## 2 Acquisition and Characterization of Pore Structure  
Characterizing the pore structure of rocks is essential for understanding the complex network of pores and throats, including their geometry, size, spatial distribution, and interconnectivity. This detailed characterization is critical for predicting and analyzing the transport behaviors, such as gas-water two-phase flow within the rock's pores.  
The pore size distribution (PSD) obtained through rock pore structure characterization plays a critical role in determining the potential for hydrogen storage or sequestration. Different types of rocks, such as sandstone and carbonate rocks, exhibit distinct pore structures that influence the behavior of hydrogen within these reservoirs. Sandstone typically has a more uniform pore structure with higher porosity, which allows hydrogen molecules to be stored and transported efficiently. Hydrogen primarily occupies medium and larger pores, minimizing hydrogen trapping. Its well-connected pore network facilitates the flow and distribution of hydrogen, making sandstone an ideal medium for underground hydrogen storage [24]. On the other hand, carbonate rocks, such as limestone, often have more heterogeneous pore structures, consisting of a mixture of macropores and micropores, with complex and irregular pore geometries. While these rocks may have lower porosity,  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Citation** & **Main Concerns** & **Research Directions** \\ \hline [PERSON] et al., 2024 [13]; & & \\ [PERSON] et al., 2025 [11] & & \\ [PERSON] et al., 2023 [14]; & & \\ [PERSON] et al., 2022 [15] & & \\ [PERSON] et al., 2022 [16]; & & \\ [PERSON] et al., 2024 [17] & & \\ [PERSON] et al., 2023 [18]; & & \\ [PERSON] et al., 2023 [19] & & \\ [PERSON] et al., 2023 [20]; & & \\ [PERSON] et al., 2023 [21] & & \\ [PERSON] et al., 2024 [22]; & & \\ [PERSON] et al., 2025 [23] & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Articles about UHS and its main concerns.  
they possess high permeability, which helps trap hydrogen gas within their pores. Additionally, the presence of natural fractures and vugs in carbonate rocks further enhances their ability to retain hydrogen, making them an ideal choice for hydrogen sequestration. The PSD of these rocks influences the distribution and retention of hydrogen, which in turn affects storage efficiency and the long-term stability of hydrogen reservoirs. Therefore, understanding the PSD of different rock types is crucial for optimizing hydrogen storage and ensuring safe and effective sequestration strategies [25].  
In recent years, with the development of experimental testing equipment and analytical techniques, laboratory methods for obtaining and characterizing pore structures have become increasingly advanced and diverse, forming an integrated analysis and testing system that includes qualitative description, semi-quantitative analysis, and quantitative characterization of pore structure features. Based on differences in testing methods and theories, existing testing technologies can be divided into two main categories: fluid invasion methods and imaging methods. Fluid invasion methods primarily include mercury intrusion porosimetry (MIP), nitrogen adsorption, and nuclear magnetic resonance (NMR). While NMR is often associated with fluid invasion techniques, it is important to note that NMR can also be used for solid-state measurements, making it versatile in characterizing both fluid-filled and solid pore spaces. On the other hand, imaging methods mainly include techniques such as thin section casting, scanning electron microscopy (SEM), and CT imaging, which use optical radiation technologies to capture images of the pore structure. These images, combined with digital image analysis, enable the quantitative characterization of pore structures, as shown in Figure 2.