To enable the energy transition, renewable energy sources such as wind and solar are playing an increasingly central role. However, their weather-dependent nature makes balancing energy supply and demand challenging. While short-term fluctuations can often be managed with batteries or other local storage solutions, many countries, such as the Netherlands, are expected to face large seasonal energy imbalances of several tens of terawatt-hours (TWh), far beyond the capabilities of conventional technologies such as batteries.

Underground Hydrogen Storage (UHS) has therefore emerged as a promising solution. Storing hydrogen in geological formations such as depleted gas reservoirs or aquifers at depths of several kilometers can provide storage capacities on the order of several terawatthours. These reservoirs are porous rocks of solid grains and interconnected voids, where hydrogen is stored in the pore space. This introduces complex flow dynamics and technical challenges across multiple scales: from the molecular interactions of hydrogen with other fluids present in the reservoir, to the flow through micrometer-scale pore channels, up to reservoir-scale processes spanning several kilometers.

This thesis advances the understanding of UHS by addressing knowledge gaps across multiple scales, combining molecular simulations, microfluidic experiments, pore-scale modeling, and the development of a site selection framework. At the molecular scale, missing datasets of thermophysical properties of hydrogen–brine systems were obtained by using molecular simulations. These properties, including densities, viscosities, interfacial tensions, solubilities, and diffusivities, are essential for accurate large-scale reservoir simulations.

At the pore scale, microfluidic experiments were conducted to measure dynamic contact angles in glass micromodels that mimic the micrometer-scale channels of porous rocks. These measurements reveal how wettability depends on pore geometry and governs flow dynamics. Complementary pore-scale simulations using the lattice Boltzmann method were performed to explore mechanisms of hydrogen trapping and bypassing under varying flow rates, pore shapes, and capillary conditions. Together, these results improve our understanding of multiphase flow processes critical for predicting and maximizing hydrogen flow behaviour and recovery efficiency.

At the reservoir scale, a systematic site selection framework was developed for identifying suitable depleted gas fields for UHS. This framework integrates multidisciplinary criteria including reservoir performance, geomechanical stability, bio-geochemistry, and techno-economic feasibility providing a practically accessible and reliable method for screening and ranking potential storage sites.

By linking insights across scales and disciplines, this thesis strengthens the scientific foundation for the safe, efficient, and reliable deployment of UHS. The findings contribute critical data, improve predictions, and support decision-making toward enabling large-scale UHS.

Underground hydrogen storage in porous media is promising for large-scale energy storage. However, its technical and financial effectiveness is heavily dependent on a reliable site selection strategy. In this review, we critically assess the available literature across disciplines to identify the most influential criteria for reliable site selection. Drawing from this evaluation, we propose a systematic, multidisciplinary framework for early stage reservoir screening, integrating key criteria from reservoir performance, geomechanics and containment, location and techno-economics, and biogeochemistry. Our framework allows for rapid identification and ranking of the most suitable reservoirs by proposing 11 elimination criteria and 15 screening criteria. The presented framework consists of practically applicable and scientifically grounded criteria to support consistent, early stage decisions based on readily available data while allowing for detailed site-specific analysis in later project development phases. By unifying diverse disciplinary insights into a structured methodology, this study contributes to more informed, inclusive, and effective site selection.

Large-scale geological storages of hydrogen (H₂) and carbon dioxide (CO₂) in saline aquifers present feasible options for a sustainable energy future. We compared the plume migration of CO₂ and H₂ in aquifers using the FluidFlower benchmark, incorporating the state-of-the-art thermophysical and petrophysical properties. The H₂ plume, with its higher buoyancy and mobility compared to CO₂, remains predominantly in the gas phase due to its lower solubility, increasing the chances of escaping through fractures or migration to distant regions. This additionally leads to a higher pressurized reservoir, which, along with higher buoyancy, increases the chance of caprock penetration. Dissolution trapping of CO₂ into brine increases over time due to its fingering, while H₂ does not show fingering. Our findings show that while geological carbon storage (GCS) benefits significantly from all structural, dissolution, and residual trapping, underground hydrogen storage (UHS) relies mainly on structural trapping, making the integrity of sealing elements of the system a key factor in its performance.

Residual trapping is a critical mechanism influencing the efficiency of Underground Hydrogen Storage (UHS). This study investigates the underlying processes of residual trapping by bypassing, through bifurcating geometries, focusing on how geometrical parameters and flow characteristics affect the trapping process. We develop a dynamic simulation framework based on the lattice Boltzmann method (LBM) to simulate full drainage/imbibition cycles. Various geometries, based on the pore doublet model, were investigated and supported by theoretical analysis. In addition, trapping behavior of hydrogen was compared to that of CO₂ and CH₄. It is found that the channel width ratio, specially across the local bifurcating geometries, and the roundness of the grains, are among the key factors which control hydrogen trapping. Results indicate that the suited reservoirs for underground hydrogen storage have narrower channel-size ratios and smoother edges at micro-scale. Operational conditions also play a significant role. Lower flow rates enhance bypassing, which increases trapping.

Data for several key thermodynamic and transport properties needed for technologies using hydrogen (H₂), such as underground H₂ storage and H₂O electrolysis are scarce or completely missing. Force field-based Molecular Dynamics (MD) and Continuous Fractional Component Monte Carlo (CFCMC) simulations are carried out in this work to cover this gap. Extensive new data sets are provided for (a) interfacial tensions of H₂ gas in contact with aqueous NaCl solutions for temperatures of (298 to 523) K, pressures of (1 to 600) bar, and molalities of (0 to 6) mol NaCl/kg H₂O, (b) self-diffusivities of infinitely diluted H₂ in aqueous NaCl solutions for temperatures of (298 to 723) K, pressures of (1 to 1000) bar, and molalities of (0 to 6) mol NaCl/kg H₂O, and (c) solubilities of H₂ in aqueous NaCl solutions for temperatures of (298 to 363) K, pressures of (1 to 1000) bar, and molalities of (0 to 6) mol NaCl/kg H₂O. The force fields used are the TIP4P/2005 for H₂O, the Madrid-2019 and the Madrid-Transport for NaCl, and the Vrabec and Marx for H₂. Excellent agreement between the simulation results and available experimental data is found with average deviations lower than 10%.

Underground Hydrogen Storage (UHS) is an attractive technology for large-scale (TWh) renewable energy storage. To ensure the safety and efficiency of the UHS, it is crucial to quantify the H₂ interactions with the reservoir fluids and rocks across scales, including the micro scale. This paper reports the experimental measurements of advancing and receding contact angles for different channel widths for a H₂/water system at P = 10 bar and T = 20 °C using a microfluidic chip. To analyse the characteristics of the H₂ flow in straight pore throats, the network is designed such that it holds several straight channels. More specifically, the width of the microchannels range between 50 μm and 130 μm. For the drainage experiments, H₂ is injected into a fully water saturated system, while for the imbibition tests, water is injected into a fully H₂-saturated system. For both scenarios, high-resolution images are captured starting the introduction of the new phase into the system, allowing for fully-dynamic transport analyses. For better insights, N₂/water and CO₂/water flows were also analysed and compared with H₂/water. Results indicate strong water-wet conditions with H₂/water advancing and receding contact angles of, respectively, 13°–39°, and 6°–23°. It was found that the contact angles decrease with increasing channel widths. The receding contact angle measured in the 50 μm channel agrees well with the results presented in the literature by conducting a core-flood test for a sandstone rock. Furthermore, the N₂/water and CO₂/water systems showed similar characteristics as the H₂/water system. In addition to the important characterization of the dynamic wettability, the results are also crucially important for accurate construction of pore-scale simulators.