A Systematic Photo Documentation of Drill Bit Forensics Applied to Motor Back-Drive Dynamics Case Caused by Auto-Driller Dysfunction and Formation Effect

Abstract

Abstract North America shale drilling is a fast-paced environment where downhole drilling equipment is pushed to the limits for maximum rate of penetration (ROP). Downhole mud motor power sections have rapidly evolved to deliver more horsepower and torque, resulting in different downhole dynamics, such as motor back-drive drilling dynamics. This paper investigates the root cause of the motor back-drive dynamics and the bit/BHA damage caused by this. High-frequency (HF) compact drilling dynamics recorders embedded in the mud motor bit box and top sub provide unique measurements to fully understand the reaction of the power section under load relative to the type of rock being drilled. 3-axis shock, gyro and temperature sensors placed above and below the power section measure the dynamic response of power transfer to the bit and associated losses caused by motor back-drive dynamics. Formations with high interfacial severity pose more of a challenge due to the rapid change in formation strength. The torsional energy stored and released in the drill string can be high due to surface rotation-speed/torque output and downhole mud-motor speed/torque. Torsional drill string energy wind-up and release results in variable power output at the bit, inconsistent ROP and rapid fatigue on downhole equipment. Detailed analysis of the high-frequency embedded downhole sensor data as well as Electronic Drilling Recorder (EDR) data provides an in-depth understanding of mud motor dynamics. In one of the "Delaware Basin" field examples from Loving County, Texas, the root cause of the motor back-drive dynamics was identified. A systematic photo documentation of drill bit forensics was performed to precisely document the bit damage from this type of drilling dynamics. The auto-driller weight-on-bit (WOB) and ROP setpoints were examined along with the downhole sensor data and EDR to pinpoint the root cause of drilling dysfunction. A drillstring analytical model was used to predict the torsional natural frequencies, which are compared against the sensor-observed torsional oscillation frequencies. This paper reports a unique case of motor back-drive drilling dynamics caused by auto-driller dysfunction and formation effects. Additionally, a systematic photo documentation of drill bit forensics was applied to thoroughly document motor driven bit damage.