This article, written by Editorial Manager Adam Wilson, contains highlights of paper SPE 156240, ’A New Model of Bit Whirl,’ by Yevhen Kovalyshen, CSIRO Earth Science and Resource Engineering, prepared for the 2012 SPE/IADC Asia Pacific Drilling Technology Conference and Exhibition, Tianjin, China, 9-11 July. The paper has not been peer reviewed. Previous analytical models of bit whirl considered bottomhole assembly (BHA) mass imbalance to model bit whirl. The model presented here takes into account the history-dependent boundary conditions at the bit/rock interface. Within this model, the geometry of the bit is characterized by three dimensionless parameters. The results show that, depending on the value of these parameters, the system can be stable or undergo forward or backward whirl. As a result, the model has potential application as a tool that assists bit design. Introduction Bit whirl is a lateral vibration of the drill bit during which the bit axis undergoes a circular motion. If the axis of the bit rotates in the same direction as the bit itself, the bit is said to undergo forward whirl. If the axis of the bit rotates in the direction opposite to that of the bit, the bit is said to undergo backward whirl. Bit whirl is the most damaging mode of vibration and is responsible for the premature damage principally of polycrystalline-diamond-compact (PDC) bits. Most whirl models are based on the assumption that the whirl instability is caused by an imbalance of the BHA. The imbalance is either from mass imbalance or from imbalance in the cutting structure. Although experiments show that BHA imbalance can increase the whirl tendencies, a perfectly balanced BHA also can generate bit vibrations. This suggests that bit whirl cannot be explained solely by BHA imbalance. According to field observations, the main factor influencing whirl is bit geometry. For example, a paper published in 1990 states, “... longer tapers, more aggressive structures, or aggressive gauge cutters all significantly increase a bit’s tendency to whirl. Conversely, higher back rake, flatter profiles, and smooth gauge all tend to reduce bit whirl. Also, features that would seem to possibly prevent whirl, such as irregular spacing blades, strongly inverted cones, or perfectly balanced bits, do not completely eliminate whirl … .” According to the same authors, shock subs do not alleviate the problem. Another factor that affects bit whirl tendency is rock strength; the harder the rock is, the higher the chance of bit whirl is. Proper bit/rock interaction has been incorporated into a numerical analysis of bit vibration, but no theories have been able to explain the field observations. In contrast to previous studies, this work takes into account bit geometry and the regenerative effect, while the BHA is modeled as a mass/spring/dashpot system. Furthermore, the investigation concentrates on stability analysis rather than on solution of an evolution problem.
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Feb 18, 2015
G.C Cudmore + 2
Open Access