## 4 Pore-Scale Visualized Experiments for Two-Phase Flow
### NMR-Based Visualized Experiment  
NMR technology, as a rapid, non-destructive, and safe measurement method, it can measure the pore characteristics of reservoir porous media and conduct in-depth studies on the seepage characteristics of fluids within rocks. Furthermore, NMR technology can effectively separate fluids from the porous medium skeleton, obtaining intuitive information about the distribution of fluids within the core that traditional measurement methods cannot achieve, as well as the distribution state of fluids during the displacement process. When NMR is applied, relaxation phenomena occur, which are closely linked to the pore structure of the material. By employing mathematical analysis techniques, the distribution of fluids with different T2 relaxation times can be determined, producing what is known as the NMR T2 spectrum. According to the NMR relaxation mechanism, signals with longer T2 relaxation times in the spectrum correspond to fluids in larger pores, while those with shorter T2 times are associated with fluids in smaller pores [63]. Nuclear magnetic resonance (NMR) technology allows for the real-time observation of fluid morphology and the dynamic seepage process within porous structures, making it particularly effective for studying the seepage laws of fluid flow as well as the fluid-solid interactions in porous media, which are crucial for applications such as underground hydrogen storage. NMR technology is utilized to study the two-phase fluid flow state in reservoir porous media, which is primarily divided into two approaches. One is to evaluate the permeability of the core by establishing a regression relationship between the saturation of mobile fluids, porosity, and permeability [64; 65]; the other is to directly observe the distribution of fluids within the core using NMR imaging [66; 67]. [PERSON] et al. 2021 [68] used nuclear magnetic imaging to analyze the gas-water two-phase flow characteristics of the core. By analyzing NMR images under different gas saturations, it was found that both gas and water preferentially flow through larger pores, but there is also flow through smaller pores during the process, leading to plugging of these smaller pores. Although this increases the  
Figure 6: Schematic of the microfluidic system [60] (**a**) and the typical fluid distribution patterns of gas–water two-phase flow after drainage (**b**) and imbibition (**c**), as well as the in situ contact angle measurement (**d**). bound water saturation, it further expands the gas phase flow channels, thus increasing the gas phase permeability and reducing the relative permeability of the water phase. NMR imaging reveals the dominant flow channels during the displacement process in the core and the distribution characteristics of gas and water within pores of different sizes. [PERSON] et al. 2025 [69] analyzed the visualization method of fluid flow and transport in porous media based on NMR imaging technology, as shown in Figure 7. As the volume fraction of the invading phase in the pores increases, the displacement front gradually stabilizes, and the saturation of the defending phase gradually decreases, as shown in Figure 7b. By comparing the saturation curves calculated by NMR and experimental testing, it is concluded that the liquid phase saturation curves obtained by both methods are essentially consistent, as shown in Figure 7c. Research on the dynamic flow experiments of gas-water two-phase fluids in underground reservoirs using NMR technology has shown that NMR imaging technology accurately and intuitively reflects the fluid distribution information in the core during the two-phase flow process. Compared with conventional methods of studying flow characteristics, it has the advantages of high experimental efficiency and the ability to obtain fluid distribution information across different sections without destroying the rock. Nuclear magnetic resonance (NMR) experiments have elucidated the distribution characteristics of hydrogen in pore structures during gas-water two-phase flow.