Diffusive Mixing Between Hydrogen and Carbon Dioxide: Implications for Underground Hydrogen Storage

The diffusive process between hydrogen (H2) and cushion gas affects the purity of H2 stored in the subsurface porous media. It is essential to understand the diffusive mass transfer and its impact on the migration of H2. Carbon dioxide (CO2) serves as a promising option for cushion gas. However, due to experimental challenges, there has been limited research conducted to quantify the diffusion between H2 and CO2 under reservoir conditions. For the first time, we quantitatively measured the horizontal diffusive process between H2 and CO2 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 CO2. 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 H2.