Domed salt

Salt Dome Salt mass that rises through sediments because of salt s lesser density. 

Sulfur is commercially recovered from wells sunk into the salt domes along the Gulf Goast of the U.S. Using the Frasch process heated water is forced into the wells to melt the sulfur, which is then brought to the surface. 

The first of these reactions takes place at temperatures of about 150°C, the second reaction proceeds at about 550—660°C. Typical furnaces used to carry out the reaction include cast-iron retorts the Mannheim mechanical furnace, which consists of an enclosed stationary circular muffle having a concave bottom pan and a domed cover and the Laury furnace, which employs a horizontal two-chambered rotating cylinder for the reaction vessel. The most recent design is the Cannon fluid-bed reactor in which the sulfuric acid vapor is injected with the combustion gases into a fluidized bed of salts. The Mannaheim furnace has also been used with potassium chloride as the feed. 

The demand for gas is highly seasonal. Thus pipeline companies economi2e by si2ing production faciUties to accommodate less than the system s maximum wintertime demand. Underground storage faciUties are used to meet seasonal and daily demand peaks. In North America, gas is stored in three main types of underground formations depleted oil or gas fields, aquifers that originally contained water, and caverns formed by salt domes or mines. 

Salt acts as a completely mobile plastic below 7600 m of overburden and at temperatures above 200°C (2). Under lesser conditions, salt domes can grow by viscous flow. Salt stmctures originate in horizontal salt beds at depths of 4000—6000 m or more beneath the earth s surface. The resulting salt dome or diapir is typically composed of relatively pure sodium chloride in a vertically elongated, roughly cylindrical, or inverted teardrop-shaped mass. 

Sulfur constitutes about 0.052 wt % of the earth s cmst. The forms in which it is ordinarily found include elemental or native sulfur in unconsohdated volcanic rocks, in anhydrite over salt-dome stmctures, and in bedded anhydrite or gypsum evaporate basin formations combined sulfur in metal sulfide ores and mineral sulfates hydrogen sulfide in natural gas organic sulfur compounds in petroleum and tar sands and a combination of both pyritic and organic sulfur compounds in coal (qv). 

Salt-Dome Sulfur Deposits. The sulfur deposits associated with salt domes in the Gulf Coast regions of the southern United States and Mexico have historically been the primary sources of U.S. sulfur. These remain an important segment of both U.S. and world sulfur supply. Although the reserves are finite, many are large and voluntary productive capacity ensures the importance of these sources for some time to come. In 1994, the output from the salt domes in the U.S. was about 2.09 million metric tons (21). 

Evaporite Basin Sulfur Deposits. Elemental sulfur occurs in another type of subsurface deposit similar to the salt-dome stmctures in that the sulfur is associated with anhydrite or gypsum. The deposits are sedimentary, however, and occur in huge evaporite basins. It is befleved that the sulfur in these deposits, like that in the Gulf Coast salt domes, was derived by hydrocarbon reduction of the sulfate material and assisted by anaerobic bacteria. The sulfur deposits in Italy (Sicily), Poland, Iraq, the CIS, and the United States (western Texas) are included in this category. 

A typical setting of equipment for a sulfur well and the principles of mining are illustrated schematically in Eigure 1. Eirst, a hole is drilled to the bottom layer of the salt-dome cap rock with equipment of the same type as that used in oil fields. Three concentric pipes within a protective casing are placed in the hole. A 20-cm pipe inside an outer casing is sunk through the cap rock to the bottom of the sulfur deposit. Its lower end is perforated with small holes. Then, a 10-cm pipe is lowered to within a short distance of the bottom. Last and innermost is a 2.5-cm pipe, which is lowered more than halfway to the bottom of the well. 

Solution mining produced nearly 23 million metric tons of salt in 1989 representing more than half of the total U.S. salt production (14). Salt brine is made from bedded salt at more than 18 different locations and from 17 salt domes (15). Bedded salt of the salina formation is the most widely and intensively exploited by solution mining. Enormous reserves of salina salt are available. Cost of solution mining salt is usually less than the cost of salt produced by dry mining. The method is particularly good where salt deposits are deep and dry mining would not be feasible. 

Fig. 2. Typical solution mining operation in a salt dome (9).

An environmental risk in solution mining is surface subsidence. This risk is greatest with embedded salt. No cases of salt subsidence have been reported in mining domes that have been mined according to standard industry approved practice in the United States, but some have been seen in other countries. One side benefit of dome solution mining is use of the cavities later for storage of industrial fluids, chiefly petroleum and natural gas. 

Elemental sulfur in the caprock of salt domes w almost certainly produced 1 the anaerobic bacterial reduction of sedimentary sulfate deposits (mainly anhydrite or gypsum, p. 648). The strata are also associated with hydrocarbtms these are consumed as a source of energy by the anaerobic bacteria, which use sulfur instead of O2 as a t drogen acceptor to produce CaC03, H2O and H2S. The H2S... 

The ingenious process of melting suhlerranean sulfur with superheated water and forcing it to the surface with compressed air was devised and perfected by Herman Frasch in the period 1891-4. Oiiginally designed to overcome the problems of recovering sulfur from the caprock of salt domes far below the swamps and quicksands of Louisiana, the method is now also extensively used elsewhere To extract native sulfiu. ... 

Flowline temperature is also an overpressure indicator. In overpressured zones the formation temperature increases. The flow line temperature gradient (increase in temperature per 100 ft) can increase by 2° to 10° over the normal gradient. However, other effects (salt domes, lithology changes) may also cause gradient changes. 

Salts are sometimes added to drilling muds to obtain certain desired mud characteristics. They can also enter the drilling fluid through contamination by addition of makeup water, formation-fluid inflow, and drilled formations such as salt domes, gypsum or anhydride formations. In freshwater systems, if salt contamination reaches undesirable levels, the following methods should be considered for control. 

This series of prohibitions restricts how wastes subject to LDR requirements are handled. The most visible aspect of the LDR program is the disposal prohibition, which includes treatment standards, variances, alternative treatment standards (ATSs), and notification requirements. Land disposal means placement in or on the land, except in a corrective action unit, and includes, but is not limited to, placement in a landfill, surface impoundment, waste pile, injection well, land treatment facility, salt dome formation, salt bed formation, underground mine or cave, or placement in a concrete vault, or bunker intended for disposal purposes. The other two components work in tandem with the disposal prohibition to guide the regulated community in proper hazardous waste management. The dilution prohibition ensures that wastes are properly treated, and the storage prohibition ensures that waste will not be stored indefinitely to avoid treatment. 

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