Most of natural gases (CH4, C2 H6, C3H8, CO2, N2, H2S, etc.) are able to form the gas hydrates stable under specific thermobaric conditions. They are contained in the sea bottom sediments and in the areas of permafrost. Methane hydrates and carbon dioxide hydrates are the most widespread. The methane hydrates are treated as a perspective source of unconventional gas.
A unique feature of methane hydrate is that one volume of water links to 207 volumes of methane by hydrogen bonds. Moreover, the specific volume of gas increases by 26%.
The necessary factors for forming the gas hydrates include water, gas, low temperature and certain pressure The stability of gas hydrate deposits depends on the intensity of the generation, migration, diffusion scattering of gases and the dynamics of temperature. The gas of hydrate deposits partially or entirely exists in the solid hydrated form.
There are two main types of has hydrates deposits, namely the primary and the secondary gas hydrate deposits. The primary gas hydrate type is formed in the bottom sediments, which are characterized by high porosity, low temperature and low strength of the surrounding rocks. Often primary deposits have no lithological seals and the hydrate becomes the impermeable horizon under which the hydrate accumulation is originated. In the offshore area the primary hydrate zone starts at the bottom of the ocean and reaches the depth of few hundred meters. Submarine deposits are confined mostly to the shelf and ocean slope at the depths of 200 to 700 m.
Secondary gas hydrate deposits are usually located onshore. They are formed from the accumulations of the free gas below the lithological seals under the bound of temperature equilibrium in permafrost areas. These gas hydrate deposits can reach thickness of 400-1000 m.
Near 97% of all gas hydrates are formed in oceans. Within the polar waters, Atlantic, Indian and Pacific oceans more than 230 deposits of gas hydrate were identified. Now U.S., Japan and Russia are mostly involved in the development of gas hydrate deposits projects. The most perspective zones for finding the gas hydrates are located at the depths of 400-1600 m. The minimum reserves are estimated of 50-60 trillion cubic meters of gas. The favorable conditions of gas hydrate forming ranges from 170 m to 480 m.
To extract the gas from the gas hydrate deposit it is necessary to transform it from solid to gaseous form and then extract using traditional technologies. The possible methods of methane hydrates degasation include the decreasing of the reservoir pressure below the equilibrium, increasing the temperature above the equilibrium and introduction of catalysts into the rock.
