Underground hydrogen storage in reservoirs: pore-scale mechanisms and optimization of storage capacity and efficiency

Underground Hydrogen Storage in Reservoirs (UHSR) is promising for long-term and large-scale renewable energy storage, yet improvements in capacity and efficiency are still highly demanded, which requires a deep understanding of the pore-scale mechanisms. Here, we investigate UHSR using micromodels and discover three pore-scale mechanisms, namely the preferential-to-uniform flow transformation, floating flow, and dead-end pore invasion. Preferential flows ensure the base storage capacity and the early transformation to uniform flows promotes the storage efficiency. Floating flow not only enhances the flow transformation but also increases the storage capacity via its facilitation for dead-end pore invasion. We also provide pore-scale mechanism-based elucidation for the effects of pore heterogeneity, injection flux, and oil/brine distribution on storage capacity and efficiency. Injection flux affects the preferential and floating flows to regulate the rate of base storage and determine whether controlling the flow profile or breaking through the outlet via inertia induced preferential flow. Pore heterogeneity affects the proportion of dynamic dead-end pores and possibility of breaking through via capillary induced preferential flow. In comparison with brine saturated condition, the oil saturated condition is unfavorable of floating flow and dead-end invasion. We propose that brine saturated initial condition with high injection flux and median pore heterogeneity are optimal for both UHSR capacity and efficiency from our micromodel study. We further optimize the capacity from 50 % to 95 % and the efficiency from 7.4 x 10(-2) kg/ (m(3)center dot s) to 2.1 x 10(-1) kg/(m(3)center dot s) according to revealed mechanisms and influencing factors. From the microscale perspective, this work brings critical insights for enhancing and broadening the application of UHSR engineering practice.