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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.
 
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Gas can be withdrawn from salt caverns at extremely high rates.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.
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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and withdrawn as brine.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be withdrawn later by expansion.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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