Water-Weakening Effects on the Failure Behavior of Mudstones

The influence of water on rock deformation and failure behavior is a critical variable to consider when developing unconventional shale reservoirs. To better understand this effect, we conducted a series of triaxial compression tests on four mudstone lithofacies, namely siliceous calcareous mudstones, siliceous mudstones, calcareous mudstones, and carbonate-rich mudstones from the Naparima Hill Formation, under water-saturated conditions at confining pressures up to 130 MPa. Our results showed that the mudstones displayed brittle, brittle–ductile transition, and ductile failure behaviors as the confining pressure increased. Mudstones with low strength, high porosity, and high silica and clay contents exhibited a strong water-weakening effect, leading to a significant reduction in failure strength. Conversely, high strength, low porosity, and carbonate-rich mudstones showed minor water-weakening effects. We also developed a failure behavior model to predict the brittle, brittle–ductile transition, and ductile zones in the mudstones. The model showed that the transition from brittle to ductile behavior occurs over a wide range of confining pressures, and it occurs at a lower pressure threshold in saturated compared to dry conditions. Additionally, the model suggests that the presence of water results in a wider transition zone between brittle and ductile behavior, as well as a larger ductile zone. Overall, our findings suggest that the water-weakening effect can significantly decrease the depth at which the brittle–ductile transition occurs in mudstones, emphasizing the importance of considering water as a critical variable in rock deformation and failure behavior.