Directional and Stability Characteristics of Anti-Whirl Bits With Non-Axisymmetric Loading

One concept to avoid downhole vibrations with PDC bits requires a net side force that rotates with the bit and pushes a low friction gage pad against the borehole (anti-whirl bit). When drilling with an angle build assembly or in an angled hole, a non-rotating side load is applied to the bit. When drilling with a steerable assembly in the rotate mode, the bit axis is tilted relative to the profile of the bottom of the hole. This axis tilt affects the bit cutting forces and may consequently affect bit stability. This paper investigates these non-axisymetric loads for their effects on the stability characteristics of anti-whirl designs as they compare with standard PDC bits.A second hypothesis holds that standard PDC bits occasionally whirl on directional bottomhole assemblies, which leads to overgage holes and ledging. The build rates, hole diameter, and ledging characteristics of anti-whirl bits are compared to standard PDC bits.Comparisons were established in lab tests using standard and anti-whirl PDC bits both in an anisotropic formation (Green River shale) and in an isotropic formation (Carthage limestone) with a non-axisymetric loading.Further, a comparison is made between two directional wells drilled at a field research facility using the same surface hole. One well was drilled with a standard PDC bit; the other with an anti-whirl PDC bit. The two wells are compared for surface vibration level, rate of penetration, build rate, dull condition of the bits, and hole diameter and ledging from calipers of the wellbore.