Identifying groundwater threats from improperly abandoned boreholes

The University of Wyoming has investigated techniques to determine the status of plugged and abandoned wells. Proper abandonment requires 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. Such techniques may eventually be used in wellhead protection programs to determine whether abandoned wells require mitigation, and by enforcement agencies to verify compliance of regulations. In this approach, a down-going acoustic pulse produces reflections at plug boundaries. Up-going reflection energy is detected by surface acoustic sensors and used to estimate plug size and location. Initial experiments used an artificial borehole constructed horizontally on the ground using 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.