrate mixtures during hydrate formation in systems: methane + diluted acetone (tetrahydrofurane, isopropanol, etc.) water solution.

In addition, metastable gas hydrates formation is possible from both moist gaseous phase and/or homogenous gas/water solution. An interesting experiment was carried out recently with gas hydrate formation from homogenous supersaturated gas solution: black-coloured solid particles of metastable hydrate with triangle cage geometries were produced.7 In this experiment, the so called "memory effect" took place due to super-saturation of gas in liquid water after full hydrate dissociation.

Under strong non-equilibrium conditions, a gas hydrate phase with non-equilibrium hydrate numbers may be formed, i.e. practically without guest molecules in small cavities. It should be mentioned that in static conditions (at temperature ~ 273 K) the hydrate formation without induction time really occurs at 5.5-6.5 MPa in comparison with equilibrium pressure 2.6 MPa.8 Our estimates (see Figure 1) indicate that at 273 К the pressure ~ 6.0 MPa corresponds to thermodynamic conditions for a hypothetic cubic crystalline (CS I) methane hydrate, with only large cavities filled with guest molecules (small cavities are empty).9

Other metastable hydrate systems are likely to be formed within stability zones of different gas hydrates when hydrate primers are used. The prospective method of metastable hydrate formation assumes employment of kinetic hydrate inhibitors.

MESOMORPHIC STATES OF GAS HYDRATES

Liquid-crystalline states of gas hydrates(gas hydrate mesophases) may also exist. There are several ways available to produce these intermediate hydrate phases - in particular, using water-soluble polymer additives known as hydrate formation kinetic inhibitors.

One of the first attempts to discuss hydrate mesophases was presented 15 years ago.10 The authors presented experimental results concerning hydrate formation from a small water droplet (about 0.2 g) in compressed methane atmosphere under slow system cooling. Slow transition of the "water droplet + compressed methane" system through the hydrate formation line (cooling at p = const) evidenced no visible changes (i.e. hydrate crystallisation), although viscosity of water droplets rose many-fold in comparison with viscosity of pure water. Large gas emissions from the droplet and many micro-droplets were observed under pressure reduction. It should be noted that no such phenomenon occurred in the helium + water reference system. In this case viscosity remained equal that of pure water. It subsequent tests, the following results were detected:

- higher water phase viscosity of larger samples (droplets and films of about 2 grams), as well as for smaller droplets;
- the same situation for methane/propane mixtures (up to 0.4% propane content);
- only 2-3 wt.% of hydrate in water phase (calculated from thermograms);
- stabilised gel-like mesophase (gas content of water phase remains unchanged after 0.5 - 1 days).

Therefore, new aspect of gas hydrates crystallisation from liquid water phase has been experimentally tested. However, the additional research in this area is believed essential.

Table 2
Phase equilibrium: CO2 + CS I hydrate + hexagonal ice, CO2 + CS I hydrate + cubic ice,
and CO2 + CS I hydrate + supercooled water



PRODUCING GAS HYDRATES FROM METASTABLE WATER PHASES

Another possibility to produce metastable gas hydrates under much softer thermal and pressure conditions (against the equilibrium with stable water phases) implies a metastable water phase environment (supercooled water at T < 273 К and cubic and/or amorphous ice at T < 240 K). In addition to metastable gas hydrates, these conditions appear to enable the experimental research into thermal and pressure parameters of metastable three-phase equilibrium. In particular, A. Hallbruker and E. Mayer" observed nitrogen and oxygen hydrates formation from amorphous (solid) water phase and their subsequent dissociation at temperatures much higher than the equilibrium with hexagonal ice. Another publication sources the hypothesis regarding the metastable gas hydrates formation in a supercooled water environment, and under fleecy and noctilucent clouds.12

METASTABLE EQUILIBRIA OF GAS HYDRATE PHASE DURING DISSOCIATION PROCESS

Metastable equilibrium can be also observed in studies of gas hydrates dissociation. For example, at temperatures below 0o!, under gas pressure drop, the initial dissociation runs through several metastable water phases: at temperatures close to 273 К - via supercooled water formation, and at lower temperatures (below 240 K) - through cubic or another metastable ice phases. Most probably, the first researcher to observe this phenomenon was E.V. Malenko,13 and more recently - Prof. V. Kuhs et al. Metastable equilibrium of propane hydrate and supercooled water was visualised by V.P. Melnikov et al.15. Quality aspects of these metastable equilibria while gas hydrates dissociation are discussed in another paper (V. Istomin et al.) presented in this issue.

THERMODYNAMIC CALCULATION OF METASTABLE EQUILIBRIA

Thermodynamic method describes not only the stable equilibria of gas hydrates (gas + water + gas hydrate, and gas + ice + gas hydrate), but also the metastable equilibria. However, each phase - gaseous (over-saturated with water vapour), water (metastable ices, liquid water with oversaturated gas), and also gas hydrate - could be found in metastable state.

With three phase equilibria calculations of C02 and CH4 gas hydrates with metastable water phases (see our results presented at Tables 1 and 2), we used literature data regarding the metastable water phases thermodynamics. For instance, Prof. H. Tanaka14 molecular dynamics data for cubic ice were used. We hope that our calculations may be very useful for setting new types of dissociation experiments.

Metastability effects in gas hydrate systems should be taken into consideration in many practical cases: improvement of kinetic hydrate inhibitors, development of gas hydrate technologies such as gas transmission in hydrate form; studies of naturally-occurring gas hydrates, etc. At present, further detailed research of metastability in gas hydrate systems is believed critical. This research was supported by INTAS Grant No. 03-51-5537.


Literature Cited:

1. Buffet, B.A. and Zatsepina, O.Y. "Metastability of Gas Hydrates," Geophys. Research Lett.., 1999, V. 26, N 19, pp. 2981 - 2984.
2. Istomin, V.A. "Superheating of gas hydrates and ice." In: Development prospects of gas, condensate and oil fields in Russia's offshore, Moscow, VNIIGAZ, 1998, pp. 131 - 140.
3. Istomin, V.A. "Gas hydrates self-preservation at positive temperatures (Celsius)." In: Gasification. Natural gas as a motor fuel. Natural gas treatment, processing and utilisation. Energy conservation, 2000, No. 10&11, pp. 15 - 20.
4. Lu H., Ripmeester J., and Das H. "Direct determination of gas hydrate stability using recovered natural gas hydrate sample." Proc. of the 5th Intern. Gas Hydrate Conf. (Trondheim, June 2005). Paper ref. 5012, V. 5, pp. 1527 - 1531.
5. Bezverkhy P.P., Kuskova N.V., Martynets V.G., and Matizen E.V. "Metastable zone and phase equilibrium curves of methane hydrate formation and decomposition," Chemistry for Consistent Industrial Development, 1999, Vol. 7, No. 6; pp. 643 - 650.
6. Stupin D.Yu., Seleznyov A.P., and Istomin V.A. "Laboratory studies of mixed methane-ethane-propane hydrate formation in aqueous methanol solutions." In: Gas and Condensate Processing Equipment and Technologies, Moscow, VNIIGAZ, 1990, pp. 68 - 79.
7. Makogon Yu. F., Holdich S.A., and Makogon T.Yu. "Kinetics and morphology of secondary gas hydrates - experimental results." Proc. of the 5th Intern. Gas Hydrate Conf. (Trondheim, June 2005). Paper ref. 1029, V. 1, pp. 185 - 192.
8. Berecz, E. and Balla-Achs, M. Gas Hydrates (Studies in Inorganic Chem., v. 4). Amsterdam, 1983, 343 pp.
9. Istomin V., Kwon V., Kolushev N., and Kulkov A. "Prevention of gas hydrate formation at field conditions in Russia." Proc. of 2nd Int. Conf. on Natural Gas Hydrates (June 2-6, 1996, Toulouse, France). Toulouse, 1996, pp. 399 - 406.
10. Prokhorov A.Yu., Sukharevsky B.Ya., Vasyukov V.N., and Leontyeva A.V. "Quasiamorphous state of methane hydrate," Zhurnal of structural Chemistry, 1998, Vol. 39, No. 1, pp. 93- 104.
11. Hallbruker, A. and Mayer, E. "Unexpectedly stable nitrogen and oxygen from vapour-deposited amorphous solid water," J. Chem. Soc. Chem. Communs, 1989, pp. 749 - 751.
12. Istomin, V.A. and Yakushev, V.S. Naturally-occurring gas hydrates, Moscow, Nedra, 1992, 236 pp.
13. Malenko, E.V. "Studies of natural gas hydrates formation and decomposition, and the use of non-electrolytic inhibitors." PhD (Thesis, Moscow State University, Moscow, 1979, 168 pp.
14. Tanaka, H. and Okabe, I. "Thermodynamic stability of hexagonal and cubic ices," Chem. Phys. Lett, 1996, V. 259, Nos. 5-6, pp. 593 - 598.
15. MelnikovV.P., Nesterov A.N., and Reshetnikov A.M. "Mechanism of gas hydrates decomposition at 0.1 MPa," RAS, 2003, Vol. 389, No. 6, pp. 803 - 806.


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