An Analysis of Noise Characteristics of Drill Bits

Abstract There have been papers that analyse the relationship between drill bit design and its vibrational characteristics. These papers typically are based on the analysis of three-axis down-hole vibration sensors located near the bit. In this paper the authors take a simpler approach. Using a standard microphone literally pointed at the bit, they record the noise of the bit/rock interaction while drilling and analyse the resulting noise for basic information on the bit. The data were gathered at the Colorado School of Mines in Golden, Colorado. The noise of a PDC, roller cone, and diamond bit were recorded under various operational parameters using microphones and a uniaxial geophone (used for confirming the data recorded on the microphones). Using a Fast Fourier Transform, the frequency spectra were extracted from the recorded data. Summarizing the results of the analysis of the roller cone bit, the PDC, and the natural diamond bit, major differences in their frequency characteristics could be detected and furthermore, for drag bits, the frequency characteristics could be related to the bit's design. The frequency spectra of the roller cone bit can best be described with a general high amplitude level that is relatively evenly distributed over the whole frequency spectrum. The drag bit data showed a strong relationship between the number of cutting elements and frequency peaks on a plot of amplitude vs. cycles per revolution. Frequency peaks were observed at multiples of the number of cutting elements. In general this relationship was strongly visible on the PDC bit data but not a strong on the diamond bit data. The conclusion is that bit characteristics can be determined using only acoustical data (ie. the noise of a bit). Potential applications of this research include detecting and diagnosing bit problems (e.g. broken teeth, bit balling) in real time using acoustic data.