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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.
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 dev...