Speaker
Mr
Waleed Dokhon
(Imperial College of London)
Description
We investigate how pressure decline interacts with displacement at the pore scale in a water-wet Bentheimer sandstone at 4 MPa and 23 °C, representing underground hydrogen storage in saline aquifers. Brine was injected at 0.01 and 0.05 ml/min while a programmed outlet pressure decline rate of 1 kPa/min was applied. Two initial states were tested: high hydrogen gas saturation (Sg = Sgi), representative of regions above the gas-water contact (GWC), and residual gas saturation (Sg = Sgr), representative of conditions below the GWC. We used micro-CT imaging at 9.6 µm/voxel to analyse the gas distribution and connectivity at different pressure drops, and to determine the pore scale displacement type when pressure decline is combined with a constant brine influx.
The results show that capillary pressure increased during withdrawal, leading to drainage displacement at the pore scale, even though brine was injected. We observed an increase in gas saturation by expansion, where the capillary pressure increased due to the reduction in brine pressure. Large ganglia were connected to the outlet and produced by expansion. When pressure decline began at Sgr, the gas saturation increased approximately in proportion to the pressure drop (e.g., 8% saturation increase for a pressure drop of 7.5% pressure). Starting pressure decline at Sgi resulted in larger residual gas clusters and a higher degree of connectivity. When large gas clusters were connected to the outlet, the expanded volume fraction was notably lower than the fractional pressure drop because part of the gas was produced by expanding towards the outlet. The maximum gas saturation reached was 0.55, and no apparent gas pathway was connected from the inlet to the outlet. No displacement of the gas via imbibition was seen during pressure decline despite the high gas saturation.
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. Gas production occurs primarily through expansion-driven drainage rather than through normal displacement.
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Author
Mr
Waleed Dokhon
(Imperial College of London)
Co-authors
Dr
Branko Bijeljic
(Imperial College)
Prof.
Martin Blunt
(Imperial College London)
