## 5 Knowledge Gaps
In addition, assessing the risks of induced seismicity in faulted reservoirs and developing mitigation strategies remains crucial for safe operations. Further research on the cumulative impact of these cycles on porosity and permeability is required, as well as the development of predictive models that integrate long-term dynamic reservoir conditions. Advanced real-time monitoring tools are also essential to detect early failures during these operations. * Regarding the reactivity between H2 and carbonate minerals, the conclusions in the literature vary, and there are significant differences, which require further research and confirmation efforts [76; 89]. The kinetic rate of various carbonate minerals and anhydrite dissolution in hydrogen-brine-mineral system could be measured through static experiments together with core-flooding experiments [62]. Experimental results show minor dissolution of carbonate minerals in static conditions, but these reflect short-term processes. The key challenge is understanding how these reactions evolve under dynamic, long-term conditions, such as cyclic hydrogen injection and withdrawal, influenced by factors like fluctuating pressure and brine chemistry. Comprehensive studies simulating real-world scenarios are needed to predict long-term geochemical behavior accurately. Future studies in geochemistry should focus on bridging the gap between laboratory-scale experiments and real reservoir conditions. This involves addressing the complexity of heterogeneous mineralogy, variations in brine composition, and temperature and pressure fluctuations in reservoir environments. Major gaps include understanding long-term reactions under cyclic injection and extraction processes, extrapolating laboratory findings to reflect reservoir-scale variability, and integrating multiphase hydrogen-brine-rock interactions. There is a need to develop advanced in situ monitoring techniques and predictive models that capture dynamic feedback mechanisms, ensuring accurate assessments of reservoir integrity and the viability of long-term hydrogen storage. * Integrity issues and geochemical and physical changes near the wellbore can seriously affect the engineering materials [69; 149]. Ninety percent of salt cavern problems are associated with H2 leakages and their migration toward the wellbore, either resulting in corrosion (from H2 embrittlement) or casing damage (salt creeping) and H2 losses [151]. [PERSON] et al. [152] found that hydrogen did not significantly alter the chemical and physical properties of the cement samples they studied, but further studies aiming at engineering materials are needed. * Lastly, to experimentally study microbial activity--one of the most significant contributors to hydrogen loss--still presents a significant challenge [30; 69]. While current research highlights the role of sulphate-reducing bacteria, methanogens, and acetogens, there are significant gaps in understanding how these processes evolve under dynamic reservoir conditions [94]. Key areas of exploration include how microbial communities adapt during prolonged periods of hydrogen storage and under cyclic injection and extraction scenarios, as well as the development of advanced monitoring techniques to detect microbial activity in real time.