Understanding rock Fatigue mechanisms in Underground hydrogen StoragE: FUSE

绿色能源或可再生能源对于实现净零碳排放至关重要，而实现绿色能源雄心的关键技术是储能基础设施。例如，超过80%的电力供应来自可再生能源的电力系统无法使用现有的存储来平衡。在英国，弃风补贴在2020年几乎翻了一番，达到2.99亿英镑，并在2021年创下5.07亿英镑的历史新高。2020年和2021年浪费的能源足以为80万户英国家庭供电。在所有能源储存手段中，地下储氢在确保持续和明确的供应流的同时，显示出大规模和长期储存的巨大潜力。地下储氢的工作原理是将可再生电力（如风力涡轮机）产生的氢气注入地下地质构造，包括枯竭的油气储层、盐洞、含水层和硬岩洞。储存的氢可以用于发电，以平衡能源使用的波动，也可以作为燃料来满足运输需求。岩洞室通常被认为是地下储氢的最佳选择，因为它们的低透气性有助于良好的密封强度和能力。一旦用混凝土和一层气密材料（如不锈钢，PE或PVC）内衬，岩洞就可以具有出色的高密度氢气储存能力，同时对环境的影响最小。然而，洞室的长期结构稳定性和使用能力取决于其材料的非均质性和复杂的几何形状，以及地应力状态。注回采过程会对岩体产生循环压力；因此，围岩在切向上受到循环拉应力，可能同时受到循环剪应力。在较低的工作压力水平下，循环拉伸和剪切应力会使岩石产生疲劳，即强度降低，导致i型、ii型和/或混合型裂纹。这对储存库的结构完整性和安全构成了极大的威胁。在这个国际合作项目中，我们的目标是解决原位岩石在氢气注入和提取产生的循环压力下是如何疲劳和破裂的，以及岩石疲劳如何影响储氢基础设施的完整性和安全性。考虑储氢工况，研究问题可归结为高地应力水平下的低周岩石疲劳断裂。此外，材料的非均质性、应力状态、复杂几何形状、材料蠕变等都会对岩石的疲劳行为产生影响。因此，为了确保氢在岩洞中的长期安全储存，透彻了解岩石疲劳机制至关重要。

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.