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 assured.
Disposal wells are of two general types—those which are considered open-hole completions and those which are “normal” completions (casing is run to total depth). Open-hole completions are common in those areas, such as along the Gulf Coast, where the disposal zones are in slightly consolidated or unconsolidated sands. These wells utilize gravel-packed screen sections and are generally similar in design to large-capacity water wells. Other open-hole completions are made in those areas where the disposal zones are in competent rocks such as limestone, dolomite, and sandstone that do not require casing. In places where the disposal liquid may attcck the cement of a sandstone or adversely effect a limestone or dolomite, casing is required for the full depth of the hole. Casing may be of either plastic materials or some of the more cosily 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 every case to avoid excessive 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. In some wells, hydraulic seals are used rather than packers. Such an installation is satisfactory if injection is always under pressure. In every case, safety of the injection-well installation is a paramount consideration.