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Self-preservation phenomenon of gas hydrates



V.A. Istomin1, V.S. Yakushev1, and N.A. Makhonina1, V.G. Kwon1; E.M. Chuvilin2

1 VNIIGAZ
2 Moscow State University, MSU

The self-preservation phenomenon means that gas hydrates may exist in metastable state for prolonged time. The self-preservation phenomenon in gas hydrates paves the way to wide-scale use of various hydrate technologies in the oil and gas sectors. At present, this area is focal for R&D centres in the United States, Norway, Canada, Russia, Japan, and other countries of the world.

Gas hydrates self-preservation phenomenon can be defined as a very slow decomposition of gas hydrates when the external pressure drops below a three-phase equilibrium pressure of the gas-ice-hydrate system at sub-zero (Celsius) temperature (below 270-271 K) as a result of thin ice film emergence on gas hydrate surface.

This effect was initially discovered and described in detail over 1986-1992, by researchers from Canada (Ottawa National R&D Center) and Russia (by the joint team formed by VNIIGAZ and Moscow State University, identified as the Moscow Gas Hydrate Group).1-9

The term "gas hydrates self-preservation" was introduced by Russian researchers after several laboratory experiments which showed that gas hydrates decomposition come to a virtually halt when hydrate particles cover by thin ice shell at the initial decomposition stage. Thus, "self-preservation" implies that gas hydrates can exist even in absence of the external pressure of hydrate-forming gas (usually, under atmospheric pressure, 0.1 MPa) with no significant changes to gas content if ice sublimation from the surface of the sample is not to occur. At present, gas hydrates decomposition kinetics has fallen into tough scrutiny by researchers worldwide, although this term was found to vary globally: it can be called either "self-preservation" or "anomalous preservation". Wherever possible, in this paper we will refer to the first definition.

It should be noted that as early as 1987, an assumption was made, based on both field and laboratory data,1" that the so-called "relic gas hydrates" could exist for quite long periods of time in permafrost (which are away from the modern hydrate stability zone). Thus, the self-preservation phenomenon applied to gas hydrates would likely have significant geological importance. Subsequently, several technologies using this phenomenon were developed, and these are discussed below.

In 1999, attempts were made to substantiate the idea of special - or "forced" - preservation of gas hydrates, under both negative and positive temperatures (Celsius).11 It was proposed to prevent gas hydrate decomposition by covering its surface not only with hexagonal ice, but rather with the thermally-stable envelope of another solid phase (by surface formation of gas hydrate with another gas composition); at that pressure could be atmospheric or above-atmospheric. In addition, it was assumed that the special preservation of gas hydrates would be possible in aqueous solution of hydrate-forming agent. Also, it was noted that the initial stage of hydrate self-preservation can occur (depending on thermo-baric conditions) through an appearance of intermediate (metastable) water phases at hydrate surface such as supercooled water, amorphous, cubic ice etc. with subsequent phase transition to more stable phase - hexagonal ice.

So, several mechanisms of gas hydrates preservation may be occur at different thermodynamic conditions. However below we will detail the effect when occurring under negative temperatures (Celsius), since other similar effects still remain practically unstudied. Metastable phase states and equilibrium conditions of gas hydrates are discussed in detail in the paper by V.A. Istomin et al. presented in this issue.

INITIAL STUDIES OF ANOMALOUS GAS HYDRATES BEHAVIOUR AT TEMPERATURES BELOW 273

As early as 20 years back the self-preservation phenomenon was discovered, following some individual studies and attempts to explain the anomalous gas hydrates behaviour under negative temperatures (Celsius). For example, in 1968 Yu.F. Makogon observed retarded decomposition of gas hydrates (T < 273 K) in a pipeline running from Ust-Vilyuisk gas/condensate field in Eastern Siberia.12 A sample of gas hydrate were recovered at second-stage of pressure reduction of Yakutsk Gas Distribution Station at 253 temperature. Pieces of this sample were kept at unchanged temperature and atmospheric pressure, and showed no visible changes for two weeks, which could be attributable to the conditions close to those of gas hydrate formation thereby showing slow rate of decomposition. Temperature rise led to partial decomposition of these samples. As a result, Makogon concluded that these gas hydrate pieces appeared to be thermodynamically stable at 253 K. However, according to our estimates, the equilibrium temperature for the Ust-Vilyuisk gas hydrate should be about 223 at 0.1 MPa pressure.

The composition of the Ust-Vilyuisk gas (Lower Jurassic producing zone, between 1,732 and 1,788 m) is shown below:13

Thus, at 253 , gas hydrate samples were preserved in metastable state when enveloped by ice.

N.V. Chersky and V.P. Tsarev14 described gas emissions from frozen cores recovered from hydrate stability zone of the Ulakhan-Yuryakhsky gas structure. These emissions occurred after samples immersion into water. Presence of gas hydrates in frozen cores was attributed by these researchers to retarded decomposition at temperatures below 273 K.

In 1975-1982, A.G. Groisman et al.15 investigated thermo-physical characteristics of mixed gas hydrates. Slow decomposition of fine-dispersed gas hydrate was observed at temperature range 253 - 263 and atmospheric pressure, but the researchers gave no physical interpretation to this fact.

Therefore, no clear understanding and explanation of gas hydrates slower decomposition kinetics at temperatures below 273 was available before 1986.

GAS HYDRATES SELF-PRESERVATION RESEARCH ACTIVITY

In the 1980s, Canadian researchers Y.P. Handa, D.W. Davidson and others were the first ones who attempted to explain high stability of gas hydrates in non-equilibrium conditions at 273 or lower temperatures.1,3,4 In their research based on calorimeter tests of gas hydrate thermal properties, they recorded partial decomposition of xenon and krypton hydrates below 273 at reduced pressures. Also, they noted the dissociation of naturally-occurring II gas hydrates recovered in the Gulf of Mexico, and I deuterated methane hydrate produced in laboratory conditions. The Canadian researchers proposed an idea that thin-film ice is formed under gas hydrates decomposition, which slows down the process at temperatures below 273 K. Partial decomposition of relatively massive hydrate samples was also observed, with most of them dissociated well before ice film formation. At the same time, fine-dispersed and/or powder-like gas hydrates dissociated completely at negative temperatures (Celsius), close to their equilibrium point (i.e. methane hydrate completely dissociates at 190 and atmospheric pressure). Though, the authors were not able to determine the "critical" size of hydrate particles allowing for self-preservation.

In Russia, the research into "anomalous" behaviour of methane hydrate and ice/methane hydrate agglomerate system at negative temperatures (Celsius) was initiated in 1986 - 1987. The first results were reported in March 1988 at the Meeting of Earth Cryology Board (of the Academy of Sciences) , and later at the 8th Joint Soviet-American Workshop on Comparative Planetology.2,5 - 9 These reports were authored by E.D. Ershov, V.S. Yakushev V.A. Istomin, E.M. Chuvilin and Yu.P. Lebedenko.

Samples of methane hydrates were produced in a pressure chamber at 274 - 278 temperatures and 3 - 8 MPa gas pressures. Then the temperature was reduced to negative temperature (at range 255 - 271 K) and after 10-16 hours storage the pressure was reduced to atmospheric value (0.1 MPa). No visual changes were observed for the samples. These samples were kept at the same temperature and atmospheric pressure during monitoring their gas content.


Figure 1. Hydrate samples stability versus their inner morphology structure


Figure 2. Self-preservation phenomenon of gas hydrate particle after depressurisation (p < peq)
at 238 K < T < 273 K

It was discovered that stability of samples strongly depends on their structure (solid monolithic hydrate, ice/hydrate agglomerate, porous hydrate) at the same conditions (in any case no ice sublimation occur). Acicular and filamentary hydrate crystals dissociated very quickly: from 2 - 3 minutes to 5 - 6 days, depending on their initial sizes. Massive porous hydrate dissociated completely within 2 weeks. Gas content of monolithic (non-porous) hydrate was reduced by about 30% within 5 months (at 263 - 268 K). Ice/ hydrate agglomerate samples containing several per cents of hydrate, produced by freezing gas-saturated water, were kept in the freezer at 267 for 18 months, with no serious change to gas content. Figure 1 illustrates hydrate samples stability depending on their structure.

Higher stability of methane hydrate at thermodynamic conditions below the three-phase gas - hexagonal ice - gas hydrate equilibrium pressure was explained by the formation of "insulating" thin film of ice on sample surface while hydrate decomposition. This effect was assigned as a "self-preservation phenomenon" of gas hydrates at negative temperatures (Celsius).2, 5 - 8

The initial stage of this process can be accompanied by formation of several intermediate metastable phases: supercooled water, cubic crystalline ice and/or amorphous ice. At temperatures higher than 238 - 242 the most probable mechanism was found to be the initial hydrate dissociation through the thin film of liquid supercooled water with subsequent freezing of supercooled water film (see Figure 2). After reducing pressure the surface dissociation of hydrate into gas and supercooled water occur (see Figure 2a). During a short time metastable water begins to crystallise forming the thin ice film (see Figure 2b). When this film reaches specific critical thickness, further hydrate decomposition is practically terminated (see Figure 2c). At the same time, re-crystallisation zone can occur in the hydrate/ice interface, which (depending on hydrate's formation and storage conditions) provides for partial hydrate-to-ice and ice-to-hydrate transformations by means of gas diffusion to the interface. This effect may be considered from the thermodynamic point of view as metastable two-phase ice/ hydrate equilibrium with no free gas phase available within the system.16 Given this consideration, chemical potentials of water at gas hydrate and ice are calculated, which allows to estimate the possibility of surface re-crystallisation. In some cases, ice/hydrate transition zone can occur, to promote the self-preservation effect.

The picture shown in Figure 2 is by all means idealised. Supercooled water formation on hydrate's surface can occur at temperatures above 240 even if ice film had been available before the decomposition starts. Water film could be formed on the surface of dissociating gas hydrate due to dynamic nature of the process; this water will freeze on the ice surface, i.e. the supercooled water can exist as a thin dynamic layer between the ice and the hydrate. The probability of supercooled water formation on dissociating hydrate's surface is dramatically reduced at temperatures below ~240 K, thus at the temperatures below 240 the ice film may be formed without intermediate supercooled water (but with formation of metastable phases of ice like cubic ice in some cases).

In this connection, it is interesting to investigate the possibilities of gas hydrates formation from metastable phases of ice with future studies of hydrate decomposition mechanism. Abnormally slow decomposition of gas hydrates formed from metastable ice phases was observed in 1988 -1989 by Austrian researchers E. Mayer and A. Hallbruker. They set up a series of experiments on N2, 02, CO and Ar hydrates formation from amorphous ice at atmospheric pressure;17-19 the ice was produced by vapour precipitation on the cooled down to 77 X-ray sample holder in vacuum. Earlier it was shown that amorphous ice produced at the same ambient conditions has a microporous structure.20 When the thickness of the amorphous ice film reached 0.5 -1 mm, hydrate-forming gas was introduced into the chamber at about 0.1 MPa. Then, the temperature was raised to 113 - 120 which, according to the authors, helped to saturate the microporous ice with huge amount of gas. At the same time pressure of hydrate-forming gas was kept constant. Re-cooled to 77 the sample was studied by X-ray diffraction. The obtained diffraction pictures showed several phase transitions: first, cubic crystalline ice formation (at about 163 K); secondly, gas (N2, 02, CO, Ar) hydrate formation (within 170 -210 range); and later, at about 223 K, cubic ice was partially re-crystallised into hexagonal ice. Hydrates decomposition occurred at 240 - 250 and, at 253 no cubic ice was observed in the samples.


Figure 3. Time relationship for decomposition rates of methane hydrate at various temperatures (K): 1 - 253; 2 - 233; 3 - 213; 4 - 205; 5 - 198; 6 - 189; 7 - 178; 8 - 168;
9 - 158; 10 - 148

Experimentally produced temperatures of N2, 02, CO and Ar hydrates decomposition were much higher than the equilibrium temperatures at 0.1 MPa (according to various authors, the equilibrium temperature of nitrogen hydrate should be within 147 - 167 range, and for oxygen hydrate - between 159-164 K).21-23The authors proposed that the hydrates formation mechanism within ice micropores is the same as observed in glaciers where ice prevents hydrate decomposition thus rising their dissociation temperatures to well above the equilibrium. The only points not clarified were: the shift of equilibrium parameters while hydrates formation from metastable phase of ice, and from what kind of ice (amorphous or cubic) hydrate formation was to occur.

The group of Norwegian researchers led by J.-S. Gudmundsson24 also observed high gas hydrates stability in non-equilibrium conditions. Samples of mixed gas hydrates composed of (mol.%): methane - 92; ethane - 5; propane - 3, were stored at 255, 263, and 268 temperatures (at barometric pressure) for 10 days. At 268 K, gas loss amounted to 11.3%, at 263 - 3.4%, and at 255 - its loss was negligible. Most probably, the last case appears to be illustrative of thermodynamically stable hydrate forms since equilibrium temperature of their formation is close to 255 K. According to the authors, at 263 and 268 gas hydrates were preserved due to surface ice film which was formed after their partial decomposition within several minutes (or hours).

Japanese researchers at Hokkaido National Research Institute (HNRI) have studied methane hydrates decomposition kinetics and self-preservation phenomenon using X-ray diffraction technique.25-28 Methane hydrate samples were studied at temperatures from 148 to 268 K, at barometric pressure. Sample temperatures were maintained by dry nitrogen flow at specified temperature produced by controlled liquid nitrogen evaporation. The flow of dry and cool nitrogen prevented hoar-frost formation on the sample surface (due to vapour condensation), though allowed ice sublimation.

In their research, the authors have discovered that at temperatures higher than 189 K, methane hydrate dissociation appears to run in two stages: within the first 10 minutes of active decomposition, hydrate dissociation slows down dramatically due to thin ice film formation (see Figure 3).26 Also, the authors showed that hexagonal ice is formed on the sample surface. Further dissociation depends on methane diffusion conditions via available pores and/or interfaces of ice crystals.

According to estimates, effective diffusion factor of methane through the ice film at 189 is 2.2·10-11 sq m/sec, and 8.7·10-11 sq m/sec at 198 K.25,26

The Japanese colleagues have also studied methane hydrate dissociation activity versus hydrate particle size.27 Samples of polycrystalline methane hydrate were heated from 135 to 263 at 1 K/min rate (at barometric pressure). Hydrate particles of up to 250 μm were shown to fully decompose at about 210 K, while 1,000 - 1,400 μm particles contained about 20% hydrate at 263 K. Fine-structural investigation of hydrate particles using a confocal scanning electron microscope (SEM) showed thin ice film on hydrate surface after partial dissociation. This proves the idea that the bigger the size of hydrate particles, the longer they remain stable at low temperatures. Further studies showed that at 193 methane hydrate surface becomes covered by ice nano-particles. Then, until 210 no visible changes would occur, though, when the temperature is elevated up to 230 K, continuous ice layer is formed on the surface, which prevents hydrate from further decomposition.28


Figure 4. Average rates of methane hydrate dissociation calculated by its half-life time after pressure drops to 0.1, 1.0 and 2.0 MPa, at various temperatures (filled-in marks - methane hydrate dissociation calculated by its half-life time at T = const; non-filled marks - extrapolated data)

Fine-structural investigation data could be compared with those obtained while the studies of methane hydrate decomposition kinetics;28 the samples were heated from 170 to the target levels within 200-268 temperature range at the rate of 5 K/min and then were kept at preset temperature for 90 minutes. Hydrate decomposition was observed at temperatures slightly above 190 K. In cases that temperatures were set 200 or 210 K, almost complete dissociation of the samples occurred. At temperatures over 230 K, only small part of the sample dissociated at this temperature, while its main portion dissociated when heated to the temperature of ice melting.

Therefore, the studies carried out by the Japanese researchers have not only proved, but specified the data regarding the two-stage mechanism of gas hydrates dissociation: the fast one - which lasts for the first several minutes, and the slow one - which starts when thin ice film is formed on the hydrate surface. In their view, dissociation rate at the slow stage is determined by the conditions of methane diffusion through pores or crystalline interfaces. Also, the bigger the hydrate particle sizes, the longer the time it remains stable at negative temperatures (Celsius).

American authors L. Stern et al. studied gas hydrates stability and decomposition which provided for another mechanism of gas hydrates preservation at negative temperatures (Celsius).29-34

Samples of polycrystalline methane hydrate were produced from fine-dispersed ice (180 to 250 μ particle size) at methane pressure.35,36 Grains of ice were loaded into a container at temperature of about 250 and saturated with methane under 27 MPa pressure, then the unit was slowly heated to about 290 K. The start-up of hydrate formation was observed at the melting point of ice, and the process ended at the temperature of about 290 and pressure of about 30 MPa, which was determined using the changes in the /T-curve slope. Usually, the process was about 7 - 8 hours long. Hydrate samples of 2.54 cm in diameter and 9.3 cm long had average porosity of 29 ± 1% and weighted about 30 g. Most of them were investigated just after formation in order to avoid structural and compositional changes. Dissociation rates were measured for more than 70 sampies. Hydrate decomposition kinetics was studied using two different techniques.30,31 The first one implied heating of pre-cooled samples (to the temperatures below 193 at equilibrium conditions) to 273 at the rate of 8 K/hr at barometric pressure. The second one - sharp samples depressurisation (first, from 30 MPa to pressures slightly above equilibrium, and then to barometric pressure within 6-10 sec) under temperatures below 273 K. At the end of each experiment the temperature was raised to positive (Celsius), to provide for hydrates full decomposition. Produced gas was then measured using a laboratory gas meter. On average, every 30 g sample produced about 6 litres of gas (at standard conditions). Sample chips were studied using SEM technique. Hydrate samples were pre-cooled with liquid nitrogen down to temperatures below 112 K, degassed, and then split under vacuum to obtain a fresh surface.

Three temperature ranges of metastable methane hydrate formation were singled out.29,31 At temperatures between 195 and 240 K, hydrate dissociation rate was found to grow constantly with temperature rise; in the 240 - 271 range, dissociation rate versus temperature relationship appeared to be of irregular character. At temperatures below 240 K, over 90% of methane hydrate would decompose. Abnormally low dissociation rates were observed within the 242 - 271 temperature range (see Figure 433). Minimum dissociation rate was observed at 268 ± 1 K. In all cases, less than 20% of hydrate decomposes, while the remaining 80% survived for at least 80 hours after depressurisation. In several experiments, about 40% of methane hydrate decomposed within 160 hours, though in the others - about 50% of hydrate decomposed within 410 hours. Abnormally low methane hydrate dissociation rates within the 242 - 271 temperature range and the effect of about 10% hydrate remaining for the time periods from several hours to several weeks was called by the authors "anomalous preservation" of gas hydrates.33


Figure 5. Time changes in methane hydrate dissociation rate at 268 and various pressures
(MPa):
1 - 0.1; 2 - 1.0; 3 - 2.0

Fine-structure studies of these abnormally-preserved methane hydrate samples showed visible structural changes in hydrate particles which is likely to explain the metastable behaviour of gas hydrates at negative temperatures (Celsius). However, ice formation on hydrate surface was not observed, even in those samples that were kept under 268 for several weeks and decomposed by 50 - 60%.32 Moreover, it is not clear from these publications, why ice could disappear following the hydrates decomposition (probably sublimated?).

Also, two samples of mixed II methane/ethane hydrate were studied. No anomalous preservation was observed: about 96% of hydrate decomposed within 3 minutes after depressurisation.32

Another series of experiments by the same research team33 was focused on pressure versus hydrates decomposition ratio. In these tests, pressure was reduced down to 1 or 2 MPa, then decomposition ratio was measured at 250 - 288 K. At the same time, temperature vs. hydrates decomposition ratio curve at 1 MPa pressure seemed "equidistant" from that obtained at 0.1 MPa using a semi-logarithmic representation tool (see Figure 4). Lower boundary of anomalous preservation was located at 250 K. Under this temperature and pressure of 1.0 MPa methane hydrate dissociation ratio was shown to decrease dramatically - by about an order of two magnitudes. Minimum dissociation ratio was observed at 268 K: over 50% remained stable for over two weeks at 1.0 MPa, and for more than a month at 1 and 2 MPa. The higher the pressure (at T = const), the lower the dissociation ratio was observed (see Figure 5). Also, hydrate porosity was increased by adding quartz sand to the samples (see Figure 6).33

Some authors32,33 believe that thin ice film formation can occur at temperatures below 242 K, though this fact (i.e. ice film) not explain the anomalous preservation (self-preservation) of methane hydrates at temperatures from 242 to melting point of ice. To understand the problem, our American colleagues tried to use data concerning a static film of quasi-liquid water on ice surface which exist above 240-242 K.37 They attributed a very special role to this quasi-liquid (and thermodynamically stable!) water film at ice surface during hydrate particles transformation, which leads (?) to a dense and smooth ice layer on the surface of hydrates as in SEM micro-photographs. To our opinion the authors32,33 overestimated the role of stable quasi-liquid water and its hydrodynamic flowability. They used data on quasi-liquid layer on the ice surface, but not on the hydrate surface though namely hydrate particles are a subject to dissociation. Up to now, there are no experimental data regarding the existence of quasi-liquid water film on gas hydrate surface (at two-phase equilibrium "gas - gas hydrate" when pressure higher than the three-phase equilibrium gas - ice - gas hydrate). Therefore, special research concerning hydrate surface is essential using modern techniques employed by physical chemistry, if such surface is being transformed and becomes "dense and smooth."


Figure 6. Volumes of gaseous methane from the samples versus time (at 268 K and 0.1 MPa)
depending on sample porosity:
1 - 32% porosity ; 2 - 47% porosity; 3 - 50% quartz sand
(layered); 4 - 50% quartz sand (mix);

In our view, the interpretation of experimental data32,33 should be involved the idea of thin metastable water film formation on hydrate surface for a short time under sharp depressurisation. Even more, the supercooled metastable water can really exist at temperatures above 240 - 242 K. In particular, it can be detected by measurements using dew point hygrometers.38 Also, several Russian papers dated back to the 1970s mentioned that hydrate decomposition occurs through the stage of liquid water phase formation.39 Not so long ago, it was observed that propane hydrate and supercooled water can really coexist without water crystallisation into ice at temperature of about 271 K.40

Analysing their data, researchers33 proposed that anomalous preservation is specific only to methane hydrates, since no abnormalities in temperature versus dissociation rate relationship were observed in the tests using C02 hydrates, and in experiments with mixed CH4/C2H6 hydrates no anomalous preservation was ever discovered.

The complex nature of methane hydrates temperature - dissociation rate relationship in the anomalous preservation area, combined with the lack of data on mixed methane/ethane hydrates behaviour, and the absence of visible thin ice film on the surface of hydrate particles in SEM photos - all these are indicative of various mechanisms of gas hydrates decomposition and preservation which could occur, depending on temperature, pressure, process initialisation conditions, and gas composition.

The paper by V.P. Melnikov, A.N. Nes-terov and A.M. Reshetnikov*i provide new data for methane hydrates, which qualitatively coincide with data produced by other authors, including those obtained for the use of surfactants while gas hydrates formation. However, complex temperature - dissociation rate relationship was not proved, since no curve like the one shown in Figure 4 was obtained. Also, the authors showed the data on propane hydrate dissociation at 268 and atmospheric pressure: 70-to-90% of hydrate would decompose within 150-200 minutes, then the dis¬sociation rate drops to zero, i.e. propane hydrates show smaller kinetic stability compared with methane hydrates. Therefore, the authors41 proposed that gas hydrates dissociation at barometric pressure depends on their crystalline structure: hydrates of cubic structure II (CS II) show less self-preservation activity then CS I hydrates. But during our recent discussion the details of decomposition process for small propane hydrate particles with one of authors (Dr. Nesterov) it was established that the part of particles may be fully decomposed into gas and supercooled water but another particles can be self-preserved. So in reality we observed a new aspect of self-preservation effect: stochastic (probability) behaviour. This stochastic behaviour may be possible when small driving force take place during decomposition process and supercooled water film may exist for a long time. The main conclusion of the above discussion is following: self-preservation effect also may exist for carbon dioxide, ethane, propane and i-butane hydrates (and their mixtures) like methane hydrate but self-preservation effect needs in appropriate conditions. No real contradict between different authors concerning self-preservation or not self-reservation. Strictly speaking, for the same hydrate sample at one conditions self preservation may exist and at another condition may not exit but at intermediate ranges of thermodynamic parameters the stochastic behaviour may occur.


Figure 7. Computed data for three-phase equilibrium of methane - water -hydrate ! I system
at temperatures below 272.95 K:
1 - methane - hexagonal ice - hydrate CS I; 2 - methane -
cubic ice - hydrate CS I (preliminary estimation); 3 - methane - supercooled water -
hydrate CS I

A German research team led by Prof. W. Kuhs42,43 made a detailed study of anomalous preservation of CH4 and C02 hydrates, and structural changes in ice at low temperatures, using neutron diffraction analysis and SEM tools. They have confirmed the American data concerning the lower boundary for methane hydrate self-preservation (240 K), and its relatively fast dissociation below this temperature. They also noted that below 240 decomposition of hydrate leads to thin cubic ice film formation, which starts to re-crystallise into a hexagonal ice form. Thus, it was proved experimentally that cubic ice transforms into hexagonal one at temperatures of about 240 K, i.e. at much higher temperatures than it had been obtained before.44 The SEM data showed that cubic ice, which emerged on the surface of hydrate particles at the initial stages of its decomposition below 240 K, has multiple crystalline flaws and, most probably, could not effectively prevent methane diffusion from the hydrate. Therefore, this temperature zone does not include an abnormally slow decomposition stage. On the contrary, hexagonal ice has no crystalline imperfections and, as it was shown by X-ray diffraction, appears to be virtually impenetrable for gas molecules.42,43

Data produced by the Prof. Kuhs team well agree with the above mentioned thermodynamic consideration: below temperatures in the 238 - 240 range, no supercooled water can be formed, but at the early stage of hydrate decomposition, metastable forms of ice could emerge on its surface (see below).

It could be also interesting to note that in W. Kuhs' experiments with C02 hydrates it was shown that their behaviour is largely different from that of CH4 hydrates, though their dissociation rates at negative temperatures (Celsius) were also very low. Moreover, under the same pressures and temperatures, i.e. at 260 and barometric pressure, C02 hydrate dissociation rate appeared to be lower than that of CH4 hydrate. The authors explained such variation in self-preservation mechanisms by different micro-structure of ice formed while hydrates decomposition: the ice formed while CH4 hydrate dissociation had more crystalline flaws and cracks than that formed while C02 hydrate dissociation.42,43

To produce a more detailed picture of the initial stage of methane hydrate decomposition we performed thermodynamic computations for three-phase (stable and metastable) equilibrium of methane hydrate at temperature below the 272.95 (quadruple point) using the in-house software.45

These computations were performed for CS I methane hydrate and helped us determine the probability zones for supercooled water and cubic ice formation (see Figure 7).

Now it would be viable to discuss the available estimates, assuming that hydrate decomposition temperature point is 263.15 K, and point A corresponds to the initial state of the methane - hydrate system (two-phase state). Pressure reduction down to 0.1 MPa (point D) leads to formation of supercooled water on the surface of the hydrate particle at the initial stage of its decomposition, which will form an ice film after some period of time. The induction time for transition of water film to ice film depends on many factors, but it should be noted again, that gas hydrate crystal can never be considered as a good matrix for ice film formation from supercooled water. Reducing the pressure down to 1.0 - 1.8 MPa under the same ambient conditions (point I) makes it practically impossible for the supercooled water (as a metastable phase) to exist on gas hydrate surface. Assuming theoretically, that the supercooled water does occur, it should very fast (i.e. without induction period) re-crystallise into hydrate phase, but an another composition. This implies that formation of supercooled water is unlikely. However, in the same case, the equilibrium thermodynamics does not contradict with the idea of amorphous or cubic ice formation as metastable phases. If we reduce the pressure to the value corresponding to point B, hexagonal ice should be formed on the hydrate surface, thereby omitting all possible intermediate phases.

As for the temperature zone below 240 - 242 K, we can conclude that lowering the pressure down to 0.1 MPa would lead to forming of surface ice phases on the hydrate surface rather than supercooled water (according to Figure 7, methane pressure should be set below barometric). Also, at temperatures below ~ 240 K, the liquid water is not exist as metastable phase at all, since no induction period for its bulk crystallisation to hexagonal ice. Thus, according to the basics of thermodynamics (and not going into details of hydrate decomposition), we can assume that lower methane pressure down to barometric at about 240 would lead to a changed mechanism of methane hydrate surface decomposition. The same approach to other gas hydrates gives us different temperature ranges for possible occurrence of the self-preservation phenomenon.

Therefore, several interesting conclusions could be made. For example, we can change the mechanism of methane hydrate surface decomposition at temperatures higher than 240 (i.e., at 253 K, pressure should be reduced only to 1.0-1.8 MPa to prevent supercooled water formation). In this case we imply that the self-preservation phenomenon studies can be split into stages: initially, lowering pressure down to 1.0 - 1.8 MPa, then the system is kept at this pressure for some time, and then the pressure is reduced to a barometric level, and decomposition kinetics of the sample is formed at this pressure which could be studied. This is likely the self-preservation phenomenon has deteriorated (in comparison to pressure reducing up to barometric level into one stage). Note that in the previously described tests the American team studied methane hydrate decomposition kinetics under some overpressure, which was never reduced to barometric levels; this implies that they never compared their samples at the same ambient conditions, i.e. under the similar decomposition drivers.

Therefore, the use of specific techniques (i.e. stepwise pressure reduction) can help constrain the boundaries of abnormally slow methane hydrate decomposition and provide for an in-depth study of cubic and/or amorphous ice formation on the dissociated hydrate surface. Now we can formulate the problem of controlling the process of gas hydrate preservation. Several technologies could be made available to ensure optimum self-preservation (or forced preservation) of gas hydrates, including the "poorly-preserved" hydrates of easy-lique-fiable gases (ethane, propane, carbon dioxide, and their mixes with methane). At present, Moscow Gas Hydrates Group is engaged in the appropriate R&D efforts.

STUDIES OF GAS HYDRATES SELF-PRESERVATION IN DISPERSED SOILS

in 1980s, Moscow Gas Hydrates Group researchers embarked on studies of gas hydrates formation in dispersed soils. At the same time, initial experimental data on methane hydrates meta-stability in the porous media (sandy soils), at barometric pressure and temperatures below 273 K, were obtained.6,9 The research implied the use of artificially hydrate-saturated sand samples which were frozen and, following their depressurising to barometric pressure, stored at T < 273 K. Partial pore hydrate decomposition was observed. For example, frozen hydrate-saturated sand samples stored in kerosene in the sealed vessel for over 2 months, under temperature below 273 K, lost only 12% of their gas. Later, the discovery of the gas hydrates self-preservation phenomenon in frozen soils helped the researchers to apply the techniques developed for frozen ground to gas hydrate-containing soils as well.46

It should be pointed out that recent experimental data on gas hydrates decomposition, within the pore volume of dispersed soils at positive temperatures (Celsius), published by prof. Yu.F. Makogon,47 show that in some cases higher temperatures and lower pressures appear to be necessary to dissociate porous hydrates, compared to those for bulk gas hydrates. This effect (which, for unknown reason, was named as "self-preservation phenomenon") was described only briefly. Therefore, additional studies are needed (in any case it is unclear how this experimental fact could be related to gas hydrates self-preservation phenomenon).

Incomplete decomposition of hydrate-containing soils, while freezing, has been frequently observed by several researchers when recovering hydrate-containing core samples. Though, so far, specific features of gas hydrates self-preservation in frozen soils and the influence of various factors on their stability remain virtually unexplored. For this reason the geocryology laboratory (Geological Department of MGU University) has recently resumed experimental studies of frozen hydrate-containing soils.48,49

Models with disrupted structure of soil were used in the experiments, i.e. quartz sand and sand/clay mixtures (7% of kaolinite or montmorillonite clay). The samples were filled with gas hydrates in the pressure chamber under methane overpressure and 273.5 - 275 temperatures. Hydrate-filled samples were refrigerated down to 265 K. Residual pore water, not used for hydrate formation, was frozen. Hydrate content and hydrate factor (percent of water transformed into the hydrate) were estimated using gas law equation for methane.

Then, the pressure in the chamber was reduced to barometric one, and after 30 minutes the sample was thoroughly studied using the available petrophysical techniques, at 265 K. The remaining piece of the sample was stored in weighing cup at 265 for future studies. To avoid sublimation the samples were covered with broken ice. Later, over specified time intervals, the samples from the stored pieces were recovered to monitor changes in their gas and gas hydrate saturation. Such sampling was continued until the complete hydrate decomposition was achieved.

Over the first few hours after depressurisation methane hydrates decomposition in the frozen soil samples was rather intensive, though with time the dissociation rate was noted to be gradually smaller and eventually to terminate (see Figure 8).

Gas hydrates self-preservation factor, Ksp, was estimated as the ratio between residual (after decomposition slows down) volumetric hydrate content, Hvsp, (at non-equilibrium conditions) and volumetric hydrate content, Hvin, (at equilibrium conditions):

The lowest methane hydrate self-preservation factor was found for kaolinite clay, which is related to the worse admission of ice into the pore volume before depressurising. Also, in this sample, gas hydrates are more dispersed and, thus, less stable against temperature and pressure changes.

Qualitative studies of metastability of frozen gas hydrate containing soils showed that methane hydrate self-preservation depends on several parameters and characteristics of frozen soil, as well as temperature conditions.

HYDRATE TECHNOLOGIES FOR NATURAL GAS STORAGE AND TRANSPORTATION

Some reseats from the Norwegian University of Science and Technology proposed two gas hydrate technologies for natural gas storage and transportation, based on the self-preservation phenomenon at barometric pressure and temperatures ranging 258 - 263 K.50-53


Figure 8. Kinetics of methane hydrate dissociation
in frozen hydrate-saturated sand sample
(Winit = 17%) containing 7% of montmorillonite
clay particles

The first technology was named "gas-in-ice" and implies the following: hydrocarbon gas is injected into the water circulated in the cooled mixer which has the following operating conditions: 5 MPa pressure under 283 temperature. The water/ice mixture is also used for cooling down the unit. Gas/water mix could pass through one-to-three mixing units to improve the hydrate content. After the third unit it is delivered into a separator. Since densities of water and gas hydrate are very close and their separation is a serious technological challenge, the authors proposed the idea of using condensate instead of hydrocarbon gas. After separation, the produced hydrate goes to the dryer, then to the freezer where it is depressurised, and its temperature is brought down to 258 K. Then gas hydrate goes to palletising or graining unit (depending on the method of its storage and tanker downloading). According to a feasibility study by J.-S. Gundmundssonet al.,52 this technology of natural gas storage and transportation in gas hydrate state, is believed to be 25% cheaper than the currently used LNG technology.

The second technology is focused on pipeline transmission of associated petroleum gas as a mixture of self-preserved gas hydrate with crude oil.52,53 This mixture is prepared in special units based on offshore platforms and is pumped (at negative temperatures) to shuttle tankers which deliver it to onshore storage facilities followed by downstream processing plants. According to J.-S. Gundmundsson et al.ss this technology will be likely the most cost effective when transporting the mix to reception points over 200 km away.

Also, a brand-new technology for underground gas hydrate storage in proximity of major gas consumers is proposed.54 This technology implies the use of pipelined methane to produce large frozen hydrate monolithic blocks at self-preservation conditions, combined with their storage in the sealed underground storage sites with temperature controlled in the -5 ... -20oC range. Methane hydrate decomposition is effected by electric heaters. Free gas is then directed to low-pressure distribution pipelines. These types of underground storage facilities can be also built in permafrost areas where no artificial freezing is required. For the moment, such technologies are under development and improvement in Japan.55-59

CONCLUSIONS

The self-preservation phenomenon enables gas hydrates to exist in metastable condition for long periods of time, at atmospheric pressure and temperatures below 271 K. At present, it has been laboratory-tested that the most likely mechanism of gas hydrates self-preservation would imply thin ice film formation on the surface of hydrate particles which develops after the stage of their initial decomposition. Gas hydrates stability after self-preservation depends on several factors (microstructure, temperature, pressure, sublimation, etc.) and can be controlled by changing ambient and storage conditions. Additional research in this area is believed essential to better understand the details of gas hydrates dissociation and self-preservation mechanisms (for instance, a Dr. Delosludov's research team recently proposed a new low temperature thermo-mechanical model for stable co-existence of the methane hydrate - ice system).

When using the idea of gas hydrates self-preservation phenomenon, we can better understand their potential presence in natural conditions. Gas hydrates can occur in their metastable form in cometary nuclei and gas-formed planets. In permafrost they can be found as the so-called "relic gas hydrates" above the modern hydrate stability zone, where it earlier was believed impossible. Gas hydrate accumulations in the permafrost environment can cause incidents while drilling and production operations in the northern fields. In addition, shallow gas hydrate pools can be used for additional improvement of available E&P gas-from-hydrate technologies. Due to self-preservation phenomenon we now can adapt standard geocryological laboratory techniques to study hydrate-saturated soils.

The gas hydrates self-preservation phenomenon opens up new prospects for using hydrate technologies in the oil and gas industry. At present, several new technologies for natural and associated gas storage and transportation in the form of gas hydrates at barometric pressure and temperatures of 253 - 268 are under development. The opportunities for storing large amounts of gas (up to 160 m3 of gas in 1 m3 of hydrate) in the hydrate form are believed to cut natural gas storage and transportation costs.

This study was supported by INTAS Grant (Project No. 03-51-4259).


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