# Hydrogen-Rock Interactions in Carbonate and Siliceous Reservoirs: A Petrophysical Perspective
## 1 Introduction  
In light of the rapid expansion of hydrogen production and its adoption as a clean energy carrier, there is a growing need for subsurface geological storage solutions capable of balancing fluctuations in supply driven by variable renewable electricity input and seasonal variations in demand, thereby enhancing long-term energy security [1]. While salt caverns are considered a promising option due to their operational flexibility [2; 3], porous reservoirs such as depleted gas fields [4] and deep saline aquifers [5], despite their broader availability, still require comprehensive investigation to assess their geochemical behavior and effectiveness as safe and efficient media for hydrogen storage [6; 7]. The study of geochemical processes within underground hydrogen storage systems is of critical importance, particularly those that may contribute to hydrogen loss during storage operations [8]. Although the past decade has seen numerous investigations ranging from geochemical modeling to experimental studies [9; 10; 11; 12] across various rock types, the current understanding of the physicochemical interactions between hydrogen and geological formations remains limited and requires further systematic development [13; 14]. Hydrogen loss during high-pressure underground hydrogen storage (UHS) is primarily attributed to geochemical and microbial interactions that typically occur under reservoir conditions characterized by temperatures below 127 \({}^{\circ}\)C and pressures below 350 bar. Understanding these interactions is essential for evaluating hydrogen mobility and reactivity within geological media, which is key to enhancing storage monitoring and ensuring long-term operational stability [15]. Hydrogen-rock interactions are influenced by various factors, including mineral composition, storage pressure and temperature, pore structure, and the rock's specific properties such as heterogeneity, mechanical stability, organic content, and wettability behavior [16]. These physical, chemical, and biological interactions can induce changes in permeability, porosity, and wettability [15; 17; 18], as well as trigger microfracture activation directly impacting the long-term safety and efficiency of geological hydrogen storage systems [19]. In this context, [PERSON] et al. [17] investigated carbonate cores exposed to hydrogen for 75 days (48 bar, 75 \({}^{\circ}\)C), revealing that calcite expansion reduced effective porosity by 47%, significantly impairing limestone storage capacity. In contrast, dolomite showed a slight (~6%) improvement due to a balance between dissolution and grain expansion. Another study by [PERSON] et al. [9] exposed calcite-rich shale samples from the EFS and WCS formations to hydrogen at 9.65 MPa and 348 K for 108 days. Calcite precipitation occurred due to organic matter interactions, with no sulfate or pyrite reduction. Minimal pore changes were observed, indicating preserved rock integrity and suitability for geological hydrogen storage. [PERSON] et al. [20] suggest that calcite-free reservoirs and caprocks are geochemically more suitable for underground hydrogen storage, as carbonate presence, particularly calcite, can induce hydrogen dissociation during dissolution, leading to losses of up to 9.5%. The study by [PERSON] et al. [21] demonstrated that hydrogen interaction with pyrite-free Buntsandstein sandstone is limited, even after exposure to 150 bar and 100 \({}^{\circ}\)C for four weeks. No significant mineralogical transformations were observed, and no noticeable changes were detected in key petrophysical properties such as porosity and permeability, confirming the rock's suitability for hydrogen storage. [PERSON] et al. [22] investigated how sandstone formations respond petrographically and petrophysically to hydrogen exposure. The study revealed a moderate decline in median porosity from 13.3% to 11.9% accompanied by a notable increase in permeability, rising from 48.78 mD to 61.53 mD.