## 4 Applications  
### The Impact of Relative Permeability Hysteresis on Recovery  
The application Section will discuss the practical applications of relative permeability hysteresis models in fields such as oilfield development and enhanced oil recovery. This may involve how these models are applied to simulate enhanced oil recovery processes, such as the significant and irreversible hysteresis phenomena of relative permeability observed in WAG (water-gas alternating injection) experiments, and how these observations can help improve the development strategies of oilfields and increase oil recovery rates. Table 3 shows the CO\({}_{2}\) injection studies with consideration of relative permeability hysteresis. The hysteresis effect may be advantageous in reducing water coning and gas coning issues, increasing production in some cyclic operations (such as steam injection), and reducing gas mobility in certain processes (such as water alternating gas or simultaneous water and gas injection projects). During continuous displacement processes, hysteresis effects may significantly reduce the residual oil saturation in some heterogeneous carbonate reservoirs. Hysteresis effects can have either positive or negative impacts on production scenarios. In ultra-low permeability reservoirs, such as tight sandstones, tight carbonates, and shale formations, hysteresis effects in development strategies may be more crucial. **Positive impacts:**  
(1) Flow field control [71; 72]  
Hysteresis effects are often utilized for flow control to preferentially reduce the mobility of one phase compared to another, thereby achieving enhanced sweep efficiency and ultimate displacement efficiency, especially in cases with unfavorable viscosity ratios, reducing gas mobility in gas-water alternating processes. The interfacial effects between gas and liquid phases are employed to selectively retard the gas migration rate. Due to the lower viscosity of water, it tends to preferentially flow into higher permeability channels within the reservoir, selectively reducing the permeability of gas. Due to hysteresis and flow control effects, it becomes much more challenging for gas to displace water from this region, improving the overall sweep efficiency and conformance, particularly in thick oil zones, high-permeability zones. (2) Reduction of water coning [71]  
Hysteresis effects form the basis for techniques aimed at mitigating water coning in heavy oil reservoirs. In situations where the viscosity ratio between many heavy oils and underlying active water is highly unfavorable, rapid water coning occurs, resulting in high water cut and marginal or uneconomic production. In such cases, the presence of a flowing gas saturation seems to preferentially reduce the water-phase permeability rather than the oil-phase permeability. (3) Enhancement of gas sequestration [73; 74; 75]  
Injecting water after gas injection forces many connected gas segments to be disconnected. The hysteresis effect of water-wetting phases capturing non-wetting phases enhances the trapping and fixation of gas. In processes with strong flow reversals, significant hysteresis occurs, making gas-water alternation a potential effective CO\({}_{2}\) sequestration strategy. (4) Gas blocking [13]  
Gas-phase relative permeability may be preferentially affected by selective treatments of non-mixed fluids. Selective injection of water or surfactants into the upper part of high gas oil ratio wells may result in gas being hysteresis-trapped and lead to a temporary or permanent reduction in gas-oil ratio. Upon contact with gas saturation areas, the addition of surfactants can produce a high-viscosity, stable foam system, which will block most of the pores available for flowing gas, once again resulting in a preferential reduction in gas-phase permeability. Foam systems degrade over time due to adsorption (especially in clastic systems) and dispersion, and such treatments are often temporary to a certain extent. (5) Water blocking [13]  
Hysteresis effects can also be used to aid in water blocking.