Abstract
Abstract
The objective of bit selection is to drill a round, ledge free hole, without patterns, with minimum vibration, minimum dog leg severity, that reaches all directional and geologic targets. And to do this in one run per section where rate of penetration (ROP) is unconstrained by the bit, and it remains vibration free in control drilling situations. The objective of this paper is to share these guidelines, accumulated from forensics investigations and sometimes costly lessons learned over the past decade.
Bit selection can be based on; offset runs, forensics analysis, and first principles. Offsets capture past learnings. Forensics analysis is applied to past and current wells, and it is the foundation for continuous improvement. First principles, based on independent thinking, are required to challenge the status quo.
These guidelines are the distillation of forensics observations, offset analysis, and physics modeling. It is important to note that these are guidelines, not rigid standards. They are presented in their current state, to be challenged, tested, and revised as new ideas and technology are developed. They are not a substitute for thinking.
The guidelines start with a description of the formation properties, drilling environment, and other requirements such as directional objectives, drill out considerations, and required life. They cover how to suppress bit whirl, stick slip, and borehole patterns; improve structural integrity, allow for predictable build rates, reduce gauge trimmer damage, and design for appropriate aggressiveness, ribbon flow, bit stability and life. These functions and dysfunctions are paired with design features such as depth of cut elements; gauge pad length and relief; cutter materials, size, and geometry; back rake; blade count and standoff; hydraulic design; and blade strength. The guidelines document specific and general lessons learned and why each rule was established. They help drilling engineers and service companies determine which design features are most important for a given application. Applying them across teams and applications highlights design elements that may be of concern and has helped avoid repeat failures. The first failure is for learning, the second failure is a failure to learn.
Design features have been matched up with associated critical performance limiters. This transparency in thinking allows the assumptions to be questioned, provides knowledge of which rules apply in each application, and provides background to evaluate when new technology may help. Some of the hard problems that remain are; consistent forensics analysis, chip flow modeling, cutter stress verses rock strength and penetration per revolution, understanding and suppressing high frequency torsional oscillation, and improvement and expansion of in-bit instrumentation.