## 1 Introduction
Chemical reactions, particularly those associated with carbonation driven by CO\({}_{2}\), can further compromise the strength of the cement and increase its permeability [16; 17]. Unlike CO\({}_{2}\), hydrogen's low density and viscosity may lead to rapid upward flow through cement seals, affecting their integrity and increasing the risk of leakage [18]. Understanding the alterations that cement undergoes during underground hydrogen storage  
Figure 1: Different leakage pathways from a well [7]. (UHS) remains limited, with only a few studies below addressing this issue. Notably, high-pressure conditions and interactions between hydrogen and rock formations can contribute to the chemical degradation of cement during UHS operations [19; 20]. Nevertheless, the potential for hydrogen leakage through various channels during production cycles, particularly in older wells, may gradually lead to cement deterioration, elastomer failure, caprock sealing failure, fluid migration through preexisting leakage routes, casing microbiological corrosion, hydrogen blistering, and hydrogen embrittlement, thereby compromising the overall integrity of the wellbore [21]. [PERSON] et al., [PERSON] et al., and [PERSON] et al. studied the effects of H\({}_{2}\) exposure on neat Class H cement, which indicated increased permeability and porosity, and a decrease in the strength of cement [22; 23; 24]. Experimental investigations by [PERSON] et al., [PERSON] et al., and [PERSON] et al. demonstrated no significant alterations in cement properties after exposure to hydrogen under reservoir conditions, suggesting potential suitability for underground hydrogen storage under specific reservoir conditions [25; 26; 27]. Investigations by [PERSON] et al. [28] demonstrated that no significant geochemical or structural alterations to cement were revealed due to exposure to hydrogen under simulated reservoir conditions, indicating potential security for underground hydrogen storage under specific conditions. Studies by [PERSON] et al. [29] and [PERSON] et al. [30] investigated the well integrity of H\({}_{2}\) storage using computed tomography and scanning electron microscopy, where no changes in the microstructure were found. However, debonding is a leakage pathway for H\({}_{2}\) migration in depleted hydrocarbon reservoirs. Experimental results by [PERSON] et al. [31] suggest increased cement strength and alterations in porosity distribution after exposure to molecular hydrogen under specific conditions, potentially enhancing wellbore integrity in underground hydrogen storage projects. All the tests performed to assess well integrity for underground hydrogen storage (UHS) have utilized standard cement formulations (1.96 g/cc), typically cured for 3-7 days. However, the depleted hydrocarbon reservoirs designated for hydrogen storage will employ aged cement, which has been in the well for several years. Therefore, this study employed cement samples cured for a prolonged duration of 12 and 18 months to better simulate the conditions of cement encountered in actual UHS scenarios. Studies in the literature have mentioned that the compressive strength of cement increases and then remains stagnant for a curing period of 6 months [32], but after an elongated period, it decreases [33; 34]. Another notable distinction is that, unlike Carbon capture and storage (CCS) applications where CO\({}_{2}\) is intended to be stored for extended periods, underground hydrogen storage (UHS) involves a cyclic seasonal process. Therefore, to comprehensively understand the impacts of UHS on well integrity, it is imperative to investigate the effects of cyclic exposure to hydrogen on cement and on its sealing ability. In the long term, interactions between hydrogen, cement, and the surrounding rock formations in the presence of formation fluids could exacerbate cement weakening and compromise containment measures. Thus, a comprehensive understanding of these processes is crucial for ensuring the long-term integrity and effectiveness of UHS operations.