Diffusive mixing between hydrogen and Carbon dioxide: Implications for underground hydrogen storage

The diffusive process between hydrogen (H 2 ) and cushion gas affects the purity of H 2 stored in the subsurface porous media. It is essential to understand the diffusive mass transfer and its impact on the migration of H 2 . Carbon dioxide (CO 2 ) serves as a promising option for cushion gas. However, due to experimental challenges, there has been limited research conducted to quantify the diffusion between H 2 and CO 2 under reservoir conditions. For the first time, we quantitatively measured the horizontal diffusive process between H 2 and CO 2 without convection interference in a high-pressure optical cell. The Raman spectroscopy is used to monitor the diffusive process in real-time, and the diffusion coefficient is determined based on the measured concentration profiles. We showed that the Fick’s second law with a constant diffusion coefficient describes adequately the observed diffusive process. The resulting diffusion coefficient scales linearly with the reciprocal viscosity of CO 2 . Based on the measured diffusion coefficient, we conducted a numerical study at field-scale. Results suggest that the dispersive mixing plays a role in the purity of produced H 2 . • The diffusive process of H 2 in CO 2 is measured under reservoir conditions. • Constant diffusion coefficient can predict experimental observations. • The diffusion coefficient for the H 2 -CO 2 system is correlated with CO 2 viscosity. • The impact of dispersive mixing on the purity of produced H 2 cannot be ignored.