# Hydrogen-Rock Interactions in Carbonate and Siliceous Reservoirs: A Petrophysical Perspective
## 2 Materials and Methods  
This research included laboratory experiments that aimed to evaluate the effect of hydrogen gas (Linde, purity 5.0) on three distinct rock types: dolomite (CaMg(CO\({}_{3}\))\({}_{2}\)), vaterite-rich limestone (CaCO\({}_{3}\)), and a siliceous sample composed predominantly of quartz. These samples were labeled A, B, and C, respectively, as shown in Figure 1. The experiments were designed to simulate subsurface underground hydrogen storage conditions. All core samples were obtained from intact and homogeneous outcrop blocks collected from representative carbonate and siliceous formations in Romania. Prior to experimentation, each sample was visually inspected and subjected to XRD and SEM analyses to ensure mineralogical consistency and structural uniformity. Samples exhibiting visible laminations, fossil inclusions, or macroporosity anomalies were excluded. Figure 1: Core samples used in study of hydrogen’s effect on geological components where A— dolomite, B—limestone and C—siliceous rock. Prior to hydrogen exposure, the intrinsic moisture content of each rock core was determined using a KERN DBS 60-3 moisture analyzer. The measurements indicated that the dolomitic sample (Core A) had a moisture content of 4.89%, the limestone sample (Core B) had a moisture content of 7.34%, and the siliceous sample (Core C) had a moisture content of 2.41%. These values confirm the presence of residual water within the cores, which may facilitate moisture-dependent geochemical reactions during hydrogen exposure. To identify geochemical alterations and variations in petrophysical properties induced by hydrogen exposure, a combination of analytical techniques was employed. The petrophysical properties of the rock cores were characterized both before and after hydrogen treatment to assess the extent of any changes. Mineralogical analysis was performed using X-ray diffraction (XRD) with a D8 Advance diffractometer (Bruker-AXS, Karlsruhe, Germany) operating with Cu-K\(\alpha\) radiation (\(\lambda\) = 1.54 A), equipped with a nickel filter, \(\theta\)-\(\theta\) geometry, and a Bragg-Brentano configuration. XRD measurements were carried out under the following conditions: 40 kV, 40 mA, a step size of 0.1\({}^{\circ}\), a scan speed of 0.1\({}^{\circ}\) per 5 s, and a 20 scan range from 5\({}^{\circ}\) to 60\({}^{\circ}\). Permeability and porosity measurements of the rock cores were conducted using a GASPERM permeameter (Vinci Technologies, Nanterre, France) and a helium porosimeter (He-Porosimeter, Vinci Technologies, France), respectively. To analyze textural changes such as BET surface area, pore size distribution, and mean pore diameter, nitrogen adsorption-desorption isotherms were recorded using a Quantachrome NOVA 2200e gas adsorption analyzer (Boynton Beach, FL, USA). Isotherms were collected at 77.35 K over a relative pressure range (p/p\({}_{0}\)) of 0.005 to 1.0, and the data were processed using the NovaWin software, version 11.03. Additionally, morphological changes and elemental composition of the rock samples before and after hydrogen treatment were examined using scanning electron microscopy (SEM). A Hitachi S-3400N SEM and an FEI SEM system (Hillsboro, OR, USA), both equipped with energy-dispersive X-ray spectroscopy (EDS), were utilized. The SEM images were acquired at an accelerating voltage of 2 kV, while EDS analyses were conducted at 30 kV. Initially, cylindrical rock samples (core plugs) were placed in a stainless steel test cell. The cell had a diameter of 3.5 cm, a wall thickness of 1 cm, and a length of 50 cm, and was designed to withstand pressures up to 250 bar. It was equipped with a gas inlet fitted with a pressure gauge and a sealed outlet connected to a flowmeter for monitoring hydrogen flow. Hydrogen gas was introduced into the cell until a pressure of 100 bar was reached, using a 50 L hydrogen cylinder pressurized to 200 bar and regulated with a pressure controller. The rock samples were exposed to a hydrogen atmosphere for 100 days under constant pressure (100 bar).