New Bit Designs

New advancements made in drill bit technology

Bit designers have made further strides in balancing, computer modeling, cutting structures and bearing and seal systems, all with an eye toward improving performance and efficiency, thereby lowering drilling costs further.

Ron Lord, Contributing Editor, Houston

Every year, bit designers, like car designers, use newly developed technology to make drill bits run faster, drill deeper and last longer in an effort to give operators more value for their bit dollar. In some cases, the added value is real, while in other cases, it is simply perceived. In the end, it is the operators who must determine which bits actually do perform better. Here are some of the latest technologies being applied to drill bits.

TECHNOLOGY OVERVIEW

Generally, the increased use of computer modeling, for roller cone and PDC bit design and manufacturing, is producing a new generation of bits that delivers breakthroughs in rates of penetration (ROP), increased durability and longer life. These computer models utilize proprietary algorithms to model forces and bit behavior to assure maximum bit performance.

Additionally, computer modeling of the dynamics of interactions between PDC bit cutters and rock allows bits to be custom-designed for specific applications. Computer-aided design tools are being employed to model, with 3D and 4D graphics, the drag, axial and radial forces acting on the bits’ cutting surfaces.

One manufacturer’s cutter/ rock interaction model divides the cutting edge into three surfaces: cutting face, chamfer and cylinder surfaces. The computer calculates the cutter’s engagement area by meshing each surface into grids, so the cutter orientation’s effects on the engagement area can be considered.

Fig. 1. This roller cone product from Security DBS is an example of a bit that is both force-balanced and load-balanced. It also has optimized tooth orientation and an anti-tracking feature.

Data on advanced cutter wear is one of the results of this modeling. Cutter wear depends on cutting force, relative speed, temperature, cutter material properties and rock properties. Previously, computer models estimated only the wear flat without considering its orientation, as well as the actual diamond thickness, the interface geometry of the diamond layer, and carbide and abrasive resistance. With the newer computer models, cutter wear can be considered three-dimensionally, and all factors neglected by previous models are now easily considered. Bit designers then use this information to devise a bit specifically for a particular job.

Bit balancing. Another development that is becoming more important is bit balancing, Fig. 1. This concept considers the forces acting on the bit to create designs in which no single cone or cutting system is overstressed. This increases cutting efficiency and extends bit service life. Two types of balancing methods are used – force balancing and load balancing.

Force balancing. Of the three forces acting on a bit – axial force, lateral force and bending moment – it has long been recognized that balancing the lateral force is very important for preventing whirl. In fact, previous concepts of PDC bit force balancing referred only to lateral force balance, due to the belief that once lateral force was balanced, the bit bending moment was balanced also.

However, further study revealed that bit bending moment contributes not only to bit lateral motion or whirl, but also to tilt motion, which significantly affects directional control. Even a perfectly force-balanced bit may exhibit tilt motion, if the axial forces are not balanced. Therefore, balancing the axial forces is equally as important as balancing lateral force.

A PDC bit that is balanced, both in terms of lateral force and bending moment, is a “global force-balanced” bit. Designing such a bit involves adjusting the cutting structure to reduce the imbalance numbers. For example, newer series bits are force-balanced according to a specific set of design criteria that consider the summation of cutter forces to a global, lateral and axial bit imbalance, resulting in a global force-balanced design.

Load balancing. A bit in which the drilling forces acting on each individual cutter are balanced and are evenly distributed across the entire cutting is said to be “load balanced.” This technique is meant to prevent cutter wear and excessive point loading that can break or damage cutters.

Roller cone bits are load balanced in two ways – by volume and by force. Volume balancing almost equalizes rock removal among all the cones, while force balancing ensures that all cones are subjected to nearly the same loads, including weight-on-cone, bending moment and force-on-bearing.

For PDC bits, load balancing, which was employed originally on roller cone bits only, is now being used to improve the PDC bit performance. The concept of load balancing is based on the fact that the amount of formation removed by each individual cutter differs 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. To avoid overloading individual cutters and blades, it is necessary to control these load distributions.

Equally distributing the forces minimizes the change in work, or force, among zones of the cutting structure. Thus, designing a “torque- and drag-balanced” PDC bit involves analyzing the distribution of work and forces acting on a cutting structure, with the goal of controlling force distribution over both the blades and cutters. By controlling the force distribution, these bits are able to reduce impact damage and uneven wear while promoting improved ROP.

Transition drilling simulation. To simulate a fixed cutter bit drilling through a change in rock strength, such as from sandstone to shale, transition drilling simulation is employed to some extent by bit makers. This drilling simulation allows evaluation of how cutting forces are affected during transitions from one rock type to another, a common occurrence in hard rock drilling environments.

The computer program models the amount of torque per revolution that each cutter experiences as it traverses through the transition layer. This gives designers the capability of evaluating how cutting forces are affected when the bits are drilling into harder or softer rock. In effect, designers can identify trouble zones, where impact damage might occur, and compensate for it in the bit design. Features, such as profile shape, blade count, start of secondary blades, cutter back rake, impact arrestor location and other considerations, can be manipulated to improve a bit’s ability to transition through differing rock types.

Fig. 2. Finite Element Analysis, as practiced by Hughes Christensen, allows modeling of residual stresses in PDC cutters, to allow bit design to be optimized for durability.

Drillstring considerations. The minimization of vibration and jarring that can occur as the drill bit makes hole is emerging as a prime consideration for bit designers and manufacturers. Vibration and bit impact can deliver a bumpy ride to the electronics packages that follow close behind in the drill string. Logging-while-drilling (LWD) and other packages installed in the drillstring that send data back to the surface require a smooth ride to function properly. To overcome or minimize these issues, software packages are available that provide platforms for optimizing bit and drilling performance through in-depth planning, bit design and implementation.

Testing capabilities. Overall, Finite Element Analysis (Fig. 2), acoustic and scanning electron microscope analysis, and destructive testing are seeing increased use by bit manufacturers to evaluate new diamond and carbide materials, manufacturing processes and bit designs. The goal of these efforts is to optimize all the performance aspects of new bit cutter designs. As a result, bits are much more efficient and last longer. Also, manufacturers are learning much more about the three biggest contributors to bit cutter failure: abrasion, impact and thermal degradation.

SPECIFIC NEW ADVANCEMENTS

No matter the design, size or shape of a roller cone bit, its basic purpose is still to grind rock as it turns, hence the term, “rock” bit. However, today’s roller cone bits utilize technical advancements that enable higher rates of penetration, lower mechanical wear and lengthen service life in tough formations.

Advances in cutting structures. One important improvement in roller cone bits is the new technology being imparted to the bit’s cutting structures or “teeth” during the manufacturing process. As the interface point with the formation, a bit’s cutting structure must both withstand and perform under the tremendous forces required to grind rock and produce a borehole. This point of contact is subjected to extreme pressure, heat and resistance as the bit rotates. New manufacturing techniques are enabling these cutting structures to become more efficient and wear-resistant.

Fig. 3. On ReedHycalog’s TuffDuty directional bits, the KerfStop feature uses wedge-crested inserts to reduce off-center wear without sacrificing rate of penetration.

Additional improvement in bit cutting structures is the recognition of the unusual loads and dynamics that occur when bits are run on directional or steerable assemblies. When a steerable assembly is rotated, the bit is subject to off-center rotation, which can lead to tracking and kerfwear, as well as high side loading. These conditions can reduce bearing life, and damage the bit’s gauge and shirttail areas.

Key design aspects of ReedHycalog TuffDuty Directional bits specifically address off-center wear with features such as KerfStop, GageGuard, progressive skip spacing and WideCut. These features are designed to significantly reduce cutting structure damage resulting from kerf build-up. As regards KerfStop, specially designed wedge-crested inserts add additional tungsten carbide where it is needed most. The insert’s wedge shape provides more carbide on the outer wear surface of the insert, resisting wear without sacrificing rate of penetration.

Progressive skip spacing is designed to reduce a bit’s tendency to track. Progressive skip spacing is critical, when a bit drills with multiple centers of rotation, as is encountered on directional assemblies. Presenting a wide cutting path to the formation is one way of overcoming kerf build-up. WideCut strategically staggers the main row of inserts for a wider insert cutting path to the formation.

GageGuard is an advancement that, by using small, round-top inserts located between conventional gauge inserts or teeth, proves ideal on soft-to-medium formations. This is especially true when drilling formation intervals of moderate-compressive strength and high abrasivity. The rounded inserts ream the wellbore gauge, relieving the gauge inserts from the abrasive reaming action. The inserts are particularly useful in applications, where gauge insert wear or gauge insert breakage limits bit performance.

The feature works by relieving the primary gauge teeth from part of their high-velocity sliding action against the hole wall. Contact between gauge tooth and hole wall starts well above the hole bottom, with a high velocity and low force reaming action. As the bit and cones rotate further, the primary gauge tooth slides down the hole wall and slightly toward the bit center until it reaches the hole bottom. During this cutting cycle, sliding velocity continually decreases, while force on the tooth increases as the gauge insert moves to the bottom.

Security DBS has developed a unique range of roller cone bits to reduce operator drilling costs. The company’s Energy Balanced Series bits balance the forces each cone will encounter with the volume of rock that each cone will remove. A computer drilling simulator models downhole dynamics and kinetic forces acting on the bit cutting elements. It calculates the direction of the scraping action on bottom and optimizes the insert orientation angle, relative to the insert travel path. This increases the volume of formation removed and the ROP while reducing vibration levels that can cause damage to other drillstring components, as well as the risk of tooth breakage or insert loss.

Patented diamond chisels for drilling in hard formations are employed on Smith Bits’ Magnum product line. New manufacturing techniques give Magnum’s diamond chisels more strength than carbide inserts, as well as increased capability for drilling varied formations at higher ROP than is possible with conventional bits. Also, the Magnum diamond chisel bits feature diamond-enhanced heel inserts for the most comprehensive gage protection currently available for maximizing life in abrasive and thermally damaging formations.

High Energy Tumbling (HET) technology (patent pending) that improves the toughness and fracture resistance of tungsten carbide inserts is being applied to Varel’s Challenger tungsten carbide insert bits, Fig. 4. The HET inserts enable the bits to experience less breakage, thereby resulting in increased ROP, extended cutting structure life and lower drilling costs. The cutting structures are designed to be durable while maintaining the highest efficiency in rock destruction. Insert row placement and insert spacing are determined by using Varel’s proprietary algorithms designed to reduce wear, promote efficient drilling and ensure uniform loading across the bit face.

Fig. 4. Varel’s High Energy Tumbling (HET) improves the toughness and fracture resistance of tungsten carbide inserts on Challenger series bits.

Advances in bearing and seal systems. Technology advancements are also appearing in the bearing and seal systems of new roller cone bits. According to most experts, reliability of the bearing and seal systems is the key to these bits’ efficiency. If these systems fail, the driller has no recourse but to pull the bit and replace it.

Fig. 5. Smith’s twin seal rock bits feature primary seals that have a geometry with very high wear-resistant material on the dynamic inner and static outer parts. A new energizing elastomer is in between.

Because seals are so important to roller cone bit reliability, Smith Bits uses two seals in its family of premium performance, Gemini Twin Seal rock bits, Fig. 5. The twin seals are larger and now consist of three distinct material elements. The primary seals have a geometry with very high wear-resistant material on the seal’s dynamic inner and static outer parts. In between, there is a new energizing elastomer.

These changes target improved, high-RPM reliability through reduced wear and revised critical seal characteristics. The secondary fabric seals have a modified bullet, and the sealing characteristics have been upgraded through the use of a static sidewear layer and revised energizer material. Inside the bearing, a synthetic lubricant provides a very capable, high-weight film that is extremely temperature-stable and is resistant to any fluid ingress.

A next-generation, single-energizer metal seal bearing package (SEM II) is featured on Hughes Christensen’s MXL series bit for extreme drilling environments. According to the company, all relative rotary motion takes place between two lubricated metal seal surfaces, extending performance and tolerances beyond the capabilities of an elastomer seal. A new, wider metal seal surface increases sealing capacity by up to 20%, and a proprietary wear-resistant surface dramatically retards seal wear. Its lower coefficient of friction allows smooth, low-torque motion for thousands of revolutions. A robust Backup Ring features a stress-engineered shape that extends operating life and improves debris resistance, while redesigned energizer seal geometry improves sealing efficiency and ensures that the optimum face load is applied to seal surfaces under all conditions.

Bits from Security DBS utilize a high-performance bearing and seal system with a dome compensating design. The latter design improves response to pressure changes and utilizes a pressure relief valve, integrated into the compensation system that acts directly into the reservoir to prevent high internal pressures. According to the company’s data sheets, the system’s large grease capacity lengthens bearing life, and grease formulation provides better load capacity.

The bits also feature sealed journal/ roller bearings that use a centralized ball race to increase journal-to-cone interface for higher load capacity, higher seal effectiveness and superior cone retention. The latest seal developments, including new gland designs and seal materials, have been implemented throughout the entire XS product line. Multiple independent compensation systems, utilizing purpose-specific lubricants, allow the use of dual positive seals in the XT bit line.

PDC bits. Unlike roller cone bits that grind rock, PDC bits use diamond cutters to shear the rock away. Ingenious wafers bonded to carbide posts perform the cutting duties, while the arrangement of these cutters on the bit face determines the bit’s efficiency and ROP performance. PDC bits have three obstacles to performance that designers continually strive to conquer. They are wear, heat and impact. Individually, each obstacle poses problems, but together, they present a formidable obstacle for bit engineers to overcome.

Heat is a considerable concern in a PDC bit. It is not uncommon for temperatures to reach temperatures of 2,000°F at the interface of the diamond insert and rock. This heat must be dissipated quickly, or it will destroy the cutter.

Impact is another issue that must be overcome. Impact is the mechanical failure that occurs, when forces are applied that overcome the strength of the bond between the diamond crystals and/or the bit’s carbide post. PDC cutters are most efficient when they aren’t subjected to high impact values that can cause significant cutter edge damage. High impact rates can also affect the “ride” that the BHA package sustains as it follows behind the bit.

Abrasion (wear) is the third cause of PDC cutter failure. Abrasion is the mechanically generated wear that occurs, due to failure of the individual diamond crystals and/or the diamond-to-diamond bonds within the wafer’s diamond table. Failure can be the result of mechanical loading and/or thermal degradation. Over the years, cutter technology has continually improved, and the newest PDC cutters are much more abrasion-resistant than their predecessors.

Laboratory testing coupled with field testing has led to improved knowledge of the subtleties of cutter performance on the part of bit manufacturers. Bit makers are also utilizing computer-design software to refine designs and model them prior to manufacturing. This knowledge has been particularly valuable in increasing the efficiency of PDC cutters in the drilling of hard rock formations. These are formations in which PDC bits have experienced accelerated wear and thermal mechanical failure in the past.

Advancements in cutter design and composition. A new thermostable layer is being employed on the leading edge of the polycrystalline diamond cutters of ReedHycalog‘s TReXbits to increase toughness to heat and abrasion. The layer is 200% more heat-tolerant and 400% more abrasion-resistant than conventional PDC cutters. This, the company says, enables their cutters to drill farther and faster than ever before in some of the hardest and most abrasive formations.

A new Z3 PDC cutter is being used on FM3000 Series bits offered by Security DBS. The Z3 cutter, the company says, is a highly abrasion-resistant claw cutter that extends effective PDC bit application to hard rock formations. In their HyperCut Hard Rock bits, the primary Z3 cutters can be backed up by secondary PDC R1 cutters. These cutters not only remove rock efficiently, but they also optimize the depth of cut for maximum drilling efficiency and add restorative forces, when lateral forces are encountered.

Fig. 6. Hughes Christensen’s Z series cutters utilize innovative diamond cutter technology that allows the cutters to maintain a sharp, efficient cutting edge under extreme drilling conditions.

Fig. 7. Advanced cutter technology research in diamond interface design, diamond table thickness and edge geometry provides new levels of impact and abrasion resistance.

Residual stresses are a known source of cutter failure. Hughes Christensen’s new Zenith series (ZS) cutter contains an interface between the diamond table and the carbide substrate that has been engineered to break up the high residual stress concentrations and remove these stresses from the portion of the cutter that endures the highest service-induced stress. The ZS cutter interface has improved the management of these stresses which helps the cutter resist spalling and impact failures, Fig. 6.

These innovative cutters are the most recent in a series that utilize the latest in the layered diamond table technology pioneered and commercially introduced by the service company several years ago, Fig. 7. The diamond table is manufactured utilizing a patented process that allows for a top diamond layer, which provides superior wear resistance at the cutting edge and interim layers, which provide for outstanding toughness and durability.

Hughes Christensen says that extended durability and expanded capabilities are resulting from its Genesis XT PDC bit because of its Zenith series cutters that maintain a sharp, efficient cutting edge under extreme downhole conditions. The company says that its Genesis XT bit now delivers a new level of high-performance drilling at a lower cost-per-foot. A full alloy head allows the bit’s cutting elements to be attached to the blades, thereby ensuring a strong cutting structure capable of drilling a variety of formations.

Innovative, high-operating-temperature cutters that provide a significant increase in cutter wear resistance, and are very cost-effective in applications where cutter wear rates are excessive, are featured on GeoDiamond PDC bits. The new technology allows the bit to remain sharp and to maintain a high penetration rate for longer periods, as compared to conventional cutter technology. The cutters are manufactured using a proprietary process that matches cutter thermal properties to temperatures that are expected to be generated during the drilling process. Granite Log Tests showed that the cutters experienced a wear improvement of 125% compared to conventional premium PDC cutters.

Other PDC advancements. Interactive Bit Solutions (IBitS), a proprietary software tool for Security DBS that is utilized by a company design specialist and the customer, is now available to optimize bit selection or to design a new bit for specific applications. The company states that the software can be used to design a bit in minutes on a laptop computer in 3D, at either the customer’s office or at his rig site. After the design is complete, the design specialist can transmit it directly to a manufacturing center for quick prototyping and delivery on a priority basis.

An advanced force balancing tool from Varel International is called SPOT 4D. This proprietary design software allows the company’s engineers to force-balance PDC bits over the widest range of revolutions-per-minute possible, resulting in a bit that is stable under nearly every drilling condition. Customer benefits include faster drilling, longer life and more directional stability.

A solution to transition drilling problems encountered when the bit transitions from soft to hard formations has been introduced by Hughes Christensen. It is called SmoothCut depth-of-cut control technology. Since nearly all of the weight-on-bit is carried by only a few cutters, drilling from soft to hard formations can induce destructive peak loads that cause fracture damage and premature cutter wear. SmoothCut technology selectively engages bearing surfaces on the bit body at precisely engineered depths-of-cut to help spread the load when interbedded formations are drilled. The technology offers the additional benefit of limiting PDC bit aggressiveness without limiting ROP. Controlled aggressiveness means more consistent drilling torque, despite changes in downhole weight-on-bit or formation strength. 

THE AUTHOR
	World Oil Contributing Editor Ron Lord is president of The Lord Group, a marketing and technical communications firm in Houston, Texas. He earned his BA degree in journalism from Southern Methodist University in Dallas, Texas. During his career, he has worked for Atlantic Richfield Company, Dowell Schlumberger and Schlumberger Oilfield Services. He has also been editor of several oil and gas industry publications.