With the increase of lateral extensions of extended-reach wells and horizontal wells, mechanical behavior of drill strings constrained in long open holes becomes more complicated and periodic sticking problems occur more frequently, which is a serious threat to safe and efficient drilling. On the basis of classic torque and drag models, an amended model is built by introducing the local sticking effect, effects of connectors on critical buckling load and contact force, etc. By combining the qualitative analysis of periodic sticking phenomenon and quantitative analysis of local sticking drags, the main reason for periodic sticking namely an open-hole key seat is identified. The results indicate that periodic sticking problems become more frequent and serious with an increase of open-hole length and the mechanical drag factor. When the mechanical drag factor is larger than a certain value, connector sticking and tubular buckling are coupled, which leads to inability to continue drilling. To alleviate periodic sticking problems measures should be adopted from two aspects: design and operation. On the design aspect,well structure, drill string components, steerable control modes, etc. should be optimized; on the operation aspect, measures such as active reaming trips and installing lateral vibration tools should be taken.
