Evaluating capillary trapping in underground hydrogen storage: A pore-scale to reservoir-scale analysis

This study aims to evaluate the impact of microscopic pore structures and networks on capillary trapping in underground hydrogen storage (UHS). UHS in subsurface porous media has gained significant interest as a largescale energy storage system to support the goal of net-zero emissions. However, selecting suitable geological sites for UHS requires a thorough understanding of the interactions among reservoir rock, in-situ fluid, and injected hydrogen. As UHS involves intermittent injection and production of the injected gas, the wettability and pore network of the reservoir rock plays a crucial role in determining the fraction of unrecoverable gas due to capillary trapping. Laboratory-based core flooding tests are required to quantify the fraction of unrecoverable gas due to capillary trapping, but as core flooding analyses are complex, costly, and time-consuming, only limited analysis has been done in the literature. Hence, this study presents a novel systematic workflow that evaluates the impact of microscopic pore structures and networks on capillary trapping using several rock samples with different pore properties, which is then extended to the reservoir-scale observation. The trapped fraction of the non-wetting phase is calculated from a pore-network model and then analyzed using Land's model. The Land constant showed a wide variation in the model constant, ranging from 0.2 to 2.7, depending on the pore properties of the rock sample. Compared to most of the sandstones, we observed that the Land's trapping coefficients for carbonate rock samples are higher, meaning less hydrogen will be trapped in carbonate reservoirs by capillary trapping. Moreover, we also investigated the correlation between the petrophysical properties like porosity and permeability of the rock and its impact on capillary trapping. Finally, field scale numeric reservoir simulation was conducted using an ensemble of heterogeneous models using the derived Land constants to quantify the fraction of injected gas trapped in various rock types. The results presented in this study provide a better understanding of hydrogen loss due to capillary trapping by connecting the pore-scale properties with reservoirscale behavior, which is critical for safe and economical UHS site selection.