Advanced Hydraulics Analysis Optimizes Performance of Roller Cone Drill Bits

Abstract

Abstract For nearly 30 years after the introduction of jet bits, all tricone bits had virtually identical nozzle systems. Nozzles were aimed toward the corner of the borehole bottom, more or less equidistant between the cones. More recently, tricone bits with unique patented nozzle systems have appeared and today drillers can choose between at least five different nozzle configurations. It is recognized that cone cleaning nozzle systems perform best in bit balling environments and borehole cleaning nozzle systems perform best in bottom balling environments. Another characteristic of bit hydraulics, which has not received much attention in the literature, is the ability of a bit to evacuate cuttings. Since one bit may encounter both bit and bottom balling environments during its life and all bits must evacuate cuttings, it is most ideal if one nozzle system can be developed which performs well in all three of these aspects. A bit with a new nozzle system, described in this paper, has been shown to achieve superior rates of penetration to all other nozzle systems currently available in both laboratory and field tests. Three dimensional Computational Fluid Dynamics (CFD) models of the flow field between tricone bits and boreholes have been used to quantify the bit cleaning, bottom cleaning and cutting evacuation characteristics of this new bit and two other tricone nozzle designs. Cone cleaning is not the only factor to be considered in designing a nozzle system for bit balling environments. Today's rock bit is expected to drill long intervals and therefore must drill through several different types of rock, which can often mean the bit may encounter several different drilling environments in its lifetime. Furthermore, nozzle systems that clean the cones may erode the cones in abrasive environments. CFD models have also been used to quantify and minimize the potential for erosive wear of the cone shell.