Fundamental Physics and Constitutive Modeling for Underground Hydrogen Storage

Part I: Transport Phenomena, Displacement Mechanisms, and Trapping

Abstract

Underground hydrogen storage (UHS) in porous geological formations requires understanding of hydrogen-specific transport physics that differs fundamentally from natural gas and CO$_2$ systems. This review, the first of a two-part series, synthesizes the subsurface physics controlling hydrogen migration, displacement, trapping, and diffusive transport. Hydrogen's extreme mobility contrast with formation brine (mobility ratios $M \sim 20$--100) promotes viscous fingering and reduces sweep efficiency during injection and withdrawal operations. We present fractional flow analysis, dimensionless scaling using capillary, gravity, Bond, and Peclet numbers, and stability criteria for displacement front characterization. Trapping mechanisms---structural, residual, solubility, and mineral---are quantified with hydrogen-specific parameterizations, including Land-type models for residual trapping hysteresis. Diffusive transport is assessed across regimes from continuum Fickian behavior in reservoir rocks to slip and Knudsen diffusion in tight caprocks, with Knudsen number criteria ($\text{Kn} = \lambda/d_p$) for regime selection. Thermal effects, particularly Joule--Thomson cooling during expansion ($\mu_\text{JT} \approx -0.03$ to $+0.05$~K/bar), are reviewed for their impact on near-wellbore processes. This review provides practitioners with the physical framework and parameter ranges needed to select appropriate models for reservoir simulation, with emphasis on the quantitative differences between hydrogen and other stored gases. Part II of this series addresses constitutive modeling, geochemical/microbial interactions, and field-scale considerations.