Soret effect on the mixing of H2 and CO2 cushion gas: Implication for underground hydrogen storage

Abstract

For underground hydrogen storage (UHS), hydrogen (H2) injection and withdrawal will create a temperature difference between the wellbore and the reservoir. The thermal diffusion driven by the temperature gradient (Soret effect) may enhance the mixing between H2 and cushion gas and affects the purity of the produced H2. In this work, the thermal diffusion process of H2 and CO2 under reservoir conditions is quantitatively observed for the first time. Results show that the Soret effect drives H2 accumulation in the high-temperature region, and is influenced by temperature, pressure, and concentration. There is significant concentration difference between the high- and low-temperature sections when minor quantities of gas are injected. From experimental observations, we obtain the Soret coefficients across different physical conditions and propose a correlation for their prediction. Additionally, while thermal diffusion slows down in porous reservoirs, the concentration difference established along the temperature gradient remains unchanged at steady state.