Rock Brittle–Ductile Transition Under Coupled Confining Pressure and Strain Rate Effects: A Model Based on Peak and Residual Strengths

The interactive influence of disturbances and ground stress cause the failure behavior of deep rocks to exhibit a dynamic adjustment of brittleness and ductility. Excessive brittle behavior results in wellbore instability and damage to the surrounding formations in petroleum engineering. Peak and residual strength criteria applicable to such environments are proposed to establish a theoretical foundation for drilling safety assessment. The strength criterion is according to the CS (CowperSymonds) and the slip-crack model. A brittleductile transition (BDT) index IB is established from the relation between residual and peak strengths. A large amount of experimental data within the ranges of 10 -5 –10 4 s -1 strain rate (SR) and 0–230 MPa confining pressure (CP) has verified the reliability of this strength criterion. The correlation coefficients are close to 0.99 for peak and residual strength fittings. The errors of the BDT index are within 10% for all rock types except coal. The results indicated that rock strength increases with both CP and SR. However, the enhancing influence of SR and CP exhibit mutual suppression under coupled conditions. This phenomenon ultimately manifests as a variation in rock brittleness and ductility. Higher SRs correspond to larger values of m′ and n whereas λ′ decreases. The rates of change of the three parameters under high SR conditions are much greater than those at other SRs. The variation trends of these parameters provide a macroscopic physical explanation for the BDT. This theoretical framework has significant value for safety and risk assessment in drilling engineering under complex geological conditions.