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WWRC 97-05o
Identifying Ground-Water Threats from Improperly Abandoned Boreholes


The University of Wyoming has been investigating techniques to determine the status of plugged and abandoned wells. Proper abandonment procedures require that cement plugs be carefully positioned within the borehole to prevent contamination of aquifers by toxic fluids from adjacent rock formations. The plugs seal off aquifer layers and prevent transmission of fluids through the borehole between formations. Many abandoned wells, especially wells closed before more stringent environmental rules were established, may have been abandoned without proper plugging. If successful, the techniques could be used by well head protection programs to determine which abandoned wells require mitigation, as well as by agencies responsible for enforcing well abandonment regulations.

In this approach, a down-going acoustic pulse generated at the surface produces reflections at each plug boundary. The up-going reflection energy is detected by acoustic sensors at the surface and used to estimate plug size and location. Initial experiments used an artificial borehole constructed horizontally on the ground using actual well casing. Computer modeling has been used to help interpret reflection signals. Field experiments were conducted during the summer of 1996 on a variety of plugged and unplugged wells. Initial results have been mixed. In some cases, possible reflections are seen from the bottom of the surface plug and the top of the next deeper plug. However, determining whether acoustic events are actual plug reflections or due to other acoustic sources is difficult.

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