Comparative analysis of impact failure modes between slotted rock and intact rock

The compression effect of geostatic stresses and hydrostatic pressure on rock increases with the depth of geothermal well, which strengthens the rocks and reduces the rate of penetration (ROP). How to overcome the strengthening effect of stress on rock and weakening rock drillability has become a key factor to improve drilling speed. Based on a novel method for improving the rate of penetration in deep drilling by employing ultra -highpressure water jets to create peripheral groove at the bottom of the hole, which could release in-situ stresses and consequently weaken the hard-to-break zone at the periphery of the bottom hole. By lab test and numerical simulation, main factors are analyzed to investigate the effects of slotted granite and slotting characteristics on failure mode, backward impacting energy, and impact force under the condition of bottom hole stress. The results reveal that the presence of grooves prevents the energy needed to break rocks from being transferred to the far field area. Instead, the stress wave reflects within the rock between the grooves, leading to tensile failure and a significant increase in the number and area of cracks inside the rock. Additionally, the specific energy exhibits greater sensitivity and a 64% reduction as the groove depth increases from 5 to 15 mm. Moreover, the backward impacting energy decreases gradually with the increase of groove depth, and when the groove depth reaches 15 mm, almost no backward impact energy is generated, and the impact force decreases by 51%. Furthermore, Groove depth has the dominant impact on rock failure, not only increases the internal crack density inside the rock, but also the occurrence of blocky rock masses facilitates more easily. the deeper the groove, the more significantly the blocky rock masses. The research could provide an effective means of reducing backward impact energy and increase the ROP for deep drilling, while also expanding the understanding of the impact failure mechanism of rocks.