The combined effects of pressure decline and gas withdrawal in underground hydrogen storage: A pore-scale experimental study

Hydrogen withdrawal from subsurface porous formations is expected to experience fast pressure decline rates, yet the pore-scale effects remain poorly explored. This study examines how pressure decline and simultaneous brine influx affect the withdrawal of stored hydrogen in a water-wet Bentheimer sandstone, representing hydrogen storage in saline aquifers. Brine was injected while the outlet pressure was reduced at a fixed rate. Two initial conditions were tested: A high gas saturation, representative of regions above the gas-water contact, and a residual gas saturation, representative of regions below it. Micro-computed tomography was used to quantify gas distribution, connectivity, and the dominant displacement mechanism during pressure decline with continued brine influx. The observations show that capillary pressure can increase during pressure decline, showing that the main displacement mechanism is drainage, even as brine is flowing. The gas saturation increased through the expansion of trapped gases, and large gas clusters connected to the outlet and were produced by expansion. No imbibition displacement was seen despite the high gas saturation reached by expansion. When pressure decline began from residual conditions, the gas saturation increase was proportional to the magnitude of pressure decline, whereas starting from a high gas saturation led to larger residual clusters and greater connectivity. These observations suggest that under continuous pressure decline, local capillary pressure can increase, preventing imbibition displacement of gas by water. This makes the interpretation of laboratory experiments to find the critical gas saturation challenging, as it depends on the displacement process. Gas production occurs primarily through expansion-driven drainage rather than through normal displacement.