Frictional Heating and Convective Cooling of Polycrystalline Diamond Drag Tools During Rock Cutting

Abstract

Abstract A numerical-analytical model is developed to predict temperatures in stud-mounted polycrystalline diamond compact (PDC) drag tools during rock cutting. Experimental measurements of the convective heat-transfer coefficient for PDC cutters are used in the model to predict temperatures under typical drilling conditions with fluid flow. The analysis compares favorably with measurements of frictional temperatures in controlled cutting tests on Tennessee marble. An equation incorporating several drilling parameters is developed to predict the mean operating temperature across the cutter wearflat, defined as that portion of the cutter surface that wears against the rock formation. It is shown that mean wearflat temperatures can be maintained below a maximum safe value of 750C [1,382F] only under conditions of low friction at the cutter/rock interface, regardless of the level of convective cooling. The ability of liquid drilling fluids to reduce interface friction is thus shown to be far more important in preventing excessive temperatures than their ability to provide cutter cooling. Because of the relatively high interface friction provide cutter cooling. Because of the relatively high interface friction developed under typical air drilling conditions, it is doubtful that temperatures can be kept subcritical at high rotary speeds in some formations when air is employed as the drilling fluid. Introduction Over the past several years, considerable interest has been focused on drag-type drill bits employing PDC elements. Although these bits have been most successful in drilling relatively soft formations, work has been under way at Sandia Natl. Laboratories to investigate the potential of PDC bits in the more severe environments typical of geothermal drilling. Unsealed roller bits have limited lives in such environments, and conventional sealed roller bits cannot be used in many geothermal reservoirs because of temperature limitations on seals and lubricants. Because PDC bits require no bearings or seals, they are particularly attractive for geothermal drilling, especially at the high rotational speeds typical of downhole motors. The high penetration rates and long lives achieved with PDC bits in certain formations further suggest that geothermal drilling costs might be reduced if PDC bits can be developed for this application. The inherent characteristics of drag bits impose high frictional heating on PDC cutters. The purpose of our study was to investigate the thermal characteristics of PDC elements mounted on tungsten carbide (WC) studs to understand the driving parameters behind the thermal response of these cutter. The incentive for understanding these parameter, is to permit action to keep cutter operating temperatures as low as possible. permit action to keep cutter operating temperatures as low as possible. The need for achieving this goal is illustrated by an examination of the wear characteristics of PDC cutters at various operating temperatures. Below 750C [1,382F], the primary mode of wear of PDC cutters is microchipping of the sintered diamond. It has been shown 3 that the intensity of this wear increases with sliding speed, presumably a result of the increased temperatures associated with higher speeds. Evidence indicates that this increased wear rate is caused by a decrease in the hot hardness of individual diamond crystals with increasing temperature. Above750C [1,382F], the wear mode changes from microchipping of individual diamond grains to a more severe form in which entire grains are pulled from the compact. This is caused by stresses resulting from differential thermal expansion between the diamond and residual metal inclusions along the diamond grain boundaries, which lead to intergranular cracking and grain boundary failure. By 800C [1,472F], the hot hardness of the WC stud is severely degraded, leading to accelerated wear of the stud itself, which even at low temperatures has a wear rate greater than that of sintered diamond. For temperatures above 950C [1,742F], WC is susceptible to plastic deformation and flow under applied surface shear. In this study, we therefore establish 750C [1,382F] as the maximum safe operating temperature of PDC/WC cutters but recognize that even below this temperature, wear rate can be significantly reduced by maintaining operating temperatures as low as possible. The major objective of our study was to identify parameters that can be controlled to achieve this goal. Approach. Numerical modeling of PDC cutters was performed to compute local temperatures for assumed frictional heating and convective cooling rates. Referring to Fig. 1, the assumptions used in this thermal modeling are the following. SPEJ p. 121