Porosity-Permeability Evolution of Carbonate Rocks under Cyclic Hydrostatic Loading: Creep-Fatigue Interaction and Implications for Hydrogen Storage

Underground hydrogen storage in porous formations (UHSP) provides a large-scale and long-term solution to balance the seasonal supply-demand mismatch of renewable energy. Assessing the long-term safety and performance of UHSP requires understanding how reservoir rocks respond to cyclic pressure variations. In this study, we conduct hydrostatic loading tests on Saint-Maximin limestone (SML) samples under various loading paths, complemented by measurements of permeability and acoustic wave velocities. The resulting evolutions of porosity, permeability and the damage are systematically investigated. Specific loading paths are applied to distinguish cycle-dependent ratcheting deformation from time-dependent creep. Experimental results reveal that for cyclic loading, mechanical behavior of SML is governed by a dual stress-threshold mechanism. The peak stress ratio (PR) governs initiation and evolution of damage, as supported by ultrasonic velocity measurements and a post-loading microstructural observation, and leads to enhanced creep and onset of ratcheting deformation when PR > 1. The valley stress ratio (VR) describes the reopening of existing fractures during unloading and induces an amplified ratcheting rate when VR < 1. Across all loading paths, porosity change serves as a robust metric for mechanical hardening and permeability evolution. These findings offer new insights into the coupled hydro-mechanical processes governing reservoir rocks under long-term cyclic operation and contribute to more reliable predictions of UHSP performance.