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.
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This publication consists of papers based on oral presentations at a symposium of the same name co-sponsored by the United States Geological Survey and the American Association of Petroleum Geologists. A wide range of technical issues are covered, as well as regulatory and liability concerns. Documentation of two areas in Colorado where earthquakes had resulted from subsurface fluid injection set the stage for modern debates regarding possible similar results elsewhere. A wide range of fluid compositions are subject to subsurface waste disposal. The largest volumes are brines separated during the production of oil and gas wells, but acid-water and industrial wastes of all types can be disposed in significant quantities in local areas. Large hydraulic fracture treatments never recover all of the injected fluids, and the chemical additives in the fluid that remains underground can be a concern. The subsurface injection of radioactive waste is a topic for three of the papers. The possible need for sequestration of carbon dioxide was not a significant concern at the time and was not covered, but many of the papers provide insight into the issues related to modern proposals. When fluids are injected under pressure into subsurface aquifers, they interact in numerous ways. The fluids can potentially migrate for long distances and potentially interfere with other uses for the native aquifer fluids. If the aquifer cannot transport all of the fluids away, the buildup in pressure can cause fracturing of the rock. Differences in composition between the injected and native fluids can cause chemical reactions to occur; in some cases these can be desirable in that they can immobilize certain solutes in mineral form. The long-term environmental consequences are a common theme in many of the papers because of the recognition that the disposed fluids would become a permanent fixture in subsurface aquifers and could have long-term consequences for their future utilization.