# 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 obtained at 77 K for core samples A and AH (Figure 3a) exhibit the characteristics of mesoporous materials, showing a type IV profile in accordance with the IUPAC classification. The presence of hysteresis loops confirms capillary condensation in pores with intricate geometries. Specifically, core A displays a hysteresis loop of the H2 type, associated with \"Ink Bottle\"-shaped pores, whereas core AH exhibits an H3-type loop, indicative of groove pores [34; 35; 36]. Hydrogen exposure at 100 bar for 100 days led to pronounced morphological alterations in Core (AH), resulting in a 61.57% increase in the adsorbed nitrogen volume at high relative pressures from 14.376 cm\({}^{3}\)/g (Core (A)) to 23.35 cm\({}^{3}\)/g (Core (AH)), as shown in Figure 3a and Table 4. Although the total porosity variation was negligible (23.95% for Core (A) vs. 23.87% for Core (AH)), the pore network in Core (AH) appears significantly more extended and interconnected. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Core** & **Porosity (\%)** & **Permeability** & **BET Surface** & **Total Pore** & **Mean Pore** & **Adsorbed N\({}_{2}\)** \\  & & **(mD)** & **Area (m\({}^{2}\)/g)** & **Volume** & **Diameter** & **Volume** \\  & & & **(cm\({}^{3}\)/g)** & **(nm)** & **(cm\({}^{3}\)/g)** \\ \hline A & 23.95 & 574.84 & 3.270 & 0.0214 & 3.3 & 14.376 \\ AH & 23.87 & 592.44 & 4.023 & 0.245 & 5.09 & 23.35 \\ B & 19.89 & 132.16 & 6.93 & 0.0175 & 3.023 & 26.11 \\ BH & 16.33 & 107.43 & 5.80 & 0.0143 & 2.853 & 21.17 \\ C & 24.50 & 581.96 & 19.048 & 0.0291 & 3.107 & 27.81 \\ CH & 24.13 & 567.43 & 18.87 & 0.0284 & 3.305 & 27.40 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Physical and textural parameters of core samples: porosity and permeability. Figure 3: Nitrogen adsorption–desorption isotherms of core samples before and after hydrogen exposure: (**a**) dolomite (A/AH), (**b**) limestone (B/BH), and (**c**) siliceous rock (C/CH). BET and BjH analyses corroborate this transformation: the specific surface area increased from 3.270 m\({}^{2}\)/g (Core (A)) to 4.023 m\({}^{2}\)/g (Core (AH)), and the total pore volume rose from 0.0214 cm\({}^{3}\)/g to 0.245 cm\({}^{3}\)/g. Additionally, the mean pore diameter (Dv(d)) increased from 3.3 nm to 5.09 nm (Figure 4a), reflecting a more open and fluid-accessible mesoporous architecture. The pore size distribution further reveals a higher density of small pores in AH, contributing to enhanced adsorption capacity and internal network expansion. These structural modifications positively influence permeability, which increased from 574.84 mD (Core (A)) to 592.44 mD (Core (AH)), suggesting improved pore connectivity and possibly reduced tortuosity. These textural and transport enhancements are primarily attributed to selective dolomite (CaMg(CO\({}_{3}\))\({}_{2}\) dissolution under reductive hydrogen exposure [17; 30; 31; 32], as described by reactions (1) and (2), along with the concurrent precipitation of ankerite (Fe-carbonate). The elevated presence of ankerite likely originates from the reduction of siderite, while the cationic substitution of Mg\({}^{2+}\) by Fe\({}^{2+}\) in the dolomite matrix led to the formation of ferroan carbonates (reaction (4)) [31]. \[CaMg(CO_{3})_{2}+Fe^{2+}+H_{2}\rightleftharpoons Ca(Fe,Mg)(CO_{3})_{2}+H_{2}O \tag{4}\]  
Accordingly, dolomitic formations emerge as promising candidates for subsurface hydrogen storage. These findings extend the current literature [17; 37], highlighting that not only the geochemical reactivity but also the physical structure of the host rock is critical in governing mineralogical transformation kinetics in reductive environments.