A static mechanical automatic vertical drilling tool with double-layer positive pushing pistons for slim well drilling
In this study, a new kind of automatic vertical drilling tool, with outer diameter of 127 mm and double-layer posi-tive pushing pistons, was designed for slim well drilling. The new tool has no independent electronic or hydraulic system, but relies on a deflective eccentric weighting block to control its straightening motion in the well borebore. When the eccentric block rotates under gravity, the flow channel will change and then an internal-external pressure difference is formed, by which the pistons are pushed out onto the upper side of the borehole to reduce the deviation and maintain the well path with high perpendicularity. During the structure design, because of the reduction of the friction in comparison to the regular disk valves, a new ring valve was adopted to improve the sensitivity and precision of the drilling tool. Distinctively, with eight double-layer positive pistons, the pushing unit can gain a higher push force and straightening moment, then a better deviation correction performance can be obtained. Three pistons located on different layers can be pushed out and a stable triangle structure is formed by the three action points, thus the stability and responsibility of the vertical drilling tool can be signifi-cantly improved. In this paper, based on the structure design of the vertical drilling tool, the functions of the eccentric unit and push unit, as well as the friction resistance of angular contact ball bearing on the straightening motion are analyzed. The results show that the push force of the newly designed vertical drilling tool is 1+ 2 times of that with only one piston, and its straightening precision reduces or gets lost only after the deviation angle is lower than 0. 154° and 0. 059°, respectively, thus the vertical drilling tool has high quality, precision and sensitivity. Moreover, due to a simple mechanical structure, its man-ufacture and maintenance cost is low, and it also has high temperature tolerance. The vertical drilling tool has a good pro-spective for field application.