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With the development of drilling technology and the needs of the larger Bulit-up, the screw drill with larger structure corner is used more widely in drilling field. Because the existence of single curved screw drill tool with larger structure corner, it have larger friction during the bend PDM in the process of internal casing, so we often need to use the wellhead adjuvant into the well. In order to understand the single bent screw drill into the state in the wellhead diameter of casing, and how the angle of screw structure impact the feasibility, This paper analysis the stress of the single curved screw drilling tools in the well, and the condition it can safely through the mouth of the well conditions, establish the single bend PDM in the calculation wellhead down model. and the finite element model is established combined with the specific parameters of a well which is in the western of SiChuan depression. Using ansys analysis a particular casing inside diameter, under wellhead vertical segment into different structure when the turn of the screw drilling tools down into characteristics and stress distribution characteristics, it is concluded how large of the structure corners and how depth that the bend PDM can rely on gravity well down into the inside diameter of 250.19 mm casing, concluded the relationship between reaction force, friction force, and maximum stress with the well depth and the change of structure corners from 1.5°to 2.75°，provide a basis for drilling site selection of the appropriate combinations of screw drill and how to regulate the radial force of the device while into the well.