Abstract

The effects of downhole vibration on drill bit performance have been discussed a great deal in the past several years. Data acquired on near-bit vibration has proved useful for assessing bit selections, design features and running parameters. Typically, engineers have had only limited access to this type of data because measurement-while-drilling (MWD) tools capable of acquiring vibration measurements are generally placed well above the bit in the bottom hole assembly (BHA), where the dynamics can be significantly different than at the bit. Also, these conventional MWD tools are expensive to operate.

The recent development of an easy-to-use, memory-mode, vibration-logging tool has made obtaining relevant “at-bit” data much more feasible. The tool is very small and can be used in collar sizes as short as 8 in. from shoulder-to-shoulder. The tool can be placed directly above the bit without disturbing the BHA. This paper highlights the development of this tool and presents data from recent field tests that show how it has been used to optimize and evaluate drill bit performance in various applications.

Introduction

Drill bit dynamics have been the focus of recent studies^1,2,3 that have identified several forms of potentially harmful downhole dynamic events, most notably stick slip and lateral vibration. While some traditional MWD equipment can identify various dynamic problems, it is typically located well above the bit in the drill string where the dynamic activity can be drastically different than the activity at the bit. The development of a small and simple-to-operate downhole tool allows detection of these dynamic motions directly at the bit.

Background

Advanced Research Sub

The precursor to the vibration tool discussed here is a larger and significantly more advanced drilling research sub used to obtain a large suite of measurements, including downhole vibration and dynamic activity data.

The advanced research sub is designed to be positioned directly above the drill bit with most of its sensors only a few inches from the bit pin. It features high-speed logging capabilities as well as downhole algorithms which calculate and store time-averaged values. The sub measures RPM, accelerations in various axes, pressures and the drilling forces acting on the bit. In several years of use, it has been used to collect valuable data that has resulted in important findings about downhole bit performance. These findings were the catalyst for the development of a smaller and simpler tool that could be developed to address many applications.

Lateral Vibration

Lateral vibration, specifically whirl and chaotic vibration, have long been recognized as detrimental to bit and BHA life. Whirl occurs when the centerline of the bit rotates in a circular pattern about the center of the hole at a non-zero radius. The whirl rotation occurs several times per revolution of the bit and can be forward (in the same direction as the bit) or, in the more common case of backward whirl, in the opposite direction.

During whirl, the bit will drill an overgage hole with a lobed cross section. An orbit plot from a whirling PDC bit is shown in Figure 1. In this case, the outer radius of the bit traces a 7- lobed pattern. Chaotic or random vibration occurs at various levels at all times while the bit is drilling. This vibration can reach severe levels. Severe chaotic lateral vibration and whirl have been recorded by the advanced research sub.