| With gradually decreasing of oil and gas resource, its price is keeping on high level; human being has met with enormous energy problem. , Because of large population, oil and gas resource depletion and the rapid development of economic, energy shortage has become a very serious problem in China. Thus, developing the new energy become more and more urgent to satisfy the need of economic development, and achieve economic and society sustainable development for China. The gas hydrate that is a kind of new energy has been a exploiting forcus for the more and more country in the past 20 years.It is estimated that the carbon volume of gas hydrates is double to the volumes of carbon in coal, oil and natural gas of the globe. So the gas hydrate has become a key factor in energy balanced development for the entire globe.The gas hydrate is regarded as the important substituting energy by the scientist in the all countries in the coming century.Exploiting gas hydrate is based on evaluate gas hydrate, and sample drilling is the most directly method to evaluate gas hydrates. The physics condition of Gas hydrates forming is 0～10℃of temperature and more tham 10MPa of pressure, It is very strict for sample drilling. At present, the pressure-tight core barrel is the main truth-preserving core sampling, and its operating principle is that maintaining pressure inhibits the gas hydrates dissociating by mechanical operating mechanism. It demands high intensity for core barrel, especially, the higher intensity and tightness for ball valve.If the tightness of ball valve has a small drop; the core will not preserve its primitive pressure, and lead to sample failure. When the design pressure of sampling device arrives at a high value, it is very difficult to raise the tight pressure for designing core barrel, and it must greatly improve the leak tightness and intensity of material.The method of freezing sample of as hydrate is the way that the temperature of the gas hydrate core is decreased by cold source. Because the critical dissociated pressure of gas hydrate is reduced with the decreasing of core temperature of active form. The result of decreasing the temperature is the same as increasing pressure to inhibit gas hydrate dissociating and retains the core of gas hydrates. But it is easy to decrease temperature of core by out cold source at the well bottom.Firstly, the feasibility of freezing method for sampling gas hydrates is analyzed in the paper. By researching the thermophysics characteristic, temperature-pressure property and the process of sampling of gas hydrates, it is proved that low temperature gas hydrate holds more stability. The method is to decrease the critical dissociated pressure of gas hydrate and inhibit gas hydrate dissociating by means of reducing core temperature.The optimum freezing temperature is -20℃.The design diffuculity of pressure-tight mechanism for truth-preserving core sampling of gas hydrates is reduced greatly; even it is possible to achieve the truth-preseving core without pressure-tight mechanism indeed.Secondly, the optimum cold source and freezing way is chosen by the indoor freezing experiment. Both Freon device and dry ice freezing device are built in the Lab. The samples of different percentage of ice sands and loess are frozen in the experiment, and the optimum freezing method is selected by measuring the variation of sample temperature. Because of the sample temperature dropping speed of 1℃/minand the limited power of freezing device .the result of former experiment with Freon freezing ways indicated that it's not enough to achieve rapid freezing, and it is difficult to apply the structure of Freon freezing ways in designing the sample tool.The latter, the dry ice is used as coolant, the alcohol as positive freezing catalyst and cooling medium. Because of dry ice phase change absorbing heat, the temperature of cold source reaches to -70℃, the temperature dropping speed of sample arrives at 6-10℃/min.The result of quick freezing is achieved. and time of the frezzing sample under the optimum temperature is long to 100-170min.So it is available to apply dry ice freezing method in the design of sample tool for freezing sample of gas hydrates in the well bottom.Thirdly, the prototype of dry ice freezing sample tool is developed.In order to inspect the function of the sample tool, the freezing sample experiment is done, and the freezing loess sample is achieved. According to the conclusion of above indoor simulate freezing experiment, the dry ice is used as coolant, the alcohol as positive freezing catalyst and cooling medium, design the prototype of gas hydrates freezing sample tool. The tool is making up of single action mechanism, control mechanism and freezing mechanism. Single action mechanism can make the inner and outer pipe assembly independent rotary to avoid disturbancing core. The control mechanism achieves alcohol enter into dry ice and accelerate the dry ice sublimation by the way of putting steel ball into the tool to change the direction of drilling fluid circulation. Freezing mechanism which is copper as the material of core barrel makes the core rapid freezed by the dry ice cold source to achieve freezing core in the bottom of hole. By the freezing inspection experiment in the Lab, it is testified that the functiones of all mechanismes of sample tool are realiable. The further freezing sample experiment is done in the loess stratum in Changchun region, the result of experiment proves that the sample tool holds same reliable in the process of drilling. It is easy to freeze core sample in the bottom of hole, and a real freezing loess core is achieved.Finally, the heat transfer simulation of freezing sample of gas hydrates is made in the paper. The ideal physical and mathematical model of freezing sample is established, and the solution of the model is got by the way of heat transfer numerical method. The temperature variation and freezing energy of freezing sample is simulated by the computer, including the simulation of freezing process and defreezing.By comparing the results of simulation and experiment, it is testified that the model is accurate. And the model is used to simulate variety of the temperature and energy of the gas hydrates core with the freezing sample tool in the bottom of hole.Futher simulating the temperature falling and raising process of gas hydrates core, the time of decreasing the optimum freezing temperature and remaining its temperature is got, and it could get conclusion that it is available to achieve the truth-preserving gas hydrates core without using pressure-tight mechanism ,and it must make the temperature decrease to more lower temperature.The research puts forward the idea of freezing method for sampling gas hydrates, and a prototype of gas hydrate freezing sample tool was developed. The truth-presering core sampling is very difficulty due to the special feature of gas hydrates existing. In the future, there are so many futher researches to be done, such as, apply the freezing method in the field experiment, improve cold insulation of freezing sample tool, and futher modify heat transfer simulation model.
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