Fracturing of Porous Sandstone by Decreasing Effective Pressure: The Roles of Stress Path and Rate Dependence

When a porous rock is subjected to compressive stress, rock failure may occur due to either an increase in pore pressure or a decrease in confining pressure. The stress path and the rate of effective pressure change can influence the initiation and propagation of fractures within brittle materials. Understanding these mechanisms is key for applications in underground engineering, as well as in geo-energy exploration and storage. We conducted triaxial compression tests on Bentheim sandstone samples under different stress paths and effective pressure change rates. First, intact cylindrical samples were loaded axially at a constant confining pressure of 35 MPa and a pore pressure of 5 MPa, up to about 85% of the peak strength. We then fixed the axial piston and either increased the pore pressure or decreased the confining pressure at two different rates (0.5 MPa/min or 2 MPa/min), leading to final macroscopic failure. Comparison of located Acoustic Emission (AE) events with post-failure microstructures of deformed samples shows a good agreement, indicating a location accuracy of AE events of about 2 mm. The AE source types, determined by P-wave first-motion polarities, indicate that shear failure mechanisms are dominant as the rock approaches failure at the expense of tensile cracking and compaction events. Approaching failure, we observe a significant decrease in Gutenberg-Richter b-values in all tests. Our results show that samples subjected to faster rates of decreasing effective confining pressure experience larger stress drops, higher slip rates, greater total breakdown work, higher rates of AE before failure, and faster post-failure AE decay rates. In contrast, the applied stress path did not significantly affect the deformation microstructures and rock failure characteristics.