Gas exposure trials of the Hall Formation, Lehen Field, Austria: Laboratory simulations of gas-caprock-brine interactions in an underground hydrogen storage system

In underground hydrogen storage (UHS) systems a reliable caprock is required to contain the stored hydrogen and prevent it from migrating out of the reservoir formation. Potential chemical reactions, which may occur upon the introduction of hydrogen into the subsurface, may change the sealing capacity of the caprock. To assess the risk of gas-caprock-brine interactions, gas exposure experiments using caprock and brine taken from an UHS site (Lehen Field, Austria) were undertaken at 50 °C and 150 bar pressure for treatment intervals of 3 months, 6 months and 12 months. Three different gas treatments were used: argon, as a non-reactive, experimental control, hydrogen and a gas mix of methane, hydrogen and carbon dioxide. Post-treatment scanning electron microscope imaging shows no significant dissolution features in the samples and the hydrochemistry before and after treatment do not indicate chemical changes attributable to reactions with hydrogen. The variations measured in porosity and mineralogy are interpreted to be due to geologic variation rather than resulting from chemical reactions in the experiments. These results confirm that large scale dissolution should not be expected in the caprock under the conditions of this UHS setting.