First assessment of hydrogen/brine/Saudi basalt wettability: implications for hydrogen geological storage

Introduction: Underground hydrogen (H 2 ) storage is a prominent technique to enable a large-scale H 2 -based economy as part of the global energy mix for net-zero carbon emission. Recently, basalts have gained interest as potential caprocks for subsurface H 2 storage due to their low permeability, vast extension, and potential volumetric capacity induced by structural entrapment of the buoyant H 2 . Wettability represents a fundamental parameter which controls the capillary-entrapment of stored gases in porous media. Methods: The present study evaluates the wettability of basalt/H 2 /brine system of two basalt samples from Harrat Uwayrid, a Cenozoic volcanic field, in Saudi Arabia. The H 2 /basalt contact angle was measured using a relevant reservoir brine (10% NaCl) under storage conditions of 323K temperature and pressure ranges from 3 to 28 MPa using the modified sessile drop method. The surface roughness of the basaltic rocks was determined to ensure accurate results. Results: The investigated Saudi basalt samples are water-wet, thereby they did not achieve a 100% hydrogen wetting phase even at 28 MPa pressure. The measured contact angles slightly decrease as pressure increases, thereby pressure did not significantly influences the height of the H 2 column. Discussion: We interpret this trend to the slight increase in H 2 density with increasing pressure as well as to the olivine-rich mineralogical composition of the Saudi basalt. Thus, from the wettability aspects, Saudi basalt has the potential to store a large volume of H 2 (>1,400 m height) and maintain its excellent storage capacity even in deep, high-pressure regimes. This study demonstrates that the basalt rock texture (pore throat radii) and mineralogy control their capacity for subsurface H 2 storage.