## 5 Comparison of Interface Tracking Methods in Pore-Scale Two-Phase Flow Simulation  
Recent advancements in digital core technology have enabled the quantitative characterization of the internal microstructure of reservoirs, offering deeper insights into the variation patterns of reservoir rock properties and the micro-scale mechanisms of seepage. On this basis, the pore-scale numerical simulation technology, compared to experiments, not only saves experimental costs, shorten experimental cycles, and overcomes the limitations of experimental conditions, but also provides intuitive and visually clear simulation  
Figure 8: Typical fluid distribution patterns of (**a**) H\({}_{2}\)-water [76] (**b**) CO\({}_{2}\)-water two-phase flow after drainage imaged by the in situ CT scanning [77]. results. Moreover, it allows for lateral comparative simulation experiments under different conditions using the reconstructed models. With the development of computer technology and computational fluid dynamics theory, pore-scale numerical simulation technology based on micro-CT images of rocks has become increasingly favored by researchers. In two-phase or multiphase flow systems within porous media, a distinct phase interface exists, and this interface is continually evolving as the system progresses. Accurately capturing the dynamics of the gas-liquid phase interface is a critical aspect of understanding two-phase seepage mechanisms. However, due to the complex flow patterns inherent in two-phase flow and the intricate coupling between the flow field and the fluid exchange mechanisms at the interface, verifying the accuracy of interface conditions remains challenging. Unlike single-phase flow, the defining characteristic of two-phase and multiphase flows is the presence of a well-defined phase interface within the mixed system. As the mixture moves through the porous media, the shape and state of these phase interfaces are continuously changing and evolving, further complicating the study of their behavior [78]. To develop an accurate pore-scale two-phase flow model, it is crucial to precisely describe the fluid interfaces. In practical applications, the evolution of fluid interfaces is closely related to the distribution of residual fluids, especially during the interaction between gas and liquid, as shown in Figure 9[79]. Researchers worldwide commonly employ two primary methods to track the motion of these interfaces: the Eulerian and Lagrangian approaches. The Lagrangian method tracks the motion of material points that reside on the interface itself, which is numerically implemented by following grid points and either the local fluid velocity or the velocity of grid movement. However, this method faces challenges, particularly in dealing with the deformation of interfaces. These challenges include the need for grid remeshing and interpolation, which can lead to increased computational costs and errors. In contrast, the Eulerian method tracks the flow of the two fluid components on a fixed grid. The interface is captured by an isosurface of a globally defined function, and at each time step, the interface is reconstructed or determined using a scalar indicator function. The Eulerian method generally outperforms the Lagrangian method, particularly in handling complex interface features, such as irregular contours, sharp corners, merging, and destructive interfaces. The most widely used Eulerian methods for two-phase flow modeling include the level set method [80], the volume of fluid (VOF) method [60], and the phase field method [81]. In addition to these methods, the pore network (PN) model is also a critical technique for pore-scale modeling in hydrogen storage, as well as in applications like enhanced oil recovery (EOR) and carbon sequestration. The PN model simulates fluid flow by discretizing the pore structure into pores and throats, which is well-suited for capturing phenomena such as capillary pressure, relative permeability, and fluid distribution at the pore scale.