Friction Reduction and Buckling Behavior of a Horizontal Drill String Under Axial Vibration

Severe drill-string–wellbore friction during slide drilling limits axial-load transfer and reduces drilling efficiency in extended-reach horizontal wells. To investigate the mechanical response of a horizontal drill string under axial vibration, a three-dimensional geometrically nonlinear finite element model was developed in Abaqus/Explicit. The model incorporates a previously validated dynamic friction formulation and directly accounts for deformation-dependent drill-string–wellbore contact, allowing the coupled effects of axial vibration, friction, load transfer, and lateral deformation to be captured. Numerical results show that axial vibration markedly alters the spatial and temporal distributions of axial force and sliding velocity. Periodic modulation and reversal of axial velocity induce synchronous reversal of the friction force, which constitutes the dominant friction-reduction mechanism under the investigated conditions. The reduced effective friction improves axial-load transfer, increases WOB, and produces a more continuous drilling response, although larger WOB fluctuations are introduced. The vibration effect attenuates with propagation distance, indicating a spatially limited effective range of the axial oscillation tool. Under high axial compression, buckling increases drill-string–wellbore contact and deteriorates load-transfer efficiency. Axial vibration further reduces the externally applied compressive load required to reach sinusoidal and helical buckling, with a maximum reduction of approximately 12.9% in the investigated cases. These results provide a mechanical basis for balancing friction reduction, dynamic bottom-hole loading, load-transfer improvement, and drill-string stability in the application of axial oscillation tools, thereby helping to improve drilling energy efficiency and enhance the development capabilities of unconventional oil and gas resources.