There is a critical need to understand the interaction between a drill bit and underreaming tool in hole opening assemblies. Problems that can result from improper matching include vibration, inability to open hole, mechanical damage to string tools or to the bit, and sub-optimal drilling performance. Matching of the bit and reamer to avoid these dysfunctions is not a simple process, and certainly is not resultant from simply having a similar cutter size in both cutting structures, or from compromising the rate of penetration potential of the bit.

There are four indices that substantially describe the performance of a bit and reamer system: Aggressivity; Stability; Durability; and Steerability. A tool that is capable of calculating the four indices for a bit and reamer allows optimal solutions to be developed for an application once the importance of the rankings are defined for that application. Accurate modeling of bit and reamer solutions must investigate the torque and weight balance in a number of different lithology scenarios. This must take into account the range of compressive strengths plus the type of transition encountered. Suitability of bits for specific reamers can be assessed using a sophisticated and novel reamer matcher tool. These can be tailored, via user input, to the specific parameters of the application. This matching tool uses logic tables based on theory, modeling, and field experience to provide a consistent, logical and quantitative approach to bit selection for reamers.

Case studies are presented from deepwater applications that clearly demonstrate the value of the use of indices and reamer matching tool. In combination, these have enabled high efficiency drilling performance, reducing cost per foot to the operator.

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