Combining sophisticated design capabilities with the most advanced cutter technology yet developed, a series of polycrystalline diamond compact (PDC) bits has been created that are uniquely qualified for "hard rock" drilling. In direct offset comparisons in the challenging Travis Peak formation, the new FM3000 series bits delivered significantly reduced drilling costs.

Designed using new tools that capitalize on better understanding of bit dynamics, FM3000 series bits incorporate a new highly abrasion-resistant Z3 claw cutter that extends effective PDC bit application to hard rock. The primary Z3 cutters are backed up by secondary PDC R1 cutters that not only remove rock efficiently but optimize depth of cut for maximum drilling efficiency and add restorative forces when lateral forces are encountered.

By using the R1 cutters to back up the primary Z3 cutters, these hard rock designs usually have one or two fewer blades than competing bits. Fewer blades mean fewer primary cutters, which translates to faster rates of penetration (ROP) and ultimately lower drilling costs. These R1 backup cutters are placed using software that allows designers to optimize the primary cutter's engagement based on drilling parameters. The R1 backup cutters also prevent the primary cutter from overengagement during moments of high instantaneous ROP. When engaged, these sharp cutters can also efficiently remove rock, which reduces drag, torque and axial vibration associated with excessive cutter overengagement.

The science behind these hard rock designs includes a better knowledge of PDC failure mechanisms, which revealed the new Thermal Mechanical Integrity qualified cutter performance criterion and led to development of the Z3 cutter, with more than 20 times the abrasion resistance of industry-standard PDC cutters.

But more than improved cutter technology, these "hard rock" PDC bits comprise a highly engineered design platform. FM3000 series bits are designed using a "toolbox of options" combining advanced modeling and sophisticated analytical capabilities to allow customized designs for hard or abrasive applications. Cutting efficiency and durability are optimized according to specific rock properties and drilling parameters, while a transitional drilling model simulation evaluates how cutting forces are affected during the transitional drilling often encountered in hard rock environments.

In addition, the FM3000 series bits are globally force-balanced with the forces equally distributed to optimize axial, lateral and torsional forces for more effective bit performance in these challenging environments.

New insights, new model

The FM3000 series bit was developed following creation of a new cutter/rock interaction model based on a better understanding of cutter wear. Cutter wear depends on a number of factors, including 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, or abrasive resistance.

Using the new cutter/rock interaction model, bit designers analyze cutter wear in three dimensions, and all factors, including those overlooked by the previous model, can be easily taken into account.

Specifically, the FM3000 series bits are designed using a Transition Drilling Model that simulates a fixed cutter bit drilling through a change in rock strength such as shale to sandstone. The program uses the new cutter/rock interaction model to calculate the amount of torque per revolution each cutter experiences through the transition layer.

This technology gives the designer the ability to evaluate how the cutting forces are affected when the bit is drilling into harder or softer rock. In effect, the designers are able during the design phase to identify trouble zones where impact damage could occur while transition drilling. Then, specific bit features such as profile shape, blade count, cutter density, cutter back rake and impact arrestor location can be manipulated to improve the bit's performance in drilling transitional formations.

Global force-balancing and energy-balanced bits

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

However, further study found that 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. Even a perfectly force-balanced bit may exhibit tilt motion caused by bending moment. Therefore, balancing the bending moment is 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, these new hard rock 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, resulting in a global force-balanced design.

The concept of energy-balanced bits 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 blades, it is necessary to control these load distributions.

Equally distributing the forces minimizes the change in work or force among regions or zones of the cutting structure. Thus, designing a "torque- and drag-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 force distribution bit design criteria has been developed, in this case considering the ratio of the average change in cutter torque across zones to the average cutter torque over the entire region. By controlling the force distribution over blades and over cutters, these bits reduce impact damage and uneven wear while promoting improved ROP.

New measure of cutter performance

The advanced Z3 PDC cutters used in these hard rock designs were developed as a result of an improved understanding of cutter failure mechanisms. Following a period of extensive research and development of advanced testing capabilities, Thermal Mechanical Integrity qualified cutter analysis was identified as crucial to understanding a PDC cutter's failure, in addition to impact and abrasion.

This failure is a result of loss of diamond that occurs due to a combination of thermal degradation and force and is a measurement of cutter toughness as wear and thermal degradation occur.

The new testing capabilities enabled cutter durability to be optimized for both abrasive and hard inter-bedded formations, resulting in development of the new highly abrasion-resistant Z3 cutter. Field tests and commercial application now confirm the Z3 is highly effective for applications in which accelerated wear typically leads to thermal mechanical failure - including hard and abrasive applications where cutting efficiency must be maintained. As a result, PDC application now extends into what is classified hard rock where IADC 4 through 7 Type roller cone insert bits typically have been used.

Putting it all together - field performance

The Travis Peak formation in East Texas comprises primarily very fine-grained sandstone inter-bedded with shale and mudstone. Traditionally the interval could be drilled only by several IADC 647Y to 817Y insert bits. Until recently, efforts to use PDC bits to drill this section produced only marginal economic improvement.

In one Travis Peak application, the challenge was to extend the bit life in order to drill well into the Travis Peak formation in an application that typically was drilled using heavier-set PDC bits.

To ensure extended bit life and provide a more robust cutting structure, an FM3000 series with Z3 and R1 backup cutters was recommended. In addition to the cutters, the hard rock design, designated FMX3655 bit, features continuous spiraling blades through the gage pad for optimized distribution of lateral forces and single-set cutting structure for increased aggressiveness.

In this case, the FMX3655 bit drilled 1,681 ft (512.7 m) of the Travis Peak formation at an average ROP of more than 40 ft/hr (12.2 m/hr), outperforming all offsets in both footage drilled and ROP. This outstanding footage performance eliminated additional bit runs and reduced cost per foot to create savings to the customer of US $14,178.

In another Travis Peak application in the Carthage field, a 77⁄8-in. FM3000 series bit designated the FMX753 bit with advanced Z3 and R1 backup cutters successfully drilled 2,757 ft (840.8 m) at 26.8 ft/hr (8.2 m/hr), drilling completely through the Travis Peak and into the Cotton Valley. This run was the lowest cost per foot of all the offsets run in that area, and several were run on the same rig.

These hard rock successes are not just limited to the Travis Peak. In Colorado in the Piceance Creek field, the Mesaverde formation is comprised primarily of tight sandstones inter-bedded with mudstones and shales. Due to the very abrasive and hard inter-bedded nature of the formations, PDC bits had little success as a viable economic solution.

With success stories coming in from other regions, a 77⁄8-in. FM3000 series bit with advanced Z3 and R1 backup cutters was called up for the challenge. This bit, designated FMX753, successfully drilled 3,200 ft (976 m) through the Mesa Verde formation at 26.6 ft/hr (8.1 m/hr), an impressive ROP for the footage drilled. This footage was more than double the footage of the offsets and replaced the two to three bits it usually takes to drill this interval. The cost per foot for this run was the lowest of the offsets, with the next best being 40% higher.

These new PDCs have revolutionized the way such formations are drilled, saving contractors and operators thousands of dollars per well. Drilling days have been cut dramatically - by up to 60% in certain fields in East Texas. A hard rock team has been formed to be a part of this rapid developing market. As these hard rock PDC bits continue to drill deeper, harder and more abrasive formations, more roller cone insert and diamond impregnated bits will be left off the drilling programs.

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