Ductile-brittle failure transition of rocks in process of drilling with high confining pressure: Model and experiment

When drilling with high ground stress in petroleum drilling engineering, understanding the ductile-brittle failure transition of rocks is crucial for improving drilling efficiency. Herein, a criterion is established to determine the depth of cut for the ductile-brittle failure transition of rocks when drilling with high confining pressure. Digital drilling experiments are carried out on six kinds of rock under different confining pressures. The effect of confining pressure on the ductile-brittle failure transition is studied. The research results show that ductile failure of rock occurs when the depth of cut is shallow, and the mechanical specific energy (MSE) decreases slightly with the increase of depth of cut. As the depth of cut increases beyond the critical depth for ductile- -brittle failure transition, the rock-failure mode changes from ductile to brittle failure. The decrease of MSE be-comes larger as the depth of cut increases. Cutting force, thrust force, cutting point and MSE are linearly positively correlated with confining pressure; critical depth of cut is linearly negatively correlated with confining pressure. The work in this paper provides a fast and simple method for predicting the critical depth of cut for rock ductile-brittle failure transition under high confining pressure.