| China is the third in the world with a permafrost area of 2.15 million square meters, which is mainly distributed in the Daxing"anling region and Qing-Tibet Plateau, while a few in the high mountains. The permafrost area accounts for 22.3% of total land area in China. The previous studies have showed that it has perfect conditions and great prospecting potential for gas hydrate in the Qiangtang Basin, Qilian Mountain region, Fenghuo Mountain region and Mohe Basin.Core drilling is not only the most direct method used to identify gas hydrate, it is also a necessary means to verify the findings of other methods (e.g.. geophysical methods) and a prerequisite for the development and utilization of gas hydrate. By means of the gas hydrate core sample analysis, it could be assessed the reservoirs, scale and other geological parameters, which are of great significance for gas hydrate exploration and exploitation.In view of the thermophysical properties and the geological conditions of occurrence, the decomposition inhibition method is commonly used in permafrost gas hydrate drilling operation. The essentials of decomposing inhibition method involves low-temperature mud is chilled by mud cooling system to prevent the temperature of the gas hydrate strata and core sample from increasing, maintaining the gas hydrates at the equilibrium state of inhibition, and maintaining borehole stability. Meanwhile, in order to obtain undisturbed core samples, a special core sampler should be used when encounter the gas hydrate strata. 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.A down-hole freezing method for gas hydrate core drilling is proposed by Jilin University. It involves that a low-temperature mud chilled by mud cooling system cycles in drilling well during the running of drilling occurrence, and a down-hole freezing core sampler (FCS) is used for gas hydrate core sample in the coring operation.Mud cooling and FCS are two key technologies in the down-hole freezing core drilling method. A mud cooling system suited for low-temperature mud is designed and developed. And. based on previous studies, a research on FCS will be studied, which involved the down-hole freezing way. the processing of down hole freezing and cold-store mechanism.According to the requirement of gas hydrate core drilling, firstly, a mud cooling system is designed, of which involves the process and principle. Considering the mud properties, mud flow, operating temperature, operating pressure and other factors, the heat exchanger for mud cooling system is selected, and the design and calculation for its specific parameters is carried out on request. The secondary refrigerant tank is one of the important devices for mud cooling system. The secondary refrigerant tank should ensure the amount of refrigerant circulation when the system is running, and the secondary refrigerant tank should contain enough refrigerant in order to carry enough cooling energy, to prevent the refrigeration units frequently start, so as to protect various components of the refrigeration units. The secondary refrigerant tank is a place where the air and impurities are carried out by refrigerant cycling and separated. Therefore, the secondary refrigerant tank volume calculation is also important, according to its functional requirement, the design calculation for the secondary refrigerant tank volume has been carried out.According to the composition of mud cooling system, the other necessary devices are selected. And then the laboratory experiments are carried out to test whether the temperature difference between the mud import and export of the heat exchanger can meet the design requirement when it is in the designed flow conditions. Then, according to gas hydrate drilling in the Muli Basin in the Qilian region in 2009. The system has been tested that whether it can meet the technological requirements of gas hydrate core drilling or not. Combined the gas hydrate core drilling in 2010 in Mohe Basin, and based on the first set of mud cooling system, a new mud cooling system has been developed, and field experiments are conducted.According to the problems occurred during the field experiments, the mud cooling system is improved. First, three possible piping connections are explored, and a comparative study of their own characteristics are conducted. According to mud clogging problem found in field experiments, its anti-blocking function is designed. The heat transfer efficiency of heat exchanger plays a great role on the heat transfer efficiency of the mud cooling system, enhancing the heat transfer efficiency can increase the heat transfer efficiency of the system. Finally, based on the analysis above, a new mud cooling system is designed, of which screwed pipe instead of smooth pipe in the heat exchanger is used. The mud cooling system have two special functions, one is the velocity of flow adapted and another is mud could prevent from blocking during the running.The FCS uses dry ice as freezing source, uses alcohol as refrigerant and catalyst. Firstly, the characteristics of the FCS is analyzed based on the analysis of cot characteristics of temperature and pressure, the freezing way of the FCS in the bottom of the hole is also analyzed. Second, the freezing process of sampler freezing institution is simulated when coring in the 200m deep in the hole. Finally, the insulated effect of cold-store mechanism is carried out by means of simulation and the cold-store mechanism is improved.This study could provide a technical support for the terrestrial permafrost gas hydrate exploration and exploitation, also provide a technical reference for seabed gas hydrate drilling in the future.
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