Improved drifting oscillator model for dynamical bit-rock interaction in percussive drilling under high-temperature condition

High cost is the main factor restricting the development and utilization of geothermal resources, and the proportion of drilling costs is even higher than 50%. Therefore, it is crucial to find an efficient drilling method and optimize drilling parameters. Percussive drilling is a widely-recognized efficient geothermal drilling method. In this study, an improved drifting oscillator model is developed to determine the optimal drilling parameters by representing the dynamical interaction between drill bit and high-temperature granite (HT-G). The Abel damper is introduced to represent the nonlinear relationship between the damping force and strain rate. Solutions of HTG Young's modulus and viscosity coefficient as a function of temperature are obtained by experimental data fitting. Then the rock response without and with bit penetration are studied theoretically and numerically, respectively. Finally, experiments are conducted to validate the effects of wave shapes and frequency of the impact load on rate of penetration (ROP). The results show that the ROP increases with the increase of temperature, which proves that percussive drilling is more suitable for HT-G formation than the conventional oil-gas reservoir. There is an optimal frequency ranging from 60 to 120 Hz where the system gives the best drilling performance when sin-shape impacts are applied. Rectangular-shape load gives the fastest ROP among the three evaluated wave shapes of excitation load, especially when the frequency is low. The research of this paper provides a basis for the application of percussive drilling in HT-G formation and is of great significance to the design of downhole impactor and formation adaptability evaluation.