Caverns in salt: Drilling and leaching
To construct a cavern, it is first necessary to drill a borehole down to the deepest point of the planned cavern. During the early days of cavern construction, these boreholes were always drilled vertically. But it has been possible in recent decades to also drill directional boreholes – enabling the boreholes to be deviated laterally away from a drill pad.
In Etzel, the horizontal distance between the top of the borehole and the bottom of the hole is around 250 to 300 m. This reduces the amount of space required on the surface.
In both cases, the external pipe is completed with the additional internal pipe strings needed for the precise leaching of the caverns in compliance with the rock mechanical specifications.
During solution mining (leaching), the salt concentration of the injected freshwater or seawater becomes more concentrated as it dissolves the salt. The salt-saturated brine is continuously replaced by fresh water to leach out the cavern in a controlled way from bottom to top. The aim is to produce a cavern with as cylindrical a horizontal cross section as possible, and to create a cavern roof that satisfies the structural specifications. The cavern sump at the deepest level in the cavern fills up with the insoluble residues incorporated in the salt deposit (e.g. anhydrite).