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№ 1 (January 2012)

Properties of Floating Layers of Natural Gas Hydrates

   In 1961, a new discovery [1, 2] led to an announcement of a new source of hydrocarbons – gas hydrates (GH) which represent chemical compounds of hydrocarbon gases and water.

By A.Ya.Khavkin, L.S.Tabakaeva

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   Methane hydrates are a huge energy source. Gas quantities in GH deposits of Earth are (16,000-14,000)·1012 cubic meters [3, 4], and humans will be able to use gas from GH for over a thousand years. Methane liberation from GH requires about 15 times less energy than thermal energy contained in methane itself [4], and 1 cubic meters  of GH contains 160 cubic meters  of methane and 850 liters of water [5]. Methane hydrate density is 913 kg/cu.m; ethane hydrate density is 967 kg/cu.m and propane hydrate density is 899 kg/cu.m [3-6]. Gas hydrates are referred to nonstoichiometric compounds i.e. compounds with variable compositions [7-9].

   Russia’s resources of GH in quantities exceeding 100·1012 cubic  meters are concentrated in the West Siberia and offshore, and Russia’s gas reserves in GH is 48·1012 cubic  meters according to international estimations. Production, transportation, and processing of GH are quite complicated processes, but with annual gas production of 700·109 cubic  meters, estimated gas reserves in natural gas fields will be sufficient for 70 years, and development of GH resources will cover Russia’s demand in natural gas for another hundred years. Therefore, development of GH deposits is an important scientific and technical problem [1-9].

   Methane is a solid hydrated form at atmospheric pressure and at a temperature below –29 С. At pressures of 70-80 MPa, natural gas hydrates exist up to temperatures of +(20÷25) С [6]. At a temperature above the critical one for water (374 С), GH formation cannot occur [10]. Since an average geothermal gradient of Earth is about 3 С for 100 meters, then at depths over 12 kilometers, the temperature can exceed the critical one, making the formation of GH impossible. A temperature of reservoirs even in permafrost zones does not drop below 0С, and at a temperature over 30 С, a pressure exceeding 80 MPa is required for GH stability (Fig. 1 [11]). Therefore, commercial GH accumulations are likely located within a temperature range of 0-30 С in case of gas-tight caps available.

   Found conditions for methane GH formation and stability made it possible to forecast potential GH deposition zones onshore, at a depth of 200-1,100 meters and temperatures ranging from –10 С to +15 С, and in bottom layers of water bodies, at depths of 1,200-1,500 meters and temperatures of +(0÷17) С. First confirmations for these forecasts were obtained in 1969. Such deposits were discovered in north areas of West Siberia, in the Far East, Alaska, and Canada, as well as offshore in many countries [1-9]. Based on the forecast made on geothermal data, GH depositions were discovered in a freshwater body, when drilling in the south depression of Baikal Lake, at a depth of 1,433 meters [12, 13].

   Being a chemical compound, methane is described by small sizes in relation to formation pores; this allows its free migration to the Earth’s surface and, without accumulating in the air, it is transported to the stratosphere where it reacts with monatomic oxygen, with a formation of carbon dioxide and water. A natural barrier for young gas streams and foremost methane, are anticline deposits of argillaceous materials, salt domes, and permafrost zones that serve as traps during methane migration. All discovered oil and gas reserves onshore and offshore are in these natural traps [14].

   One conventional theory of GH formation considers a mechanistic approach, when a methane molecule gets in a water crystal cavity and remains there (since it is impossible to get out), with formation of inclusion compounds that are referred to as clathrates. Based on such GH concept, there is a long discussion about such methods of gas production from GH deposits as temperature rise or pressure reduction in such deposits [2-9].

   If for GH formation, water molecules hosting a methane molecule must preliminarily freeze (crystallize), then at a positive temperature, methane GH molecules cannot be formed, since ice cannot be formed preliminarily. Alongside with that, there are known facts of GH existing at positive temperatures, for example, in gas pipelines at a temperature above the water freezing temperature [1, 6, 7].

   The above makes it possible to state [15] that GHs are formed without preliminary water crystallization. A СН4 molecule is electrically neutral, since inside a regular tetrahedral pyramid, it has an increased electron charge and four hydrogen ions compensate for this charge, but due to non-uniform electron density, a dipolar moment is formed between molecule atoms. Interplanar spacing of three hydrogen atoms in a methane tetragonal molecule exceeds 0.22 nanometers, which makes it possible for a proton with sizes less than 0.05 nanometers to penetrate this tetrahedral cavity, and results in formation of a metastable methanium ion СН5+ [16] that can exist only in presence of the liquid water phase due to dissociation. During subsequent hydration, the methanium ion forms methane GH that is a metastable molecular compound of СН4·nН2О type where n can exceed 3 [10].

   A physical-and-chemical mechanism of applying many chemicals to prevent GH formation consists in donor capacities of functional amino-groups of an inhibitor molecule. Thus, based on the Lewis theory [10], a conclusion can be made that alkali solutions are one of the most effective GH inhibitors. Researches confirmed that alkali solutions are really the most effective inhibitors of GH formation from industrially applied chemicals [15], which is proved by the proposed donor-acceptor mechanism of GH formation and decomposition.

   Researches carried out in 1997-2000 in the area of the Selenga River delta front established that along the depression of the Baikal rift zone, gas liberation occurs in fault and fracture systems over the whole area, with methane being a basic component of the liberating gas. Gas seep flowrates were measured subject to a number of springs and a total area of thaw holes. The measurement results made it possible to estimate cumulative quantity of methane liberating into the air within the Selenga River delta front at 20 million cubic meters per year [17].

   In water environments, temperature at depths do not drop below 3-4 С, because the water density at 4С is maximum [10]. Hydrate formation in water environments with initial ice formation could hardly be assumed, but the “methanium” mechanism of GH [15] formation allows assuming that floating layers (suspension) of gas hydrates (suspended gas hydrates, SGH) do exist in water environments including Baikal Lake. This is connected with the fact that some GHs, as mentioned above, have a density close to water density, and inflow of natural gas from geological structures ensures a balance of natural gas inflow for SGH formation and gas outflow due to SGH decomposition.

   Baikal Lake temperature at a depth of 100 meters is 3-4 С and in the bottom layers is 3.1 С [18]. Based on the equilibrium graph of methane GH formation [11] (Fig. 1), one can conclude that the upper boundary of methane GH formation in Baikal is at a depth of 380-400 meters (Fig. 2a). Thus, at the bottom methane liberating in Baikal is first converted into hydrate and then, at a depth of 400 meters, is again converted into a gas.

   Forming GH with a density close to that of water will generate floating layers in a water environment (Fig. 2b), and GH with density exceeding water density (for example, when GHs are formed from a gas mixture [11]) will gravitate to the bottom. So, H2S hydrate has a density of 1,046 kg/cu.m, and СО2 hydrate, 1,107 kg/cu.m. Due to convective heat transfer with rocks, the water temperature directly in the bottom zone can be elevated and the water perhaps will not contain GH floating layers, near the bottom.

   The Mining Institute of the North, the Siberian Branch of the Russian Academy of Sciences (RAS SB MIN) carried out experimental studies of equilibrium conditions for GH formation subject to water salinity; their results demonstrated that pressure of GH formation in saline water is 2.5 MPa higher [6]. Therefore, it can be expected that GH floating layers (accumulations) exist only at depths exceeding 500 meters. SGHs are a significant localized source of methane all over the world.

   It is especially important to take into account the existing floating layers while interpretation of marine geophysics methods.   

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