Sojwal Manoorkar , Gulce Kalyoncu Pakkaner, Hamdi Omar, Soetkin Barbaix, Dominique Ceursters, Maxime Latinis, Stefanie Van Offenwert, Tom Bultreys 

Repurposing natural gas (methane) storage facilities to hydrogen storage leverages existing infrastructure to address seasonal energy demand-supply fluctuations. However, the differences in the injection-withdrawal cycle between hydrogen and methane-brine systems remain poorly understood. Therefore, we investigate the pore-scale two-phase flow dynamics of hydrogen (H$_2$), methane (CH$_4$), and their mixtures (50\% H$_2$-50\% CH$_4$, 95\% H$_2$-5\% CH$_4$) in fractured limestone from the Loenhout natural gas storage aquifer in Belgium. The controlled single-cycle primary drainage and imbibition experiments were conducted at 10 MPa and 65$^{\circ}$C, typical reservoir conditions for three different rock samples. Our results show that H$_2$, CH$_4$, and their mixtures exhibit similar average gas saturations after drainage, though invasion patterns depend on fracture geometry. Rougher fractures exhibit discontinuous invasion, dominated by roof snap-off events, with H$_2$ forming a greater number of smaller ganglia compared to CH$_4$, likely due to hydrogen’s lower viscosity. Fractured rock with a wider aperture achieves higher gas saturation during drainage but shows lower recovery efficiency due to increased residual trapping. During imbibition, gas type plays a more significant role, unlike drainage. In a pore-scale sample (D = \SI{6}{\milli\meter}), hydrogen achieves 100\% recovery in smoother, narrower fractures, whereas methane and its mixtures experience notable trapping. In rougher fractures, both gases are retained, but H$_2$ forms fewer, more connected clusters, while CH$_4$ forms smaller, fragmented ganglia, influencing storage capacity, leakage risk, and recovery efficiency. These findings highlight uncertainties in current methodologies for cyclical operations in underground hydrogen storage and emphasize the need for improved predictive models.