Hydrogen solution exposure at a seasonal timescale does not affect the geomechanical properties of clay-rich sandstones

Underground hydrogen storage (UHS) in underground geological reservoirs is a promising solution for large-scale energy storage. However, several challenges, particularly geomechanical ones, must be resolved before UHS can be widely and safely deployed. The interactions between hydrogen, brine, and reservoir rock, combined with the cyclic stresses resulting from hydrogen injection and withdrawal may affect the mechanical integrity of the reservoir, the caprock, as well as its surrounding formations. This is an experimental investigation into the geomechanical impact of a 6 month exposure of clay-rich sandstone (Yellow Felser) rocks to hydrogen and/or brine. Cm-scale samples were exposed to hydrogen-saturated brine at 150 bar and [Formula: see text] in an autoclave for the period of six months. Afterwards, triaxial cyclic loading experiments were conducted on the samples under confining pressures of 10, 20, and 30 MPa. The results are compared with those from the reference samples, which have been exposed to brine only, for the same time period. Each mechanical test included eight stress cycles in the linear stress regime (below the brittle yield point), followed by loading to failure. The frequency, amplitude, and stress conditions were tailored to each confining pressure. The results showed that six months of hydrogen-saturated brine exposure had no noticeable effect on the failure envelope, elastic properties, inelastic strain, and acoustic properties of the Yellow Felser sandstone compared to exposure to brine alone. Internal friction, P-wave velocity, and Young's modulus each showed a change of around 3%, which is on the same order as the repeatability and therefore indicating minimal geomechanical alteration. Complementary qualitative and quantitative scanning electron microscopy (SEM) analyses revealed negligible microstructural changes. When eight stress cycles were applied within the linear stress regime, the majority of inelastic strain occurred during the first cycle, with no progressive accumulation thereafter. A comparison with samples tested under monotonic loading to failure confirmed that cyclic loading under these conditions does not affect the rock strength of Yellow Felser sandstone. These findings provide new insights into the combined effects of cyclic stress and hydrogen/brine/rock interactions on the geomechanical behavior of clay-rich sandstones under reservoir-relevant pressure and temperature conditions.