Salinity influence on underground hydrogen storage: Insights from molecular dynamics and pore-scale analysis

The main objective of this study is to investigate underground hydrogen storage in H-2/brine systems, considering the different brines and the effect of hydrogen flow. This study is divided into four stages, including i) estimation of hydrogen-brine interfacial tension (IFT) varying type of brine, salinity, and pressure, ii) hydrogen drainage and imbibition experiments in a micromodel device to describe hydrogen displacement and trapping mechanisms, iii) quantification of hydrogen dissolution kinetics and contact angles in H-2/brine/glass microfluidic system, and iv) hydrogen hysteresis cycles. H-2/brine IFT was slightly modified with salinity and type of brine, while in the pressure range evaluated, there were no significant variations. The highest IFTs were found at 74.4, 77.9, and 83.0 mN m(-1), for KCl + NaCl at 0.5, 2.0 and 4.0 M, respectively. Capillary numbers during drainage and imbibition were calculated according to IFTs. The capillary number is decreased with salinity and fell in the order KCl + NaCl > NaCl > KCl. Considering microfluidic experiments, the initial hydrogen saturation after drainage is affected by salinity, type of brine, and pressure, obtaining the increasing trends: KCl + NaCl < NaCl < KCl, 4.0 < 2.0 < 0.5 M, and 60 < 45 < 30 < 10 bar, respectively. Hydrogen dissolution during withdrawal depended on the type of ions, salinity, and pressure. Diffusion coefficients were found in the same line as hydrogen dissolution increased and a competing effect between three factors: i) the diffusion capacity, ii) the average bubble size, and iii) capillary pressure on the dissolution time. Contact angle measurements show pressure-dependent behavior. With a pressure increase from 10 to 60 bar, the contact angles for KCl at salinities of 0.5, 2.0, and 4.0 M increased from 22 degrees, 23 degrees, and 23 degrees-27 degrees, 29 degrees, and 31 degrees, respectively, for drainage stage. In contrast, the contact angles at salinities of 0.5, 2.0, and 4.0 M increased from 36 degrees, 37 degrees, and 39 degrees-44 degrees, 46 degrees, and 50 degrees, respectively, for the imbibition stage. Cyclic injections revealed that KCl brines represent the best scenario for cyclic hydrogen injection in saline aquifers, considering the initial and residual hydrogen saturation and the recovery. After drainage in cyclic injections, hydrogen saturation (Sgi) fluctuated between 26% and 42%. Our findings offer crucial UHS experimental data to advance knowledge of UHS on saline aquifers and aid in determining whether large-scale implementations are feasible regarding H-2 storage, H-2 recovery, H-2 dissolved, and H-2 dissolution times.