The Effects of Pressure and Rotary Speed on the Drag Bit Drilling Strength of Deep Formations

Shear induced dilatancy of rock during drag bit cutting in deep formations can result in dynamic confining pressures that equal the borehole pressure and increase the drilling strength. Rock cutting experiments were carried out in five sedimentary rock types with widely varying permeability, porosity and strength to test a model of dynamic confinement. Indentation and shear loads on thermally stabilized diamond (TSD) cutters were obtained at pressures of up to 70 MPa and at traverse rates ranging from 0.5 to 8 m/s. Ambient pressure causes rate-insensitive drilling strength hardening in the low-permeability shale and carbonate and rate-sensitive hardening in the permeable sandstones. These results are predicted by the dynamic confinement theory. An analysis of indentation load versus cut depth shows that cutter wear in all five rock types should be minimized at high rotary speeds and shallow depths of cut in deep formations.