Application of the drilling mechanical properties of rock to determine confining pressure in shallow geological formations

Abstract

Confining pressure is an essential factor to consider in geological disasters in geothermal reservoirs and deep geological formations. Assessing the confining pressure holds practical importance, particularly in mitigating high-risk disasters associated with resource production. This study focuses on the development of a theoretical model for the mechanical properties of drilled rocks, specifically considering the impact of elevated confining pressure during the drilling process. Digital drilling experiments with different confining pressures are carried out to investigate the effect of confining pressure on the drilling mechanical properties of rock. A new method for identifying confining pressure and strength parameters using digital drilling is proposed. A comparison between the predicted confining pressure and the measured confining pressure is conducted to verify the accuracy of the model. The results show that the cutting force and the normal thrust at the cutting point increase with an increase in confining pressure. The drilling parameters (the bit rake angle a, the contact friction angle θ, the friction angle of rock φ, the cohesion of rock c, the effective stress generated by the drill torque St, the effective stress generated by the thrust along the normal axis Sn.) also increase gradually as confining pressure increases. Digital drilling technology offers shorter testing cycles and lower costs in comparison to traditional confining pressure testing techniques such as stress relief and hydraulic fracturing. The predicted confining pressure is consistent with the actual value. In summary, the method of determining confining pressure based on digital drilling has practical engineering significance.