Digital drilling response and mechanical parameter prediction of rocks under confining pressure conditions

To investigate the digital drilling response of rocks under confining pressure and establish a method for predicting the mechanical parameters, laboratory digital drilling tests were conducted on red sandstone, sandstone, limestone, slate, granite, and diorite under different confining pressures. Based on the axial penetration and rotary cutting processes of the drill bit, a bit-rock interaction model was established by considering confining pressure, drill-bit geometric parameters, and the stress state of the fracture surface. The relationship between drilling parameters and rock mechanical parameters was derived, and a rock cutting strength ratio was proposed to characterize the correlation between drilling responses and mechanical parameters. The results show that the cutting force of the six rock types increases with increasing thrust force, indicating a clear positive correlation between them. Confining pressure increases both thrust force and cutting force. The internal friction angles predicted from digital drilling parameters agree well with those measured by triaxial compression tests, demonstrating high prediction accuracy. The confining pressures inverted from drilling parameters are generally consistent with the preset confining pressures. The rock cutting strength ratio exhibits strong correlations with the internal friction angle, cohesion, and rock strength; specifically, it shows a functional relationship with the internal friction angle and linear relationships with cohesion and strength. These findings provide a reference for the rapid identification of rock mechanical properties and in-situ stress environments using digital drilling technology.