Acoustic emission insights into rock salt damage under humidity cycling

Salt caverns used as compressed air energy storage (CAES) reservoirs undergo humidity cycling during operation, introducing moisture in the surrounding rock salt's cracks and influencing its mechanical properties. This study quantitatively examines the evolution of mesoscale (2-50 nm) structures and damage mechanisms associated with this effect. Acoustic emission (AE) was monitored during uniaxial compression tests on pre-damaged rock salt specimens subjected to humidity cycling. Real-time AE characteristics, such as count density, frequency, and amplitude, were utilized to identify sound sources within the rock, facilitating the analysis of the distribution and evolution of the associated structural damage throughout the macroscopic compression. The findings indicate that humidity cycling weakens the Felicity effect in damaged rock salt, with AE event counts decreasing by 59.2 % while the outbreak of AE was significantly delayed. AE signal frequency and amplitude progressively decreased, indicating a transition from high-energy transgranular cracking to lower-intensity damage such as intergranular cracking and void closure. RA-AF analysis revealed an increase in tensile crack proportion from 44.39 % to 59.11 %. Combined with mesoscale observation, self-healing effects in damaged rock salt were confirmed, and the relationship between the mesoscale damage mechanisms and macroscopic mechanical responses of rock salt under humidity cycling was elucidated. Recrystallized salt structures bridged the cracks, mitigating stress concentration at crack tips and suppressing secondary crack propagation. These structures facilitated releasing strain energy in small increments during the later loading stage, delaying high-energy fracture events. Additionally, brine inclusion within cracks reduced intergranular damage and enhanced rock deformation. Consequently, humidity cycling improves the compressive strength and ductility of damaged rock salt. These insights facilitate predictions of the mechanical properties of rocks surrounding CAES salt caverns, enabling a more accurate assessment of reservoirs' geological safety under long-term operation.