## 1 Introduction and Aims  
Underground hydrogen storage (UHS) is used to store large amounts of hydrogen generated from renewable energy sources (such as wind and solar) to compensate for seasonal fluctuations in the supply and demand of energy [1; 2]. Large amounts of hydrogen can be stored in depleted oil and gas fields, in salt caverns, and in aquifers [1; 3]. Nevertheless, practical experience of underground hydrogen storage is still rare. In the US and UK, hydrogen is currently stored in salt caverns [3; 4; 5; 6], but hydrogen storage in depleted oil and gas fields is still under research and discussion. Depleted oil and gas fields have a huge storage capacity, are well known from former exploration and production, and qualify therefore for hydrogen storage. However, the existing underground gas storages (UGS) are designed for the storage of natural gas, which does not contain hydrogen (or only very low amounts). Because the chemical and physical properties of hydrogen are different to those of methane (CH\({}_{4}\))--the main component of natural gas--the effects of hydrogen on the reservoir rock, cap rock, and storage facilities must be analyzed before injecting hydrogen into these storages [2]. Compared to methane, hydrogen has a higher diffusivity in pure water. The diffusion coefficient for hydrogen is 5.13 \(\times\) 10\({}^{-9}\) m\({}^{2}\) s\({}^{-1}\) and for CH\({}_{4}\) it is 1.85 \(\times\) 10\({}^{-9}\) m\({}^{2}\) s\({}^{-1}\), both in pure water at 25 \({}^{\circ}\)C. The data are given in the database phreeqc.dat, that is provided by the US Geological Survey. Generally, the diffusion process of an aqueous species is described by its diffusion coefficient in pure water (m\({}^{2}\) s\({}^{-1}\)). However, in any porous system, where solids are available, the influence of porosity on the diffusion must be considered by scaling the diffusion coefficient with tortuosity [7], resulting in an effective diffusion coefficient (Equation (1)), where \(d^{\prime}\)m is the effective diffusion coefficient, \(d_{\text{m}}\) is the diffusion coefficient in pure water, and \(\tau\) is the tortuosity. The tortuosity is the ratio of the effective diffusion mass fluxes in a porous medium to ideal diffusion mass fluxes in solutions without a rock matrix. \[d^{\prime}_{\text{m}}=\frac{d_{\text{m}}}{\tau^{2}} \tag{1}\]  
Therefore, the effective diffusion coefficient for methane is 2.35\(-\)2.49 \(\times\) 10\({}^{-10}\) m\({}^{2}\) s\({}^{-1}\) in clayey host rocks at 21 \({}^{\circ}\)C [8]. The effective diffusion coefficient for hydrogen is 3.0 \(\times\) 10\({}^{-11}\) m\({}^{2}\) s\({}^{-1}\) in clayey host rocks saturated with water at 25 \({}^{\circ}\)C [9]. However, there is a lack of data regarding effective diffusion coefficients of hydrogen at underground storage conditions with higher temperatures and pressures [10]. Compared to methane, hydrogen has a lower viscosity. The viscosity of hydrogen at the two-phase boundary at 17 MPa and 350 K is 1.01 \(\times\) 10\({}^{-5}\) kg m\({}^{-1}\) s\({}^{-1}\)[11] and methane fluid at 20 MPa and 350 K is 1.81 \(\times\) 10\({}^{-5}\) kg m\({}^{-1}\) s\({}^{-1}\)[12]. Furthermore, hydrogen has a lower density than methane. At normal conditions, the density of hydrogen is eight times smaller than that of methane and the difference increases by 20-30% under storage conditions [2]. The solubility of H\({}_{\text{2(g)}}\) in pure water is smaller than the solubility of CH\({}_{\text{4(g)}}\). At 25 \({}^{\circ}\)C and 1.0 atm, 7.9 \(\times\) 10\({}^{-4}\) mol kgw\({}^{-1}\) H\({}_{\text{2(g)}}\) and 1.4 \(\times\) 10\({}^{-3}\) mol kgw\({}^{-1}\) CH\({}_{\text{4(g)}}\) dissolve in pure water (data based on phreeqc.dat). The properties of hydrogen--being different from those of methane--could lead to risks when storing hydrogen in depleted gas fields, which were constructed for storing methane (underground gas storage).