# Hydrogen-Rock Interactions in Carbonate and Siliceous Reservoirs: A Petrophysical Perspective
## 3 Results and Discussion  
### X-Ray Diffraction (XRD) Analysis  
The X-ray diffraction (XRD) analysis conducted on the untreated dolomite sample (Core A) revealed distinct and intense reflections at a 20 value equal to 24.10\({}^{\circ}\), 30.88\({}^{\circ}\), 37.25\({}^{\circ}\), 41.05\({}^{\circ}\), 44.85\({}^{\circ}\), and 50.90\({}^{\circ}\), corresponding to the characteristic crystallographic planes of dolomite [24; 25; 26], as presented in Figure 2a. Additionally, minor peaks related to chlorite and calcite were identified at 6.2\({}^{\circ}\) and 29.5\({}^{\circ}\), attributed to the (001) and (104) planes, respectively [26; 27]. Quartz-related peaks were also observed at 20.8\({}^{\circ}\) and 26.6\({}^{\circ}\), assigned to the (100) and (101) planes, respectively [28], alongside a weak reflection for illite at 8.9\({}^{\circ}\) (the 001 plane) [29]. Following hydrogen exposure at 100 bar and 70 \({}^{\circ}\)C, the XRD patterns of the treated dolomite sample (Core AH) (Figure 2a) revealed significant mineralogical and structural transformations. A marked reduction in the intensity of the characteristic reflections at 24.1\({}^{\circ}\) and 41.05\({}^{\circ}\), corresponding to the (012) and (113) planes, was observed. Additionally, reflections associated with calcite, chlorite, and illite diminished or disappeared entirely. These changes are attributed to the ability of hydrogen to induce partial dissolution and destabilization of the mineral's crystalline structure, possibly due to the phase transformation or local rearrangement of the crystal lattice [30]. The process is illustrated by the dissolution mechanism of dolomite, which leads to the release of constituent ions and rearrangement within the mineral matrix, and the following reaction describes the hydrogen-induced breakdown of dolomite in the presence of moisture [17; 30; 31; 32]:  
\[CaMg(CO_{3})_{2(s)}+4H_{2(g)}\rightleftharpoons CaCO_{3(s)}+Mg(OH)_{2(s)}+CH_{4(g)}+H_{2 }O_{(l)} \tag{1}\]  
\[CaMg(CO_{3})_{2(s)}+10H_{2(g)}\rightleftharpoons Ca^{2+}+Mg^{2+}+2 CH_{4(g)}+6H_ {2}O_{(l)}+4e^{-} \tag{2}\]  
These pathways suggest that dolomite undergoes hydrogen-driven destabilization, releasing Ca\({}^{2+}\) and Mg\({}^{2+}\) ions, hydroxides, and methane, thus altering its crystalline framework. It is important to note that, although the injected hydrogen gas was dry, the reactions described by Equations (1) and (2) are still feasible due to the inherent moisture content within the rock samples. Moisture analysis confirmed the presence of residual water in all cores prior to hydrogen exposure, with values ranging from 2.41% to 7.34%. This internal moisture acted as a reactant medium, enabling the partial dissolution and transformation of carbonate minerals observed in the XRD results. In the case of the limestone sample (Core B), the XRD pattern depicted in Figure 2b confirmed the dominance of carbonates (calcite and dolomite) as the primary crystalline phases. Calcite (CaCO\({}_{3}\)) shows sharp peaks at 29.4\({}^{\circ}\), 36.10\({}^{\circ}\), 39.38\({}^{\circ}\), 43.2\({}^{\circ}\), 47.4\({}^{\circ}\), and 48.50\({}^{\circ}\) matching the (104), (110), (113), (202), (018), and (116) planes. On the other hand, dolomite show significant peaks at 30.88\({}^{\circ}\), 56.2\({}^{\circ}\), and 57.3\({}^{\circ}\), corresponding to the (104), (018), and (116) planes [24; 26; 27]. Furthermore, vaterite (the CaCO\({}_{3}\) biogenic polymorph) was detected through additional peaks at 24.8\({}^{\circ}\), 27.1\({}^{\circ}\), 32.7\({}^{\circ}\), 43.8\({}^{\circ}\), and 55.7\({}^{\circ}\), corresponding to the (110), (112), (114), (300), and (224) planes [27].