Mechanism of Penetration Rate Improvement in Hot Dry Rock Under the Coupling of Impact Load and Confining Pressure Release

Deep geothermal resources are environmentally friendly and represent a highly competitive form of clean energy. However, low rock-breaking energy combined with high rock strength results in a low rate of penetration (ROP), which significantly restricts the efficient utilization of geothermal resources. Previous studies have shown that rock failure is primarily caused by shear stress. Therefore, this paper aims to enhance the shear stress level by increasing the impact load and releasing the confining pressure, thereby improving the ROP. Specifically, the rock-breaking efficiency under the coupling of impact load and confining pressure releasing is analyzed to reveal the influence of confining pressure releasing on shear stress. Furthermore, a rock-breaking model is established, and an impact load generator is employed to validate the proposed model, enabling the evaluation of rock-breaking efficiency under the coupled action of impact load and confining pressure releasing. The results indicate that the ratio of shear stress to $I_1$ dominates the rock-breaking process. When this ratio is low, the rock tends to remain in a compressed state, the hydrostatic pressure effect is enhanced, the shear stress effect is relatively weakened, and the rock-breaking efficiency decreases. The coupling of impact load with confining pressure releasing can achieve effective rock breaking under relatively low weight-on-bit conditions in deep wells, thereby providing theoretical support for improving rock-breaking efficiency in hot dry rock geothermal development.