| Recently, the energy problem is more and more extrusive along with the development of the national economy, because of the speedily increase of energy demand. Besides, the coal-focus energy configuration would increase the pollution of the environment. For keeping continuable development of national economy, replaceable energy-sources must be exploited, which is good for environment. Natural gas hydrate has been recognized of a new energy that has best development potential. There is a mass of gas-hydrate in the nature. Thereinto, hydrocarbon, such as CH4, can provide energy twice of other fossil fuels. In the elementary analyses, according to prophase reconnaissance, South-China-Sea and Tibet altiplano frozen earth area (mostly C6H14s) and some part of the East-Sea area are primary storage area in China. The storage of gas-hydrate in South-China-Sea is estimated about 70 billion ton. Therefore, the increase of reconnaissance and development of gas-hydrate has an important strategic meaning for changing Chinese energy structure and developing exuberance.When exploration is being done in low temperature stratum where is abounding in gas-hydrate, not only does the drill-liquid have fine rheology and miss-water peculiarity that a conventional drill-liquid must be provided with, but also can the drill-liquid hold its good fluidity below zero. In addition, it is required that the temperature of the liquid must below the melting point of the drilled stratum, because heat-heat permutation exist between drill-liquid and the stratum in the circle process of drilling slurry. It will change the temperature of the stratum and destroy original occurrence state.According to the method of using drill-liquid in frozen stratum and some experiences, some work has been done in design and manufacture about polymer drill-liquid which can be used in the frozen stratum. Combining with existing condition of our laboratory, studies about rheology and water-loss peculiarity of drill-liquid have been done in different system. Meanwhile, according to the temperature peculiarity of altiplano frozen stratum, some researches have been carried out about correlative peculiarity in low temperature. Based on comprehensive analysis of the drill-liquid peculiarity and economic peculiarity, 11 prescriptions have been selected for practical use, as shown following in detail.⑴PVA(2.5%)+ bentonite(2%)+ ETH(15%)+NaCl(7.5%);⑵PVA(1.875%)+PHPA(0.03125%)+ bentonite (3%)+ ETH(20%) +NaCl (7.5%);⑶PVA(1.875%)+ PAC-141(0.0625%)+ bentonite (3%)+ ETH(20%) +NaCl (7.5%);⑷PVA(1.875%)+ KPA(0.0625%)+ bentonite (3%)+ ETH(20%) +NaCl (7.5%);⑸PVA(3%)+PHPA(0.0375%)+PAC-141(0.1%)+KPA(0.05%)+Na-CMC(0.075%)+NaCl(10%);⑹PVA(3%)+PAM(0.075%)+PAC-141(0.1%)+KPA(0.05%)+Na-CMC(0.075%)+NaCl(10%);⑺PVA(3%)+PHPA(0.09375%)+Na-CMC(0.075%)+NaCl(10%);⑻PVA(3%)+PAM(0.1875%)+Na-CMC(0.075%)+NaCl(10%);⑼PVA(3%)+PAC-141(0.1875%)+Na-CMC(0.075%)+NaCl(10%);⑽PVA(3%)+KPA(0.1875%)+Na-CMC(0.075%)+NaCl(10%);⑾PVA(2%)+PAC-141(0.15%)+bentonite(3%)+NaCl(10%)。The researched drilling fluid system is a pollution-free drilling fluid system with good rheological properties and anti-collapse features. The former four optimized low-temperature -resistant formula is to meet the anti-low temperature requirements under the conditions of -20℃, and the later seven ones is to meet the temperature of -5 to 10℃resistance under conditions of low temperature requirements. Of the former four, in the actual use of anti-low temperature according to different requirements, appropriately adjust the volume of ethylene glycol. The result of tests shows that the anti-low temperature of drilling could get about -5℃when the percentage of the ethylene glycol is 7.5%; the anti-low temperature of drilling could get about -10℃when the percentage of the ethylene glycol is 12.5%; the anti-low temperature of drilling could get about -16℃when the percentage of the ethylene glycol is 17.5%; the anti-low temperature of drilling could get about -20℃when the percentage of the ethylene glycol is 25%. In the test system which takes NaCl in cryoprotectants as a major, when the amount of NaCl takes 10%, the anti-drilling capacity can achieve at a low temperature of -10 to -5℃to meet the exploration requirements of the permafrost hydrate occurrence layer. When the temperature gets lower, viscosity change of PAC-141 is smallest, namely, the PAC-141 temperature sensitivity of the minimum, PAM is close to KPA, get greatest temperature sensitivity and PHPA in the middle.The anti-collapse ability of the drilling fluid is strong, which is mainly due to the corporation of polyvinyl alcohol (PVA) and other polymers synergy. Polyvinyl alcohol becomes to film slowly by absorption, but good dense membrane. PVA with other polymers in the drilling fluid has good compatibility, which formats the adsorbed film that would be denser. As the corporation of PHPA, PAM, PAC-141 and the KPA increase, the capacity of drilling fluid to anti-collapse will improve. In accordance with the temperature of -10 to -5℃accounting requirements of the drilling fluid costs of permafrost layer, the former four glycol low-temperature-resistant polymer-based drilling fluid costs about 1,100 ~ 1,600 yuan per stere, the later seven NaCl-based low-temperature-resistant costs about 600 ~ 800 yuan per stere, the drilling fluid is economic.In order to smoothly carry out drilling tundra, it is certain that the temperature of the flushing medium into the hole by drilling is below the temperature of formation. Taking into account the specificity of the exploration of gas hydrate, to avoid gas hydrate samples decomposition in the process of drilling and sampling, drilling fluid temperature should be controlled to be -5℃. Therefore, drilling fluid must be coolled in the surface. The preliminary design of the refrigeration system has been done, of which the basic principles is to cool drilling fluid using the condensation principle of snake-shape refrigeration Condenser. In the exploration, drilling fluid is in a state of constant flow, it is not the way to the use of a refrigerant, and refrigeration cycle can be used step by step approach.For Surface cooling, the key question is to determine specific heat of fluid, because it directly affects the calculation of heat when drilling. According to available information, combined with the liquid specific heat of the calculation method and formula, select the appropriate slurry recipe to meet tundra drilling, at the same time the specific heat has been fit and calculated for different drilling temperature, providing for the basis of the surface cooling system's selection.Low-temperature-resistant polymer drilling fluid system is a complex system, and its performance is affected by various factors.According to the formation of different conditions and problems encountered, choose suitable drilling fluid formulation and adjust the component of drilling fluid at the right time to meet the require of drilling.
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