Dynamics of Rock-Chip Removal by Turbulent Jetting

Abstract

Summary The efficiency of the drilling process is largely governed by the efficiency with which the available hydraulics removes rock chips created by the mechanical action of the bit. To date, a physical understanding of the process associated with the hydraulic removal of the chips remains unknown. Rock chips mechanically freed from the parent rock by the drilling action of the bit are generally held in place by overbearing pressures. These pressures must be overcome either by hydraulic action or by mechanical regrinding before the chip may be removed. This work presents the experimental results of a study designed to examine the effects of dynamic forces brought about by jet turbulence on the removal of loose rock chips. Synthetic chips of known shape and size, embedded in a simulated hole bottom and held in place by hydrostatic pressure, were removed solely by the jetting action of a vertically impinging jet. The synthetic chips were flush-mounted into the plate, rendering the shear forces on the surface of the chips at least two orders of magnitude less than the hold-down forces. Measurements of the static jet-impingement pressure and the dynamic fluctuating pressure caused by the jet turbulence were related to turbulent time-scale measurements made using a laser Doppler anemometer, producing an analytical tool to predict the necessary conditions for chip removal. Tests were conducted in both water and an optically clear synthetic clay mixture having non-Newtonian viscosity characteristics similar to a bentonitic fluid. The results indicate that chip hold-down forces can be overcome by the turbulent action of the jet nozzle. Correlations are given indicating the conditions necessary (i.e., jet standoff, radial chip location, chip size, and viscosity) to permit chip removal. These results can be used to place and size jets optimally to maximize chip removal and bottomhole cleaning.