## 3 Results and Discussion
### Effect of Wettability on Flow in Pore Media  
Wettability characteristics reflect the adhesion difference between gas and water on the coal pore surface, which is decisive for capillary pressure. The evolution of capillary pressure mainly reflects the interactive displacement ability between gas and water (i.e., the ability of water to displace gas during high-pressure water injection and the ability of gas to push water during the later depressurization and extraction process). The microscopic migration patterns of multiphase media in coal pores reflect not only the effects of wettability and capillary pressure but also the influence of the pore network structure on high-pressure water injection and later extraction. The behavioral differences exhibited by the immiscible interface during the displacement of the two multiphase fluids can be attributed to the effect of pore wall wettability. Wettability refers to the tendency of a fluid when in contact with a solid surface and is mainly measured by the contact angle. Generally, when \(\theta_{c}<90^{\circ}\) (wetting angle), the liquid tends to spread on the solid surface, and the solid particles have a high affinity for the liquid. In this case, we consider the particle surface to be wetting. When \(\theta_{c}>90^{\circ}\), the liquid tends to form small droplets on the particles, and the solid has a low affinity for the liquid. At this time, the particles are considered non-wetting (hydrophobic) [44]. This section discusses the impact of wettability on CH\({}_{4}\)-water displacement under the conditions of \(lgCa=-5.79\), \(lgM=-1.95\), and inlet velocity \(v=0.01\) m/s. Figure 8 shows the simulation results of fluid distribution at the breakthrough of CH\({}_{4}\) in coal with contact angles of \(\pi/6\), \(\pi/3\), \(\pi/2\), \(2\pi/3\), and \(5\pi/6\), corresponding to strongly hydrophilic, weakly hydrophilic, neutral, weakly hydrophobic, and strongly hydrophobic conditions, respectively. When the porous media is hydrophilic, i.e., \(\theta_{c}=\pi/6\) and \(\theta_{c}=\pi/3\), the invading CH\({}_{4}\) forms 2-3 main flow channels during invasion, each about 1-2 pore widths wide. Figure 9 displays velocity snapshots at breakthrough and stability in hydrophilic porous media. It shows that at the initial breakthrough, the flow rate is higher, and the fluid tends to follow low-resistance paths. Over time, each main channel develops branching paths, but only a few side channels break through the porous media. When displacement stabilizes, some branching paths formed at breakthrough disappear. This is because they become preferential paths after some branches break through, causing some stagnant branches to flow back into the preferential paths, eventually leading to the disappearance of branching channels. When the porous medium is neutral, i.e., \(\theta_{c}=\pi/2\), five fingering channels with an average width of 2-3 pore widths form. The top and bottom two channels break through the porous media, while the middle fingering channels stagnate. The flow of fluid in the pores is hindered in porous media, forming an uneven fluid pressure distribution. This uneven pressure distribution causes the top and bottom pores to be under greater pressure, making it easier to form fingering channels and break through the porous media. For hydrophobic porous media, i.e., \(\theta_{c}=2\pi/3\) and \(\theta_{c}=5\pi/6\), the width of the fingering channels significantly increases to 4-5 pore widths compared to hydrophilic media. At this time, capillary forces gradually weaken, and viscous forces become the main driving force. Capillary fingering gradually transitions to viscous fingering, and the water-locking effect appears. A lot of elongated and spot-shaped residual water is trapped in the porous media by CH\({}_{4}\), making it difficult to expel. This is consistent with the conclusion drawn by [PERSON]'s work [26] through experiments, which found that viscous fingering is more significant in strongly oil-wet media than in weakly oil-wet or water-wet media.