Laboratory Drilling Performance of PDC Bits

Abstract

Summary A laboratory study of polycrystalline diamond compact (PDC) bit designs has generated data that give an insight into PDC-bit performance in the field. The tests reviewed in this paper include those for rate of penetration (ROP), torque response, hydraulic energy sensitivity, balling tendency, dull-pit performance, and bit performance after the removal of selected cutters. A total of four bit designs was tested. The designs included flat-faced profiles and parabolic profiles. Introduction PDC bits have gained increasing favor because of advancements in the materials used in bit manufacturing and improvements in bit designs. These advancements have relaxed some of the requirements limiting PDC-bit use (e.g., use of oil muds to get good bit performance). In some areas, PDC bits are being used to drill more than 50% of the intermediate and production hole footage. The results of a laboratory study designed to evaluate various factors affecting bit performance are presented in this paper. The bits were chosen from several manufacturers on the basis of their present or prospective use in Louisiana gulf coast drilling. Because comparable bit designs can be obtained from more than one manufacturer, it is not intended to compare the performance of different manufacturers' bits, but rather to compare the general designs. The performance of a particular bit design may vary, depending on subtle design variations, manufacturing quality control, and cutter attachment procedures, but these differences may not be detected in the short-duration testing described here. Rarely is there a perfect bit design to drill a particular hole interval. Therefore, the bit selection for a specific application involves a compromise between positive and negative attributes of the available designs. Considerations must be made for variations in lithology, as well as for operational limitations of other drilling-system components that affect bit performance. This paper is intended to help quantify the effect of various design features, formation types, bit wear, and fluid types on the overall PDC-bit performance. It is recognized that to do this properly, much more information is needed than can be presented in a single paper. Even so, the material presented here is useful in improving bit selections. The results documented here were obtained from a laboratory PDC-bit testing program over a 2-year period. These tests were conducted in different sets designed to investigatethe effect of bit dullness on ROP,performance differences in water- and oil-based muds,the effect of number of cutters on the bit performance, andthe performance of current bit designs. The purpose of the testing was to understand better the strengths and weaknesses of the different bit designs, thereby improving the bit-selection process and helping to define operating guidelines. Bit Designs Four basic bit styles, shown in Figs. 1 and 2, were used in the test program. All the bits were 8 1/2-in. [21.6-cm] diameter. The two bits in Fig. 1, designated as Bits A and B, are similar designs built with 1/2-in. [1.27-cm] -diameter cutters. Bit A is a matrix-body bit with 32 cutters and four jets on the bit face. Bit B, designed with a slightly more rounded face, is a steel-body bit with 39 cutters and five jets. Two bladed bits with tapered or parabolic profiles were also tested. The major design difference in these bits is the number and size of the cutters. These bits are shown as Bits C and D in Fig. 2 and are referred to as the large-cutter bladed bit and small-cutter bladed bit, respectively. The cutting structure of the large-cutter bladed bit is made up of seven 1 1/2-in.. [3.8-cm] -diameter PDC cutters arranged on three blades with six 1/2-in. [1.27-cm] -diameter cutters for gauge protection. The small-cutter bladed bit has 41 conventional 1/2-in. [1.27-cm] -diameter round cutters set in six radial blades. In addition to these bits, three bits similar to Bit B were also tested. Two of these bits were field-worn and had significant wear flats on the cutters. The third bit was new and had chisel-shaped cutters. Testing Procedures Bit-performance tests were conducted by drilling 14-in. [36-cm] -diameter cores that were about 3 ft [0.9 m] long. The cores were held in a pressure vessel with a constant borehole pressure of 1,250 psi [8.6 MPa]. Data for each bit were collected by varying the weight on bit (WOB) during drilling at a fixed rotary speed and flow rate. The WOB was computer-controlled and incremented according to a preprogrammed schedule. Time averages of 21 performance variables, including ROP and torque, were recorded at each level of WOB. Tests were conducted at flow rates, varying from 250 to 460 gal/min [0.9 to 1.7 m3/min] to determine the effect of hydraulics. Rotary speeds of 60 to 175 rev/min were tested. In addition to the bit-performance tests, ROP and torque data were collected at normal and reduced hydraulic levels while the tendency of a bit to ball was studied under stabilized operating conditions. The tests were conducted by drilling all but the top 6 to 10 in. [15 to 25 cm] of the core under constant operating conditions. The first few inches of the core was drilled to establish a bottomhole pattern and to build the WOB to the desired level. The bits were examined visually and photographed at the completion of each test to understand the balling mechanism better. Unless otherwise indicated, the tests were run with an unweighted bentonite/water mud. Typical properties for this mud were as follows: mud-weight, 9.2 lbm/gal [1102 kg/m3]; plastic viscosity, 10 cp [10 mPa·s]; yield point, 12 lbf/100 ft2 [5.7 Pa]; and solids content, 4%. The actual properties for any particular test may have been somewhat different. Bit Performance Bit performance is strongly influenced by rock properties (see descriptions of the test rocks in the Appendix). Figs. 3 and 4 show the performance of each bit in the Berea sandstone. No appreciable difference is seen in either the ROP or the bit torque in this rock. There is a more pronounced difference in the performance of the bits in the Carthage limestone (see Fig. 5). Three of the bits performed similarly with only a minor difference between the two flat-faced bits. Bit D drilled much faster than the other bits for the same operating conditions. Its performance was confirmed by three separate tests that proved it did drill faster than the other bits.