High-Frequency At-Bit Measurements Provide New Insights Into Torsional Dynamics When Drilling With Steerable Mud Motors in Unconventional Horizontal Wells

Abstract

Summary New insights into torsional bottomhole-assembly (BHA) dynamics have been gained from high–frequency at–bit measurements when drilling with steerable mud motors in horizontal wells. Steerable mud motors are traditionally thought to decouple the bit from “sticking” as the BHA and drillstring are subject to torsional fluctuations. However, newly gathered data show that the bit can actually come to a complete stop while the motor is forced into the backward rotation. Motor twistoffs and back–offs while drilling can be the costly consequence of such behavior. Using high–frequency, at–bit measurements, dynamic and environmental characteristics of the bit were recorded in a dedicated bit sub at 200 Hz while drilling a horizontal well in the Granite Wash Formation in the Texas Panhandle. The recorded variables include azimuthal gamma, triaxial accelerations, inclination, revolutions per minute (RPM), and temperature. Recorded data were examined after each interval drilled, and correlated with standard measurement–while–drilling (MWD) data (measured and recorded above the motor), as well as surface–recorded variables. Examination of the data revealed that the bit experienced full stick/slip behavior and, instead of being decoupled from the BHA as is traditionally thought, drove the BHA into backward rotation by means of the action of the mud motor's power section. It is believed that, owing to the tortuosity of the wellbore and the relative hardness of the formation being drilled, the lower end of the motor periodically “caught” and was hung up while rotating such that the negative–driving torque generated in the power section (during the “bit–stick” intervals) loosened the connections below the motor's power section and ultimately led to back–offs of the connections downhole. This new high–frequency at–bit measurement tool has led to clear insights into the stick/slip behavior while drilling horizontal wells with steerable mud motors. Because this new understanding contradicts the traditional thinking of torsional oscillations in motor BHAs, the authors believe it is worth sharing with the industry because it can help prevent nonproductive time related to this mechanism in the future.