Research on the helium seepage mechanism in the strata surrounding bedded salt cavern helium storage and its tightness evaluation

To improve the safety and scale of helium storage, large-scale or strategic helium storage in underground salt caverns can be implemented. Salt caverns leached from bedded salt formations have been considered as potential underground storage sites for helium in China. However, the salt formations are typical lacustrine sedimentary rocks with an interbedded structure, thus the sealing feasibility of such salt caverns must be evaluated before they can be used. To this end, the helium seepage equation in deep strata and permeability evolution function are derived with the help of experimental results and theoretical analysis. Based on these, a helium seepage model considering slippage effect and threshold pressure gradient is proposed to evaluate the sealing properties of salt caverns for helium storage. The analysis results indicate that it is feasible to use bedded salt mine formations for helium storage when the formations permeability parameters and reservoir design parameters meet the long-term tightness evaluation criteria, including permeability, porosity, salt pillar width, salt roof thickness and operating pressure and duration. More importantly, specific recommendations are provided for the operation parameters, design parameters and formation permeability thresholds of salt caverns for ensuring helium storage safety while maintaining stable cavern operations. The threshold permeability of the target salt formations should be approximately 1×10-18 m2 to ensure tightness. The minimum and maximum operating pressure are recommended to be 10 MPa and 18 MPa respectively for 50 years of this helium storage. The salt pillar width should be 300 m, and the salt roof thickness should be 40 m.