Torsional Resonance - An Understanding Based on Field and Laboratory Tests with Latest Generation Point-the-Bit Rotary Steerable System

Abstract

Abstract Torsional dynamics can be extremely damaging to downhole drilling tools and can significantly affect drilling efficiency. Industry focus on combating this has led to the development of various downhole mitigation tools, improved bit technologies, BHA designs and better management of drilling parameters. The aforementioned focus primarily on combating stick-slip, or at least the conventional form of it, with its distinctive stick and slip phases. This paper outlines two lesser known types of torsional dynamics observed whilst drilling inter-bedded carbonate formations in the middle-east with the latest generation in point-the-bit Rotary Steerable Systems (RSS). One of these phenomena has the potential to be extremely damaging, more so than conventional stick-slip, and is of such high frequency that conventional logging rates are incapable of detecting it. This paper highlights the presence of a low frequency (<0.5Hz) and a high frequency (~70Hz) periodic oscillation in rpm observed using one of the industries most advanced downhole dynamics sensors. The low frequency oscillation was found to occur at the fundamental torsional natural frequency of the drillstring, with RPM fluctuating harmonically by as much as three times the average surface rpm. The high frequency torsional oscillation, attributed to a torsional resonance within the bottom hole assembly (BHA) itself, resulted in extremely damaging high frequency, high magnitude (10,000rad/s2) torsional oscillations. The loading environment resulting from this high frequency torsional resonance has the ability to fatigue downhole tools in a matter of hours! This paper outlines findings from both field and laboratory tests, attempting to better understand both the low and high frequency torsional phenomenon. This paper goes one step further than existing literature by attibuting the high frequency torsional oscillations to structural resonances within the BHA. This is substantiated both analytically and experimentally. Understanding the underlying characteristics of these phenomena paves the way for new, more effective, prevention systems. The paper concludes with the development of a unique sensor which monitors subtle changes in the industry standard Stick-slip Index to detect different torsional dynamic modes in realtime and send a text based description and severity rating to surface.