Increasing interest in the use of the deep subsurface for disposal of industrial waste requires that both the practitioner and the governmental regulatory body be assured that injection is not harming the environment.
There are three principal areas of interest in monitoring subsurface injection systems: (1) the well, (2) the surface equipment, and (3) the subsurface.
Minimum monitoring function for the well requires measurement of wellhead injection pressure and of injection tube-casing annulus pressure; definition of corrosive effects of the waste on the well materials; in some cases, bottomhole monitoring of injection pressure; and the location of a conducfor-insulator interface.
Monitoring of the surface equipment should include records of the injection-pump discharge pressure, fhe rate and cumulative measurement of injected volume, injecta temperature and quality, and the corrosive-erosive effects of the injected stream upon the materials of construction.
Because the real purpose of the monitoring process is to establish that the waste is going where it is intended to go—and remaining there—an examination of the subsurface takes on special importance. The requirements will vary depending on the geographic location, the properties of the waste, the subsurface geology, and the design and construction of the disposal well itself. An occasional monitoring requirement is the drilling of one or more wells to the disposal formation to obtain pressure data and, perhaps, fluid samples. Although there is some purpose for monitor wells of this type where relatively shallow formations are used for disposal, the use of such wells to obtain measurements in deep aquifers may not serve a purpose commensurate with the expense and possible hazards that may result.
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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.