Temperature Distribution in Synthetic Diamond Cutters During Orthogonal Rock Cutting
- V. Prakash (Kansas State U.) | F.C. Appl (Kansas State U.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- June 1989
- Document Type
- Journal Paper
- 137 - 143
- 1989. Society of Petroleum Engineers
- 1.5 Drill Bits, 4.3.4 Scale, 5.1.5 Geologic Modeling, 1.6.9 Coring, Fishing, 1.5.4 Bit hydraulics, 5.3.4 Integration of geomechanics in models, 1.5.1 Bit Design, 1.2.3 Rock properties, 1.6 Drilling Operations
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A heat-transfer model based on the finite-element method is developed for synthetic polycrystalline-diamond-compact (PDC) cutters used in oilwell drill bits. The model is verified by comparison with single-cutter experiments. Effects of process variables, such as film coefficient and diamond thickness, on the temperature distribution in the cutter are studied.
The introduction of synthetic PDC cutters for use as cutters on oilwell drill bits by General Electric in 1973 was a significant advancement in drilling technology. The use of these bits with synthetic diamond cutting elements grew because of their effectiveness in drilling soft to medium formations at relatively high penetration rates. Most of the research work related to these bits was devoted to experimental and theoretical work related to the mechanical aspects of cutting. It was recognized, however, that the synthetic diamond compact material that is currently used deteriorates (loses strength and abrasion resistance) rapidly at temperatures exceeding 1,290 to 1,380F [700 to 750C]. This is thought to be one of the main reasons that successful drilling of harder formations with PDC bits has not been possible. What is needed to continue the improvement of PDC bits is the development and experimental verification of a theoretical model for the heat transfer and the temperature distributions in the rock and cutters near the cutting tips of the cutters. Some work was done on this problem, but the thermal analysis was not combined with a cutting theory from which the heat generation caused by shearing of the rock and the heat caused by sliding friction of the rock on the rake face of the cutter could be obtained. The analysis was based on heat input to the cutter on the wear-flat only and thus is not complete. Zijsling did experimental and theoretical work on PDC bits under field drilling conditions. The thermal analysis shows that the bulk of the heat flows into the diamond, but the effects of the heat generated along the shear plane and along the rake face were not included. No experimental temperature measurements were made. The development of an improved cutting and heat-transfer model appears to be timely and realistic, in view of the significant R and D work done in this field and in the related field of metal cutting. The basic problem of cutting rock with PDC has been studied both experimentally and theoretically since the introduction of these cutters to drilling in 1973. Eaton et al. conducted abrasion, impact, and rotary-drilling tests with PDC cutters. These tests demonstrated that the cutters could be used to cut rock, but that the cutters experienced abrasive wear that produced wear-flats on the cutters and that the diamond layer was subject to fracture because of impact. Single-cutter tests were made by Heller by cutting a cylindrical rock on a lathe. Results of these tests were also reported by Feenstra et al. These tests showed that, because of the flat cutting face and the relatively sharp cutting edge, the cutting forces for PDC cutters are much smaller than for natural diamonds, which are essentially spherical in shape. Some single-cutter experiments, made by cutting Georgia granite with a shaper, were reported by Radtke et al. Single-cutter turning tests and three cutter-core drill tests were conducted by Bailey and Bex, and an attempt to correlate results of the two types of cutting was made. The correlation was not good, but Bailey and Bex found that the load, torque, and wear were the lowest for a negative rake of 15. Dunn and Lee studied experimentally the fracture and fatigue of synthetic diamond compact material and found that the fracture stress of the diamond compact was reduced after cyclic loading. They also postulated that the failure load necessary to cause fatigue failure is much greater than the loads encountered in rock cutting. This may not be the case, however, because fracture does occur and is still a major problem in some types of rock. Hibbs and Lee conducted an in-depth experimental study of the wear processes of sintered PDC's during the cutting of a cylindrical rock with a single cutter on a lathe. The cutting velocity varied from 100 to 300 ft/min [30.48 to 91.44 m/min], and it was found that, under the cutting conditions investigated, the wear and breakage mechanisms were essentially mechanical in nature and thus were not thermally activated. Hibbs and Flom continued the single-cutter experiments and measured cutting forces, which were found to be largely independent of cutting geometry and depth of cut. This work was continued by Lee and Hibbs, who used cutting velocities up to 650 ft/min [198.12 m/min]. With this extended velocity range, it was found that there was an abrupt increase in the wear rate of the diamond at 410 ft/min [124.97 m/min]. The wear rate increased by roughly an order of magnitude. It was suspected that this was related to the effects of increased cutter temperature, so a series of microhardness-vs.-temperature tests was made for the same type of synthetic diamond. These tests indicated that the micro-hardness decreased abruptly in the temperature range of 1,290 to 1,380F [700 to 750C]. This suggests that the abrupt increase in wear rate in the cutting tests is caused by decreased mechanical strength, resulting from the increased cutter temperature. Unfortunately, no measurements of cutter temperature were made, so a direct correlation with the hardness was not possible. Nevertheless, this is the only known published experimental work in which strength-vs.-temperature data and rock cutting and wear data were measured for the same diamond material. These data provide the opportunity for initial verification of the heat-transfer model. The first step in developing the heat-transfer model is to analyze the stresses in both the rock and cutter near the cutting tip for orthogonal cutting with a PDC cutter. The cutting model developed by Prakash provides this first step. Then, a two-dimensional (2D) heat-transfer model is developed in much the same way as was done for orthogonal metal cutting. Although the heat-transfer problem is three dimensional, it is believed that the 2D model will be adequate because the theoretical temperatures predicted for metal cutting have been found to correlate well with experimental results.
The Cutting Model
The cutting theory developed by Prakash is based on a Merchant's-type single-shear-plane analysis and is used as a first step toward developing the heat-transfer model. Figs. 1a and 1b show a schematic of the cutting process. It involves the movement of a single-edged cutter in the transverse direction over a stationary rock to produce a chip. The analysis is done for a 2D plane-strain, rigid, plastic-flow field. The tool has a rake angle, alpha which is negative, and cuts with a depth of cut D. The friction between the rake face and chip is determined by a method based on studies by Wanheim et al. and by Abebe. The cutting model is used to define the geometry of the cutting process and to determine the heat generated along the rake face, shear plane, and the wear-flat. The heat generation on the shear plane cannot be precisely determined, but the work done because of mechanical shearing can be used to approximate the heat generation by estimating the proportion of work that is converted to heat.
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