Drilling into the depth
Prior to drilling the cavern borehole proper, a conductor pipe with a diameter of 28” is rammed-in to a depth of approx. 50 m to help guide the drill bit and to stabilise the top of the borehole.
The conductor pipe is then drilled out with a 23” drill bit down as far as the caprock above the salt. An 18 5/8” casing string is then run-in to the bottom of the hole and cemented all the way up to the surface. This anchor casing withstands the lithostatic pressure acting on the borehole, and protects any water-bearing horizons which have been drilled through.
A 17 1/2” drill bit is then used to drill through the salt, followed by the installation of a 13 3/8” / 13 5/8” casing string, which is cemented all the way up to the surface. The sections of this “last cemented casing” are welded together before being run into the hole. The last cemented casing is set in the upper part of the salt so that the lower part of the borehole remains open and uncased.
A 10 3/4” string and a 7” leaching string are then hung into the hole. The 10 3/4’’ string is hung at the height of the planned cavern roof. The 7’’ string is set deeper, almost to the bottom of the borehole, i.e. almost to the base of the planned cavern. Enough space is left between the end of the string and the base of the planned cavern to accommodate the insolubles which fall down to the floor of the planned cavern.