Analysis of the Toolface Stability of a Motor-Fully-Rotating Directional Drilling System

The reduction in longitudinal drag and increase in rate of penetration have recently been a hot topic in the drilling field. In this study, a motor-fully-rotating directional drilling system was proposed. Compared to the conventional system of motor/MWD (measurement while drilling), the advantage of the new system is that all drill string except the guiding bent housing rotates during slide drilling, which meets the two requirements of reducing longitudinal drag and increasing the rate of penetration. The stability of the toolface is the premise for the new proposed system to realize its working principle. Lateral force between the guiding bent housing and wellbore under different well conditions was evaluated by a model established by the continuous beam method. On this basis, combined with the principle of rolling friction, and through indoor experiments, it was shown that when the wellbore curvature is less than 6°/30 m and friction coefficient between the guiding bent housing and wellbore is bigger than 0.2, the toolface can be kept stable. Field tests showed that the well deviation angle continued to increase, while the azimuth remained unchanged in the well section where the tool was used, and the maximum change rate of inclination angle was 7.886°/30 m, verified the toolface stability of the proposed system during slide drilling.