New Bit Designs

Bit technology keeps pace with operator activity

As operator preference shifts toward fixed cutter bits, manufacturers have rushed to develop new products to fill ever-expanding needs. Specialized roller cone designs still occur to fill niche applications.

Ron Lord, Contributing Editor, Houston

During the past year, the drilling of oil and gas wells worldwide has mirrored the skyrocketing, overall increase in crude prices. And, as one might expect, all the manufacturers of roller cone and fixed cutter bits have experienced strong demand for their wares.

Yet, while the volume of orders for both roller cone and fixed cutter bits has increased, an underlying shift in operator preference from roller cone to fixed cutter bits that began years ago continues. In 2003, PDC bits drilled 50% of total footage, as compared to 26% in 2000. Now, three years later, PDC bits drill 60% of total footage.

The increase in PDC bit preference can be attributed to continuing performance improvements, but sheer volume is credited for keeping the roller cone bit market steady. Also, roller cone bits have traditionally been less costly, and they are better at drilling certain formations. This is because they are often the only bits that can provide both the tool face control and build rates required.

No one expects the roller cone bit to fade into the sunset, but much of the shift to PDCs can be explained by the larger changes that are occurring in operational philosophies and bit design technology.

A SYNERGISTIC APPROACH

Most bit experts agree that a fundamental sea change is occurring in drill bit design and manufacturing that is propelling the industry toward consideration of the impact of the entire drillstring and its various components (including the bit) on the overall drilling application. No longer can bit performance be isolated from the rest of the drillstring. No longer can the bit be selected independently from other drillstring components. Instead, it must be selected, based on what it can contribute as an integral part of the system creating the borehole. As such, it must be matched to both the drilling system and the formation.

This synergistic approach for optimization of overall drillstring system performance utilizes compromise to obtain its goal. It requires that three factors be considered, and ultimately balanced, to optimize directional control, vibration control and borehole quality. These three factors are drilling efficiency (optimized ROP, maximized durability and drilling at the lowest cost per foot), steerability (optimized angle-build capability and the force to deviate the bit), and stability (minimization of stick-slip, bit whirl and axial vibration).

In this approach, optimized vibration control can be obtained by balancing drilling efficiency and stability. In addition to increasing bit life, lower vibration levels will protect sensitive BHA components and measuring devices, and maximize productive time and footage. To obtain high-quality hole, balancing steerability and stability will result in a smooth, consistent wellbore with little or no spiraling or ledging. Finally, by balancing drilling efficiency and steerability, ideal directional control can be obtained that will produce excellent ROP and durability.

Today's bit designers strive for three factors – drilling efficiency, steerability and stability – by using computer modeling and innovative designs, and by tweaking features, such as cutting structure layout, cutter or tooth composition and design, bearing and seal systems, inserts, hard face abrasion-resistant coatings, bit hydraulics and bit body design.

COMPUTER DESIGN MODELING

The use of computer modeling for both roller cone and PDC bit design and manufacturing has grown over the past 12 months, and software programs are becoming more sophisticated. These programs use proprietary algorithms to model forces and bit behavior during simulated drilling operations that lead to optimization of bit designs. They have contributed greatly to increases in ROP, toughness and wear resistance, stability and reliability. Most importantly, modeling programs allow bit manufacturers to custom-design bits for specific drilling applications.

ROTARY STEERABLE SYSTEMS

Much emphasis is being given to bits for rotary steerable drilling systems (RSS's) that allow operators to plan complex wellbore geometries, including horizontal and extended-reach wells that could not previously be drilled efficiently or effectively with conventional drilling methods. They accomplish this by enabling full directional drilling control in three dimensions while drilling with continuous drillstring rotation from the surface.

These systems can be divided into two groups, "push-the-bit" and "point-the-bit" systems. No matter which RSS type is selected by the operator, the drill bit selection must be matched to the operating methodology of the RSS for maximum drilling optimization. By matching the most capable bit with the RSS and the formation, key drilling factors, such as efficiency, steerability and stability, can be obtained. These factors maximize, directional control, hole quality and vibration, as well.

A FOCUS ON HARD ROCK

Dealing with hard rock formations is getting renewed interest in the bit industry because one of the greatest challenges for any PDC bit application is hard rock drilling, where impact damage, heat damage and abrasive wear of PDC cutters limit bit performance. A common misconception held by many is that hard rock formations consist of high, compressive strength rock when, in fact, they are actually hard-to-drill rock and formations. Since drillers can spend as much as 80% of their budget drilling hard rock footage that represents only 20% of a well's total footage, bit selection becomes critical, because poor bit choices can be costly.

Due to the inability of past PDC bits to effectively deal with various destructive forces of hard rock formations, roller cone bits have dominated hard rock drilling for years. Now, new developments in PDC technology are extending PDC bit applications to hard rock, and, as a result, PDC bits are routinely replacing roller cone bits in drilling these formations.

BITS FOR DRILLING WITH CASING

Fig. 1. Hughes Christensen's Casing Bit System.

Development of drill bits for drilling in casing and liners continues to be of interest. Casing bit systems help reduce risks inherent in casing and liner running operations. Challenges, such as hole instability, lost circulation, getting casing to total depth and drilling into trouble zones, can be actively controlled by drilling the liner and casing down through the trouble zone, either in one operation or after the hole has been drilled conventionally. The use of a full PDC cutting structure on the bit addresses the need to drill or ream harder formations and longer intervals than was previously possible.

One company offering a casing bit system is Hughes Christensen, Fig. 1. According to the firm, its new EZCase system comprises a special alloy crown fitted with a full PDC cutting structure. This tool allows operators to combine drilling and casing in a single run, thereby reducing flat time and lowering risks associated with problematic wells.

The system also reduces risks where casing/ liner running may be problematic, and it reduces the risk of not being able to get the casing or liner to TD. The company also says that in cases of severe losses or unstable formations, including rubble zones, lost circulation zones, depleted reservoirs and underground blowouts, the new drilling system provides a means to complete a well that otherwise might not have been possible. Also, the system can drill out the EZCase with a PDC bit, thereby ensuring that additional milling operations are not required.

BITS FOR UNDERBALANCED APPLICATIONS

Fig. 2. Cutting structure of Varel International's bit for underbalanced drilling.

Increasingly, oil and gas operators that want to prolong the lives of their older oil and gas reservoirs, and allow the development of new fields by lowering life cycle costs, are choosing to drill underbalanced. This is because it provides distinct advantages for reducing formation damage and drilling problems (lost circulation and differential sticking), and it increases drilling performance through higher ROP and longer bit life. Varel International recently announced a new journal bearing bit to fill the needs of operators, who want a bit designed especially for underbalanced drilling applications where air, water mist or foam circulation is used, Fig. 2.

According to the company, its Jet Air Journal Bearing Bit uses a tungsten carbide insert cutting structure that is specifically engineered for efficient drilling of the formation, thorough hole cleaning and the most economical drilling solution available. The bit has a cutting structure that is designed to achieve high penetration rates in underbalanced applications while reducing sliding and wear. The result, says the company, is longer bit life and lower drilling costs.

An interesting feature of the bit is its special shirttail edge contouring around the seal area that prevents shale packing and prolongs bearing life. To protect the seal, a generous application of proprietary hard metal has been applied to the shirttail edge and leading edge of the bit leg. Also, shirttail inserts that extend bit life and improve in-hole drilling time are available as an option.

NEW TECHNOLOGY OVERVIEW

During the past year, Hughes Christensen (Hughes) continued development of its Genesis series PDC bits by adding a new family of Genesis HCM bits for steerable motors and a family of Genesis HCR bits specifically designed to work in tandem with rotary steerable systems, Fig. 3. The company also introduced a new long-life motor bit and a total hardfacing package.

Fig. 3. Designs of Hughes Christensen HCR bits vary depending on the application for the bit.

Hughes says its new HCM bit family offers the penetration rate and longevity of traditional PDC bits, coupled with the control and precision of roller cone bits. These advantages are possible, the company says, because of its EZSteer depth-of-cut control technology that utilizes bearing surfaces near the center of the bit designed to engage the bottom hole in slide mode, but not in rotate mode. This tempers the aggressiveness when steering and preserves ROP while rotating. Additional features, such as Genesis cutters, Computational Fluid Dynamics and enhanced stabilization, enable the new HCM bits to drill a smooth wellbore at high ROP, making them ideal for use in steerable motor applications.

Another new family introduced by Hughes is the HCR bit. Each of these bits is designed and engineered for a specific application and offers a shorter profile for efficient sidecutting and high dogleg wells, said the company. Conventional profiles are also available for an optimum balance of durabililty, stability and borehole quality. All designs are available in 53D4-in. through 181D4-in. sizes.

Gauge configuration for each HCR PDC bit is chosen on an application-specific basis. Very short-gauge configurations are used when high DLS is required to complete a complex, demanding directional plan or when RSS is required. Standard gauge configurations are used for most RSS applications. The longer-gauge configurations are used for certain RSS applications, or when hole quality is the primary concern with moderate directional work.

Another Hughes Christensen offering is its MXL long-life motor bits – a line of high-speed roller cone bits designed for demanding applications and accelerated time to completion. Completely redesigned with a new cutting structure, configurable hydraulics and very reliable bearings, the MXL series, said Hughes, stays in the hole longer than other bits. The bit uses a next-generation single energizer metal seal bearing package that is reliable in extreme drilling environments. All relative rotary motion takes place between two lubricated metal seal surfaces, extending performance and tolerances beyond the capabilities of an elastomer seal.

Hughes also announced its new MaxLife total hardfacing package, Fig. 4. Aimed at increasing steel tooth bit durability, HCC said it responded to operators who wanted a tougher steel tooth bit. By strategically applying 100% more hardfacing compared to standard steel tooth bits, a large reduction in the amount of exposed steel around the gauge and teeth was realized. This delivered more robust cutting, greater shirttail protection and increased seal life.

Fig. 4. MaxLife hardfacing process from Hughes Christensen increases tooth strength and wear resistance.

Reed-Hycalog. Great drilling performances often depend on focusing on issues beyond the drill bit, says Reed-Hycalog (Reed). By characterizing and focusing on four key indices – ROP, durability, stability and steerability – the company claims it has developed new technical solutions to a range of drilling challenges that have resulted in better drilling performance and lower cost-per-foot.

Fig. 5. The Reed-Hycalog V-Stab vibration dampening tool improves overall bit performance, including ROP.

For example, when durability was a priority focus for a new bit design, Reed says it turned to its latest generation of computer modeling tools to design its new RockForce series of roller cone bits. The computer modeling tools ensured that the bits had both an efficient cutting pattern and optimized hydraulics. In addition, the bits utilize a completely redesigned bearing system, tighter tolerances, and a redesigned lubrication and pressure compensation system. All of these items have contributed to the bit's outstanding performance in rotary and motor applications.

Another Reed technology solution involves vibration. Because conventional BHAs are often unable to mitigate vibration during drilling, instability often results. And, in studies, operators have said that vibration has become one of their leading causes of lost time and poor drilling performance.

To address downhole vibration issues, Reed has introduced its V-Stab vibration dampening tool which, the company claims, has proven effective at diminishing both the magnitude and frequency of drilling shocks, reducing damage to expensive BHAs and improving overall bit performance, including ROP, Fig. 5. With no moving parts and integral blade design, this tool gives the drilling engineer an excellent, low-risk manner in which to reduce drilling cost.

Fig. 6. The BlackBox downhole vibration recorder enables post-run analysis that details dynamic behavior of the bit.

As another solution to vibration during drilling, Reed introduced a newly developed BlackBox downhole vibration recorder that enables detailed post-run analysis to determine a bit's specific dynamic behavior, Fig. 6. Drilling events that can have a high risk for inducing bit and BHA damage can be identified. Analyses of the recorded data enable the company to make recommendations for bit, BHA and/or operational changes to improve drilling performance. The BlackBox sub has flexible placement, but it is typically run immediately above the bit.

Finally, because many directional products now include steerable motors with high power output, RSS's that utilize a variety of push, point and push-to-point steering mechanisms, extended gauge systems and verticality systems, Reed said it has become vital that these systems' distinct mechanisms be taken into account to obtain optimal bit performance. Therefore, the company employs its new SystemMatchertool technology, which takes both the directional profile and the drive system into account in optimizing bit selection for an application where steerability is a priority need.

Security DBS. Recent new technology from Security DBS (SDBS) includes advancements in its proven FS2000 bit design technology. This led to the recent introduction of a new FS3000 Series steel-body PDC bit that retains the ROP and cleaning advantages of the FS2000, and also incorporates continuous spiraling along bit cutter blades through the gage pad for optimized distribution of lateral forces. In addition, the company says its new bits employ SDBS' proprietary Z3 cutters that provide 20 times the abrasion resistance of standard cutters, and more than four times the abrasion resistance of premium cutters.

Advanced cutter technology also characterizes the evolution of SDBS' HyperCut hard rock bits. This new line of PDC bits for hard-to-drill formations features dual-row cutting structures with various combinations of cutting elements configured according to a particular application's requirements. For example, the ZZ configuration uses Z3 cutters exclusively, making it extremely effective in highly abrasive hard rock, where the dual rows of Z3 cutters reduce how fast wear flats are generated, thereby allowing for significantly longer bit runs.

Fig. 7. SDBS XS Series roller cone bit for use in slimhole applications.

The ZR dual-row configuration incorporates non-planer R1 PDC cutters to back up primary Z3 cutters and prevent primary cutter over-engagement in heavily interbedded formations with stringers. The ZZ configuration combines Z3 PDCs with impregnated backup cutters to effectively target softer upper sections that end up in a hard or abrasive reservoir sand or basement. For highly abrasive applications that also require significant impact resistance, a ZR hybrid combination has been designed specifically to drill those formations.

Security has also introduced new design developments across its line of X Series roller cone bits, Fig. 7. SDBS stated that a number of enhancements were made to its small size XS bits following The Slimhole Project, a focused effort launched by the company in early 2005 to advance roller cone bit design for slimhole applications. Addressing those design aspects most susceptible to the high energy levels generated in small bit sizes used for slimhole drilling, the project led to such specific design improvements as beefier arm forgings, new O-ring seals, and a fully integrated dome vent compensation system.

Additionally, the improved geometry of a high-energy bearing design provides significant increases in load capacity, and affords greater flexibility in shirttail protection and cutting structure design. In the field, SDBS claimed that its enhanced slimhole XS Series designs are running longer with a higher rate of reliability.

At the other end of the spectrum, Security has developed a new series of 9-7/8-in. to 12-1/4-in. XL bits to meet the increasing total energy requirements of today's high-end, large bit market. With an improved roller bearing design that features double positive seals and a patented dual reservoir system, the new XL bits incorporate full length integral lug pads for added stability, and use a cutting structure that equalizes forces and rock removal among all three cones, increasing ROP and durability, and reducing DHTF.

Fig. 8. SDBS FullDrift bit for rotary steerable systems.

In the RSS arena, Security has moved forward the design of its application-specific RSS bits by optimizing the design according to the type of drilling for which it is used. The company's new HyperSteer bit design features a reduced lateral area for greater directional responsiveness to the side force of push-the-bit systems, while extended-gauge FullDrift bit designs provide the stability to produce "gun barrel" boreholes on point-the-bit systems, Fig. 8. Customized using SDBS' IBitS design technology, both RSS bit lines deliver optimal directional control, minimum vibration and excellent borehole quality, said the company.

Smith Bits. Over the past year, Smith Bits (Smith) has continued the refinement and further upgrading of IDEAS (Integrated Dynamic Engineering Analysis System). The system process certifies the performance capabilities of each bit design through a dynamic simulation and modeling methodology that takes into account not just the lithology at the rock/ cutter interface, but also the drillstring, the drive system, the BHA and the total system's influence on the bit's behavior. As a result, the design system works synergistically with the drilling system, and performance is optimized.

Smith pioneered the new system with roller cone bits, and the result was a leap in truly understanding the dynamics of the rock/ cutter interface. They then used this knowledge base and sophisticated modeling capability to design and build an array of roller cone products. Now the company says it can be applied to fixed cutter bits, too.

Designs from the IDEAS system go from concept to field-proven performance much faster than bits developed using traditional design methods, because the process uses a simulation model that predicts real-world behavior of the bit very accurately. Now all new Smith bit designs, for both roller cone and fixed cutter product lines, are developed using the new integrated design system, Fig. 9.

Fig. 9. Smith Bits IDEAS modeling is used to design and model all bits produced by the company.

The process starts with bit performance data, geological information, BHA details and dull bit analysis. From these parameters, actual laboratory rock/ cutter tests are devised and carried out. The laboratory data from IDEAS quantify actual cutter forces and rock removal rates. These are then used for the design analysis in lithologies that compare to the particular field application, rather than estimated rock/ cutter behaviors generated by other design tools.

Once the rock/ cutter data are obtained, they are integrated into a full bit design model to determine the bit characteristics in actual drilling conditions. This model allows examination of the bit in a confined mode for initial design development. It also delivers an accurate projection of a bit's performance, considering the actual BHA under actual well characteristics (geometry, parameters and lithology variations) in a fully dynamic simulation mode, where influences on the bit are identical to those encountered in the real drilling environment. Outputs from these simulations enable the designer to tune the bit's desired performance, be it ROP or footage drilled, with the highest degree of dynamic stability engineered into the bit.

United Diamond. The firm's Edge Series, soft-formation PDC bit line sports a dramatic new profile and unique cutter layout. Engineered to drill with less WOB and high RPM, the result is a fast, straight well. These bits are typically identified as being ugly, but in this case, looks aren't everything. Engineered cutter placement and accurate CNC machining of cutter pockets allow designers to set each cutter with exacting tolerances to achieve the best performance.

Field testing has shown substantial increases in ROP. In fact, the bit prefers higher RPM applications. In a recent test to determine compatibility with RSS's, United Diamond said the bit achieved a 37% increase in ROP, when rotary speed was increased by 50%. The new bit series uses an ultra-short, single-piece body with unsymmetrical blade layout and spiraled blades and gauges.

Another new development from the company is its TorkBuster Torsional Impact Generator. Often, when drilling difficult formations with PDC bits, there is insufficient torque available to fail the formation, causing the bit to momentarily stop rotating. The drill string winds up as torsional energy is stored in its length. Once the torque required to shear the formation is generated, the drillstring releases with higher-than-normal impact loads on the PDC cutters. This eventually breaks down the diamond inserts and results in bit failure.

Run directly above a United Diamond PDC in rotary or directional assemblies, the tool applies a high-frequency torsional impact to the bit that, when combined with steady-state drillstring torque, results in a dramatic increase in horsepower – directly at the bit. This drastically reduces stick-slip and significantly improves rates of penetration and bit life. The tool's compact length has minimal effect on BHA characteristics and has been proven in rotary, directional, performance drilling and rotary steerable applications.

United Diamond also continues to develop and improve its in-house, custom business intelligence software called Piranha. It keeps functions, such as engineering, manufacturing, operations, sales and accounting, working together in the manufacture of both its fixed cutter and roller cone bit offerings. The software closes gaps that typical accounting software hasn't been designed to handle. For instance, engineers directly interface with the software system to configure bills of materials, while shop personnel generate work orders, and operations specialists record tool performance details. Purchasing, sales orders, book values and depreciation, quality assurance, and even competitor data analysis are handled by Piranha.

Perhaps the software's most beneficial feature is being able to tie everything together in a real-time dashboard that consists of various graphs. Even further, since the system is designed to handle such an intense level of informational detail, a user can quickly pull the complete history of any serialized drilling tool. One can immediately see all activity related to the specific item, along with summarized performance details and charts with only a few mouse clicks.

Fig. 10. Varel's TOUGH-DRILL Bit for drilling applications in hard and abrasive rock.

Varel International. In addition to the Jet Air Journal Bearing Bits, Varel has announced the development of a new generation of PDC bits that are especially designed for tough drilling applications, Fig. 10. The company said that due to the improved stabilization characteristics of its new TOUGH-DRILL bits, both the ROP and durability challenges of hard and/or abrasive formations are addressed. Varel says the bits are able to meet these challenges by employing both advanced cutting structures and high-performance PDC cutters.

As part of product development, the bits undergo elaborate computational fluid dynamics evaluations. Re-grinding and re-circulation of drilled cuttings, a common occurrence in hard and abrasive applications, are eliminated in this process. Also, possible stagnation zones are identified and eliminated. This process improves cleaning efficiency, cutter cooling and ROP.

The bit's application-specific cutters incorporate advanced carbide interface geometries and materials science technologies. The carbide interface geometrics are modeled using finite element analysis to minimize damaging residual stresses (axial, hoop and radial). This process greatly improves the PDC cutters' impact resistance. Diamond grades are then selected, analyzed and prepared to maximize abrasion resistance.

According to the company, the new drill bits were developed using Varel's proprietary SPOT design software. Developed after extensive single-cutter and full-scale bit testing under laboratory and field conditions, the design software ensures cutting structure optimization for specific conditions.

The software models complete bit behavior in different rock types during the drilling process. In addition, it evaluates cutter failure as a result of friction and heat transfer. Using appropriate rock, geologic and mechanical properties, the software also performs sensitivity analysis for various drilling parameters (WOB and RPM) to evaluate their effects on ROP, bit wear, vibrations and footage, thereby improving the ROP and run length of the new bits.

Fig. 11. The Particle Impact Drilling System bit features particle and fluid exit nozzles.

Particle Impact Drilling. The patented Particle Impact Drilling (PID) technology is a mobile system that readily adapts to typical conventional drilling rigs, Fig. 11. This system is not intended to drill entire wells from top to bottom. Rather, it is designed to drill sections that are very difficult, time-consuming and costly. Intended intervals are very hard and/or abrasive formations.

The system will be provided and operated by Particle Drilling Technologies, Inc. (PDTI), as a service to oil and gas companies for drilling certain rock intervals; specifically in wells where penetration rates decrease, due to extreme compressive strength of the formations being drilled. Tests have shown that the PID system is capable of drilling through rock formations at rates multiple times faster than current conventional techniques.

The system relies on readily available hydraulic energy combined with round hardened steel particles entrained in the drilling fluid to rapidly excavate the rock formation being drilled. Unlike conventional drilling techniques, the process does not rely on weight-on-bit and torque to mechanically break the rock. Steel particles are introduced into the drilling fluid at a point downstream from the rig pumps, thus not interfering with the normal operation of the rig pumps.

These particles then flow down the drill stem and are accelerated through the nozzles of a specially designed fixed-cutter bit, striking the formation at more than 4 million times per minute and veolocities of ~ 500 ft/sec. Particles and cuttings are circulated to the surface by the drilling fluid, where a shot-extraction device captures the particles for re-injection. The volume of particles required is relatively small at only 2 – 3% of total fluid volume.

The PID application reportedly results in significant rate-of-penetration gains, thereby reducing the amount of time the operator spends on location to drill. Value is generated by savings in labor, fuel, rentals, rig time and other variable well costs, which should result in much more cost-effective drilling.

An initial field trial was completed by PDTI in the US, in Wyoming, and it verified penetration rates. Pump pressure events and the PID shot trap limited that trial's duration, so the shot recovery system was completely redesigned and successfully implemented on a follow-up trial. Continuation of field trial 2 was slated for late this year. 

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.