Investigation of the cutting force response to a PDC cutter in rock using the discrete element method

Abstract

In the drilling industry, Polycrystalline Diamond Compact (PDC) bits with multiple fixed PDC cutters are widely applied to cut rock under high bottom hole pressure and high temperature. The bit-rock interaction is the main excitation source of drill string vibrations and needs to be understood and controlled better to improve the performance of the drilling process. Rock cutting mechanisms can be examined best for a single PDC cutter: Limitations of dynamic control and available sensors in experimental setups are eliminated in numerical simulations. Using the Discrete Element Method (DEM), deformation and strength of rock can be modeled through its full degradation in the PDC cutting process under typical environmental conditions. Stresses and strains in the rock material and on the cutter faces become available for examination.In this paper, numerical simulations of the cutting process are conducted using a modified bonded-particle model (specialized DEM), which is calibrated to a pressure series of high-strain triaxial laboratory tests. The simulation results are interpreted from the perspective of continuum mechanics providing stress and strain rate fields within the rock. These serve to identify two primary deformation modes predominant in two distinct zones moving with the PDC cutter: A crush zone below the chamfer and a shear zone ahead of the cutter face. The influences of two cutter design parameters, the back rake angle and the chamfer size, on these zones and the cutting force, are investigated. The shear angle is found to decrease with increasing back rake angle, resulting in a wider shear band. Moreover, the back rake angle has a considerable influence on the cutting forces both in the normal and tangential direction, whereas the chamfer size affects the normal cutting force predominantly. These results are in good accordance with experimental observations in the literature. Furthermore, the relationship between the cutting force and underlying rock failure mechanisms is clarified.This study enhances the understanding of the rock cutting process and gives first indications for dynamic cutting force control through cutter design.