Measurement of Mud Motor Back-Drive Dynamics, Associated Risks and Benefits of Real-Time Detection and Mitigation Measures
Authors: Sugiura Junichi , Jones Steve 
Published: March 2021

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 advanced to deliver more horsepower and torque, resulting in different downhole dynamics that have not been identified in the past.

High-frequency (HF) compact drilling dynamics recorders embedded in the drill bit, mud-motor bit box, and motor top sub (sub-assembly) provide unique measurements to fully understand the reaction of the steerable-motor 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 back-drive dynamics. Detection of back-drive from surface measurements is not possible, and many measurement-while-drilling (MWD) systems do not have the measurement capability to identify the problem. Motor back-drive dynamics severity is dependent on many factors, including formation type, bit type, power section, WOB (weight on bit) and drill pipe size. The torsional energy stored and released in the drill string can be high due to the interaction between surface RPM (revolutions per minute)/torque output and mud-motor downhole RPM/torque. Torsional drill string energy wind-up and release results in variable power output at the bit, inconsistent rate of penetration (ROP), rapid fatigue on downhole equipment, and motor or drillstring back-offs and twist-offs.

A new mechanism of motor back-drive dynamics due to the use of an MWD pulser above a steerable motor is discovered. HF continuous gyro sensors and pressure sensors were deployed to capture the mechanism in which a positive mud pulser reduces as much as one third of the mud flow in the motor and bit rotation speed, creating a propensity for a bit to come to a complete stop in certain conditions and for the motor to rotate the drillstring backward. We have observed the backward rotation of a PDC drill bit during severe stick-slip and back-drive events (-50 RPM above the motor), confirming that the bit rotated backward for 9 mS every 133.3 mS (at 7.5Hz), using a 1000-Hz continuous sampling/recording in-bit gyro. In one field test, multiple drillstring dynamics recorders were used to measure the motor back-drive severity along the drillstring. It is discovered that the back-drive dynamics are worse at the drillstring, approximately 1110 ft behind the bit, than these measured at the motor top-sub position. These dynamics caused drillstring back-offs and twist-offs in a particular field. A motor back-drive mitigation tool was used in the field to compare the runs with and without the mitigation tool, while keeping the surface drilling parameters nearly the same. The downhole drilling dynamics sensors were used to confirm that the mitigation tool significantly reduced stick-slip and eliminated the motor back-drive dynamics in the same depth interval.

Detailed analysis of the HF embedded downhole sensor data provides an in-depth understanding of mud-motor back-drive dynamics. The cause, severity, reduction in drilling performance and risk of incident can be identified, allowing performance and cost gains to be realized. This paper will detail the advantages to understanding and reducing motor back-drive dynamics, a topic that has not commonly been discussed in the past.