Abstract
Vibration analysis on downhole forces and motions of roller bits could be a new realtime diagnosis method of bit state and lithology to be drilled.Drilling experiment and data acquisition were performed using atmospheric full-scale bit test machine. Data include weight on bit, bit torque and acceleration of axial bit motion at the bit sub for a couple of new and worn 8-1/2″ roller bits drilling samples of granite and andesite. Measured data were analyzed through power spectral densities and cross correlations.The power spectral densities of measured data presented characteristic peaks in frequency domain due to successive impacts of bit teeth on the rock surface. Bit tooth wear causes shifts of locations of these spectral peaks to higher frequencies. Variations of rock type and penetration rate have effect on vibration amplitude of axial bit motion rather than on the power spectral peak frequencies within the limits of the experiments. Dominant frequencies are quite sensitive to cone rotary speeds. Considerable correlation between weight on bit and bit torque vibrations was observed, and cross-correlation function showed that weight on bit is delayed in time from bit torque.
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