Drag-Reduction and Resonance Problems of a Jointed Drillstring in the Presence of an Axial Excitation Tool

Axial excitation tools (AETs) have the ability to improve slide-drilling efficiency by reducing the friction between the drillstring and the wellbore wall. However, drag-reduction effects are not always satisfactory, and excessive vibration may cause failures of downhole tools in some cases. Thus, a mathematical model was proposed to simulate the vibration responses of a drillstring. In the model, velocity-dependent friction is adopted to calculate the friction-reduction effect. The effect of drillstring joints on the weight on bit (WOB) was first investigated. The simulation results indicate that the joints intensify the stick-slip motion of the drillstring system. The effect of the location of an AET was then examined. The results show that it is better to place an AET near the drill bit rather than near the rear of a build section. Because the frictional drag acting on the lower portion of the drillstring dominates the axial stick-slip motion of a drill bit. Finally, the resonance responses were examined in terms of the drillstring system acceleration. The results show that resonance moderately increases the accelerations of a long horizontal drillstring system in a heavy-damping environment but that the growth of the exciting force can profoundly increase the accelerations.