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A significant potential for the pollution of fresh-water aquifers exists due to oilfield waterflood operations. The sources of potential pollution are surface spills, a lack of mechanical integrity of injection wells, and improperly plugged wells which are in communication with the injection zone. Surface spills are relatively easy to detect and control. Procedures for checking the mechanical integrity of a properly constructed injection well are available. Making a determination in the absence of good records as to whether or not a well is improperly plugged, providing a conduit for the vertical migration of formation brines from the production zone to shallower fresh-water aquifers, is very difficult.
Electrical surface geophysical methods offer considerable promise in detecting the movement of formation brines into fresh-water aquifers, through improperly abandoned or plugged wells.
An electrical surface geophysical technique, Controlled Source Audio-Frequency Magnetotellurics (CSAMT) has been applied to locate the presence of anomalies resulting from the upward movement of formation brines through improperly plugged wells. The primary objective in a CSAMT survey is to provide apparent resistivity and the phase angle between the electric and magnetic fields over a prospect area. CSAMT has the advantages of excellent lateral resolution, good depth penetration (a kilometre or more) and is relatively inexpensive. The frequency and resistivity of the subsurface control the depth of penetration. The lower the frequency, the deeper the penetration.
A CSAMT survey was run in an oil-producing field in east central Oklahoma, which is currently on waterflood and has many abandoned and apparently improperly plugged wells. The water in the Vamoosa aquifer underlying the study area has a high chloride content. The objective of running the CSAMT survey was to locate suspected low-resistivity anomalies due to formation brines in the vicinity of improperly plugged wells and to attempt to map their extent.
ground-water pollution, surface electrical geophysics, audio-magnetotellurics, oilfield waterfloods, improperly plugged injection wells
Vice president, Geraghty and Miller, Inc., Phoenix, AZ
Geophysicist, Zonge Engineering, Tucson, AZ
Roger Anzzolin, A
Environmental Protection Agency, Washington, DC