In the present paper an attempt is made to demonstrate, by means of several selected examples, the present technological status of the establishment of underground storage facilities by leaching cavities in salt, indicating the technical and economical improvements achieved in the:
planning and solution-mining of caverns in saltdomes upon striking salts of varying solubility,
planning and solution-mining of the roof area of a high pressure natural gas storage cavern,
application of processes for achieving high salt concentrations in the initial cavern project phase.
Caverns for storing high pressure natural gas and liquid hydrocarbons can be set up at a favourable cost in comparison to rock mined caverns and surface steel tanks (Fig. 1) and within a relatively short period by leaching a cavity in a salt formation (Fig. 2). To produce caverns a well is sunk in a suitable salt formation and cased until shortly above the planned cavern roof. The brine pipes necessary for the leaching process are lowered into this firmly cemented casing. Normally two concentrically inter-suspended casings are used. A protective medium is lowered into the annulus between the cemented casing and the outer brine string, e.g. a propane/butane mixture delimiting the well area being leached from above. Solution-mining with this installation usually requires two methods.
Using this method freshwater is injected through the inner tube and the brine drained off via the annulus between the inner and outer casing. This solution-mining method has, to name just one, the advantage of allowing the inner casing string to be landed in the well bottom at the commencement of leaching, although the lower part of the injection string can already be buried from the outside due to the precipitation of insoluable solids from the salt within a short leaching time. Nevertheless it is still possible to inject freshwater because the heap of solids - as experience has shown - is loosely distributed resp. permeable. Therefore nowadays the direct solution-mining method is generally applied in the initial phase of cavern establishment. The disadvantage of the direct solution-mining method is the relatively slight salt concentration achievable in the initial phase.
Using indirect solution-mining the freshwater is injected via the annulus between inner and outer casing and the brine drained off via the inner casing. By this method saturated brine is produced with an adequate dwell time in the cavern, which is then reflected in a shorter establishment time for the caverns and lower energy costs, since the water quantity necessary for solution-mining decreases with increasing saturation. This method is very frequently employed after completion of the initial phase, in which cavity must be created for the accommodation of the solids sedimenting from the salt (sump cavern). This is a more complicated technical method demanding more accurate precalculations of the cavity development dependent on solution rates, positions for freshwater injection and brine withdrawal in the cavern as well as the blanket level depth.