Improved Methods to Understand and Mitigate Stick-Slip Torsional Vibrations

Abstract

Abstract In this paper, it will be shown how stick-slip vibration distributions can be used to evaluate drill string and parameter redesign options to mitigate stick-slip on the next well. It will also be shown how stick-slip vibration data can be used to estimate the time interval during which the bit is stuck when operating at or beyond full stick-slip. The torsional vibration example to be discussed in this paper is based on the distribution of stick-slip vibration data when downhole data is available, or alternatively using surface torque swing data. The operator's frequency-domain dynamic model relates torque swing at the surface to the variation in bit rotary speed, as a function of the drill string design and surface RPM. Linear relationships between this distribution function and several variables are investigated, including the critical torque swing at full stick-slip, rotary speed (RPM), and downhole torque. The operator drilled several wells in a vibrations-prone formation, and one well had extreme stick-slip vibrations whereas another well experienced minimal stick-slip as a result of limiter redesign. An understanding of the differences in the factors that led to the vibration reduction has been generalized into a procedure that can be applied to future wells. Although prediction of stick-slip in advance of drilling is difficult, with the benefit of data from one well it is possible to determine new parameters for a second well that will have significantly less stick-slip tendency. In a well in a different field, high frequency torsional measurements at the bit provided data illustrating the rotary speed swing of the bit stick-slip cycle, highlighting the duration of time that the bit was in the stuck condition. Results from the dynamic drill string torsional model have been compared with downhole data, and application of the dynamic model and some new relationships enable stuck time to be related to the drilling parameters. The variation in stored potential energy on each stick-slip cycle can also be calculated. These new understandings improve efforts to redesign relevant drilling parameters to mitigate stick-slip vibrations. Historically, prediction of stick-slip vibrations in advance of drilling has been fraught with challenges. Although we know what factors influence stick-slip, and what directional changes are required to mitigate this dysfunction, there is a need for a simple methodology to quantitatively determine the degree of dysfunction and the effects of one or more redesign parameters.