## 4 Pore-Scale Visualized Experiments for Two-Phase Flow
### CT-Based Visualized Experiment  
Computed tomography (CT) imaging technology, which is fast, non-destructive, and safe, provides a three-dimensional visualization tool for characterizing rock pore structures and gas-water two-phase flow dynamics. It also delivers real-time, visualized results. These technologies have been widely applied in various fields, overcoming the resolution and dimensionality constraints of traditional planar micro-scale models to visualize fluid distribution at the pore scale. It allows can investigate microscale two-phase flow behaviors in reservoir cores while preserving their original geometry and internal structure. For example, by studying the displacement process between hydrogen and brine, researchers found that hydrogen experiences a certain degree of retention effect in the pore media [70]. By imaging different parts of the core and various displacement stages within the experimental setup, the dynamic evolution characteristics of the core's internal microstructure, fluid flow, and distribution during the displacement experiment can be obtained [71]. Combined with monitoring information such as pressure and flow rate in the experiment, as well as CT cross-sectional imaging technology, the distribution characteristics of fluid saturation and heterogeneity within the core can be acquired. This enables a qualitative description and quantitative characterization of micro-displacement efficiency and the distribution of two-phase fluids. To investigate the seepage pathways of gas-water two-phase flow in underground rock reservoirs, Researchers utilize insights from CT technology, including wet and dry images, to better inform pore-scale models. By visualizing the throats and pores within the rock matrix, we can gain insights into the factors influencing the efficiency of gas-water displacement within the rock's pore structure. [PERSON] et al. 2022 [72] used CT imaging to study the displacement between hydrogen gas and brine. The study revealed that after a 12-h period without flow, trapped hydrogen ganglia underwent significant rearrangement. Although the total mass of hydrogen remained unchanged, smaller gas ganglia tended to disappear while larger ones grew. Additionally, researchers can utilize CT imaging technology to assess the impact of rock physical properties on gas-water two-phase seepage dynamics. The distribution of two-phase fluids within porous media is affected by several factors, including rock type, surface wettability, and the capillary number et al. [73]. Furthermore, researchers conducted a quantitative and comprehensive study on the influence of rock properties on two-phase seepage characteristics, employing CT scanning and pore-scale multiphase flow modeling. They analyzed how rock surface wettability and heterogeneity influenced hydrogen transport in porous media [74]. [PERSON] et al. 2025 [75] conducted core displacement experiments involving hydrogen injection and extraction in sandstone samples, two sets of micro-CT images were captured during the primary drainage and imbibition stages. Image analysis revealed that after the hydrogen injection stage, hydrogen was distributed across both large and small pores and throats, but after water injection, hydrogen predominantly remained in larger pores, where capillary pressure barriers were lower. Thanks to the in situ CT scanning imaging technology, which allows for three-dimensional imaging of the heterogeneity of rock pore structures and revealsthe differences in pore structure at different slice locations, the volume fraction of fluid at each slice position can be obtained. This results in an uneven distribution of fluid volume fractions across different strata of underground hydrogen storage pores, as shown in Figure 8a. The hydrogen volume fraction remained virtually unchanged between the H\({}_{2}\) imbibition and the post-12-h storage period.