Drillstring Stability Based on Variable Material Specific Force and Using a Sharp Three-Insert Polycrystalline Diamond Compact (PDC) Coring Bit

Abstract

Previous research in stability of drillstrings was based on the assumption of constant material specific force, i.e., a bit force that is proportional to the area of cut. Moreover, earlier work used a simplified bit model that consisted of planar radial blades. In this paper, correlation between the material specific force and area of cut is obtained for Sierra White granite and Berea sandstone from test data developed at Sandia National Laboratories in Albuquerque, NM. These correlations, together with an improved bit model in which cylindrical cutters are arrayed in an overlapping pattern over a flat surface, are used to obtain the stability equations. Laboratory testing shows good correlation between measured bit vibrations and relative instability as predicted by the stability equations. These results are useful in predicting the appropriate operating conditions for stable drilling and serve as a basis for future development of more accurate models of PDC bits.