A patented roller cone balancing process introduced in 2002 began to manipulate forces to increase cutting efficiency. This yielded more aggressive bit designs with tougher cutting structures that extend bit service life. The improvements were immediately seen in the field, when the bits consistently exhibited as much as 27% greater durability. Looking forward, it became clear that capitalizing on the benefits would require comparable breakthroughs in other aspects of roller cone bit design.

In 2006, following a 2-year study of bit design, materials and manufacturing, four specific longevity design features were introduced that dramatically increased roller cone reliability. Comprising an improved bit design platform, this “quad-pack” of features includes
1. larger bearing load capacities;
2. greater seal dependability;
3. a more responsive pressure compensation system; and
4. new, stronger body forgings, all of which combine to extend the longevity of roller cone cutting structures.

Hundreds of the new designs were field-tested in a variety of applications over a period of

Figure 1. (Left) Predicted cutter wear on rotary steerable drill bit using patented modeling software (Right) Actual cutter wear on bit after RSS run. (Images courtesy of Halliburton Security DBS)
18 months and delivered an overall 80% reliability rate at 92 hours, compared to 69% by the next-best bit. Now field-tested and proven in a range of applications, roller-cone bits with the new longevity features enable operators to realize the benefits of more robust and reliable roller cone performance.

For example, one recent UK offshore well typically required two bit runs and 99 drilling hours to drill the Cretaceous tophole section. By contrast, one of the new long-life bits drilled the entire 4,018-ft (1,224-m) section in a single run at an average rate of penetration (ROP) of 54 ft/hr (16 m/hr), achieving greater footage in a single run than all comparable offsets and increasing ROP by more than 32%. In this case, the longer bit life resulted in a two-fold savings to the customer: eliminating the cost of a second bit and trip time, and saving some 30 hours drilling and trip time.

Advancements in fluid flow optimization enabled the development of a new design platform to overcome problems caused by turbulent flow near cutting structures, which can inhibit cuttings removal and lift. Examining flow impact on the hole and interaction with the bit as well as flow impact on the environment around the hole and up the annulus, testing demonstrated that hydraulic performance could be enhanced by creating directed flow channels through optimized arm geometry.

The new forging design uses computational fluid dynamics (CFD) software to aid in creating unique arm geometries that incorporate lifting surfaces and trailing wedges to effectively guide fluid flow. In addition, the support arms of the bit are designed to work together to generate an optimum fluid spiral that entrains and lifts cuttings while nozzles are directed toward the leading edge of the cutting structure to ensure unrestricted fluid flow. As a result, stagnant fluid zones are eliminated on the cutting structure, and hole cleaning is improved.

New fixed cutters set records

Advances in testing and modeling have accelerated cutter development cycle times to just months and have led to a new, highly wear-resistant PDC cutter. Research has confirmed the performance benefits of this PDC cutter. In the latest laboratory VTL Heavy Wear Test, the new cutter showed virtually no cutter wear when tested under extreme conditions. This performance has since been repeated in ongoing field testing.

In the PA field in Grand Junction, Colo., a FM3555 bit with the new cutters set a field record by drilling approximately 5,500 ft (1,676 m) at an average ROP of almost 105 ft/hr
(32 m/hr) to total-depth (TD) the section. Exhibiting virtually no wear after this field record performance, the bit was re-run without repair. Similarly, in the Jonah field of Wyoming, an FM3655Z incorporating the new PDC cutters was pulled with no wear after drilling 6,584 ft (2,006 m) at an average ROP of 123 ft/hr (37 m/hr), deeper and faster than all comparable bits in the field.

But this type of cutter performance is of no benefit unless the bit design is capable of achieving all drilling objectives. Concurrent with material and cutter development, there has been a continuous exploration of drilling optimization through sophisticated dynamic modeling for rotary steerable applications.

In general, rotary steerable system (RSS) bit design is a matter of balancing what’s needed from a directional standpoint with what’s needed for optimal bit performance. In these terms, drilling efficiency — as determined by steerability and stability — is what defines bit performance in RSS applications.

Directional drilling with RSS requires an understanding of bit directional behaviors, especially
Figure 2. PDC cutters undergo abrasion testing in a research and development facility.
with regard to the maximal dogleg severity for a given drilling system. A point-the-bit rotary steerable system, for example, orients the bit by deflecting an internal driveshaft. The bit is pointed in the desired direction, and only the bit face cuts the formation. By contrast, the more active gauge of push-the-bit designs provides optimum directional response to a side force, emphasizing steerablity.

Specifically, extended-gauge bits provide the stability to produce “gun barrel” boreholes when matched with a point-the-bit system, while those bit designs featuring a reduced lateral area afford greater directional responsiveness to the side force of push-the-bit systems.

Furthermore, rotary drill bits designed or selected for use with a directional drilling system have an optimum directional control for a desired wellbore profile and anticipated downhole drilling conditions. To determine that optimum, a patented new bit/formation interaction model was developed that more fully explores the effects of bit geometry parameters on bit steerability.

The new model simulates drilling various directional well geometries using both push- and point-the-bit directional drilling systems in a wide variety of formations including combination soft, medium and hard formations; multiple layers; and relatively hard interbedded stringers.
While various types of computer-based systems, software applications and computer programs have previously been used to simulate forming well bores, this new method meshes design of the cutting structure, impact arrestors, gauge pads and formation in three dimensions.

Simultaneously using bit rotation, bit axial penetration, required dogleg severity and formation properties to simulate the bit/formation interaction, the model accurately predicts the required amount of bit side force, amount of bit walk force created, speed of bit walk in azimuth direction and variance of bit torque during directional drilling. As a result, rotary drill bits and bottomhole assemblies can be designed with optimum directional characteristics for drilling a specific wellbore profile with a given drilling system.

These developments reflect that products are built on a technological foundation based on an extensive understanding of fundamental downhole drilling dynamics. In order to bring the benefit of these advances to the customer, a unique “customer-centric” design process is utilized.

Incorporating first-hand customer input into the bit design function, the design process is a continuous-improvement loop that uses a global network of specialists who work directly with the customer to define application-specific bit solutions.

Working with customer personnel, service specialists use these sophisticated well planning tools to analyze formation properties and precisely define the application, then match design to application using powerful 3-D design software to optimize the bit design. During simulation, the drilling optimization software then applies a proprietary bit model to the specific downhole environment to determine optimum drilling parameters and bit usage, while post-well analysis provides direction for further refinements.

From analyzing formations and designing application-specific bits to optimizing and evaluating performance, engineers follow this continuous improvement loop to ensure customers benefit fully from the latest developments in bit technology.

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