Pressure decline and gas expansion in underground hydrogen storage: A pore-scale percolation study

Using high-resolution micro-CT imaging at 2.98 μm/voxel, we compared the percolation of hydrogen in gas injection with gas expansion for a hydrogen-brine system in Bentheimer sandstone at 1 MPa and 20 °C, representing hydrogen storage in an aquifer. We introduced dimensionless numbers to quantify the contribution of advection and expansion to displacement. We analysed the 3D spatial distribution of gas and its displacement in both cases and demonstrated that in gas injection, hydrogen can only advance from a connected cluster in an invasion-percolation type process, while in gas expansion, hydrogen can access more of the pore space even from disconnected clusters. The average gas saturation in the sample increased from 30% to 50% by gas expansion, and we estimated that 10% of the expanded volume is attributed to hydrogen exsolution from the brine. This work emphasises the importance of studying the combined effects of pressure decline and gas withdrawal in hydrogen storage to assess the influence of gas expansion on remobilising trapped gases. • High-resolution X-ray imaging studied hydrogen injection and expansion in a sandstone. • Disconnected gas clusters can expand and reconnect during pressure decline. • Gas expansion increased the average gas saturation from 30% to 50% as pressure declined. • Hydrogen exsolution from brine increased the expanded gas volume by 10%. • Hydrogen accessed more of the larger pores by expansion than by injection.