A comparison of porous sandstone fracturing induced by increasing fluid pressure or decreasing confining pressure: Stress-path and rate-dependence effects

When a porous rock is subjected to overall compressive loading, either increasing pore pressure or decreasing confining pressure could result in rock failure. The stress path and the applied pressure change rate may affect the initiation and propagation of fractures within brittle materials. Understanding the physical mechanisms leading to failure is crucial for underground engineering applications and geo-energy exploration and storage. We conducted triaxial compression experiments on porous Bentheim sandstone samples at different stress paths and pressure change rates. First, at a constant confining pressure of 35 MPa and pore pressure of 5 MPa, intact cylindrical samples were axially loaded up to about 85% of the peak strength. Subsequently, the axial piston position was fixed, and then either the pore pressure was increased or the confining pressure was decreased at two different rates (0.5 MPa/min or 2 MPa/min), leading to final catastrophic failure. The mechanical results revealed that samples subjected to higher rates of decreasing effective confining pressure exhibited larger stress drop rates, higher slip rates, higher total breakdown work, higher rates of acoustic emissions (AEs) before failure, and higher post-failure AE decay rates. In contrast, the applied stress path did not significantly affect rock failure characteristics. Comparison of located AE events with post-mortem microstructures of deformed samples shows a good agreement. The AE source type determined from the P-wave first-motion polarity shows that shear failure dominated the fracture process when approaching failure. Gutenberg-Richter b -values revealed a significant decrease before failure in all tests. Our results indicate that, in contrast to the stress path, the rate of effective stress change strongly affects fracturing behavior and AE rate changes.