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Bureau of Mines engineers have investigated the applicability
and limitations of the underground disposal of liquid industrial wastes by observing installations at industrial plants, cities, and oil fields. About 10 million b/d of oil-field brines are being injected into formations from which no fluids are withdrawn. In addition, about 175 deep wells are being used by the chemical process industry to inject approximately 1 million b/d of aqueous waste solutions that may be classified in 5 distinct categories: (1) inorganic salt solutions, (2) mineral and organic acids, (3) basic solutions, (4) chlorinated and oxygenated hydrocarbons, and (5) municipal sewage. In many cases, underground disposal is the most economical method for disposal of liquid wastes that are not amenable to surface treatment.
The wells, ranging from 1,000 to 8,000 ft deep, are completed in 4 general types of formations: (1) unconsolidated sand, (2) consolidated sandstones, (3) vugular carbonate rocks, and (4) fractured granite. The hydrology and physical characteristics of the disposal formation often dictate the design of the underground disposal system and govern its operation. Because of the widely diverse parameters, almost every new system presents unique problems of design and operation. For a given rate of waste injection, the wellhead pressure usually depends on the reservoir permeability and fluid pressure. Some wastes can be injected at 20,000 b/d with zero pressure at the wellhead, whereas others require a 1,000-psi wellhead pressure for the same rate of waste injection.
Unconsolidated sands tend to enter the casing and restrict fluid flow. Suspended solids may plug sandstone and sandy carbonate formations that have small pores. Injection into fractured granite under tectonic stress may lead to earthquakes. Thus, each waste-disposal system must be considered separately, although a few general principles of design and operation are applicable to all underground systems.
One primary indicator of well behavior is the injectivity index. It is specific for an individual well, remaining the same as long as the permeability and porosity of the formation do not change. The injectivity index is used to distinguish between plugging and the normal pressure buildup within the formation, and to examine the effectiveness of well-stimulation procedures.
There are many advantages of subsurface over surface methods of waste disposal. Capital investment and operating costs are lower, the surface area required for the plant is less, seasonal temperature variations have less effect on the system, chemical treatment of the waste is minimal, and generally the only physical treatment required is filtration. However, inadequate knowledge of how the waste constituents interact with the subsurface formation imposes a potential for the creation of a severe environmental hazard. The safety hazards of underground waste disposal should receive careful consideration in the planning stages of a waste-disposal well.
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