Years of research and experience show that optimal bit selection results from careful consideration of the four fundamental, interdependent indices that describe bit performance:
• Rate of penetration (ROP) — how fast the bit will drill for a given weight on bit;
• Stability — how resistant the bit is to lateral vibration;
• Durability — how resistant the bit is to abrasive wear; and
• Steerability — how the bit responds to side forces and therefore how steerable it is on rotary steerable systems.

A bit designed for maximum durability will need a very dense cutting structure to resist

Ranking the four fundamental indices is critical to application- optimized bit selection. (Image courtesy of ReedHycalog)
abrasive wear, which will limit its ROP potential. In addition, a durable bit design will consider stability, since shocks break cutters and inserts and limit the bearing life of roller cone bits. Because of this interdependence, ranking the relative importance of each index in a specific application enables the selection of optimal technologies to improve overall drilling performance.

Improved roller bearing durability
The completely redesigned bearing system in the RockForce series of roller cone bits has been developed with maximum bearing reliability as a top priority to give excellent durability. The improved bearing package features:
• A high-energy cone retention
system;
• Tighter machining tolerances;
• Reformulated synthetic lubrication; and
• A completely new pressure compensation system.
These new designs, which also incorporate computer modeled cutting structures and optimized hydraulics are setting bearing reliability records around the world in both rotary and motor applications.

System-specific steerability
In response to the proliferation of steerable motors with higher power output and rotary steerable systems with different steering mechanisms, the SystemMatched line of directional products combines the critical design features for each specific tool, lithology and directional objective for optimum bit performance. The SystemMatcher application optimization software tool takes the directional profile and the drive system into account for optimal bit selection.

Addressing drilling stability
Customers have identified drilling vibration as one of the leading causes of lost time and poor drilling performance. Historically, evaluating bit performance, particularly with respect
to downhole dynamic behavior was achieved by dull evaluation of the bit after the run. This indirect indication has proven to be inadequate.

In order to address downhole vibration, where stability is the most critical index, ReedHycalog has taken a holistic approach with tools and techniques that focus on:
Prediction — VibraSCOPE bottomhole assembly (BHA) and drillstring vibration modeling software predicts critical parameters likely to cause excessive axial, torsional and lateral vibration modes.

Mitigation — V-Stab vibration dampening tool 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.

Validation — BlackBox downhole vibration recorder can be deployed in single or multiple locations along the drillstring to record up to 200 hours of high frequency lateral and torsional vibration data. This detailed post-run analysis to determine the specific dynamic behavior of the bit and BHA in the context of the formation and drilling parameters.
Great drilling performance often means focusing on issues beyond the drill bit. Focusing on and characterizing the four fundamental indices — ROP, stability, durability and steerability — is the best way to meet a range of drilling challenges and provide better performance and lowest cost-per-foot.

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