Numerical simulation of rock breaking by PDC bit in hot dry rocks

Penetrating through hot dry rocks for geothermal resources with high geothermal gradients, high degrees of hardness and abrasivity will be extremely difficult, and a reasonable tooth distribution is a key factor to improve its rock-breaking efficiency. In order to explore an available cutter arrangement design of PDC cutters in hot dry rock drilling, we based on elastic–plastic mechanics and rock mechanics, established a dynamic 3D numerical simulation model of rock breaking with PDC cutters by using the Drucker–Prager yield criterion as the rock strength judgment principle. On the basis of this, we studied, under the confining pressure of 60 MPa, the effects of cutting depth, temperature, back rake angle and cutting speed on the mechanical specific energy of PDC cutters. The following results were achieved. (1) When a PDC cutter cuts a rock with the speed of 0.5 m/s and the back rake angle of 5–25°, the rock destruction specific energy of the cutter decreases significantly with the rise of cutting depth, while increases first and then decreases with the increasing temperature. And the critical temperature is 200 °C. (2) When a PDC cutter cuts a rock with the speed of 0.5 m/s and the cutting depth of 1–3 mm, the rock destruction specific energy of the cutter decreases first and then increases with the increase of back rack angle. And the optimal rack angle is 20°. (3) Within the temperature range of 20–300 °C, a PDC cutter cuts a rock with the back rake angle of 5°, the rock destruction specific energy of the cutter increases with the cutting speed but decreases with the increase of cutting depth. Keywords: Hot dry rock, Geothermal exploitation, PDC cutter, Mechanical specific energy, Rock-breaking efficiency, Numerical simulation, Drucker–Prager yield criteria, Rack angle