Recent Research on Properties of Gas Hydrates

Abstract

Abstract Gas hydrates are metastable minerals whose formation, stable existence and decomposition depend on pressure, temperature and composition of the gas and water. Gas hydrates are widespread in nature; they exist in sedimentary rocks on land in sub-Arctic regions and in the cold sediments of the ocean at depths of 200 m or more. Potential gas resources concentrated on earth in the hydrate state exceed 1.5 × 1 0 16m3. Although gas hydrates can be a tremendous energy source, they can also be an expensive problem during the drilling and operation of deep wells and during the operation of oil and gas pipelines. Analysis of research results indicates that the properties of hydrate and hydrate saturated sediments remains one of the most important and least studies parts of the hydrate problem. The properties of natural gas hydrates depend on the conditions of formation, dynamics of pressure, temperature in the sediments in geological time, migration of gas-saturated water, structure and hydrate saturation of sediments, position of hydrate-saturated stratum in the hydrate formation zone, supercooling d hydrates, and many other factors. Recent research results of the authors into the properties of hydrates are discussed with particular reference to the factors mentioned above. Introduction Deep drilling and operation of oil and gas wells, and operation of oil and gas pipelines in the deep sea are vastly complicated by the presence of natural gas hydrates. Gas hydrate is a metastable mineral. Its formation, stable existence and decomposition depend on pressure, temperature, and composition of the gas and water. Gas hydrates are inclusion compounds, solid solutions, in which water is the solvent. Molecules of water tied together by hydrogen bonds form a space-filling lattice. Mobile molecules of gasses are absorbed or incorporated in the cavities of the water lattice. Gas hydrates are widespread in nature A in sedimentary rock on land in the sub-Arctic regions and in the cold bottom rock of the ocean at depths of 200-500 m or more. Potential gas resources concentrated on earth in the hydrate state exceed 15x1015m3. Explored reserves of methane in the hydrate state are over 700 trillion m3. Several countries have started research aimed at the discovery and production of gas from offshore gas hydrate deposits. For example, Japan discovered 12 gas hydrate deposits around the Japan islands with total reserves of methane estimated at 6 trillion m3. A program of testing production of gas from the hydrate deposits will start in 1999 (Okuda, 1996). Similar programs have started in India, where potential resources in the Bay of Bengal can be over 60 trillion m3, and in the Ukraine (Black Sea opposite the Crimea) where potential resources are over 25 trillion m3. The amount throughout the entire Black Sea bottom is about 75-100 trillion m3 (Smirnov, 1996). On one hand, gas hydrate represents a tremendous amount of mineral energy, but on the other hand, a large expensive problem.