I. Introduction
Slide drilling is a typical technique of directional drilling, which is popular in the directional drilling market because of its low expenses. In field, drillers adjust the tool-face by manually rotating the top drive or changing the weight-on-bit (WOB). For example, to initiate a slide, the driller must first stop drilling, pull the bit off bottom and reciprocate the drill string to release the build-up torque within it. Then, the bent housing of the PDM is turned to the desired direction by rotating the top drive according to the real-time tool-face measurements of a surveying instrument called measurement-while-drilling (MWD). During the practical sliding process, there are two ways to maintain the desired tool-face to keep the trajectory on course. First, drillers can make minor shifts to the tool-face by changing the WOB, since when weight is applied to the bit, reactive torque, which is applied by the formation, will develop in the opposite direction and affect the tool-face. To produce larger changes, drillers can lift the bit off bottom and reorient the tool-face by rotating the top drive directly. It is not difficult to manually set or maintain the tool-face of the SDS in a shallow- or middle-depth well. However, it becomes an extremely challenging task for a deep-depth well, which can be as long as several kilometers, due to the large friction and drag between the drill string and wellbore and ultra-flexibility of the drilling assembly resulted from its large slenderness ratio.