Underground storage - the understudy
Industrial plants do not generally win awards for aesthetics, and tank terminals are no exception. It is hard to see a future where refineries are hidden underground, but at the same time, the use of rock and salt caverns to store various types of fuels is well known.
Use of the underground per se is nothing new, for living in, for tunnelling to get from A to B in secret, for taking refuge, for secret government bunkers or laboratories, to house museums or private collections, for storage of food, archives, …and yes, of fuel.
There are numerous caverns, of rock or salt, in various locations around the world that have been, or are being, used to house crude oil and liquid gases safely and securely, with the added social and environmental benefits that going underground brings. Crude caverns in particular are becoming more attractive with the attention paid to the need for strategic stocks, good examples being found along the Texas and Louisiana Gulf coast for the US government’s Strategic Petroleum Reserve (SPR).
Canada opened its first underground crude storage cavern in 2003. Caverns storing crude are also to be found in locations including Japan, Korea, Norway, Sweden, France, Zimbabwe and South Africa, and cavern storage projects for strategic reserves are also believed to be underway in India.
Underground storage tanks (USTs) are still largely in the realm of retail outlets, where they are of course the norm, but a number of studies have examined the feasibility of installing bulk size tanks underground in various ways. Some have had success where particular conditions mean going subsurface makes sense but practical applications of bulk USTs are few and far between.
Cavernous capacity Construction of an underground cavern generally comprises of an initial geological investigation; extensive cavern stability analysis; initial rock support design; excavation works; and redesign of rock support as necessary. According to the International Tunnelling Association (ITA), salt formations offer a low cost environmentally secure way to store crude oil for long periods, and it compares the circa $3.50 (€2.6) per barrel cost of storing crude in artificially-created underground salt caverns with the $15-18/barrel cost of using conventional ASTs. Artificially-created salt caverns are made by drilling a well into a salt formation and injecting huge volumes of fresh water, which dissolves the salt, creating a void. The exact dimensions of the cavern can be controlled by controlling the pressure and direction of the water injection. A major benefit of storing crude in a cavern is the natural temperature difference between the top and bottom of the cavern, which keeps crude inside constantly moving and maintains a consistent quality. Geological pressures seal any cracks that might develop in the salt formation so that crude oil cannot leak from the cavern and, providing the site is below the level of the ground water table, oil can be stored directly against the rock surface without any steel or concrete lining. Stored product is extracted from salt caverns by pumping brine into the cavern. Because of density differences, the brine does not mix with the stored product and forces the product out of the cavity. When brine is removed from the cavern, it is stored in specially-constructed brine storage ponds and can be used over and over again, minimising environmental impact.
