Analysis of the Stick-Slip Phenomenon Using Downhole Drillstring Rotation Data

Abstract

Abstract The stick-slip phenomenon has been identified in the industry as an inefficient and often damaging drilling vibrational condition. Numerous studies, predominantly focusing on measurements of drillstring torque, have detailed the nature of this phenomenon, and have shown its detrimental effects on drillstring components and especially on PDC bits. Using surface rotary motor current, drilling contractors and surface logging companies have attempted to monitor drillstring torque so as to recognize and correct for this torsional instability. Recent MWD tool developments using downhole drillstring rotational speed have established a superior method to identify stick-slip and establish its severity. Measuring actual rotational behavior of the lower BHA gives a better insight to the torsional movement of the drill bit during the drilling process, without the problems involved when inferring this behavior from the surface torque readings, obtained at the opposite end of the drillstring. This paper describes the method of determining the stickslip behavior downhole from magnetometer readings. The sensors are sampled at a high enough frequency to compute nearinstantaneous values of downhole drillstring rotation, and variations in the RPM are then used to establish the incidence and severity of stick-slip. This information is telemetered to the surface and provided to the driller in order to allow for real-time changes in drilling parameters to correct for this inefficiency. Also presented in the paper are several examples using actual field data that detail stick-slip behavior and demonstrate how real-time information about downhole RPM variations can help to reduce the problems associated with this torsional drilling dysfunction.