Effects of Thermal and Mechanical Loading on PDC Bit Life

Abstract

Summary Results of numerical analyses to determine the effects of thermal and mechanical loading on the integrity of polycrystalline-diamond-compact (PDC) cutters during deep-hole drilling polycrystalline-diamond-compact (PDC) cutters during deep-hole drilling are presented. Cutter-temperature distributions for several drilling environments are combined with the appropriate mechanical cutting loads to predict stress distributions for the study of cutter wear mechanisms. A predict stress distributions for the study of cutter wear mechanisms. A discussion of wear mechanisms acting on the cutting structure is presented. Suggestions based on these analyses are presented for the presented. Suggestions based on these analyses are presented for the operation of PDC bits to increase bit life in hard, abrasive rock drilling. Microstructural design considerations for the cemented carbide that supports the diamond layer are also discussed. Introduction In deep drilling for geothermal and fossil fuel resources, rate of penetration (ROP) and bit life are important factors in the determination of well cost. The use of PDC bits in oil and gas drilling can result in substantial well cost reduction in certain formations. Drilling cost savings of more than $100,000 have been reported for single PDC bit funs, and total drilling cost savings of 30 to 50% are common in some areas. Generally, such savings result from greatly improved bit life (two- to ten-fold) and smaller improvements (zero to two-fold) in ROP. In more severe applications, such as geothermal drilling and hard-rock drilling for oil and gas, a reduction in bit life and ROP is experienced. Because of their aggressive nature, however, PDC bits can generally drill as fast as or faster than roller bits in any environment. Bit life then becomes the primary factor in the determination of the economics of a given bit run, in the absence of other drilling problems. The impacts of ROP and bit life on typical geothermal-well costs have been determined with a well-cost simulator. Results show that if ROP and bit life can be doubled by the use of PDC bits, the total well cost should be reduced by 10 to 15%. If bit life is reduced by 50% as a result of PDC bits, however, savings resulting from improved ROP are cut in half. In particularly severe environments, PDC bits can increase drilling costs because of poor life. Thus, there is an incentive to improve bit life for any potential application of PDC bits to take advantage of the inherently higher ROP's. Although bit life and ROP are both variables in the determination of drilling costs, the two are actually interrelated. Limited experimental data with hard, abrasive sandstone show that PDC-cutter wear rates increase one to two orders of magnitude when cutter-wearflat temperatures exceed a critical value of about 350C. [660F]. Yet wearflat temperatures in a given environment are controlled by weight on bit (WOB) and rotary speed. Because these two variables are also the primary controllable factors influencing ROP, the trade-off between ROP and bit life is evident. This paper examines the effects of controllable drilling variables on PDC bit life. It is an extension of earlier work and is aimed at identifying conditions that lead to rapid cutter wear and uneconomic bit life. General Wear Considerations It has been shown that in the absence of thermal effects, the volumetric wear, Vw, of a metal specimen sliding on an abrasive surface is proportional to the load, F, on the specimen and the sliding distance, Ls. Vw FLs......................................(1) Thus in the absence of thermal effects, 1 dVw = c. ................................(2)F dLs PDC-cutter-wear data presented in our earlier work are replotted in Fig. 1 in the form (1/F)(dVw/dLs) vs. Tw, where Vw is the measured wear volume while cutting hard, abrasive sandstone and Tw is the mean temperature across the cutter wearflat. If thermal effects are not important, Eq. 2 indicates that the data in Fig. 1 should be relatively constant with temperature. Although there is considerable scatter, the data seem to follow a different trend. This trend is consistent with the finding that adhesive wear of cemented carbides in machining steel decreases with sliding speed until a wearflat temperature of 450C [840F] is reached, after which wear increases. The decrease is attributed to thermal healing of defects in the microstructure and the increase to general thermal softening at higher temperatures. Although abrasive-wear mechanisms for PDC cutters in rock drilling are different from those for adhesive wear, these general thermal effects are evidently similar, at least qualitatively. SPEDE