Time-dependent deformation and permeability evolution in porous sandstones: Implications for Underground Hydrogen Storage

During Underground Hydrogen Storage (UHS) operations, reservoir rocks often experience time-dependent deformation under long-term stress, which can alter the microstructure and subsequently affect the stability and hydrogen storage efficiency. Therefore, understanding and predicting these time-dependent deformation of reservoir rocks under in situ conditions and its impact on rock properties are crucial for ensuring the long-term safe operations of UHS. This study investigates the time-dependent mechanical and transport behaviour of three representative porous sandstones—St Bees, Castlegate, and Zigong—through constant stress (creep) and multi-level stress creep experiments. These tests were designed to simulate the in situ conditions (1.3-2.6km depth) of the underground hydrogen storage process at a laboratory scale. In the constant stress experiments, permeability and porosity were measured concurrently to reveal the impact of time-dependent deformation on the transport properties of porous sandstones. In the multi-level stress creep tests, long-term pore pressure cycling was applied to simulate hydrogen injection and withdrawal, and the results were compared with those from experiments under constant pore pressure. This allowed for a systematic assessment of the influence of pore pressure fluctuations on the mechanical response and transport characteristics of the sandstones. The research results indicate that all three sandstones exhibit stable creep behaviour, with the steady-state creep rate increasing as temperature and stress increased. The high-porosity St Bees and Castlegate Sandstones show higher steady-state creep rates under the same conditions compared to the low-porosity Zigong Sandstone. The creep behaviours of the three sandstones under in situ conditions can be well described by Burgers model. The permeability of the three sandstones gradually decreased during the experiments, and this trend become more obvious as the stress and temperature increases. Microstructural analysis reveals that the deformation mechanism of the high-porosity St Bees Sandstone is dominated by dilatancy. Although shear-induced deformation causes the feldspar and quartz clusters to fracture, creating new voids and increasing the overall porosity, the fractured debris from these clusters block the throats, complicating the pore structure and leading to a significant permeability loss. The deformation mechanisms of Castlegate and Zigong Sandstone, on the other hand, are dominated by compaction, with pore compression and microcrack closure being the primary causes of porosity, permeability losses. Pore pressure cycling increases the creep rate of sandstones, accumulating more inelastic strain especially in St Bees Sandstone, but has limited effect on the properties of Castlegate and Zigong Sandstones.