Implications of Thermal Wear Phenomena for PDC Bit Design and Operation

Abstract

Abstract Previous work has suggested that thermal phenomena can greatly accelerate the wear of phenomena can greatly accelerate the wear of polycrystalline diamond compact (PDC) drag cutters polycrystalline diamond compact (PDC) drag cutters under certain conditions. In this paper, the implications of that finding for PDC bit design and operation are examined. An abrasive wear model for cutters is developed and applied to rotary drilling applications to demonstrate the significance of thermally-accelerated wear on bit life. Under conditions where thermal effects are not important, it is concluded that in order to maximize bit life, greater bit weights and lower rotary speeds are preferable over shallow cuts and high speeds. It is preferable over shallow cuts and high speeds. It is found that even under normal abrasive wear conditions, a PDC cutter develops a significant wearflat before one-half its useful life is expended, suggesting that PDC bits should be designed to operate effectively in the worn condition. A cutter placement criterion is presented for designing bits placement criterion is presented for designing bits for equal cutter wear. Through analysis of new experimental data, a technique is developed for approximating cutter penetrating forces in the presence of cutter interactions on a bit face and for presence of cutter interactions on a bit face and for estimating relative cutter wear. A hypothetical bit designed for equal rock removal volume per cutter is examined to determine the relationship between the equal volume and equal wear criteria for cutter placement. The complex role of thermal wear placement. The complex role of thermal wear phenomena in this relationship is also examined for phenomena in this relationship is also examined for the hypothetical bit in three different rock types. Introduction The success of PDC drag bits in the petroleum drilling industry is well documented in the literature. New performance records continue to be set as operating experience with these bits increases and new areas of application are found. Because of the inherently long life of PDC bits relative to roller cone bits under certain conditions, significant savings in total drilling costs are possible in some rock formations. The record published hole length drilled by a single PDC bit currently stands at over 6100 meters (20,000 feet), and savings of over $100k have been reported for a single PDC bit run. Such remarkable results, however, are not universal. Many rock formations are apparently not suitable for conventional PDC bits. The extreme stresses required to penetrate hard formations produce excessive frictional heating of the cutters, produce excessive frictional heating of the cutters, which leads to thermally-accelerated cutter wear. Adverse downhole conditions such as abrasive rock formations, elevated temperatures, and dynamic bit loading complicate and intensify the problem. The objective of Sandia's PDC bit research has been to determine the cause and effects of these phenomena on bit performance in order to extend the phenomena on bit performance in order to extend the inherently rapid and efficient drag cutting process to harder rocks and more severe environments such as those found in geothermal drilling. Increased understanding of wear processes under these conditions should help to improve PDC bit life in soft and moderately hard formations as well. REVIEW OF THERMAL WEAR PHENOMENA In earlier work, we developed an analytical/ numerical model for determining temperature distributions in stud-mounted PDC cutters under downhole conditions (see Figure 1 and Nomenclature for terminology). From this model, an equation for the mean cutter wear flat temperature was derived: T = T + 1 + fk 1/2 -1 (1) w fl hf By applying this equation to the results of cutter wear measurements under controlled conditions, a correlation was obtained between cutter wear (based on volume) and mean wearflat temperature.