Abstract: During horizontal wellbore drilling, factors like the stratigraphy, vibrations of the drilling tool, lateral forces on the drill bit, and beds of cuttings tend to cause slight but continuous deflections in the trajectory of the drilled wellbore. Although these deflections will not induce severe deviations in the trajectory of the drilled wellbore, they will nonetheless alter the nature of the contact between the drill string and the walls of the well. This form of deflection is often neglected due to the limitations of current well-logging methods. Through theoretical analyses and simulations, we have studied the nature of the contact between a drill string and the walls of a well when a horizontal wellbore trajectory continuously fluctuates in small deflective fluctuations within the well' s plane of inclination, as well as its effects on frictional forces. This study reveals that "suspended spans" often occurs in the contact between the drill string and a horizontal wellbore undergoing deflective fluctuation, and this questions the rationality of the assumption that the drill string and the walls of the well are continuously in contact, which is often invoked in current analytical methods for solving questions related to drill string mechanics. The drill string' s weight and axial forces are the most important factors in determining the bending displacements and maximum critical span of a drill string in "suspended span" contact. These "suspended span" contacts will decrease the contact area between a drill string and the walls of the well, and increase drilling friction.

Key words: horizontal wells, suspended drill strings, deflections in wellbore trajectories, axial forces, friction