Mechanical Responses and Permeability Evolution in Porous Sandstones Under Cyclic Loading Conditions: Implications for Subsurface Hydrogen Storage

Abstract In underground hydrogen storage operations, reservoir rocks often experience periodic pore pressure fluctuations due to annual or more frequent gas extraction and injection cycles. These fluctuations subject the reservoir rocks to cyclic effective stress changes, causing their mechanical and transport behaviors to differ from those under static conditions. However, understanding how porous rocks react to cyclic loading conditions is still limited. To bridge previous research gaps, cyclic loading tests were conducted on Castlegate and St Bees Sandstone, with applied stress amplitudes ranging from 70 to 90% of their monotonic peak strength. This experimental approach was designed to replicate the in situ stress conditions experienced by reservoir rocks during gas operations. Concurrently, we utilised the steady-state method to measure permeability changes under cyclic loading. By comparing the micro-CT features of the sandstones before and after cyclic loading tests, we quantitatively analysed the microscopic mechanisms driving these alterations in sandstone samples. Our results show that under cyclic loading conditions, the inelastic axial strain and Young’s Modulus initially increase for both sandstones, with the most significant changes occurring within the 1st cycle, followed by a trend towards stability. Permeability decreases with increasing stress and loading cycles. For the Castlegate Sandstone, elevated confining pressure intensified permeability loss, while in St Bees Sandstone, high confining pressure resulted in less permeability loss compared to low confining pressure, which was related to shear band development. Microstructural analysis showed grain movement, rotation, and rearrangement in Castlegate Sandstone under external forces, leading to pore/throat compression and reduced porosity/permeability. In contrast, St Bees Sandstone microstructure changes under low stress involved grain cracking from shear dilatancy, increasing porosity but blocking throats, complicating pore structure, then reducing permeability. Under high confining pressure, the strength of St Bees Sandstone rose without sufficient differential stress for shear dilatancy. Decreased permeability and pore volume were linked to compaction-dominated deformation.