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Salt Caverns

Under certain conditions, salt rock of considerable thickness can appear below the surface of the earth in the form of bedded salt layers or salt domes.

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If the salt is of sufficient thickness, caverns can be created for storage pur-poses in those formations. Salt caverns are large cavities in the rock salt formation which are artificially created by solution mining - also known as leaching. In order to create such a cavern by leaching it is necessary to drill and to case a borehole down to the depth of the salt formation. Water is then injected through the borehole, rock salt is dissolved and a brine-filled cavity is created at the lower end of the borehole. The brine produced is, in turn, pumped up to the surface by another string, concentrically installed within the well. In this way caverns can be created with a volume of up to several 100,000 m³. During the leaching period and the subsequent storage operations, the cavern shape is periodically monitored by means of sonar surveys. The stability of the caverns is ascertained by means of geomechanical investigations.

Salt caverns are suitable for storing gas (particularly natural gas) under high pressure as well as liquids that are inert to both water and salt (e.g. crude oil, oil products, fuels, propane or butane). The tightness of salt caverns is a direct result of the plastic creep behaviour of the salt rock. Gas can be withdrawn from salt caverns at extremely high rates. This is why such caverns are particularly suitable for peak shaving purposes.





For the creation of a cavern volume of 1 m³, on average, 8 m³ freshwater has to be injected and withdrawn as brine. This ratio creates one of the major problems that confront solution mining: What do we do with the enormous quantities of brine produced? If the brine cannot be processed locally (e.g. in the production of salt or in the chemical industry) an environmentally friendly alternative is to inject the brine into deep porous or fractured rocks which are already filled with mineralised water and have a tight cap rock cover.

When salt caverns are used for gas storage, the gas is injected into the cavern under pressure. It can be withdrawn later by expansion. However, for the gas storage operation to be carried out, all brine must be removed from the cavern. This is achieved by installing a concentric tube in the well that reaches down to the bottom of the cavern. Gas is then injected through the annulus while simultaneously an equivalent volume of brine is withdrawn through the central tube.

When storing liquid products, the liquid injected through the annulus replaces the brine which has been removed from the cavern through the withdrawal string. Conversely, for product discharge, the same process takes place in the opposite direction. If unsaturated brine is used during product withdrawal, more rock salt is dissolved and the volume of the cavern is increased.

 
 
 
 
 
 
 









































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