Salt Caverns

There are numerous natural underground cavities, such as caves in limestone formations, which might conceivably be used for the containment of carbon dioxide, although there is always the risk that pressurized gas will leak out 

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The Waste Isolation Pilot Plant (WIPP) is in an excavated salt cavern in southern New Mexico, twenty-seven miles from Carlsbad. The WIPP site is 2,000 yards underground, and defense waste is being placed. There are plans to place there about 6 million cubic feet of material there containing fewer than five million curies of radio activity. 

The main problem of hydrate formation will arise in pipelines transporting natural gas, because gas hydrates are solids and will leave deposits. The solid deposits reduce the effective diameter of the pipeline and can therefore restrict or even clog the flow properties. Furthermore, the formation of condensates, hydrates, or ice may occur in the course of decompression of natural gas stored in natural reservoirs (e.g., in salt caverns). The operation of oil and gas pipelines in the deep sea is significantly complicated by the formation of gas hydrates [1204]. Experience indicates that large gas hydrate plugs in gas and oil pipelines form most actively during the period of an unforeseen long shut-down. In static conditions, three types of hydrate crystals can be formed [1153] ... 

Probably the most important options for geological storage are storage in oil and gas fields, storage in saline aquifers and storage in coal seams. Further, there are options like the storage in abandoned mines or in salt caverns (see Table 6.1). 

Storage in coal seams Storage in salt caverns Storage in abandoned mines Enhanced coal-bed methane production Storage in unminable coal seams... 

Crotogino, F. and Huebner, S. (2008). Energy Storage in Salt Caverns - Developments and Concrete Projects for Adiabatic Compressed Air and for Hydrogen Storage. www.kbbnet.de. 

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. 

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. 

Frequently, hydrates become important in natural gas storage in salt caverns for peak shaving, or seasonal or diurnal volume averaging delivery of gases. The work by deRoo et al. (1983) discusses this process, regarding hydrate formation in high salt concentration, with their data provided in Chapter 6 on methane hydrate inhibited by sodium chloride. 

Storage in porous reservoir is the most convenient way for storing gas in order to meet winter demand (seasonal modulation). The cheaper means of storage is often in depleted fields. In the absence of such structures, however, natural gas can be stored in aquifers or in salt caverns. Examples of natural gas storage in abandoned mines can also be found. Other alternatives have been investigated, such as lined rock caverns. 

Fig. 3 - Schematic cross section of an aquifer converted into UGS 1.2.3 Salt caverns...

Salt caverns have been used to store Liquefied Petroleum Gas (LPG) for a long time, but the technique is relatively recent for natural gas. It was introduced in the United States in 1961, in Saint-Clair County, Michigan. Today, there are around 60 storage facilities of this type world-wide, 27 are located in the US. The number of this type of storage is increasing rapidly (a lot of new projects especially in Western Europe). 

Salt caverns which are also a useful complement to the large porous reservoirs, offer other advantages ... 

Thus the combination of the two types of storage, in porous reservoirs which are generally used to guarantee basic demand to meet seasonal modulation, and storage in salt caverns, which are generally operated to cover peak demand, makes for high withdrawal rates even at the end of the withdrawal period. 

Costs Depleted Fields Aquifers Salt Caverns... 

Unlike storage in reservoirs or aquifers which rely on natural voids in porous and permeable rocks, with storage in salt caverns, the gas is stored in man-made, solution-mined caverns. Geology is only the starting point, and engineers design and construct the project. 

The optimisation studies for solution mining techniques which were performed using these computation tools made possible a considerable reduction in the number of steps and therefore of round trips and echographic surveys. This explains why in the 1960 s, the first 100,000 m3 salt caverns created by Gaz de France had required 10 steps whereas now a 500,000 m3 would only need 5 steps. 

For salt caverns in salt domes, where mining is easier because of the larger available height, the results are not as easily perceptible. 

This process has been successfully used in the US to develop salt caverns for storage at Moss Bluff (Texas) and Egan (Louisiana). 

Even though it may be relatively easy to characterize the short-term performance of a salt cavern facility by a relation directly relating the maximum possible flow rate to the in-situ gas volume and the number of cavities available the same is not true for porous reservoirs where the dynamic aspect (effect of injection/withdrawal profiles) is important. 

The investments required to develop and operate storage facilities represent a major share of the cost of gas supply. Sustained efforts from industrial R D are devoted to the widely used conventional techniques (storage in oil and gas fields, storage in aquifers, and storage in solution mined salt caverns) with the aim to both improve their performances and reduce their costs. Beside the potential of improvement in the performance on the existing sites, the development of new technologies continues to guarantee the consumer security of supply at a reduced cost. 

Increasing Maximum pressure in Salt Caverns", P. Desgree M. Fauveau, Gaz de France, 20th World Gas Conference. Copenhagen, 10 - 13 June 1997. 

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