Stability Analysis of Drill-String in Directional Drilling

In deviated deep hole directional drilling operation, drill collar and heavy drill pipe are used to provide enough weight on bit in order that penetration of rock can be conducted and drill-string stability can be maintained. However, as a result, this practice usually puts overloaded torque and tension force on the drill-string and hence results in early fatigue failure of drill-string. Based on the elastic deformation and stability analysis of the bottom hole assembly of drill-string, a discussion on the critical condition to keep drill-string in stabilization and the planning criterion to determine bottom hole assembly is presented . Optimized planning of BHA can be achieved by making the drill pipe partially work in compression state, which may considerably reduce the torque and tension force on the drill-string with the weight reduction of drill collar, and therefore reduce the failure possibility of drill-string.