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The rise in gas prices, accelerated by the war in Ukraine, has exposed the volatility of the energy market and Europe’s need to achieve independence from external supplies from politically sensitive areas. Reaching such independence, while simultaneously complying with climate agreements, will inevitably require a substantial expansion of subsurface energy and byproducts storage options., JMM is funded by the DeepNL research programme (project DEEP.NL.2019.003) which is financed by the Dutch Research Council. JA is funded by the Spanish Ministry of Science and Innovation through the Juan de la Cierva fellowship (IJC 2018-036074-I), funded by MCIN/AEI/10.13039/501100011033. NH is funded by the UK Engineering and Physical Sciences Research Council funded research project “HyStorPor” (grant number EP/S027815/1).

Expectations for energy storage are high but large-scale underground hydrogen storage in porous media (UHSP) remains largely untested. This article identifies and discusses the scientific challenges of hydrogen storage in porous media for safe and efficient large-scale energy storage to enable a global hydrogen economy. To facilitate hydrogen supply on the scales required for a zero-carbon future, it must be stored in porous geological formations, such as saline aquifers and depleted hydrocarbon reservoirs. Large-scale UHSP offers the much-needed capacity to balance inter-seasonal discrepancies between demand and supply, decouple energy generation from demand and decarbonise heating and transport, supporting decarbonisation of the entire energy system. Despite the vast opportunity provided by UHSP, the maturity is considered low and as such UHSP is associated with several uncertainties and challenges. Here, the safety and economic impacts triggered by poorly understood key processes are identified, such as the formation of corrosive hydrogen sulfide gas, hydrogen loss due to the activity of microbes or permeability changes due to geochemical interactions impacting on the predictability of hydrogen flow through porous media. The wide range of scientific challenges facing UHSP are outlined to improve procedures and workflows for the hydrogen storage cycle, from site selection to storage site operation. Multidisciplinary research, including reservoir engineering, chemistry, geology and microbiology, more complex than required for CH4 or CO2 storage is required in order to implement the safe, efficient and much needed large-scale commercial deployment of UHSP.

Expectations for energy storage are high but large-scale underground hydrogen storage in porous media (UHSP) remains largely untested. This article identifies and discusses the scientific challenges of hydrogen storage in porous media for safe and efficient large-scale energy storage to enable a global hydrogen economy. To facilitate hydrogen supply on the scales required for a zero-carbon future, it must be stored in porous geological formations, such as saline aquifers and depleted hydrocarbon reservoirs. Large-scale UHSP offers the much-needed capacity to balance inter-seasonal discrepancies between demand and supply, decouple energy generation from demand and decarbonise heating and transport, supporting decarbonisation of the entire energy system. Despite the vast opportunity provided by UHSP, the maturity is considered low and as such UHSP is associated with several uncertainties and challenges. Here, the safety and economic impacts triggered by poorly understood key processes are identified, such as the formation of corrosive hydrogen sulfide gas, hydrogen loss due to the activity of microbes or permeability changes due to geochemical interactions impacting on the predictability of hydrogen flow through porous media. The wide range of scientific challenges facing UHSP are outlined to improve procedures and workflows for the hydrogen storage cycle, from site selection to storage site operation. Multidisciplinary research, including reservoir engineering, chemistry, geology and microbiology, more complex than required for CH4 or CO2 storage is required in order to implement the safe, efficient and much needed large-scale commercial deployment of UHSP., This work was stimulated by the GEO*8 Workshop on “Hydrogen Storage in Porous Media”, November 2019 at the GFZ in Potsdam (Germany). NH, AH, ET, KE, MW and SH are funded by the Engineering and Physical Sciences Research Council (EPSRC) funded research project “HyStorPor” (grant number EP/S027815/1). JA is funded by the Spanish MICINN (Juan de la Cierva fellowship-IJC2018-036074-I). JM is co-funded by EU INTERREG V project RES-TMO (Ref: 4726 / 6.3). COH acknowledges funding by the Federal Ministry of Education and Research (BMBF, Germany) in the context of project H2_ReacT (03G0870C).

Travertine deposits present above the St. Johns Dome natural CO2 reservoir in Arizona, USA, document a long (>400 kyr) history of surface leakage of CO2 from a subsurface reservoir. These deposits are concentrated along surface traces of faults, implying that there has been a structural control on the migration pathway of CO2-rich fluids. Here, we combine slip tendency and fracture stability to analyse the geomechanical stability of the reservoir-bounding Coyote Wash Fault for three different stress fields and two interpreted fault rock types to predict areas with high leakage risks. We find that these areas coincide with the travertine deposits on the surface, indicating that high-permeability pathways as a result of critically stressed fracture networks exist in both a fault damage zone and around a fault tip. We conclude that these structural features control leakage. Importantly, we find that even without in situ stress field data, the known leakage points can be predicted using geomechanical analyses, despite the unconstrained tectonic setting. Whilst acquiring high-quality stress field data for secure subsurface CO2 or energy storage remains critical, we shown that a first-order assessment of leakage risks during site selection can be made with limited stress field knowledge.

Travertine deposits at the St. Johns Dome natural CO2 reservoir in Arizona, USA, document a long (> 400 ka) history of surface leakage of CO2 from a subsurface reservoir. Travertine deposits are concentrated along surface traces of faults implying that there has been a structural control on the migration pathway of CO2 rich fluids. Here, for the first time, we combine slip tendency and fracture stability to analyse the geomechanical stability of the reservoir-bounding Coyote Wash Fault for three different stress fields and predict areas with high leakage risks. We find that these areas coincide with the travertine deposits on the surface indicating that high permeability pathways as a result of critically stressed fracture networks exist in both fault damage zones and around a fault tip. We conclude that these structural features control leakage. Importantly, we find that even without in-situ stress field data the known leakage points can be predicted using geomechanical analyses, despite the complex tectonic setting. Thus, even though acquiring high quality stress field data for secure subsurface CO2 or energy storage is a must, a first order assessment of leakage risks during site selection can be made with limited stress field knowledge.

Faults can act as barriers to fluid flow in sedimentary basins, hindering the migration of buoyant fluids in the subsurface, trapping them in reservoirs, and facilitating the build-up of vertical fluid columns. The maximum height of these columns is reliant on the retention potential of the sealing fault with regards to the trapped fluid. Several different approaches for the calculation of maximum supported column height exist for hydrocarbon systems. Here, we translate these approaches to the trapping of carbon dioxide by faults and assess the impact of uncertainties in (i) the wettability properties of the fault rock, (ii) fault rock composition, and (iii) reservoir depth on retention potential. As with hydrocarbon systems, uncertainties associated with the wettability of a CO2–brine–fault rock system for a given reservoir have less of an impact on column heights than uncertainties of fault rock composition. In contrast to hydrocarbon systems, higher phyllosilicate entrainment into the fault rock may reduce the amount of carbon dioxide that can be securely retained due a preferred CO2 wettability of clay minerals. The wettability of the carbon dioxide system is highly sensitive to depth, with a large variation in possible column height predicted at 1000 and 2000 m of depth, which is the likely depth range for carbon storage sites. Our results show that if approaches developed for fault seals in hydrocarbon systems are translated, without modification, to carbon dioxide systems the capacity of carbon storage sites will be inaccurate and the predicted security of storage sites erroneous.