Stability and economic evaluation of multi-step horizontal salt caverns with different step distances in bedded salt formations

Abstract

A multi-step horizontal salt cavern (MSHSC) for energy storage has been drawing more attention, using retractable water-injection tubes to mine larger storage spaces between two boreholes. The retract distance per step best determines the shape of the cavern roof, and thus has an important influence on the stability of MSHSCs. This paper discusses the impact of the step distance on the stability and economy of MSHSCs. The geometry of MSHSCs with different step distances is obtained using our previously developed solution mining model. Geo-mechanical models are developed based on the geometry and the geo-conditions of Jintan salt mine in China. Then a series of static creep analyses are performed using FLAC3D software. Results show that smaller step distances mean higher stability risks during long-term operation. As the step distance is reduced, both the volume shrinkage and plastic zone volume increase. In the displacement contours after 30 years of operation, the large displacement zone increases significantly with decreasing step distance. However, the cavern volume and expected revenue increase with decreasing step distance, contrary to the stability parameters. Economic analysis with integrated stability considerations is conducted to give a balanced step distance. Volume shrinkage is considered in the calculation of effective cavern volume. Plastic zone volume is reflected in the space cost of MSHSCs, since it influences the thickness of the cavern top plate and the pillar width between adjacent caverns. Further, a comprehensive indicator of economic cost/expected revenue per unit volume of rock salt versus time is proposed, calculated, and compared. According to the comparison results, a medium step distance of 100 m or 125 m is recommended for the design of MSHSCs in a given salt layer, with the stability and economic considerations best balanced.