## 3 Experimental Studies
### Summary of Experimental Research  
To study the hysteresis phenomenon of relative permeability under different conditions, experts and scholars have conducted extensive experiments and numerical simulations. According to statistics in Table 2, it has been found that the hysteresis of relative permeability is influenced by various factors, such as rock wettability, permeability, and porosity.  
Permeability, as one of the important properties of rocks, also has a significant impact on the hysteresis effect of relative permeability. Current research indicates that in reservoirs, low-permeability reservoirs compared to high-permeability reservoirs exhibit increased difficulty in gas-water alternating drive with increasing numbers of cyclic periods, leading to a stronger hysteresis effect of relative permeability [13; 65; 70]. A lower \(\rm K_{v}/K_{h}\) ratio (vertical permeability to horizontal permeability ratio) results in earlier water breakthrough during the water injection phase, earlier gas breakthrough during the water injection period, lower ultimate recovery, greater reduction in recovery caused by the hysteresis effect of relative permeability, and stronger hysteresis effect [12].  
Porosity  
The larger the matrix porosity, the larger the geological reserves of natural gas, the more stable the reservoir pressure, the longer the stable production time, and the higher the cumulative gas production. However, the lower the ultimate recovery, the larger the matrix porosity, the greater the reduction in recovery caused by the hysteresis effect of relative permeability, and the stronger the hysteresis effect.  
The impact of fracture porosity and matrix porosity on the ultimate recovery and hysteresis effect of relative permeability exhibits two opposite trends. The larger the fracture porosity, the higher the recovery, and the smaller the reduction in recovery caused by the hysteresis effect of relative permeability, resulting in a weaker hysteresis effect.  
Wettability  
Compared to the water-wet system, the three-phase water relative permeability of the mixed-wet system exhibits less hysteresis during the gas injection cycle. In the water-wet system, the hysteresis effect of gas phase (non-wet phase) relative permeability is more pronounced than that of water phase (wet phase); compared to the drainage process, the wetting phase relative permeability increases during the imbibition process, while the non-wetting phase relative permeability during the imbibition process is lower than during the drainage process.  
Miscibility  
Whether there is miscible flow does not affect the trend of relative permeability curves. Compared to miscible and near-miscible drives, a more significant decrease in gas relative permeability, greater decrease in recovery, and stronger hysteresis effect are observed in immiscible drives [32].  
Cyclic times  
With the increase in gas-water alternating cycle times, the water phase relative permeability continuously decreases as the saturation direction changes from imbibition to drainage, and continuously decreases with increasing cycle counts, resulting in a reduction in the hysteresis effect. Similarly, the gas phase relative permeability decreases continuously during drainage and imbibition cycles. In water and gas injection cycles, early-cycle gas phase relative permeability is higher, and late-cycle gas phase relative permeability is lower and decreases with increasing cycle count. The hysteresis phenomenon gradually decreases with cycle period and eventually stabilizes.