ANALYSES OF GAS PRODUCTION METHODS AND OFFSHORE PRODUCTION TESTS OF NATURAL GAS HYDRATES

Natural gas hydrate, a pollution-free clean energy, is buried in sediments of the shallow geosphere and widely distributed, therefore considered as the most likely potential substitute resource in the oil and gas sector. Field scale exploitation of gas hydrate have been tried of 5 countries including China, especially the successful offshore production tests in Shenhu area, South China Sea, making a record of gas producing time and cumulative production. Nevertheless, there is no mature solution to realize economical and large-scale exploitation because of the special physical properties and occurrence environment of gas hydrate. In this paper, the offshore production tests carried out by China and Japan are introduced and analyzed according to fundamental understanding of gas hydrate. Our results indicated that:(1)the current production methods could be summarized into two types:in situ decomposition and stratum excavation methods. (2)The exploitation data suggested the temperature and pressure are the main constrain conditions for gas production and they works differently in different stages of production. (3)The first exploitation data of Japan showed that there's fine relevance between production rate and pressure/temperature driving force. The decomposition model based on coupled driving force were proposed using a comprehensive parameter named "distance to phase equilibrium curve". (4)According to the judgments of drastic-rapid and phased-slow depressurization methods, we presented that efficient heat injection are possibly one necessary condition for long-term exploitation of gas hydrate. Furthermore, convection heat supply and depressurization method were proposed according to the integration of hydraulic fracturing horizontal well technology in shale exploitation and the heat convection exchange cycle in hot dry rock exploitation.