Comparative Geomechanical and Flow Behaviour of Bentheimer and Corsehill Sandstones Under Underground Hydrogen Storage (UHS) Conditions

UHS represents viable option for enabling large-scale and long-term energy storage in Net Zero energy systems. However, UHS involves several uncertainties, particularly from a geomechanical perspective. To ensure safe operation in porous reservoirs, storage integrity must be maintained under cyclic injection and production. This study investigates the effects of hydrogen exposure and pore pressure cycling on the geomechanical and flow properties of two sandstones: Bentheimer sandstone, which is 99% quartz, and Corsehill sandstone, which is clay rich. Core plugs were exposed to hydrogen at 70 °C and 18 MPa for 50 days, while nitrogen-exposed and unexposed samples were used as controls to isolate hydrogen-specific effects. Triaxial and flow tests were conducted before and after each pore pressure cycle under in-situ stresses and temperatures representative of North Sea reservoir conditions.

Results show a reduction in stiffness, measured as Young’s modulus, of about 6% in Corsehill sandstone after five pore pressure cycles, whereas Bentheimer sandstone showed no significant change. The reduction in Young’s modulus was slightly higher in hydrogen exposed Corsehill samples compared to nitrogen exposed and unexposed control samples. This difference may reflect sample variability, as Corsehill sandstone exhibits a degree of heterogeneity. The reduction in Young’s modulus of Corsehill sandstone may result from several factors, including fracture development, grain grinding, and mineral dissolution facilitated by deionized water. Clay-focused XRD analysis confirmed the absence of swelling clay minerals in Corsehill sandstone. Batch geochemical and core flood experiments revealed mineral dissolution, which likely contributed to the observed mechanical degradation. The finer and more angular grains of Corsehill sandstone increase the reactive surface area in contact with deionized water, enhancing dissolution under cyclic effective stress. Flow tests showed negligible changes in permeability for both sandstones after pore pressure cycling, indicating that the observed mechanical changes did not significantly affect flow properties. This implies that permeability trends alone are not sufficient to assess integrity during cyclic operation, and that rock-specific mechanical criteria are required.