gas hydrates of siberia

101

Gas Hydrates of Siberia

F.A. KUZNETSOV

a

Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences, Lavrentiev Avenue 3, 630090 Novosibirsk, Russia

A

BSTRACT

: Natural gas hydrates were first observed on 21 July 1961 (the dis-covery was officially registered on 24 December 1969).

1

It soon became clearthat these compounds are not just chemical curiosities, or a nuisance for gastransportation in pipe lines, but rather that they are related significant naturalphenomena. It is possible that in the past natural gas hydrates have manifestedthemselves in marine and land incidents. In future gas hydrates may offer aninexhaustible source of energy and chemical raw material. However, if notaccessed with sufficient knowledge, gas hydrates may be destabilized to createmany local problems and possibly global problems as well. In the USSR andmore recently in Russia, a number of groups are traditionally involved ininvestigation of different aspects of gas hydrates problems. These groups havemade significant contributions to present knowledge about the nature andproperties of the gas hydrates. The Siberian branch of the Russian Academyof Sciences recently started an interdisciplinary program to comprehensivelyinvestigate all major aspects of gas hydrates, with special emphasis on naturalgas hydrates and gas hydrates deposits in Siberia and its adjacent regions.The program consists of three components: (1) Geology of gas hydrates in thecryolito-zone of Siberia and in the bottom deposits of the Arctic and East seas,and of Lake Baikal. (2) Physicochemical study of hydrates of different gases.(3) Ecological monitoring. In this paper we offer a short summary of the mainresults of these three components of the program. We emphasize the secondcomponent, since it is close to the scientific interests of the author.

GEOLOGY OF GAS HYDRATES IN CRYOLITO-ZONE OF SIBERIA,IN BOTTOM DEPOSITS OF ARCTIC SEAS AND OF LAKE BAIKAL

Gas-hydrate stability zones have been outlined (see F

IGURE

1) based on analysisof data collected to date. The data include geothermal conditions, pressure, mineral-ization of underground waters, and presence of gas (required for formation ofhydrates). The scheme outlines the most favorable areas for the search for gashydrate accumulations that formed from free gas that existed in these areas prior torock and sediment freezing, or from gas that migrated to the hydrate-formationzone from lower layers. The analysis shows that early conclusions need to bereconsidered.

More detailed mapping of the gas-hydrate stability zones (GHSZ) in NorthernSiberian regions was performed with use of computer technology. The aim was todetermine the upper and lower boundaries of the zone. F

IGURE

2 shows the results

a

Telecommunication. Voice: (7 3832) 34 44 88; fax (7 3832) 34 44 [email protected]

102 ANNALS NEW YORK ACADEMY OF SCIENCES

FIGURE 1. Map of GHSZ in boundaries of oil-gas fields in Northern Siberia. 1,boundaries of oil-gas fields; 2, isolines of thickness of the zones, with possible presence ofgas hydrates; area with different thickness of the zones in meters; 3, up to 300; 4, 300–600;5, more then 1000; 7, absence of hydrates.

FIGURE 2. Estimated thickness of gas-hydrate stability zones.

103KUZNETSOV: GAS HYDRATES OF SIBERIA

FIGURE 3. GHSZ in Okhotsk Sea. ΓΓ indicates point of direct visual observationsof hydrates in bottom probes. Thickness of hydrates deposits in meters: 1, less then 100;2, 100–200; 3, 200–300; 4, 300-400; 5, areas with no hydrates.


from these calculations. The upper boundary of the GHSZ over almost all Siberia isat the depth of 100 meters. The depth of the lower boundary varies. In West Siberiait occurs at depths around 1,000 meters; in East Siberia, which is the coolest part ofthe region, the lower boundary is down to 2,000 meters. These data demonstrate agreater extent of the GHSZ then was previously accepted.

New data about the concentration of helium in natural gas has been obtained forthe Myasoyakh gas field. An increase in He concentration is attributed to the forma-tion of hydrates because the formation of gas hydrate would, in effect, scavengemethane and other hydrocarbon gases, leaving helium. It follows from variation inHe concentration that the central part of the Myasoyuakh gas field most probablydoes not contain hydrates, whereas peripheral areas (characterized by higher con-centration of He) may contain hydrate accumulations.

F

IGURE

3 shows GHSZ at the bottom of Okhotsk Sea Derived from work of Prof.G. Ginsburg.

2,3

Finally, concerning hydrates in Lake Baikal bottom deposits; for anumber of years a team from the South Branch of Institute of Oceonology RAS hasperformed seismic investigation of the Lake Baikal bottom.

4

As a result of thisinvestigation a map of positions of lower boundary of gas hydrates was constructed(see F

IGURE

4).In March 1997 international expedition working on the

Baikal-drilling

programtook a bottom probe in the center of the depression south of the Lake Baikal.

5

Onheating this probe, an intensive evolution of gas from the sample was observed. Vol-umetric measurements and chemical and X-ray analysis proved the presence ofmethane hydrate in the sample. On the basis of isotope analysis, it was concludedthat methane in the hydrate had a biogenic origin.

FIGURE 4. Map of the lower boundary for gas-hydrate deposits in Lake Baikalobtained from seismic data (depths of BSR). 1, position of the lower boundary in meters;2, year and number of the profile.


PHYSICO-CHEMICAL STUDY OF HYDRATES OF DIFFERENT GASES

Phase Equilibrium in Systems with Gas Hydrate Formation

We report on new results from a program of systematic investigations of clathratecompounds (including clathrate hydrates), obtained by a team headed by Prof. Yu.A. Dyadin.

All the hydrophobic gases can be divided into two groups relative to formationof clathrate hydrates.

6,7

The first group includes three gases: hydrogen, helium, andneon with molecular diameter less then 3.5Å. For these molecules classical clathratehydrates are not known. However, due to the small size of the molecules they can beincluded within small voids in ice-Ih, -Ic, and -II (only these modifications of icehave voids within which such small molecules can be included, with formation ofsolid solutions or clathrate hydrates

8–11

).The second group includes gases with molecules that are 3.8–9.2 Å in size. These

gases do not dissolve in ice, but individually or in mixtures they can form classicalhydrates: cubic-I and -II (CS-I and CS-II), hexagonal-III (HS-III or “Structure H”).It is one example of tetragonal structure (Br

2

·8·6H

2

O) formation.Our recent investigations show that argon and krypton at sufficiently high pres-

sure can form hydrates based on ice-II (analagous to hydrogen or neon). Experimen-tal study of the phase equilibrium under pressures up to 15 kbar was performed usingthe technique described previously.

14

Results for noble-gas hydrate-equilibrium are shown in F

IGURE

5.

12–15

For thecase of heavy noble gases, it can clearly be seen that decomposition curves of clas-sical hydrate structures have a dome-like form. Stability of the hydrates decreaseswith decreasing guest molecule size. The low pressure section of the diagrams forhydrogen and neon shows the same feature (dome-like shape). This indicates, that inthis pressure interval, hydrogen and neon form hydrates in which a pair of moleculesoccupies large voids. For the case of nitrogen, inclusion of two molecules in the largevoids of CS-I and CS-II structures was reported by Kuhs and others.

16

The same pro-cess apparently operates for even smaller hydrogen molecules. This conclusion isalso supported by the experimental observation that hydrogen and neon establishesequilibrium much slower then other gases for which the voids are occupied by onlyone guest molecule.

As it can be seen from F

IGURE

5, to the right of the dome-like section, the higherpressure parts of the curves show that decomposition temperature sharply increaseswith pressure (in all the systems except xenon). It was shown, by using X-raydiffraction, that for hydrogen systems this part of the diagram corresponds to decom-position of a hydrate phase based on the ice-II crystal lattice. The similar characterof the high pressure part of the curves for other systems leads to the conclusionthat the hydrates based on the of ice-II crystal lattice are also formed in systemswith neon, argon and krypton at pressures of 4 kbars, 9.6 kbars, and 13.4 kbars,respectively.

Thus, we conclude that in systems with neon, hydrogen, argon, krypton, andxenon classical clathrate hydrates are formed. Their stability decreases in directionfrom xenon to neon. For all the gases, except xenon, at high pressure hydrates basedon the ice-II crystal lattice are formed. The pressure required for formation ofthis phase increases with increasing molecule size. For the case of xenon, the gas


molecule is obviously too large for inclusion in voids of the ice-II crystal lattice andhydrate CS-I is stable at least for pressures up to 15 kbar.

The same experimental technique was used to investigate of phase diagrams ofthe binary systems methane–water and propane–water, and a section of the ternarysystem C

3

H

8

-CH

4

-H

2

O. The results from this study are shown in F

IGURE

6.In the system with methane (described previously

17

) two hydrate phases areformed: at pressures less than 6.2 kbars, hydrate with CS-I structure is stable. Athigher pressure another hydrate phase is formed having higher density. The structureof this phase is not known yet. In the water-propane system three different hydratephases are formed. At low pressure a hydrate with CS-II structure forms. Stability ofthis phase decreases with pressure. At a pressure of 1.45 kbars this structure is trans-formed into hydrate with higher density. Stability of this phase increases with pres-sure. At 6.45 kbars and 31.5

°

C a new hydrate phase with even higher density isformed.

In the propane system nothing but clathrate structure can be expected. Thus, tak-ing into consideration what is know about the water clathrate structure, we proposethe following sequence of transformations in this system at elevated pressures:

FIGURE 5. Decomposition of of noble gas and hydrogen hydrates. The thin lines showfield of crystallization of different modifications of ice.


Hydrate CS-II

→

Hydrate TS-I

→

Hydrate CS-I

where TS-I is tetragonal structure I.In methane–propane mixtures, even at low concentrations of propane hydrate,

CS-II with composition C

3

H

8

·CH

4

·17H

2

O is formed.

18

As can be seen fromF

IGURE

6 there are no phase transitions in the system within the range of pressurevalues used (up to 15 kbars). This results from the fact that the structure is wellpacked from the beginning and thus phase transformation is not required for addi-tional densification.

Physical Aspects of Structure and Interactions in Gas Hydrates

The theoretical part of the project aims at the construction of a quantitative theoryof clathrate compounds. Work in this direction was started by van der Waals andPlatteuw

18

and continues to attract the attention of researchers.

19–21

Problems, thatwe try to solve within the framework of the project, are related to microscopic mech-anisms of phase transformations in clathrate hydrates and in different modificationsof ice. Using the method of crystal network dynamics, we demonstrate the signifi-cant role played by guest molecules in the stability of hydrate structures. Another

FIGURE 6. Decomposition of hydrates of methane, propane, and binary hydrateC3H8CH4·17H2O l and l2, liquid water and propane phases, respectively; h1, h2, hd, hT, hC,hydrates of cubic I and II, binary hydrate, tetragonal, and cubic I phases, respectively.


problem of interest to us is the relation between various forms of ice and hydrates,and the structure and mechanical instability of these compounds. Despite of the sim-ilarity of short-range order in hydrates and ice the hydrates demonstrate very pecu-liar mechanical and thermodynamic properties. Clathrate hydrate transforms at highpressure into a high-density amorphous phase. The transformation is reversible—atreduced pressures the initial structure returns. This is known as structure memoryeffect.

22

In the case of ice amorphisation under high pressure is irreversible.

23,24

FIGURE 7. Conditions of stability of methane hydrate.


The methods of lattice dynamics permit investigation of the dynamic propertiesof both guest molecules and the network formed by host molecules. Thus, the roleof the guest molecules in the stability of the clathrate structure can be studied. Themethod can also be used to calculate dynamic and thermodynamic properties of theclathrates and to find their boundary for mechanical stability.

Previously, the molecular dynamics approach was used to analyze various prop-erties of the clathrate hydrates:

(a)

The effect of filling of host lattice voids with guest molecules on the stabilityof the structure was investigated and a phase diagram for the H

2

O-Xe system wascalculated.

25,26

(b)

A strong interaction between the acoustic and optical branches, associatedwith motion of the guest molecules, was revealed.

27

(c)

The pressure dependence of thermal expansion and the stability boundariesfor hydrates and different modifications of ice were found.

28,29

New results have now been obtained that support the hypothesis of a relation be-tween amorphization of clathrates with mechanical instability of crystal structures athigh pressures and low temperatures. For methane and xenon hydrates and for anempty network of hydrate with CS-I structure, the elastic modules,

C

ij

(

P

) in Voigh’snotation (

i,j

= 1, 2, …, 6) for

T

= 10 K were calculated. The mechanical stabilityboundaries for the structures were determined from the pressure dependence of mainminors

D

k

(

k

= 2, 3, …, 6) of the matrix

C

ij

(

P

). According to Born criteria all valuesof

D

k

must be positive in the range of stability.

30

It was shown that the stabilityof hydrates strongly depends on presence of a guest molecule. The effect of stabili-zation also significantly depends on the type of proton ordering in the host crystalnetwork.

F

IGURE

7 shows results obtained for pressure dependence of principal minorsfor two different versions of proton ordering for methane hydrate. For the firstversion the hydrate becomes unstable at pressure of 13.0 kbar, for the second versionat

P

= 16.5 kbar. Respective values for xenon hydrate are 16.5 kbar and 24.5 kbar. Infuture we plan to make an X-ray study of the gas hydrates at high pressure. This willpermit us to demonstrate phenomena that have been theoretically predicted.

ECOLOGICAL MONITORING

In this paper we report on our results for modeling methane emission to theatmosphere as a result of marine gas hydrate decomposition caused by global warm-ing. In our model, hydrate deposits were assumed to be located at Arctic andAntarctic coastal lines. It was accepted that decomposition starts when the tempera-ture exceeds the thermodynamic decomposition temperature by 0.1 to 2.0

°

C. At thatmoment, the concentration of methane is assumed to increase to 100 times higherthat of the background concentration of methane in ocean water (50 ppb). Three-dimensional diffusion in the ocean was calculated. Two different warming scenarioswere considered: (a) instantaneous increase of ocean surface temperature by 3

°

C and(b) gradual warming by 0.08

°

C a year. Distribution of methane concentration in timeat different depths was obtained by calculation. An example of these results is shownin F

IGURE

8.


For these two warming scenarios emission of methane was also calculated. Forthe case of 3

°

C instantaneous warming and a decomposition threshold of 0.1

°

C, overa period of four years the total flow of methane into the atmosphere exceeds 1terragram (TG) and reaches 14 TG in 50 years. In the case of gradual warming by0.08

°

C/year the methane flow reaches a value of 1 TG in 17 years and 3.8 TG in 50years. More details of these calculations can be found in.

31–33

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gas hydrates of siberia

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