Storage assurance for hydrogen

A key unknown with geological storage of hydrogen is the mechanisms by which gas would be retained in the sub-surface. One aspect to this is the behaviour of the geological seals of the storage formation. While there is an extensive understanding of the properties of cap rocks in terms of retaining petroleum fluids, hydrogen is a novel gas in the sub-surface. In general, hydrogen tends to have a high diffusivity and thus can migrate through a range of materials. In the sub-surface this could mean that hydrogen could migrate through the cap rock with the potential to escape the storage formation with unforeseen consequences.  

Another key property determining the retention of gases by cap rocks is the threshold or entry pressure. This is the pressure (caused by the buoyancy of the stored gas) at which the gases commence flowing through the cap rock. The key property that determines the threshold pressure is the gas wettability and for hydrogen with cap rock shales this is unknown.