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 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 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 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 he used to place and size jets optimally to maximize chip removal and bottomhole cleaning. Introduction A major factor controlling the efficiency of rotary drilling operations is the rate at which loose rock chips are removed from the bottom of the hole. Ideally, rock chips would be hydraulically removed as rapidly as they were created by the mechanical action of the bit. Strong evidence suggests, however, that chips mechanically loosened from the parent formation are frequently held to the hole bottom by large differential pressures, a phenomenon historically termed the "chip hold-down effect." Chips held to the hole bottom in this manner are then reground until they are either mechanically dislodged or ground small enough to render the holddown forces ineffective against the available hydraulic forces. If the chip hold-down effects are minimized, higher drilling rates may be realized by reduction of the amount of energy wasted on regrinding loose rock chips. Hydraulics at the bit traditionally has been optimized by maximizing either bit hydraulic horsepower or impact force. The total discharge through the bit, however, is restricted to an envelope of velocity (maximum or minimum) in the well annulus to ensure effective cuttings transport. Any further enhancements to the hydraulic efficiency require alterations to the contours, orientation, or location of the nozzles. To make adjustments to these parameters effectively, a knowledge of the physical phenomenon responsible for hole cleaning is essential. In particular, an understanding of the hydraulic forces available beneath the bit to lift chips from the bottom of the wellbore is required. One force available to overcome chip hold-down effects is the positive fluctuating pressure (dynamic pressure) resulting from the positive fluctuating pressure (dynamic pressure) resulting from the turbulent flow conditions adjacent to the bottom of the hole. This study examines the conditions that promote lifting, by turbulent pressure fluctuations, of particles held to the hole bottom by pressure fluctuations, of particles held to the hole bottom by differential pressure. Experiments were conducted with a single jet impinging on a flat permeable plate. Synthetic rock "chips" of known size and shape were flush-mounted in the plate. Measurements were made of the hold-down forces (chip weight, differential pressure, and jet-impingement pressure) and available lifting forces (shear stress and pressure fluctuations along the hole bottom) on the individual chips to evaluate quantitatively the conditions necessary for their removal. Two working fluids, water and an optically clear non-Newtonian solution, were used to evaluate the effects of non-Newtonian viscosity on the chip-removal rate. Background The task of investigating the phenomena associated with bit hydraulics, specifically the chip hold-down problem, historically has taken several diverse approaches. Early investigators focused on such areas as the bottomhole shear forces, scour rates of packed chip beds, and visualization of the crossflow beneath bits. The following approach concentrates on the random fluctuating lift forces, generated by the turbulent flow around the bit, that are available to assist in lifting loose rock chips from the bottom of the borehole. The basic concepts of the chip hold-down phenomenon are illustrated in Fig. 1. In this ideal case, a rock chip, fractured in the formation by the mechanical bit action, is held in place by a differential pressure across the chip caused by the hydrostatic pressure of the fluid column in the wellbore. These pressures typically exceed the formation pore pressure by as much as 500 psi [34.5 MPa]. Clay particles in the drilling fluid tend to fill the small cracks around the loose chip, preventing the differential pressure across the chip from being equalized. The forces acting on the chip may be divided into hold-down forces (differential and jet-impingement pressures and chip weight) and chip-lifting forces (shear-stress-moment pressures and chip weight) and chip-lifting forces (shear-stress-moment forces and positive fluctuating pressure forces resulting from the turbulent flow field adjacent to the chip). This last lifting force is the object of the present investigation.