Hydrogen vs Methane: Microscopic Flow Dynamics in Fractured Reservoir Rocks for Energy Storage

Underground hydrogen storage in saline aquifers offers a promising solution to address seasonal fluctuations in renewable energy supply. Repurposing natural gas storage facilities for hydrogen leverages existing infrastructure; however, the distinct flow behaviors of hydrogen-brine and methane-brine systems, particularly in fractured reservoirs and sealing caprocks, remain poorly understood. This study investigates the microscopic two-phase flow dynamics of hydrogen (H₂), methane (CH₄), and their mixtures in fractured karstic limestone from the  Loenhout natural gas storage facility in Belgium. Experiments on primary drainage (gas injection) and imbibition (withdrawal) were conducted under reservoir conditions (10 MPa, 65°C) using three different rock samples to examine the influence of fracture geometry on fluid invasion and recovery efficiency. Our findings reveal that while H₂ and CH₄ reach similar gas saturations after primary drainage, H₂ forms a greater number of smaller ganglia due to its discontinuous invasion in rough fractures. Fracture aperture variability and roughness significantly affect flow dynamics, gas trapping, and recovery. Furthermore, steady-state relative permeability experiments demonstrate that hydrogen’s relative permeability closely matches that of methane but is substantially lower than nitrogen, emphasizing nitrogen’s inadequacy as a proxy for hydrogen in reservoir simulations. These results highlight the importance of precise pore-scale modeling to improve field-scale predictions, ensuring effective and secure hydrogen storage in fractured reservoirs like Loenhout.