The first factor in considering the feasibility of underground waste disposal is the quality of the waste stream. A practicable method or methods of removing suspended solids must be planned. Equally important is that the effluent be chemically stable, after filtration, under elevated temperature conditions of the injection zone. Compatibility of the waste with the indigenous brine is necessary to avoid plugging. The disposal well is the final filter in the waste-disposal system; it is the nature of filters to become plugged, and a filter several thousand feet underground is difficult to clean and the cleaning process is usually expensive.

Once the suitability of the waste stream for underground waste disposal has been determined, the reservoir must be selected. Existing knowledge of the subsurface gained from oil and gas exploration will provide enough data to plan the well depth at which several probable reservoirs will have been penetrated. Sand parameters measured in the disposal well will permit selection of the most suitable reservoir. The geologic subsurface study will provide information as to the areal extent and thickness of probable reservoirs.

Well design must meet state requirements for protecting surface freshwater sands and confining the waste to the selected reservoir. Drilling and well-completion techniques, including casing and cement selection to meet corrosion protection needs, should all be planned so as to offer maximum protection against failure of any part of the waste system.

The quantity and quality of the waste stream, the type and size of drilling equipment, and the type of contract used are the principal factors affecting the cost of a disposal well. Area experience with drilling conditions and potential problems, together with a good equipment and material-inspection program, will produce surprising cost reducing results.

Operating an underground disposal well properly is just as important to success as good well design and good reservoir selection. Operating personnel should receive careful training in how to handle new waste sources and maintain good instrumentation and records. A dependable underground disposal system should include a standby or alternate well.

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