Examining Gas Expansion at the Pore-Scale for Underground Hydrogen Storage

Abstract The impact of hydrogen expansion during pressure decline on remobilizing trapped gases was examined within Bentheimer sandstone. The invasion mechanisms in gas expansion and injection were differentiated. Previous studies focused on gas trapping through displacement by water, whereas pressure decline is the primary mechanism for extraction in field operations. A dimensionless ratio was derived to compare advective displacement and expansion to determine the dominant mechanism during gas withdrawal and pressure decline. High-resolution micro-CT images at a 3 μm/voxel were used to analyze dynamic changes in gas distribution and remobilization. A water-wet porous plate was used prevent hydrogen production during displacement. A pressure decline rate of 100 kPa/day from 1 MPa to 0.35 MPa was applied during gas expansion, while a constant differential pressure of 11 kPa was imposed during gas injection. Images were acquired every 24 hours to study dynamic changes in gas distribution and fluid occupancy in pores. Distinct hydrogen behaviors were observed during expansion compared to injection. It was demonstrated that in gas injection, hydrogen advances from a connected cluster in an invasion-percolation type process, whereas in gas expansion, hydrogen accesses more pore space, including from disconnected clusters. The average gas saturation in the sample increased from 0.3 to 0.5 by gas expansion, with an estimated 10% of the expanded volume attributed to hydrogen exsolution from the brine. The spatial distribution of hydrogen during expansion showed a broader lateral spread, while in gas injection, the path was more preferential to one side. Valuable insights into hydrogen behavior under dynamic pressure conditions in remobilizing trapped gases were introduced, potentially linking to higher withdrawal efficiency. This research can be extended to simultaneously include imbibition and pressure decline to assess the relationship between gas withdrawal and gas expansion.