Abstract
Measurements of flow field characteristics across the face of a stud-mounted PDC bit have been obtained in a new test facility which simulates downhole flow fields. This facility and the measurement techniques employed to obtain the data are described. Flow tracer tracking and cutter heat transfer coefficient measurements are shown to be valuable in determining flow field characteristics, whereas the utility of the dye injection system tested is low. Three hydraulic configurations for the bit have been tested to determine the effects of nozzle location on bottomhole crossflow velocities and cutter cooling rates. The configuration that provides higher velocities and cooling rates is determined to be the one that employs smaller nozzles located near the center of the bit.
Introduction
Bit hydraulics is recognized throughout the industry to have significant effects on drilling performance. The major functions of hydraulics in the drilling environment include bottomhole scavenging, chip removal, bit cleaning, and, for drag bits, cutter cooling. Failure to perform these functions adequately leads to reduced drilling rates, excessive energy consumption, decreased bit life, and, consequently, increased drilling costs.
The development in recent years of drag bits employing polycrystalline diamond compact (PDC) cutters has created the need for examining the hydraulic designs of these bits in more detail. The flow fields of PDC bits differ significantly from those of roller cone bits, which have been studied extensively for hydraulic performance. For instance, the rock-breaking mechanism of PDC cutters results in the generation of a significant amount of frictional heat. The removal of this heat by hydraulics is essential to prevent thermal
The purposes of this paper areto describe a new test facility for assessing, the hydraulic performance of PDC bits,a) discuss the utility of the various measurement techniques employed in this facility,to illustrate the use of acquired data by providing, preliminary optimization of the hydraulic design of a specific bit, andto evaluate the data in terms of the guidance they provide in building analytical and numerical models of the flow field.
Approach
The complexity of the flow field across the face of a typical multiple-cutter PDC bit significantly reduces the probability of optimizing, hydraulic design through purely analytical means. Measurement of flow field characteristics, therefore, has been assumed necessary in this research to obtain essential data around which optimization procedures can be developed. A test facility has been designed and built for this purpose. As desscribed in the following section, the facility simulates downhole flow fields using a full-scale bit.
To simulate bottomhole flow fields adequately in the laboratory, one of two approaches must be taken. The first and most straight forward approach is to use fluids and flow rates identical to those used in the field. The capacity of drilling muds, however, renders these fluids useless in flow visualization tests, which are extremely useful in the investigation of complex flow fields. The second approach circumvents this problem by using the principles of similitude.
Similitude is the principle used in fluid mechanics to simulate flow fields around full-scale geometries with smaller-scale models and/or different test fluids. This principle is illustrated by an examination of the nondimensional Navier-Stokes equation for the flow of an incompressible, Newtonian fluid.
SPEJ
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Optimization of Bit Hydraulic Configurations