Development and application of screw structure of miniature drilling tool for super-deep wells

A miniature drilling tool with special screw (TMXT36) structure was developed to cope with unfavorable exploration and development conditions in deep and super-deep wells in the Tarim Oilfield and to prevent fatigue failure of miniature drilling tool in such wells. The special screw has the number of teeth in every inch reduced, and the distance between the end of external screwed joint and the external shoulder increased. Moreover, with three threads more than conventional screws, the new screw has rigidity reduced and elasticity enhanced to promote the fatigue-resistant service life. The new screw structure can effectively transfer the maximum stress onto the relatively wide guiding surface of screw to minimize fatigue notch sensitivity coefficients at the root section of the screw. Finite-element comparative analysis was made for performances of the new screw with NC38, TX60 and TH90 screws currently used in the oilfield. The results show that TMXT36 has the minimum stress concentration coefficient and fatigue notch sensitivity coefficient, whereas the fatigue-resistant service life is 2.5 times longer than other screwed joints. Prototypes of the screw have been deployed and vilified in the Tarim Oilfield.