# Hydrogen-Rock Interactions in Carbonate and Siliceous Reservoirs: A Petrophysical Perspective
## 3 Results and Discussion
### Porosity, Permeability, and Textural Characteristics
The nitrogen adsorption-desorption isotherms at 77 K for all studied core samples (B/BH and C/CH) reveal mesoporous characteristics with type IV profiles and H3-type hysteresis loops and H4-type hysteresis, respectively, according to the IUPAC classification, denoting capillary condensation within groove pores and silt pores, thus indicating the presence of a developed but poorly connected pore network typical of carbonate rocks with irregular medium porosity [34; 35; 36]. However, the impact of hydrogen exposure (100 bar for 100 days) differed significantly depending on the mineralogical composition. Upon hydrogen exposure (100 bar for 100 days), Core BH exhibited a considerable reduction in adsorption capacity throughout the relative pressure range. At the maximum relative pressure, the nitrogen adsorption volume dropped by approximately 19%, from 26.11 cm\({}^{3}\)/g (B) to 21.17 cm\({}^{3}\)/g (BH), as observed in Figure 3b, suggesting significant structural alterations in the pore network caused by the hydrogen treatment. Textural parameter analysis confirmed this trend: the BET specific surface area decreased from 6.93 m\({}^{2}\)/g (B) to 5.80 m\({}^{2}\)/g (BH), while the total pore volume (BJH) declined from 0.0175 cm\({}^{3}\)/g to 0.0143 cm\({}^{3}\)/g (Table 4). Additionally, the mean pore diameter (Dv(d))  
Figure 4: Pore size distribution and total pore volume of core samples before and after hydrogen exposure: (**a**) dolomite (A/AH), (**b**) limestone (B/BH), and (**c**) siliceous rock (C/CH). experienced a slight reduction from 3.023 nm to 2.853 nm. These variations align with the decrease in total porosity (from 19.89% to 16.33%) and permeability (from 132.16 mD to 107.43 mD), indicating diminished pore connectivity and a potential partial collapse of the porous framework. The pore size distribution (Figure 4b) further supports this observation, revealing a reduced density of active pores especially within the 3-6 nm range in the hydrogen-treated sample. Such transformations are likely attributable to mineral reprecipitation phenomena or localized pore blockage, possibly induced by the formation of secondary carbonate phases (e.g., redeposited calcite), triggered by solid gas reactions under high hydrogen pressure [32; 38]. These findings underscore the negative impact of prolonged hydrogen exposure on the structural integrity of calcite rocks. Unlike dolomite, which exhibited pore network enhancement, calcite underwent compaction and occlusion, leading to reduced storage potential. Consequently, calcite formations appear less favorable for long-term hydrogen containment due to their susceptibility to morphological degradation under reductive conditions. In contrast, core samples C and CH, composed primarily of quartz, exhibited remarkable structural resilience following exposure to hydrogen at 100 bar for 100 days. However, the structural response of quartz was notably more stable. The adsorbed nitrogen volume, as shown in Figure 3c, showed only a marginal decrease of \(\sim\)1.5%, from 27.81 cm\({}^{3}\)/g (C) to 27.40 cm\({}^{3}\)/g (CH), significantly lower than the changes observed in dolomite (+61.57%) and calcite (\(-\)19%). This indicates that hydrogen has a minimal effect on the microstructural integrity of quartz. This conclusion is further supported by the textural parameters: the BET surface area slightly decreased from 19.048 m\({}^{2}\)/g to 18.87 m\({}^{2}\)/g, the total pore volume (BJH) declined marginally from 0.0291 cm\({}^{3}\)/g to 0.0284 cm\({}^{3}\)/g, and the mean pore diameter (Dv(d)) increased moderately from 3.107 nm to 3.305 nm. Total porosity and permeability also exhibited only slight reductions, from 24.5% to 24.13% and from 581.96 mD to 567.43 mD, respectively. The pore size distributions (Figure 4c) for samples C and CH are nearly superimposed, indicating negligible structural changes.