Ostwald Ripening in Underground Gas Storage

Abstract Underground gas storage supports the energy transition, enabling long‐term sequestration and seasonal storage. A key process shaping the fate of injected gases is Ostwald ripening—the curvature‐driven mass transfer between trapped ganglia—yet its behavior in confined porous structures remains poorly constrained. We present ultra‐high‐resolution microfluidic experiments that track residually trapped hydrogen for weeks in realistic heterogeneous pore networks. The data show rapid local equilibration among neighboring bubbles, followed by slow global depletion driven by long‐range diffusion. We develop a continuum model that couples pore‐scale capillary pressure–saturation relationship, derived using the pore‐morphology method, with macroscopic diffusion. The model predicts saturation evolution without fitting parameters and collapses results across diverse conditions. Reservoir‐scale estimates indicate that local equilibration far outpaces convective dissolution for and occurs on timescales comparable to seasonal storage. Because minimal redistribution is required to reach local capillary equilibrium, residual trapping remains stable in the absence of sinks. Plain Language Summary Storing gases deep underground is an important strategy for advancing clean energy. Carbon dioxide can be injected and permanently stored to reduce emissions, while hydrogen can be held for later use when renewable energy supply is low. After injection, much of the gas becomes trapped as tiny bubbles within the rock's narrow pore spaces. How these trapped bubbles evolve over months to years influences the safety and effectiveness of underground storage. One important process is Ostwald ripening, in which bubbles grow or shrink as gas moves from regions of high curvature to low curvature. Although well understood in simple laboratory systems, this behavior is far less understood in complex, rock‐like environments. In this study, we created transparent “miniature rocks” using microfluidics to directly observe how hydrogen bubbles change over several weeks. We found that bubbles quickly reach a local balance with their immediate neighbors, and then evolve much more slowly as gas diffuses over longer distances. Building on these insights, we developed a model that accurately predicts how trapped gas changes over time. Our findings show that trapped gas remains stable underground, strengthening confidence in both carbon storage and seasonal hydrogen storage technologies. Key Points Long‐term, ultra‐high‐resolution microfluidic imaging reveals two‐stage Ostwald ripening governed by pore heterogeneity and diffusion A parameter‐free continuum model couples PMM‐derived Pc‐S relations with diffusive mass transfer to predict gas evolution across conditions Local equilibration occurs on timescales comparable to gas storage yet induces negligible volume redistribution and preserves trapping