Flow Patterns Under a Tricone Bit Determined From Pulsed Wire Anemometry and Flow Visualization

Abstract

ABSTRACT At present, the optimization of drilling hydraulics consists of selecting nozzle sizes that maximize either jet impact or hydraulic power at the nozzle, whilst ensuring that the flow rate is sufficient to transport cuttings to the surface. If this crude approach is to be improved upon, what is required is an understanding of the cleaning mechanism at the work face, i.e. the action of hydraulic forces on the crushed rock. Previous investigations of the problem have focussed on the measurement of the mean and fluctuating pressure field under the bit, and on the distribution of fluid velocities. We report here measurements of the mean and fluctuating shear stresses on the surface under a 8 1/2 in. roller J jet bit for a number of different nozzle geometries. The shear-stress maps obtained are shown to be consistent with the results of surface flow visualisation performed for the same bit. For a given configuration, the flow patterns observed were independent of nozzle size, and the measured shear stresses scaled with the momentum flux (at the nozzle). The values of shear stress measured are two orders of magnitude greater than that required to move a chip and suggest that it is an important mechanism for bit cleaning. In a two-nozzle configuration, a major stagnation region is found to be situated at, about 2/3 radius whereas in the three nozzle case the stagnation zone is at the center of the hole. Since the flow pattern rotates with the bit, in the two nozzle case the areas of low shear stress are immediately followed by high shear stress and this sequence may prevent the build up of cuttings. This suggests a possible explanation for the observation that two nozzles can lead to faster drilling rates than three.