Eike Marie Thaysen , Zaid Jangda , Aliakbar Hassanpouryouzband, Hannah Menke, Kamal Singh, Ian Butler, Niklas Heinemann, Katriona Edlmann 

Hydrogen (H2) can support the transition to net-zero carbon (C) emissions by facilitating increased renewable energy use by acting as an energy store to balance supply and demand. Underground H2 storage in porous media is investigated due to its high capacity and economical price. An important unknown in underground porous media H2 storage is the volume of recoverable H2 which is partly controlled by the H2 wettability. Current H2 contact angle data in sandstone systems span large ranges and fall short of clarifying if H2 wettability changes with pressure.
We computed novel in-situ receding and advancing contact angles for the H2-brine-Clashach sandstone system at pore fluid pressures of 2-7 MPa and for nitrogen (N2)-brine-Clashach sandstone at 5 MPa, based on X-ray microtomography images of gas displacement and trapping in Clashach sandstone. A centrifuge analysis of the capillary pressure (Pc) at varying water saturations was conducted for N2. The H2 Pc curve was derived from the N2 Pc, the N2 wettability measurements, and existing information on the density differential between brine and H2 and N2 and the interfacial tensions of these gases.
The results show no change of the H2-brine-Clashach sandstone contact angles within the examined pressure range, with mean receding (drainage) and advancing (imbibition) contact angles of 61°± 24-26° and 58°± 20-22°, respectively, at all pore fluid pressures, indicating a water-wet rock and implying that based on the wettability alone, no decrease in H2 recovery with increasing pressure (i.e. reservoir depth) is expected. While residual trapping was consistent with trapping in water-wet systems, the observed increase in residual trapping at 7 MPa requires further investigation. Alignment with other wettability studies in sandstone systems indicates that for contact angles around 60-70°, wettability may not always be the main control for the H2 saturation in the pore space but that H2 dissolution and channeling events may significantly affect those parameters. Further, contact angle measurements in artificial systems significantly underestimate in-situ contact angles as provided by this study, highlighting the need for microtomography-based wettability investigations. We found relatively low irreducible water saturations of 12.6-14.0% at H2 Pc of 0.43 MPa, suggesting a favorable H2 relative permeability in Clashach and high H2 storage capacity. Our results provide detailed insights into the controls on H2 displacement and capillary trapping as well as crucial input parameters for the modelling and design of H2 storage operations in porous media.