Brine caverns

Salt caverns are developed by solution mining, a process (leaching) in which water is injected to dissolve the salt. Approximately 7 to 10 units of fresh water are required to leach 1 unit of cavern volume. Figure 10-190 illustrates the leaching process for two caverns. Modern salt dome caverns are shaped as relatively tall, slender cylinders. The leaching process produces nearly saturated brine from the cavern. Brine may be disposed into nearby disposal wells or offshore disposal fields, or it may be supplied to nearby plants as a feedstock for manufacturing of caustic (NaOH) and chlorine (CI2). The final portion of the produced brine is retained and stored in artificial surface ponds or tanks to be used to displace the stored liquid from the cavern. 

Brine-Compensated Storage As the stored product is pumped into the cavern, brine is displaced into an aboveground brine storage reservoir. To withdraw the product from the cavern, brine is pumped back into the cavern, displacing the stored hquid. This method of product transfer is termed hrine-compensated, and caverns that operate in this fashion remain hquid-filled at all times. Figure 10-191 illustrates brine-compensated storage operations. 

Uncompensated Storage Hard rock caverns and a few bedded salt caverns do not use brine for product displacement. This type of storage operation is referred to as pumpout or uncompensated storage operations. When the cavern is partially empty of liquid, the void space is filled with the vapor that is in equihbrium with the stored hquid. When liquid is introduced into the cavern, it compresses and condenses this saturated vapor phase. In some cases, vapor may be vented to the surface where it may be refrigerated and recycled to the cavern. 

Since salt caverns contain brine and other contaminants, the type of gas to be stored should not be sensitive to the presence of contaminants. If the gas is determined suitable for cavern storage, then cavern storage may not offer only economic benefits and enhanced safety and security salt caverns also offer relatively high rates of deliverability compared to reservoir and aquifer storage fields. Solution-mined gas storage caverns in salt formations operate as uncompensated storage—no fluid is injected into the well to displace the compressed gas. 

Solution mining the caverns represents about 25 - 35 % of the investments. Taking one to several years to complete, it is a long process which requires large water resources (7 - 9 m3 per m3 mined) and which produces just as much brine with a salt concentration of 260 - 310 kg/m3, which is used by chlorine and sodium chemistry or re-injected into the subsoil or even pumped into the estuaries or the sea. 

The upper section of the cavern is then dewatered and used for gas storage operation while solution mining is resumed in the lower. Gas store, in the upper section acts as a blanket for continued solution mining of the lower section. The gas-brine interface is closely controlled during the process and maintained at mid-cavern. 

Dissolution, precipitation, and deposition processes (e.g., those present during the formation of limestone caverns or in the formation of brines or high-salinity waters). 

A suitably thick salt layer is located at sufficient depth below the surface. This layer is drilled and water injected and brine extracted which excavates a cavern. Several stages are usually involved. Sometimes this brine is used for caustic-chlorine production. 

Natural brines flowing through salt-dissolution cavities at Big Salt Plain commonly are saturated with respect to salt. The concentration of NaCl in brine from one well is 337 g/1, and (Na + Cl) constitute 98% of all dissolved solids in the brine (Table I). The large quantity of brine locally present in the cavern system is evident from the fact that this brine is pumped at rates of 2000—40001/min. from wells 12—30 m deep for commercial production of salt by solar evaporation (Johnson, 1970). Not all brines in the area are salt-saturated mixing of brine with near-surface fresh water dilutes the brine in some parts of the salt plains. 

Cavities, caverns, or other openings in salt beds are evidence of past or present dissolution. The cavities commonly are filled with low-salinity to saturated brines that may or may not be under artesian pressure. The cavities may also be partly or totally filled with clay or other sediment deposited from water that had passed through the openings. Cavities may also contain brecciated rock from overlying formations that collapsed into the openings. 

After the sump leaching phase is complete, the system is converted to indirect leaching in order to undercut the deposit. Freshwater is pumped into the cavern via the inner annulus. As it rises to the roof of the cavity, it mixes with the brine. This partly saturated brine then sinks to the bottom of the cavity while becoming more saturated as it dissolves salt from the cavern walls. Brine with the greatest concentration is withdrawn from the bottom of the cavity via the tubing. This leaching process creates a disk-shaped space in the salt deposit, which is further raised vertically during the production phase. 

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