## 4 Applications
### Effect of Relative Permeability Hysteresis on Hydrogen Storage  
Unlike the injection process of carbon capture, utilization, and sequestration (CCUS), hydrogen storage is a cyclic process of injection and production. The cyclical nature of underground hydrogen storage processes makes them relevant to the concept of relative permeability hysteresis. Underground hydrogen storage refers to storing energy in the form of hydrogen in underground reservoirs, which can be salt caverns, aquifers, depleted oil and gas reservoirs, etc. Hydrogen is stored during periods when energy production exceeds demand, and when demand exceeds production, stored hydrogen will be recovered to the surface. Within the range of hydrogen storage conditions, the relative permeability of the hydrogen-water system is not significantly affected by changes in temperature and pressure, as the viscosity of hydrogen changes little. [PERSON] et al. [99] first used a non-steady-state method to obtain relative permeability curves of hydrogen-water systems under various temperatures, pressures, salinities, and rock types. The results showed that due to capillary \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Author**} & \multirow{2}{*}{**Time**} & \multirow{2}{*}{**Model**} & \multicolumn{2}{c}{**Research Target**} \\ \cline{3-4}  & & & **Displacement** & **CO2 Sequestration** \\ \hline [PERSON] et al. [88] & 1981 & [PERSON] & \(\surd\) & \\ \hline [PERSON] et al [89] & 2000 & Larsen and Skauge & \(\surd\) & \\ \hline [PERSON] et al. [26] & 2003 & UTCHEM & \(\surd\) & \\ \hline [PERSON] et al. [14] & 2004 & Land & & \(\surd\) \\ \hline [PERSON] et al. [90] & 2006 & [PERSON], Kilough, [PERSON] and Skauge & \(\surd\) & \\ \hline [PERSON] et al. [91] & 2008 & [PERSON] & & \(\surd\) \\ \hline [PERSON] et al. [92] & 2008 & [PERSON] & \(\surd\) & \\ \hline [PERSON] et al. [32] & 2010 & [PERSON] & \(\surd\) & \\ \hline [PERSON] et al. [93] & 2009 & \(\backslash\) & \(\surd\) & \\ \hline [PERSON] et al. [75] & 2011 & [PERSON] & & \(\surd\) \\ \hline [PERSON] et al. [94] & 2011 & [PERSON] and Skauge & \(\surd\) & \\ \hline [PERSON] et al. [15] & 2011 & [PERSON], [PERSON], Larsen and Skauge & \(\surd\) & \\ \hline [PERSON] et al. [95] & 2014 & Carlson, Kilough and Jargon & \(\surd\) & \\ \hline [PERSON] et al. [68] & 2015 & [PERSON] and [PERSON] & \(\surd\) & \\ \hline [PERSON] et al. [59] & 2016 & [PERSON] & \(\surd\) & \\ \hline [PERSON] et al. [96] & 2017 & [PERSON] and [PERSON] & \(\surd\) & \\ \hline [PERSON] et al. [74] & 2018 & [PERSON], [PERSON], [PERSON] & & \(\surd\) \\ \hline [PERSON] et al. [97] & 2018 & \(\backslash\) & \(\surd\) & \\ \hline [PERSON], [PERSON] et al. [11] & 2019 & [PERSON] and [PERSON] & \(\surd\) & \\ \hline [PERSON] et al. [98] & 2020 & [PERSON] and [PERSON] & \(\surd\) & \\ \hline [PERSON], [PERSON] et al. [99] & 2021 & [PERSON] & & \(\surd\) \\ \hline [PERSON] et al. [56] & 2021 & [PERSON], [PERSON] & \(\surd\) & \\ \hline [PERSON] et al. [57] & 2021 & [PERSON] & \(\surd\) & \\ \hline [PERSON] et al. [83] & 2022 & [PERSON], [PERSON] & \(\surd\) & \\ \hline [PERSON] et al. [100] & 2022 & [PERSON] & & \(\surd\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Relative permeability hysteresis applications with CO2. forces overcoming viscous forces, higher pressures exhibited higher relative permeability of hydrogen. Relative permeability measurements of hydrogen-water system in Berea sandstone, conducted by [PERSON] et al. [101], showed a significant effect of hysteresis on the curves of water and hydrogen relative permeability. [PERSON] et al. 's [102] PNM results indicate that due to the disconnect of hydrogen phase after residual water and secondary imbibition in porous media, the relative permeability values of secondary drainage are lower than primary drainage. However, hysteresis effects remain unchanged with more cycle occurrences. From Table 5, the relative permeability and hysteresis effects in hydrogen-water systems differ from those in other gas and oil-water-gas systems.