Optimum Magnetometer Transect Spacing to Locate Legacy Oil and Gas Wells: Preliminary Results

The purpose of this project was to examine the optimum transect spacing to locate legacy oil and gas wells using an Overhauser magnetometer. Widely known to be a potential environmental hazard, legacy oil and gas wells may act as a conduit for methane and/or deeper subsurface fluids (naturally occurring brines, injected waste fluids, or injected CO 2 ) to the surface or shallow subsurface. Many plugged wells have all surface equipment removed leaving no visible trace at the surface and thus making the environmental assessment of these wells difficult. Using a magnetometer along a set of predefined transects, magnetic anomalies from the metal casing can be detected. In order to assess large numbers of wells, understanding the typical anomaly size is critical to maximize the transect spacing and therefore minimize magnetometer field work time. Here we briefly review the wide range of transect spacings reported in the literature and show the results of five wells with an initial survey grid at two meter spacing. Although there is significant variation in the anomaly size (X, Y, and Z), transect spacing of 20 m was sufficient to identify all buried wells using the method described herein. The anomalies associated with four of the wells ranged from approximately 1000-4000 nanoteslas (nT), while one well anomaly exhibited more than 10,000 nT above background.