Deformation bands as flow barriers in an underground hydrogen storage system

To enable hydrogen (H2) to substantially contribute to the energy transition, a better understanding of the efficacy of large-scale underground hydrogen storage (UHS) is needed. Storage options in porous rocks such as reservoirs and saline aquifers offer the largest potential by volume. However, sandstone reservoirs are typically characterised by different degrees of heterogeneity that affect storage. Heterogeneities include deformation bands, often found in well-sorted aeolian sandstone with good UHS potential. Deformation bands are planar, low-permeability features that can develop through grainsize reduction and fusing in response to changes in stress. Little is known about the effect of heterogeneities on the injection, storage and recovery of H2. Therefore, this study examines the effect of permeability contrasts caused by deformation bands on UHS. To do this we undertake two cycles of H2 injection and production in a two-phase (H2-brine) flow experiment on a Sherwood Sandstone sample containing a deformation band at 50 °C and 10 MPa in an X-ray micro Computed Tomography scanner. In addition, we repeat the experiment with helium to further evaluate effects of the permeability contrasts and understand whether helium can be a useful proxy for H2 in laboratory-based experiments. The results show that the deformation band analysed is a baffle to flow, increasing the required injection pressure for gas transit, and there are few pathways for H2 across it. In the first cycle very little H2 is trapped below and in the deformation band. But when the pathways become blocked during imbibition, due to brine snap off, there is a substantial increase in H2 trapping, reducing H2 flow and production. We also see trapping of H2 in larger pores and post injection movement of H2. Both processes exhibit similar characteristics, with H2 occupying the pore centres and brine on the rock surfaces. Our observations will assist in understanding processes within UHS reservoirs and enable strategies to be developed to deal with fluid movement and pressure changes resulting from permeability contrasts. There are many similarities in the behaviour of H2 and helium in our experiment, confirming our observations with H2. As such, we find that helium can be used as a proxy in these conditions, although adjustments may need to be made for the viscosity differences.