Laboratory Investigation of Fault Reactivation and Acoustic Emission Patterns in Entrada Sandstone Under Triaxial Compression: Implications for CO2 Injection-Induced Seismicity

ABSTRACT: Understanding fault reactivation mechanisms during subsurface fluid injection is critical for safe geological carbon sequestration operations. This laboratory study investigates the hydro-mechanical-acoustic behavior of pre-existing faults in Entrada sandstone under triaxial compression conditions representative of the San Juan Basin. We conducted experiments on saw cut on an Entrada sandstone specimens with 60° fault orientations under 10 MPa confining pressure, employing synchronized acoustic emission (AE) monitoring to capture micro seismic activity during loading. Results revealed five distinct behavioral regimes: elastic loading, microcrack initiation, pre failure acceleration, fault slip, and post-failure relaxation. Peak differential stresses ranged from 100.9 to 105.5 MPa, with apparent elastic moduli of 8.38–13.55 GPa. Assuming zero cohesion (due to the saw‑cut interface), the calculated apparent friction angles are 51.13°–51.47°, indicating critically stressed fault conditions. High energy AE bursts (amplitude >100) correlated strongly with the transition from stable to unstable slip, with maximum energy release occurring during fault activation at 225–245 seconds. The zero-cohesion, high friction behavior suggests that pre-existing faults in sedimentary basins may be highly susceptible to reactivation under small pore pressure perturbations induced by CO2 injection. These findings provide essential constraints for developing risk-informed injection protocols and establishing safe operational limits for carbon sequestration projects in fault bearing formations.