Measurement of H2 diffusivity in water and NaCl brines using the pressure-decay method

Underground hydrogen storage in saline aquifers, salt caverns and depleted reservoirs requires reliable knowledge of hydrogen transport properties in aqueous phases. In this work, H 2 diffusivity was measured in pure water and NaCl brines with molalities of 1 and 3 mol kg −1 using the pressure-decay method at 295.15 K and pressures of approximately 10 and 100 bar. Six experiments were conducted, and the pressure-decay profiles were interpreted using finite- and semi-infinite-domain solutions derived from Fick's second law. The finite-domain model was fitted to the complete selected quiescent pressure records using the diffusion coefficient as the only adjustable parameter, while the semi-infinite solution was subsequently evaluated using the same diffusion coefficients without additional fitting. The finite-domain model reproduced the experimental pressures with average absolute relative deviations ranging from 0.06 to 0.20%. At approximately 10 bar, the mean H 2 diffusion coefficient decreased from 5.99 × 10 −9 m 2 s −1 in pure water to 5.34 × 10 −9 m 2 s −1 at 1 mol kg −1 NaCl and 4.82 × 10 −9 m 2 s −1 at 3 mol kg −1 NaCl, corresponding to reductions of 11 and 20%, respectively. At 96.6 bar, a value of 5.56 × 10 −9 m 2 s −1 was obtained in pure water, suggesting a comparatively weak pressure effect. Finally, the modified Arrhenius-type correlation reproduced the present salinity dependence and the literature-based temperature dependence in pure water with average absolute deviations of 1.8 and 4.96%, respectively. This study confirms that temperature has a stronger influence on H 2 diffusivity than pressure or salinity and demonstrates the applicability of the pressure-decay method under well-controlled, diffusion-dominated conditions.