Identifying Ground Water Threats from Improperly Abandoned Boreholes

Principal Investigators
R.F. K ubichek, W.P. Iverson, and J.J. Cupal, University of Wyoming

Abstract

Goal: This research will explore the possibility of using sonic pulse propagation, combined with advanced signal processing techniques, to determine the depth o f coherent cement plugs in abandoned wells.

Rationale: Each year many wells are plugged and abandoned throughout the United States. These include water wells, mineral exploration wells, and oil and gas production wells. Many wells penetrate one or more aquifers. The wells also pierce formations containing oil and gas reservoirs, mineral deposits such as uranium and lead, and water contaminated with salt, iron, selenium, sulfates, and radon. The well borehole provides a mechanism for communication of fluids and gasses between formations. When aquifers are involved, this poses a severe pollution threat. For example, if the borehole passes through both an aquifer and a brine-bearing formation, the brine can invade the aquifer and compromise the quality and purity of the water. The problem escalates if the brine layer is pressurized with respect to the aquifer, causing continuous flow of brine into the fresh-water formation. Conversely, water will escape from the aquifer if its hydrostatic pressure exceeds the pressure in other porous layers. Improperly plugged wells can compromise the integrity of the aquifer layer since this natural isolation is destroyed, allowing water to come in contact with these potentially toxic materials.

Approach: In this project, investigators will develop, instrument, and test a borehole scale model. Research will be undertaken to understand wave propagation and plug reflections in the model. Investigators will simulate responses for selected borehole scenarios and evaluate various models and receiver configurations. They will develop a sensor system, analog-to-digital conversion, and portable computer-based analysis system for measuring plug reflections, develop signal processing methods to extract plug information from reflection data, and conduct field tests to characterize performance of the prototype system.

Status: Several preliminary tests to measure noise and signal levels have been conducted on a 900-foot-deep cased well. The signal-to-noise ratio was found to be too large for the planned experiments. An artificial borehole is being developed that will accommodate the experiments. The artificial borehole will consist of 100 to 200 feet of 5.25" steel well casing positioned horizontally on the ground. This setup can be readily extended to any required length. The resulting horizontal borehole will be easily accessible at all points along its length for attaching acoustic sensors. Ten sections of casing have currently been set up on the site. If needed, the horizontal borehole can easily be covered with sand and filled with fluid to better simulate actual borehole conditions. Collaboration with the Bureau of Land Management to locate wells on Federal property that could be used in field tests has begun. A finite-difference modeling software program to simulate the subsurface wavefield is being developed. Geophones are being investigated as sensors to detect low-amplitude reflection energy. Initial experiments on the artificial borehole are promising and show adequate reflection energy to resolve plugs in actual boreholes. This project is in its first year.

Clients/Users: This project will be of interest to agencies responsible for protecting subsurface aquifers. It will also be of interest to the Department of Justice, U.S. Environmental Protection Agency, and U.S. Department of Defense. Private oil and gas exploration companies may also be interested.

Key words: boreholes, aquifers, oil wells, gas wells, cement plugs.

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