Theoretical Vs. Actual Mud Motor Performance in Unconventional Formations and Strategies to Extend Motor Life

Abstract

The continued growth in unconventional drilling has resulted in downhole motors being pushed to drill more complex geometries faster, often higher downhole temperatures. This has resulted in increased rates of failure for downhole motors. The most common method to postpone motor failure has been to select a larger motor. This option ignores the root causes of motor failure and is shown to actually not improve failure rates. Impetus for this work centered on an operator who was experiencing high rates of motor failure, however in the process of investigating the failures, important findings were made that apply to any BHA with a motor. Damage occurs in all motors in the data set, regardless of whether or not they are considered "failed." This points to a potential, unrecognized industry-wide issue wherein motor performance may be degrading over a run and is not recognized by operations teams. The result is a version of "hidden NPT" which is limiting drilling performance in wells drilled with downhole motors. Calculating the RPM factors on the operator's wells using actual motor speeds measured by downhole sensors show that motor degradation is a significant problem, resulting in a reduction of up to 40% in bit RPM. In order to measure performance, motor properties are used to convert the traditional surface MSE measurement into a more representative downhole MSE. Further refinement of this metric was tested as downhole MSE is calculated using a simple factor and a more exact but complicated multi-dimensional correlation. Drilling data is used to show that the simplicity of the simple factor more than offsets minor loses in MSE accuracy. As noted, the real time RPM factors are also computed using downhole data showing the stark differences between the actual factors and the originally reported factors. The difference is used to determine the degree of motor damage while drilling. A suite of downhole sensors was placed above and below the motor to monitor the rate of motor damage. Differential pressure was also analyzed in the frequency domain spectrum to demonstrate the damaging effect of bottom hole assembly (BHA) whirl during rotating. The use of depth of cut control (DOCC) is analyzed as a method to improve motor efficiency and decrease damage. Two identical wells and BHA's are compared, one with and one without DOCC. The uses of the first derivative of differential pressure is suggested as a method to better quantify and visualize the effects of DOCC. Ultimately, the research presented provides quantitative insights into the operation of a motor, and shows that motor performance is not as expected based specification sheet values.

Positive Displacement Motor Condition and Performance Prediction Using Surface and Downhole Sensor Data

“…In case the increase in pressure is due to an increase in flow rate, then these changes in flow rate at the selected peak are used to disregard the corresponding value as a valid stall. Motor stalling detection requires a high-frequency downhole, allowing us to evaluate the duration of the stall and its impact on motor efficiency [36][37][38]. Depending on the duration, it can also be detected from surface measurements but requires downhole data for validation.…”

Drilling Monitoring System: Mud Motor Condition and Performance Evaluation

“…Part of the BHA for both vertical and inclined sections is a positive displacement motor or mud motor, which converts the hydraulic energy the drilling fluid provides to mechanical energy, which is rotation and torque. Mud motor design and performance have been a subject of research for many years since adding an additional mechanical tool in the BHA, on the other hand, provides performance but, on the contrary, depending on the duty cycle (which is a function of the operating parameters) might fail and is associated with non-productive time (El-sayed et al, 2020).…”

Embedded Force Sensing at the Cutter

“…For the past 20 years, the concept of detection, estimation and prediction of bit wear by utilizing drilling data analytics and physics-based modelling has demonstrated improvements in drilling efficiency (Rashidi et al, 2010;Lakhanpal & Samuel, 2017;Witt-Doerring et al, 2021). Several researchers are focusing on utilizing mechanical specific energy (MSE) to enhance drilling efficiency (El-sayed et al, 2020; and observe bit wear in real-time (Al-Sudani, 2017;Dupriest et al, 2020). Since MSE is the required energy to drill a volume of rock, a significant increase from the baseline can indicate bit wear.…”