The Origin and Mechanism of Severe Stick-Slip

Abstract

Abstract A drill bit is subjected to rotational speed variation during drilling, ranging from minor rotational speed oscillations to severe speed variation, including stuck phase. The severe speed variation, referred to as stick-slip, is known to be a major source of problems, such as fatigue failures, bit wear, and poor drilling rates. This paper will provide new insight into the mechanisms that drive severe stick-slip based on continuous high-frequency downhole measurements and 3D transient dynamic drilling simulation. To understand the mechanism of severe stick-slip, a series of drilling tests were conducted at a full-scale drilling test facility. An advanced downhole measurement tool was placed in the bottom hole assembly (BHA) to record three-axis shock and vibration, RPM, bending moment, downhole weight on bit (DWOB), downhole torque (DTOR) and internal and annular pressure at high frequency. The drilling system was then modeled on a 3D transient drilling simulation platform, including detailed bit-rock interaction based on single-cutter tests, the exact BHA, and the wellbore geometry. The downhole recorded data showed clear coupling of severe stick-slip, axial load, and bending moment conditions. Interesting patterns observed between RPM, DWOB, DTOR, and bending moment will be presented in detail. The recorded severe stick-slip condition was successfully reproduced by 3D transient dynamic simulation, and the torque, axial load, and bending moment variations along the BHA revealed coupling between torsional, axial, and lateral motions of the drilling system from simulation. Bit-rock interaction and drillstring-wellbore contact are the drivers supporting the coupling. It was found that the coupling of three motions of the drilling system is the most reasonable explanation to the self-sustained severe stick-slip condition. This mechanism can explain the field observed stick-slip trend (i.e., the higher WOB and lower RPM tend to increase the risk of severe stick-slip tendency). Downhole measurements and transient 3D dynamic simulation of the entire drilling system are essential to fully understand this mechanism. Understanding the stick-slip process opens additional opportunities for controlling severe stick-slip. Because the stick-slip mechanism is driven by the coupling of three motions, it is possible to mitigate this condition by breaking the coupling mechanism. For example, a shock sub possibly will reduce the severity of axial coupling due to added axial compliance. Simulation shows breaking the mechanism could reduce the severity of stick-slip. The study being reported in this paper also proved the validity of applying advanced 3D transient dynamic model to obtain a better understanding of drilling system behavior. Drilling simulation might well be an effective way to plan a drilling system and drilling parameters.