| It is well-known that natural gas hydrate is a new energy source with great exploitation prospect.If people can accelerate the development of marine gas hydrates development technology and realize the commercial exploitation of marine gas hydrates,these will provide a strong guarantee for China’s energy strategy.At present,the United States,Russia,Japan,and Canada are all accelerating the exploration of commercial exploitation of marine gas hydrates.In 2017,China conducted the first experimental test of natural gas hydrate in the South China Sea and achieved a completely success.However,the commercial exploitation of marine gas hydrate is still blank,especially the exploitation and utilization of marine natural gas hydrate are still in the technical exploration stage.Submarine surface hydrates usually exist in the seabed sediments with the forms of veins,blocks,fine grains which may cause decomposition,gasification,and release of natural gas hydrates due to changes in temperature and pressure conditions.Compared to other types of hydrate deposits,submarine surface hydrates are easier to mine and have a certain development prospect.In this paper,a kind of seabed surface gas hydrate mining device based on Top Cover Subpressure Method is designed.The device uses cap shell and sealing baffle to cover the hydrate reservoir and to form a closed environment for depressurization,which has the function of controlling hydrate decomposition and preventing methane gas leakage.The specific research content of this article includes the following aspects:Based on a large number of investigations about the basic properties of gas hydrate and experimental exploitation,combined with the characteristics of the surface gas hydrate,the top cover subpressure process flow of gas hydrate mining is designed and the mining device based on this process is also designed.This paper analyzes the stress condition of pressure-relief mining device with lid pressure in deep water environment and designs a cap top shell with tension rod-membrane shell structure,adopts adjustable tension rod,modular shell plate,support the column constitutes a stress system,which balances the pressure difference between the inside and outside of the shell caused by the decompression mining,improves the structural strength of the mining device,and ensures the stable production of hydrates.The finite element software is used to analyze the force of the mining device in the 1000m deep sea working condition under the production pressures of 9MPa,8.5MPa and 8MPa.It is verified that the tensile rod-shell membrane structure can guarantee the stable operation of the mining device at 9MPa,8.5MPa and 8MPa(1MPa,1.5MPa,2MPa negative pressure conditions).The mathematical model of temperature field in steady production of cap top pressure lowering mining device is established,and the influence of 20m diameter mining device on steady mining under different heating temperature and different production pressure is studied.The effects of heating temperature and production pressure on gas production rate per unit time and cumulative natural gas production are obtained.It is found that increasing the input hot water temperature can significantly accelerate the hydrate decomposition near the heating pipeline.The maximum gas yield of the heating water temperature 20 ℃ and 40 ℃ is 7462.5 m3/h and 9983.3 m3/h,respectively,when the production pressure is 8MPa,and the early gas recovery efficiency is improved.But at the same time,it is found that the farther away the reservoir decomposition area from the heating pipeline,the weaker the effect is,and the effect of input hot water temperature on the gas production rate in the later stage of production is not significant.The simulated production pressure of the production device is heated by 40 ℃ hot water under 8MPa.The accumulative amount of natural gas running for 300 hours is 22.67 million square meters of natural gas per day. |
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