What is Salt Cavern Storage?

Many people have asked me to explain what salt cavern storage means. It’s a new term that’s popping up in the news, especially when the issue is renewable energy or hydrocarbon storage.  To best explain it, I will break it down into three parts: salt, cavern, and, storage.

Salt
The salt deposits are a naturally occurring formation below the Earth’s surface.  The deposits are located only in certain areas throughout the world.  These deposits are also where the world gets its potash; a key ingredient in fertilizer.  The salt deposits are found in two forms:  salt beds, or salt domes.  Salt beds are more prominent and consist of a thinner layer of salt.  They are usually not as dense as a salt dome nor as deep, for these reasons salt beds are less popular than salt domes for salt cavern construction.  Salt domes are more sought after for they are much thicker and extremely deep, sometimes to 25ft high by a half mile wide, or more.

Cavern
Once a suitable salt bed or dome is found, it needs to be hollowed out into a cavern or basically a very large holding tank.  The cavern walls are impermeable and can withstand extreme pressure.  The caverns can be shaped to be as big as needed and that of which will allow.  How is this man-made cavern created in salt?  In most case, it is by a process of solution mining.  This just means highly pressurized water flow is pumped through a pre-drilled well in the layer of salt that will dissolve the salt.  The dissolved salt is then cycled back up the well and piped off to another location.  

This process brings much of the debate over the construction of salt caverns.  First off, millions of gallons of water must be used to create the cavern, and secondly the location as to where the saltwater goes.  People are concerned it may dry up their water sources or even pollute them.

Storage
Now that we have a large cavern deep in a salt deposit, what do we put in it?  The salt cavern were first used to store oil waste that couldn’t be processed or used, so it was stored for eternity in the caverns and capped off.  But we are finding more and more valuable uses for these caverns.  The biggest is probably natural gas.  

Natural gas can be stored in great volumes for time of peak demand.  Rather than increasing the work load of a power plant and risk power outages, stored natural gas can be leached from the salt caverns as needed.  This will also keep one’s utility bills consistent.  

Next we have is CAES or Compressed Air Energy Storage.  Air is compressed into the salt caverns and released under pressure to power gas turbines for electricity.  It uses the same concept as with the natural gas, in that it is used in times of peak demand, as more of a back up energy source.  Hydropower can create the pressure or more popular wind energy.  Wind farms can create the air to be compressed in the salt caverns, and therefore no wind generated power is wasted.

Finally, salt caverns can store CO2 from big industry.  Though a newer idea, projects are being put together where heavy carbon producing plants are having their CO2 piped into these salt caverns for storage instead of releasing it into the atmosphere.  Governments are rewarding these companies with carbon credits.