No abstract

During the drilling process, high-strength rock can lead to various issues such as drilling suppression, bit wear, and increased operational costs. To ensure safe and efficient drilling operations, it is crucial to accurately predict the strength parameters of the rock and recommend modifications to operational procedures. This paper proposes a low-cost and fast measurement method for predicting the strength parameters of rock in the field. To evaluate the effectiveness of this method, a drilling process monitoring experiment was conducted on sandstone, limestone, and granite. The experiment studied the effect of confining pressure on the response of cutting with an impregnated diamond bit. By analyzing the relationship between the thrust force, torque force, and penetration depth under different confining pressures, the researchers developed an analytical model for drilling that considers confining pressure, compressed crushed zone, and bit geometry. The results show that the confining pressure has a significant effect on the cutting response. As the confining pressure increases, the thrust force, torque force, and penetration depth at the cutting point also increase. Furthermore, a new measurement method was proposed to determine the strength parameters, such as cohesion, internal friction angle, and unconfined compressive strength. The estimated strength parameters for the three rock types using the drilling method were in good agreement with those of the standard laboratory test, with an error range of 10%. This method of estimating rock strength parameters is a practical tool for engineers. It can continuously and quickly obtain the drilling parameters of in-situ rocks.

No abstract

Anisotropy affects the mechanical behaviours of rock, especially for application in rock engineering. In this study, a digital drilling method is proposed to evaluate the mechanical anisotropy of rock. In consideration with the critical friction, the cutting efficiency and contact stress are determined from the revised drilling model to characterize the drilling process. For six types of rock, a series of drilling tests are conducted on three axial directions using the coring bit. The anisotropy of rock strength is obtained from the point load test to compare with the anisotropy of drilling characteristics. Correspondingly, an anisotropy criterion is established. A critical point is identified in the evolution of contact stress and the plot of drilling parameters, corresponding to the critical friction. Result indicates that the evolution of contact stress with inclination angle suggests the similar elastic and plastic stages (inclination angles of 5 and 12, respectively). The typical evolution is also confirmed by the critical depth of the friction point. Moreover, the cutting efficiency and contact stress at the critical point show the evident anisotropic characteristic. A comparison of A 1 and A 2 is conducted to determine the anisotropy index of drilling characteristics. Contact stress present the anisotropy sequence as shale (22.45) > gneiss (14.21) > schist 302 (10.74) and blue sandstone (10.07) > granite (7.29) > red sandstone (5.09). The consistency examination with strength anisotropy index suggests that the contact stress has a fitting correlation with an accuracy of 91 %. In summary, the digital drilling-based method provides a reliable evaluation for rock anisotropy, showing potential in practical application.