Assessment of rock-bit interaction and drilling performance using elastic waves propagated by the drilling system

A novel passive Vibration Assisted Rotary Drilling (pVARD) tool was designed and tested to improve drilling performance or rate of penetration (ROP) both in laboratory and field trials. This paper focuses on characterizing drilling performance by means of seismic while drilling (SWD) method and bit vibration analysis. The field scale pVARD tool was applied in drill-off test (DOT) with an array of geophones (1C) spread along drill site. Rotary drilling using a polycrystalline diamond compact (PDC) bit was conducted and bit-rock interaction acted as the seismic source for reverse vertical seismic profile (RVSP). The rig waves were selected for characterizing drilling performance due to limited body waves observed during the experiment. The frequency spectra of the rig waves were determined which provided two effective seismic parameters: rig wave amplitude and frequency bandwidth for drilling performance analysis. These spectrums varied in response to variation in drilling conditions, i.e. weight-on-bit (WOB), pVARD tool use and configuration, and rock type. Bit vibration was assessed by means of vibration accelerations measured with a downhole SensorSub. The whole available data used for characterizing drilling performance included WOB, bit vibration accelerations, seismic wave amplitude and frequency bandwidth and rock type. Three groups of DOT tests were conducted: 1) conventional drilling in red shale, 2) pVARD drilling and conventional drilling in red shale, and 3) pVARD drilling and conventional drilling in grey shale. Analysis of the data shows that the seismic wave amplitude and frequency bandwidth decreased with increase of drilling performance. This is explained as more energy being partitioned for improved drilling performance with less energy partitioned to longitudinal wave and rig-ground motion. This phenomenon existed for both conventional drilling and pVARD tool drilling, independent of rock type. For comparable WOB, seismic wave amplitude and frequency bandwidth varied in response to drilling with or without pVARD tool, from which the pVARD tool mechanism was further investigated.