What is it about?
Hydrogen can be stored underground in rocks that contain water, such as deep saline aquifers. During withdrawal, the pressure in the storage site drops as hydrogen is produced. This study looks at what happens inside the tiny pores of the rock during this process. We used 3D X-ray imaging to see how hydrogen and brine move inside a sandstone sample while pressure was reduced and brine was still flowing. The main finding is that hydrogen was not pushed out by water in the usual way. Instead, trapped hydrogen expanded as pressure declined. Some larger hydrogen clusters connected to the outlet and were produced, but much of the gas remained trapped. This means that hydrogen withdrawal is mainly controlled by gas expansion and pore-scale connectivity, not simply by water pushing gas out of the rock.
Featured Image
Photo by Neeqolah Creative Works on Unsplash
Why is it important?
This is important because underground hydrogen storage will need repeated injection and withdrawal cycles. To design these systems safely and efficiently, we need to understand how much hydrogen can be recovered and how much may remain trapped. The study shows that pressure decline can make trapped hydrogen expand, but this does not always mean it becomes mobile. Even at relatively high gas saturation, hydrogen may remain disconnected inside the rock. This challenges the simple use of a single “critical gas saturation” value, because gas mobility depends on how the pressure decline happens, the initial gas distribution, and the pore-scale structure. In practice, this means hydrogen recovery models should include gas expansion, connectivity, and the timing of pressure decline.
Perspectives
My perspective is that hydrogen withdrawal from porous storage formations should not be interpreted only as a conventional two-phase displacement problem. The pore-scale mechanism is more subtle. When pressure declines, trapped hydrogen can expand locally, reconnect with other clusters, or remain isolated depending on the pore structure and initial gas state. This work shows that the pathway matters. Starting pressure decline from high gas saturation gives different behaviour from starting at residual gas saturation. Therefore, laboratory measurements and field-scale predictions should account for displacement history, not just final saturation. Future work should extend this approach to more complex rocks, different wettability conditions, and relative permeability measurements during pressure decline. This will help improve predictions of hydrogen recovery during real underground storage operations.
Waleed Dokhon
Imperial College London
Read the Original
This page is a summary of: The combined effects of pressure decline and gas withdrawal in underground hydrogen storage: A pore-scale experimental study, ADVANCES IN GEO-ENERGY RESEARCH, March 2026, Tsinghua University Press,
DOI: 10.46690/ager.2026.04.06.
You can read the full text:
Contributors
The following have contributed to this page
