New Insights on the Interaction of Stick-Slip and HFTO Shown in a Case Study in North Sea

Abstract

Abstract Drilling tools are subjected to mechanical loads while drilling, resulting in loss of performance due to accelerated wear and subsequent tool damage. Two special phenomena are well known to generate high mechanical loads in torsional direction: Stick-Slip and High Frequency Torsional Oscillations (HFTO). Preceeding studies have explained the nature of the interaction of these phenomena, how they can superimpose and how stability maps define safe combinations of operational parameters. This paper will provide a systematic and quantitative study of these effects based on high-frequency measurements recorded during a North Sea deployment. In this paper, for the first time, an in-bit high frequency measuring device is used to validate the theoretical modeling of the stick-slip-HFTO interaction. Field data from the North Sea including surface and MWD measurements are used in a broad study to investigate the influence of operational parameters, bit and formation on the torsional dynamics over the increasing depth of a well. Pure Stick-Slip, pure HFTO as well as the development of HFTO within a slip-phase of stick-slip is observed, described and compared to the theoretical model describing self-excitation. Several hundred instances of stick-slip are used to determine characteristic RPM thresholds for combinations of RPM and WOB. The totality of these thresholds compose a stability map indicating zones of reduced torsional loads. The study shows that stick-slip amplifies torsional loads generated by HFTO up to a certain RPM threshold. Further increasing the RPM leads to reduced loads for certain combinations of bit, formation and operational parameters. In contrast to common belief, higher RPM can lead to lower torsional loads in certain scenarios while also increasing ROP. The stability map summarizes the learnings, indicating zones of safe operation in an accessible manner. The resulting stability map can be used by operators to navigate in stable regimes of operational parameters. This reduces wear, tool damages and enhances drilling performance.