Influence of Bit Design on the Stability of the Rotary Drilling System

Abstract

Rotary drilling systems used to drill deep boreholes for hydrocarbon exploration and production experience self-excited vibrations, which can lead to drilling dysfunctions such as bit bounce, stick-slip and whirling. These dysfunctions cause premature failure of drill bits and other components within the system, ultimately leading to increased cost of the drilling operation. It is of interest, therefore, to understand the mechanism of self-excited vibration, with the objective of mitigating the drilling dysfunctions. This thesis focuses on establishing the link between the stability of the system in regard to coupled axial-torsional vibrations and features of PDC bits, which consist of multiple fixed blades or cutters mounted on a bit body. It has been established that a state-dependent delay system can depict the mechanism of the stick-slip drillstring vibrations. The complex arrangement of cutters of a realistic PDC bit, however, are responsible not only for multiple delays but also for a delay distribution that depends on the bit motion history. Since the delay distribution controls the stability of the bit, it is imperative to develop a model capable of capturing the exact design of PDC bits. This additional consideration, although greatly complicating the analysis, is expected to give more realistic insights into the mechanism of the stick-slip vibrations of a rotary drilling system.