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Along with the exploration science of deep earth, Scientific ultra-deep well drilling techniques formed and developed a new technology areas. Drill stems, is the key factors in the ultra-deep well, not only pass drilling pressure to the bit, but also deliver the drilling fluid.Drill stems bear the axial tension and compression, shear stress, and collapse pressure in the ultra-deep drill. The axial tension plays the leading role in determining the using limit depth of the drill stems. Be based on the axial tension, the article sets up the strength conditions of the drill stem, and derive the calculation formulas for getting the using limit depth. With the formula, the article calculated the using limit depth of the common drill pipe used in the ultra-deep well. At the same time, drilling fluids, formation and temperature, are researched in the article.Using the ANSYS software, we set up a model for the drill string assembly, including the 127mm drill pipe and 177.8mm drill collar, and make static analysis to the model. At last, the conclusions including: 1. In the ultra-deep wells, because the weight of drill stem is much bigger than the drilling pressure, the force will be taken to lift the drill stem to get the need drilling pressure. In this case, the upper of the drill stem is in tension state, and the lower is in compression state. There will be a neutral cross-section in the drill stem. The location of the neutral cross-section changes with the drilling pressure, which leading to this section will bear large alternating stress. For the safety of the drill stem, the neutral cross-section must be in the drill collars; 2. In the ultra-deep wells, the compound stress in the top of the drill stem is maximum, which gradually decrease along the drill pipes and sharply decrease in the drill collars; 3. Under the ultra-deep wells conditions, the torque plays a little role in the compound stress in the drill stem, especially in the drill pipe; 4. The drill collars are much heavier and rigid, so their compound more stress is much smaller . It improves the safety of the drill stem, and help to control deviation.Finally, the article researched the new type of drill pipes, and discussed its future of using in the ultra-deep wells. The conclusions of this article are important to increase the using limit depth and ensure its safety, and provide a foundation for further study in the related fields.
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