Microfluidic experiments on hydrogen behavior in heterogeneous rocks during underground hydrogen storage in saline aquifers

Hydrogen (H2) plays a crucial role in the global transition to a low-carbon economy. Underground H2 storage (UHS) provides a practical solution for large-scale storage of H2 generated with renewable energy. Due to the complexity of subsurface environments, understanding the behavior of H2 in porous rocks remains a pressing need. This study investigated the displacement process of H2 and water in heterogeneous or fractured porous media which is a common characteristic in geologic formations. Micromodels were fabricated to reproduce the H2-brine-rock system in saline aquifers for UHS. Three groups of micromodels were fabricated: homogeneous porous media, layered heterogeneous porous media, and fractured porous media. Three cycles of injection and withdrawal were performed for each scenario. The results suggested that for homogeneous porous media with larger pore sizes were beneficial for increasing H2 storage capacity and reducing residual H2. In layered heterogeneous porous media, the heterogeneity structure dominated the displacement process. The H2 accumulated in larger pores and penetrated low-permeability layers through preferential channels. The residual H2 was much larger than homogeneous porous media and can be trapped on both the upstream and downstream sides of the low-permeability layer. In fractured porous media, the fracture with a width larger than the average pore size of the porous medium acted as a high-connectivity channel which changed the flow path of fluids. The H2 storage capacity and residual H2 were also increased. The fracture with a width smaller than the average pore size of the porous medium acted as a barrier in the withdrawal process and trapped up to three times more H2 after the withdrawal process. The findings provided novel insights on the role of layered heterogeneity structure and fractures in the cyclic injection-withdrawal operations and offered guidance in the site selection and operation optimization for UHS in saline aquifers.