Experimental Study on the Hydromechanical Behaviours of Porous Sandstone under Cyclic Proportional Triaxial Loading: Implications for Cyclic Underground Hydrogen Storage

To better understand how cyclic effective stress variations affect the mechanical and transport properties of reservoir rocks during underground hydrogen storage (UHS), this study conducted proportional loading experiments on three porous sandstones varying in porosity and permeability. Proportional loading experiments were conducted under different stress paths, simulating variations in effective stress caused by cyclic UHS. Mechanical properties, porosity, and permeability were evaluated based on the mean effective stress and the number of loading cycles. Scanning electron microscopy (SEM) analysis provided insights into the microscopic deformation processes responsible for observed macroscopic behaviours. Results indicate distinct deformation mechanisms influenced by stress paths. Under cyclic triaxial loading conditions (K = 0), high-porosity sandstones initially exhibit compaction but transition to dilatancy-dominated deformation, characterised by microcrack development and grain rearrangement. This dilatancy behaviour paradoxically results in a negative correlation between porosity and permeability. This phenomenon arises because fragmented grains obstruct pore throats, modifying the pore structure and causing localised variations in porosity distribution, which in turn adversely impacts permeability. Conversely, the low-porosity sandstone consistently exhibits compaction-driven deformation, with porosity loss closely correlating with permeability reduction. Under cyclic proportional loading conditions (K > 0), all sandstones exhibit predominant compaction, particularly under repeated cyclic loading. The mechanical and transport properties initially evolve mainly with increasing mean effective stress irrespective of stress paths. However, during cyclic loading, both bulk and pore compressibilities significantly depend on the applied stress paths, becoming notably larger at higher stress path values. Consequently, greater accumulations of inelastic strain and subsequent porosity and permeability loss occur under elevated stress path conditions. SEM observations revealed that these inelastic strains predominantly originate from grain fracturing, contact wear, and compaction or consolidation of clay-rich grain boundaries under cyclic loading. Furthermore, permeability evolution across all samples follows an exponential decay trend, emphasising the cumulative impact of cyclic loading-induced microstructural changes. These findings elucidate critical process-driven mechanisms governing mechanical and transport property evolution in reservoir rocks under cyclic stress conditions, thereby informing the design and operational safety assessments of underground hydrogen storage facilities.