High Frequency Vibration Measurement Coupled with Time-Based Dynamic Simulations: New System to Predict/Solve Instability Issues
Authors: Wang Yaou , Shen Yuelin , Charter Marc , Skoff Gregory 
Published: October 2014

Abstract
Downhole vibration is detrimental to drilling efficiency and can cause MWD/LWD tool failure, drillstring fatigue, bit/PDM damage. Preventing or mitigating BHA instability is critical to improve drilling efficiency, increase ROP and reduce drilling costs. To prevent dynamic instability requires an in-depth understanding of the BHA's downhole behavior in terms of the actual phenomenon and ability to determine root cause. Downhole RPM and vibration data sent to surface from MWD/LWD usually have very low frequency limited by telemetry. Because vibrations are measured at a distance from the bit, the data may not capture detailed bit behavior patterns. The low frequency data is typically used in combination with steady-state response or static analysis models developed to predict interaction between the drillstring and wellbore. Results produced by these type systems have been less than optimal.
To obtain superior quality downhole vibration data, a high-frequency drilling dynamics measurement module (DDMM) was recently developed to capture and measure bit/BHA dynamic response. The authors will describe the application of the new module to measure and record downhole RPM, acceleration at bit/BHA at sampling frequencies up to 2048-Hz. Analysis verified the high-frequency data produces more specific details about dynamic behavior compared to standard data. The module can be positioned at any location within the BHA or directly above the bit.
The high-frequency data is used in conjunction with a unique time-based dynamic simulation system to verify and confirm the modeling's prediction of ROP/RPM, acceleration and drillstring instability including stick-slip/whirl. When drilling dysfunction is observed, the time-based modeling system has the potential to ascertain the root cause not typically identified in measured data. This system approach is cost effective and can be highly effective at preventing or mitigating complex instability issues. Three DDMM field tests are presented that document the capability of the combined testing and modeling system approach to achieve better understanding of downhole dynamics:

Case Study 1
Rotary BHA with one DDMM positioned at top of bit. Analysis of actual downhole data from DDMM shows large RPM variation at the bit which was confirmed by the time-based modeling system. The two data plots showed good correlation between axial/lateral acceleration (acceleration/g's vs time/sec).

Case Study 2
Motor BHA with two DDMMs installed. One positioned at top of motor another at top of the bit. Analysis of actual data documents stick-slip at the bit and top of motor which is confirmed by the modeling system. Detailed analysis of the two data plots revealed good correlation between actual and modeled data with both depicting negative RPM at the top of the motor with increased acceleration at the occurrence of stick-slip (RPM vs time/sec).

Case Study 3
Motor BHA with two DDMMs installed. One positioned at top of motor, another at top of bit. Analysis of actual data documents stick slip at the top of motor which was confirmed by the modeling system. Further analysis of the modeling data plot identified stick-slip coupled with BHA whirl.
The case studies document accurate modeling predictions of ROP/RPM and acceleration corroborated by the measured data. Modeling also successfully predicted dynamic instability issues identified by actual DDMM measurement and gave specific details about coupling of stick-slip and BHA whirl not observed in the measured data.