The Stress-Strain State of the Drill String at the Section of the Borehole with a Cavern

This study analyses the stress-strain state of a drill string at the section of the borehole with a cavern/chute. The study was conducted to obtain analytical dependencies to determine normal bending stress in the cross section of the drill string and its downforces to the walls of the well. This will allow to compare these values with the critical ones, and draw conclusions about the possibility and duration of the drill string operation under these conditions. The study is based on modelling the drill string as a beam, which indicates longitudinal and transverse load and deforms in-plane. The formulas obtained to determine stresses and pressing forces apply to the following cases of the relative position of the drill string in a straight borehole and a curved borehole with a cavern/chute: a – the drill string touches only the bottom of the borehole; b – the drill string touches only the bottom of the borehole and the bottom of the cavern/chute; c – the drill string touches the top and the bottom of the borehole; d – the drill string touches the top and the bottom of the borehole and the cavern/chute. The calculations based on the dependencies obtained lead to the following conclusions: a – the cavern/chute in the inclined straight borehole causes bending stress value in the cross section of drill pipes proportional to the fatigue margin of the material; b – the cavern/chute in the curved borehole may increase normal bending stress in the cross section of the borehole up to five times.