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Two factors, safety and utility, are basic in the design of disposal wells. Every means must be taken to insure the safety of the installation so that the environment is protected against inadvertent pollution. Also, the well must be designed for maximum utility so that continued disposal of the waste is insured.
Disposal wells are of two general types--those which are considered openhole completions, and those utilizing casing for their entire depth. Openhole completions
are common in those areas, such as along the Gulf Coast, where only lightly consolidated or unconsolidated sands are the disposal zones. These wells employ gravel-packed screen sections and are generally similar to large-capacity water wells in design. Other openhole completions are made in those areas where the disposal zones are in competent rocks, such as limestone, dolomite, and sandstone, and do not require casing. In places where the disposal liquid may attack the cementation of a sandstone or adversely affect a limestone or dolomite, casing is required for the full depth of the hole. Casing may be either of plastic materials or some of the more costly metals, such as stainless steel, Hastelloy, Carpenter 20, or zirconium.
Tubing and packer requirements vary depending on the nature of the waste stream. Lined tubing is required in almost all wells to avoid excess corrosion. Tubing lining may be either sprayed-on plastic or thin-gage metallic alloys swedged to the base metal. Packers must be made of the same materials as the tubing to insure longevity. Some wells employ hydraulic seals rather than packers. This is a good installation if injection is always under pressure.
Detailed designs of each type of injection well are shown, and in every case, safety of the installation is a paramount consideration.
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