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