Stochastic Gassmann fluid substitution for rapid assessment of hydrogen and carbon dioxide saturations in clastic storage reservoirs.

Safe subsurface storage of carbon dioxide and hydrogen will likely be essential to decarbonising global energy systems. Such storage requires monitoring capable of detecting dynamic changes and unintended migration pathways; seismic methods are expected to form a key component. We investigate the sensitivity of seismic responses to fluid substitution and saturation changes in storage reservoirs on the UK Continental Shelf.

We undertake fluid substitution modelling where brine is progressively replaced by carbon dioxide or hydrogen over a range of water saturations and reservoir pressures. New elastic properties are calculated and propagated into synthetic seismic responses. Rapid scenario evaluation was undertaken for three proposed reservoirs: the Triassic Bunter Sandstone, the Permian Leman Sandstone, and the Triassic Helsby (Ormskirk) Sandstone formations. A stochastic workflow quantified the influence of geological uncertainty on predicted responses.

Seismic responses are strongly reservoir dependent. At 20% water saturation, acoustic impedance changes range from increases of approximately 10% in the Leman Sandstone to decreases of up to 12% in the Bunter Sandstone, while the Helsby Sandstone exhibits only minor changes (<2%). Hydrogen and carbon dioxide produce similar responses, although magnitudes differ, particularly at higher porosities. Modelled AVO signatures and time shifts of approximately 4–8 ms indicate that fluid substitution should be detectable in the Leman and Bunter reservoirs with appropriately designed time-lapse surveys, whereas monitoring sensitivity may be more limited in the Helsby Sandstone. Reservoir geology therefore exerts a first-order control on seismic detectability, highlighting the need for reservoir-specific monitoring strategies for carbon dioxide and hydrogen storage.