Understanding rock Fatigue mechanisms in Underground hydrogen StoragE: FUSE

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Green or renewable energy has become vital in achieving net zero carbon and the key enabling technology to realise green energy ambitions is energy storage infrastructure. For instance, power systems in which more than 80% of the supply is generated from renewable sources cannot be balanced using existing storage. In the UK, wind curtailment payments almost doubled in 2020 to a total of £299M and hit a record high of £507M in 2021. The energy wasted in 2020 and 2021 is enough to power 800,000 British homes. Amongst all energy storage means, underground hydrogen storage has shown great potential for large-scale and long-term storage while securing a continuous and well-defined supply stream. Underground hydrogen storage works by injecting hydrogen that is produced from renewable electricity, e.g., wind turbines, into underground geological formations, including depleted oil and gas reservoirs, salt caverns, aquifers and hard rock caverns. The stored hydrogen can then be used for power generation to balance the fluctuation in energy use as well as for fuel to meet transportation demands. Rock caverns are often regarded as the best option for underground hydrogen storage due to their low gas permeability which contributes to excellent sealing strength and capability. Once lined with concrete and a layer of gas-tight material such as stainless steel, PE or PVC, rock caverns can have excellent storage capability for high-density hydrogen with minimum environmental impact. However, the caverns' long-term structural stability and serviceability depend on their material heterogeneity and complex geometries, and the in-situ stress state. The injection and withdrawal process will generate cyclic pressure on the rock mass; as a result, the surrounding rock is subjected to cyclic tensile stress in the tangential direction, possibly together with cyclic shear stress. The cyclic tensile and shear stresses will generate fatigue of rock, i.e., strength reduction, leading to mode-I, mode-II and/or the mixed mode cracks, at (possibly much) lower level of operational pressure. This poses a great threat to the structural integrity and safety of the storage site.In this international partnership project, we aim to address how the in-situ rock is fatigued and fractured under the cyclic pressure that will be generated from the injection and withdrawal of hydrogen, and how rock fatigue may affect the integrity and safety of the hydrogen storage infrastructure. Considering the hydrogen storage working conditions, the research problem can be summarized into low-cycle rock fatigue fracture under high in-situ stress level. Moreover, material heterogeneity, stress state, complex geometries, material creep, etc. can all have effects on the fatigue behavior of rock. To ensure the safe long-term storage of hydrogen in rock caverns it is therefore critically important to have a thorough understanding of rock fatigue mechanisms.