Impact of Large-Scale Porosity Variability on Hydrogen Transport in Porous Reservoirs

The representation of reservoir heterogeneity is a key source of uncertainty in the numerical modelling of underground hydrogen storage. Most studies assume either spatially uniform porosity or small-scale random variability, whereas large-scale porosity trends commonly encountered in storage formations are rarely assessed separately. This study investigates this form of variability through a controlled numerical framework in which porosity is higher near the injection well and decreases with radial distance. A compositional transport model implemented in the X2H module is used to define four simulation cases that distinguish the direct effect of porosity variability from its indirect effects through intrinsic permeability and air-entry pressure. The results show that the influence of large-scale porosity variability is not negligible and is controlled primarily by the porosity–permeability relationship. Higher porosity and permeability near the well initially accelerate plume migration, whereas the subsequent advance into regions where porosity and permeability are lower than in the homogeneous reference configuration progressively reduces this initial difference. Spatial porosity variability alone has a limited effect, and the contribution associated with porosity-dependent air-entry pressure is minor within the range examined. If the porosity–permeability relationship is neglected, numerical simulations may therefore misrepresent hydrogen migration and the resulting gas-saturation distribution. The study provides a controlled reference configuration for evaluating the consequences of porosity-related modelling assumptions and highlights the importance of representing large-scale petrophysical heterogeneity consistently in underground hydrogen storage models.