Background Artificial salt cavern underground helium storage is the optimal approach for achieving long-term, large-scale strategic helium storage and represents an essential solution for establishing an autonomous, controllable helium supply system. The core challenge currently constraining China’s development of salt cavern underground helium storage facilities lies in the sealing capacity of salt rock against high-concentration helium (helium volume fraction ≥70%).

Methods Molecular dynamics simulation was used in this study, a composite model of salt rock slit pore and channel pore was constructed to reveal the influences of temperature-pressure conditions, helium concentration, and pore fluid properties on the occurrence state and diffusion patterns of helium in mixed gas systems.And the variations in helium self-diffusion coefficient and Fick diffusion coefficient under multi-factor coupling conditions were investigated.

Results and Conclusions  The simulation results indicate that helium primarily occurs in the free state in nanopores of salt rocks, with a minority distributed on mineral surfaces in an adsorbed state (heat adsorbed: 2.56-3.05 kJ/mol). This suggests an extremely weak competitive adsorption capacity of helium. When the helium volume fraction reaches 90% in narrow slit-like pores, helium occurs as larger helium clusters, facilitating the underground helium storage. Compared to pure helium, the helium in helium-methane mixed gas systems shows a significant decrease in the self-diffusion coefficient. Moreover, the self-diffusion coefficient of helium gradually decreases with increasing methane proportion, indicating that carrier gas can effectively inhibit the helium diffusion and migration. Besides, fluid properties in micropores in salt rocks serve as an important factor influencing the sealing performance of salt rocks. When pores in salt rocks are saturated with high-salinity formation water, the amount of helium escaping can be almost negligible on the timescale of helium storage facility operation compared to environments with a single gas phase.