To ensure success of a subsurface waste-disposal operation, surface pretreatment of the waste water is generally required. Pretreatment can be quite expensive, but it can make the difference between a successful operation and one subject to repeated difficulties and even failure.
Reduction of formation permeabilities and porosity, face plugging, and precipitation and polymerization reactions will all lead to diminished acceptance rates and excessive backpressure levels. Injection compatibility is directly influenced by formation structure, in-terstitial-water properties, and waste characteristics, including particle size of solids, pH, corrosiveness, viscosity, bacterial content, dissolved gases, temperature, and specific gravity.
Each disposal problem and its related solution must be evaluated separately. Basic pretreatment designs vary considerably and are usually tailored to the particular operation. Of basic importance is the minimizing of precipitate-producing reactions and the removal of suspended solids before injection into unconsolidated formations. The latter is less important in vugular or fractured hard-rock areas.
Usually, a pretreatment operation will involve waste storage, separation of oil and/or suspended solids through flotation or gravity means, filtration through coarse sand or fine cartridge and diatomaceous earth, chemical fixation of pH, and treatment to correct for corrosiveness or biologic growths. These procedures are followed by additional storage and final pumping to fhe disposal well.
A thorough chemical and physical analysis of the waste water and the receiving formation will result in an optimum design. Simplicity should be the aim. Although difficult, it may be possible to define and classify the various types of wastes that are deemed suitable for deep-well injection.
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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.