|
|
 |
| Salt domes, solution mining, and storage caverns |
In addition to descriptions of the three common types of Domes/Caverns, we have provided downloadable schematics of each.
TYPICAL SALT DOME
There are over 500 known underground salt domes in the Gulf Coast region stretching from Mexico to the Florida panhandle. These domes originate from a thick bed of salt as deep as 4 to 5 miles below the surface. Because salt is less dense or "buoyant" compared to the surrounding sediments and because salt can flow at higher temperatures and pressures, these salt domes push their way up through the sediment layers almost to the surface. These "piercement" salt domes are made of mostly pure salt "halite" and serve as a valuable resource for several industries. A typical salt dome is roughly circular in shape and averages about 1 to 2 miles in diameter. The tops of most domes that are commercially mined, either by solution mining or by "dry" room and pillar mining, are only a few hundred to a few thousand feet below the surface and their base extends downwards 20,000 or more feet below the ground.
SOLUTION MINING CAVERNS
The chemical industry uses salt "sodium chloride" as a basic raw material in many of their manufacturing processes. Salt saturated brine is produced by solution mining the rock salt from inside these underground salt domes. A large diameter borehole is drilled to about 4000 - 5000' deep using standard oil industry drilling methods. A series of pipes or "casings" are cemented into the borehole to provide environmental protection to the surrounding sediments. Next an injection pipe and a production pipe are lowered to the bottom of the borehole. Fresh water is pumped through the injection pipe, the salt is dissolved, and the resulting salt saturated brine is pumped to the surface through the production pipe. A pad oil material, typically " diesel" is pumped into the roof of the salt cavern that is formed in the mining process. This pad oil floats on top of the fresh water and brine and acts to control the mining and protect the cavern roof from mining upwards instead of outwards. Significant geomechanical design work is engineered into these caverns to ensure long term safety and stability.
R.L. Thoms and R.M. Gehle's paper, A Brief History of Salt Cavern Use, is available for download.
STORAGE CAVERNS
Once the caverns are mined out to their desired size and shape, they are ready to be converted to underground storage wells.
One of the most beneficial characteristics of these underground caverns is their ability to store large quantities of hydrocarbon products. Salt caverns are essentially impermeable, meaning no fluid or gas can escape through the surrounding rock salt. This makes them ideal for storing high pressure hydrocarbon products ranging from LPG's (Liquefied Petroleum Gasses: ethane, propane, butane, ect,) to Natural Gas (methane).
Because these hydrocarbon products are lighter than brine, they will float on top of the brine within the caverns. To inject products into the cavern, the products are pumped down the outer annulus of the pipe casings and the brine is displaced up and out of the inner brine pipe. The reverse operation brings product out o f the cavern by pumping in the brine and retrieving out the product.
The size of the caverns average around 300 to 600 feet in diameter and can be up to 2000 - 3000' tall. To help visualize the size of a cavern, picture the Houston Astrodome buried about 3 000' below the ground. The size of the Astrodome will contain about 10 million barrels of volume, whereas these underground salt caverns range in size from 1 or 2 million barrels to over 20 million barrels of storage volume.
|
|
|
|
|
|
|
