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Abstract
Well known in the oil and gas industry is the importance of understanding drilling system vibrations and bit-reamer weight transfer when drilling with hole openers.

A field testing program was carried out on a full-scale experimental test rig in the state of Oklahoma, USA, with known lithological formations, in order to evaluate underreaming system designs. In this context, the underreaming system comprised the bit, the drive system, and the underreaming element.

To fully understand how the system interacts with the formation and reacts to inputs from the surface, drilling mechanics measurements were taken above and below the reamer element. Drilling dynamics measurements were also taken at three places in the BHA, with two drilling dynamics packages spaced out below the reamer and one positioned directly above it. That way, dynamics on the bit and reamer could be studied separately to understand how bit and reamer performance affect each other and the overall system dynamics.

The first well was directionally drilled with a rotary steerable system, concentric reamer, and vibration monitoring equipment set up in an industry-standard arrangement. This paper will describe how the findings and learning from the initial field test led to bit redesign and operational techniques implemented to enhance the drilling system stability. Another borehole was then drilled through identical formations with this improved system, offering a unique detailed comparison between the two field tests. The new bit design allowed the bit to control the drilling rate with better weight distribution between bit and reamer, significantly reducing vibration, and in this study, without impacting penetration rate.

A concentric reaming tool new to the drilling industry was used in both wells as a part of this study and will also be described. This tool demonstrated good steerability with a rotary steerable tool system and operated properly, drilling closed, activating, reaming, and finally, closing for retrieval from the hole.

Introduction
The purpose of this paper is to describe testing done under downhole-controlled conditions in identical, side-by-side wellbores with heavily instrumented BHAs. The testing compared the results of drilling with a conventional bit/reamer system compared to a specially designed pilot bit synchronized to the reamer. In both cases, the BHA was designed with dynamics modeling software with the objective of minimizing vibration. The test results proved a methodology that matched the reamer and pilot bit to substantially reduce vibration.