Drilling Hydraulics-A Study of Chip Removal Force Under a Full-Size jet Bit
Authors: Sutko A.A. 
Published: December 2007

Bit hydraulics play an important role in the drilling process. The beneficial action of the fluid's impacting, cleaning the bottom of the hole and the bit teeth, and carrying particles into the annulus is well established. What is not well known or commonly agreed upon is bow this beneficial action can be made most effective. To understand better the drilling hydraulics phenomenon, laboratory tests were carried out with phenomenon, laboratory tests were carried out with full-scale bits and a simulated hole bottom. An earlier paper described the pressure distribution on the hole bottom as it varied with nozzle size, number, and extension. This paper discusses the way the force acting on a stationary chip varies with these same parameters and also includes The effect of chip size and fluid density and viscosity. The results show that closing one or two nozzles or extending the nozzles increases chip removal force and should assist is cleaning the hole and, in some cases, increase penetration rate. Fluid velocity, nozzle diameter, and chip size are the most important factors affecting chip removal force. Fluid viscosity plays a minor role. These findings reinforce and broaden the conclusions reached in the pressure distribution studies. It was also found that chip removal force correlates more closely with jet impact and horse power than with velocity or nozzle Reynolds number.

Introduction

Bit hydraulics are difficult to study. If tests are carried out in the field, there are uncertainties regarding down-hole conditions and whether these conditions change during a test. If the tests are carried out in the laboratory, results are usually more clear-cut, but then there is the question of how well the tests simulate field conditions. The most reliable insight into the over-all hydraulics problem is probably reached by blending the results problem is probably reached by blending the results of all types of work; no single study fits together more than a few pieces of the intricate puzzle. Past work in drilling hydraulics has been in the following categories: field testing, similar to that of Thompson, Spear, and Eckel; drilling tests in the laboratory, similar to those of Horner el al, van Lingen, Feenstra and van Leeuwen, and Eckel; and laboratory nondrilling tests like those of McLean and Cheatham and Yarbrough. other significant work includes the Kendall and Goins theoretical paper on optimization and that portion of Bingham's series on "a new approach portion of Bingham's series on "a new approach to interpreting rock drillability" that deals with bit hydraulics. The work reported here fits in the category of laboratory nondrilling. It is aimed at explaining how nozzle size, jet velocity, fluid properties, etc., affect what takes place as the fluid leaves the nozzle impinges on the hole bottom, and begins to return to the surface. This work differs from Refs. 4 through 7 in that no actual drilling took place. In a sense, this detracts from its usefulness; on the other hand, we feel that we were able to maintain closer control over test conditions because we did not have the problems of rock variability, drilling fluid problems of rock variability, drilling fluid contamination, and the other phenomena involved in drilling tests. The work in Refs. 8 and 9 described the measurement of velocities and pressures under a jet bit. Maximum jet velocity was 171 ft/sec. The work reported in this paper describes the forces caused by fluid velocities and pressures as they act on a chip. Maximum jet velocity was 286 ft/sec. An earlier paper, described the pressure distribution on the bottom of the hole as it is affected by nozzle size, number, and extension. Conclusions included the desirability of plugging one or two nozzles, using extended nozzles and a correlation of pressure distribution with jet velocity, nozzle diameter, and the distance between the nozzle and hole bottom. The present work is a continuation of the same study and was set up to find out what happens when fluid moving across the hole bottom meets an object in its path. In a sense, it a basic attempt to study hole cleaning under closely controlled conditions.