Special design and construction criteria for cavern storage wells

— Cavern wells can be drilled vertically as stand-alone wells or as directional wells from cluster wellpads. — When drilling directional wells from cluster wellpads, costs are saved in particular for site set-up, relocation at the drilling location involved, piping construction and in all operational phases. On the other hand, slightly more costs are incurred for continuous directional drilling service. However for the final decision it has to be borne in mind that repair works on one cavern well using a workover rig might seriously endanger the storage operation of the other wells or necessitate costly preventive measures. — Both geological knowledge as well as technical evaluations of comparable wells are required for overall optimisation of the drilling procedure, determination of the casing shoe depths, mud usage and cementation. — The dimensions (determination of casing diameters) depend on the leaching time to be adhered to as well as the stipulated production tubing. — The last cemented casing (production casing) should only be installed and cemented after complete exploration of the cavern interval. — During drilling, any stipulated caprock investigations (open-hole measurements) as well as the necessary salt body investigations in the cavern are to be performed to a considerable extent (open-hole measurements, core recovery, in-situ pressure tests). — Production casings are to be fitted with screwed gas-tight joints or with welded joints. — The production casing is always to be designed with respect to external pressure gradients of 0.23 bar/in acting in the salt as well as the empty casing. Externally thicker-walled casings can be used in order to comply with these loads. — The last cemented casing is to be tested for gas-tightness prior to initial gas injection at the very latest. If a gaseous blanket is used, tightness testing has to be performed prior to usage of the blanket. — Casing cementation on all casings is performed as surface cementation via the cementing string. The density is to be adjusted in line with the rock conditions. — Cement slurries with non-shrinking hardened cement are to be used for the casing shoe area of the production casing. The hardened cement must feature a high level of compressive strength and adhesive shear strength. — In order to avoid any bending stress at the leaching strings in the brine area and to ensure unobstructed accessibility of the cavern chimney during gas operation, the defection within the entire cavern chimney and approx. 50 m above the casing shoe should be <1,5°.