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Copyright 2005, SPE/IADC Drilling Conference This paper was prepared for presentation at the SPE/IADC Drilling Conference held in Amsterdam, The Netherlands, 23-25 February 2005. This paper was selected for presentation by an SPE/IADC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers or the International Association of Drilling Contractors and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the SPE, IADC, their officers, or members. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers or the International Association of Drilling Contractors is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.

Abstract An advanced series of PDC drill bits incorporating a new highly abrasion-resistant PDC cutter has extended effective PDC bit application to hard rock drilling. In direct offset comparisons, the advanced series of PDC bits fitted with the new cutters delivered significant increases in footage drilled and rate of penetration.

To achieve an optimum match in drilling efficiency and bit life to lower costs and mitigate risk in hard rock environments, the series is designed using a combination of advanced modeling capabilities and sophisticated analytical tools. These tools allow the designs to be "customized" for specific applications, optimizing cutting efficiency and durability according to specific rock properties and drilling parameters. A transitional drilling model simulation allows evaluation of how cutting forces are affected during transitional drilling, common in hard rock environments. The bit design is globally balanced to optimize axial, lateral, and torsional forces, and can be modified by adjusting features such as profile shape, cutter rake angles, impact arrestors, and cutter type, to optimize bit performance when drilling in hard and transitional environments.

In addition, recognition of a third dimension of PDC performance Thermal Mechanical Integrity (TMI) -- has lead to development of a new PDC cutter that provides 13.5 times the abrasion resistance of the industry standard, without sacrificing impact resistance. This improved understanding of PDC cutter failure provides a different way of looking at the traditional characteristics of abrasion and impact, enabling cutter durability to be optimized in both abrasive and hard, inter-bedded formations.

The paper discusses the science behind the advanced series of bits, including the impact of TMI on cutter performance. New laboratory capabilities and testing results are described,

and actual field case histories presented to demonstrate performance improvements of these PDC bits in hard rock applications.

Introduction One of the greatest challenges that any PDC bit manufacturer faces today is the extension of PDC bit application into hard rock drilling, where impact damage, heat damage and abrasive wear of PDC cutters limits performance. Research and development have been focused on better understanding of cutter/formation interaction, cutter performance, bit dynamics and BHA dynamics.

Since the first modeling studies conducted by Sandia Laboratories in the late 1980s, analysis of the interaction between the cutting elements of a PDC drill bit and the formation it is drilling has been widely investigated. One of the predominant developments from these early investigations was the first reliable kinematics cutter force and wear prediction model. These models were helpful for bit manufacturers to better understand the mechanism of cutter/formation interaction and to design the cutter layout of a PDC bit so load and wear of cutters over the bit face can be balanced.

Perhaps the most significant advancements in understanding how this interaction affects bit performance were the result of research conducted by the Amoco Research Center during the late 80s. 1 Laboratory tests demonstrated that conventional PDC bits whirl backwards during drilling, and backward whirl was a primary cause of PDC cutter damage. This important finding led to extensive studies in bit dynamics and drillstring dynamics. Bit dynamics models, including BHA dynamics models were developed and were able to repeat the backward whirl phenomenon under specific conditions. However these dynamics models were rarely used by bit manufacturers in the bit design process due to their complexity and limited ability to consider the effects of cutter layout on bit dynamics.

There were two design principles identified; namely, anti-whirl PDC bit design and force balanced PDC bit design. The anti-whirl PDC bit design principle 1, 2 incorporated a specific design concept, namely low friction gauge, to minimize the effect of bit backward whirl. According to this design principle, cutters were so arranged over the bit face so that a net resultant radial force (around 12% of weight on bit) was directed toward a specified portion of the bit with less friction.

SPE/IADC 91840

New Bit Design, Cutter Technology Extend PDC Applications to Hard Rock Drilling Robert Clayton, Shilin Chen, and Guy Lefort, HES-Security DBS

2 CLAYTON, SHILIN, LEFORT SPE/IADC 91840

Since introduction of anti-whirl technologies in the late 80s, PDC bit utilization has made significant inroads into roller cone markets, but has consistently faltered when drilling the traditionally more difficult insert roller cone applications, such as hard rock. In these applications, where roller cone bits suffer short bit life and slow ROP as well as risk loss of cones, PDC bits typically suffer short life as a result of high impact damage, large vibration and abrasion.

The force balanced PDC bit design principle 3 was developed based on the understanding of PDC bit dynamics and the mechanism of cutter/formation interaction. Unlike an anti-whirl PDC bit, the cutters on a force balanced PDC bit were arranged so that a net resultant radial force was minimized or balanced. The use of tracking cutters, where multiple cutters located within a groove, provided a restoring force to keep the bit rotating around hole center. The design of a force balanced PDC bit allows a higher density of cutters on the gauge, which is usually required in hard formation drilling, than does the anti-whirl PDC bit. The use of force balancing, tracking cutters and asymmetrical spiraled blades improved bit performance significantly, further expanding the range of applications for PDC bits.

Recently, significant performance improvements have been achieved with a new series of highly-engineered PDC bit designs utilizing advanced cutter technology to expand the range of PDC applications into hard rock drilling. This paper describes several key concepts and features including global balancing, energy balancing, transitional impact prediction model, and the development of highly abrasion-resistant PDC cutters based on an improved understanding of thermal cutter failure mechanisms.

Improved Cutter/Rock Interaction Model Advanced Cutter/Rock Interaction Model. In order to accurately calculate the forces acting on a PDC bit, it is necessary to have a model presenting the cutter/rock interaction. 4-6 Such a model should be able to predict the forces acting on a cutter, as well as the force distributions over cutting area when the cutter is engaged with the formation. Previous models used in either kinematics calculation or in dynamics calculation considered only three summarized forces on a cutter based on the engaged area: drag force, normal force and side force. 7,8 Fig. 1. Such a model is no longer valid with the introduction of PDC cutters with a chamfered geometry, which significantly affects bit ROP and WOB/TOB relationship. Furthermore, laboratory tests have found that the orientation of PDC cutter relative to rock surface, defined by back rake angle, side rake angle and helical motion, play a significant role in the determination of cutting forces.

As shown in Fig. 2, a new cutter/rock interaction model is developed in which the cutting edge is divided into three surfaces: cutting face surface, chamfer surface and cylinder surface. There are three forces acting on each surface, namely, drag force, axial force and radial force. In order to accurately calculate the engagement area of the cutter, each surface is meshed into very small grids. In this way, the effects of cutter orientation on the engagement area can be considered. Depending on the cutting depth, forces on each surface may be significantly different.

Advanced cutter wear model. It is well known that cutter wear depends on cutting force, relative speed, temperature, cutter material properties and rock properties. Previous wear models estimated only the wear flat without considering the orientation of the wear flat, the actual diamond thickness, the interface geometry of diamond layer and carbide, and abrasive resistance.

With the new cutter/rock interaction model described above, cutter wear can be considered three dimensionally and all neglected factors in the previous model can be easily considered. (Fig. 3).

Global Balancing Concept for PDC Bits There are three forces acting on a bit: bit axial force, bit lateral force and a bending moment. It has been believed that balancing lateral force is very important to prevent whirl. Previous concepts of PDC bit force balancing referred only to lateral force balance. It was also believed that once lateral force was balanced, bit bending moment was also balanced.

A further study on bit forces found that even a perfectly force balanced bit may exhibit tilt motion caused by bending moment. In fact, bit bending moment consists of two parts: one contributed by lateral force and another contributed by axial force distribution over bit face. Fig. 4 shows an example of bending moment generated by uneven distribution of axial forces over cutters.

Bit bending moment contributes not only to bit lateral motion or whirl, but also to bit tilt motion, which has a significant effect on bit directional control. Therefore, bending moment balancing becomes equally important as lateral force balancing. A PDC bit featured with both lateral force and bending moment balanced is a "global force balanced" bit.

The design of a global force balanced PDC bit involves adjusting the cutting structure to reduce the imbalance numbers. The new series of bits are force balanced according to a specific set of design criteria, which considers the summation of cutter forces to a global lateral and axial bit imbalance. The global lateral bit imbalance consists of the total, radial, and drag imbalances. Energy Balancing Concept for PDC Bits The energy balanced concept was initially developed for roller cone bits, where forces and rock removed by each cone (cutter) were evenly distributed among cones. Significant performance improvement of energy balanced roller cone bits has been consistently observed in the field worldwide. 9

The development of an energy balanced PDC bit is based on the fact that the amount of formation removed by each cutter on a bit is different, and as a result, the force acting on each cutter also differs. Furthermore, the number of cutters differs from blade to blade, therefore, the forces acting on each blade differ. In order to avoid overloading individual cutters and blade, it is necessary to control the load distributions over the cutters and blades.

An element of an efficiently running bit is a cutting structure that evenly distributes work among the cutters during normal drilling. Energy balancing accomplishes even disribution by minimizing the change in work or force among regions or zones of the cutting structure. By controlling the

SPE/IADC 91840 NEW BIT DESIGN, CUTTER TECHNOLOGY EXTEND PDC APPLICATIONS TO HARD ROCK DRILLING 3

force distribution over blades and over cutters, energy balancing reduces impact damage and uneven wear while promoting improved ROP.

The design of an energy balanced PDC bit involves analyzing the distribution of work and forces on a cutting structure with the aim of controlling force distribution over blades and cutters. As with force balancing, a specific set of design criteria has been developed which considers the ratio of the average change in cutter torque across zones, to the average cutter torque over the entire region.

Transition Drilling Model The Transition Drilling Model simulates a fixed cutter bit drilling through a change in rock strength such as sandstone to shale. This transitional drilling model simulation allows evaluation of how cutting forces are affected during transitional drilling, common in hard rock environments.

The program uses the new cutter/rock interaction model to calculate the amount of torque per revolution each cutter experiences through the transition layer (Fig. 4). The model data is used in the design stage, providing the ability to easily evaluate lateral, torsional, and axial cutter forces and work to control impact damage, further enhancing cutter performance.

This technology gives the designer the capability to evaluate how the cutting forces are affected when the bit is drilling into a harder or softer rock. In effect, this technology enables designers, during bit design phase, to identify trouble zones where impact damage could occur while transition drilling. Bit features such as profile shape, blade count, start of secondary blades, cutter back rake, impact arrestor location, can all be manipulated to improve the bits ability to drill transitional formations. (Fig. 5).

Development of New Cutter Technology In addition to advanced models and design tools, the new series of bits incorporate advanced PDC cutters, developed as a result of an improved understanding of cutter failure mechanisms. Understanding a PDC cutters failure characteristic is the key to understanding how to improve its performance. Historically, abrasion and impact have been the two characteristics observed and studied.

Abrasion refers to the mechanically generated wear that occurs due to failure of the individual diamond crystals and/or the diamond-to-diamond bonds within the diamond table. The diamond failure can be a result of mechanical loading and/or thermal degradation.

Impact wear is a mechanical failure that occurs when forces are applied which overcome the strength of the bond between diamond crystals and/or to the carbide of the PDC.

As a result of extensive research and development, a third dimension of PDC cutter failure has been identified, Thermal Mechanical Integrity (TMI). TMI failure is defined as loss of diamond that occurs due to a combination of thermal degradation and force, and is a measurement of the cutters toughness as wear and thermal degradation occur.

This improved understanding of cutter failure has brought about a different way of looking at the traditional characteristics of abrasion and impact. Advanced testing capabilities now enable optimization of cutter durability in both abrasive and hard inter-bedded formations, and led to

development of a cutter more suitable for the tougher environments of hard rock drilling. (Fig. 6).

Testing Capabilities. Finite Element Analysis (FEA), acoustic and scanning electron microscopic (SEM) analysis, and destructive testing (DT) are utilized to evaluate new diamond and carbide materials, new manufacturing processes, and new designs in order to optimize all performance aspects of a new PDC cutter.

Finite Element Analysis (FEA) evaluates stresses that occur between the diamond table and the carbide substrate as a result of the differences in rate of thermal expansion between the two materials. Significant changes in temperature can occur during manufacturing or due to drilling temperatures down hole. FEA images allow the residual stresses to be analyzed under different parameters. (Fig. 7). By manipulating the interface geometry between the diamond table and the carbide, the stresses can be managed to optimize the cutter's performance. (Fig. 8).

The acoustic and SEM analyses are utilized to evaluate changes in material properties and characteristics throughout the manufacturing process and to qualify the final product.

DT is utilized to validate the overall development for abrasion resistance, impact resistance, and thermal mechanical integrity in order to provide results that closely resemble cutter failure characteristics observed in the field.

Among DT tests, the G-ratio abrasion test addresses the diamond abrasion resistance, while heavy wear testing extends not only into the diamond table but also into the carbide. Extension tests the ability of the entire cutter to remove rock, and provides the ability to test all three characteristics of PDC failure modes: abrasion, impact and thermal degradation.

Cutter Testing Results. Laboratory testing showed the new PDC cutters to be 20 times more abrasion-resistant than the industry standard and 3.9 times that of industry premium cutters, with no loss of impact resistance. (Fig. 9)

Over an 18-month period, 22 different specific cutter iterations were developed, manufactured, and laboratory tested. Many of these were also taken to the field for extensive testing under actual drilling conditions. Field-testing provided additional knowledge and drove additional insights and improvements in the new technology cutters.

The result was a highly abrasion-resistant PDC cutter suitable for applications in which accelerated wear typically leads to thermal mechanical failure. This includes hard and abrasive applications where cutting efficiency must be maintained, and has allowed expansion of PDC applications into Hard Rock where IADC 4 thru 7 Type Insert RC bits currently are used.

Field Applications The innovations incorporated into the new series of bits and described in this paper provide a number of benefits in terms of drilling performance. Global force balancing reduces lateral and axial vibration to maximize ROP, while energy balancing evenly distributes cutter forces, reducing impact damage and uneven wear. Use of new PDC cutters with greater TMI offers significantly increased abrasion-resistance with no loss of impact-resistance, effectively drilling even hard rock formations.


While specific case histories will be detailed in forthcoming papers, the following summaries describe typical performance to date of the new series PDC bits, designated FM3000 with the new highly abrasion-resistant PDC cutters, designated Z3:

Travis Peak Formation. The Travis Peak formation in the Oak Hill field in East Texas comprises primarily very fine grained sandstone interbedded with shale and mudstone. Traditionally the interval could be drilled only by several IADC 647Y to 817Y insert bits. Recent efforts of using PDC bits to drill this section have been made with only marginal economic improvement. 10-11

A new PDC bit designed based on the principle described in this paper, namely, a 7-7/8 FMX753 with the advanced Z3 cutters, successfully drilled 1213 feet of at 29.6 feet/hour, outperforming the next-best offset by 34% more footage and 7% better ROP.

The FMX753 drilled 58% more footage at a 40% higher ROP than the average of all competitor offsets in the field, including one offset that utilized two competitor bits. Even when combined, those two competitor bits did not equal this single run performance, which dropped cost per foot from a high of $73.54 to just $19.20 per foot. (Fig. 10).

In addition, the bit was pulled in repairable condition with a dull grade of 1-2-WT-S-X-I-CT-PR. By comparison, competitor designs were not repairable and had an average dull condition of 6-8-RO-N/S-X-3-BT-PR. (Fig. 11).

Almond Formation. This new technology has also successfully drilled to TD in the Almond formation, an extremely tough, interbedded formation at the end of a long tough run. For this application, an FMX655 was designed with an aggressive cutting structure incorporating the new Z3 cutters, as well as continuous spiraling along bit cutter blades through the gage pad for optimized distribution of lateral forces.

The bit successfully drilled the Almond formation to well TD at an average ROP of 50 ft/hr, establishing a new ROP performance record. In addition to achieving the best penetration rate, the bit also resulted in the best dull condition of any bit run in the Almond, and was pulled in re-runnable condition.

Parkland Field Formation. In this case a 7-7/8-in. FM3645 drilled 1162 meters in 44.5 hours to achieve the fastest ROP in the Parkland Field to date, 26.11 m/hr. The bit outperformed all offset bits in this application, achieving a record average ROP of 26.11 m/hr. As a result of this outstanding ROP performance, the customer realized approximate savings of $31,000.

Red Rock Formation. In this case, an 8-3/4-in. FM3541 drilled significantly greater footage than all offsets, while maintaining a competitive ROP. The FM3541 bit drilled a total 874 meters, compared to the next-best offset performance of 240 meters. The bit also maintained an average ROP of 5.67 m/hr to outperform the next-fastest ROP of 3.8 m/hr. As a result, the operator saved the cost of another bit plus trip time, for approximate total savings of $28,900.

Conclusions A series of PDC drill bits incorporating a new highly abrasion-resistant PDC cutter has extended effective PDC bit

application to hard rock drilling. In direct offset comparisons, the advanced series of PDC bits fitted with the new cutters delivered significant increases in footage drilled and rate of penetration.

The innovations incorporated into the new series of bits and described in this paper provide a number of benefits in terms of drilling performance:

Global force balancing provides maximized ROP with reduced lateral and axial vibration.

Energy balancing evenly distributes cutter forces, reducing impact damage and uneven wear while promoting improved ROP.

Transition Drilling Modeling allows for faster transitioning with reduced impacts and increased toolface stability.

New PDC cutters with greater TMI offer significantly increased abrasion-resistance with no loss of impact-resistance, effectively drilling even hard rock formations.

Designers now have the ability to easily evaluate lateral, torsional, and axial cutter forces and work. Analysis of the distribution of these parameters from cutter to cutter, blade to blade, and between different bit profile segments has aided in the discovery of features that will continue to improve overall bit performance.

References 1. T.M. Warren, J.F. Brett and L.A. Sinor, "Development of a Whirl

Resistant Bit", SPE Drilliing Engineering, 5 (1990) 267 274. 2. T.M.Warren and L.A.Sinor, "PDC Bits: Whats Needed to Meet

Tomorrows Challenge", paper SPE 27978, presented1994. 3. G.E.Weaver, R.I.Clayton, "A New PDC Cutting Structure

Improves Bit Stabilization and Extends Application into Harder Rock Types", paper SPE/IADC 25734, presented 1993.

4. Ortega and D. A. Glowka, "Frictional Heating and Convective Cooling of Polycrystalline Diamond Drag Tools During Rock Cutting", SPE Journal of Petroleum Technology, 24 (1984) pp.121-128.

5. D.A. Glowka, and C.M. Stone, "Effects of Thermal and Mechanical Loading on PDC Bit Life", SPE Drilling Engineering, 1 (1986) 201-204.

6. D.A. Glowka, "Use of Single-Cutter Data in the Analysis of PDC Bit Designs: Part 1 Development of a PDC Cutting Force Model," SPE Journal of Petroleum Technology, 41 (1989) pp.797-849.

7. C.J. Langeveld, "PDC Bit Dynamics", paper IADC/SPE 23867, presented 1992.

8. J.M.Hanson, W.R.Hansen, "Dynamics Modeling of PDC Bits", paper SPE/IADC 29401, presented 1995.

9. S. Chen, and J. Dahlem: Development and Field Applications of Roller Cone Bits with Balanced Cutting Structure, paper SPE 71393 presented at the 2001 SPE / ATCE Annual Meeting, New Orleans, 30 September 2001.

10. E.J.Schell, D. Phillippi, ect, New, Stable PDC Technology Significantly Reduces Hard Rock Cost Per Foot, paper SPE/IADC 79797 presented at the SPE/IADC Drilling Conference held in Amsterdam, The Netherlands, Feb., 2003.

11. R. Fabian, S.Johnson, ect, Enhancements in Design Technology and Performance of Stable PDC Bits Revolutionize Hard Rock Drilling in East Texas Fields, Addendum to SPE/IADC Paper No.79797, IADC/SPE 87098 presented at the IADC/SPE Drilling Conference held in Dallas, Texas, USA, March,2004.


Acknowledgments The authors would to thank Halliburton and Security DBS for permission to present this paper. Collective thanks to all the operators, contractors and field personnel who collaborated to make these runs possible. Special thanks go to Larry Eichman, Robert Gum, Charlie Cogdill, Brian Davies, and Dave Herman for their efforts on the case histories used in this paper.


Fig. 1 Previously used cutter/rock interaction model.

Fig. 2New cutter/rock interaction model divides cutting edge into three surfaces, each with its own force model.

Fig. 3Wear profile predicted with numerical bit model.

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Fig. 4Bending moment generated by uneven distribution of axial forces over cutter Fig. 5 Cutter locations are manipulated to reduce maximum change in torque for individual cutter and to distribute change in torgue equally over more cutters. Fig. 6Thermal Mechanical Integrity is the third dimension of PDC cutter per formance.

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Fig. 7Interface geometry between diamond layer and carbide is optimized to reduce residual stress. Fig. 8Interface geometry of new Z3 PDC cutter. Fig. 9New cutter offers 20 times the abrasion resistance of industry standard cutters.

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Fig. 10Travis Peak interval drilled by new PDC bit and recent comparable offset PDC bit performance. Fig. 11Dull condition after drilling 1213 ft of Travis Peak formation at almost 30 ft/hr, outperforming offset by 58% more footage and 40% higher ROP.

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New PDC