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IDENTIFYING GROUND WATER THREATS FROM IMPROPERLY ABANDONED BOREHOLES

Many abandoned wells, especially wells closed before more stringent environmental rules were established, may have been abandoned without proper plugging. If successful, the technique 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 will produce reflections at each plug boundary. The up-going reflection energy will be detected by acoustic sensors at the surface and used to estimate plug size and location. Initially, an artificial borehole was constructed to evaluate acoustic sensors, data acquisition techniques, and signal processing methods, and allow experiments to be performed in an environment where plug dimensions and locations are know exactly.

The simulated cased borehole was constructed using approximately 200 feet of 5.25 inch well casing. The casing includes cement plugs at each end, and a single cement plug near the center. Acoustic sensors have been installed in each plug to measure vibrations traveling along the casing.

The structure of the recorded geophone signals is very complex, comprising both primary reflections from plug surfaces, and secondary reverberations from energy reflecting back and forth between plugs. To help understand the nature of various reflection events, two different computer modeling programs have been developed to simulate the flow of sound energy within the borehole.

The first program treats the casing as a simple layered medium, and predicts the times and amplitudes of simple reflections. The second program uses finite difference techniques to provide a much more complete simulation of possible wave propagation modes.

During the summer of 1996, seven wells were acoustically tested. Two wells were unplugged water wells, two were plugged without witnessing, and the remaining three were plugged with BLM representatives present to ensure correct plug placement. An effort was made to select wells with relatively simple structure, and four of the five plugged wells meet this criterion.

The wells typically have a 100 foot-deep surface plug, with a second plug located at about 1000-foot depth. The fifth plugged well is more complicated, and has nested surface casings that may produce unpredictable effects in the recorded data. Overall, findings based on the raw data have been mixed. The best results were seen on tests of an unplugged water well, where strong reflections were seen from the 3000-foot-deep well bottom. Acoustic records from the plugged wells have been more challenging to interpret. In some cases, possible reflections are seen from the bottom of the surface plug and the top of the next deeper plug.

However, it is difficult to determine whether a particular acoustic event is an actual plug reflection, or due to other sound sources such as reflections from subsurface strata. Based on these results, the paper will discuss the feasibility of acoustic techniques for plug detection, and makes recommendations for future research.

Key words: borehole, acoustic sensors, reflection events, plug detection