Pore-scale insights on mixed-wettability and its impact on underground hydrogen storage in aquifers

The efficient storage and extraction of hydrogen from underground porous formations are essential for advancing a sustainable energy transition. However, the pore-scale effects of wettability distribution on hydrogen displacement, trapping, and recovery remain insufficiently characterized. Spatial distribution of wettability (contact angle)—not just the average value—strongly influences fluid configurations and flow pathways, and neglecting these differences can lead to unrealistic storage predictions. This study employs direct numerical simulations to systematically examine how uniform, random, and correlated wettability distributions affect compressible flow of hydrogen in porous media. Results show that decreasing wettability toward a less water-wet state increases hydrogen saturation during drainage across nearly all capillary numbers, enhancing pore space utilization. Moreover, it is found that drainage is primarily governed by invasion percolation, while imbibition is dominated by I₁ and I₂ mechanisms. In strongly water-wet systems, optimal recovery occurs at low capillary numbers (on order of 10⁻⁷) in drainage, and high capillary numbers (on order of 10⁻⁵) in imbibition. Weakly water-wet systems require higher capillary numbers (on order of 10⁻⁶ in drainage, and 10⁻⁴ in imbibition) to suppress capillary fingering. Random wettability distributions reduce hydrogen recovery compared to uniform systems, highlighting the negative impact of wettability heterogeneity. In correlated wettability models, where wettability is linked to pore size, hydrogen trapping significantly increases during imbibition despite similar saturation levels in drainage. These findings enhance our understanding of how mixed wettability affects hydrogen storage and recovery in aquifers, revealing novel insights for identifying suitable reservoirs through wettability classification and predicting flow dynamics.