Salt caverns are considered in several European countries as a promising tool for storing hydrogen. These caverns act as a buffer between electrolyzers converting "green" or "blue" hydrogen and a hydrogen distribution grid. These caverns must be tight. Tightness results from the properties of the rock formation, the design of wellbore completion, the quality of the cementing job and steel equipment, and the pressure selected for storage operation. Much information and experience are available from the 2000+ salt caverns used worldwide for hydrocarbon storage. Generally speaking, salt permeability is exceedingly small. However, in-situ tests proved that the overall cavern permeability experiences a significant increase when fluid pressure at cavern depth is larger than 80-85 % of the geostatic pressure. Several incidents proved that breaches or conduits can be created between a cavern and a neighboring cavern, or between a cavern and the boundaries of the salt formation, through Anomalous Zones.
However, as in most pressure vessels, it is the "piping" (the access well) that most often is the weakest point. Several incidents are described in this paper. Most of them are due to the presence of a single casing between the stored product and the rock formation.
In this context, tightness tests are mandatory. It is suggested to test the wellbore after drilling and to test the cavern before commissioning. The Nitrogen Leak Test, a high-resolution measurement technique, has become a standard. It consists of filling the annular space with pressurized nitrogen, setting the gas-brine interface below the last casing shoe, and tracking this interface with a logging tool. The mass of gas is very small when compared to brine mass, and accurate measurement of gas-mass changes during the test is possible. A complementary interpretation method, based on the analysis of wellhead-pressure evolutions during the test rather than on interface tracking, can be used. In the case of hydrogen storage, it is proposed in this paper to test the well with nitrogen and hydrogen successively, and to set the gas-brine interface at various depths to check the tightness of different parts of the well.