## 3 Results
### Geomechanical Considerations  
The understanding and characterization of the mechanical response/behavior of heterogeneous reservoir rocks and caprocks during the cyclic injection and production of H\({}_{2}\) and, complementary, the interaction (abiotic and biotic) of H\({}_{2}\) with rock and brine, in reservoir conditions, are key to ensuring the safety of the H\({}_{2}\) storage [57]. Based on the review by [PERSON] et al. [90], cyclic H\({}_{2}\) injection and withdrawal can significantly impact storage integrity. This is due to cyclic pressure changes affecting intact rocks and fault zones, short- and long-term chemical interactions of H\({}_{2}\) with these rocks and faults, and the influence of stress, strain, and sorption on mechanical and transport behavior [66; 90]. Unlike CO\({}_{2}\) storage, where Joule-Thomson cooling near the wellbore poses a storage integrity issue (>2 MPa; >40 \({}^{\circ}\)C) (e.g., [102]), H\({}_{2}\) injection causes minor temperature fluctuations near the wellbore. However, this injection could induce pressure and stress changes, potentially causing deformation beyond the pressure change area (e.g., [103]). The gas injection-withdrawal cycle alters the stress regime in the storage complex (e.g., sandstone), leading to reservoir compaction porosity reduction (e.g., porosity decreases slightly from 5.80 to 5.57% for Sample 1, 60 MPa) and fluid flow impediment (e.g., permeability decreases slightly from 2.47 to 1.53 mD for Sample 1 to 60 MPa) [104]. According to [PERSON] et al. [105], a significant decrease in porosity and pore connectivity of sandstones after consecutive cycles of loading due to the closure of microcracks in the reservoir was found. This may result in subsidence, fault reactivation, or micro-seismicity(e.g., the Wilmington oil field or the earthquake series near Coalinga, in which a connection between seismicity and hydrocarbon production was suspected) [106; 107]. According to [PERSON] et al. [57], insufficient knowledge about the subsurface reservoir and stress conditions, especially in faulted reservoirs, can lead to injection-induced seismicity. Reservoir compaction could bend the caprock, creating fractures and potential leakage pathways (e.g., mineralogical changes due to gas-brine-rock interactions can impact the frictional behavior and seismogenic potential of faults) [108]. Load-bearing minerals and cement may be removed through dissolution-precipitation reactions, causing reversible and permanent deformation [109]. According to [PERSON] and [PERSON] [59], repeated injection of dry gas (e.g., CO\({}_{2}\)) may dehydrate the reservoir over time (dehydration can lead to both shrinkage and cracks), affecting the mechanical properties and integrity, if the hydration of the rock is also considered after those dehydration cycles. For example, clays may shrink, reopening fracture seals and creating leakage pathways in the caprock (dehydrate-hydrate cycles) [59]. Salt caverns, with their viscoplastic behavior, resist fracture formation and spreading, ensuring tightness during gaseous H\({}_{2}\) storage. However, UHS in salt caverns or detritic reservoirs has risks related to gas compression and expansion cycles. Altering the original salt stresses during cavern construction and operation can lead to damage or rupture, primarily due to tensile and shear failures (short and long term) caused by thermal effects and stress changes [59]. The cyclical injection and reproduction of hydrogen itself lead to (i) cyclic pressure changes in the behavior of intact rocks and faults, (ii) short- and long-term chemical interaction of hydrogen with intact rocks and faults, and (iii) stress-strain-sorption in mechanical and transport behavior, all of which can have a significant influence on the mechanical integrity of storage [66]. Certain researchers have noted minimal reservoir porosity and permeability alterations due to the slow reaction rates between hydrogen (H\({}_{2}\)) and iron-bearing minerals [110].