Sonic logging tools are commonly used in wireline logging and logging-while-drilling (LWD) operations for determining formation lithology along the wellbore. Having an understanding of the near-wellbore rock with high certainty can significantly reduce risk when planning well stimulation and completion. The quality of sonic logging data is critical for decision making, especially in offshore applications for which mistakes can be costly. While much of the progress towards improving sonic logging data quality has been pursued via mechanical and electrical design along with post-process signal analysis, little has been done with using advanced control systems to boost performance of the sonic logging tools.
In this paper, the authors demonstrate noticeable improvement in the sonic logging tool's performance by producing high-quality acoustic waves from the acoustic transmitter using a novel control system design. While with proper mechanical and electronic design a transmitter can generate acoustic waves with high-quality frequency characteristics for measurement, there can exist sustained resonance vibration that degrades the data. This paper introduces a control method that mitigates excessive transmitter vibrations, releasing the full potential of the transmitter mechanical design. The robustness of this control system allows for peak performance over the range of high-pressure/high-temperature (HP/HT) environments the tool might experience downhole.
The control method described in this paper operates on similar principle as noise-cancelling headphones; however, because there is no external microphone or other sensing device, the control system is challenging to design. Essentially, it cancels out undesired vibration by generating a second waveform of equal strength but opposite direction and superimposing it to the existing waveform. The challenge of generating the proper cancelation waveform is overcome by a multistep learning algorithm. To sense the vibration, it uses the back-electromotive force (back-EMF); a voltage signal determined from the actuation device itself. One of the issues with using the back-EMF is it is substantially smaller than the actuation voltage, making reading this value during firing difficult. The benefits of this approach include reducing sensor costs and improved robustness by reducing sensor failure modes. The only modification required for the implementation of this advanced control system is a firmware update, making it an effectively zero-cost solution for a step-change boost in performance. Both laboratory test results and field data are presented to demonstrate the effectiveness of the novel control algorithm.