Experimental study on shape expansion and construction efficiency of horizontal hydrogen storage salt caverns in bedded salt

Abstract

Horizontal salt caverns are ideal places for hydrogen storage, utilizing bedded salt fully to increase the storage capacity. However, the shape expansion laws of this new horizontal construction method are not clear, leading to the risks of leakage or even collapse and delaying the construction of the hydrogen storage salt caverns seriously. This paper aims to reveal the shape expansion laws and propose a new strategy to improve the construction efficiency through reduced-scale experiments of horizontal multi-step construction in the laboratory. Results show that at a constant small flow rate (equivalent to 20 m³/h on site), the cavern develops unevenly. A prominent hump-like bulge forms above the inlet, and the vertical well is essentially undissolved. After the retreat, a new hump-like bulge forms above the inlet, and the horizontal section has a saddle-like shape. The vertical well remains undissolved, but the previous bulge still develops. The construction efficiency is only 65.76 m³/day. At a constant high flow rate (equivalent to 100 m³/h on site), the cavern develops evenly. No bulge occurs above the inlet, and the dissolution amount of the vertical well increases significantly. The roof height of the vertical well is even higher than that directly above the inlet. After the retreat, the effect of freshwater upward floating on the shape is essentially negligible. The construction efficiency is as high as 261.36 m³/day. High flow rates enable a large cavern with a regular roof. However, each bulge also dissolves and expands in the next step, ensuring adequate margins by accurately designing and closely monitoring the cavern height is necessary to avoid the roof being dissolved through. When the roof height is close to the extreme height, the proposed strategy of dynamically adjusting the flow rate is recommended. A small flow rate is used initially and then the flow rate is gradually increased while ensuring that the discharged brine concentration is saturated. The shape of this approach is close to that of a constant small flow rate but with improved construction efficiency (in this case it is 101.26 m³/day).