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Increasing interest in the use of the deep subsurface for disposal of industrial waste requires that both the practitioner and the government regulatory body be assured injection is not harming the environment.
Webster defines the verb "monitor" in the subject context as, "To watch, observe or check upon, especially
for some special purpose," and, "To keep track of, regulate or control." There are three principal areas of interest in monitoring subsurface injection systems: (1) the well, (2) the surface equipment, and (3) the subsurface. The importance of performing the function on active systems should be apparent, but the monitoring requirement for "abandoned" installations can, on occasion, be equally important.
The well structure is usually possessed of a string of cemented surface casing to protect the fresh groundwaters, a fully cemented string of casing to the disposal stratum, and an injection tube to conduct the waste stream to the formation. The minimum monitoring function for the well requires measurement of wellhead injection pressure, injection tube-casing annulus pressure, definition of corrosive effects of the waste on the well materials and, on occasion, bottom-hole monitoring of injection pressure and the location of a conductor-insulator interface. Several techniques are available that are useful to the injector.
The monitoring of the surface equipment should include records of the injection-pump discharge pressure, the 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.
As 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 a special importance. The requirement will vary depending upon 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, sometimes, to obtain fluid samples. Although there is some purpose for monitor wells of this type where relatively shallow formations are used, their employment for measurements in deep aquifers may not serve a purpose commensurate with the expense and possible hazards that may result.
Once the disposal-well system is no longer needed sound practice dictates that the hole be effectively plugged. Although this technique has been well developed for abandoned oil and gas wells, some additional care is required where industrial injection systems are concerned.
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